Research ArticleCELL BIOLOGY AND SIGNAL TRANSDUCTION
Open Access

Propidium Iodide Competes with Ca2+ to Label Pectin in Pollen Tubes and Arabidopsis Root Hairs

Caleb M. Rounds, Eric Lubeck, Peter K. Hepler, Lawrence J. Winship

Published September 2011. DOI: https://doi.org/10.1104/pp.111.182196

Abstract

We have used propidium iodide (PI) to investigate the dynamic properties of the primary cell wall at the apex of Arabidopsis (Arabidopsis thaliana) root hairs and pollen tubes and in lily (Lilium formosanum) pollen tubes. Our results show that in root hairs, as in pollen tubes, oscillatory peaks in PI fluorescence precede growth rate oscillations. Pectin forms the primary component of the cell wall at the tip of both root hairs and pollen tubes. Given the electronic structure of PI, we investigated whether PI binds to pectins in a manner analogous to Ca2+ binding. We first show that Ca2+ is able to abrogate PI growth inhibition in a dose-dependent manner. PI fluorescence itself also relies directly on the amount of Ca2+ in the growth solution. Exogenous pectin methyl esterase treatment of pollen tubes, which demethoxylates pectins, freeing more Ca2+-binding sites, leads to a dramatic increase in PI fluorescence. Treatment with pectinase leads to a corresponding decrease in fluorescence. These results are consistent with the hypothesis that PI binds to demethoxylated pectins. Unlike other pectin stains, PI at low yet useful concentration is vital and specifically does not alter the tip-focused Ca2+ gradient or growth oscillations. These data suggest that pectin secretion at the apex of tip-growing plant cells plays a critical role in regulating growth, and PI represents an excellent tool for examining the role of pectin and of Ca2+ in tip growth.

The apical wall of tip-growing cells participates directly in the process of growth regulation (McKenna et al., 2009; Winship et al., 2010), yet few methods permit monitoring the wall properties of living cells. Despite this, several recent studies have enhanced our understanding of the apical cell wall. Chemical analyses of isolated pollen tube wall material have revealed a complex mixture of pectic polysaccharides with regions comprising long sequences of polygalacturonic acid. Important patterns of pectin methoxylation have been detected using immunocytochemical approaches, but these are limited to fixed cells (Dardelle et al., 2010). In a recent study, Parre and Geitmann (2005) used microindentation to show significant correlations between wall strength and growth rate. None of these techniques allow for easy investigation of the cell wall during growth.

In an earlier study, we found that propidium iodide (PI) vitally stains pollen tubes of lily (Lilium formosanum) and tobacco (Nicotiana tabacum) and in particular reveals with great clarity the thickened apical cell wall (Fig. 1; McKenna et al., 2009). In addition, the apical PI fluorescence oscillates and in lily pollen tubes correlates tightly with oscillations in wall thickness measured by differential interference contrast (DIC) optics. Finally, these studies indicated that the PI fluorescence predicted cell growth rates with high confidence, suggesting that PI binding may provide useful information about the physical and chemical properties of the cell wall.

Figure 1.
Figure 1.

PI fluorescence and growth rate oscillate in lily pollen tubes (A and B), Arabidopsis root hairs (C–E), and Arabidopsis pollen tubes (F and G). A, The top panel shows a DIC image of a lily pollen tube, and the bottom panel shows PI fluorescence of the same tube. The PI fluorescence is pseudocolored, with white representing high signal and blue representing low signal. Bar = 10 μm. B, Growth rate (blue) and PI fluorescence (red) are plotted on a line graph. Both oscillate with the same period but with different phases. C, DIC image (top panel) and PI fluorescence image (bottom panel) of an Arabidopsis root hair. Bar = 10 μm. D, Two PI fluorescence images of the same root hair focused on the apex representing peak (top) and trough (bottom) PI signals. Bar = 5 μm. E, A line graph showing the growth rate (blue) and peak PI fluorescence at the apex (red) for the same root hair shown in C and D. F, The top panel shows a DIC image of an Arabidopsis pollen tube, and the bottom panel shows PI fluorescence of the same tube. The PI fluorescence is pseudocolored, with white representing high signal and blue representing low signal. Bar = 5 μm. G, Growth rate (blue) and PI fluorescence (red) are plotted on a line graph. Both oscillate with the same period but with different phases. The growth rate between individual 3-s frames was smaller than the pixel size for our optics in both Arabidopsis cell types; to remove the noise this generated, a four-image (pollen) or five-image (root hair) running average is shown. A.U., Arbitrary units.

PI is commonly used to visualize plant cell walls by wide-field fluorescence and confocal microscopy (Fiers et al., 2005; Tian et al., 2006; Estevez et al., 2008) and to select viable cells during cell sorting (Deitch et al., 1982; Jones and Senft, 1985). A positively charged phenanthridine derivative, the propidium ion stains cell walls but does not pass through the intact cell membranes of living cells. It readily diffuses into dead cells and forms highly fluorescent complexes by intercalation between base pairs of double-stranded nucleic acids, thus acting as an excellent indicator for cell vitality (Hudson et al., 1969). Binding to cell walls presumably occurs by a different mechanism, since it is not accompanied by the dramatic increase in fluorescence and shift in absorption and emission maxima observed when PI binds to nucleic acids. The mechanism of PI binding needs further exploration, as does the potential for broader use in other tip-growing plant cells.

In this report, we test two hypotheses: first, that PI stains other tip-growing cells with pectin-containing cell walls; and second, that PI and Ca2+ bind to the same sites in these walls. This binding would occur through the interaction of partial positive charges caused by localized deficits in π-orbital electrons associated with three of the four nitrogen atoms of PI (Luedtke et al., 2005) coordinating with negatively charged carboxyl and hydroxyl groups on homogalacturonans (HGs), as has been suggested in Oedogonium bharuchae (Estevez et al., 2008).Our findings indicate that both hypotheses are satisfied. Notably, oscillatory changes in apical PI fluorescence occur and are observed to anticipate oscillations in growth rate in Arabidopsis (Arabidopsis thaliana) root hairs and Arabidopsis pollen tubes. In addition, competition studies indicate that PI and Ca2+ bind to the same sites in cell walls. Supporting these studies, we demonstrate that pectin methyl esterase (PME) creates more sites for PI binding, presumably by demethoxylating HGs as they are secreted, and that pectinase reduces PI fluorescence dramatically. However, unlike other pectin-binding dyes, PI does not block Ca2+ channels at the concentration used in live cell studies, nor does it alter oscillatory growth characteristics. Our findings provide evidence that PI may be employed as a quantitative measure of Ca2+-binding sites and thus may have use as an indicator of the degree of cross-linking of HGs and of cell wall extensibility.

RESULTS

Apical PI Labeling of Arabidopsis Root Hairs and Pollen Tubes Oscillates

Because PI vitally labels lily and tobacco pollen cell walls and predicts growth rate, we asked whether a similar phenomenon existed in two other well-studied tip-growing plant cells: the Arabidopsis root hair and pollen tube. Monshausen et al. (2007) have shown that root hair growth rate oscillates, and they speculated that changes in cell wall properties lead to these oscillations. The specific composition of root hair cell walls remains unknown (Knox, 2008), although it is clear that HGs are important to growth (Diet et al., 2006) and that arabinogalactan proteins, frequently associated with pectin deposits, localize to the tip of maize (Zea mays) root hairs (Šamaj et al., 1999).

We germinated Arabidopsis seeds on vertically oriented agar pads affixed to microscope slides. Before imaging, we added water with or without supplemental PI. Root hairs displayed a markedly different distribution of PI fluorescence from pollen tubes (Fig. 1, A, C, and D; Supplemental Movies S1 and S2). In pollen tubes, the shanks are fluorescent, but the predominant labeling is at the cell apex. It is this apical fluorescence that has been shown to oscillate and predict growth (Fig. 1, A and B; Supplemental Movie S1; McKenna et al., 2009). In root hairs, in contrast, the predominant labeling occurs along the shanks. Although the tip is fluorescent, it is much dimmer than the rest of the cell wall (Fig. 1, C and D; Supplemental Movie S2). Also in root hairs, the DIC image shows no dramatic differences in wall thickness at the apex versus the shank. Nevertheless, ultrastructural studies of root hairs show a thick cell wall at the apex, although it is unclear how different the thickness is from the wall of the shank (Galway et al., 1997).

Using time-lapse recordings of PI fluorescence and DIC images of root hairs, we examined changes in the PI fluorescence at the root hair tip and compared these with the growth rate (Fig. 1, D and E). Notably, we observed distinct rhythmic fluctuations in the apical fluorescence (Fig. 1, D and E). Root hairs grow more slowly than lily pollen tubes; thus, when collecting images at a rate of one per 3 s, the changes in growth are less than the resolution of the microscope, resulting in growth rate data that are noisy. We averaged the growth rate over five successive frames to correct for this high-frequency noise. The resulting plots reveal a clear oscillatory pattern to the growth rate (Fig. 1E, blue line) and an equally clear and dramatic oscillatory pattern in the apical PI fluorescence (Fig. 1E, red line). While the growth rate period and PI fluorescence period are similar, they are out of phase.

We next investigated the fluorescence pattern of PI in Arabidopsis pollen tubes. Several lines of evidence demonstrate the presence of pectin at the Arabidopsis pollen tube tip. PME mutants have been identified that show significant fertility or growth deficits (Jiang et al., 2005; Tian et al., 2006). Furthermore, transcriptome analysis demonstrates that pectin-related enzymes are up-regulated substantially in growing pollen tubes (Honys and Twell, 2003). Arabidopsis pollen tubes exhibit more erratic growth rate changes than lily pollen tubes and Arabidopsis root hairs. Roughly 33% of the pollen tubes we imaged sustained growth rate oscillations over several minutes (n = 14; Supplemental Fig. S1; Supplemental Movie S3). The pattern of PI fluorescence in Arabidopsis pollen tubes is remarkably similar to that of both tobacco (McKenna et al., 2009) and lily pollen tubes (Fig. 1, compare A and F). All show higher signal at the tip and a more uniform signal along the shank. We examined the changes in this fluorescence over time and compared it with growth rate changes (Fig. 1G). Both the growth rate and the fluorescence oscillate, although both signals are more erratic than in root hairs and lily pollen tubes.

PI Fluorescence Oscillations Precede Growth Rate Oscillations in Arabidopsis Root Hairs and Pollen Tubes

Given the oscillations in PI fluorescence and growth rate in Arabidopsis root hairs and pollen tubes, we felt it was important to explore their phase relationship. For this, we performed cross-correlation analysis to evaluate whether these two observed oscillatory phenomena were correlated as they are in lily and tobacco pollen tubes (McKenna et al., 2009).

We collected growth rate and corresponding PI fluorescence data from 10 growing root hairs. Growth rates varied from as slow as 17 nm s−1 to as fast as 28 nm s−1, with an average of 22 ± 1.3 nm s−1 (n = 10). Similarly, oscillatory period varied from 72 s to as much as 104 s, or roughly 30% of the mean growth rate of 92 ± 2.0 s (Table I). Cross-correlation analysis revealed that the PI fluorescence peaks precede the growth rate oscillation peaks by −112°. These data are remarkably similar to the offsets of −99° for lily and −124° for tobacco pollen tubes (McKenna et al., 2009).

View this table:
Table I. Cross-correlation analysis between growth rate and maximal PI fluorescence was performed for 10 Arabidopsis root hairs and pollen tubes

The data show that maximal PI fluorescence leads growth by 112.3° ± 3.9°.

For Arabidopsis pollen tubes, the data are less consistent. Growth rates (Supplemental Fig. S1; Supplemental Table S1) ranged from 29.8 nm s−1 to as fast as 65 nm s−1, with a mean rate of 52 ± 7 nm s−1 (n = 5). The oscillation period for growth rate was surprisingly similar to that in both lily and tobacco pollen tubes, with an average of 32 ± 4 s. Cross-correlation analysis showed that peaks in the PI signal preceded growth rate peaks by −47° ± 8°.

We express the phase shift in degrees or fractions of a single cycle, where one complete cycle is 360°, rather than in an absolute value in seconds (McKenna et al., 2009). Support for this decision appears in Supplemental Figure S2, which shows comparisons between growth rate and the phase shift in seconds as well as between the growth rate and period in Arabidopsis root hairs (Supplemental Fig. 2, A and B). The changes in period do not correlate with changes in growth rate; neither does the phase shift in seconds. However, the longer the period, the longer the phase shift in seconds (Supplemental Fig. 2C), indicating that maximum PI staining consistently occurs at a well-regulated fraction of a cycle before the peak in growth rate, regardless of the actual rate.

Figure 2.
Figure 2.

PI fluorescence decreases with increasing Ca2+ concentration. A, Arabidopsis roots incubated in 6.4 μm PI in water with increasing amounts of Ca2+. B and C, Fluorescence of PI in different concentrations of PI and Ca2+ (B) or PI, Mg2+, and Ca2+ (C). In B, squares represent 100 μm Ca2+ (dashed line), triangles represent 10 μm Ca2+ (dotted and dashed line), and circles represent 1 mm Ca2+ (solid line). In B and C, regression lines represent Hill plots. In B, fluorescence depends on the concentration of Ca2+. Decreasing Ca2+ results in more fluorescence at a given PI concentration. Each point represents the average of at least 20 pollen tubes, and error bars represent se. In C, triangles represent 10 μm Ca2+ (dotted and dashed line), squares represent 100 μm Ca2+ (dashed line), diamonds represent 10 μm Ca2+ + 90 μm Mg2+ (dotted line), and circles represent 10 μm Ca2+ plus 1 mm Mg2+ (solid line). A.U., Arbitrary units.

PI Fluorescence Is Modulated by Ca2+ and Mg2+

PI has long been used as a root cell wall stain, although the mechanism of action is unclear (Fiers et al., 2005; Tian et al., 2006; Estevez et al., 2008). Since the PI molecule carries two positive charges, we hypothesize that PI is binding to the same sites in the cell wall as Ca2+. While there are no published studies on the electronic structure of PI, we can look to investigations of ethidium bromide, which contains an identical phenanthridine core. X-ray crystallography, UV/visible and IR absorption, fluorescence emission, and NMR spectroscopy studies (Luedtke et al., 2005) show that the carbon and hydrogen atoms of the phenanthridium structure have relatively high electron densities, while nitrogen atoms are electron deficient, resulting in three positively charged regions in the molecule spaced 6 to 7 Å apart. We reasoned that if PI were interacting with the negative carboxyl groups in HGs, as our pollen tube and root hair data suggested (McKenna et al., 2009), then Ca2+ and Mg2+, two ions that interact with negative charges in the cell wall, might affect PI fluorescence. In this scenario, if we increased the Ca2+ or Mg2+ in solution, we should see a corresponding decrease in PI fluorescence. Similarly, an increase in PI concentration should have less effect on the observed fluorescence in a solution with higher Ca2+ than in one with relatively lower Ca2+.

As a first step in investigating this hypothesis, we transferred Arabidopsis roots that had been grown on Hoagland medium in vertically oriented plates to water supplemented with 6.4 μm PI and increasing concentrations of Ca2+ (Fig. 2A). The microscope and exposure settings were kept constant and the look-up table is identical for each image, yet clearly, as the Ca2+ concentration increased, the PI fluorescence decreased markedly. These images were collected from above the expansion zone of the roots. The root hairs in the 100 mm image are too dim to visualize, whereas in the 1 mm image, they are shorter. The images were taken within 20 min of immersion in PI, so a growth effect does not account for the difference in root hair density. These results suggested that Ca2+ and PI compete for the same sites in the cell walls.

To further characterize this interaction, we chose to work with lily pollen because controlling growth conditions is more efficacious and larger numbers are easier to gather, compared with root hairs or Arabidopsis pollen. We grew pollen tubes in our standard medium and then prepared them for imaging. We then supplemented the growth medium with six different concentrations of PI and three of Ca2+. To assess fluorescence, we measured a defined area at the tip of the pollen tube and subtracted background fluorescence. Figure 2B shows that in a low-Ca2+ growth medium of 10 μm (triangles), PI fluorescence is strongly dependent on concentration. In contrast, at a higher Ca2+ concentration (1 mm), the fluorescence increases only slightly as PI concentration is increased (Fig. 2B, circles). The standard solution (100 μm Ca2+) shows an intermediate response (Fig. 2B, squares).

To investigate whether increasing concentrations of Mg2+ would alter PI fluorescence, we performed the same experiment outlined above with the following ion concentrations (Fig. 2C): 10 μm Ca2+ (triangles), 100 μm Ca2+ (squares), 10 μm Ca2+ + 90 μm Mg2+ (diamonds), and 100 μm Ca2+ + 1 mm Mg2+ (crosses). As expected, the medium supplemented with 10 μm Ca2+ showed the most dramatic change in PI fluorescence as PI concentration increased. In keeping with our hypothesis, if Ca2+ and Mg2+ were both competing with PI for the same sites in the cell wall, we saw that 100 μm Ca2+ and 10 μm Ca2+ + 90 μm Mg2+, which contain the same molarity of bivalent cation, showed the same response. When the Mg2+ concentration was increased to 1 mm, the PI fluorescence showed the same response visible when 1 mm Ca2+ was added (compare Fig. 2B, circles, with Fig. 2C, crosses).

Analysis of the dependence of fluorescence on Ca2+, Mg2+, and PI concentrations by nonlinear least-squares fitting showed that the best-fit model was a Hill equation with a Hill coefficient between 1.8 and 2, consistent with previously reported cooperative binding of Ca2+ to pectins during gelation (Garnier et al., 1994). Although cooperative binding kinetics usually is evidence of a conformational change during the sequential binding of ligands, in the case of linear association of HGs we may be seeing the effect of the filling and unfilling of sequences of Ca2+-binding sites (Michel, 2007). Linear arrays of eight to 15 GalUA subunits, as described in the modified egg-box model for pectin Ca2+ binding (Braccini and Pérez, 2001; Vincent and Williams, 2009), are thought to result from the action of plant PME on esterified HGs and constitute the main binding regions for Ca2+ ions.

PI Growth Inhibition Can Be Ameliorated with Ca2+ But Not Mg2+

High Ca2+ concentrations (greater than 10 mm) cause pollen tube growth to arrest (Holdaway-Clarke et al., 2003). Although similar experiments have not been published with regard to Mg2+, we expected that high concentrations would interfere with growth through binding to carboxyls in the HGs. We reasoned that this would also be true of PI. Furthermore, Ca2+ and Mg2+ might be able to partially offset growth inhibition at high PI concentrations.

We germinated lily pollen and then transferred them to multiwell plates containing concentrations of PI from 0 to 120 μm. The cells were allowed to recover for approximately 10 min and then imaged once. After 20 min, we imaged them again. Average lengths for more than 40 pollen tubes were determined for each time point and concentration, and then the mean change in growth was determined (Fig. 3A). In 100 μm Ca2+, our standard lily growth medium (squares), pollen tubes grew very close to the control growth rate up to 60 μm PI, where they grew at 70% of control. At 120 μm PI, the pollen tubes showed almost no growth. When the Ca2+ in the medium was raised to 1 mm (circles in Fig. 3A), cells grew at 70% of the control growth rate up to 120 μm PI. Conversely, when grown in 10 μm Ca2+, pollen ceased growing with 60 μm PI in solution. That is, the magnitude of the growth inhibition caused by PI depends on the Ca2+ concentration.

Figure 3.
Figure 3.

PI growth inhibition depends on Ca2+ concentration (A) but not Mg2+ (B). Increasing concentrations of PI inhibit pollen tube growth. This can be mitigated by increasing the concentration of Ca2+ or exacerbated by decreasing the concentration. In A, squares represent 100 μm Ca2+ (dashed line), triangles represent 10 μm Ca2+ (dotted and dashed line), and circles represent 1 mm Ca2+ (solid line). Regression lines in A represent sigmoidal plots. Each point represents the average of at least 40 pollen tubes, and error bars represent se. B shows a similar experiment performed with Mg2+. An increase in Mg2+ concentration does not reverse PI inhibition, although a dramatic decrease in growth rate can be seen at the higher Mg2+ concentration.

We do not see this effect in medium supplemented with Mg2+. At 100 μm Mg2+, 60 μm PI inhibits pollen tube growth approximately 50%, whereas pollen tubes in 30 μm PI grow as well as pollen tubes in standard medium without PI (Fig. 3B). Increasing the concentration of Mg2+ in solution to 1 mm inhibits growth in 30 μm PI, suggesting that Mg2+ itself has an inhibitory effect. When the PI concentration is increased to 60 μm, no additive effect is seen (Fig. 3B). The fluorescence data (Fig. 2) presented above suggest that all three ions are competing for the same binding sites on HGs. As Mg2+ or PI concentrations increase, there is a detrimental effect on growth as they bind to carboxyls but do not cross-link the HGs. These data support our hypothesis that PI is binding to the same sites in the cell wall as both Ca2+ and Mg2+.

PI Does Not Alter Oscillatory Behavior

The oscillatory growth of the lily pollen tube provides an excellent tool for ordering the processes that regulate tip growth (Holdaway-Clarke and Hepler, 2003; Moreno et al., 2007; McKenna et al., 2009; Rounds et al., 2010). Recent work has shown that during oscillatory growth in lily pollen tubes, the apical cell wall thickness increases before growth rate peaks (McKenna et al., 2009). Given our data suggesting that PI at high concentrations inhibits pollen tube growth, we investigated whether PI altered growth oscillations.

We grew lily pollen tubes in our standard growth medium supplemented with concentrations of PI ranging from 0 to 80 μm in 20 μm increments. Our bulk growth studies (Fig. 4A) show that at 80 μm PI in 100 μm Ca2+, we would expect significant growth inhibition in the entire population. To compare the effect on oscillations, we chose pollen tubes that were growing at 50 nm s−1 or greater and had appreciable clear zones; we selected for healthy tubes, thus excluding those that were not oscillating. We collected the growth rate data for each pollen tube and performed a fast Fourier transformation to generate a periodogram using R (see “Materials and Methods”). This allowed us to examine the oscillatory components for each pollen tube and determine the predominant period. As shown above (Supplemental Fig. S1) for root hairs, growth rate does not correlate with period (Fig. 4A) in lily pollen tubes. The growth rates vary between just over 50 nm s−1 to as fast as 250 nm s−1, and the periods vary between 20 and 100 s. No trend is observed, as an attempted linear model gives an r2 value of 0.16. This suggests that the oscillatory period and growth rate are not related. Similarly, when PI concentration is plotted against average period, no trend is apparent (r2 = 0.07; Fig. 4B). PI affects average growth rate as concentrations increase in large pollen tube populations. If one examines growing pollen tubes within this population, PI does not directly affect growth rate oscillations.

Figure 4.
Figure 4.

Increasing PI does not alter growth oscillations. Pollen tubes were grown in LPGM and then plated, and PI was added. Time series from growing tubes with appreciable clear zones were collected, and the growth rates and periods were collected. A, The entire data set at all concentrations shows no significant correlation between growth rate and period. Plus signs represent no PI, squares represent 10 μm PI, diamonds represent 20 μm PI, triangles represent 40 μm PI, and white circles represent 80 μm PI. r2 = 0.16. B, The average period is plotted against the increasing PI concentrations. Error bars represent se. n > 8 for each concentration. No linear correlation is seen, as the r2 value is 0.07.

PME Increases While Pectinase Decreases Binding Sites for PI

PME is secreted by pollen tubes at the cell apex, where it demethoxylates HGs (Bosch et al., 2005; Bosch and Hepler, 2005; Parre and Geitmann, 2005). It has already been demonstrated that addition of PME causes a dramatic increase in the cell wall thickness at the apex (Bosch et al., 2005; Parre and Geitmann, 2005). As there is no evidence that exocytosis has ceased, the wall thickening also may be attributed at least in part to continued deposition of wall material. Parre and Geitmann (2005) showed that the stiffness of the pollen tube cell wall increased upon treatment with PME. In the same study, it was shown that JIM5 (Knox, 2008), an antibody specific to acidic pectins, which usually occur only along the shank, labeled the tip of pollen tubes after PME treatment (Parre and Geitmann, 2005). We reasoned that if PI binds to the same sites in HGs that Ca2+ does, then PI fluorescence should increase upon treatment with PME. The source of the demethoxylated HG would be a mixture of those created by the exogenous PME and those demethoxylated by endogenous PME.

We labeled growing pollen tubes with PI and then added PME to the slides. After the addition of PME, cells grew normally for approximately 10 min and then quickly stopped. We never observed growth recovery (Fig. 5; Supplemental Movie S4). Concomitantly, the PI signal doubled at the tip (Fig. 5; Supplemental Movie S3). Note that the thickened cell wall at the apex and the PI signal overlay one another. This finding is consistent with the hypothesis that PI binds to newly demethoxylated sites created by PME.

Figure 5.
Figure 5.

PME causes a dramatic increase in PI fluorescence at the pollen tube apex. A, DIC (top) and PI fluorescence (bottom) images of a pollen tube before (0 min) and after (20 min) treatment with PME. At 20 min, the tip is clearly thicker and the fluorescence has increased dramatically. Bar = 10 μm. B, A line graph representing the time course for the same pollen tube shown in A. Blue represents the growth rate, and red represents the PI fluorescence at the pollen tube apex. Note that both growth rate and PI apex fluorescence continue to oscillate until the pollen tube stops, and then growth rate plummets and PI fluorescence surges in tandem. The arrow in B shows where PME was added to the slide. A.U., Arbitrary units.

As a complement to this experiment, we investigated the effect of pectinase on the PI fluorescence of growing pollen tubes. Pectinase is widely used for enzymatic digestion of cell walls. In Solanum chacoense, pectinase has been shown to increase growth rates at concentrations up to approximately 5 mg mL−1 (Parre and Geitmann, 2005). Parre and Geitmann (2005) showed that Aspergillus niger pectinase specifically digested both acidic and esterified HGs in the pollen tube. If PI is indeed interacting with carboxyl groups on the HGs, we would expect PI fluorescence to dissipate as the enzyme digested the cell wall. Disruption of the cell wall could lead to cytolysis, especially in a hypotonic solution, so we added Suc to a final concentration of 10.5% along with 1 mg mL−1 pectinase.

Images of PI-labeled pollen tubes were collected during treatment with pectinase. Suc at 10.5% alone can induce incipient plasmolysis, but it does not cause alteration in PI fluorescence (data not shown). Line scans down the center of the PI images were collected from before treatment and after treatment. The signals from six pollen tubes were normalized to the peak fluorescence prior to the addition of pectinase. Before treatment, there is a clear PI signal at the tip that is several times brighter than background or the shank walls (Fig. 6, left panels; Supplemental Movie S5). As pectinase takes effect, the PI fluorescence declines markedly at the tip and along the shank of the pollen tube. Ultimately, the signals at the tip and along the shank are roughly equivalent (Fig. 6, right panels). The tip swells somewhat and growth has stopped but cyclosis continues, until after several minutes the pollen tube explodes when turgor pressure overwhelms the weakened wall. Presumably, exocytosis continues, but the PI signal still drops markedly, suggesting that the pectinase is digesting possible sites for PI binding.

Figure 6.
Figure 6.

Pectinase decreases PI fluorescence and stops growth. A, Pectinase was added to growing lily pollen tubes. The top panels show DIC images before and after pectinase addition. The bottom panels show the PI fluorescence of the same pollen tube at the same time points with the same look-up table. B, Mean intensity of a line scan down the middle of six pollen tubes before treatment with pectinase on the left and after treatment on the right. The gray lines represent se.

PI Does Not Interfere with the Tip-Focused Ca2+ Gradient in Pollen Tubes

The tip-focused Ca2+ gradient in pollen tubes has been studied extensively for nearly two decades (Rathore et al., 1991; Miller et al., 1992). Inhibition of growth has usually been shown to block the Ca2+ gradient (Hepler et al., 2001; Cole and Fowler, 2006; Cárdenas, 2009). The pectin dyes ruthenium red (Picton and Steer, 1985; Bednarska, 1989; Ischebeck et al., 2008; Szumlanski and Nielsen, 2009) and coriphosphine O (Weis et al., 1988) have been used on plant cells but inhibit growth and block Ca2+ channels; they are not useful as vital stains of pectin. We have already shown that at working concentrations of PI and Ca2+, cells grow at control rates. We wished to investigate whether there would be changes in the Ca2+gradient.

We injected pollen tubes with the ratiometric Ca2+-sensitive dye fura-2 dextran. After removing the needle, we added PI to the growth solution and then imaged the cells (Fig. 7). The DIC image shows that the cell retains standard morphology with a rounded tip and an apical clear zone (Fig. 7A; Supplemental Movie S6). The PI fluorescence also appears normal for a growing pollen tube (compare with Fig. 1; McKenna et al., 2009). The Ca2+ gradient has a tip high peak and decreases to a basal level of approximately 150 nm. If PI were interfering with Ca2+ channels, we would expect an alteration in the magnitude of the gradient. Furthermore, as shown in Figure 5B, the oscillatory profiles of growth rate, PI, and Ca2+ correspond to published data. These data give us confidence that PI is not interfering with Ca2+ uptake (Pierson et al., 1996; McKenna et al., 2009).

Figure 7.
Figure 7.

PI does not interfere with tip-focused cytosolic Ca2+ gradient ([Ca2+]i). A, DIC (top), PI fluorescence (middle), and [Ca2+]i (bottom) images of a growing pollen tube that has been injected with fura-2 dextran and labeled with PI. Bar = 10 μm, and the Ca2+ gradient scale is in nanomolar. B, Line graphs showing the growth rate (top), apical PI signal (middle), and [Ca2+]i (bottom) at the apex of the pollen tube shown in A. Growth rate, PI, and [Ca2+]i continue to oscillate in the same manner as each of the phenomena measured individually. A.U., Arbitrary units.

DISCUSSION

In this study, we provide evidence that PI, which vitally stains plant cell walls, competes with divalent cations, especially Ca2+, and binds to carboxyl residues on HGs. With these properties, PI staining allows analysis of the link between wall deposition and cell growth. Notably, we expand our understanding of oscillatory growth and show that in root hairs, as well as pollen tubes, an increase in apical HG accumulation precedes peaks in growth rate.

As PI represents an excellent tool for understanding the physiology of wall extensibility in divergent tip-growing plant cells, we invested considerable effort in determining the mechanism of cell wall binding. The competition studies, for example, indicate that Ca2+, Mg2+, and PI interact with a common wall component, namely the HGs. Several previous studies indicate that Ca2+ cross-links GalUA residues, lending strength to the cell wall (Thibault and Rinaudo, 1985). Although Mg2+ also interacts with these negative charges, it does not create links between the pectin chains (Thibault and Rinaudo, 1985; Malovíková et al., 2004). Models of the electrostatic charge on the surface of the PI molecule show three partial positive charges associated with tertiary nitrogen atoms that are 6 to 7 Å apart. The negative charges on the GalUA residues occur at similar intervals, suggesting that Ca2+ and PI bind to the same sites in cell walls.

Because Ca2+ itself inhibits growth at high concentrations, we explored the interactions of Ca2+, Mg2+, and PI on growth and PI fluorescence in lily pollen tubes. We show that in low-Ca2+ medium, PI dramatically increases in fluorescence as its concentration increases (Fig. 2B). In a higher Ca2+ concentration, the effect is much more gradual, and in the range tested the fluorescence is never as bright. The effect of increasing Mg2+ concentration is quite similar, supporting the hypothesis that all three ions are interacting with the same charges. Initially, this suggested competitive binding of Ca2+ and PI for HGs. However, the regression model that most closely fits the data suggests cooperative binding. In this scenario, PI and Ca2+ interact with the same binding sites, but only Ca2+ effectively cross-links HGs, so that a higher proportion of binding sites occupied by Ca2+ limits the ability of PI to intercalate between HGs (for model, see Fig. 8). At sufficiently high PI concentrations, cross-linking by Ca2+ is reduced, and the wall loses strength so that tubes burst at or near the tip. Thus, increased cross-linking due to high Ca2+ concentration simultaneously makes PI binding energetically less favorable and stiffens the cell wall. This model is supported by findings showing that Ca2+ binds to HGs in this manner in vitro (Malovíková et al., 2004).

Figure 8.
Figure 8.

Model for binding of Ca2+ and PI to HGs during tip growth. Disorganized chains of highly methoxylated HG leave the growing cell by exocytosis. In the cell wall, PME demethoxylates the chains, creating linear regions of negatively charged carboxyl groups. In the presence of Ca2+, cooperative binding occurs between HG chains, creating a stiff gel. In the presence of Ca2+ and PI, both ions bind to carboxyls, although PI cannot cross-link.

The interplay between cell wall extensibility and the constant of turgor pressure (Winship et al., 2010) ultimately regulates tip growth. This study shows that the amount of demethoxylated HG available for Ca2+ binding oscillates at the tips of root hairs as it does in pollen tubes. These data suggest a fundamental mechanism for tip growth based upon two counterpoised processes. First, exocytosis of largely methoxylated HG replaces wall material carried away from the tip by expansion, locally reducing the relative concentration of binding sites for Ca2+, thus making the wall more extensible. Second, demethoxylation by PME increases the relative concentration of Ca2+-binding sites, leading to a progressively less extensible wall due to Ca2+ cross-linking (Fig. 8). A previous study showed that PME exocytosis itself oscillates (McKenna et al., 2009), suggesting that, along with the exocytosis of HGs, this could account for changes in cell wall extensibility and oscillations in growth rate. PI, by sampling a small fraction of the total Ca2+-binding sites available, reports on the deposition of cell wall (Fig. 8).

These data focus our attention on the control of exocytosis in our effort to understand the mechanisms of both oscillations and growth. There has been much compelling work concerning the targeting of exocytosis (Preuss et al., 2004; Lee et al., 2008; Nielsen et al., 2008; Szumlanski and Nielsen, 2009; Cheung et al., 2010). For pollen tubes in particular, external signals influence the directionality of growth (Okuda et al., 2009; Chae and Lord, 2011). How can these two factors be coordinated? It seems unlikely that wall extensibility itself changes first. Somehow, external signals are linked through intracellular processes to changes in cell wall extensibility. It is unclear what the intracellular signals are that could lead to directional growth changes. In addition to the molecular tools of overexpressed proteins and the probes for assessing the ionic changes in the cytoplasm, with PI we have a reporter for changes within the cell wall, which should allow further understanding of the basic mechanism of tip-growing plant cells as they respond to their environment.

MATERIALS AND METHODS

Pollen Tube Growth Conditions

Pollen was grown as described previously (Rounds et al., 2010). Briefly, all pollen was from lily (Lilium formosanum) stocks stored at –80°C and germinated for 1 to 1.5 h on a rotator at room temperature in a standard growth medium (LPGM): 7% (w/v) Suc, 1.6 mm H3BO3, 0.1 mm CaCl2, and 15 mm MES buffer adjusted to pH 5.7 with KOH (a final concentration of KOH of approximately 6 mm); all reagents were from Fisher Scientific unless otherwise noted. For microscopic observations, a pollen suspension was spread on custom-made well slides with a growth medium solution containing a final concentration of 0.7% (w/v) low-melting agarose (Sigma-Aldrich). The immobilized pollen was then covered with fresh growth medium for imaging. Cells were allowed to recover for at least 0.5 h before imaging or further manipulations.

PI Staining of Pollen Tubes

For PI staining of the primary cell wall, the growth medium covering the immobilized and recovered cells on microscope slides was replaced with LPGM supplemented with 20 to 40 μm PI (Sigma). Cells were imaged more than 10 min after addition of the stain.

Root Hair Growth Conditions

Prestratified, surface-sterilized Arabidopsis (Arabidopsis thaliana Columbia ecotype) seeds were germinated on agar pads affixed to microscope slides. The agar medium consisted of 1% (w/v) Bacto-agar (Difco) and 1% (w/v) Suc in a modified Hoagland medium (Bannigan et al., 2006). The slides were placed in standard petri dishes and oriented so that the roots would grow across the slide. Seeds were germinated and grown in constant yellow light (approximately 100 μmol m−2 s−1) at 22°C. For root hair growth analysis, seedlings were analyzed between 2 and 4 d. Slides were removed and the preparation was covered in modified Hoagland medium supplemented with 10 μm PI. The preparation was then covered with a coverslip and sealed on three sides with a 1:1:1 mixture of petroleum jelly, lanolin, and paraffin (Vidali et al., 2009). Roots were allowed to recover for at least 3 h before imaging. Newly emerging root hairs were selected for imaging.

Growth Inhibition Experiments

PI growth inhibition experiments were performed largely as described (Rounds et al., 2010). Briefly, pollen was germinated in control LPGM for approximately 1 h. The pollen suspension was then divided into aliquots on 12-well plates with the indicated amounts of Ca2+ and PI. The cells were allowed to recover for 20 min and then imaged. After 20 min, the cells were imaged again. All images were calibrated with a micrometer. Imaging was performed using a 1× lens on a stereomicroscope (Leica MZ16FA) using a CCD camera (Leica DF300FX). Images were analyzed using ImageJ (sbweb.nih.gove/ij), and analysis was performed in Excel. Regression curves were generated using the nonlinear least-squares regression analysis command in R (Ihaka and Gentleman, 1996).

PI/Ca2+ Fluorescence Competition

The experimental setup was performed largely as described for the inhibition experiments. Data acquisition was performed with the Nikon TE300 microscope (see below) using a 20× objective and the light path settings described in “Microscopy and Imaging” below. PI fluorescence exposure time was 1 s. Data analysis was performed using ImageJ (Abramoff et al., 2004). The average fluorescence in a 3- × 3-pixel box at the pollen tube tip was measured. Background was calculated by taking the average fluorescence in a 20-pixel box. Approximately 40 pollen tubes were measured for each treatment. Data were tabulated in Excel, and regression curves were generated using the nonlinear least-squares regression analysis command in R (Ihaka and Gentleman, 1996).

Microscopy and Imaging

PI and DIC images were acquired using a CCD camera (Quantix Cool Snap HQ; Roper Scientific) attached to a Nikon TE300 inverted microscope (Nikon Instruments) with a 40×/1.3 numerical aperture oil-immersion objective lens for pollen and a 60×/1.4 numerical aperture oil-immersion lens for root hairs. All the equipment was operated with MetaMorph/MetaFluor (Molecular Devices) software. A filter wheel system (Λ10-2; Sutter Instruments), mounted immediately before the CCD camera, controlled the position of a polarizing filter for DIC or an emission filter for fluorescence imaging. Fluorescence excitation light was provided by a 175-W ozone-free xenon lamp in a DG-4 switching system (Sutter Instruments). Transmitted light was provided by a low-voltage halogen lamp. We used the following filter setup for PI imaging: emission, 495/10×; a 565 dichroic long pass; and excitation 509 long pass (all filters from Chroma). Exposure times varied but were generally approximately 25 ms for DIC and approximately 800 ms for PI unless otherwise stated. For imaging fura-2 dextran simultaneously with PI, we employed the following filters: for fura emission, 340 and 380 nm; for PI, 490 nm. A triple band (UV/DAPI/FITC/rhodamine) dichroic was employed. No emission filter was used for fura, but a 640/25 emission filter was employed for PI. Excitations were as follows: 150 ms at 340 nm, 50 ms at 380 nm, 15 ms for DIC, and 850 ms at 490 nm. The DG-4 excitation was set to 25% for fura-2 dextran imaging, as brighter intensities tended to harm the cell.

In Vitro Growth Rate Experiment with PME and Pectinase

Exogenous orange peel PME (Sigma-Aldrich) treatment was carried out largely as described (Bosch et al., 2005). Briefly, pollen tubes were germinated and then plated for microscopy with PI staining as described above. Once the pollen tubes had recovered, 1 well volume of a solution containing orange peel PME at 30 units mL−1 was added for a final concentration of 15 units mL−1. Images (both DIC and fluorescence) were collected at 3-s intervals. Pectinase experiments were carried out in much the same way using pectinase (USB) from Aspergillus niger. The enzyme was added along with LPGM with extra Suc to bring the final concentration to 10.5% Suc and 1 mg mL−1 pectinase. Line scans were performed using the linescan feature of MetaMorph, and statistical analysis was performed using Excel. The images used for analysis were from the tenth frame of image collection, before the addition of pectinase, and the last frame of image collection, after incubation in pectinase.

Intracellular Ca2+ Measurement

Cells were pressure injected with fura-2 dextran as described (Cárdenas et al., 2008). After image collection, analysis was performed using ImageJ. Masks were made for each frame in entire movies and then background was subtracted. The images were Gaussian filtered, and then the entire movies were ratioed using the ratioplus plugin for ImageJ at 32 bits so as to obtain fractional ratio values. These were then calibrated using standard methods described elsewhere (Roy et al., 1999).

Growth Rate Measurement and Cross-Correlation Analysis

Growth rate was measured using the tip-tracking feature of the MetaMorph software package (Molecular Devices). For both PI and ratioed fura-2 dextran images, the intensity along line scans (20 pixels wide) down the center of the tube was collected for each image in entire movies. A custom R script (Ihaka and Gentleman, 1996) was then used to find the peak PI or Ca2+ values near the X,Y position of the tip of the pollen tube (or root hair) based on the MetaMorph tracking file. The resulting traces were detrended to remove bleaching artifacts, and cross-correlation was performed using a custom R script (Supplemental Protocols S1 and S2) and as described previously (Cárdenas et al., 2006; Lovy-Wheeler et al., 2006; McKenna et al., 2009).

Supplemental Data

The following materials are available in the online version of this article.

Acknowledgments

We thank Dr. M. Bezanilla and Dr. T.I. Baskin and members of their respective laboratories for helpful discussions.

Footnotes

  • The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Peter K. Hepler (hepler{at}bio.umass.edu).

  • www.plantphysiol.org/cgi/doi/10.1104/pp.111.182196

  • ↵1 This work was supported by the National Science Foundation (grant no. MCB–0847876 to P.K.H.).

  • ↵2 Present address: Department of Biology, Mount Holyoke College, South Hadley, MA 01075.

  • ↵3 Present address: Department of Chemistry, California Institute of Technology, Pasadena, CA 91106.

  • ↵[OA] Open Access articles can be viewed online without a subscription.

  • ↵[W] The online version of this article contains Web-only data.

  • Received June 21, 2011.
  • Accepted July 14, 2011.
  • Published July 18, 2011.

REFERENCES

  1. ↵
    1. Abramoff MD,
    2. Magelhaes PJ,
    3. Ram SJ
    (2004) Image processing with ImageJ. Biophotonics Int 11: 36–42
    OpenUrl
  2. ↵
    1. Bannigan A,
    2. Wiedemeier AMD,
    3. Williamson RE,
    4. Overall RL,
    5. Baskin TI
    (2006) Cortical microtubule arrays lose uniform alignment between cells and are oryzalin resistant in the Arabidopsis mutant, radially swollen 6. Plant Cell Physiol 47: 949–958
    OpenUrlAbstract/FREE Full Text
  3. ↵
    1. Bednarska E
    (1989) The effect of exogenous Ca2+ ions on pollen grain germination and pollen tube growth. Sex Plant Reprod 2: 53–58
    OpenUrl
  4. ↵
    1. Bosch M,
    2. Cheung AY,
    3. Hepler PK
    (2005) Pectin methylesterase, a regulator of pollen tube growth. Plant Physiol 138: 1334–1346
    OpenUrlAbstract/FREE Full Text
  5. ↵
    1. Bosch M,
    2. Hepler PK
    (2005) Pectin methylesterases and pectin dynamics in pollen tubes. Plant Cell 17: 3219–3226
    OpenUrlFREE Full Text
  6. ↵
    1. Braccini I,
    2. Pérez S
    (2001) Molecular basis of C(2+)-induced gelation in alginates and pectins: the egg-box model revisited. Biomacromolecules 2: 1089–1096
    OpenUrlCrossRefPubMed
  7. ↵
    1. Cárdenas L
    (2009) New findings in the mechanisms regulating polar growth in root hair cells. Plant Signal Behav 4: 4–8
    OpenUrlCrossRefPubMed
  8. ↵
    1. Cárdenas L,
    2. Lovy-Wheeler A,
    3. Kunkel JG,
    4. Hepler PK
    (2008) Pollen tube growth oscillations and intracellular calcium levels are reversibly modulated by actin polymerization. Plant Physiol 146: 1611–1621
    OpenUrlAbstract/FREE Full Text
  9. ↵
    1. Cárdenas L,
    2. McKenna ST,
    3. Kunkel JG,
    4. Hepler PK
    (2006) NAD(P)H oscillates in pollen tubes and is correlated with tip growth. Plant Physiol 142: 1460–1468
    OpenUrlAbstract/FREE Full Text
  10. ↵
    1. Chae K,
    2. Lord EM
    (2011) Pollen tube growth and guidance: roles of small, secreted proteins. Ann Bot (in press)
  11. ↵
    1. Cheung AY,
    2. Niroomand S,
    3. Zou Y,
    4. Wu HM
    (2010) A transmembrane formin nucleates subapical actin assembly and controls tip-focused growth in pollen tubes. Proc Natl Acad Sci USA 107: 16390–16395
    OpenUrlAbstract/FREE Full Text
  12. ↵
    1. Cole RA,
    2. Fowler JE
    (2006) Polarized growth: maintaining focus on the tip. Curr Opin Plant Biol 9: 579–588
    OpenUrlCrossRefPubMed
  13. ↵
    1. Dardelle F,
    2. Lehner A,
    3. Ramdani Y,
    4. Bardor M,
    5. Lerouge P,
    6. Driouich A,
    7. Mollet J-C
    (2010) Biochemical and immunocytological characterizations of Arabidopsis pollen tube cell wall. Plant Physiol 153: 1563–1576
    OpenUrlAbstract/FREE Full Text
  14. ↵
    1. Deitch AD,
    2. Law H,
    3. deVere White R
    (1982) A stable propidium iodide staining procedure for flow cytometry. J Histochem Cytochem 30: 967–972
    OpenUrlAbstract/FREE Full Text
  15. ↵
    1. Diet A,
    2. Link B,
    3. Seifert GJ,
    4. Schellenberg B,
    5. Wagner U,
    6. Pauly M,
    7. Reiter WD,
    8. Ringli C
    (2006) The Arabidopsis root hair cell wall formation mutant lrx1 is suppressed by mutations in the RHM1 gene encoding a UDP-l-rhamnose synthase. Plant Cell 18: 1630–1641
    OpenUrlAbstract/FREE Full Text
  16. ↵
    1. Estevez JM,
    2. Leonardi PI,
    3. Alberghina JS
    (2008) Cell wall carbohydrate epitopes in the green alga Oedogonium bharuchae f. minor (Oedogoniales, Chlorophyta). J Phycol 44: 1257–1268
    OpenUrlCrossRef
  17. ↵
    1. Fiers M,
    2. Golemiec E,
    3. Xu J,
    4. van der Geest L,
    5. Heidstra R,
    6. Stiekema W,
    7. Liu CM
    (2005) The 14-amino acid CLV3, CLE19, and CLE40 peptides trigger consumption of the root meristem in Arabidopsis through a CLAVATA2-dependent pathway. Plant Cell 17: 2542–2553
    OpenUrlAbstract/FREE Full Text
  18. ↵
    1. Galway ME,
    2. Heckman JW Jr.,
    3. Schiefelbein JW
    (1997) Growth and ultrastructure of Arabidopsis root hairs: the rhd3 mutation alters vacuole enlargement and tip growth. Planta 201: 209–218
    OpenUrlCrossRefPubMed
  19. ↵
    1. Garnier C,
    2. Axelos MAV,
    3. Thibault J-F
    (1994) Selectivity and cooperativity in the binding of calcium ions by pectins. Carbohydr Res 256: 71–81
    OpenUrlCrossRef
  20. ↵
    1. Hepler PK,
    2. Vidali L,
    3. Cheung AY
    (2001) Polarized cell growth in higher plants. Annu Rev Cell Dev Biol 17: 159–187
    OpenUrlCrossRefPubMed
  21. ↵
    1. Holdaway-Clarke TL,
    2. Hepler PK
    (2003) Control of pollen tube growth: role of ion gradients and fluxes. New Phytol 159: 539–563
    OpenUrlCrossRef
  22. ↵
    1. Holdaway-Clarke TL,
    2. Weddle NM,
    3. Kim S,
    4. Robi A,
    5. Parris C,
    6. Kunkel JG,
    7. Hepler PK
    (2003) Effect of extracellular calcium, pH and borate on growth oscillations in Lilium formosanum pollen tubes. J Exp Bot 54: 65–72
    OpenUrlAbstract/FREE Full Text
  23. ↵
    1. Honys D,
    2. Twell D
    (2003) Comparative analysis of the Arabidopsis pollen transcriptome. Plant Physiol 132: 640–652
    OpenUrlAbstract/FREE Full Text
  24. ↵
    1. Hudson B,
    2. Upholt WB,
    3. Devinny J,
    4. Vinograd J
    (1969) The use of an ethidium analogue in the dye-buoyant density procedure for the isolation of closed circular DNA: the variation of the superhelix density of mitochondrial DNA. Proc Natl Acad Sci USA 62: 813–820
    OpenUrlAbstract/FREE Full Text
  25. ↵
    1. Ihaka R,
    2. Gentleman R
    (1996) R: a language for data analysis and graphics. J Comput Graph Statist 5: 299–314
    OpenUrlCrossRef
  26. ↵
    1. Ischebeck T,
    2. Stenzel I,
    3. Heilmann I
    (2008) Type B phosphatidylinositol-4-phosphate 5-kinases mediate Arabidopsis and Nicotiana tabacum pollen tube growth by regulating apical pectin secretion. Plant Cell 20: 3312–3330
    OpenUrlAbstract/FREE Full Text
  27. ↵
    1. Jiang L,
    2. Yang SL,
    3. Xie LF,
    4. Puah CS,
    5. Zhang XQ,
    6. Yang WC,
    7. Sundaresan V,
    8. Ye D
    (2005) VANGUARD1 encodes a pectin methylesterase that enhances pollen tube growth in the Arabidopsis style and transmitting tract. Plant Cell 17: 584–596
    OpenUrlAbstract/FREE Full Text
  28. ↵
    1. Jones KH,
    2. Senft JA
    (1985) An improved method to determine cell viability by simultaneous staining with fluorescein diacetate-propidium iodide. J Histochem Cytochem 33: 77–79
    OpenUrlAbstract/FREE Full Text
  29. ↵
    1. Knox JP
    (2008) Revealing the structural and functional diversity of plant cell walls. Curr Opin Plant Biol 11: 308–313
    OpenUrlCrossRefPubMed
  30. ↵
    1. Lee YJ,
    2. Szumlanski A,
    3. Nielsen E,
    4. Yang Z
    (2008) Rho-GTPase-dependent filamentous actin dynamics coordinate vesicle targeting and exocytosis during tip growth. J Cell Biol 181: 1155–1168
    OpenUrlAbstract/FREE Full Text
  31. ↵
    1. Lovy-Wheeler A,
    2. Kunkel JG,
    3. Allwood EG,
    4. Hussey PJ,
    5. Hepler PK
    (2006) Oscillatory increases in alkalinity anticipate growth and may regulate actin dynamics in pollen tubes of lily. Plant Cell 18: 2182–2193
    OpenUrlAbstract/FREE Full Text
  32. ↵
    1. Luedtke NW,
    2. Liu Q,
    3. Tor Y
    (2005) On the electronic structure of ethidium. Chemistry 11: 495–508
    OpenUrlCrossRefPubMed
  33. ↵
    1. Malovíková A,
    2. Rinaudo M,
    3. Milas M
    (2004) Comparative interactions of magnesium and calcium counterions with polygalacturonic acid. Biopolymers 34: 1059–1064
    OpenUrl
  34. ↵
    1. McKenna ST,
    2. Kunkel JG,
    3. Bosch M,
    4. Rounds CM,
    5. Vidali L,
    6. Winship LJ,
    7. Hepler PK
    (2009) Exocytosis precedes and predicts the increase in growth in oscillating pollen tubes. Plant Cell 21: 3026–3040
    OpenUrlAbstract/FREE Full Text
  35. ↵
    1. Michel D
    (2007) Cooperative equilibrium curves generated by ordered ligand binding to multi-site molecules. Biophys Chem 129: 284–288
    OpenUrlCrossRefPubMed
  36. ↵
    1. Miller DD,
    2. Callaham DA,
    3. Gross DJ,
    4. Hepler PK
    (1992) Free Ca2+ gradient in growing pollen tubes of Lilium. J Cell Sci 101: 7–12
    OpenUrlAbstract/FREE Full Text
  37. ↵
    1. Monshausen GB,
    2. Bibikova TN,
    3. Messerli MA,
    4. Shi C,
    5. Gilroy S
    (2007) Oscillations in extracellular pH and reactive oxygen species modulate tip growth of Arabidopsis root hairs. Proc Natl Acad Sci USA 104: 20996–21001
    OpenUrlAbstract/FREE Full Text
  38. ↵
    1. Mancuso S,
    2. Shabala S
    1. Moreno N,
    2. Colaco R,
    3. Feijó JA
    (2007). The pollen tube oscillator: integrating biophysics and biochemistry into cellular growth and morphogenesis. In Mancuso S, Shabala S, eds, Rhythms in Plants: Phenomenology, Mechanisms, and Adaptive Significance. Springer-Verlag, Berlin, pp39–62
  39. ↵
    1. Nielsen E,
    2. Cheung AY,
    3. Ueda T
    (2008) The regulatory RAB and ARF GTPases for vesicular trafficking. Plant Physiol 147: 1516–1526
    OpenUrlFREE Full Text
  40. ↵
    1. Okuda S,
    2. Tsutsui H,
    3. Shiina K,
    4. Sprunck S,
    5. Takeuchi H,
    6. Yui R,
    7. Kasahara RD,
    8. Hamamura Y,
    9. Mizukami A,
    10. Susaki D,
    11. et al.
    (2009) Defensin-like polypeptide LUREs are pollen tube attractants secreted from synergid cells. Nature 458: 357–361
    OpenUrlCrossRefPubMed
  41. ↵
    1. Parre E,
    2. Geitmann A
    (2005) Pectin and the role of the physical properties of the cell wall in pollen tube growth of Solanum chacoense. Planta 220: 582–592
    OpenUrlCrossRefPubMed
  42. ↵
    1. Picton JM,
    2. Steer MW
    (1985) The effects of ruthenium red, lanthanum, fluorescein isothiocyanate and trifluoperazine on vesicle transport, vesicle fusion and tip extension in pollen tubes. Planta 163: 20–26
    OpenUrlCrossRef
  43. ↵
    1. Pierson ES,
    2. Miller DD,
    3. Callaham DA,
    4. van Aken J,
    5. Hackett G,
    6. Hepler PK
    (1996) Tip-localized calcium entry fluctuates during pollen tube growth. Dev Biol 174: 160–173
    OpenUrlCrossRefPubMed
  44. ↵
    1. Preuss ML,
    2. Serna J,
    3. Falbel TG,
    4. Bednarek SY,
    5. Nielsen E
    (2004) The Arabidopsis Rab GTPase RabA4b localizes to the tips of growing root hair cells. Plant Cell 16: 1589–1603
    OpenUrlAbstract/FREE Full Text
  45. ↵
    1. Rathore KS,
    2. Cork RJ,
    3. Robinson KR
    (1991) A cytoplasmic gradient of Ca2+ is correlated with the growth of lily pollen tubes. Dev Biol 148: 612–619
    OpenUrlCrossRefPubMed
  46. ↵
    1. Rounds CM,
    2. Hepler PK,
    3. Fuller SJ,
    4. Winship LJ
    (2010) Oscillatory growth in lily pollen tubes does not require aerobic energy metabolism. Plant Physiol 152: 736–746
    OpenUrlAbstract/FREE Full Text
  47. ↵
    1. Roy SJ,
    2. Holdaway-Clarke TL,
    3. Hackett GR,
    4. Kunkel JG,
    5. Lord EM,
    6. Hepler PK
    (1999) Uncoupling secretion and tip growth in lily pollen tubes: evidence for the role of calcium in exocytosis. Plant J 19: 379–386
    OpenUrlCrossRefPubMed
  48. ↵
    1. Šamaj J,
    2. Braun M,
    3. Baluška F,
    4. Ensikat H,
    5. Tsumuraya Y,
    6. Volkmann D
    (1999) Specific localization of arabinogalactan-protein epitopes at the surface of maize root hairs. Plant Cell Physiol 40: 874–883
    OpenUrlAbstract/FREE Full Text
  49. ↵
    1. Szumlanski AL,
    2. Nielsen E
    (2009) The Rab GTPase RabA4d regulates pollen tube tip growth in Arabidopsis thaliana. Plant Cell 21: 526–544
    OpenUrlAbstract/FREE Full Text
  50. ↵
    1. Thibault JF,
    2. Rinaudo M
    (1985) Interactions of mono- and divalent counterions with alkali- and enzyme-deesterified pectins in salt-free solutions. Biopolymers 24: 2131–2143
    OpenUrlCrossRef
  51. ↵
    1. Tian GW,
    2. Chen MH,
    3. Zaltsman A,
    4. Citovsky V
    (2006) Pollen-specific pectin methylesterase involved in pollen tube growth. Dev Biol 294: 83–91
    OpenUrlCrossRefPubMed
  52. ↵
    1. Vidali L,
    2. van Gisbergen PAC,
    3. Guérin C,
    4. Franco P,
    5. Li M,
    6. Burkart GM,
    7. Augustine RC,
    8. Blanchoin L,
    9. Bezanilla M
    (2009) Rapid formin-mediated actin-filament elongation is essential for polarized plant cell growth. Proc Natl Acad Sci USA 106: 13341–13346
    OpenUrlAbstract/FREE Full Text
  53. ↵
    1. Vincent RR,
    2. Williams MA
    (2009) Microrheological investigations give insights into the microstructure and functionality of pectin gels. Carbohydr Res 344: 1863–1871
    OpenUrlCrossRefPubMed
  54. ↵
    1. Weis KG,
    2. Polito VS,
    3. Labavitch JM
    (1988) Microfluorometry of pectic materials in the dehiscence zone of almond (Prunus dulcis [Mill.] DA Webb) fruits. J Histochem Cytochem 36: 1037–1041
    OpenUrlAbstract/FREE Full Text
  55. ↵
    1. Winship LJ,
    2. Obermeyer G,
    3. Geitmann A,
    4. Hepler PK
    (2010) Under pressure, cell walls set the pace. Trends Plant Sci 15: 363–369
    OpenUrlCrossRefPubMed
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Propidium Iodide Competes with Ca2+ to Label Pectin in Pollen Tubes and Arabidopsis Root Hairs
Caleb M. Rounds, Eric Lubeck, Peter K. Hepler, Lawrence J. Winship
Plant Physiology Sep 2011, 157 (1) 175-187; DOI: 10.1104/pp.111.182196
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