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First published online December 22, 2006; 10.1104/pp.106.087023 Plant Physiology 143:1024-1036 (2007) © 2007 American Society of Plant Biologists OPEN ACCESS ARTICLE
Xylella fastidiosa Infection and Ethylene Exposure Result in Xylem and Water Movement Disruption in Grapevine Shoots1,[OA]Department of Plant Sciences (A.G.P.-D., L.C.G., K.A.S., J.M.L.) and Nuclear Magnetic Resonance Facility (J.H.W.), University of California, Davis, California 95616
It is conventionally thought that multiplication of the xylem-limited bacterium Xylella fastidiosa (Xf) within xylem vessels is the sole factor responsible for the blockage of water movement in grapevines (Vitis vinifera) affected by Pierce's disease. However, results from our studies have provided substantial support for the idea that vessel obstructions, and likely other aspects of the Pierce's disease syndrome, result from the grapevine's active responses to the presence of Xf, rather than to the direct action of the bacterium. The use of magnetic resonance imaging (MRI) to observe the distribution of water within the xylem has allowed us to follow nondestructively the development of vascular system obstructions subsequent to inoculation of grapevines with Xf. Because we have hypothesized a role for ethylene produced in vines following infection, the impact of vine ethylene exposure on obstruction development was also followed using MRI. In both infected and ethylene-exposed plants, MRI shows that an important proportion of the xylem vessels become progressively air embolized after the treatments. The loss of xylem water-transporting function, assessed by MRI, has been also correlated with a decrease in stem-specific hydraulic conductivity (KS) and the presence of tyloses in the lumens of obstructed water conduits. We have observed that the ethylene production of leaves from infected grapevines is greater than that from healthy vines and, therefore, propose that ethylene may be involved in a series of cellular events that coordinates the vine's response to the pathogen.
Pierce's disease (PD) is a disease of grapevines (Vitis vinifera) caused by the xylem-limited bacterium Xylella fastidiosa (Xf). Vines affected by PD become nonproductive, declining in vigor and ultimately dying from the disease, generally within 2 years (Varela et al., 2001  ). PD emerged as a serious problem in California in the early 1990s when the glassy-winged sharpshooter (Homalodisca coagulata Say), a highly efficient vector native to the east coast, reached the southern part of the state. It became clear when several vineyards were lost to the disease, that PD posed a major statewide threat to the valuable grape and wine industries.
Hopkins (1989)
The occurrence of vessel cavitations during the progression of PD could also partially explain the loss of water-conducting capacity in Xf-infected vines. Classical anatomical methods are not useful to reveal the presence of cavitations because they are destructive. Usually, two consecutive hydraulic conductivity (KS) measurements, the first made at a low and the second at high pressure are used to estimate the percentage of embolized vessels in a stem or petiole segment. This technique was used in a study that did not find evidence of cavitations contributing to the loss of water conduction described in PD (McElrone et al., 2003
This study was performed as part of an effort to describe events in the development of PD. The specific objective was to use MRI to assess visually and nondestructively the occurrence of nonfunctional vessels as disease development progressed. There is a relatively long and somewhat variable time delay between Xf introduction and the appearance of visible vine symptoms (Varela et al., 2001
Stem Morphology The vines used in the Xf-inoculation and ethylene-exposure experiments developed between fall and spring time and, as a consequence, grew continuously but slowly. Only one inoculated vine, analyzed at the end of the evaluation period (spring, 2004), presented external leaf PD symptoms at the base of the shoot. All the other vines did not show external symptoms or signs in shoot apices, leaves, or stems that could be attributed to the Xf population or response to ethylene. However, after the experiments were concluded, all the new shoots that grew after pruning the Xf-inoculated plants (shoots that had not been directly inoculated) exhibited typical PD symptoms. The lack of external symptoms in Xf-inoculated vines, even 7 months after inoculation, indicates also a slow infective colonization process by the bacteria. Therefore the images, hydraulic measurements, and other results from the inoculation experiment most likely describe the development of early PD symptoms in grapevine stems. The total stem lengths of the shoots used in these experiments can be seen in Table I . Only the mean number of internodes for the Xf-inoculated vines (designated I) was statistically different from the means of the mock-inoculated controls (designated IC). There were no significant differences between infected vines and ethylene-treated vines (designated E) and their controls (IC and EC, respectively) with respect to the average lengths and diameters of the internode segments used for KS measurements (Table I). In general, the vines used in the ethylene experiment were evaluated later in the growth season and they tended to exhibit larger internode diameters (P = 0.0731) and xylem cross-sectional areas (P = 0.0205) than the vines from the inoculation experiment. We found a linear relationship between the cross-sectional area of the stem (AS) and the xylem cross-sectional area (AX) measured in the same internodes. The cross-section area of the xylem is about half of that of the stem (Fig. 1 ). There were no statistical differences in AS and AX between the treated and control vines (P > 0.05). AX was also analyzed as a function of the relative position at which it was measured along the stem; i.e. the number of internodes away from the tip (of the total length of the shoot). In all treatments AX increased linearly away from the tip (P < 0.001 for all the treatments).
Use of MRI to Visualize Xylem Vessel Content
Previous studies have described the ability of MRI to visualize in vivo the occurrence of individual vessel cavitations caused by large pressure gradients in the xylem during water deficit (Holbrook et al., 2001
To understand the limits of the interpretations of changes in vessel functionality that could be made from examination of the MRIs, the signal intensities were compared for glass tubes filled with pure water, a saline solution (KCl), and a pectin gel. The signal intensities of glass tubes filled with these three materials were quite similar (Fig. 3 ). This suggests that MRI cannot differentiate between vessels filled with regular xylem sap and vessels filled with pectic materials from plant gels. It also indicates that the occurrence of dark zones where water-filled vessels are expected should be interpreted as regions where vessels are air filled, not gel filled.
MRI Reveals Vessel Cavitation in Externally Asymptomatic Xf-Infected and Ethylene-Gassed Vines MRI is capable of showing xylem disruption and nonfunctional vessels well before external symptoms appear in Xf-infected plants. Dark zones, indicative of vessel embolisms, are typically observed in images of infected vines. The image in Figure 4A shows an infected plant at a basal internode where leaf symptoms of PD were apparent. Closer to the stem apex, at a point where the leaves showed no PD symptoms, MRI also reveals the presence of several empty vessels (Fig. 4B). The contrast of these images with those for a healthy stem is evident; in healthy stems the xylem appears as a hollow disc full of bright images indicating water-filled vessels (Fig. 2A).
A time course series of images taken after imposing the treatments revealed the progressive development of dark zones in both Xf-inoculated and ethylene-treated vines. Initial signs of embolisms in the xylem can be seen 18 to 54 d after inoculating the vines with Xf (Fig. 5 ). The dark, cavitated zones increase in size over time until they become an important proportion of the total area of the xylem. A similar progression was observed in ethylene-gassed vines from two independent experiments (Fig. 6 ). MRI showed the presence of cavitations between 19 to 47 d in young vines (Fig. 6, A–C) and between 10 to 19 d in mature vines gassed with ethylene (Fig. 6, D–F).
A higher proportion of the images taken along the stems of Xf-inoculated (I) and ethylene-treated vines (E) was categorized as exhibiting large cavitations (L) in comparison with the images of the controls (IC and EC; Fig. 7 ). This increase in the proportion of images depicting large, dark zones implied a reduction in the proportion of images classified as presenting normal, water-filled, and presumably functional xylem (N). The  2 likelihood-ratio test indicated that the distributions of the proportions of images exhibiting large, dark zones or normal-looking xylem were significantly different among the group treatments (Fig. 7). A correspondence analysis confirmed that Xf-inoculated and ethylene-treated vines were more closely associated with accumulations of empty (dark) vessels in the xylem, whereas the control groups were clearly associated with images of normal, water-filled xylem conduits (Fig. 8
). A comparison by internodes shows that both Xf- and mock-inoculated plants presented a high proportion of images exhibiting large, dark zones near to the base of the stem where inoculation or needle penetration during mock inoculation had occurred (internode zero in Fig. 9
). Even 7 months after treatment, MRI showed inoculation-related cavitations extending frequently up to two internodes above the inoculation site (Figs. 9 and 10
). If an internode was present below the inoculation point, generally, it also exhibited cavitated vessels (Fig. 9). In most of the other internodes, Xf-infected vines presented a higher proportion of large, dark zones and a lower proportion of normal-looking xylem than did the controls (IC). These differences were significant at internodes 10 and 12 (Fig. 9). A similar trend was observed for ethylene-gassed plants and their controls (EC) in which the biggest differences were also found in the distal halves of the stems, at internodes 14, 18, and 20 (Fig. 11
).
Two of the vines that had been inoculated with Xf, and then were examined using MRI prior to the appearance of external PD symptoms and subsequent dissection for KS assessments, exhibited only minor, internal xylem disruptions. The stems of these vines (designated I-N in the result description for KS measurements) presented a distribution of the category proportions intermediate between Xf- and mock-inoculated plants (N = 35.29%, S = 47.06%, and L = 17.65%). This proportion was not significantly different from that of mock-inoculated vines (P = 0.0682, 2 likelihood ratio).
The impression of a loss in xylem water-transporting function that is given by the MRIs of Xf-inoculated and ethylene-gassed vines was confirmed by subsequent destructive measurements of water flow rates and the calculations for the specific hydraulic resistivity (
In comparison with the controls (IC and EC), Xf-inoculated and ethylene-gassed vines (I and E) exhibited a reduction in KS for individual internodes at specific internode positions along the stem (Fig. 13 ). Xf- and mock-inoculated vines presented a sharp drop in KS at the inoculation point (internode 0) due to cavitations produced by the pin-prick inoculation technique (Figs. 9 and 10). I-N represents KS for two vines that were inoculated with Xf but did not show external or internal (MRI) disease symptoms. The I-N vines had a higher mean KS than the mock-inoculated control vines, presenting the highest KS values for individual plants in the inoculation experiment (Fig. 13A). I-N vines were removed from the statistical analysis for KS, and the comparison was done only using Xf-inoculated and control (mock-inoculated) vines. Xf-inoculated vines demonstrated a major reduction in KS with respect to the controls from internodes 3 to 7, and at internode 16. Following treatment, ethylene-gassed vines presented significantly lower KS than control plants (EC) from the base of the stem (internode 0) up to internode 5, and also at internode 15 (Fig. 13B).
Xf Infection and Ethylene Exposure Induce Vascular Occlusion Formation
Light microscopy observation of stem sections revealed that tyloses and pectin gels were more common in Xf-inoculated and ethylene-gassed vines than in the controls (data not shown). This is consistent with previous observations made on ethylene-gassed vines (S. Lurie, L.C. Greve, P. Bates, and J.M. Labavitch, unpublished data) and with reported data from Xf infection (Fry and Milholland, 1990
Symptomatic and asymptomatic leaves taken from Xf-infected vines generated more ethylene than leaves taken from healthy vines (Fig. 14 ). Only the amount of ethylene produced by visually symptomatic leaves was statistically different from the amount produced by healthy leaves over a 24 h period.
MRI is very useful for visualizing the progression of changes in the internal vascular anatomy of grapevine stems because it is nondestructive. In MRI cross sections, the vascular cambium and water-filled xylem conduits located between the parenchyma rays appear bright and clearly distinguishable as individual conducting elements. The phloem tissue also appears bright and is readily discernible in young internodes prior to periderm differentiation. Once the periderm has formed, the vascular cambium and, especially, the phloem tissue are more difficult to distinguish. The parenchyma cells from the rays and pith are usually dark, but in the younger, more apical internodes both pith and xylem rays often appear bright, making it more difficult to distinguish individual xylem conduits (Figs. 5 and 6). Although the precise cause of the differences in brightness of stems imaged at different developmental stages is not known, it is likely that differences in tissue water content and water distribution might be related to the phenomenon. The epidermis appears very bright in young stems but as the more suberized and less hydrated periderm develops the brightness at the stem's periphery decreases (Fig. 6, I–H). Similarly, the brightness that can be observed in the pith and most of the xylem of younger, more apical internodes could be explained because young xylem conduits are mostly protoxylem, with walls composed mostly of primary cell wall. Thus, it is likely that these conduits are more permeable to radial water movement than older conduits; perhaps this allows the space between cells to become filled with water producing the brighter images observed with MRI.
The utility of MRI for visualizing cavitations in xylem conduits has been demonstrated in our experiments as well as in previous work (Holbrook et al., 2001
An observation that has not been described before is that when imaging artificially induced cavitations (Fig. 2) or cavitations in diseased stems (Fig. 4), we frequently found that the weaker signal from tissue that surrounded the vessels as well as the stronger, well-defined signals from individual vessels disappeared. This weaker signal may have originated from water stored in intercellular spaces and xylem fibers located between the water conduits (Tyree and Zimmermann, 2002
Despite the high variation in KS and
Mock-inoculated vines exhibited the development-related changes in KS along the stem that have been described previously for grapevine internodes. KS is usually higher in the middle portion of the stem and it decreases toward the base and tip of the shoot, being especially clear in the apical internodes (Schultz and Matthews, 1993
However, MRI revealed that in relationship to the controls (IC and EC), Xf-inoculated and ethylene-gassed vines (I and E) had a higher proportion of the dark zones associated with cavitations in the images taken at the apical portions of the stems. The discrepancy in the location of the xylem anomalies detected by MRI and KS tests indicates that physical occlusions that obstruct the water flow throughout the vessels are more likely to be located in the internodes at the base of the stems, and that cavitations are predominantly located in the more apical internodes. In vines that have been needle inoculated at the stem base, PD symptoms develop sequentially from the inoculation point to the upper parts of the shoots, although traditional anatomical studies indicate that tyloses seem to develop in the stems only after bacteria, gum, and tyloses have been detected in leaves (Stevenson et al., 2004
Xf has several genes whose sequences are similar to genes in other bacteria that encode cell wall-degrading polyglacturonase and
There has been disagreement on whether or not the amount of vascular occlusions observed in Xf-infected vines is enough to explain the reduction in water-conductive capacity and the other symptoms characteristic of PD (Hopkins, 1989
Tyloses, and particularly pectin gels have been observed near or encapsulating bacterial cells in xylem vessels of resistant and tolerant grapevine and muscadine (Muscadinia rotundifolia) cultivars (Mollenhauer and Hopkins, 1976
In our experiments, grapevines exposed to ethylene demonstrated some of the symptoms observed in PD-infected stems, exhibiting a greater proportion of cavitated vessels and tylose formation, as well as a reduction in the water-conducting capacity. This suggests that, like in other vascular diseases (VanderMolen et al., 1983
Plant Material The experiments were carried out using grapevines (Vitis vinifera cv Chardonnay, clone no. 4), grafted on 110-R rootstocks. The vines were grown in a greenhouse between August, 2003 and June, 2004 in 3.8-L pots. The greenhouse was located at the main campus of the University of California at Davis. No supplementary artificial light was provided, and the average temperature of the room during the period was 21.3°C (minimum 15.2°C and maximum 35.0°C). Plants were supplemented with 100 g of slow-release fertilizer (Osmocote, Scotts-Sierra Horticultural Products) and hand watered as needed, when the surface of the soil had dried out. Most vines had one main shoot with about 15 expanding internodes at the time treatments were initiated.
Seven grapevines were inoculated on September 5, 2003 with Xf Temecula strain cultured on PD3 broth and solid medium (Davis et al., 1981
A custom-built chamber was used to enclose and expose five vines to ethylene (C2H4) on September 13, 2003 (E vines). The chamber consisted of a wooden frame (1.2 x 0.6 x 0.6 m) and clear polyethylene walls. A small opening at the top of the chamber provided exchange with the external environment. A mix of ethylene and air (10 µL L–1) was kept flowing into the chamber during 48 h at a rate of 30 L h–1. Small fans were installed at the base of the chamber to ensure adequate gas mixing and circulation throughout the chamber. The room where the chamber was located remained in relative darkness during the treatment (approximately 10 µE m–2 s–1). A set of five nontreated (EC) vines, kept in the greenhouse, was used as a control group. Similar treatments were imposed in a previous experiment (January, 2003) on mature Chardonnay vines. Results from this experiment are also presented.
1H-MRI was performed in a 7 Tesla (300 MHz), horizontal wide-bore magnet (180 mm diameter), with a BioSpec spectrometer (Bruker Instruments) located at the Nuclear Magnetic Resonance Facility (University of California at Davis). A microgradient set (60 mm i.d.), capable of approximately 950 mT m–1, was mounted in the magnet bore and a volume coil (35 mm i.d.) was used for both radio frequency (RF) excitation and reception. The leaves of the vine shoot were held close to the stem and the entire shoot was wrapped with plastic film to allow the introduction of the shoot through the opening of the coil. Then the internode to be imaged was positioned at the center of the volume coil and this assembly placed at the center of the magnet. Ten transverse image slices were simultaneously acquired at the center of the internodes using a gradient-echo pulse sequence, with a tip angle of 30°, a repetition time (TR) of 500 ms, and an echo time (TE) of 6.0 ms. Later, only the slice in which the stem was most perpendicularly oriented to the magnetized plane was selected for processing due to its higher image quality. The slices were 1.0 mm thick with a separation of 2.0 mm between them. Adequate signal-to-noise ratio was obtained by averaging four acquisitions per slice. The images were acquired as data arrays of 256 x 256 pixels with a field of view of 12.8 mm (in-plane resolution of 50 µm). Each image required a scanning time of 8 min and 32 s. The gradient-echo method was chosen over spin echo because it provides better contrast due to local field heterogeneity at the boundary of anatomical structures (Blümich, 2000
The plants were watered and taken to the NMR facility several hours in advance (usually the day before) of being positioned in the magnet. Because of the low evapotranspirational demand of the ambient atmosphere in the facility, this time allowed the water potentials of the plant and the wet soil to equilibrate. Thus any difference observed between the images of treated and control vines (e.g. presence of cavitations) should not be the result of large differences in xylem tensions at the moment of taking the images (Holbrook et al., 2001
MRI raw data sets were zero filled to a matrix of 1,024 x 1,024 pixels and Fourier transformed to produce images with a digital resolution of 12.5 µm, which were converted into TIFF files. These images were further processed and analyzed with the public-domain software package ImageJ version 1.33k (National Institutes of Health, Washington, DC). The background noise was removed from the images using the math tool by subtracting the average background signal intensity +1 SD from the overall signal intensity of the image. Then the tissues to the outside of the vascular cambium, as well as the central pith, were outlined and removed from the images. The area calculator tool was used to quantify the area of the entire xylem disc (AX). The images taken along the vines in the inoculation and ethylene experiments were classified in three categories. If the xylem appeared as a disc full of bright vessels, the image was categorized as normal (N). However, if the image was showing one or a few small dark zones indicating that a few vessels were not filled, the image was categorized as having small cavitations (S). If one or more areas of several unfilled vessels were evident, the image was categorized as presenting large cavitations (L).
A series of artificially induced cavitation followed by vessel-refilling experiments was conducted to test whether cavitations could be specifically revealed by our MRI protocol. The stem of a healthy, well-hydrated vine was imaged at a specific, labeled internode. Then the vine shoot was taken out of the magnet to cut and remove a sector accounting for approximately 70% of the stem's cross-sectional area immediately below the imaging site. A second image was then taken at the same position as the first image, where it was anticipated that air would have entered and embolized many vessels. The shoot was again removed from the magnet and the stem segment containing the embolisms and the cut region was excised. This stem segment was refilled with pressurized water introduced through the complete cross section at the base of the segment. An image was made to confirm the refilling of the vessels. Then the stem segment was flushed, in both directions, with pressurized air to completely empty the vessels, and a last image was taken. We performed an experiment to clarify whether MRI can differentiate between a dilute saline solution (similar to the xylem sap) and pectic materials (components of primary cell walls and plant gels) that can plug water conduits during PD development. Glass tubes containing either a pectin gel, distilled water, or a 10 mM KCl solution were arrayed together in the magnet and imaged using the MRI parameters described previously, except for the field of view that, in this case, was 25.6 mm. The pectin gel was obtained from commercial fruit pectin jelly powder prepared according the instructions of the manufacturer (Sure-Jell, Kraft Foods).
Measurements were made within 24 h of finishing the MRI assessments. The shoots were cut at the base and under water to prevent the occurrence of cavitations. While still under water, the cut end was smoothed with a sharp razor blade and fitted tightly to Tygon tubing connected to a plastic reservoir filled with a filtered (0.2 µm) and degassed 10 mM KCl solution. The apical ends of shoots were trimmed back up to internode 17 or 18 from the base, for the ethylene and inoculation experiments, respectively. If the shoots did not have enough internodes they were trimmed up to the last fully expanded internode. The leaf blades were cut and the petioles were sealed using hot plastic glue. Then the reservoir was pressurized at about 103 kPa, forcing the solution into the stem. After waiting 10 to 20 min for a stable flow to be established, the fluid coming out of the apical end was collected in a 10 mL beaker that was weighed after each of three 30 min periods to determine the flow rate (F, kg s–1). F values for whole stems were used to calculate their average specific hydraulic conductivities (KS, kg m–1 s–1 MPa–1) according to Equation 1.
The reciprocal of KS (1/KS) is defined as the specific hydraulic resistivity (
Normalizing RSTEM by the effective cross-section area (ASTEM), calculated as the length-weighted average of the cross sections for all the internodes in the stem, and the length of the stem (LSTEM) results in
RINT is compounded from the resistances in series for each internode in the stem. Changing RSTEM by using the definition of hydraulic resistance and expressing RINT as the summation of the resistances of all the individual internodes allowed us to obtain Equation 4, where
Equation 5 was obtained by rearranging terms in Equation 4 and expressing some of them as resistivities. Equation 5 shows the calculation procedure used to estimate
Hydraulic resistance values for all internodes are required to compute
Excised internodes from treated and control plants were fixed in formalin, acetic acid, and alcohol (Johansen, 1940
Healthy leaves from mock-inoculated plants (control), nonsymptomatic (asymp), and visually symptomatic (symp) leaves from infected plants, were detached and immediately incubated in custom-built, flat, Plexiglas chambers (200 x 205 x 20 mm, 450 mL internal volume). The leaves weighed about 4 g each. In visually symptomatic leaves approximately 30% of the lamina presented chlorosis, some leaves also presented small necrotic spots. One milliliter gas samples from the chamber's headspace were taken through a septum fitted in the wall of the chambers and injected into a gas chromatograph Carl 211 (Chandler Engineering) equipped with an F-1 alumina column and a flame ionization detector. Nitrogen gas at 30 mL min–1, 80°C was used as carrier.
The data were analyzed using the statistical package JMP, version 5.1 (SAS Institute). A one-way ANOVA test was used to analyze the differences between the means of inoculation and ethylene treatments, and the controls. The one-sided version of the test was used in Figures 12 and 13 to determine if the treatments had higher
We thank Duarte Nurseries for their generosity in providing the grapevines used in this research, Dr. Caroline Roper for assistance in preparing the Xf cultures used for inoculation of grapevines, Dr. Katie Pinney for invaluable guidance in performing the histochemical analysis of stem tissues, Mr. Joshua Lenhof for assistance with sample handling and histochemical analysis, and Professor Wendy Silk for her careful reading of the manuscript and useful suggestions. Received July 21, 2006; accepted December 1, 2006; published December 22, 2006.
1 This work was supported by the Pierce's Disease Control Program and the Glassy-Winged Sharpshooter Board (California Department of Food and Agriculture grant no. 03–0283 to K.A.S. and J.M.L.). 
2 Present address: Departamento de Fruticultura y Enología, Facultad de Agronomía e Ingeniería Forestal, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago, Chile. 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: John M. Labavitch (jmlabavitch{at}ucdavis.edu).
[OA] Open Access articles can be viewed online without a subscription. www.plantphysiol.org/cgi/doi/10.1104/pp.106.087023 * Corresponding author; e-mail jmlabavitch{at}ucdavis.edu; fax 530–752–2278.
Agüero CB, Uratsu SL, Greve C, Powell ALT, Labavitch JM, Meredith CP, Dandekar AM (2005) Evaluation of tolerance to Pierce's disease and Botrytis in transgenic plants of Vitis vinifera L. expressing the pear PGIP gene. Mol Plant Pathol 6: 43–51[CrossRef] Blümich B (2000) NMR Imaging of Materials. Oxford Science Publications, New York Brooks RM, Bradley MV, Anderson TI (1950) Plant Microtechnique Manual. University of California, Davis, CA Clearwater MJ, Clark CJ (2003) In vivo magnetic resonance imaging of xylem vessel contents in woody lianas. Plant Cell Environ 26: 1205–1214[CrossRef] da Silva FR, Vettore AL, Kemper EL, Leite A, Arruda P (2001) Fastidian gum: the Xylella fastidiosa exopolysaccharide possibly involved in bacterial pathogenicity. FEMS Microbiol Lett 203: 165–171[Web of Science][Medline] Davis MJ, French WJ, Schaad NW (1981) Axenic culture of the bacteria associated with phony disease of peach and plum leaf scald. Curr Microbiol 6: 309–314[CrossRef][Web of Science] Fry SM, Milholland RD (1990) Response of resistant, tolerant and susceptible grapevine tissues to invasion by the Pierce's bacterium, Xylella fastidiosa. Phytopathology 80: 66–69[CrossRef][Web of Science] Hacke UG, Sperry JS, Pittermann J (2000) Drought experience and cavitation resistance in six shrubs from the Great Basin, Utah. Basic Appl Ecol 1: 31–41[CrossRef] Holbrook NM, Ahrens ET, Burns MJ, Zwieniecki MA (2001) In vivo observation of cavitation and embolism repair using magnetic resonance imaging. Plant Physiol 126: 27–31 Hopkins DL (1989) Xylella fastidiosa: xylem-limited bacterial pathogen of plants. Annu Rev Phytopathol 27: 271–290[CrossRef][Web of Science] Johansen DA (1940) Plant Microtechnique. McGraw-Hill, New York Krivanek AF, Stevenson JF, Walker MA (2005) Development and comparison of symptom indices for quantifying grapevine resistance to Pierce's disease. Phytopathology 95: 36–43[Medline] Lo Gullo MA, Trifilo P, Raimondo F (2000) Hydraulic architecture and water relations of Spartium junceum branches affected by a mycoplasm disease. Plant Cell Environ 23: 1079–1088[CrossRef] McElrone AJ, Sherald JL, Forseth IN (2003) Interactive effects of water stress and xylem-limited bacterial infection on the water relations of a host vine. J Exp Bot 54: 419–430 Mollenhauer HA, Hopkins DL (1976) Xylem morphology of Pierce's disease-infected grapevines with different levels of tolerance. Physiol Plant Pathol 9: 95–100 Mollenhauer HH, Hopkins DL (1974) Ultraestructural study of Pierce's disease bacterium in grape xylem tissue. J Bacteriol 119: 612–618 Newbanks D, Bosch A, Zimmermann MH (1983) Evidence of xylem dysfunction by embolization in Dutch elm disease. Phytopathology 73: 1060–1063[Web of Science] Newman KL, Almeida RPP, Purcell AH, Lindow SE (2003) Use of a green fluorescent strain for analysis of Xylella fastidiosa colonization of Vitis vinifera. Appl Environ Microbiol 69: 7319–7327 Newman KL, Almeida RPP, Purcell AH, Lindow SE (2004) Cell-to-cell signaling controls Xylella fastidiosa interactions with both insects and plants. Proc Natl Acad Sci USA 101: 1737–1742 Osiro D, Colnago LA, Otoboni AMMB, Lemos EGM, de Souza AA, Filho HDC, Machado MA (2004) A kinetic model for Xylella fastidiosa adhesion, biofilm formation, and virulence. FEMS Microbiol Lett 236: 313–318[CrossRef][Web of Science][Medline] Scarpari LM, Lambais MR, Silva DS, Carraro DM, Carrer H (2003) Expression of putative pathogenicity-related genes in Xylella fastidiosa grown at low and high cell density conditions in vitro. FEMS Microbiol Lett 222: 83–92[CrossRef][Web of Science][Medline] Schultz HR, Matthews MA (1993) Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.): evidence that low light uncouples water transport capacity from leaf area. Planta 190: 393–406[Web of Science] Simpson AJG, Reinach FC, Arruda P, Abreu FA, Acencio M, Alvarenga R, Alves LMC, Araya JE, Baia GS, Baptista CS, et al (2000) The genome sequence of the plant pathogen Xylella fastidiosa. Nature 406: 151–157[CrossRef][Medline] Sokal RR, Rohlf FJ (1995) Biometry. The Principles and Practice of Statistics in Biological Research, Ed 3. Freeman, New York Sperry JS (2003) Evolution of water transport and xylem structure. Int J Plant Sci 164: S115–S127[CrossRef][Web of Science] Stevenson JF, Matthews MA, Greve LC, Labavitch JM, Rost TL (2004) Grapevine susceptibility to Pierce's disease II: progression of anatomical symptoms. Am J Enol Vitic 55: 238–245 Tyree MT, Ewers FW (1991) Tansley review no. 34: the hydraulic architecture of trees and other woody plants. New Phytol 119: 345–360[CrossRef][Web of Science] Tyree MT, Zimmermann MH (2002) Xylem Structure and the Ascent of Sap, Ed 2. Springer, Berlin van Ieperen W, van Meeteren U, van Gelder H (2000) Fluid ionic composition influences hydraulic conductance of xylem conduits. J Exp Bot 51: 769–776 VanderMolen GE, Labavitch JM, Strand LL, DeVay JE (1983) Pathogen-induced vascular gels: ethylene as a host intermediate. Physiol Plant 59: 573–580[CrossRef] Varela LG, Smith RJ, Phillips PA (2001) Pierce's Disease. Publication 21600. University of California, Division of Agricultural and Natural Resources, Oakland, CA Weis KG, Polito VS, 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[Abstract] Wulff NA, Carrer H, Pascholati SF (2003) Cloning and expression of cellulase XF-818 of Xylella fastidiosa in Escherichia coli. Scientia Agricola 60: 715–721 Zwieniecki MA, Melcher PJ, Holbrook NM (2001) Hydrogel control of xylem hydraulic resistance in plants. Science 291: 1059–1062 This article has been cited by other articles:
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= 0.05) is also shown. AX = –0.058114 + 0.4708913 AS (P < 0.0001; r2 = 0.8038); the slope is not significantly different from 0.5.
S) of whole stems (
-1,4-glucanase (Wulff et al., 2003
P (MPa) represents the pressure differential between the two ends of the stem segment and 
where AINT is the average cross section of all the internodes and Li is the length of the individual internodes. This method yielded values very similar to those obtained using the far-right side of 
