| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
First published online February 11, 2009; 10.1104/pp.109.135301 Plant Physiology 149:1896-1905 (2009) © 2009 American Society of Plant Biologists OPEN ACCESS ARTICLE
A Role for the TOC Complex in Arabidopsis Root Gravitropism1,[W],[OA]Laboratory of Genetics (J.P.S., K.B., J.C.S., P.H.M.) and Department of Botany (M.S.O.), University of Wisconsin, Madison, Wisconsin 53706
Arabidopsis (Arabidopsis thaliana) roots perceive gravity and reorient their growth accordingly. Starch-dense amyloplasts within the columella cells of the root cap are important for gravitropism, and starchless mutants such as pgm1 display an attenuated response to gravistimulation. The altered response to gravity1 (arg1) mutant is known to be involved with the early phases of gravity signal transduction. arg1 responds slowly to gravistimulation and is in a genetically distinct pathway from pgm1, as pgm1 mutants enhance the gravitropic defect of arg1. arg1 seeds were mutagenized with ethylmethane sulfonate to identify new mutants that enhance the gravitropic defect of arg1. Two modifier of arg1 mutants (mar1 and mar2) grow in random directions only when arg1 is present, do not affect phototropism, and respond like the wild type to application of phytohormones. Both have mutations affecting different components of the Translocon of Outer Membrane of Chloroplasts (TOC) complex. mar1 possesses a mutation in the TOC75-III gene; mar2 possesses a mutation in the TOC132 gene. Overexpression of TOC132 rescues the random growth phenotype of mar2 arg1 roots. Root cap amyloplasts in mar2 arg1 appear ultrastructurally normal. They saltate like the wild type and sediment at wild-type rates upon gravistimulation. These data point to a role for the plastidic TOC complex in gravity signal transduction within the statocytes.
Auxin and its asymmetrical redistribution across organs following gravistimulation are essential for normal gravitropic curvature responses (Chen et al., 2002  ). Yet the mechanisms that allow plant organs to perceive their orientation within the gravity field remain poorly understood (for review, see Harrison and Masson, 2008). The starch-statolith hypothesis posits that the sedimentation of starch-filled amyloplasts in root cap columella cells or endodermis cells provides directional cues to the root or hypocotyl and shoot, respectively (for reviews, see Boonsirichai et al., 2002; Morita and Tasaka, 2004). Importantly, starch-deficient pgm1 mutants lack a normal response to gravistimulation (Caspar and Pickard, 1989; Kiss et al., 1989), while the starch excess mutant sex1 displays an increased sensitivity to gravistimulation (Vitha et al., 2007). Likewise, mutants containing intermediate levels of starch display gravitropic sensitivity corresponding to the relative amount of starch present in either the roots or hypocotyls (Kiss et al., 1997). In the root cap in particular, laser ablation studies have demonstrated that the second story (S2) central columella cells contribute most to gravitropism (Blancaflor et al., 1998). Within these cells, the sedimentation of the mass of amyloplasts or a subset thereof is critical for gravity perception.
The sedimentation of the amyloplasts is relayed through an unknown mechanism, although several models have been proposed. The actin tensegrity model posits that sedimenting amyloplasts disrupt a dense actin network that is tethered to the plasma membrane. This disruption alters the tension applied to mechanosensitive ion channels, thereby activating them (Yoder et al., 2001
Forward genetic screens have been utilized to help unravel these complexities. We previously reported on ARG1, a J-domain protein that participates in early gravity signal transduction in statocytes, along with its paralog ARL2 (Sedbrook et al., 1999 We mutagenized arg1-2 seeds of Arabidopsis (Arabidopsis thaliana) with ethylmethane sulfonate (EMS) to identify mutants with enhanced root gravitropic defects. In particular, we hoped to find mutants that exacerbate the root phenotypes of arg1-2 while retaining wild-type amyloplast morphology and behavior as well as normal responses to phytohormones and polar auxin transport inhibitors. Here, we report the identification of two recessive mutants, modifier of arg1 (mar1) and mar2, both of which affect proteins in the Translocon of Outer Membrane of Chloroplasts (TOC) complex.
Most plastid-targeted proteins are encoded in the nuclear genome and translated in the cytosol (for review, see Jarvis, 2008
Mutagenesis of arg1-2 Identifies Two Genetic Enhancers
arg1-2 seeds were mutagenized with EMS to identify novel extragenic mutations that modify the phenotype of arg1-2. Plants were screened at the M2 generation to isolate mutants with extreme root phenotypes. Several individual lines were then transformed with Agrobacterium tumefaciens with genomic ARG1 DNA (Sedbrook et al., 1999
TOC Complex Genes Are Mutated in mar1-1 and mar2-1
Mapping populations for both mar1-1 and mar2-1 were created by crossing the mutant (Wassilewskija [Ws]) with Landsberg erecta (Ler). Recombinant breakpoint analysis of 1,113 segregating F2 or F3 plants using existing and newly derived PCR-based markers demonstrated that mar1-1 maps to a region on chromosome III containing 24 predicted genes (Fig. 2
), whereas mar2-1 maps to chromosome II between nga1145 and nga1126 (Fig. 2). Sequence analysis of candidate genes in mar1-1 revealed a C-T transition mutation in At3g46740, resulting in a G658R change. Amino acid 658 is a highly conserved amino acid in a pore-forming β-sheet of TOC75-III (Schleiff et al., 2003
TOC132 Rescues the Gravitropic Defect of mar2-1 arg1-2
arg1-2 mar2-1 plants expressing TOC132 cDNA under control of the cauliflower mosaic virus 35S promoter (Kubis et al., 2004
To determine if mar2-1 is also a null allele of TOC132, we analyzed its expression. Reverse transcription-PCR analysis of TOC132 expression using RNA derived from 10-d-old seedlings indicates that mar2 plants still produce TOC132 RNA. However, when proteins extracted from whole 10-d-old seedlings were probed by western blot using an antibody raised against the N-terminal 431 amino acids of TOC132 (Ivanova et al., 2004
mar2-1 Resembles the Wild Type Gravitropically
mar2-1 and mar1-1 produce randomly oriented roots and hypocotyls only when paired with the arg1-2 mutation. When mar2-1 single mutant seedlings are grown in the light embedded in 0.8% agar growth medium, they show no gravitropic defects following 90° reorientation (Fig. 4A
). Unlike mar2-1, the reorientation kinetics of the mar1-1 single mutant are slightly delayed following 90° reorientation (Fig. 4A). Furthermore, the mar1-1 single mutant plants are noticeably pale and smaller than wild-type plants. The paleness seen represents a 30% decrease in chlorophyll content (Supplemental Fig. S2). Although mar2-1 plants appear to accumulate slightly less chlorophyll than Ws plants, in agreement with previous findings, the difference is not statistically significant (P < 0.05) in our experiment (Kubis et al., 2004
arg1-2 mar2-1 Is Affected in Gravity Signal Transduction To test the possibility that mar2-1 might affect the differential growth response to lateral stimulation in an arg1-2 mutant background, we investigated the phototropic response of wild-type and mutant seedlings. Although the main focus of this project is on root gravitropism, we chose to analyze phototropism in the hypocotyl because this organ develops a stronger curvature response to lateral light stimulation than roots do under our growth conditions. We found that all single and double mutants have a normal phototropic response (Fig. 4C). Although these mutants retain apparently normal, starch-filled amyloplasts in the root cap (Fig. 5 ), they are likely to be deficient in the early phases of gravity signaling, because arg1-2 mar2-1 responds like the wild type (Ws) to exogenous application of brassinolide, cytokinin, auxin, the polar auxin transport inhibitor 1-N-naphthylphthalamic acid, and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (Supplemental Fig. S3).
arg1-2 mar2-1 Root Cap Amyloplast Morphology and Sedimentation Resemble Those of the Wild Type Because mutations in the TOC protein import machinery may affect plastid morphology (notably starch content) and/or behavior, we investigated the morphology and behavior of columella amyloplasts. Although iodine staining of the root statocytes showed starch accumulation in Ws, mar2-1, mar2-1 arg1-2, and arg1-2 seedlings (Fig. 5A), it was still possible that the overall size or morphology of the amyloplasts might be substantially different between mutant and the wild type. However, this was not the case, as transmission electron microscopy revealed similarly shaped plastids in Ws, arg1-2, mar2-1, and arg1-2 mar2-1 root tips (Fig. 5C). Next, we wanted to test the possibility that although the mar2-1 arg1-2 and Ws amyloplasts appeared indistinguishable, they might be sedimenting differently. Because the sedimentation of plastids after gravistimulation is a complex and dynamic process, we opted to observe it in living roots. Not only does this approach allow for careful observation of plastids over a time course, it also allows for the same root to be monitored over time. Therefore, we analyzed the behavior of amyloplasts in S2 columella cells in growing roots after 90° reorientation. Bright-field images of mar2-1 arg1-2 and Ws amyloplasts were collected every 10 s for about 12 min after gravistimulation. In these studies, mar2-1 arg1-2 amyloplasts were found to saltate like the wild type in columella cells of the root cap (Supplemental Movies S1 and S2; data not shown).
Preliminary examination of amyloplast movement upon gravistimulation also indicated that columella plastids sediment as a group. Therefore, the distances along both the x and y axes from the distal lower cell corner to the upper and proximal edges of the upper and leading amyloplast, respectively, were measured for every frame (Fig. 6A
). These measurements were chosen because it can be difficult to discriminate individual plastids in every frame, as they often move behind other plastids. A t test at each time point using a 95% probability threshold showed only five of 79 time comparisons as significantly different between Ws and arg1-2 mar2-1 for the x axis measurement (Fig. 6B) and zero of 79 for the y axis measurement (Fig. 6C), well within the range of expected type I error. As a control, we also included a mutant with intermediate levels of starch in its roots, ACG20. Under our experimental conditions, ACG20 roots do not respond to 90° gravistimulation slower than Ws roots (Supplemental Fig. S4). However, previous experiments detected slightly slower root reorientation and demonstrated a slower rate of amyloplast sedimentation in ACG20 (Kiss et al., 1997
mar2-1 Enhances the Phenotype of arl2-3; toc120-3 Does Not Enhance the Phenotype of arg1-2
By double mutant analyses, both the ARG1 and TOC132 genes have been shown to share some functional redundancy with their respective paralogs, ARL2 and TOC120 (Kubis et al., 2004
In this study, we took advantage of the relatively subtle gravitropic phenotype of arg1-2 roots in a genetic screen to identify mutants that have more severe root phenotypes. We were able to successfully identify two independent mutations in different members of the same protein complex. Interestingly, these mutants display a nearly identical random growth phenotype in the arg1-2 mutant background despite the toc75 mutant also having pleiotropic growth defects. These additional defects are likely due to the difference in ascribed functions of each component: TOC75 is the central pore in the outer membrane of plastids through which vital preproteins must pass in order to perform their functions. As such, insertional alleles at this locus are lethal (Baldwin et al., 2005 ). Unlike previously described toc75 lethal mutants, the toc75 mutation that we describe, mar1-1, is sufficient to attenuate, but not to eliminate, the function of TOC75. In contrast, TOC132 is one of a four-member family of receptors (TOC90, TOC120, TOC132, and TOC159) that displays functional specialization. Although specialized, these receptors also demonstrate functional redundancy. In particular, toc120 toc132 double mutants are more abnormal than either single mutant (Kubis et al., 2003). However, the toc120 mutant does not enhance the root phenotype of arg1-2, suggesting that TOC132 and TOC120 retain functional specialization despite sharing some functional redundancy.
The TOC complex likely participates in early gravity signaling, as the mutants respond like the wild type to phytohormones and drugs that affect the auxin-transport phase of the gravity response as well as to exogenously applied brassinolide, cytokinin, and an ethylene precursor. These hormones have all been implicated in various aspects of gravitropism (Li et al., 2005 As a plastid-based protein complex, there is a compelling possibility that mutants of the TOC complex may produce amyloplasts that are aberrant in their morphology or sedimentation. However, when we examined amyloplasts of the root columella cells by iodine staining and electron microscopy, we were unable to detect significant differences in starch accumulation or amyloplast morphology between mutant and the wild type. Furthermore, our in vivo analysis of plastid movements in central S2 columella cells revealed no behavioral differences: arg1-2 mar2-1 mutant amyloplasts saltate like the wild type, and they sediment at wild-type rates upon gravistimulation (Fig. 6; Supplemental Movies S1 and S2). Therefore, it is unlikely that the TOC complex participates in gravity signal transduction by altering the structure or density of amyloplasts. Likewise, the Ws-like sedimentation and saltation behaviors of the mutant plastids suggest that the actin cytoskeleton is not structurally altered.
Within the cell, the ARG1 and ARL2 proteins were shown to be associated with both the plasma membrane and components of the vesicle trafficking pathway, suggesting a role in mediating the targeting or activity of gravity signal transducers at the plasma membrane or at organelles of the secretory pathway (Boonsirichai et al., 2003
In the context of the ligand-interaction model of gravity sensing discussed above, it is important to note, once again, that TOC75 also contributes to the proper targeting of plastid outer envelope proteins, including members of the TOC159 family (Tu et al., 2004 ). Therefore, the missense mutation in TOC75 associated with mar1-1 may affect the proper targeting of the proposed plastid-associated transducer, which, in view of the latter observation, could conceivably be TOC132 itself, as illustrated in Figure 8B.
To our knowledge, while fully compatible with our genetic studies of the interaction between arg1, arl2, and mar mutations, the interaction model of gravity sensing discussed above has not been considered as a possible mechanism for gravity sensing in higher plants. It should be noted, however, that a ligand-receptor model of gravity sensing has been proposed to function in Chara rhizoids based on careful evaluations of gravity sensing in the context of parabolic-flight microgravity environments (Limbach et al., 2005
While the ligand interaction model of gravity sensing is quite attractive, we cannot rule out the possibility that the TOC complex affects gravitropism through an unidentified regulatory molecule that contributes to gravity signal transduction. Indeed, plastids are known to play a central role in the manufacture and/or storage of many diverse and important biological products (Neuhaus and Emes, 2000
Finally, our observations are in stark contrast to a recent report that details the plastid sedimentation of arg1-2 amyloplasts in endodermal cells of hypocotyls (Kumar et al., 2008 This report details, to our knowledge, an exciting new component of gravity signal transduction. Our current experiments are directed at uncovering the mechanism of TOC132 action in gravity signal transduction and its relation to the ARG1/ARL2 signaling pathway.
Plant Growth Conditions
Arabidopsis (Arabidopsis thaliana) seeds were sterilized in 20% bleach solution containing 0.01% SDS for 10 min followed by four rinses with distilled water, then plated with the appropriate medium. Plants were grown as described previously (Rutherford and Masson, 1996 Procedures used to test plant growth responses to phytohormones and polar auxin transport inhibitor are described in Supplemental Materials and Methods S1.
Unless otherwise indicated in the text, statistical comparisons were performed using unpaired two-tailed Student's t tests assuming equal variance where P < 0.05 was deemed significant.
arg1-2 seeds (Ws ecotype) were mutagenized (Joy, 2001
mar1-1 arg1-2 and mar2-1 arg1-2 plants were crossed with Ler to create a mapping population. DNA was extracted from leaves of F2 seedlings and used with PCR to genotype both the ARG1 locus and mapping markers throughout the genome. The MAR genotype could be inferred from the F3 generation root phenotypes if the arg1-2 allele was also present. Initial mapping linked mar1-1 to TOPP5 (chromosome III), while mar2-1 was linked to nga1145 and nga1126 (chromosome II). Additional molecular markers linked to mar1-1 include the simple sequence length polymorphism markers F1P2 (5'-AAGTGGTGGTTGGTTTTGTC-3' and 5'-ACCCCACTCTTCATTATTGTTAC-3') and F16L2 (5'-CATTAGTAACCAAAGACCAAAGAGACAC-3' and 5'-TTTTTGCAGGTACATAGAGC-3') and the cleaved-amplified polymorphic sequence (CAPS) markers SC10 (5'-CTTGCCACTCAGGTAGACGA-3' and 5'-ACTGAGGGAAGACTGGCGTA-3'; Sau3a), MMM2 (5'-GCTCGTATTGACAAAGCTAACG-3' and 5'-CATACGAACACCACCGTTCA-3'; RsaI), and MMM3 (5'-TGGAAAATCGTAATCTTGTGGA-3' and 5'-GCGTGCATGTATGGATTAGG-3'; DdeI). CAPS markers were developed by sequencing noncoding regions of Ws and Ler to identify polymorphisms and then selecting appropriate restriction enzymes using New England Biolabs NEBcutter version 2.0 (http://www.neb.com). Using these molecular markers, recombinant breakpoint analysis mapped mar1-1 between At3g46590 and At3g46820. Candidate genes were then sequenced and compared with Ws sequence to identify a C-T transition mutation in TOC75-III (At3g46740). To facilitate genotyping of mar1-1 plants, this polymorphism was used with derived CAPS (dCAPS) finder (Neff et al., 2002 The transgenic rescue experiment aimed at verifying the molecular identity of mar2-1 is described in Supplemental Materials and Methods S1.
For the root gravitropism assay, seedlings were grown for 4 d embedded in 0.8% agar-containing GM in vertically oriented square petri dishes incubated within a Conviron TC16 growth chamber under our standard conditions. Plates were rotated 90°, and images were collected with a digital camera at various times over a 24-h period. For the hypocotyl gravitropism assay, seeds were sown on the surfaces of 0.8% agar-containing GM plates, wrapped in aluminum foil, and allowed to grow in a Conviron TC16 growth chamber under our standard conditions for 4 d. All plates were rotated 90° simultaneously and then unwrapped, and digital photographs were taken at various times over a 24-h period. Using NIH Image J, root and hypocotyl angles were then measured as described previously (Rutherford and Masson, 1996
The kinetics of hypocotyl phototropism were determined by growing Ws and mutant seedlings on vertically oriented 0.8% agar-containing GM plates wrapped in black paper and aluminum foil. After 3 d of growth, the plates were unwrapped on the top side only, and a 75-W incandescent lamp was shone on the exposed side for 18 h to allow all of the hypocotyls to orient upward. Next, the plates were unwrapped in the dark and mounted on an infrared backlight (Advanced Illumination). A 75-W incandescent lamp was positioned horizontally 12 inches away. Images of the hypocotyls were collected using an infrared-sensitive Marlin F146B CCD camera (Allied Vision Technologies) controlled by AVT SmartView software. Images were collected every 20 to 60 min over the course of 6 h. Using NIH Image J or Photoshop CS3 (Adobe), the angles between planes tangential to the hypocotyl tips and vertical planes were measured.
Five-day-old seedlings were stained in 10%/5% KI/I solution (Lugol). Stained roots were cleared with chloral hydrate prior to observation under bright-field microscopy using a Leica DM LB2 microscope and a 40x objective.
Three independent replicates of Ws, arg1-2, and ACG20 seeds (n = approximately 250–400) were plated on the surface of 0.8% agar GM plates. Plates were wrapped in foil and placed at 4°C in the dark for 48 h before moving to a Conviron TC16 growth chamber set to our standard conditions. Seeds were dark grown vertically for 4 d before collecting, weighing, and freezing the etiolated hypocotyls. Frozen samples were ground and used with the Megazyme Amyloglucosidase/ Procedures used to quantify chlorophyll levels in wild-type and mutant plants are described in Supplemental Materials and Methods S1.
Six-day-old seedlings grown on 1.5% agar-containing GM were loaded into sample holders filled with 0.1 M Suc and then frozen in a Baltec HPM 010 high-pressure freezer (Technotrade). Cryosubstitution and sample embedding were performed as described by Otegui and Staehelin (2004)
Following 2 to 3 d of stratification at 4°C in the dark, seeds (Ws, ACG20, or mar2-1 arg1-2) were germinated on a glass microscope slide containing approximately 500 µL of GM with 0.75% agar and grown in the light in a petri dish sealed with Parafilm. The slides were then mounted and sealed onto a custom-made chamber that allows the live, growing roots to be mounted on a microscope stage. Roots that were growing along the slide at 4 to 7 d after germination were analyzed with bright-field microscopy using a Nikon microscope placed on its side equipped with a rotatable stage. Roots were oriented in the vertical position for at least 20 min prior to gravistimulation. At time 0, the stage was rotated 90° and images were collected with a 40x objective as soon as possible following gravistimulation, typically 40 to 70 s. Images were collected every 10 s for approximately 13 min using a CCD camera and software (Spot RT-slider; Diagnostic Instruments). Sequential images were combined into a stack using NIH Image J. Movement of plastids in central S2 columella cells was then analyzed. For each image, plastid movement was measured relative to projections through a cell corner along either the old bottom of the cell or the new bottom of the cell. Two values were measured: X-movement, which is the distance in micrometers of the leading edge of the plastid located farthest from the cell edge (former cell bottom) at any given time point; and Y-movement, which is the distance in micrometers of the top-most edge of the plastid located farthest from the new cell bottom at any given time point. Measurements from seven individual Ws roots, nine individual mar2 arg1 roots, and five ACG20 roots were collected and grouped into 10-s bins.
The following materials are available in the online version of this article.
We thank Paul Jarvis for providing us with the 35S::TOC132 plasmid and toc132-3 and toc120-3 seeds. We thank Danny Schnell for providing us with anti-TOC132 antibody. We thank Chris Staiger for providing us with anti-actin antibody. We thank Carolyn Neal, John Kiss, and Katherine Baldwin for comments on the text and Katherine Baldwin for figure development. Received January 5, 2009; accepted January 28, 2009; published February 11, 2009.
1 This work was supported by the National Science Foundation (grant nos. MCB–0240084 and IOS–0642865 to P.H.M. and grant no. MCB–0619736 to M.S.O.), the National Aeronautics and Space Administration (grant no. NAG2–1602 to P.H.M.), and a fellowship to J.P.S. from the National Institutes of Health Training Grant in Genetics at the University of Wisconsin-Madison. This is manuscript 3642 of the Laboratory of Genetics. 
2 Present address: Thailand Institute of Nuclear Technology, Nakornnayok 26120, Thailand.
3 Present address: Department of Biological Sciences, Illinois State University, Normal, IL 61790. 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: Patrick H. Masson (phmasson{at}wisc.edu).
[W] The online version of this article contains Web-only data.
[OA] Open Access articles can be viewed online without a subscription. www.plantphysiol.org/cgi/doi/10.1104/pp.109.135301 * Corresponding author; e-mail phmasson{at}wisc.edu.
Aloni R, Aloni E, Langhans M, Ullrich C (2006) Role of cytokinin and auxin in shaping root architecture: regulating vascular differentiation, lateral root initiation, root apical dominance and root gravitropism. Ann Bot (Lond) 97: 883–893 Baldwin A, Wardle A, Patel R, Dudley P, Park S, Twell D, Inoue K, Jarvis P (2005) A molecular-genetic study of the Arabidopsis Toc75 gene family. Plant Physiol 138: 715–733 Blancaflor E, Fasano J, Gilroy S (1998) Mapping the functional roles of cap cells in the response of Arabidopsis primary roots to gravity. Plant Physiol 116: 213–222 Boonsirichai K, Guan C, Chen R, Masson P (2002) Root gravitropism: an experimental tool to investigate basic cellular and molecular processes underlying mechanosensing and signal transmission in plants. Annu Rev Plant Biol 53: 421–447[CrossRef][Medline] Boonsirichai K, Sedbrook J, Chen R, Gilroy S, Masson P (2003) ARG1 is a peripheral membrane protein that modulates gravity-induced cytoplasmic alkalinization and lateral auxin transport in plant statocytes. Plant Cell 15: 2612–2625 Buer C, Sukumar P, Muday G (2006) Ethylene modulates flavonoid accumulation and gravitropic responses in roots of Arabidopsis. Plant Physiol 140: 1384–1396 Caspar T, Pickard B (1989) Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing. Planta 177: 185–197[CrossRef][Web of Science][Medline] Chen R, Guan C, Boonsirichai K, Masson P (2002) Complex physiological and molecular processes underlying root gravitropism. Plant Mol Biol 49: 305–317[CrossRef][Web of Science][Medline] Clough S, Bent A (1998) Floral dip: a simplified method for Agrobacterium-mediated transformation of Arabidopsis thaliana. Plant J 16: 735–743[CrossRef][Web of Science][Medline] Guan C, Rosen E, Boonsirichai K, Poff K, Masson P (2003) The ARG1-LIKE2 (ARL2) gene of Arabidopsis thaliana functions in a gravity signal transduction pathway that is genetically distinct from the PGM pathway. Plant Physiol 133: 100–112 Harrison B (2007) Investigating the function of ARG1 and ARL2 as gravity signal transducers in Arabidopsis thaliana. PhD thesis. University of Wisconsin, Madison, WI Harrison B, Masson P (2008) ARL2, ARG1 and PIN3 define a gravity signal transduction pathway in root statocytes. Plant J 53: 380–392[CrossRef][Web of Science][Medline] Ivanova Y, Smith M, Chen K, Schnell D (2004) Members of the Toc159 import receptor family represent distinct pathways for protein targeting to plastids. Mol Biol Cell 15: 3379–3392 Jarvis P (2008) Targeting of nucleus-encoded proteins to chloroplasts in plants. New Phytol 179: 257–285[CrossRef][Web of Science][Medline] Jelic M, Soll J, Schleiff E (2003) Two Toc34 homologues with different properties. Biochemistry 42: 5906–5916[CrossRef][Web of Science][Medline] Joy R (2001) Arabidopsis embryonic development: a screen for mutants disrupted in pattern formation, and analysis of BOBBER, a gene required for proper specification of the apical region of the Arabidopsis embryo. PhD thesis. University of Wisconsin, Madison, WI Kessler F, Schnell D (2004) Chloroplast protein import: solve the GTPase riddle for entry. Trends Cell Biol 14: 334–338[CrossRef][Web of Science][Medline] Kiss J, Guisinger M, Miller A, Stackhouse K (1997) Reduced gravitropism in hypocotyls of starch-deficient mutants of Arabidopsis. Plant Cell Physiol 38: 518–525 Kiss J, Hertel R, Sack F (1989) Amyloplasts are necessary for full gravitropic sensitivity in roots of Arabidopsis thaliana. Planta 177: 198–206[CrossRef][Web of Science][Medline] Kubis S, Baldwin A, Patel R, Razzaq A, Dupree P, Lilley K, Kurth J, Leister D, Jarvis P (2003) The Arabidopsis ppi1 mutant is specifically defective in the expression, chloroplast import, and accumulation of photosynthetic proteins. Plant Cell 15: 1859–1871 Kubis S, Patel R, Combe J, Bedard J, Kovacheva S, Lilley K, Biehl A, Leister D, Rios G, Koncz C, et al (2004) Functional specialization amongst the Arabidopsis Toc159 family of chloroplast protein import receptors. Plant Cell 16: 2059–2077 Kumar N, Stevens M, Kiss J (2008) Plastid movement in statocytes of the ARG1 (ALTERED RESPONSE TO GRAVITY) mutant. Am J Bot 95: 177–184 Li L, Xu J, Xu Z, Xue H (2005) Brassinosteroids stimulate plant tropisms through modulation of polar auxin transport in Brassica and Arabidopsis. Plant Cell 17: 2738–2753 Limbach C, Hauslage J, Schäfer C, Braun M (2005) How to activate a plant gravireceptor: early mechanisms of gravity sensing studied in Characean rhizoids during parabolic flights. Plant Physiol 139: 1030–1040 Morita M, Tasaka M (2004) Gravity sensing and signaling. Curr Opin Plant Biol 7: 712–718[CrossRef][Web of Science][Medline] Neff MM, Turk E, Kalishman M (2002) Web-based primer design for single nucleotide polymorphism analysis. Trends Genet 18: 613–615[CrossRef][Web of Science][Medline] Neuhaus H, Emes M (2000) Nonphotosynthetic metabolism in plastids. Annu Rev Plant Physiol Plant Mol Biol 51: 111–140[CrossRef][Web of Science] Nomura H, Komori T, Kobori M, Nakahira Y, Shiina T (2008) Evidence for chloroplast control of external Ca2+-induced cytosolic Ca2+ transients and stomatal closure. Plant J 53: 988–998[CrossRef][Web of Science][Medline] Otegui M, Staehelin L (2004) Electron tomographic analysis of post-meiotic cytokinesis during pollen development in Arabidopsis thaliana. Planta 218: 501–515[CrossRef][Web of Science][Medline] Rutherford R, Masson P (1996) Arabidopsis thaliana sku mutant seedlings show exaggerated surface-dependent alteration in root growth vector. Plant Physiol 111: 987–998[Abstract] Sack F (1991) Plant gravity sensing. Int Rev Cytol 127: 193–252[Web of Science][Medline] Schleiff E, Eichacker L, Eckart K, Becker T, Mirus O, Stahl T, Soll J (2003) Prediction of the plant β-barrel proteome: a case study of the chloroplast outer envelope. Protein Sci 12: 748–759[CrossRef][Web of Science][Medline] Sedbrook J, Chen R, Masson P (1999) ARG1 (Altered Response to Gravity) encodes a DnaJ-like protein that potentially interacts with the cytoskeleton. Proc Natl Acad Sci USA 96: 1140–1145 Staves M (1997) Cytoplasmic streaming and gravity sensing in Chara internodal cells. Planta (Suppl) 203: S79–S84[CrossRef][Web of Science][Medline] Tu S, Chen L, Smith M, Su Y, Schnell D, Li H (2004) Import pathways of chloroplast interior proteins and the outer-membrane protein OEP14 converge at Toc75. Plant Cell 16: 2078–2088 Vitha S, Yang M, Sack F, Kiss J (2007) Gravitropism in starch-excess mutant of Arabidopsis thaliana. Am J Bot 94: 590–598 Wolverton C, Ishikawa H, Evans M (2002) The kinetics of root gravitropism: dual motor and sensors. J Plant Growth Regul 21: 102–112[Medline] Yoder T, Zheng H, Todd P, Staehelin L (2001) Amyloplast sedimentation dynamics in maize columella cells support a new model for the gravity-sensing apparatus of roots. Plant Physiol 125: 1045–1060 Related articles in Plant Physiol.:
This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ASPB Publications | PLANT PHYSIOLOGY® | THE PLANT CELL | |
|---|---|---|---|
TOP
ABSTRACT
RESULTS
-Amylase Total Starch Kit (Megazyme; catalog code K-TSTA) following the dimethyl sulfoxide-modified procedure for samples containing resistant starch. 
