A Role for the TOC Complex in Arabidopsis Root Gravitropism

John P Stanga,Patrick H Masson,Kanokporn Boonsirichai,Marisa S Otegui,John C Sedbrook

doi:10.1104/pp.109.135301

John P Stanga, Patrick H Masson + Show 3 more

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https://doi.org/10.1104/pp.109.135301

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Journal: Plant Physiology	Publication Date: Feb 11, 2009
Citations: 83	License type: CC BY 4.0

Affiliation: University of Wisconsin–Madison

Abstract

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.

Highlights

Arabidopsis (Arabidopsis thaliana) roots perceive gravity and reorient their growth
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; Boonsirichai et al, 2003; Guan et al, 2003; Harrison and Masson, 2008). arg1 and arl2 mutants affect gravitropism without altering starch accumulation, root growth response to phytohormones or polar auxin transport inhibitors, or phototropism (Sedbrook et al, 1999; Boonsirichai et al, 2003; Guan et al, 2003)
The roots and hypocotyls of arg1-2 mar1-1 and arg1-2 mar2-1 double mutants grow in random directions (Fig. 1; Supplemental Fig. S1)

Summary

Introduction

Arabidopsis (Arabidopsis thaliana) roots perceive gravity and reorient their growth . Starch-dense amyloplasts within the columella cells of the root cap are important for gravitropism, and starchless mutants such as pgm display an attenuated response to gravistimulation. Root cap amyloplasts in mar arg appear ultrastructurally normal They saltate like the wild type and sediment at wild-type rates upon gravistimulation. Close associations between sedimenting amyloplasts and the endoplasmic reticulum in root statocytes may activate mechanosensitive ion channels (Sack, 1991) Another model proposes that the cell can sense the pressure exerted by the entire protoplast. Arg and arl mutants affect gravitropism without altering starch accumulation, root growth response to phytohormones or polar auxin transport inhibitors, or phototropism (Sedbrook et al, 1999; Boonsirichai et al, 2003; Guan et al, 2003). Arg and arl are ideally sensitized backgrounds that could be used in a genetic modifier screen to identify new gravity signal transducers functioning in the PGM1 pathway

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