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Published on February 11, 2009; 10.1104/pp.109.135301

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Received January 5, 2009
Accepted January 28, 2009

A Role for the TOC Complex in Arabidopsis Root Gravitropism

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

Laboratory of Genetics and Department of Botany, University of Wisconsin-Madison, Wisconsin, 53706

* Corresponding author; email: phmasson{at}wisc.edu.

Arabidopsis 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 (pgm1) display an attenuated response to gravistimulation. The altered response to gravity 1 (arg1) mutant is known to be involved with the early phases of gravity signal transduction [Boonsirichai et al., 2003, Plant Cell 15: 2612-2625]. arg1 responds slowly to gravistimulation, and is in a genetically distinct pathway from pgm1, as pgm1 mutants enhance the gravitropic defect of arg1 [Guan et al., 2003, Plant Physiol. 133: 100-112]. arg1 seeds were mutagenized with EMS to identify new mutants that enhance the gravitropic defect of arg1. Two modifier of arg1 mutants (mar1; mar2) grow in random directions only when arg1 is present, do not affect phototropism, and respond like wild-type to application of phytohormones. Both have mutations affecting different components of the TOC complex. mar1 possesses a mutation in the Translocon of Outer Membrane of Chloroplast 75 (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 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.




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J. Biol. Chem., July 10, 2009; 284(28): 19132 - 19141.
[Abstract] [Full Text] [PDF]


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