A Role for the TOC Complex in Arabidopsis Root Gravitropism    

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.

In Vivo Analyses of the Roles of Essential Omp85-Related Proteins in the Chloroplast Outer Envelope Membrane

“…In agreement with our results on atToc75-IIIY ( Fig. 2A), the mar1 mutant was observed to exhibit a significant chlorophyll deficiency, although pigment concentrations (at approximately 70% of the wild-type level; Stanga et al, 2009) were considerably higher than those in the atToc75-IIIY number 6 line. To further corroborate the results obtained using atToc75-IIIY RNAi lines, we conducted a series of experiments on mar1, which shall henceforth be referred to as toc75-III-3 in accordance with the previously established nomenclature for atToc75-III mutants (Baldwin et al, 2005).…”

“…The modifier of altered response to gravity1 (mar1) mutant was recently identified in a screen for modifiers of the altered response to gravity1 mutation (Stanga et al, 2009). The mar1 mutant was found to carry a point mutation in the atTOC75-III gene, which causes a single amino acid change (G658R) within the seventh predicted transmembrane b-strand of the channel domain (Ertel et al, 2005).…”

Phytochromes inhibit hypocotyl negative gravitropism by regulating the development of endodermal amyloplasts through phytochrome-interacting factors

“…Among these steps, gravity sensing requires starch-filled amyloplasts in root columella cells for root gravitropism or in shoot endodermis for hypocotyl and shoot negative gravitropism (14,15). Consistent with the critical role of endodermis in shoot gravity sensing, the scarecrow (scr) and the shortroot (shr) mutants, which do not develop an endodermis, do not display shoot negative gravitropism (16,17).…”

Suppressors of the Chloroplast Protein Import Mutant tic40 Reveal a Genetic Link between Protein Import and Thylakoid Biogenesis

“…All of these mutants display a degree of chlorosis linked to defective import, with the lesions occurring at either the outer membrane (ppi1 is a null mutant for the Toc33 receptor; toc75-III-3 is a point mutant of the Toc75 channel) or the inner membrane (hsp93-V is a null mutant for the Hsp93/ ClpC chaperone; tic110 is a heterozygous mutant for a chaperone recruitment scaffold protein) (Jarvis et al, 1998;Kubis et al, 2003;Kovacheva et al, 2005;Stanga et al, 2009;Huang et al, 2011). Double mutants were generated and their phenotypes were (B) and (C) Chloroplast subfractionation analysis.…”

“…The stic1 and stic2 T-DNA mutants, alb4-1 (SALK_136199), stic2-3 (SALK_001500), and stic2-4 (WiscDsLox445D01), were all identified using the T-DNA Express website provided by the Salk Institute Genomic Analysis Laboratory (SIGnAL) and obtained from the Nottingham Arabidopsis Stock Centre or the ABRC (Alonso et al, 2003;Woody et al, 2007). The mutants tic40-3 (Koncz line N33230), tic110-1 (SAIL_896_D08), hsp93-V-1 (SAIL_873_G11) (Kovacheva et al, 2005), ppi1 (Jarvis et al, 1998), and toc75-III-3 (mar1; introgressed into the Col-0 ecotype) (Stanga et al, 2009;Huang et al, 2011) were all previously described. The thylakoid targeting pathway component mutants cpsrp54-3 (WiscDsLox289_292B14), cpftsY-1 (SALK_049077), cpftsY-2 (SALK_112451), and secA-1 (SALK_063371) were also previously characterized (Asakura et al, 2008;Liu et al, 2010;Yu et al, 2012).…”