Abstract
Although plastid sedimentation has long been recognized as important for a plant's perception of gravity, it was recently shown that plastids play an additional function in gravitropism. The Translocon at the Outer envelope membrane of Chloroplasts (TOC) complex transports nuclear-encoded proteins into plastids, and a receptor of this complex, Toc132, was previously hypothesized to contribute to gravitropism either by directly functioning as a gravity signal transducer or by indirectly mediating the plastid localization of a gravity signal transducer. Here we show that mutations in multiple genes encoding TOC complex components affect gravitropism in a genetically sensitized background and that the cytoplasmic acidic domain of Toc132 is not required for its involvement in this process. Furthermore, mutations in TOC132 enhance the gravitropic defect of a mutant whose amyloplasts lack starch. Finally, we show that the levels of several nuclear-encoded root proteins are altered in toc132 mutants. These data suggest that the TOC complex indirectly mediates gravity signal transduction in Arabidopsis and support the idea that plastids are involved in gravitropism not only through their ability to sediment but also as part of the signal transduction mechanism.
Highlights
Root gravitropism allows plants to anchor themselves while exploring their environments to gain access to water and nutrients and to avoid obstacles and toxins
MUTATIONS IN TOC120 AND TOC34/PPI3 ENHANCE THE arg1 PHENOTYPE If Toc132 acts as a ligand as proposed in the direct interaction model, mutations in other the Outer envelope membrane of Chloroplasts (TOC) complex receptors are unlikely to enhance the arg1 gravitropic defect
If Toc132 mediates the gravitropic response through its role in plastid protein import, mutations in other TOC complex receptor genes such as TOC120 and TOC34/PPI3 may enhance the arg1 mutant phenotype
Summary
Root gravitropism allows plants to anchor themselves while exploring their environments to gain access to water and nutrients and to avoid obstacles and toxins. Amyloplast sedimentation triggers changes in the localization of plasma membraneassociated auxin efflux facilitators, leading to the accumulation of auxin on the lower side of the root. Upon transmission to the elongation zones, the resulting auxin gradient promotes differential cellular elongation between the upper and lower flanks, resulting in downward curvature. Possible second messengers in this process include Ca2+, inositol 1,4,5-triphosphate, and protons. It is still unknown how amyloplast sedimentation leads to an auxin gradient (reviewed in Strohm et al, 2012)
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