Abstract
Starch granules in the endodermis of plant hypocotyls act as statoliths that promote hypocotyl negative gravitropism—the directional growth of hypocotyls against gravity—in the dark. To identify the molecular components that regulate hypocotyl negative gravitropism, we performed a mutagenesis screen and isolated reduced gravitropic 1 (rgv1) mutants that lack starch granules in their hypocotyl endodermis and show reduced hypocotyl negative gravitropism in the dark. Using whole genome sequencing, we identified three different rgv1 mutants that are allelic to the previously reported early starvation 1 mutant, which is rapidly depleted of starch just before the dawn. ESV1 orthologs are present in starch-producing green organisms, suggesting ESV1 is a functionally conserved protein necessary for the formation of starch granules. Consistent with this, we found that liverwort and rice ESV1 can complement the Arabidopsis ESV1 mutant phenotype for both starch granules and hypocotyl negative gravitropism. To further investigate the function of ESV1 in other plants, we isolated rice ESV1 mutants and found that they show reduced levels of starch in their leaves and loosely packed starch granules in their grains. Both Arabidopsis and rice ESV1 mutants also lack starch granules in root columella and show reduced root gravitropism. Together, these results indicate ESV1 is a functionally conserved protein that promotes gravitropic responses in plants via its role in starch granule formation.
Highlights
IntroductionPlants need to adjust their growth and development in response to various environmental factors
As sessile organisms, plants need to adjust their growth and development in response to various environmental factors
We previously showed that the endodermis-specific expression of PIF1 is sufficient to rescue the endodermal starch granule and hypocotyl negative gravitropism phenotypes of pifQ mutants in the dark (Kim et al, 2011)
Summary
Plants need to adjust their growth and development in response to various environmental factors. ESV1 Maintains Statoliths in Statocytes sedimentation of amyloplasts, which are plastids filled with starch granules (Sack, 1991; Toyota et al, 2013) These amyloplasts move with gravity inside statocytes, gravity-sensing cells that include the columella cells in roots and the endodermal cells in hypocotyls and stems (Hashiguchi et al, 2013; Nakamura et al, 2019). Endodermal cells contain a large central vacuole that limits the movement of amyloplasts to transvacuolar strands and to the narrow spaces between the vacuolar and plasma membranes (Saito et al, 2005) Such constriction makes proper vacuolar membrane dynamics essential for shoot gravitropism. Several shoot gravitropism mutants that are defective in vacuolar membrane dynamics show abnormal localization and sedimentation of amyloplasts (Kato et al, 2002; Morita et al, 2002; Yano et al, 2003; Hashiguchi et al, 2014)
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