EARLY STARVATION 1 Is a Functionally Conserved Protein Promoting Gravitropic Responses in Plants by Forming Starch Granules.

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 () 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 mutants that are allelic to the previously reported mutant, which is rapidly depleted of starch just before the dawn. orthologs are present in starch-producing green organisms, suggesting is a functionally conserved protein necessary for the formation of starch granules. Consistent with this, we found that liverwort and rice can complement the Arabidopsis mutant phenotype for both starch granules and hypocotyl negative gravitropism. To further investigate the function of in other plants, we isolated rice 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 mutants also lack starch granules in root columella and show reduced root gravitropism. Together, these results indicate is a functionally conserved protein that promotes gravitropic responses in plants via its role in starch granule formation.