Abstract
We have isolated a number of shoot gravitropism (sgr) mutants of Arabidopsis to elucidate the molecular mechanism of shoot gravitropism. We have previously shown that the endodermal cells, which contain sedimented amyloplasts, are essential for shoot gravitropism. The sgr6 recessive mutant exhibited reduced gravitropic response in the inflorescence stem. Amyloplasts were normally sedimented to the direction of gravity in the endodermal cell of sgr6. In addition, sgr6 exhibited normal phototropism. The SGR6 gene encodes a protein with 1703 amino acids containing two HEAT repeat motifs. Endodermis-specific expression of the SGR6 using the SCR promoter restored shoot gravitropism of sgr6-1. These results suggest that SGR6, which is a novel big protein with unknown function, is involved in the gravi-perception and the signal transduction process which take place in the encodermal cells.
