A SNARE complex containing SGR3/AtVAM3 and ZIG/VTI11 in gravity-sensing cells is important for Arabidopsis shoot gravitropism

Plants can sense the direction of gravity and change the growth orientation of their organs. The molecular mechanisms of gravity sensing and signal transduction during gravitropism are not well known. We have isolated several shoot gravitropism (sgr) mutants of Arabidopsis. The sgr3-1 mutant exhibits a reduced gravitropic response in the inflorescence stems. in the inflorescence stems of Arabidopsis, gravity is sensed in endodermal cells that contain sedimentable amyloplasts. In sgr3-1, some amyloplasts in the endodermis failed to sediment in the direction of gravity. SGR3 encodes a syntaxin, AtVAM3, which had previously been cloned as a homologue of yeast Vam3p. AtVAM3 is localized to the prevacuolar compartment and vacuole and is suggested to function in vesicle transport to the vacuole. We have also cloned another soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), ZIG/AtVTI11, a mutation that causes abnormal gravitropism. This mutant displayed an abnormal distribution of amyloplasts in the endodermal cells similar to that in sgr3-1. Endodermis-specific expression of SGR3 and ZIG by using the SCR promoter could complement the abnormal shoot gravitropism of each mutant. Protein-protein interaction between AtVAM3 and AtVTI11 in the endodermal cells was detected immunologically. The sgr3-1 mutation appeared to reduce the affinity of AtVAM3 for AtVTI11 or SYP5. These results suggest that vesicle transport to the prevacuolar compartment/vacuole in the endodermal cells, mediated by a specific SNARE complex containing AtVAM3 and AtVTI11, plays an important role in shoot gravitropism.