EHB1 and AGD12, two calcium-dependent proteins affect gravitropism antagonistically in Arabidopsis thaliana

The ADP-RIBOSYLATION FACTOR GTPase-ACTIVATING PROTEIN (AGD) 12, a member of the ARF-GAP protein family, affects gravitropism in Arabidopsis thaliana. A loss-of-function mutant lacking AGD12 displayed diminished gravitropism in roots and hypocotyls indicating that both organs are affected by this regulator. AGD12 is structurally related to ENHANCED BENDING (EHB) 1, previously described as a negative effector of gravitropism. In contrast to agd12 mutants, ehb1 loss-of function seedlings displayed enhanced gravitropic bending. While EHB1 and AGD12 both possess a C-terminal C2/CaLB-domain, EHB1 lacks the N-terminal ARF-GAP domain present in AGD12. Subcellular localization analysis using Brefeldin A indicated that both proteins are elements of the trans Golgi network. Physiological analyses provided evidence that gravitropic signaling might operate via an antagonistic interaction of ARF-GAP (AGD12) and EHB1 in their Ca2+-activated states.