Altered ARA2 (RABA1a) expression in Arabidopsis reveals the involvement of a Rab/YPT family member in auxin-mediated responses

Ras super family proteins serve as molecular switches regulating many different cellular processes. However, given the large number of family members, sequence information has provided little insight into the function of individual proteins. This study examined phenotypic alterations in an Arabidopsis ara2 mutant, in which a Ras super family member-encoding gene is disrupted by a T-DNA insertion. Although one mutant line (Salk-013811) was hypersensitive to auxin, its T-DNA insertion was in the 5-UTR of ARA2. Thus, we examined a true ARA2 knock-out mutant (Salk-077747) which contains an insertion in the first exon of ARA2. We found that ARA2 expression is responsive to auxin and at low concentrations, ara2 mutant plants exhibit increased numbers of lateral roots. ARA2 overexpression causes plants to exhibit hypersensitivity to auxin, due to altered expression of auxin-responsive genes, and these plants exhibited reduced numbers of lateral roots. A GFP-ARA2 fusion protein localized to the endosomes, suggesting that ARA2 may play a role in vesicle trafficking of components involved in polar auxin transport. Taken together, these results show that ARA2 is an essential component of a pathway that couples auxin signaling to plant growth and development.