Arabidopsis ROP11 Small GTPase, Activated By ROPGEF1 And ROPGEF4, Is A Negative Regulator Of Abscisic Acid Responses

ROP GTPase(or ROP)is a group of plant-specific small G proteins. Their activities are regulated through alternative binding to GTP and GDP. The ROPs play important roles in plant growth and development, as well as their responses to environmental stresses. In this work, we performed a detailed functional analysis of ROP11, a member of ROP gene family in Arabidopsis, in regulating ABA signaling pathway.ROP11 is mainly expressed in guard cells of most plant organs. The expression of a constitutively active form of ROP11 (CA-ROP11) negatively regulate ABA-induced stomata closure, ABA-inhibited seed germination, seedling growth, and ABA-induced gene expression. The CA-ROP11 plants are also hypersensitive to drought, salt, and osmotic stresses. In contrast, rop11 knockout mutant and the plants expressing a dominant-negative form of ROP11 (DN-ROP11) exhibit opposite phenotypes compared to that of CA-ROP11 plants.ABA treatment does not affect ROP11 gene expression, but induces the translocation of CA-ROP11 from plasma membrane to nucleus. RopGEFs are plant-specific guanine exchange factors which regulate the activity of ROPs. Using various techniques, we demonstrated that, among the 14 RopGEFs in Arabidopsis, RopGEF1 and RopGEF4 have specific interactions with ROP11 in guard cells. The plants overexpressing RopGEF1 or RopGEF4 are relatively insensitive to ABA-induced stomata closure, a phenotype similar to CA-ROP11 plants. In contrast, the gef1gef4 double knockout mutant exhibits enhanced ABA sensitivity which is similar to rop11 and DN-ROP11 plants. Furthermore, we showed that ROP11 can interact with negative regulators of ABA signaling, ABI1 and ABI2, and ABA treatment reduce such interactions. RopGEF1 and RopGEF4 may interact with putative ABA receptor, RCAR1, on plasma membranes. This work has laid a good foundation for further elucidating the molecular mechanism of ROP11-regulated...