Mitogen-activated protein kinase 3/6 regulates the stability of AtIAA3 and AtIAA7 during auxin signaling in Arabidopsis

Abstract

Auxin mediated Aux/IAA degradation is required to release the ARFs from the control of IAAs, and ARFs in the free forms perform their role of transcription activation or suppression in response to developmental ques. Auxin is known to tag IAAs for proteasomal degradation, but how this tagging is regulated has not been widely explored. Here we report that, in Arabidopsis, exogenous application of auxin activates MPK3/6 which in turn phosphorylate IAA3 and IAA7 at Ser-58 and Ser-26, respectively. Further, incubation of IAA3 and IAA7 with the protein extracts from auxin treated mpk3 or mpk6 single mutants increase the rate of degradation of IAAs. Consequently, the phospho-null mutants, IAA3S58A and IAA7S26A were observed to be comparatively more stable. Thus, MAP kinase-mediated phosphorylation destabilised IAA3 and IAA7 leading to their degradation. Additionally, over-expression of the phospho-dead mutant of IAA3 (35S:IAA3S58A) and complementation of iaa3 mutant with this phospho-dead mutant resulted in reduced primary root length because of increased stability and accumulation of IAA3. Interestingly, we found that a member of ARF, ARF7 regulated the expression of MPKs by binding to their respective promoters.