Regulation Of Auxin Signaling And Root Growth By Aux/IAA Transcription Factor OsIAA9 In Arabidopsis

Auxin regulates a varity processes of plant growth and development. Auxin signaling is extensively studied in dicots plant Arabidopsis thaliana but little is known about auxin signaling in monocot plant rice.Two differsent classes of transcript factors are involved in the regulation of auxin signaling, AUXIN/INDOLE ACETIC ACID (Aux/IAAs) and AUXIN RESPONSE FACTOR (ARFs). ARFs bind directly to auxin response elements (AuxRE) in the promoter region of auxin-responsive genes via their conserved DNA-binding-domain (DBD. When the IAA level is low, Aux/IAA proteins bind to ARFs, preventing the expression of auxin-responsive genes. High level of auxin promotes ubiquitination and the degradation of Aux/IAAs through SCFTIR1/AFB and the 26S proteasome, leading to the action of auxin-response genes by ARFs.Most Aux/IAA proteins have for conserved amino acid domains, namely domain Ⅰ, Ⅱ Ⅲ and IV. Domain I is a respression domain, which contains a LxLxL motif. Domain Ⅱ functions as a degron, and is responsible for auxin-dependent degradation of Aux/IAA proteins. Domain III and IV mediate homo- and heterodimerization between Aux/IAA proteins and heterodimerzation between Aux/IAA proteins and ARF proteins.OsIAA9 is a canonical Aux/IAA protein. We examined subcellular localization of OsIAA9 by generating transgenic Arabidopsis plants expressiong GFP-OsIAA,, and found that OsIAA9 protein is localized in the nucleus. By using protoplast transfection assays, we found OsIAA9 repressed reporter gene expression, and the repression was ratically relased by IAA. These results suggest that OsIAA9 is a transcription repressor and its stability is affected by IAA.Based on protoplast assay results, we found that OsIAA9CTD can specific bind to ARF5, but not ARF6,7,8 and 19.RT-PCR results showed that the expression of the auxin-responsive genes tested in wild-type and OsIAA9 transgenetic plants remained largely unchanged. However, detailed analyses of DR5:GFP and DR5:GUS activity in wild-type and OsIAA9 transgenetic plants seedlings in response to IAA indicated that OsIAA9 may regulate auxin response in root.Transgenic plants expression OsIAA9 have increased number of lateral roots, and reduced gravitropic reponse.Taken together, our stuty showed that OsIAA9 is a transcriptional repressor, and it regulates lateral root formation and gravitropic response when expressed in Arabidopsis.