AUTHOR=Luo Sha , Li Qianqian , Liu Shanda , Pinas Nicholaas M. , Tian Hainan , Wang Shucai 

TITLE=Constitutive Expression of OsIAA9 Affects Starch Granules Accumulation and Root Gravitropic Response in Arabidopsis

JOURNAL=Frontiers in Plant Science

VOLUME=6

YEAR=2015

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.01156

DOI=10.3389/fpls.2015.01156

ISSN=1664-462X

ABSTRACT=<p><italic>Auxin</italic>/<italic>Indole-3-Acetic Acid</italic> (<italic>Aux</italic>/<italic>IAA</italic>) genes are early auxin response genes ecoding short-lived transcriptional repressors, which regulate auxin signaling in plants by interplay with Auxin Response Factors (ARFs). Most of the Aux/IAA proteins contain four different domains, namely Domain I, Domain II, Domain III, and Domain IV. So far all <italic>Aux</italic>/<italic>IAA</italic> mutants with auxin-related phenotypes identified in both <italic>Arabidopsis</italic> and rice (<italic>Oryza sativa</italic>) are dominant gain-of-function mutants with mutations in Domain II of the corresponding Aux/IAA proteins, suggest that Aux/IAA proteins in both <italic>Arabidopsis</italic> and rice are largely functional redundantly, and they may have conserved functions. We report here the functional characterization of a rice <italic>Aux</italic>/<italic>IAA</italic> gene, <italic>OsIAA9</italic>. RT-PCR results showed that expression of <italic>OsIAA9</italic> was induced by exogenously applied auxin, suggesting that <italic>OsIAA9</italic> is an auxin response gene. Bioinformatic analysis showed that OsIAA9 has a repressor motif in Domain I, a degron in Domain II, and the conserved amino acid signatures for protein-protein interactions in Domain III and Domain IV. By generating transgenic plants expressing <italic>GFP-OsIAA9</italic> and examining florescence in the transgenic plants, we found that OsIAA9 is localized in the nucleus. When transfected into protoplasts isolated from rosette leaves of <italic>Arabidopsis</italic>, OsIAA9 repressed reporter gene expression, and the repression was partially released by exogenously IAA. These results suggest that OsIAA9 is a canonical Aux/IAA protein. Protoplast transfection assays showed that OsIAA9 interacted ARF5, but not ARF6, 7, 8 and 19. Transgenic <italic>Arabidopsis</italic> plants expressing <italic>OsIAA9</italic> have increased number of lateral roots, and reduced gravitropic response. Further analysis showed that <italic>OsIAA9</italic> transgenic <italic>Arabidopsis</italic> plants accumulated fewer granules in their root tips and the distribution of granules was also affected. Taken together, our study showed that OsIAA9 is a transcriptional repressor, and it regulates gravitropic response when expressed in <italic>Arabidopsis</italic> by regulating granules accumulation and distribution in root tips.</p>