OsABCB14Functions In Auxin Transport And Iron Homeostasis In Rice (Oryza Sativa.L)

Polar auxin transport (PAT) in the plant organs to form a concentration gradient as the center of the top and maintain the differences of different tissues auxin concentration, thus regulates plant development. It plays an essential role in developmental processes, including embryonic morphogenesis, vascular differentiation, apical dominance and tropic growth. PAT is supposed to be directed by auxin influx and efflux carriers distributing polar in cells and the carriers can be divided into three families: Pin-formed (PIN) family, Auxin resistant1/like AUX1(AUX1/LAX) family and ATP binding cassette B/Multidrug-resistance/P-glycoprotein (ABCB/MDR/PGP) family. Of the three families, ABCBs are the least reported proteins in plants. However, to perfect auxin pathway and elaborate the exact regulation mechanism of PAT, the researches of ABCBs are essential.In this study, we characterized a novel member of ABCB family in rice, named OsABCB14. The main results were displayed as follows:first, osabcbl4mutants were insensitive to2,4-D or IAA, but not responsive to NAA. This phenotype could be reverted by the complemented transgenic lines. Second, using transgeneic rice expressing the GUS reporter gene driven by the OsABCB14endogenous promoter, it was found that OsABCB14was expressed in the root tip, stele of PR, leaf blade, leaf sheath, node, internode, root-stem transition region and flower. Cross section of the organs showed that OsABCB14was expressed in vascular. qRT-PCR analysis showed that the expression of OsABCB14was greatly induced by2,4-D, IAA,6-BA, ABA, SA or NOA treatment, whereas only slightly up-regulated under NAA, GA3and NPA treatment. Third, transient expression of OsABCB14in the epidermis cells of Nicotiana benthamiana (N. benthamiana) leaves and onion indicated that OsABCB14was localized at the plasma membrane. Fourth, MS-GAS and DR5-GUS staining showed that osabcbl4mutants owned reduced auxin concentrations and qRT-PCR analysis showed that mRNA levels of OsIAA3, OsIAA9, OsIAA23and OsSAUR39were down-regulated in osabcb14mutants. Fifth, analysis of auxin transport activity in yeast and protoplasts of osabcbl4mutants confirmed auxin influx directionality of OsABCB14. Analysis of auxin transport activity in root tips showed that acropetal auxin transport of osabcbl4mutants was decreased, while basipetal auxin transport was not different from WT.In addition, we also found that the Fe concentrations in the seeds, shoots and roots of osabcb14mutants were all significantly higher than that in WT, and the concentrations of the other metals, such as Mn, Cu, Zn, and Mg were also enhanced in the osabcb14mutants. The mutants were insensitive to-Fe and the significant differential expression of a number of Fe-deficiency responsive genes was also seen in osabcbl4mutants compared to WT. Therefore, OsABCB14was involved in iron homeostasis.Taken together, our results clarify that OsABCB14functions in auxin transport and iron homeostasis in rice. To our knowledge, this is the first report to functionally characterize ABCB in rice and to demonstrate ABCB as an auxin uptake transporter in plant system. The functional characterization of OsABCB14not only enriches us the understanding of ABCBs, but also provides novel insights in monocot auxin transport and crosstalk of hormone and mineral nutrition.