The Lotus japonicus AFB6 Gene Is Involved in the Auxin Dependent Root Developmental Program

Auxin is essential for root development, and its regulatory action is exerted at different steps from perception of the hormone up to transcriptional regulation of target genes. In legume plants there is an overlap between the developmental programs governing lateral root and N₂-fixing nodule organogenesis, the latter induced as the result of the symbiotic interaction with rhizobia. Here we report the characterization of a member of the L. japonicus TIR1/AFB auxin receptor family, LjAFB6. A preferential expression of the LjAFB6 gene in the aerial portion of L. japonicus plants was observed. Significant regulation of the expression was not observed during the symbiotic interaction with Mesorhizobium loti and the nodule organogenesis process. In roots, the LjAFB6 expression was induced in response to nitrate supply and was mainly localized in the meristematic regions of both primary and lateral roots. The phenotypic analyses conducted on two independent null mutants indicated a specialized role in the control of primary and lateral root elongation processes in response to auxin, whereas no involvement in the nodulation process was found. We also report the involvement of LjAFB6 in the hypocotyl elongation process and in the control of the expression profile of an auxin-responsive gene.