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 differentsteps from perception of the hormone up to transcriptional regulation of target genes. In legumeplants there is an overlap between the developmental programs governing lateral root and N2-fixingnodule 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 wasobserved. Significant regulation of the expression was not observed during the symbiotic interactionwith Mesorhizobium loti and the nodule organogenesis process. In roots, the LjAFB6 expressionwas induced in response to nitrate supply and was mainly localized in the meristematic regionsof both primary and lateral roots. The phenotypic analyses conducted on two independent nullmutants indicated a specialized role in the control of primary and lateral root elongation processesin response to auxin, whereas no involvement in the nodulation process was found. We also reportthe involvement of LjAFB6 in the hypocotyl elongation process and in the control of the expressionprofile of an auxin-responsive gene