A shared gene drives lateral root development and root nodule symbiosis pathways in <i>Lotus</i>

Abstract

Legumes develop root nodules in symbiosis with nitrogen-fixing rhizobial bacteria. Rhizobia evoke cell division of differentiated cortical cells into root nodule primordia for accommodating bacterial symbionts. In this study, we show that NODULE INCEPTION (NIN), a transcription factor in <i>Lotus japonicus</i> that is essential for initiating cortical cell divisions during nodulation, regulates the gene <i>ASYMMETRIC LEAVES 2-LIKE</i> <i>18/LATERAL ORGAN BOUNDARIES DOMAIN</i> <i>16a</i> (<i>ASL18</i>/<i>LBD16a</i>)<i>.</i> Orthologs of <i>ASL18</i>/<i>LBD16a</i> in nonlegume plants are required for lateral root development. Coexpression of <i>ASL18a</i> and the <i>CCAAT box-binding protein Nuclear Factor-Y</i> (<i>NF-Y</i>) subunits, which are also directly targeted by NIN, partially suppressed the nodulation-defective phenotype of <i>L. japonicus</i> <i>daphne</i> mutants, in which cortical expression of <i>NIN</i> was attenuated. Our results demonstrate that ASL18a and NF-Y together regulate nodule organogenesis. Thus, a lateral root developmental pathway is incorporated downstream of NIN to drive nodule symbiosis.