A Cytokinin Signaling Type-B Response Regulator Transcription Factor Acting in Early Nodulation.

Nitrogen-fixing root nodulation in legumes challenged with nitrogen-limiting conditions requires infection of the root hairs by soil symbiotic bacteria, collectively referred to as rhizobia, and the initiation of cell divisions in the root cortex. Cytokinin hormones are critical for early nodulation to coordinate root nodule organogenesis and the progression of bacterial infections. Cytokinin signaling involves regulation of the expression of cytokinin primary response genes by type-B response regulator (RRB) transcription factors. RNA interference or mutation of , the RRB-encoding gene most strongly expressed in roots and nodules, significantly decreased the number of nodules formed, indicating a function of this RRB in nodulation initiation. Fewer infection events were also observed in mutant roots associated with a reduced Nod factor induction of the () infection marker, and of the cytokinin-regulated ( ) gene. Rhizobial infections correlate with an expansion of the nuclear area, suggesting the activation of endoreduplication cycles linked to the cytokinin-regulated ( ) gene. Although no significant difference in nucleus size and endoreduplication were detected in rhizobia-infected mutant roots, expression of the endoreduplication marker was reduced. As the expression pattern overlaps with those of and in roots and nodule primordia, chromatin immunoprecipitation-quantitative PCR and protoplast trans-activation assays were used to show that MtRRB3 can interact with and trans-activate and promoters. Overall, we highlight that the MtRRB3 cytokinin signaling transcription factor coordinates the expression of key early nodulation genes.