IN BRIEF Medicago truncatula CRE1 Cytokinin Receptor Regulates Nodulation and Lateral Root Development

The plant hormone cytokinin is implicated in the control of root architecture and development, including legume root nodulation. Gonzalez-Rizzo et al. (pages 2680–2693) identified a Medicago truncatula homolog of Arabidopsis, CYTOKININ RESPONSE1 (CRE1), which encodes a cytokinin receptor histidine kinase, and made use of RNA interference of Mt CRE1 to investigate the role of cytokinin in the nodulation process in this legume. The results demonstrate that Mt CRE1 acts as a negative regulator of lateral root formation and as a positive regulator of nodulation. The authors identified two other putative cytokinin signaling genes in M. truncatula based on homology to Arabidopsis RR4 and RR1, which encode Aand B-type response regulators, respectively, that act downstream of CRE1 in cytokinin signaling. Expression analysis of these genes and the early nodulin gene Nodule Inception 1 (Mt NIN) in M. truncatula suggests that Mt NIN, Mt RR1, and Mt RR4 are involved in crosstalk between the Nod factor and cytokinin signaling pathways depending on CRE1.