Abstract
Host legumes control nodulation by sensing external and internal cues. A major external cue is soil nitrate and an important internal cue is earlier formed nodules that suppress further nodulation through shoot-root communication. This feedback regulation is known as autoregulation of nodulation (AUT), and is believed to consist of two long-distance signals: a root-derived signal that is generated in infected roots and transmitted to shoot; and a shoot-derived signal that systemically inhibits nodulation. In Lotus japonicus, the LRR receptor-like kinase, HAR1, mediates AUT and nitrate inhibition of nodulation, and is hypothesized to recognize the root-derived signal. Here we identify LjCLE-RS1 and -RS2 as strong candidates for the root-derived signal. Overexpression of LjCLE-RS1 and -RS2 in roots inhibited nodulation systemically in a HAR1-dependent manner. Moreover, LjCLE-RS2 was strongly up-regulated by nitrate. Accordingly, we propose a simple model for AUT and nitrate inhibition of nodulation mediated by LjCLE-RS1, -RS2 and HAR1.
