Root nodulation is controlled by sensing the R/ FR ratio through phytochrome signaling

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Light is critical for supplying carbon to the energetically expensive, nitrogen-fixing symbiosis between legumes and rhizobia. In this study, I show that Phytochrome B(phyB) is part of the monitoring system to detect suboptimal light conditions, which normally suppress Lotus japonicus nodule development following Mesorhizobium loti inoculation. I found that the number of nodules produced by L. japonicus phyB mutants is significantly reduced compared to that of wild-type MG20. To explore causes other than photoassimilate production, the possibility that local control by the root genotype occurred was investigated by grafting experiments. The results showed that the shoot and not the root genotype is responsible for root nodule formation. To explore systemic control mechanisms exclusive of photoassimilation, we moved wild-type MG20 plants from white light to conditions that differed in their ratios of low or high red(R)/ far-red(FR) light. In low R/ FR light, the number of MG20 root nodules dramatically decreased compared to plants grown in high R/ FR even though photoassimilate content was higher for plants grown under low R/ FR. Also, the expression of jasmonic acid(JA)-responsive genes decreased in both low R/ FR light-grown wild-type and in white light-grown phyB mutant plants, and correlated with decreased JA-Ile content in the phyB mutant. Moreover, both infection thread formation and root nodule formation were positively influenced by JA treatment of wild-type plants grown in low