Light is critical for supplying carbon to the energetically expensive, nitrogen-fixing symbiosis between legumes and rhizobia. Here, we show that phytochrome B (phyB) is part of the monitoring system to detect suboptimal light conditions, which normally suppress Lotus japonicus nodule development after Mesorhizobium loti inoculation. We found that the number of nodules produced by L. japonicus phyB mutants is significantly reduced compared with the number produced of WT Miyakojima 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 WT MG20 plants from white light to conditions that differed in their ratios of low or high red/far red (R/FR) light. In low R/FR light, the number of MG20 root nodules dramatically decreased compared with plants grown in high R/FR, although 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 WT and white light-grown phyB mutant plants, and it correlated with decreased jasmonoyl-isoleucine content in the phyB mutant. Moreover, both infection thread formation and root nodule formation were positively influenced by JA treatment of WT plants grown in low R/FR light and white lightgrown phyB mutants. Together, these results indicate that root nodule formation is photomorphogenetically controlled by sensing the R/FR ratio through JA signaling.symbiotic nitrogen fixation | shade avoidance syndrome

Establishment of a nitrogen-fixing symbiosis between legumes and rhizobia not only requires sufficient photosynthate, but also the sensing of the ratio of red to far red (R/FR) light. Here, we show that R/FR light sensing also positively influences the arbuscular mycorrhizal (AM) symbiosis of a legume and a non-legume through jasmonic acid (JA) and strigolactone (SL) signaling. The level of AM colonization in high R/FR light-grown tomato and Lotus japonicus significantly increased compared with that determined for low R/FR light-grown plants. Transcripts for JA-related genes were also elevated under high R/FR conditions. The root exudates derived from high R/FR light-grown plants contained more (+)-5-deoxystrigol, an AM-fungal hyphal branching inducer, than those from low R/FR light-grown plants. In summary, high R/FR light changes not only the levels of JA and SL synthesis, but also the composition of plant root exudates released into the rhizosphere, in this way augmenting the AM symbiosis.

Light is critical for supplying carbon for use in the energetically expensive process of nitrogen-fixing symbiosis between legumes and rhizobia. We recently showed that root nodule formation in phyB mutants [which have a constitutive shade avoidance syndrome (SAS) phenotype] was suppressed in white light, and that nodulation in wild-type is controlled by sensing the R/FR ratio through jasmonic acid (JA) signaling. We concluded that the cause of reduced root nodule formation in phyB mutants was the inhibition of JA-Ile production in root. Here we show that the shoot JA-Ile level of phyB mutants is higher than that of the wild-type strain MG20, suggesting that translocation of JA-Ile from shoot to root is impeded in the mutant. These results indicate that root nodule formation in phyB mutants is suppressed both by decreased JA-Ile production, caused by reduced JAR1 activity in root, and by reduced JA-Ile translocation from shoot to root.Light is an important environmental factor controlling plant growth. It is well known that plants require light for photosynthesis and are able to monitor both light quality and quantity for optimal survival. Plants have photoreceptors that sense the presence of their neighbors by monitoring the ratio of red light (R), which is absorbed by chlorophyll, and far red light (FR), which is not. A low R/FR ratio indicates the presence of neighbors that may compete for photosynthetically active radiation (PAR) and initiates the shade avoidance syndrome (SAS), causing plants to grow taller or bend to the light to avoid shade. [1][2][3][4] Many leguminous plants establish a symbiosis with nitrogenfixing soil bacteria called rhizobia. Inside the root nodules, the rhizobia differentiate into nitrogen-fixing bacteroids. The bacteroids convert atmospheric nitrogen into ammonia, a source of fixed nitrogen for the host plant. Because assimilates from photosynthesis provide energy to fuel the symbiosis between legumes and rhizobia, the light conditions under which host plants grow are very important. 5-8Previously we reported that root nodule formation was suppressed in a Lotus japonicus phytochrome B (phyB) mutant having a constitutive SAS phenotype. 9 In that paper, we concluded that the cause of reduced root nodule formation in low-R/FR-grown MG20 (wild-type) plants and white-light-grown phyB mutants is inhibition of JA-Ile (an active JA derivative) production in root. By using grafted plants prepared from MG20 and phyB mutant plants, we also showed that shoot genotype controls root nodule formation. 9 Here we report additional data confirms that root nodulation is controlled by shoot genotype. The expression level of marker gene NIN, 10 which is required for infection thread formation and nodule primordium initiation, was analyzed in the root of grafted plants by using real time RT-PCR by the methods described in Tominaga et al. 11 The roots and shoots of five-day-old MG20 and phyB mutant plants were grafted in various combinations, as described by Magori et al. 12 After 7 d, the grafted plant...