Red/Far Red Light Controls Arbuscular Mycorrhizal Colonization via Jasmonic Acid and Strigolactone Signaling

Abstract

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.

Enhanced hyphal growth of arbuscular mycorrhizae by root exudates derived from high R/FR treated Lotus japonicus

“…Taken together, the combination of results strongly supports the possibility that JA in root exudates derived from high R/FR light-grown plants induced hyphal elongataion. In our previous study, 9 we showed that root exudates obtained from high R/FR-grown plants contained more strigolactone than exudates from low R/FR-grown plants. However, crude root exudates contain many compounds including amino acids, organic acids, sugars, phenolics, and other secondary metabolites.…”

“…Earlier, we tried to measure the amounts of JA and JA derivatives directly from root exudates prepared from either high or low R/FR light-treated uninoculated plants. 9 However, even though exudates from 50 plants were analyzed by LC/MS, we could not definitely find any JA-related compounds, most likely due to their very low concentrations and the presence of impurities. Therefore, to evaluate JA and JA derivative content, we took an indirect approach and studied the expression levels of JAresponsive genes in WT MG20 plants treated with root exudates prepared from either high or low R/FR light-treated plants.…”

“…8 We determined in AM-inoculated Solanum lycopersicum that the expression of SlAOS (allene oxide synthase) and SlLOX (lipoxygenase), which are involved in JA biosynthesis, increased in high R/FR light-grown plant roots compared to low R/FR light-grown plant roots. 9 An additional finding was that the root exudates derived from high R/FR light-treated plants contained more strigolactone, an AM-fungal hyphal branching inducer, than did the low R/FR light-grown plants. 9 Because previous studies reported that wheat seedling root exudates contain JA, 10 we predicted that root exudates from L. japonicus also contain JA.…”

“…9 An additional finding was that the root exudates derived from high R/FR light-treated plants contained more strigolactone, an AM-fungal hyphal branching inducer, than did the low R/FR light-grown plants. 9 Because previous studies reported that wheat seedling root exudates contain JA, 10 we predicted that root exudates from L. japonicus also contain JA. Moreover, because JA affects AM fungal colonization, 14 we further predicted that light would influence the amount of JA secreted into the rhizosphere.…”

Blue Light Perception by Both Roots and Rhizobia Inhibits Nodule Formation in Lotus japonicus

“…In the shade avoidance syndrome, plants grow taller or bend toward the light to avoid shade through the action of phytochromes (Smith and Whitelam 1997). In shade avoidance in the case of root nodulation and mycorrhization, host plants suppress these interactions under suboptimal light conditions through the phytochrome system to avoid expending energy (Nagata et al 2015;Suzuki et al 2011). Therefore, we can consider inhibition of nodulation by light as one of several avoidance responses plants use to conserve energy in response to environmental stress.…”