Abstract
Associative nitrogen fixation contributes large portion of N input to agro-ecosystems through monocot-diazotrophic associations. However, the contribution of associative nitrogen fixation is usually neglected in modern agriculture, and the underlying mechanisms of association between monocot and diazotrophs remain elusive. Here, we demonstrated that monocot crops employ mucilage and associated benzoic acid to specially enrich diazotrophic partners in response to nitrogen deficiency, which could be used for enhancing associative nitrogen fixation in monocot crops. To be specific, mucilage and benzoic acid induced in sugarcane roots by nitrogen deficiency mediated enrichment of nitrogen-fixing Paraburkholderia through specific recruitment whereas other bacteria were simultaneously repelled. Further studies suggest maize employs a similar strategy in promoting associations with diazotrophs. In addition, our results also suggest that benzoic acid application significantly increases copy numbers of the nifH gene in soils and enhances associative nitrogen fixation in maize using 15N enrichment assay. Taken together, these results reveal a mechanism regulating the association between monocot crops and nitrogen-fixing bacteria, and, thereby point towards ways to harness these beneficial microbes in efforts to increase nitrogen efficiency in monocot crops through pathways regulated by a specific signaling molecule.