Plants associate with beneficial arbuscular mycorrhizal fungi facilitating nutrient acquisition. Arbuscular mycorrhizal fungi produce chitooligosaccharides (COs) and lipo-chitooligosaccharides (LCOs), that promote symbiosis signalling with resultant oscillations in nuclear-associated calcium. The activation of symbiosis signalling must be balanced with activation of immunity signalling, which in fungal interactions is promoted by COs resulting from the chitinaceous fungal cell wall. Here we demonstrate that COs ranging from CO4-CO8 can induce symbiosis signalling in Medicago truncatula. CO perception is a function of the receptor-like kinases MtCERK1 and LYR4, that activate both immunity and symbiosis signalling. A combination of LCOs and COs act synergistically to enhance symbiosis signalling and suppress immunity signalling and receptors involved in both CO and LCO perception are necessary for mycorrhizal establishment. We conclude that LCOs, when present in a mix with COs, drive a symbiotic outcome and this mix of signals is essential for arbuscular mycorrhizal establishment.

The soybean leghaemoglobin lbc3 gene promoter was analysed in transgenic Lotus corniculatus plants. Hybrid‐promoter constructions and 5′ deletions were studied using chimeric genes composed of the various promoters, the chloramphenicol acetyltransferase (CAT) coding sequence and the lbc3 3′ flanking region. A 5′ Bal31 deletion series mapped a strong positive regulatory element between −1100 and −950. A weaker element located between −230 and −170 defined the minimum 5′ region required for detectable promoter activity. Reactivation of inactive promoters with deletion endpoints between −230 and the transcription initiation site was obtained employing the constitutive cauliflower mosaic virus (CaMV) 35S enhancer. The position of cis regulatory element(s) required for nodule‐specific expression was defined to 37 bp between −139 and −102. This region contains sequences conserved in other leghaemoglobin and nodulin genes. No indispensable control elements were found on the lbc3 3′ flanking region.

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Plants evolved lysine motif (LysM) receptors to recognize and parse microbial elicitors and drive intracellular signaling to limit or facilitate microbial colonization. We investigated how chitin and nodulation (Nod) factor receptors of Lotus japonicus initiate differential signaling of immunity or root nodule symbiosis. Two motifs in the LysM1 domains of these receptors determine specific recognition of ligands and discriminate between their in planta functions. These motifs define the ligand-binding site and make up the most structurally divergent regions in cognate Nod factor receptors. An adjacent motif modulates the specificity for Nod factor recognition and determines the selection of compatible rhizobial symbionts in legumes. We also identified how binding specificities in LysM receptors can be altered to facilitate Nod factor recognition and signaling from a chitin receptor, advancing the prospects of engineering rhizobial symbiosis into nonlegumes.

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