Receptor-ligand interaction
The complex and reiterative dialogue between plants and soil microorganisms involves extensive local and systemic communication systems that are based, at least in part, on plant receptors that detect microbe-associated molecular patterns (MAMPs) and pathogen-associated molecular patterns (PAMPs). These molecules are restricted to pathogens and are not generally conserved and shared by a wide range of microbes, trigger IR and play a prominent role in determining the success or failure of any plant/microbe interaction (Lü, Liu, Yu, Shi, & Liu, 2022). Chitin, which is a fungal cell wall homopolymer of unbranched β-1,4-linked N-acetylglucosamine (GlcNAc), is a key MAMP component that is recognized by plasma-membrane pattern recognition receptors (PRRs) (Lu, Liu, Yu, Shi, & Liu, 2022). The breakdown products of chitin (chitooligosaccharides: COs) are perceived by plant lysin motif (LysM) plasma membrane receptors triggering defence-gene induction, secretion of chitinases, a plasma membrane-derived oxidative burst and restriction of fungal growth (Chen et al., 2021; X. He et al., 2021; Morella et al., 2020; X. Zhou et al., 2022). The secretion of LysM-containing effector proteins that sequester AMF cell-wall derived COs plays a major role in the control of chitin-triggered host immunity (Zeng et al., 2020). Beneficial microbes, including rhizobia and AMF, which inhabit plant tissues can establish a mutualistic relationship with their hosts by either evading or suppressing host immunity (Fig. 2). The molecular dialogue between plant immune system and rhizosphere microbiota not only promotes colonization by beneficial microbes but it also prevents growth-defense trade-offs triggered by the MAMP-rich rhizosphere environment.
Plants secrete a range of chemicals such as flavonoids and strigolactones that are recognized by symbiotic rhizobia, which in turn secrete bacterial lipo-chitooligosaccharides (LCOs or Nod factors) that have a core structure of three to five N-acetyl glucosamines with an acyl chain attached to the non-reducing residue (Escudero-Martinez & Bulgarelli, 2023). Rhizobium Nod factors that are perceived by legumes and Parasponia  species trigger a common symbiotic signaling (SYM) pathway that involves genes that are also required for AMF symbiosis (Escudero-Martinez et al., 2022; Oyserman et al., 2022). LCO receptor has a well-conserved hydrophobic structural signature that monitors the composition of amphiphilic LCO molecules. Receptor-ligand interactions provide a kinetic proof-reading mechanism for the selective recognition-guided activation of symbiotic signaling in legumes, determining rhizobia-legume compatibility (Gysel et al., 2021). Rhizobial species either have a broad host range (e.g.Sinorhizobium fredii NGR234 interactions with over 200 distantly related legumes) a narrow host range like S. meliloti that primarily nodulates M. truncatula or M. sativa (Kelly, Radutoiu, & Stougaard, 2017). Many S. Meliloti strains induce incompatible signaling due to two Nodulation Specificity 1 locus (NS1a and NS1b ) genes that encode malectin-like leucine-rich repeat receptor kinases, which block tissue invasion and root nodule induction. NS1 activation is dependent upon the bacterial gene rns1 (Root nodule symbiosis) that encodes a type I-secreted protein (Liu et al., 2022).
Unlike the relatively well-characterized interactions between plants, AMF, and rhizobia, the molecular basis for cooperative interactions between PBBs and plants remains poorly characterized. Some PBBs display MAMPs, while others appear to bypass this high-affinity protein receptor-mediated surveillance system that triggers host immune responses. These are then recognized by cell-surface receptors, PRRs. For example, membrane lipid-dependent elicitation by surfactin induces an atypical early immune response including an intra-cellular burst of reactive oxygen species (ROS) and low calcium influx that triggers systemic resistance to Botrytis cinerea infection inArabidopsis thaliana (Pršić et al., 2023; Hoff et al., 2021; Pršić & Ongena, 2020). The surfactin-induced activation of enhanced immune responses in A. thaliana is mediated by docking into specific sphingolipid-enriched domains leading to host membrane deformation and activation of mechanosensitive ion channels (Pršić et al., 2023).