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).