A combination of chitooligosaccharide and lipochitooligosaccharide recognition promotes arbuscular mycorrhizal associations in Medicago truncatula

Abstract

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.

Stomatal immunity against fungal invasion comprises not only chitin-induced stomatal closure but also chitosan-induced guard cell death

“…Probably, it is from the differences in nature of the signaling triggered by mixture and a particular CTOS as seen in previous studies and discussed by Bi and Zhou (61) and by Yamaguchi et al (62). Studies have shown that interactions between different oligosaccharides play important roles in determining signaling specificity (63). An interesting possibility is that CSOS in the crude chitin preparation may interact with CTOS to define CTOS signaling specificity.…”

Population‐level variation in host plant response to multiple microbial mutualists

“…Thus, it is possible that the two species interact in the wild, but it is unlikely that the specific temperate genotype of R. irregularis isolate we obtained co-evolved with M. truncatula populations. We note however, that temperate isolates of R irregularis have previously been used to study plant mycorrhizal interactions in M. truncatula (Kiirika et al, 2012;Afkhami and Stinchcombe, 2016;Feng et al, 2019). For those pots receiving AM fungi, pots were first filled with 300 mL of sterile soil, and then the remaining 300 mL of soil was mixed with 1.5 g of inoculum powder (800 spores/g), and this mixture was then added to the pot.…”

Functional and structural characterization of LysM proteins in interactions between fungi and plants

“…The above-described chitin, Myc factors and Nod factors, β-glucans and peptidoglycan are the most well-characterized microbial cell wall polysaccharides that act as IPs. Besides these, more cell wall components are recognized by plants, such as fungal COs, cyclic β-glucans (Pettolino et al, 2009;Feng et al, 2019) and bacterial lipopolysaccharides (LPS) (Wang and Quinn, 2010;Erbs and Newman, 2012), and probably many more remain to be discovered. Therefore, microbial cell walls can be considered a rich source of IPs, some of which are not only perceived by plants, but also by human and animal immune systems (Ariizumi et al, 2000;Kumar et al, 2009;Bozza et al, 2009;Brown, 2011;Rosadini and Kagan, 2017).…”

“…Besides chitin, plant LysM-containing receptors play essential roles in perceiving chitin derivatives such as lipochitooligosaccharides (LCOs), molecules with a chitin backbone that generally consists of four or five residues and an acyl chain attached to the non-reducing terminal GlcNAc that are known to act as Nod factors, rhizobial signal molecules that are essential for the initiation of a symbiosis with legume plants (Gough, 2003;Gust et al, 2012). Similarly, arbuscular mycorrhizal (AM) fungi secrete LCOs that are known as Myc factors in combination with chitooligosaccharides (COs) to initiate their symbiosis (Gust et al, 2012;Liang et al, 2014;Limpens et al, 2015;Feng et al, 2019). The perception of Nod or Myc factors in various plant species involves receptor complexes with LysM-RLKs (Gough, 2003;Maillet et al, 2011;Genre et al, 2013;Liang et al, 2014;Zipfel and Oldroyd, 2017).…”

“…Like OsCERK1, AtCERK1 has been shown to be required for peptidoglycan-triggered immunity (Willmann et al, 2011). In addition to its role in immunity, OsCERK1 has also been implicated in AM symbiosis that is established upon perception of fungal LCOs in combination with COs (Miyata et al, 2014;Zhang et al, 2015;Feng et al, 2019). These findings suggest that CERK1 likely acts as a co-receptor in different receptor complexes (Sánchez-Vallet et al, 2015).…”