**2. Roles of LysM-RLKs in perceiving symbiotic signals in AMS and RNS**

In nature, only a few species of microbes can establish compatible interactions with host plants to cause either pathogenic or mutualistic symbiosis. More and more data suggest that plant innate immunity plays a key role in distinguishing invading microbes to establish different interactions. Hence, how plants recognize and distinguish signals from different microbes could be precisely regulated. The existing data indicate that LysM-RLKs play such roles in distinguishing different microbes and initiating different physiological responses in plants. N-Acetylglucosamine (GlcNAc)-containing molecules are conserved components of cell walls for different microbes. For example, chitin, the major component of fungal cell wall, and bacterial peptidoglycan (PGN), function as microbe-associated molecular patterns (MAMPs) perceived by LysM-containing proteins to trigger plant immunity against invading pathogens.

Whereas lipo-chitooligosaccharides (LCOs), for example, rhizobial NFs and mycorrhizal Myc factors are key signals recognized by two LysM-RLKs to induce symbiotic signaling transduction in plants [33]. Rhizobial NF, a short-chain of chitin with different modifications at the terminal residues, plays an important role in specific recognition between rhizobial and legumes [34]. In AMS, Myc factors that contain Myc-LCOs, and Myc-COs can activate the CSSP with resultant calcium oscillations

#### *Receptor Kinases and Signal Pathway in the Arbuscular Mycorrhizal Symbiosis DOI: http://dx.doi.org/10.5772/intechopen.107261*

in root epidermal cells [35–37]. Myc factors were proposed to be mixtures of CO4/5 and LCO, while the function of CO4 appears to be the predominant molecule activating symbiotic responses in rice. Thus, symbiotic signaling pathways induced by Myc-LCOs and COs seem to be a little bit different since AMF produces a mixture of molecules during the symbiotic interaction with hosts [18, 37, 38]. In this process, a class of LysM receptor kinases (LYKs) participate in the discrimination of these GlcNAc molecules and determine the outcomes of the downstream signaling pathway to immunity or symbiosis [39–41].

In the establishment of symbiosis between rhizobial and legume host plants, LjNFR1 (Nod Factor Receptor 1) and LjNFR5 in *Lotus japonicus*, also named MtLYK3 (LysM containing receptor Kinase 3) and MtNFP (Nod Factor Perception) in *Medicago truncatula* are two essential LYKs regulating NFs specific perception [42–44]. In non-legume species, for example, rice OsCERK1 (Chitin-elicitor receptor kinase 1), a LysM-RLK, is a necessary receptor involved in COs-induced immunity and mycorrhizal symbiotic responses. CERK1 was originally identified as an essential receptor for chitin elicitor signaling in *Arabidopsis thaliana*, and the KO mutant for *AtCERK1* completely lost the ability to respond to chitin [45]. Subsequent studies have demonstrated that OsCERK1 and a LysM protein OsCEBiP (Chitin Elicitor binding protein) could cooperatively regulate chitin elicitor signaling in rice [39, 46]. But, interestingly, OsCERK1 plays a dual role in mediating both AMS and immunity [47, 48]. The AMS in the rice *Oscerk1* mutant plants was severely diminished but was normal in the *Oscebip* mutant, the function of OsCERK1 in mediating symbiosis or immune responses seems to be dependent on specific interaction with different receptors in response to either symbiotic signals or pathogenic signals [19]. Recent breakthroughs have revealed OsCERK1DY from Dongxiang wild rice with two amino acids substitutions in the second LysM domain exhibited stronger colonization with AMF than the rice cultivar Zhongzao 35 (ZZ35), as well as promoting phosphorus acquisition [49].

CO4/CO5 are necessary signals for symbiotic interactions between AMF and host plants, however, rice OsCERK1 does not seem to bind to CO4 directly [39, 50]. It was implied that another component was needed to perceive these signals, just like the sandwich models of OsCERK1/OsCEBiP in mediating chitin-triggered immunity and LjNFR1/LjNFR5 complex for rhizobial symbiosis in *L. japonicus* [44, 46]. OsMYR1/ OsLYK2/OsNFR5/OsRLK2, grouped in the same clade as LjNFR5/MtNFP/SILYK10, is the co-receptor of OsCERK1 required for AMS. OsMYR1 directly binds to CO4 but not Nod factors or lipopolysaccharides (LPS) [20]. Significant reduction of AMF colonization, as well as transcription levels of AM-specific marker genes and calcium spiking, were observed in the *Osmyr1-1*/*Oslyk2-1* mutant compared to wide type (WT) inoculated with *Rhizophagus irregularis* spores [20]. However, the AM colonization in a high dosage did not show too many differences between *Osmyr1-1*/*Osnfr5* and WT plants, but a significant decrease in transcript levels of AM-responsive gene was detected in the *Osmyr1-1*/*Osnfr5* mutant plants [51]. Whether OsMYR1/OsNFR5 responds differentially to different dosages of *R. irregularis* is unclear. But all these data indicated that OsMYR1 seems to be a binding receptor for sensing CO4, and the subsequent dimerization and phosphorylation between OsMYR1 and OsCERK1 activate symbiotic signaling pathway [20].

It was identified that only the long chain of COs with polymer of degree between 6 and 8 (CO6, CO7, and CO8) but not CO4 or CO5 could trigger a plant immunity [52, 53]. A recent study has shown that CO8 has a similar function as CO4 to induce symbiotic nuclear calcium oscillations and activates some of the symbiosis-related

genes expression [18], raising a question that nuclear calcium oscillation might not be a specific signal representing symbiosis. In *Medicago truncatula*, MtCERK1 and MtLYR4 can bind both CO4 and CO8, in which MtCERK1 is required in both COs induced immune and symbiotic signaling pathways [18]. Interestingly, the plant defense triggered by CO8 could be suppressed by additional LCOs in both legumes and non-legumes, and CO4 could reduce CO8-triggered ROS generation via OsMYR1 in rice or AtLYK3 in Arabidopsis [18, 19, 54], suggesting that non-legumes could still respond to LCOs. Hence, a Single Pole Double Throw (SPDT) switch model in symbiosis and defense signaling pathways was proposed [19]. In this model, OsMYR1 can recognize CO4 from symbiotic fungi and then associates with OsCERK1, which suppresses the formation of chitin-induced OsCEBiP/OsCERK1 complex and transphosphorylation of immune-associated substrates, then activate mycorrhizal colonization. When OsCEBiP senses CO8, it competes with OsMYR1 to bind OsCERK1, which induces immunity and negatively regulates AMS [19].

#### **Figure 1.**

*Phylogenetic tree of LysM-containing receptor-like kinases. Molecular phylogeny of OsCERK1 and OsMYR1 with homologs from Oryza sativa (Os), Medicago truncatula (Mt), Glycine max (Gm), Lotus Japonicus (Lj), Arabidopsis thaliana (At), Zea mays (Zm), Solanum lycopersicum (SI) was constructed using a maximumlikelihood algorithm in MEGA-X with 1000 bootstraps value.*

#### *Receptor Kinases and Signal Pathway in the Arbuscular Mycorrhizal Symbiosis DOI: http://dx.doi.org/10.5772/intechopen.107261*

Rice is a very-well studied model species used for AMS study. The dual function of OsCERK1 homologs in both symbiosis and immunity was also studied in other plant species. Similar to rice OsCERK1 in symbiosis and immunity, OsCERK1 homologs in leguminous plants also play a dual role in both symbiosis and immunity (**Figure 1**). For example, MtLYK9 in *M. truncatula* and PsLYK9, a close ortholog of CERK1 in *Pisum sativum* regulate both plant immunity and AMS [55, 56]. Likewise, in *Parasponia andersonii*, the only known non-legume plant with an ability of RNS, PanLYK1, and PanLYK3 were essential for intracellular arbuscule formation, while PanLYK3 also acted as a chitin receptor for innate immunity signaling [57]. Nonetheless, LYKs in some species, such as the CERK1 homologs in tomato (SlLYK1, SlLYK11, SlLYK12, and SlLYK13) are also functional in AMS and chitin-triggered defense. It has been found that knockdown of *SILYK12* would significantly reduce AMF colonization, but the chitin-induced defense response was unaffected, whereas SILYK1 and SILYK13 only participate in immune signaling but not AMS [58]. Hence, plant CERK1 is such a receptor that works as a shared kinase mediating both chitin-triggered immunity and AMF symbiotic signals. The LYKs receptor model might at least support the hypothesis that symbiotic interaction might be a result of a graduate attenuation from plant-pathogens interaction until a condition that both symbionts and plants could benefit.
