**4. Conclusions and future perspectives**

Plants establish mutualistic symbiosis with AMF and rhizobial for nutrient uptake. In AMS, mineral nutrients, especially Pi, are supplied by AMF via AM fungal hyphae, and host plants mainly concurrently transfer fatty acids to fungi as carbon resources. Although AMS facilitates the uptake of Pi, the concentration of Pi, in turn, impairs the colonization of AMF. Thus, the status of Pi is essential in AMS establishment, and some TFs involved in CSSP like PHRs play important roles in the regulation of this process. As a central regulator of Pi homeostasis, PHR2 is required for the activation of AMS-associated genes under Pi-deficient conditions. While in RNS, legumes

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

interact with rhizobial to fix N, and N homeostasis is closely related to NIN, a member of NIN-like protein (NLP) family.

The opening question is how plants discriminate chitin, NFs and Myc factors signals that are structurally similar and then promote different signaling pathways. Receptor kinases play critical roles in primary signal recognition in immunity and AMS and/or RNS. A part of LYKs like OsCERK1 play a dual role in both immunity and symbiosis, while some others show subfunctional in regulating these two signaling pathways. What's more, duplication of LysM genes has happened, compared with 10 LysM genes in rice, *M. truncatula* has 21 and *L. japonicus* has 26 genes that are predicted to encode LYKs. Therefore, the versatile combination of LYKs receptors plays an essential role in sensing structurally similar polysaccharides to initiate symbiosis or immunity thereby discriminating symbionts or pathogens.

SymRK functions as a vital component of the genetic basis for both plant-fungal and plant-bacterial endosymbiosis. It perceives signals dependent on extracellular malectin domain and LRR domain, whose sequences vary wildly between nonlegumes and legumes, but are much more conserved in legumes. It is suggested that the diversity among them may be one of the reasons for different responses to Myc factors or NFs. SymRK activates signaling pathways downstream mediating the

#### **Figure 3.**

*The chitin-triggered immunity and common symbiotic signaling pathway. AtCERK1 and AtLYK5 form a receptor complex perceiving CO8 to induce innate immunity in A. thaliana. In AMS, the complex of OsCERK1/ OsMYR1 receives Myc factors in rice. And LjNFR1/LjNFR5 receptor complex in L. japonicus perceive Nod factors (structurally similar to Myc factors) in RNS. Upon recognition of symbiotic signals of AMS/RNS, SymRK, as the coreceptor of LysM receptors, then interacts with some key proteins such as SIP1 and SIE3. Phosphorylated SIP1 could specifically bind to the promoter of NIN gene to positively regulate symbiosis. Another SymRK interacting protein, HMGR1 is a common signaling component in both AMS/RNS, which participates in MVA pathway and triggers calcium spiking in nucleus. CCaMK-CYCLOPS-DELLA complex interacts with RAM1 or NSP1/ NSP2, inducing the expression of RAM1 or NIN to regulate AMS and RNS. The activation of symbiosis signaling inhibits innate immunity signaling at certain levels.*

post-translational modification of the interacting proteins; however, whether these interacting proteins function in RNS also participate in AMS remains to be further investigated.

It is probable that a single pathway mediating both AMS and RNS after the recognition of signal molecules, for some homolog proteins such as CCaMK and CYCLOPS play the same role in both AMS and RNS. There may also be some parallel signaling pathways to regulate the TFs in nucleus. The complex of CCaMK-CYCLOPS directly regulates at least 3 genes in different pathways: *NIN*(RNS), *RAM1*(AMS), *CBP1*(RNS and AMS) [103]. In addition, NIN and RAM1 may be the first divergence for AMS or RNS in CSSP, and we wonder whether there are other TFs involved or not.

In summary, receptor kinases are essential in the specifical recognition of signals, and OsCERK1/OsMYR1 were confirmed to be the receptor complex perceiving Myc factors in recent research, but the pivotal receptors for Myc factors in other species remain to be studied. As SymRK could receive symbiotic signals from NFRs, it is of great interest whether SymRK could receive Myc-factor signals to participate in symbiotic signal transduction between AMF and plants. Activated by the calcium spiking, CCaMK-CYCLOPS-DELLA complex could regulate TFs which promote the expression of AMS-related or RNS-related genes (**Figure 3**). Therefore, further study on the difference in signal recognition and signaling pathways between AMS and RNS may help us to apply RNS in non-legumes.
