Figure 2. Molecular mechanisms mediating mycorrhizal and rhizobial symbioses. (IMAGE)
Caption
Plant roots secrete various flavonoids that are vital in establishing mutually beneficial relationships with rhizobial and mycorrhizal symbionts. Additionally, plants release other signaling molecules such as strigolactones (SL) and 2-hydroxy fatty acids, which play crucial roles in facilitating these symbiotic interactions. These signals control the expression of numerous microbial genes, including nod genes and stimulate fungal hyphal branching. They also stimulate the production of signaling molecules by the symbionts, such as rhizobial lipo-chitooligosaccharides (LCOs) (e.g. Nod factors) and mycorrhizal LCOs (e.g. Myc factors). These LCOs are detected by different receptor complexes. LCOs produced by rhizobia bacteria are perceived at the plasma membrane by receptor like kinases (RLKs) (e.g. NFR1 and NFR5). The perception of mycorrhizal LCOs also involve plant RLKs. LysM-RLKs are the most studied in symbioses with beneficial mycorrhizal fungi. Recently, a LysM-RLK receptor complex consisting of OsMYR1/OsLYK2 and OsCERK1 has been identified in rice mediating AMF perception [59]. These examples are highlighted because the key receptors for mycorrhizal LCOs have yet to be characterized in other plant species. In addition to LysM-RLKs, a G-type Lectin-RKs (LecRK) that mediates the symbiotic interaction between Populus and Laccaria bicolor has been reported. The downstream signaling cascades for lecRKs have not been extensively studied and remain largely unknown compared to LysM-RLKs. The signal emitted by LysM receptors activates the symbiosis receptor kinase (SymRK) that associates with essential proteins like SymRK interacting protein 1 and 2 (SIP1 and 2), SymRK-interacting E3 ligase (SIE3), and the 3-hydroxy-3-methylglutaryl coenzyme A reductase1 (HMGR1) involved in mevalonate biosynthesis, which triggers calcium oscillations in the nuclear region, also known as calcium spiking. Downstream of calcium spiking, a module involving calcium/CaM-dependent protein kinase (CCaMK) and Cyclops promotes mycorrhizal symbiosis by activating the expression of Reduced Arbuscular Mycorrhiza1 (RAM1) and promotes rhizobial symbiosis by forming a complex with Nodulation Signaling Pathways 1 and 2 (NSP1/NSP2) to regulate the expression of Nodule Inception (NIN). Created with BioRender.com.
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