Legume Genomics and Genetics 2024, Vol.15, No.3, 140-151 http://cropscipublisher.com/index.php/lgg 142 expression in the symbiont, further elucidating the regulatory mechanisms of nitrogen fixation (Marx et al., 2016; Allito et al., 2020). 3.2 Host plant genetics The formation of nodules in legumes is a genetically complex process involving multiple genes. Quantitative trait loci (QTL) and candidate genes underlying nodule formation have been identified in various legumes, including soybean and common bean (Dwivedi et al., 2015). These genes are crucial for the establishment and maintenance of the symbiotic relationship between the host plant andRhizobium(Wang et al., 2018). Figure 1 Symbiosis signaling and plant immunity involved in recognition specificity in the legume-rhizobial interactions (indicated by the red stars) (Adopted from Wang et al., 2018) Image caption: (A) The process of infection and nodule development. A mature indeterminate nodule contains a meristem zone (I), an infection zone (II), an interzone (IZ), a nitrogen fixing zone (III), and a senescent zone (IV). (B) The host secretes flavonoids to induce the expression of bacterial nodulation (nod) gene through the activation of NodD proteins. The enzymes encoded by the nod genes lead to the synthesis of Nod factors (NF) that are recognized by host Nod factor receptors (NFRs). Recognition specificity occurs both between flavonoids and NodDs and between NF and NFRs. (C) In addition to NF signaling, bacteria also produce extracellular polysaccharides (EPS) and type III effectors to facilitate their infection in compatible interactions, but these molecules may also induce immune responses causing resistance to infection in incompatible interactions. (D) Certain legumes such as Medicago encode antimicrobial nodule-specific cysteine-rich (NCR) peptides to drive their bacterial partners to terminal differentiation that is required for nitrogen fixation. However, some rhizobial strains cannot survive the antibacterial activity of certain peptide isoforms, leading to formation of nodules defective in nitrogen fixation (Adopted from Wang et al., 2018)
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