Legume Genomics and Genetics 2024, Vol.15, No.4, 163-175 http://cropscipublisher.com/index.php/lgg 165 lipochitooligosaccharide core, is essential for the recognition and activity, indicating a co-evolutionary relationship between the symbiotic partners (Figure 1) (Zhao et al., 2021). Figure 1 An overview of proposed evolutionary histories of the rhizobial nitrogen-fixing symbiosis in Fabaceae (Adopted from Zhao et al., 2021) Image caption: This summary tree is a simplified version with tips collapsed as noted in the tip names. Three slightly different hypotheses for switch(es) from actinorhizal to rhizobial nodulation with subsequent multiple losses are marked as SH1, SH2, and SH3 next to the affected nodes. NFN indicates the nitrogen-fixing clade. The origin of rhizobial nodulation for the nitrogen-fixing clade and multiple losses of rhizobial nodulation are marked with green and red asterisks, respectively (Adopted from Zhao et al., 2021) Upon receiving flavonoid signals from the plant, rhizobium bacteria respond by producing Nod factors, which are recognized by specific receptors on the plant root cells. This recognition triggers a cascade of events, including polarized root-hair tip growth and invagination associated with bacterial infection. The plant enforces strict control over the infection process, ensuring that only compatible rhizobium strains can successfully initiate symbiosis. This control extends to the enforcement of terminal differentiation of bacteria within nodules, which is crucial for effective nitrogen fixation (Bellés-Sancho et al., 2022). 3.2 Nodule formation and development The early stages of nodule formation involve significant architectural changes in the plant root, such as root hair morphogenesis, cortical cell enlargement, and vascular patterning. These changes are driven by the recognition of Nod factors, leading to the formation of infection threads that guide the bacteria into the root cortex. The specificity and timing of these early events are critical for successful symbiosis, with the plant exerting control over the process to ensure compatibility and efficiency (Figure 2) (Bellés-Sancho et al., 2022; Lepetit and Brouquisse, 2023).
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