Legume Genomics and Genetics 2024, Vol.15, No.3, 140-151 http://cropscipublisher.com/index.php/lgg 143 Plant receptors and signaling pathways play a pivotal role in the recognition and establishment of symbiosis with Rhizobium. For instance, calmodulin-binding proteins have been identified as key regulators in the host plant, mediating the signaling pathways that control nodule formation and function (Dwivedi et al., 2015). Additionally, microRNAs (miRNAs) are essential for coordinating various plant processes required for nodule formation and function, impacting hormone signaling and spatial regulation of gene expression (Hoang et al., 2020; Abdelkhalek et al., 2022). 3.3 Advances in genomic techniques Advances in sequencing technologies have significantly contributed to our understanding of rhizobium-legume interactions. The genomes of several legume species, such as chickpea, pigeonpea, and soybean, as well as multiple Rhizobiumspecies, have been sequenced, providing valuable genetic information for improving SNF and legume productivity (Dwivedi et al., 2015). These genomic resources facilitate the identification of DNA markers and the exploration of genotype-phenotype relationships in SNF. Gene editing technologies, such as CRISPR/Cas9, have opened new avenues for functional genomics studies in rhizobium-legume symbiosis. These techniques allow precise manipulation of genes involved in SNF, enabling researchers to dissect their roles and improve symbiotic efficiency. For example, synthetic biology approaches are being employed to engineer new symbiotic interactions or enhance existing ones, leveraging the knowledge gained from genome- and systems-level studies (Figure 2) (diCenzo et al., 2018). By integrating these genetic insights and advanced genomic techniques, researchers can develop legume cultivars with enhanced symbiotic efficiency, ultimately contributing to sustainable agricultural practices and improved crop yields. Figure 2 Schematic representation of the life cycle of rhizobia (Adopted from diCenzo et al., 2018) Image caption: Inoculant strains are typically grown at fermenter scale and used to coat seeds of compatible legumes. In the soil, the rhizobia must compete with the indigenous microbes and initiate symbiosis with an exchange of specific nodulation signals. Curling of a host root hair traps a rhizobiumcell, allowing colonization to proceed in a growing infection thread that progresses inward to the cortical cells. Here, the rhizobia are released into the cytoplasm of specialized nodule cells, where they are enclosed in a plant-derived membrane (peribacteroid membrane). The rhizobia and plant undergo a differentiation process that involves massive transcriptional and metabolic shifts, resulting in the formation of a nitrogen-fixing nodule. Five distinct developmental zones of an indeterminate-type nodule are shown (not drawn to scale): zone I, apical meristem; zone II, infection and differentiation zone; interzone II–III, a small region between zone II and zone III; zone III, the nitrogen-fixing zone; zone IV, the senescence zone found in mature nodules. We note that this figure shows an indeterminate nodule, that many of the features shown in this image are not universal, and that many types of nodulation exist (Adopted from diCenzo et al., 2018)
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