Legume Research - An International Journal

The article has been written in view of the importance attached to symbiotic N₂-fixation taking place in unique organs called legume root nodules under tropics and subtropics. Nodules on legume roots are formed in interaction with soil bacteria a and b rhizobia. Symbiotic interactions between compatible legume host plant and rhizobia involve a fine tuned, molecular communication between the two partners. Calcium has been reported to play a crucial role in symbiotic signaling. Nod factors are central to the initial establishment of legume - rhizobial symbiosis. Production of these signaling molecules is activated by the release of plant phenolics, mainly flavonoids, in the rhizosphere, where they induce a set of nod genes in the appropriate rhizobial strain, leading to synthesis of Nod factors. The nature of both flavonoid signal from plant and Nod factor from the microbial partner are central to the maintenance of specificity in this symbiosis, ensuring that plant accommodates only the useful bacterium. Generally the invasion of plant root occurs through an invagination of root hair cell, called infection thread, at the primary site of interaction. The infection thread spans through the entire root cortex allowing rhizobial invasion into dividing cells of nodule primordium. Rhizobia are released from infection thread into membrane enclosed compartments, where they convert from free living form to N₂-fixing form named the bacteroids. Development of functional nodules requires differentiation of both plant and microbial cells. Transcriptomics and proteomics reveal truly great extent of plant and microbial differentiation. Symbiotic N₂-fixation is a finely regulated process that involves carbon and energy metabolism of the host plant significantly. The process is also under regulation by N-feedback and O2 supply within the nodules. Redox balance and antioxidant defense system play important roles in establishment of legume-rhizobial symbiosis as well as nodule functioning. Longevity and N₂-fixing efficiency of nodules are hugely dependent on environmental conditions prevailing in tropical and subtropical conditions.