Approaches for enhancement of N2 fixation efficiency of chickpea (Cicer arietinum L.) under limiting nitrogen conditions

Chickpea (Cicer arietinum) is an important pulse crop in many countries in the world. The symbioses between chickpea and Mesorhizobia, which fix N-2 inside the root nodules, are of particular importance for chickpea's productivity. With the aim of enhancing symbiotic efficiency in chickpea, we compared the symbiotic efficiency of C-15, Ch-191 and CP-36 strains of Mesorhizobium ciceri in association with the local elite chickpea cultivar Bivanij' as well as studied the mechanism underlying the improvement of N-2 fixation efficiency. Our data revealed that C-15 strain manifested the most efficient N-2 fixation in comparison with Ch-191 or CP-36. This finding was supported by higher plant productivity and expression levels of the nifHDK genes in C-15 nodules. Nodule specific activity was significantly higher in C-15 combination, partially as a result of higher electron allocation to N(2)versus H+. Interestingly, a striking difference in nodule carbon and nitrogen composition was observed. Sucrose cleavage enzymes displayed comparatively lower activity in nodules established by either Ch-191 or CP-36. Organic acid formation, particularly that of malate, was remarkably higher in nodules induced by C-15 strain. As a result, the best symbiotic efficiency observed with C-15-induced nodules was reflected in a higher concentration of the total and several major amino metabolites, namely asparagine, glutamine, glutamate and aspartate. Collectively, our findings demonstrated that the improved efficiency in chickpea symbiotic system, established with C-15, was associated with the enhanced capacity of organic acid formation and the activities of the key enzymes connected to the nodule carbon and nitrogen metabolism.