Molecular Characterization of GS2 and Fd-GOGAT Homeologues and Their Biased Response to Nitrogen Stress in Bread Wheat ( Triticum aestivum L.)

Nitrogen (N) is one of the major limiting elements affecting the growth and productivity of cereal crops including bread wheat. Identification and detailed characterization of the candidate genes involved in the assimilation of N from inorganic to organic form is very important for better N utilization. Glutamine synthetase 2 ( GS2) and Ferredoxin-dependent-glutamate synthase ( Fd-GOGAT) are the two critical enzymes involved in GS/GOGAT cycle, which are necessary for primary N-assimilation. In the present study, GS 2 and Fd-GOGAT genes were cloned from a popular bread wheat variety HD-2967 and their individual homeologues ( A , B and D ) were characterized under different levels of N-stress. Homeologous cDNAs of GS2 and Fd-GOGAT showed > 93% and > 98% sequence similarity, respectively, in the coding region but were distinguishable at UTRs. The expression patterns and contribution of each homeologue of both the genes in HD-2967 under different N treatments showed downregulation in the shoot and upregulation in the root tissues. However, the contribution of the individual homeologue varied with the tissue, time point and level of N-stress. Further, when the expression pattern and contribution of each homeologue were studied in a diverse set of wheat genotypes, a generalised contribution pattern of each homeologue of wheat was observed. While the transcript ratio of A and D homeologues of both the genes demonstrated to be compensating each other, Fd-GOGAT-B showed the least contribution in all the conditions and at all stages of growth. Substantial variation in cis -regulatory elements was observed among all three homeologues of both the genes. Overall, characterization and spatio-temporal contribution of GS2 and Fd-GOGAT homeologues would provide applied value for further detailed studies as well as their exploration in primary N-assimilation process in wheat.