Expression of nodule-specific glutamine synthetase genes during nodule development in soybeans

A cDNA clone (pcPvNGS-01) to glutamine synthetase (GS) mRNA from root nodules of Phaseolus vulgaris showed cross-hybridization to GS and mRNA from soybean root nodules, thus allowing its use as a probe to study the expression of GS genes during root nodule development in soybeans. Hybrid-select translation of root and nodule RNA of soybean with DNA from pcPvNGS-01, followed by 2D gel electrophoresis, showed six peptides in the root and an additional four peptides in the nodule which represent nodule-specific glutamine synthetase (GSn) gene products. The GSn gene products appeared for the first time between day 11 and 12 after infection, either concomitant with the onset of nitrogenase activity or immediately following it. The levels of expression of the GSn and leghemoglobin genes were not affected in young Fix(-) nodules formed by Bradyrhizobium japonicum strains that are defective in nitrogenase activity, suggesting that the induction of these two sets of host genes take place independent of nitrogenase activity. However, in Fix(-) nodules that are incapable of maintaining the peribacteroid membrane, GSn gene products were not detected while 1ba, 1bc2 and 1bc3 appeared. In both the timing of appearance during root nodule development and the effect of different bacterial mutations on the expression, GSn genes differ from most other nodulin genes examined (30), suggesting different regulatory mechanisms.