Genotypes of the Common Bean (Phaseolus vulgaris L.) Lacking the Nodule-Enhanced Isoform of Glutamine Synthetase

Glutamine synthetase (GS) is an octameric enzyme. The nodule cytosol of the common bean (Phaseolus vulgaris L.) has two major types of GS subunit polypeptides ([beta] and [gamma]). As a result, nine different isozymes containing varied proportions of [beta] and [gamma] can be generated. The isozymes are resolvable by native polyacrylamide gel electrophoresis. Staining the gel for GS activity reveals two isoforms, GSn1, which is nodule enhanced and is composed of the eight [gamma] polypeptide-containing isozymes, and GSn2, which is the isozyme [beta]8. We screened 104 cultivars and genotypes of common beans for variations in isozyme formation and found two, PI317350 and PI326054, that had no GSn1. The PI beans appeared to nodulate normally and had cytosolic protein concentrations and total GS activities similar to those of the cultivar UI-111, which has GSn1. They accumulated the [gamma] polypeptide, which had the same molecular weight (46,000) and isoelectric point (6.3) as the [gamma] polypeptide of UI-111. Experiments with extracts prepared by mixing UI-111 and the PI bean nodules suggested that the PI bean nodule extracts did not have an inhibitor or a proteolytic system that specifically inhibited or degraded GSn1. Nodules from UI-111 and the PI beans were dissected into cortex and central infection zone tissue fractions. GSn2 was found in the cortex and the central infection zone tissue of all beans. Our results suggested that the reason we were unable to detect GSn1 from the PI beans was not because their GSn1 and GSn2 had an identical electrophoretic mobility, nor was it due to an inhibited or unstable GSn1. Our results suggested that either their [gamma] gene had mutated in the region that is essential for the [gamma] polypeptide to assemble or the assembly of GS may require a chaperone. In the two PI beans, the chaperone accumulated to a lower level than it did in UI-111. This lower amount limited the assembly of the [gamma] polypeptide into GS.