Heterogeneity of sucrose synthase genes in bean (Phaseolus vulgaris L.): evidence for a nodule-enhanced sucrose synthase gene.

Sucrose synthase (SS), the key sucrose hydrolytic enzyme (EC 2.4.1.13), plays an important role in N(2)-fixing nodule metabolism. It has also been proposed that N(2) fixation in soybean nodules could be mediated by the potential to metabolize sucrose. The isolation and characterization of a nodule-enhanced SS full-length cDNA clone from the bean Phaseolus vulgaris is reported here. Southern blot analysis indicated that there are at least two SS genes in beans. Using a 3' specific probe from this SS cDNA clone, it was possible to identify a nodule-enhanced SS gene (PvSSn), which is expressed almost exclusively in nodules. A second gene (PvSS), which is expressed in all tissues tested, was detected using a coding region probe. Nodule-enhanced PvSSn transcript levels, but not the enzyme activity or protein amount, is reduced during nodule development. These data indicated that this reduction could be due to a limitation on the carbon availability in the nodule. PvSSn expression is reduced in the asparagine-treated nodules. By contrast, PvSSn transcript levels in nodules increased in the presence of glutamine, allantoin and allopurinol. This result suggests a relationship between ureide transport and SS regulation and could help in understanding why the ureide transport mechanism is activated during nitrogen fixation in bean.