Abstract

Pathogenesis of Sclerotinia sclerotiorum mainly depends on production of oxalate (Lib.) de Bary, but the mechanism by which the oxalate is produced is not well understood. Control of Sclerotinia stem rot (SSR) in soybean which is caused by this ubiquitous phytopathogenic fungus requires the knowledge of oxalate synthesis by S. sclerotiorum. Nine isolates from infected soybeans and soils were obtained and used to study the various organic acids and enzymes involved in oxalate metabolism. The study showed that S. sclerorotiorum produces malate, succinate, oxaloacetate and acetate during the synthesis of oxalate. The activities of oxaloacetate hydrolyase (OAH), malate dehydrogenase (MDH), glyoxylate dehydrogenase (GDH) were detected in all the isolates but activities of oxalate decarboxylase (ODH), and formate dehydrogenase (FDH) were not detected. The high specific activity of MDH suggests an important role that this enzyme could be playing in oxalate biosynthesis. Absence of FDH and ODH could be the reason why the oxalate concentration was found to be high in the filtrates. This study reports the significance of GDH and OAH in pathogenesis of S. sclerotiorum. Overall, these results suggest metabolism of oxalate by S. sclerotiorum through a tricarboxylic cycle.

Key words: Biosynthesis, pathogenesis, stem rot, organic acid, enzyme.