Review: Dark-induced changes in legume nodule functioning

Abstract

Exposure of nodulated leguminous plants to prolonged periods of continuous darkness has been used as a convenient tool to investigate host plant control on nitrogen fixing systems in legume root nodules. Foliar dark treatment of plants results in a rapid decline in N₂ -fixation in terms of acetylene reduction activity (ARA) and predisposes the nodules to metabolic and structural senescence. After 2 d of darkness, a significant decrease is seen in nitrogenase (N₂ -ase) proteins of common bean nodules. The effect of dark treatments on nodule respiration varies with plant species. A variable decrease in nodule carbohydrates is observed in different plant species under dark treatments, but no direct correlation between N₂ -ase activity and gross levels of carbohydrates present in the nodules has been detected. Usually nodule leghemoglobin (Lb) shows a decrease of variable intensity depending on plant species. The mRNA of Lb, sucrose synthase and glutamine synthetase shows a significant decline within 24 h of complete darkness. Dark-induced acceleration of proteolytic activity and decreased plant growth are reflected in decreased nodule proteins and accumulation of free amino acids following a drop in ARA. Antioxidants such as ascorbic acid and glutathione, along with the enzymes of their oxidation–reduction cycle, show a considerable decrease in their content and activity in nodules from dark-treated plants. Among H₂ O₂ scavengers, nodule catalase activity decreases in most of the plants studied, but peroxidase activity shows an increase. Dark-induced adverse effects on N₂ -fixation are completely or partially reversible on shifting the plants back to a normal light/dark regime. Significant changes in nodule ultrastructure are induced by dark treatment. Attempts have been made to explain the mechanisms underlying dark-induced changes in nodule functioning.