Changes in the antioxidants of mitochondria during natural senescence of legume nodules

Abstract

Legume nodules have elevated respiratory rates in mitochondria and bacteroids to meet the energy requirements for N2 fixation. However, respiration inevitably produces reactive oxygen species (ROS) such as superoxide radicals and hydrogen peroxide (H2O2). We have studied the changes associated with natural senescence (aging) in the antioxidant system of mitochondria purified from common bean (Phaseolus vulgaris) nodules. During nodule aging, the concentrations of lipid peroxides and oxidized proteins increased in mitochondria, but not in the cytosolic fraction. Also, the activities of three enzymes involved in ascorbate synthesis and regeneration were down-regulated. By contrast, qRT-PCR analysis, immunoblots, and enzyme activity assays reveal a significant up-regulation of manganese superoxide dismutase (MnSOD) activity during aging, whereas the protein level of peroxiredoxin IIF (PrxIIF) remained unchanged. Overall, our results suggest that, in legume nodules, natural senescence is associated with a decline in the capacity of mitochondria to synthesize and regenerate ascorbate, and that this organelle is a preferential target of oxidative stress.