Arbuscular mycorrhiza and reactive oxygen species

Abstract

The accrual of reactive oxygen species (ROS) is a common biochemical response to all abiotic and biotic stresses in plants. ROS are extremely lethal to biological cells causing oxidative damage to DNA, lipids and proteins. Plants have developed many strategies to overcome oxidative stress to restore the redox homeostasis. One of the strategies is to establish symbiotic association in roots with arbuscular mycorrhizal (AM) fungi to improve host resistance to stress. Initial stages of AM fungus colonization trigger intracellular ROS burst in host plant; however, this effect is transient and is overcome by enhanced activities of antioxidant enzymes and molecules such as carotenoid. Accumulation of ROS in cortical cells has also been related to arbuscule digestion. Improvement of stress resistance has usually been associated with AM-induced escalation in P acquisition and plant growth. Nevertheless, non-nutritional effects of AM on host plants have attracted increasing attention. Under stress conditions, AM plants show reduced lipid peroxidation and lower levels of hydrogen peroxide and superoxide. The formation of AM reinforces the antioxidant defence system of the plant for the prevention of oxidative damage. AM symbiosis is capable of increasing activities of enzymes involved directly in removal of ROS such as superoxide dismutase, catalase (CAT) and ascorbate (ASH)- or thiol-dependent peroxidases (POX) and indirectly by generation of two redox molecules ascorbate and glutathione such as glutathione reductase (GR), dehydroascorbate reductase and monodehydroascorbate reductase. AM helps in augmenting the concentrations of non-enzymatic antioxidants such as a-tocopherol, proline, carotenoids, glutathione and ascorbic acid. This review summarizes current knowledge on the effects of AM symbiosis on the accumulation of ROS and correspondingly on the antioxidant defence system. New perspectives and challenges in molecular studies on oxidative stress alleviation by AM symbiosis are proposed.