4. Are antioxidants a magic bullet for reducing oxidative stress during cryopreservation?

Abstract

The development of effective cryopreservation protocols for vegetatively propagated plants is highly important for the long-term conservation of important horticultural crops as well as endangered species. Cryopreservation protocols are already in place for many plant species, especially for temperate plants and provide a stable, long-term and low-cost backup that is shielded from diseases or environmental damage. However, many tropical plants are recalcitrant to cryopreservation due to lack of tolerance to low temperatures or osmotic stress. Storing plants at low temperatures has some well known deleterious effects. Chilling and freezing injury promote many sub-lethal changes that can lead to increased production of free radical species and cellular damage. Reactive oxygen species (ROS) include superoxide radicals (O 2 ), hydroxyl radicals (OH), hydrogen peroxide (H 2 O 2 ) and singlet oxygen ( 1 O 2 ). Abiotic stresses resulting from low temperature, water deficit or nutrient deprivation can increase ROS formation to toxic levels in cells and tissues. Plants have evolved natural antioxidant defense mechanisms to combat the effects of ROS that are produced during physiological stress. These ROS scavenging mechanisms include antioxidant enzymes such as superoxide dismutase, peroxidases, mono- and dehydroascorbate reductase, glutathione reductase and catalase. The steps in cryopreservation protocols impose a series of stresses on plants. The osmotic and temperature stresses involved in cryopreservation increase oxidative stress and a number of recent studies show that the addition of antioxidants during the cryopreservation process reduces oxidation and results in improved regrowth of both control and cryopreserved plant tissues.