Do free radicals have a role in plant tissue culture recalcitrance?

Free radicals have an important role in the metabolism and development of aerobic organisms; however, their uncontrolled production leads to oxidative stress. This paper explores the possibility that free radical-mediated stress has a role in tissue culture recalcitrance. In the context of this paper, recalcitrance is considered to be the inability of plant tissue cultures to respond to culture manipulations; in its broadest terms. this study also concerns the time-related decline (i.e. in vitro aging) and loss of morphogenetic competence and totipotent capacity. Studies on a diverse range of in vitro plant systems have shown that tissue cultures produce free radicals, lipid peroxides and toxic, aldehydic lipid peroxidation products. Levels of these compounds vary in response to different tissue culture manipulations, but their production is enhanced during dedifferentation and antioxidant profiles also vary throughout different phases of culture. A hypothesis is presented which suggests that tissue culture manipulations cause major metabolic and developmental changes, some of which may predispose in vitro cultures to increased free radical formation. If antioxidant protection is compromised, oxidative stress ensues and free radicals and their reaction products react with macromolecules such as DNA, proteins and enzymes, causing cellular dysfunction and, as a result, the cultures become recalcitrant.