PHOTOOXIDATIVE STRESS IN PLANTS

The light-dependent generation of active oxygen species is termed phot ooxidative stress. This can occur in two ways: (1) the donation of ene rgy or electrons directly to oxygen as a result of photosynthetic acti vity; (2) exposure of tissues to ultraviolet irradiation. The light-de pendent destruction of catalase compounds the problem. Although genera lly detrimental to metabolism, superoxide and hydrogen peroxide may se rve useful functions if rigorously controlled and compartmentalised. D uring photosynthesis the formation of active oxygen species is minimis ed by a number of complex and refined regulatory mechanisms. When prod uced, active oxygen species are eliminated rapidly by efficient antiox idative systems. The chloroplast is able to use the production and des truction of hydrogen peroxide to regulate the thermal dissipation of e xcess excitation energy. This is an intrinsic feature of the regulatio n of photosynthetic electron transport. Photoinhibition and photooxida tion only usually occur when plants are exposed to stress. Active oxyg en species are part of the alarm-signalling processes in plants. These serve to modify metabolism and gene expression so that the plant can respond to adverse environmental conditions, invading organisms and ul traviolet irradiation. The capacity of the antioxidative defense syste m is often increased at such times but if the response is not sufficie nt, radical production will exceed scavenging and ultimately lead to t he disruption of metabolism. Oxidative damage arises in high light pri ncipally when the latter is in synergy with additional stress factors such as chilling temperatures or pollution. Environmental stress can m odify the photooxidative processes in various ways ranging from direct involvement in light-induced free radical formation to the inhibition of metabolism that renders previously optimal light levels excessive. It is in just such situations that the capacity for the production of active oxygen species can exceed that for scavenging by the antioxida tive defense systems. The advent of plant transformation, however, may have placed within our grasp the possibility of engineering greater s tress tolerance in plants by enhancement of the antioxidative defence system.