Kinetics of antioxidative defence responses to photosensitisation in porphyrin-accumulating tobacco plants

Tobacco (Nicotiana tabacum L.) plants expressing antisense genes for uroporphyrinogen decarboxylase or coproporphyrinogen oxidase suffered from photodynamic damage due to their accumulating respective tetrapyrrolic substrates. In the light, unbound pigments caused massive formation of reactive molecules that triggered a cascade of deleterious reaction in these photosensitised plants. Photosensitisation strongly depended on environmental conditions. The formation of necrotic leaf lesions was suppressed by low light dose or decreased oxygen concentration, triggered by extended light periods and enhanced by elevated temperature. An experimental system was established in which the responses to tetrapyrrole-induced oxidative stress were resolved in time during extended photoperiods. Initial differences in antioxidant responses between transgenic and WT plants were observed on enzyme activity level, followed by changes in antioxidant transcript and protein accumulation. Induction of the defence system was accompanied by changes in the redox state of low molecular mass antioxidants (ascorbate, glutathione). In response to spreading porphyrins and derived reactive oxygen species, the enhanced antioxidant capacity in different cellular compartments did not prevent cellular damage. The results are discussed particularly in view of the known antioxidant responses to photosensitisation by porphyrins in plants.