A recent strategy for pest control in plants has involved transformation with genes encoding cysteine proteinase inhibitors (cystatins). Little is known, however, about the effects of constitutive cystatin expression on whole plant physiology. The present study using oryzacystatin I (OC-I) expression in transformed tobacco was designed to resolve this issue and also to test the effects on abiotic stress tolerance. All transformed plants expressing OC-I showed a conditional phenotype. A marked effect on stem elongation was observed in plants grown under low light intensities. After 7 weeks of growth at low light, the plants expressing OC-I were smaller with fewer expanded leaves and a slightly lower total biomass than empty vector controls or wild type plants. Maximal rates of photosynthesis (A(max)) were also decreased, the inhibitory effect being greatest in the plants with highest OC-I expression. After 12 weeks of growth at low light, however, the plants expressing OC-I performed better in terms of shoot biomass production, which was nearly double that of the empty vector or wild type controls. All plants showed similar responses to drought, however photosynthesis was better protected against chilling injury in plants constitutively expressing OC-I. Photosynthetic CO(2) assimilation was decreased in all plants following exposure to 5 degrees C, but the inhibition was significantly less in the OC-I expressing plants than in controls. The transformed tobacco plants expressing OC-I therefore show a phenotype-environment interaction with important implications for biotechnological applications.