Enhanced accumulation of BiP in transgenic plants confers tolerance to water stress

The binding protein (BiP) is an important component of endoplasmic reticulu m stress response of cells. Despite extensive studies in cultured cells, a protective function of BiP against stress has not yet been demonstrated in whole multicellular organisms. Here, we have obtained transgenic tobacco (N icotiana tabacum L. cv Havana) plants constitutively expressing elevated le vels of BiP or its antisense cDNA to analyze the protective role of this en doplasmic reticulum lumenal stress protein at the whole plant level. Elevat ed levels of BiP in transgenic sense lines conferred tolerance to the glyco sylation inhibitor tunicamycin during germination and tolerance to water de ficit during plant growth. Under progressive drought, the leaf BiP levels c orrelated with the maintenance of the shoot turgidity and water content. Th e protective effect of: BiP overexpression against water stress was disrupt ed by expression of an antisense BiP cDNA construct. Although overexpressio n of BiP prevented cellular dehydration, the stomatal conductance and trans piration rate in droughted sense leaves were higher than in control and ant isense leaves. The rate of photosynthesis under water deficit might have ca used a degree of greater osmotic adjustment in sense leaves because it rema ined unaffected during water deprivation, which was in marked contrast with the severe drought-induced decrease in the CO, assimilation in control and antisense leaves. in antisense plants, the water stress stimulation of the antioxidative defenses was higher than in control plants, whereas in droug hted sense leaves an induction of superoxide dismutase activity was not obs erved. These results suggest that overexpression of BiP in plants may preve nt endogenous oxidative stress.