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.