INCREASED RESISTANCE TO OXIDATIVE STRESS IN TRANSGENIC PLANTS BY TARGETING MANNITOL BIOSYNTHESIS TO CHLOROPLASTS

To investigate the potential role of a polyol, mannitol, in oxidative stress protection, a bacterial mannitol-1-phosphate dehydrogenase gene was targeted to chloroplasts by the addition of an aminoterminal tran sit peptide. Transgenic tobacco (Nicotiana tabacum) lines accumulate m annitol at concentrations ranging from 2.5 to 7 mu mol/g fresh weight. Line BS1-31 accumulated approximately 100 mM mannitol in chloroplasts and was identical to the wild type in phenotype and photosynthetic pe rformance. The presence of mannitol in chloroplasts resulted in an inc reased resistance to methyl viologen (MV)-induced oxidative stress, do cumented by the increased retention of chlorophyll in transgenic leaf tissue following MV treatment. In the presence of MV, isolated mesophy ll cells of BS1-31 exhibited higher CO, fixation than the wild type. W hen the hydroxyl radical probe dimethyl sulfoxide was introduced into cells, the initial formation rate of methane sulfinic acid was signifi cantly lower in cells containing mannitol in the chloroplast compartme nt than in wild-type cells, indicating an increased hydroxyl radical-s cavenging capacity in BS1-31 tobacco. We suggest that the chloroplast location of mannitol can supplement endogenous radical-scavenging mech anisms and reduce oxidative damage of cells by hydroxyl radicals.