Development of practically applicable stress-tolerant plants by transformation of stress response genes identified and characterized from microalgae

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Phytochelatins (PCs) are heavy-metal-binding peptides that play important roles in the detoxification of toxic heavy metals in higher plants. In this study, we investigated the regulation mechanism of PC biosynthesis in microalgae such as green algae and cyanobacteria. In higher plants, it is well known that Cd is a more effective inducer of PC synthesis than other heavy metals. On the other hand, we have observed PC synthesis is more strongly induced by zinc than Cd in a marine green alga, Dunaliella. This result suggests Dunaliella PC synthase gene may be applicable to develop practical hyperaccumulator for toxic heavy metals.PC and PC synthase genes have never been identified in prokaryotes. In this study, however, we found a gene encoding a PC synthase-like protein in a prokaryote, Nostoc sp.PCC7120. This cyanobacterial protein contained the conserved N-terminal domain but not the variable C-terminal domain found in eukaryotic PC synthases. Furthermore, this protein strongly cataly … More zed the first step of PC synthesis but could not catalyze the second step, although eukaryotic enzymes can catalyze both reactions. These results suggest this prokaryotic PC synthase may be a more primitive form of the eukaryotic ones.Abscisic acid (ABA) is a well known signal molecule for induction of various stress response reactions in higher plants. In this study, we investigated the function of ABA in microalgae. Although ABA biosynthesis is generally accelerated by exposure to various environmental stresses in higher plants, the biosynthesis was specifically accelerated by oxidative stress in freshwater green alga, Chlamydomonas. Furthermore, exogenously added ABA specifically induced anti-oxidant reactions. On the other hand, in a cyanobacterium, Synechocystis, ABA biosynthesis never occurred under exposure to any stresses and any response reactions were not induced by ABA. These results suggest the ABA biosynthetic pathway and ABA-dependent stress response systems found in higher plants also exist in green algae but do not exist in cyanobacteria. Less