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

植物螯合素（Phytochelatins, PCs）是一种重金属结合肽，在高等植物解毒中起重要作用。在本研究中，我们研究了微藻（如绿藻和蓝藻）中PC生物合成的调控机制。在高等植物中，众所周知Cd是比其他重金属更有效的PC合成诱导剂。另一方面，我们观察到锌对海洋绿藻杜氏藻PC合成的诱导作用强于Cd。该结果提示杜氏藻PC合成酶基因可用于开发实用的有毒重金属超蓄积器。PC和PC合成酶基因从未在原核生物中被鉴定。然而，在这项研究中，我们在原核生物Nostoc sp.PCC7120中发现了一个编码PC合成酶样蛋白的基因。这种蓝藻蛋白含有保守的n端结构域，而不含有真核PC合成酶中发现的可变c端结构域。此外，该蛋白对PC合成的第一步有较强的催化作用，但对第二步没有催化作用，而真核酶可以催化这两个反应。这些结果表明这种原核PC合成酶可能是真核生物合成酶的一种更原始的形式。脱落酸（ABA）是高等植物诱导各种胁迫反应的信号分子。在本研究中，我们研究了ABA在微藻中的作用。虽然高等植物暴露于各种环境胁迫下通常会加速ABA的生物合成，但淡水绿藻衣藻的氧化胁迫特别加速了ABA的生物合成。此外，外源添加ABA特异性诱导抗氧化反应。另一方面，藻青菌聚囊藻（Synechocystis）在暴露于任何胁迫下均未发生ABA生物合成，也未发生ABA诱导的任何应答反应。这些结果表明，在高等植物中发现的ABA生物合成途径和ABA依赖的胁迫反应系统在绿藻中也存在，但在蓝藻中不存在。少

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