Overexpression Of TaPCS1in Rice Leads To Cadmium Hypersensitivity And Physiological Mechanisms

As one of the most deleterious heavy metals, cadmium (Cd) is toxicity and impairs a great deal of physiological functions in plants. Rice (Oryza sativa L.) is one of the most tolerant plants to Cd, it absorbs and accumulates more Cd than any other crops without obvious deleterious symptoms, so high accumulation of Cd in rice is dangerous and hard to discern. Cd conceals potential perils and risks to rice production and human health. Phytochelatin synthase (PCS) catalyzes the final step in the biosynthesis of phytochelatins, which are a family of cysteine-rich thiol-reactive and heavy metal binding peptides that play an important role in sequestration and detoxification of heavy metals in plants. Previous studies on plants overexpressing PCS genes yielded contrasting phenotypes, ranging from enhanced cadmium tolerance to cadmium hypersensitivity. Despite the vast literature on the overexpression of PCS gene in Arabidopsis, tobacco, Brassica juncea, to the best of our knowledge, there are no reports on overexpression of a heteologous PCS gene in rice. We expressed a wheat phtochelatin synthase (TaPCS1) gene in wild-type rice (Oryza sativa L., cv. Zhonghua11), want to study the PCs and the Cd tolerance in the rice. The major results and conclusions were summarized as follows:First of all, Overexpression of TaPCS1results in increased sensitivity to cadmium。Transgenic lines not only had severe leaf chlorosis and leaf wilting but also had low fresh weight, in contrast to the wild-type plants when in500uM Cd2+.Secondly, Overexpression of TaPCS1effected the Cd distribution:the first one is that expression of the wheat TaPCS1gene in rice enhanced Cd accumulation in shoots but not increased Cd accumulation in roots tissue, Cd2+saturation in roots activates transport to the shoot in a PC-dependent process; the other one is that TaPCS1-overexpression rice increased the Cd content in soluble fraction of shoots, may be that due to increase PCs and NPT production, which may increase chelation of the metal then compartmentalization of the PC-Cd complex into vacuoles. The third, In this study, we found a higher NPT content and PCs content in shoots of TaPCS1-overexpressing plants but a significant GSH depletion than wild type, the efficiency ofAsA-GSH cycle decreased. TaPCS1-over-expressing plants under Cd stress shown the GSH pool deplete and the AsA declines, moreover, the ratios of AsA/DAsA, GSH/GSSG, NADPH/NADP+was also significantly declined, effecting the cellular redox balance, increase the accumulation of hydrogen peroxide and led to the hypersensitive of TaPCS1-overexpression rice eventually.The last one, The content of Cd in the cell organelle fraction of shoots was high in the TaPCS1-overexpression rice, effecting the metabolic processes of the cells then causing the cell death. Hence, for the TaPCS1-overexpression rice plants, the decrease in biomass and Cd hypersensitive may have resulted from higher Cd distributed in the cell organelle fraction of leaves, which showed stronger symptoms when compared with the wild type that was under same Cd treatment.