Effects And Physiological Mechanisms Of Nitric Oxide In Alleviating Cadmium Toxicity To Rice

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. Nitric oxide (NO) is a significant gas signaling molecule taking more and more attention in recent decade, it plays a number of crucial roles in plants signaling and in plant defense response. And now, the number of researches on the effects of NO in alleviating heavy metal toxicity to plants is increasing, although the physiological processes and mechanisms of NO in alleviating heavy metal toxicity are still far from clear. It is necessary and meaningful to investigate the physiological processes and mechanisms of NO in alleviating Cd toxicity to rice, the result will be useful to alleviate Cd-caused decrease of rice production, and the result also will provide significant theory basis to prevent and control high accumulation of Cd in rice seeds.A Japonica rice Zhonghua 11 was used in the studies, the effects of Cd and NO (in the form of Sodium nitroprussidum, SNP) on rice growth were studied and more attentions was paid to roots growth and Cd distribution in rice. In ord to investigate the physiological processes and mechanisms of NO in alleviating Cd toxicity to rice, the effects of NO on oxidative stress, transpiration rate, root cell wall composition and expression of metallothionein (MT) genes were researched completely. The major results and conclusions were summarized as follows:First of all, in phenotype, primary root (PR) and crown root (CR) elongation were inhibited and the number of CR was decreased by treatment with 0.1 mM Cd in rice seedlings. In physiology, the activity of NOS which is the pivotal enzyme responsible for endogenous NO generation was inhibited by treatment with 0.1 mM., Cd inhibited CR initiation by decreasing endogenous NO content. Based on the results from microscopic analysis, it was concluded that NO is indispensable for rice CR primordia initiation rather than for rice PR and CR elongation, Cd-caused decrease of endogenous NO content has on effect on inhibiting PR and CR elongation in rice. Although a suitable concentration of NO plays a role in alleviating Cd toxicity to rice root growth, high concentration of NO inhibits rice CR elongation obviously. Based on all these results, it indicates that CR primordia initiation and CR elongation belongs to different physiological processes, the roles of NO in these physiological processes are obviously different.Secondly, low concentrations of SNP (≤0.1 mM) alleviated the toxicity of 0.2 mM Cd to 5-leaves-old rice, high concentrations of SNP (≥0.1 mM) deteriorated the toxicity of 0.2 mM Cd to rice. In the rice under 0.2 mM Cd stress, treatment with 0.1 mM SNP increased Cd accumulation in the roots and stems but decreased Cd accumulation in the leaves. Subcellular analysis indicated that treatment with 0.1 mM SNP increased Cd accumulation in cell wall and cell organelle of roots, decreased Cd accumulation in cytoplasm of leaves. All these results indicate that NO alleviates Cd toxicity by regulating Cd distribution in cell wall, cell organelle and cytoplasm of different rice organs, decreased Cd contents in cytoplasm of roots and leaves is responsible for alleviation of Cd toxicity in rice.The third, in the 5-leaves-old rice under 0.2 mM Cd stress, treatment with 0.1 mM SNP decreased ROS content and alleviated oxidative stress in both roots and leaves, it is one of the mechanisms account for NO-induced alleviation of Cd toxicity in rice. Treatment with 0.1 mM SNP alleviated Cd-caused inhibition on transpiration rate and reduced Cd accumulation in the shoots of rice, however, treatment with 0.1 mM SNP alone had no visible effect on rice transpiration rate. This result indicates that NO-induced decrease of Cd accumulation in rice shoots is not subject to decrease of transpiration which effects Cd accumulation. Treatment with 0.1 mM SNP decreased cellulose and increased pectin and hemicellulose content in the roots of 5-leaves-old rice. Polysaccharides in cell wall play crucial roles in absorb Cd, so NO-induced increase of polysaccharides is one of the most important mechanisms on alleviating Cd toxicity to rice. And more, treatment with 0.1 mM SNP or 0.2 mM Cd increased the expression of MT gene U43529 but decreased the expression of MT gene U43530 in the roots, stems and leaves of 5-leaves-old rice, in contrast, neither 0.1 mM SNP nor 0.2 mM Cd had visible effect on expression of MT gene AF001396. This result indicates that different types of MTs play various roles in rice, the MT gene U43529 which expression was enhanced by treatment with 0.1 SNP and 0.2 mM Cd possibly takes part in Cd tolerance in rice.