Effects Of S-Allyl-L-Cysteine On Cadmium Transport In Rice

With the increase of production activities of human society,the toxic heavy metal cadmium（Cd）is constantly released into the natural environment,leading to the problem of cadmium（Cd）pollution in rice,one of the main crops in China.Therefore,it is necessary to explore an effective way to reduce Cd content in rice grains for improving rice quality and ensuring food security.S-allyl-L-cysteine（SAC）is one of the active components of garlic,which is commonly used in medicine for cancer treatment and heavy metal detoxification.In this study,we explored the mitigation mechanism of SAC on Cd toxicity of rice seedlings and the mechanism of blocking the transport of heavy metal Cd ions to rice shoots and grains in rice by seed germination experiment,seedling experiment and field experiment.The main research results are as follows:1.Seed germination experiments showed that SAC could significantly alleviate the stress effect of Cd on young roots and shoots by regulating the relative expression of Cd transporter-encoded genes in young roots and shoots,thereby reducing the absorption and transport of Cd to shoots.Treatment of SAC could significantly alleviate the inhibitory effect of Cd stress.The activities of CAT and SOD in young roots and shoots were significantly increased,while the contents of MDA and GSH were significantly decreased.Cd content in young roots and shoots decreased by 35.91%and 28.86%,respectively,significantly alleviating Cd stress.Cd stress significantly inhibited the expression levels of OsNRAMP5,OsHMA2 and OsHMA3 genes,and the relative expression levels of Cd transporter-encoding genes OsNRAMP5 and OsHMA2 were significantly decreased after SAC treatment compared with the Cd²⁺treatment group,respectively.The relative expression level of OsHMA3 was significantly higher than that of Cd²⁺treatment group.2.Seedling experiments showed that foliar spraying of SAC could significantly reduce Cd accumulation in shoots of rice seedlings by regulating the expression of Cd transporter-encoded genes,and at the same time significantly alleviate Cd stress in seedlings.When SAC spraying concentration was0.05～0.4 mmol·L^-1,Cd accumulation in shoot of rice seedlings gradually decreased with the increase of SAC spraying concentration,and the maximum decrease was 49.02%compared with the CK.Meanwhile,the accumulation of Zn in shoot of rice seedlings also showed a significant decreasing trend.The distribution of subcellular cells in roots changed,and the accumulation of Cd in vacuoles increased significantly.After SAC spraying,the expression of OsHMA2 gene,which is responsible for Cd transport to shoot,was inhibited,and the expression of OsHMA3 gene,which is responsible for Cd transport to vacuole,was promoted,but the expression of OsNRAMP5 gene,which is responsible for Cd uptake from outside,was not significantly affected.The results show that spraying SAC on the leaves could inhibit the transport of Cd from roots to shoots and promote the transport of Cd to vacuoles,thus alleviating the stress effect of Cd on rice seedlings.3.Field experiments showed that the Cd content in rice grains could be significantly reduced by spraying SAC on the leaves surface of Cd contaminated paddy field at the early flowering stage.After spraying SAC,the accumulation of Cd in grains was significantly reduced by 66.13%.At the same time,the accumulation of Cd in flag leaves,rachises,nodes,internodes,second leaves and stalks decreased significantly with the increase of SAC spraying concentration,but there was no significant effect on the content of Cd in root.The results of field experiments showed that increasing the concentration of SAC spraying did not reduce the Cd content in rice grains and shoot organs.Therefore,it is speculated that SAC spraying may be related to the regulation of the expression of related heavy metal transporters in the grains.4.Field experiments showed that spraying SAC on the leaf surface of Cd-contaminated paddy fields during late flowering stage could significantly reduce Cd content in grains by regulating the expression of related heavy metal transporter proteins.When the concentration of SAC was 0.1 mmol·L^-1,the relative expression levels of OsLCD in rachises,flag leaves and spike nodes were significantly decreased.The content of Cd in grains,rachises,flag leaves and spike nodes also showed a significant decreasing trend,which was consistent with the rule of early rice.By down-regulating the expression of OsLCD in the upper organs of rice,spraying SAC regulated the transport of Cd from phloem to grain,thus reducing the accumulation of Cd in grains and reducing heavy metal toxicity.