| According to the United Nations Food and Agriculture Orgnization(FAO),about more than 800 million hectares of land in the world are affected by salinity.Even worse,more and more agricultural land are subjected to salinity,therefore,salinity is one of the main factors limiting the sustainable development of agriculture.Rice is one of the crops that is sensitive to salinity.Therefore,it is of great scientific significance and application prospect to study the response of rice to salinity.Previous researches indicate that salinity inhibits seed germination,but the inhibitory mechanism of salinity on seed germination is still unclear.It is reported that the plant hormone gibberellin(GA)can promote seed germination and also alleviate the inhibition of salinity on seed germination.However,the mechanism of GA on rice seed germination in response to salinity is still unclear.The first part of our study(the second chapter of the paper)is to explore the regulation and mechanism of GA on rice seed germination under salinity.The results indicated that salinity inhibits seed germination by decreasing the bioactive GA content,as a result of an increase in bioactive GA inactivation.Furthermore,bioactive GA deficiency inhibits seed germination by decreasing ?-amylase activity via down-regulation of ?-amylase gene expression.Root is one of the most important organs of plants.It is in direct contact with soil and so susceptible to external environment.It has been shown that salinity inhibits root growth;ethylene and jasmonic acid(JA)are two important stress phytohormones.They can regulate not only the normal growth and development of plants,but also responses to abiotic stresses.However,the regulation and mechanism of ethylene and jasmonic acid on the root growth of rice seedling under salinity is still unclear.The second part of our study is to explore the regulaion and mechanism of ethylene and jasmonic acid on root growth of rice seeding under salinity.The results indicated that ethylene and jasmonic acid play roles in the inhibition of root growth by salinity,salinity promotes the synthesis of jasmonic acid through ethylene-dependent and ethylene-independent pathway,which in turn inhibits cell division and elongation,resulting in inhibition of root growth.The major results of the first part of the research(displayed in the second chapter of the paper)are as follows:1.Salinity inhibits rice seed germination by reducing the endogenous bioactive GA content.The results showed that Na Cl reduced the content of active GA(GA1 and GA4)in seed embryos by 24% and 60% respectively,resulting in a 27% reduction of seed germination percentage.Exogenous GA3 can relieve the inhibition of Na Cl on seed germination.2.Salinity reduces the content of bioactive GA by promoting the bioactive GA inactivation.Na Cl can significantly up-regulate the expression of GA inactivated genes,promote the inactivation of bioactive GA,and then decreases the content of bioactive GA(GA1 and GA4).3.Salinity-induced GA deficiency decreases ?-amylase activity by inhibiting the expression of ?-amylase genes.The results indicated that Na Cl can significantly reduce the expression of Os Amy1A?Os Amy1C?Os Amy3C?Os Amy3 E genes and ?-amylase activity,while exogenous application of GA3 can alleviate the inhibition of Na Cl on ?-amylase genes expression and ?-amylase activity.The major results of the second part of the research(displayed in the third chapter of the paper)are as follows:1.Ethylene and jasmonic acid are involved in regulating the salinity-induced root growth inhibition.Na Cl inhibited rice seedling root growth and the degree of inhibition is positively related to the concentration of Na Cl.Na Cl promotes the expression of ethylene and jasmonic acid biosynthetic genes,and increases the content of ethylene and jasmonic acid.Ethylene synthesis inhibitor(AVG)and ethylene perception inhibitor(Ag+)and jasmonic acid synthesis inhibitor(IBU)can alleviate the inhibitory effect of Na Cl on rice root growth.2.Ethylene and jasmonic acid interact with each other in regulating rice root growth under salinity.The inhibitory effect of Na Cl on root growth can be partially alleviated by single application of AVG or Ag+,and can be completely alleviated by single application of IBU;Single application of one kind of hormone(jasmonic acid)synthesis inhibitor IBU and simultaneous application of two kinds of hormones(ethylene and jasmonic acid)inhibitors(AVG + IBU,Ag++ IBU)had the same mitigating effect on inhibition of Na Cl on root growth;these results indicated that there is upstream and downstream relationship between ethylene and jasmonic acid in inhibiting rice root growth under salt stress.Further studies showed that IBU could alleviate the inhibitory effect of ethylene on seminal roots growth,while AVG and Ag+ could not alleviate the inhibitory effect of jasmonic acid on seminal root growth.Therefore,under salinity,ethylene inhibits rice seminal root growth through jasmonic acid.Meanwhile,the inhibitory effect of Na Cl on seminal root growth was partially alleviated by ethylene synthesis and action inhibitors,and completely alleviated by jasmonic acid synthesis inhibitor.The results indicated that salinity can also promote jasmonic acid synthesis to inhibit root growth through an ethylene-independent pathway.It shows that salinity promotes jasmonic acid synthesis to inhibit seminal root growth through ethylene-dependent and ethylene-independent pathways.3.Salinity inhibits expressions of cell division and elongation related genes via jasmonic acid,and then inhibits cell division in the meristem zone and cell elongation in the elongation zone,thereby inhibiting rice seedlings root growth.RNA-seq and q RT-PCR confirmed that Na Cl and jasmonic acid can inhibit cell division(such as PCNA)and elongation related genes(such as EXP and XTH)expression,resulting in fewer number of meristem cells and shorter length of mature cell,thereby inhibiting root growth;IBU can alleviate the inhibition of Na Cl on expression of PCNA,EXP and XTH genes,and alleviate the inhibitory effect of Na Cl on seminal root growth.These results indicate that salinity inhibits root growth by inhibiting cell division in the meristem zone and cell elongation in the elongation zone through jasmonic acid.In summary,salinity promotes the deactivation of active GA in rice seeds,reduces the content of active GA,thereby inhibiting the expressions of ?-amylase genes,resulting in decreased ?-amylase activity,and ultimately inhibiting rice seed germination.Ethylene and jasmonic acid are involved in the inhibition of rice seedling root growth under salinity.Salinity promotes the synthesis of jasmonic acid through the ethylene-dependent and ethylene-independent pathways,which in turn inhibits cell division and elongation,leading to inhibitory root growth. |
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