Efects Of Microcystins On Growth,Hormone Level And Mineral Nutrient Absorption Of Rice(Oryza Sativa)

Microcystins?MCs?are the most productive and harmful toxins in cyanobacterial blooms.MCs can enter the farmland system through irrigation,thus affect the growth and development of plants.The accumulation in edible part of crops will pose a potential threat to human health through the food chain.Therefore,the effect of MCs on crop yield and food safety has attracted more attention.Hormone levels and nutrient uptake play key roles in regulating crop growth,yield and grain quality.In order to comprehensively evaluate the damage of MCs to plants and clarify the internal mechanism,this study took the important food crop rice?Oryza sativa?as the object for studying the effects of MCs on rice growth,final yield and grain quality at different growth stages exposure by enzyme-linked immunosorbent assay,two-phase separation,quantitative real-time PCR,high performance liquid chromatography and inductively coupled plasma.Meanwhile this study clarified the regulating mechanism of MCs on the level and balance of hormone and nutrient absorption at the molecular level.All results can provide a new direction for mitigating the damage of MCs to plants.The main results are as follows:?1?Results on effects of MCs?1,10,100 and 1000?g/L?on rice growth,yield and grain quality show that 1?g/L MCs promoted the growth?root volume,root surface area and root-shoot ratio?of rice seedling,and had no effect on the growth of rice at booting stage and filling stage after 7 days of exposure to MCs.MCs at concentration of 10?g/L inhibited the growth of rice?root volume,total root length?at seedling stage,and had no effect on the growth of rice at booting and filling stage.MCs?100 and 1000?g/L?inhibited the growth of rice at three growth stages,and the degree of inhibition was seedling stage>booting stage>filling stage.After the above treated rice was recovered?without MCs?to the final harvest,there was no change in the yield and grain quality of rice treated with 1?g/L MCs at three growth stages.In addition,there was no change in yield and quality of rice treated with 10?g/L MCs at seedling and booting stage whereas the soluble protein and Mg²⁺contents in grains were increased treated with 10?g/L MCs at filling stage.The yield?number of grains per panicle,setting percentage and per panicle weigh?and nutritional quality?soluble protein,soluble sugar,starch and mineral elements?of grains treated with 100?g/L MCs at three growth stages were decreased,the degree of decrease was generally presented as booting stage>seedling stage>filling stage.The accumulation of MCs in grains in 100?g/L treatment was booting stage>seedling stage>filling stage,and the estimated daily intake?EDI?of MCs for adults exceeded the tolerance daily intake?TDI?value.The rice treated with 1000?g/L MCs at seedling and booting stages did not produce grains.The yield and grain quality of rice treated with 1000?g/L MCs at filling stages were decreased although the rice produced grains,and might pose potential health risks.?2?Results on effects of MCs on hormones level,biosynthesis and metabolic pathways show that 1?g/L MCs increased contents of abscisic acid?ABA?,Indole-3-acetic acid?IAA?,gibberellin?GA₃?and zeatin?ZT?in rice seedlings after 7 days of exposure,being beneficial to promoting the growth of rice seedlings.MCs?10?g/L?caused an increase in the ABA content and decreases in of IAA and ZT contents.High concentrations MCs?100 and 1000?g/L?increased the ABA content?except for ABA content in the leave treated with 1000?g/L MCs?and decreased IAA,GA₃ and ZT contents,disturbed the balance of hormones,which was not conducive to regulating the growth of rice seedlings.Combining with the results of biosynthesis and metabolic pathways of ABA and IAA,we found that the relative expression of genes?OsNCED1 and OsNCED3?encoding nine cisepoxycarotenoiddioxygenase and gene?OsZEP?encoding zeaxanthin epoxidase for ABA synthesis were up-regulated treated with MCs??10?g/L?.In addition,the relative expression of genes?OsABAox1 and OsABAox2?encoding ABA8'-hydroxylase for ABA metabolic were up-regulated treated with MCs??10?g/L?.The metabolic pathway of stress hormone“ABA”can negatively regulate ABA content,and the increase of ABA content in rice seedlings was one of the adaptive mechanisms of plants to MCs stress.The relative expression of genes?OsYUCCA1,OsYUCCA2,OsYUCCA3 and OsYUCCA6?encoding flavinmonooxygenase enzyme for IAA synthesis were down-regulated,and the activity of IAA oxidase was increased.Both could lead to the decrease in IAA content in rice seedling treated with MCs??10?g/L?.After 7 days of recovery,the hormone contents and balance in rice seedlings treated with 10 and 100?g/L MCs were better than those measured during the exposure period.The hormonal disorder caused by 1000?g/L MCs were even worse than those measured during the exposure period,showing that such stress intensity was beyond the self-regulation of rice,and could lead to the decline of rice growth.?3?Results on effects of MCs on nutrient absorption of rice seedlings show that low concentration of MCs?1?g/L?had no effect on the nutrient absorption of rice seedlings after 7days of exposure.The content of NO₃^-,NH₄⁺,Mg²⁺and Fe²⁺treated with 10?g/L MCs were increased,the increase of root activity and plasma membrane H⁺-ATPase activity was beneficial to maintain the nutrient absorption of rice seedling treated with 10?g/L MCs.Combining with changes in expression level of genes encoding plasma membrane H⁺-ATPase,we found that 10?g/L MCs exposure regulated H⁺-ATPase activity mainly by up-regulating the expression level of subfamily I?OSA1,2,3?,III?OSA9?,IV?OSA4,6?and V?OSA8?.However,high concentrations MCs?100 and 1000?g/L?reduced the content of nutrient elements(except NH₄⁺and Ca²⁺)in rice seedlings,being related to the decrease in root activity and plasma membrane H⁺-ATPase activity.After 7 days of recovery,the nutrient absorption of rice treated with 10?g/L MCs were recovered to the control level.The nutrient absorption capacity of rice seedlings treated with 100?g/L MCs had a certain degree of recovery.The inhibition of nutrient absorption in 1000?g/L MCs group exceeded the self-repairing ability of the plants,and the recovery of rice growth was inhibited.In summary,the "low promotion" effect of MCs was related to the increase of IAA,GA₃ and ZT content in plants treated with low concentration of MCs.The "high inhibition" effect of MCs was related to the imbalance of hormones biosynthesis and metabolism and the inhibition on nutrient absorption of roots induced by high concentration of MCs.In addition,the "low promotion and high inhibition" effect of MCs was closely related to the growth stage of plants,and the recovery degree of plants after stress removed was limited by the exposure concentration of MCs.The above results can provide evidences for objective and comprehensive assessment of the ecological toxicity of MCs,and also provide new directions for mitigating the damage of MCs on plants.