Study On The Physiological Mechanism Of Resistance To Nicosulfuron In Waxy Maize

Nicosulfuron is a post-emergence herbicide used for weed control in maize（Zea mays L.）fields.Field and pot experiments were conducted using nearly isogenic inbred lines Shennuo509-R（nicosulfuron resistant）and Shennuo509-S（nicosulfuron sensitive）to study the effect of nicosulfuron on physiology and detoxification mechanism in waxy maize seedling.In this study,the differences in resistance to nicosulfuron among different resistant maize varieties were elucidated from the perspective of detoxification pathway and target enzyme.The expression levels of key enzyme genes in the metabolic pathway of nicosulfuron were determined by real-time quantitative PCR,in order toreveal the resistance mechanism of nicosulfuron to waxy corn from physiological,biochemical and molecular levels.The results were as follows:1.After nicosulfuron treatment,net photosynthetic rate（P_n）,stomatal conductance,transpiration rate,leaf maximum photochemical efficiency of PSII,photochemical quenching of chlorophyll fluorescence and the actual photochemical efficiency of PSII of Shennuo509-S were significantly lower than those of Shennuo509-R.However,in contrast the intercellular CO₂concentration,stomatal limitation（L_s）and non-photochemical quenching.The study showed that L_s might be the main reason for the decline in P_n in Shennuo509-S,and it was also the main response of Shennuo509-S to the nicosulfuron treatment.After spraying with nicosulfuron,the resistant inbred line Shennuo509-R maintained a higher photosynthetic capacity on completing its normal growth and development process compared to the susceptible inbred line Shennuo509-S.2.The reduction of the electron transport activity of the sensitive inbred line Shennuo509-S causes nicosulfuron as a toxic factor to cause an increase in the oxidative stress in the plant through the same electron transport pathway.After nicosulfuron treatment,compared to Shennuo509-R,the death of Shennuo509-S might be associated with increased oxidative stress,since higher O₂·^-and H₂O₂ accumulations were observed in Shennuo509-S.This in turn might have caused severe damage to lipids and proteins,thus reducing membrane stability.After nicosulfuron treatment,superoxide dismutase（SOD）,peroxidase,catalase（CAT）and glutathione peroxidase of Shennuo509-R were significantly higher than those of Shennuo509-S.The genes SOD4,SOD9,and CAT1 showed significantly increased level of expression in Shennuo509-R compared with control,while the genes SOD4,SOD9,and CAT1showed significantly decreased level of expression in Shennuo509-S.3.Ascorbate-glutathione（AA-GSH）pathway is the main pathway of H₂O₂ metabolism.After nicosulfuron treatment,ascorbate peroxidase（APX）,monodehydroascorbate reductase（MDHAR）,dehydroascorbate reductase（DHAR）,and glutathione reductase（GR）of Shennuo509-S were significantly lower than those of Shennuo509-R.Compared to Shennuo509-R,dehydroascorbate content,glutathione（GSH）content,and GSH to glutathione disulphide ratios significantly declined with increasing exposure time in SN509-S.Gene expression analysis results showed that compared with control,the expression level of APX2,APX3,MDHAR1,MDHAR2,MDHAR3,MDHAR4,DHAR1,DHAR2,DHAR3,DHAR4,GR1,and GR2 increased significantly in Shennuo509-R,while the expression level of APX3,MDHAR1,MDHAR2,MDHAR3,MDHAR4,DHAR1,DHAR2,DHAR3,and DHAR4decreased significantly in Shennuo509-S.There was no significant change in the expression level of GR1 and GR2 genes in Shennuo509-S.These results collectively suggest that on transcription level,and in accordance with enzymatic responses,enzymes of APX,MDHAR,DHAR,and GR play a vital role in the H₂O₂ detoxification in maize seedlings on nicosulfuron exposure.4.Nicosulfuron-treated acetolactate synthase（ALS）and glutathione-S-transferase（GST）activities and gene expression levels were determined.After treatment with nicosulfuron,the ALS and GST activities of Shennuo509-R were significantly higher than that of Shennuo509-S.The expression level of ALS1 and GST1 genes of Shennuo509-R were significantly up-regulated,while ALS and GST1 genes of Shennuo509-S did not change significantly compared to control.