| Cassava(Manihot esculenta Crantz)is an important industrial raw material,feed,bioenergy and emergency crop in the world.It is also the staple food for more than 900 million people in the world.Papaya mealybug(Paracoccus marginatus Williams and granara de Willink)is an important pest in the world.It has a short life history,high fecundity,serious outbreak and difficult to control with chemicals.Cultivation and utilization of insect resistant cassava cultivars is an effective way to control P.marginatus.Studies show that the contents of nutrients,secondary metabolites and protective enzymes play an important role in plant resistance to insect pests.Therefore,considering the development of modern cassava industry and the actual needs,this study carry out the research on the resistance mechanism of cassava to P.marginatus by using physiological,biochemical and molecular biology methods,and clarified the differences of nutrient content,secondary metabolite content,protective enzyme activity and related insect resistant gene expression of resistant and susceptible cassava cultivars,while being infested by P.marginatus.This study provides a theoretical basis for exploring excellent insect resistant cassava germplasm resources and further exploring the mechanism of insect resistance of cassava.The main results are as follows:1.It was found that the cassava resistance to P.marginatus was significantly negatively correlated with the contents of free amino acids(FAA),soluble nitrogen(N),water soluble sugar(WSS)and free proline(Fpro)in leaf tissue,and significantly positively correlated with the ratio of sugar to nitrogen(S/N).These results preliminarily elucidated the nutrient-based mechanism of cassava resistance to P.marginatus.There was no significant difference in the contents of FAA,N and WSS between resistant and susceptible cassava cultivars before feeding,but the contents of FAA,N and WSS in susceptible cassava were significantly higher than those in resistant cassava after feeding.In addition,the contents of FAA and Fpro in leaf tissues of susceptible cassava cultivars BRA900,Bread and SC205 were significantly increased after feeding for 1,4 and8 days,.However,there were no significant difference in N,WSS and S/N.Furthermore,the contents of FAA,N and WSS in leaf tissues of resistant cassava cultivars C1115 and Myanmar decreased,when S/N increased.The Fpro content of insect resistant cassava cultivars increased slightly,and it was significantly lower than that of insect susceptible cassava cultivars being infested.Correlation analysis showed that the resistance of cassava to P.marginatus was negatively correlated with the contents of FAA,N,WSS and Fpro in leaf tissues(P<0.05,R = 0.845,0.866,0.846 and 0.876,respectively),and positively correlated with the S/N(P<0.05,R = 0.862).2.It was found that the cassava resistance to P.marginatus was positively correlated with the contents of total phenols and tannins,and negatively correlated with the contents of malondialdehyde(MDA).These results preliminarily elucidated the secondary metabolites-based mechanism of cassava resistance to P.marginatus.There was no significant difference between the contents of total phenol,tannins and MDA in the leaves of the cassava cultivars BRA900,Bread and SC205 before and after feeding.In addition,the contents of total phenols and tannins in the leaves of insect resistant cassava cultivars did not showed significantly difference compared with those of insect susceptible cassava cultivars before feeding,while the content of MDA was significantly lower than those susceptible ones.Furthermore,the contents of total phenols and tannins in the leaves of insect resistant cassava cultivars were significantly higher than those susceptible ones after being infested by mealybug for 1,4 and 8 days,and MDA content was significantly lower than those susceptible ones.Correlation analysis showed that the resistance of cassava to P.marginatus was positively correlated with the contents of total phenols and tannins in leaf tissues(P<0.05,R = 0.898 and 0.757),and negatively correlated with the content of MDA(P < 0.05,R = 0.813).3.It was found that the cassava resistance to P.marginatus was positively correlated with the polyphenol oxidase(PPO),peroxidase(POD),catalase(CAT)and superoxide dismutase(SOD)activity in leaf tissue.These results preliminarily elucidated the protective enzyme-based cassava resistance to P.marginatus.The activities of PPO,POD,CAT and SOD were significantly lower in the insect resistant cassava cultivars than those susceptible ones before feeding,but were significantly higher after being infested for 1,4 and 8 days.Compared with those before feeding,the activities of PPO,POD,CAT and SOD in leaf tissue of susceptible cassava cultivars BRA900,Bread and SC205 were significantly decreased,while the activities of PPO,POD,CAT and SOD in leaf tissues of resistant cassava cultivars C1115 and Myanmar increased significantly.Correlation analysis showed that the resistance of cassava to P.marginatus was positively correlated with PPO,POD,CAT and SOD activities in leaf tissue(P < 0.05,R = 0.831,0.771,0.791 and 0.851,respectively).4.It was found that the cassava resistance to P.marginatus was significantly negatively correlated with the expression levels of cassava soluble nitrogen synthesis genes Me NR and Me GS,the water soluble sugar synthesis gene Me Susy,the free proline synthesis genes Me P5 CS and Me OAT,and positively correlated with the tannins synthesis genes Me LAR and Me ANR,the POD coding gene Me POD,the CAT coding gene Me CAT and the SOD coding gene Me Cu/Zn SOD.These above results further validated the defensive mechanism of cassava to P.marginatus based on the transcription of key genes involved in nutrient synthesis,secondary metabolites synthesis and protective enzymes functioning.Compared with those before being infested,the expression of Me NR,Me GS,Me Susy,Me P5 CS and Me OAT in leaf tissue of susceptible cassava cultivars BRA900,Bread and SC205 were significantly increased after being infested for 1,4 and 8 days.In addition,the expression levels of Me LAR,Me ANR,Me POD,Me CAT and Me Cu/Zn SOD were significantly decreased.Moreover,the expression levels of Me NR,Me GS and Me Susy in leaf tissues of resistant cassava cultivars C1115 and Myanmar were significantly decreased,while the expression levels of Me P5 CS,Me OAT,Me LAR,Me ANR,Me POD,Me CAT and Me Cu/Zn SOD increased significantly.Correlation analysis showed that the cassava resistance to P.marginatus was negatively correlated with the expression levels of Me NR,Me GS,Me Susy,Me P5 CS and Me OAT(P<0.05,R = 0.762,0.757,0.783,0.796 and 0.826respectively),and positively correlated with the expression levels of Me LAR,Me ANR,Me POD,Me CAT and Me Cu/Zn SOD(P<0.05,R= 0.779,0.825,0.811,0.842 and 0.822respectively). |
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