Mutual Effect Of Psyllid Behavior And Citrus Leaf Nutrition

The Citrus,a tropical and subtropical evergreen fruit tree,is widely popular for its special flavor,delicate taste,and high nutritional value.Huanglongbing(HLB)is one of the most destructive diseases of citrus.The citrus psyllid(Diaphorina citri Kuwayama)is the main vector of pathogenic bacteria responsible for HLB.The psyllid recognizes and locates host plants by sensing olfactory and visual cues,and then ingests phloem sap by their stylet.Mineral nutrients enhancement has been widely used to maintain the productivities of HLB-infected citrus for the high similarities between HLB characteristic symptoms and nutrients deficiency symptoms including Zinc(Zn),Manganese(Mn),and Boron(B).It has been proved to effectively relieve HLB and to indirectly affect psyllid feeding behaviors by mediating citrus leaf development.However,the physiological mechanisms of how plant nutrients participate in the interactions between citrus and citrus psyllid,occurring before citrus are infected by HLB,are still unclear.Hence,this study will discuss the mechanisms of interactions between citrus mineral nutrients and psyllids by field experiments and pot experiments.The main results are as following:(1)At the field,HLB infection decreased the transfer coefficients of Mg and K from leaf to phloem tissues by up-regulating the transporter genes Cs3g03790.1 and downregulating PtrMGT5,respectively,finally resulted in the increased contents of leaf Mg and K;HLB infection also decreased leaf Zn,xylem Ca,and phloem Ca and Zn contents.With the development of symptoms from asymptomatic to serious,leaf B,Mn,Mg,Zn contents gradually decreased,while leaf K increased;The transport of P from leaf to phloem tissues,as evaluated by the transfer coefficient,was regulated by the transporter Csi PT2,resulting in irregular levels of leaf P.It suggested that the psyllid could affect leaf mineral nutrients by disseminating HLB.(2)At the orchard,the leaf with psyllid feeding showed two prominent chemical characteristics: lower Boron(B)level and abnormal cell wall structure.Compared with the leaf without psyllids feeding,the cell wall components including cellulose,hemicellulose,and alkali-soluble pectin in the leaf with psyllids feeding significantly increased,while chelator-soluble pectin decreased;By using Fourier transform infrared spectroscopy(FTIR)and X-ray diffractometer,decreased cellulose crystallinity and ionically bound pectin were observed in leaves with psyllids feeding,indicating degraded cell wall properties.In addition,the B content,highly correlated with different cell wall components and their B contents,was significantly decreased in the leaf with psyllids feeding.Taken together,this evidence showed that there was a strong relationship between psyllid feeding behaviors and the abnormal cell wall structure causing by decreased B contents.(3)In the pot experiment,the citrus with B deficiency was more attractive to psyllids because of the altered shikimic acid metabolic pathway in leaves,and B deficiency facilitated psyllids feeding process for the weakened mechanical property of the cell wall.B deficiency significantly affected the shikimic acid pathway with an increased synthesis of shikimic acid,L-phenylalanine,3-phenylpyruvic acid,and salicylic acid,resulted in increased contents of methyl salicylate,and attracted more than 70%psyllids.B deficiency significantly increased cellulose and hemicellulose contents of leaf cell wall but decreased cellulose crystallinity resulted in the significant decreases of diameter and the increased breakages in cellulose microfibril structure.B deficiency evidently decreased the B concentration of protoplast and cell wall fractions,thereby,weakening the structural,stability of cell wall,with regard to reduced pectin and cellulose content.The Young’s modulus of B deficiency was 573.09 MPa,significantly lower than the control,933.498 MPa,this indicating the reduced leaf cell wall mechanical strength.The citrus with B deficiency predisposed the psyllid adults to prefer feeding through increased feeding efficiency,with regard to increased phloem ingestion and decreased piercing process.(4)In the pot experiment,foliar spraying of nutrients down-regulated the amino acids secondary metabolism including phenylalanine,serine,and glutamic acid,by affecting leaf nutrition: on the one hand,the down-regulated methyl salicylate decreased the attraction to psyllid: on the other hand,the increased thickness of fibrous ring tissue significantly decreased psyllid feeding efficiency.The psyllid feeding behavior upregulated the amino acid metabolisms such as phenylalanine,serine,and glutamate,as well as increased leaf Mo,Zn,Ca contents,and decreased K contents to maintain the high level of amino acid metabolism.The synthesis of methyl salicylate in the phenylalanine metabolic pathway was up-regulated,increasing the attraction of plants to Psylla.The psyllid feeding behavior significantly affected the immunofluorescence intensity of monoclonal antibody JIM5 and atomic percentages of elements in different tissues,affecting tissues structures,finally resulted in increased leaf thickness,leaf weight,and leaf dry mass per area(LMA).The abnormal leaf structure significantly decreased piercing process duration by 60.53%,and increased phloem ingestion duration by165.94%,suggesting that the psyllid had a higher feeding efficiency on the leaf with psyllid feeding.Foliar spraying of nutrients down-regulated the amino acids secondary metabolism including phenylalanine,serine,and glutamic acid,by affecting leaf nutrition,relieving the effects of psyllid feeding behaviors on leaf secondary metabolism.In addition,the effects of psyllid feeding behaviors on leaf tissue structure were significantly relieved by nutrients enhancement,and the leaf structure of psyllids feeding+nutrition treatment was similar to that of the control.There was a remarkable interaction between nutrition and pesticide(psyllid number: F=11.57,P <0.01;population reduction rate:F=17.205,P<0.01).Compared with the pesticide treatment,pesticide+nutrition treatment showed a higher population reduction rate,which was increased by 15.28%.However,the treatment with only nutrition enhances an increased 12.71% psyllid number than the control treatment.In a word,there was a significant interaction between mineral nutrients and psyllid feeding behavior.On the one hand,HLB,disseminated by psyllid feeding,increased leaf K contents,but decreased leaf B,Zn,Mg contents;on the other hand,lower B contents and fragile cell wall structure were the two important characteristics of the leaf with psyllids feeding.It is revealed that B deficiency facilitated the feeding behavior of psyllid adults through the elevated release of Me SA,coupled with weakened cell wall mechanical properties.