Effect On Carbohydrate Metabolism In Citrus Sinensis Leaves Infected With’Candidatus Liberibacter Asiaticus’

Citrus Huanglongbing (HLB) caused by Candidatus Liberibacter spp. is one of the most devastating diseases of citrus industry, which can infect nearly all commercial citrus species. It has been most harmful plant quarantine disease both at home and abroad. Trees infected with HLB typically develop symptoms of leaf blotchy mottle, yellow shoots and lopsided fruit with colour inversion, which seriously affect the quality and yield of fruit. Neither effective chemicals nor HLB resistant cultivars have been found, so the most useful control methods are to spread insecticides in a large scale to control vector psyllids, to dig out HLB-infected trees promptly, and to use HLB-free nursery seedlings.Since no toxins, extracellular degrading enzymes or specialized secretion systems were found in the genome of Ca. L. asiaticus, it is more likely Ca. L. asiaticus causes host imbalances, which in turn resulting in disease symptoms. Several researches have shown that many genes involved in carbohydrate metabolism are differentially regulated in the infected citrus leaves. Apparently, further researches on the plant-pathogen interaction mechanisms would help to understand the host pathogenesis and may contribute to developing new solutions for HLB management. On that base, sweet orange was addressed to investigate the effect of HLB infection on carbohydrate metabolism. Compared to healthy controls, the contents of soluble sugar and starch, carbohydrate metabolism-related key enzyme activities were measured. At the same time, the expression levels of pathogenic bacteria content and carbohydrate metabolism-related key enzymes were compared by using Real-time quantitative polymerase chain reaction analysis, aiming to futher understand the effect of HLB infection on carbohydrate metabolism in Citrus sinensis, and provide a theoretical basis for HLB pathogenesis. Main results are shown as follows:(1) The contents of soluble sugar and starch in Citrus sinensis leaves are greatly affected by HLB infection, tending to increase with time extended and slightly decrease later, which indicate that HLB infection has an effect on the synthesis and transportation of photosynthate product, and promote their accumulation. The ratio of soluble sugar to starch drops down, it shows that the abnormal energy metabolism has affected plant normal life activities, and maybe weaken its resistance to the pathogen.(2) Various key enzymes impose different hands on carbohydrate metabolism at different stages of infection. Sucrose phosphate synthase activity rapidly rises to the top, then drops down gradually, and has become lower than the control in late stage; invertase has been consistently high, and soluble starch synthase is closely related to granule-bond starch synthase, coordinating the synthesis of acting on the starch; while amylase activities have dropped to some extend in each stage. The results may have implications for carbohydrate metabolism disorder linked to the HLB infection, and starch accumulation is not only about the retardation of starch decomposition metabolism, it is also related to starch synthesis in the early stage.(3) The transcription level of key enzymes:there was significant relationship between the expression level of sucrose phosphate synthase and its enzyme activity (p<0.05). Among the sucrose decomposition related genes, sucrose synthase is not changed obviously and has lower expression level while cell wall-bound invertase CSCWI is up-regulated and expresses abundantly during the whole process. Compared with CSCWI, The level of vacuolar invertase CUAI1is lower and the extent of variation is changed slightly. Starch decomposition related genes BAM、MEX1and DPE2have a relatively reduced expression at different periods.