Suppressing Canker Symptom Formation And Basic Research For Application By Xanthomonas Effector AvrXa7

The AvrXa7 of Xanthomonas oryzae pv.oryzae is one Tale effector protein,which is translocated into host plant cells through type ?secretion system.The PthA4 of Xanthomonas citri subsp.citri is the major eirulence effector protein determining canker disease development on plants,and transient expression of pthA4 gene can induce the formation of canker-like symptom.In this paper,AvrXa7 of X.oryzae pv.oryzae was transformed into X.citri subsp.citri,resulting in a yellowing symptom without any canker in the inoculation sites.The results of transcriptomics showed that the expression level of 138 citrus genes was down-regulated and 177 genes were up-regulated by comparison with wild type-infected leaf tissues.Among the up-regulated genes,5 genes were involved in ethylene synthesis or signal transduction.Two genes involved in auxin signal transduction was up-regulated,as well as the genes participated in photosynthesis and linolenic acid synthesis.Based on the bioinformatics analysis,the promoter sequences of a total of 315 differentially expressed genes were analyzed.According to the molecular code of TALE-DNA binding,8 possible target genes of PthA4 in citrus plant were deduced.The expression levels of these 8 genes in citrus were verified by qRT-PCR.All find the promoter of four target genes(Cs9g12620.1?LOB1?Cs4g16550.1 and Cs4g17050.1)up-regulated expression in PthA4,but down regulated expression in AvrXa7.The binding activity of PthA4 to the promoter of four target genes(Cs9g12620.1,LOB1,Cs4g16550.1,Cs4g17050.1)was confirmed by EMSA experiments,which was used to test the inhibitory effect of AvrXa7 on the PthA4 binding to promoters.To verify the roles of four genes in canker development,RNA interfering constructs were made to silence those genes in citrus.In order to explore the application of Xanthomonas AvrXa7 in citrus canker prevention,AvrXa7 was also expressed in an isolated Pseudomonas putida A1,which showed inhibitory effect on plant pathogenic bacteria.The expression of AvrXa7 in P.putida A1 showed suppression effect from 57.8%improved to 95.4%on canker lesion formation.This increased 37.6%of the suppression effect by comparison with the usage of P.putida A1 alone to control canker disease.