Analysis of virulence-associated genes in plant pathogenic bacteria Erwinia chrysanthemi and Xylella fastidiosa.

摘要

Apoplast-inhabiting bacteria such as Erwinia chrysanthemi and xylem-limited bacteria such as X. fastidiosa use different strategies to survive inside the apoplast and the xylem of plant tissue. hrp-associated genes are critical for pathogenicity of E. chrysanthemi due to the type III secretion system (T3SS) that is responsible for extracellular secretion of effectors directly into plant cells. A GFP-based Escherichia coli pPROBE-AT/pCPP3042 system and flow cytometry was used to genome-wide screen potential hrp-regulated genes of E. chrysanthemi. Distinct gene expression patterns of hrpA (a T3SS apparatus component), hrpK (a putative T3SS helper protein), dspE (a T3SS effector), yijC (a transcriptional repressor), and yecF (unknown function) were initially identified as upregulated by hrpL and further confirmed in a hrpL mutant of E. chrysanthemi . Further investigation demonstrated that all of mutated hrpA, hrpK, dspE, yijC, and yecF strains showed significantly reduced virulence in African violet comparing with wild-type E. chrysanthemi.; The insect transmitted and non-flagellated bacterium X. fastidiosa causes Pierce's disease (PD) of grapes, which is caused by the bacterial biofilm-like colonies clogging the host's vascular system. Previous studies showed that a sigma factor algU plays a role in biofilm-related phenomenon in P. aeruginosa, and an active response regulator gacA is responsible for biofilm formation in P. syringae . The potential biofilm-related algU and gacA homologues were mutated in X. fastidiosa. The algU:: nptII and gacADeltaGm mutants were shown to have reduced abilities to attach, aggregate and form biofilms, and have a reduced pathogenicity on grapevines. DNA microarray analyses of X. fastidiosa wild type, algU:: nptII and gacADeltaGm mutants showed that many extracellular surface protein genes such as mopB, xadA, hsf, and a fimbrial protein were regulated by AlgU and GacA. These results suggested that attachment and biofilm formation play crucial roles for X. fastidiosa in attaching and multiplying inside the foregut of the insect and interior xylem.; The different virulence factors identified between E. chrysanthemi and X. fastidiosa determine the pathogenicity mechanisms in differential ecological niches. Understanding these distinct molecular mechanisms of pathogenicity of these plant pathogens helps to identify targets for the development of specific and effective management strategies for controlling the diseases they cause.