The GacA-GacS Type Two-Component System Modulates the Pathogenicity of Dickeya oryzae EC1 Mainly by Regulating the Production of Zeamines

Characterization of the Arn lipopolysaccharide modification system essential for zeamine resistance unveils its new roles in Dickeya oryzae physiology and virulence

The zeamines produced by Dickeya oryzae are potent polyamine antibiotics and phytotoxins that are essential for bacterial virulence. We recently showed that the RND efflux pump DesABC in D. oryzae confers partial resistance to zeamines. To fully elucidate the bacterial self-protection mechanisms, in this study we used transposon mutagenesis to identify the genes encoding proteins involved in zeamine resistance in D. oryzae EC1. This led to the identification of a seven-gene operon, arnEC1 , that encodes enzyme homologues associated with lipopolysaccharide modification. Deletion of the arnEC1 genes in strain EC1 compromised its zeamine resistance 8- to 16-fold. Further deletion of the des gene in the arnEC1 mutant background reduced zeamine resistance to a level similar to that of the zeamine-sensitive Escherichia coli DH5α. Intriguingly, the arnEC1 mutants showed varied bacterial virulence on rice, potato, and Chinese cabbage. Further analyses demonstrated that ArnBCATEC1 are involved in maintenance of the bacterial nonmucoid morphotype by repressing the expression of capsular polysaccharide genes and that ArnBEC1 is a bacterial virulence determinant, influencing transcriptional expression of over 650 genes and playing a key role in modulating bacterial motility and virulence. Taken together, these findings decipher a novel zeamine resistance mechanism in D. oryzae and document new roles of the Arn enzymes in modulation of bacterial physiology and virulence.

Two-component system ArcBA modulates cell motility and biofilm formation in Dickeya oryzae

Phytopathogen Dickeya oryzae is a causal agent of rice foot rot disease and the pathogen has an array of virulence factors, such as phytotoxin zeamines, plant cell wall degrading enzymes, cell motility, and biofilms, collectively contributing to the bacterial pathogenesis. In this study, through deletion analysis of predicted regulatory genes in D. oryzae EC1, we identified a two-component system associated with the regulation of bacterial virulence. The two-component system contains a histidine kinase ArcB and a response regulator ArcA, and deletion of their coding genes resulted in changed phenotypes in cell motility, biofilm formation, and bacterial virulence. Electrophoretic mobility shift assay revealed that ArcA bound to the promoters of the bcs operon and bssS, which respectively encode enzymes for the synthesis of celluloses and a biofilm formation regulatory protein. ArcA could also bind to the promoters of three virulence associated transcriptional regulatory genes, i.e., fis, slyA and ohrR. Surprisingly, although these three regulators were shown to modulate the production of cell wall degrading enzymes and zeamines, deletion of arcB and arcA did not seem to affect these phenotypes. Taken together, the findings from this study unveiled a new two-component system associated with the bacterial pathogenesis, which contributes to the virulence of D. oryzae mainly through its action on bacterial motility and biofilm formation.

Hfq Is a Critical Modulator of Pathogenicity of Dickeya oryzae in Rice Seeds and Potato Tubers

The frequent outbreaks of soft-rot diseases caused by Dickeya oryzae have emerged as severe problems in plant production in recent years and urgently require the elucidation of the virulence mechanisms of D. oryzae. Here, we report that Hfq, a conserved RNA chaperone protein in bacteria, is involved in modulating a series of virulence-related traits and bacterial virulence in D. oryzae EC1. The findings show that the null mutation of the hfqEC1 gene totally abolished the production of zeamine phytotoxins and protease, significantly attenuated the production of two other types of cell wall degrading enzymes, i.e., pectate lyase and cellulase, as well as attenuating swarming motility, biofilm formation, the development of hypersensitive response to Nicotiana benthamiana, and bacterial infections in rice seeds and potato tubers. QRT-PCR analysis and promoter reporter assay further indicated that HfqEC1 regulates zeamine production via modulating the expression of the key zeamine biosynthesis (zms) cluster genes. Taken together, these findings highlight that the Hfq of D. oryzae is one of the key regulators in modulating the production of virulence determinants and bacterial virulence in rice seeds and potato tubers.