Abstract
SummaryThe Gram‐negative soil‐borne bacterium Ralstonia solanacearum first infects roots of host plants and then invades xylem vessels. In xylem vessels, the bacteria grow vigorously and produce exopolysaccharides (EPSs) to cause a wilt symptom on host plants. The EPSs are thus the main virulence factors of R. solanacearum. The strain OE1‐1 of R. solanacearum produces methyl 3‐hydroxymyristate as a quorum‐sensing (QS) signal, and senses this QS signal, activating QS. The QS‐activated LysR‐type transcriptional regulator PhcA induces the production of virulence‐related metabolites including ralfuranone and the major EPS, EPS I. To elucidate the function of EPS I, the transcriptomes of R. solanacearum strains were analysed using RNA sequencing technology. The expression of 97.2% of the positively QS‐regulated genes was down‐regulated in the epsB‐deleted mutant ΔepsB, which lost its EPS I productivity. Furthermore, expression of 98.0% of the negatively QS‐regulated genes was up‐regulated in ΔepsB. The deficiency to produce EPS I led to a significantly suppressed ralfuranone productivity and significantly enhanced swimming motility, which are suppressed by QS, but did not affect the expression levels of phcA and phcB, which encode a methyltransferase required for methyl 3‐hydroxymyristate production. Overall, QS‐dependently produced EPS I may be associated with the feedback loop of QS.
Highlights
SUMMARYThe Gram-negative soil-borne bacterium Ralstonia solanacearum first infects roots of host plants and invades xylem vessels
We identified 4501 protein-coding transcripts from the RNA sequencing (RNA-Seq) reads of strain OE1-1 by mapping to the GMI1000 genome (Table S1, see Supporting information)
The expression levels of phcB and phcA were analysed in R. solanacearum strains ΔepsB, epsB-comp and OE1-1 grown in 1/4 M63 by quantitative reverse transcription-PCR
Summary
The Gram-negative soil-borne bacterium Ralstonia solanacearum first infects roots of host plants and invades xylem vessels. To elucidate the functions of EPS I, transcriptome profiles of R. solanacearum strains, including the epsB-deletion mutant ΔepsB that lost its EPS I productivity (Mori et al, 2018a), were first examined using RNA sequencing (RNA-Seq) technology. We observed significantly lower expression levels of 411 genes (positively PhcA-regulated genes) in ΔphcA than in strain OE1-1 (Fig. 1A and Table S1). The expression levels of 140 genes (negatively PhcA-regulated genes) in ΔphcA were significantly greater than in strain OE1-1 (Fig. 1B and Table S1). We detected 477 genes (EPS I-positively regulated genes) and 198 genes (EPS I-negatively regulated genes) that were expressed at significantly lower and higher levels, respectively, in ΔepsB than in OE1-1 (Fig. 1A and Table S1). The expression levels of phcB and phcA were analysed in R. solanacearum strains ΔepsB, epsB-comp and OE1-1 grown in 1/4 M63 (until OD600 = 0.3) by quantitative reverse transcription-PCR (qRT-PCR) assays. Wild-type strain, phylotype I, race 1, biovar 4 epsB-deletion mutant of OE1–1 A transformant of ΔepsB with pCepsB containing native epsB, Gmr phcB-deletion mutant of OE1–1 phcA-deletion mutant of OE1–1
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