Abstract
SummaryThe bacterial stringent response is a response to nutrition deprivation and other stress conditions. In Gram‐negative bacteria, this process is mediated by the small signal molecules guanosine pentaphosphate pppGpp and guanosine tetraphosphate ppGpp (collectively referred to as (p)ppGpp), and the RNA polymerase‐binding transcription factor DksA. The (p)ppGpp synthetase RelA and the bifunctional (p)ppGpp synthase/hydrolase SpoT are responsible for cellular (p)ppGpp levels. Here, we investigated the roles of DksA and (p)ppGpp in the virulence traits of Xanthomonas citri subsp. citri (Xcc), the causal agent of citrus canker. ΔdksA and (p)ppGpp‐deficient ΔspoTΔrelA strains caused reduced virulence and compromised growth in host plants, indicating that DksA and (p)ppGpp are required for full virulence of Xcc. To characterize the effect of stringent response regulators on gene expression, RNA‐seq was conducted using ΔdksA and ΔspoTΔrelA mutant strains grown in hrp‐inducing XVM2 medium. Transcriptome analyses showed that DksA and (p)ppGpp repressed the expression of genes encoding tRNAs, ribosome proteins, iron acquisition and flagellum assembly, and enhanced the expression of genes for histidine metabolism, type 3 secretion system (T3SS), type 2 secretion system (T2SS) and TonB‐dependent transporters. Phenotypically, the ΔdksA and ΔspoTΔrelA strains displayed altered motility, enhanced siderophore production and were unable to cause the hypersensitive response on non‐host plants. In conclusion, stringent response regulators DksA and (p)ppGpp play an important role in virulence, nutrition uptake and host adaptation of Xcc.
Highlights
Xanthomonas spp. cause diseases in approximately 400 species of plant hosts, including many economically important crops (RyanGram-negative bacterial pathogens, including Xanthomonas citri subsp. citri (Xcc), deliver numerous effectors into the host cell via the type 3 secretion system (T3SS) to manipulate host signalling, suppress immune responses and/or induce plant susceptibility genes (Jacques et al, 2016; Tsuge et al, 2014; White et al, 2009)
The T3SS is encoded by hrp genes and the regulation of hrp genes in Xanthomonas mainly depends on two key transcriptional regulators, HrpG and HrpX (Wengelnik and Bonas, 1996; Wengelnik et al, )
We examined stringent response regulators by generating ∆dksA and (p)ppGpp-deficient ∆strain ΔspoTΔrelA (spoT)∆relA strains of Xcc
Summary
The bacterial stringent response is a response to nutrition deprivation and other stress conditions. We investigated the roles of DksA and (p)ppGpp in the virulence traits of Xanthomonas citri subsp. ∆dksA and (p)ppGpp-deficient ∆spoTΔrelA strains caused reduced virulence and compromised growth in host plants, indicating that DksA and (p)ppGpp are required for full virulence of Xcc. To characterize the effect of stringent response regulators on gene expression, RNA-seq was conducted using ∆dksA and ∆spoT∆relA mutant strains grown in hrp-inducing XVM2 medium. The ∆dksA and ∆spoT∆relA strains displayed altered motility, enhanced siderophore production and were unable to cause the hypersensitive response on non-host plants. Stringent response regulators DksA and (p)ppGpp play an important role in virulence, nutrition uptake and host adaptation of Xcc
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