Dickeya zeaeis the causal agent of rice foot rot disease, which has recently become a great threat to rice planting countries and regions. The pathogen produces a family of phytotoxins named zeamines that is critical for bacterial virulence, but little is known about the signaling pathways and regulatory mechanisms that govern zeamine production. In this study, we showed that a conserved transcriptional regulator Fis is involved in the regulation of zeamine production inD. zeaestrain EC1. Deletion mutants were markedly attenuated in the virulence against rice seed germination. Transcriptome and phenotype analyses showed that Fis is a potent global transcriptional regulator modulating various virulence traits, including production of extracellular enzymes and exopolysaccharides, swimming and swarming motility, biofilm formation and cell aggregation. DNA gel retardation analysis showed that Fis directly regulates the transcription of key virulence genes and the genes encoding Vfm quorum sensing system through DNA/protein interaction. Our findings unveil a key regulator associated with the virulence ofD. zeaeEC1, and present useful clues for further elucidation of the regulatory complex and signaling pathways which govern the virulence of this important pathogen.

Type III secretion system (T3SS) is one of the pivotal virulence factors of Pseudomonas aeruginosa responsible for evading phagocytosis, and secreting and translocating effectors into host cells. Previous studies underline the complicated and elaborate regulatory mechanisms of T3SS for the accurate, fast, and malicious pathogenicity of P. aeruginosa . Among these regulatory mechanisms, our previous study indicated that the spermidine from the host was vital to the host-pathogen interaction.

Dickeya zeae is the etiological agent of bacterial foot rot disease, which can cause massive economic losses in banana and rice plantations. Genome sequence analysis showed that D. zeae strain EC1 contains multiple c-di-GMP turnover genes, but their roles and regulatory mechanisms in bacterial physiology and virulence remain vague. By generating consecutive in-frame deletion mutants of the genes encoding c-di-GMP biosynthesis and degradation, respectively, we analyzed the individual and collective impacts of these c-di-GMP metabolic genes on the c-di-GMP global pool, bacterial physiology, and virulence. The significance of our study is in identifying the mechanism of c-di-GMP signaling in strain EC1 more clearly, which expands the c-di-GMP regulating patterns in Gram-negative species. The methods and experimental designs in this research will provide a valuable reference for the exploration of the complex c-di-GMP regulation mechanisms in other bacteria.

Summary: Sexual mating of compatible sporida is essential for Sporisorium scitamineum to form dikaryotic mycelia and then cause infection on sugarcane. Our previous work identified a Pseudomonas sp. ST 4 from a soil sample, which showed a promising biocontrol potential by inhibiting the mating of S. scitamineum sporida and hyphal growth. In this study, we set to isolate the active compounds from Pseudomonas sp. ST 4 through solid fermentation. High‐performance liquid chromatography ( HPLC ) separation coupling with bioassay showed that Pseudomonas sp. ST 4 produced a range of antimicrobial compounds. Two of the major components were purified following acetate extraction, silica gel and HPLC separation. Nuclear magnetic resonance ( NMR ) and liquid chromatography–mass spectrometry ( LC ‐ MS ) analysis identified these active compounds are 4‐hydroxybenzaldehyde and indole‐3‐carbaldehyde respectively. Further analysis showed that the former compound only inhibited the hyphal growth of the fungus at a concentration of 3 mM , while the latter interfered the fungal sexual mating at a concentration of 0.6 mM and affected hyphal growth at a concentration of 2 mM . Treatment of corn plants with 3 mM indole‐3‐carbaldehyde significantly inhibited corn smut infection, with a control rate up to 94%. Further analysis of the structure and activity relationship revealed that indole has a much stronger inhibitory activity against the fungal sexual mating than indole‐3‐carbaldehyde. The results from this study provide new agents for control and prevention of the sugarcane smut disease, and the active compounds could also be used to probe the molecular mechanisms of fungal sexual mating.

A cell death pathway, ferroptosis, occurs in conidial cells and is critical for formation and function of the infection structure, the appressorium, in the rice blast fungus Magnaporthe oryzae . In this study, we identified an orthologous lysophosphatidic acid acyltransferase (Lpaat) acting at upstream of phosphatidylethanolamines (PEs) biosynthesis and which is required for such fungal ferroptosis and pathogenicity. Two PE species, DOPE and SLPE, that depend on Lpaat function for production were sufficient for induction of lipid peroxidation and the consequent ferroptosis, thus positively regulating fungal pathogenicity. On the other hand, both DOPE and SLPE positively regulated autophagy. Loss of the LPAAT gene led to a decrease in the lipidated form of the autophagy protein Atg8, which is probably responsible for the autophagy defect of the lpaat Δ mutant. GFP‐Lpaat was mostly localized on the membrane of lipid droplets (LDs) that were stained by the fluorescent dye monodansylpentane (MDH), suggesting that LDs serve as a source of lipids for membrane PE biosynthesis and probably as a membrane source of autophagosome. Overall, our results reveal novel intracellular membrane‐bound organelle dynamics based on Lpaat‐mediated lipid metabolism, providing a temporal and spatial link of ferroptosis and autophagy.

Zeamines are a family of newly identified phytotoxins and potent antibiotics produced by D. zeae EC1. Unlike most bacterial organisms, which are highly sensitive, D. zeae EC1 is tolerant to zeamines, but the mechanisms involved are unknown. Our study showed, for the first time, that a new RND efflux pump, DesABC, is indispensable for D. zeae EC1 against zeamines. We found that the DesABC efflux pump was zeamine specific and appeared to be conserved only in the Dickeya species, which may explain the high potency of zeamines against a wide range of bacterial pathogens. We also showed that expression of DesABC efflux system genes was induced by zeamines. These findings not only provide an answer to why D. zeae EC1 is much more tolerant to zeamines than other bacterial pathogens but also document a signaling role of zeamines in modulation of gene expression.