Ralstonia solanacearum is an important bacterial pathogen that can infect a broad range of plants worldwide. A previous study showed that R. solanacearum could respond to exogenous organic acids or amino acids to modulate cell motility. However, it was unclear whether R. solanacearum uses these compounds to control infection. In this study, we found that R. solanacearum GMI1000 uses host plant metabolites to enhance the biosynthesis of virulence factors. We demonstrated that l‐glutamic acid from host plants is the key active component associated with increased extracellular polysaccharide production, cellulase activity, swimming motility, and biofilm formation in R. solanacearum GMI1000. In addition, l‐glutamic acid also promoted colonization of R. solanacearum cells in the roots and stems of tomato plants and accelerated disease incidence. Furthermore, genetic screening and biochemical analysis suggested that RS01577, a hybrid sensor histidine kinase/response regulator, is involved in l‐glutamic acid signalling in R. solanacearum. Mutations in RS01577 and exogenous addition of l‐glutamic acid to the GMI1000 wild‐type strain had overlapping effects on both the transcriptome and biological functions of R. solanacearum, including on motility, biofilm formation, and virulence. Thus, our results have established a new interaction mechanism between R. solanacearum and host plants that highlights the complexity of the virulence regulation mechanism and may provide new insight into disease control.

Background: Ralstonia solanacearum is one of the most notorious soil-borne phytopathogens. It causes a severe wilt disease with deadly effects on many economically important crops. The microbita of disease-suppressive soils are thought that they can contribute to the disease resistance of crop plants, thus, evaluation of the microbial community and their interaction characteristics between suppressive soil (SS) and conducive soil (CS) will help to understand resistance mechanism. To do this, the bacterial community structure, correlation analysis with soil chemical properties, interaction network of SS (nearly no disease in three years), and CS (suffered heavy bacterial wilt disease) were analyzed. Results: A higher bacterial community diversity index was found in SS, the relative abundance of Nocardioides, Gaiella and norank_f_Anaerolineaceae were significantly more than that of the CS. Moreover, the relative abundance of main genera Bacillus, norank_o_Gaiellales, Roseiflexus, and norank_o_Gemmatimonadaceae were significantly more than that of the CS. Redundancy analysis at the genus level indicated that the available phosphate played a key role in the bacterial community distribution, and its role was negatively correlated with soil pH, organic matter content, alkali-hydrolyzable nitrogen, and available potassium contents. Interaction network analysis further demonstrated that greater diversity at the genus level existed in the SS network and formed a stable network. Additionally, the species of Mycobacterium, Cyanobacteria, and Rhodobiaceae are the key components that sustain the network stability. Seven clusters of orthologous groups exhibited significant differences between SS and CS. Moreover, 55 bacterial strains with distinct antagonistic activities to R. solancearum were isolated and identified from the healthy tomato plant rhizosphere soil of the CS. Conclusions: Our findings indicate that the bacterial diversity and interaction network differed between the CS and SS samples, providing a good foundation in the study of bacterial wilt.

Ralstonia solanacearum is a causative agent of bacterial wilt in many important crops throughout the world. How to control bacterial wilt caused by R. solanacearum is a major problem in agriculture. In this study, we aim to isolate the biocontrol agents that have high efficacy in the control of bacterial wilt. Three new bacterial strains with high antimicrobial activity against R. solanacearum GMI1000 were isolated and identified. Our results demonstrated that these bacteria could remarkably inhibit the disease index of host plant infected by R. solanacearum. It was indicated that strain GZ-34 (CCTCC No. M 2016353) showed an excellent protective effect to tomato under greenhouse conditions. Strain GZ-34 was characterized as Escherichia coli based on morphology, biochemistry, and 16S rRNA analysis. We identified that the main antimicrobial compounds produced by E. coli GZ-34 were cyclo(l-Pro-d-Ile) and cyclo(l-Pro-l-Phe) using electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) analysis. The two active compounds also interfered with the expression levels of some pathogenicity-contributors of R. solanacearum. Furthermore, cyclo(l-Pro-l-Phe) effectively inhibited spore formation of Magnaporthe grisea, which is a vital pathogenesis process of the fungal pathogen, suggesting cyclic dipeptides from E. coli are promising potential antimicrobial agents with broad-spectrum activity to kill pathogens or interfere with their pathogenesis.

As one of the most notorious and successful phytopathogenic bacteria, Ralstonia solanacearum controls the transition between long-term survival and pathogenic modes through an intricate regulatory network, the understanding of which remains incomplete despite years of effort. In this study, we identified PhcX, a previously uncharacterized response regulator in R. solanacearum , and uncovered its essential functions in modulating virulence and metabolism. The phcX deletion mutant exhibited substantial phenotypic alterations, including slower initial growth, altered response to host extract, reduced motilities, polygalacturonase activity, and exopolysaccharide production, elevated biofilm formation, delayed hypersensitive response, and impaired virulence. Moreover, ~16% of all genes were differentially expressed in the mutant, among which the genes associated with virulence, nitrogen metabolism, and regulation were overrepresented (e.g., most T3SS/T3Es genes). Many of these traits and genes were regulated by PhcX and the global virulence regulator PhcA, but 81.4% of the genes were regulated in opposite directions. The functions of PhcX were largely conserved in R. solanacearum EP1 and GMI1000 strains. Apparent orthologs of PhcX are widely distributed in Proteobacteria, including the LqsR quorum sensing (QS) response regulator in Legionella pneumophilia . Notably, our data suggest that phcX was originally part of the Lqs QS system but was decoupled from Lqs in Ralstonia / Cupriavidus , physically linked to the phc QS genes, and connected with the virulence regulatory network in Ralstonia during its evolution. The findings of this study contribute to a better understanding of the virulence and metabolism regulation mechanisms in R. solanacearum and shed light on the evolution of its complex regulatory network. IMPORTANCE: The bacterial wilt caused by the soil-borne phytopathogen Ralstonia solanacearum is one of the most destructive crop diseases. To achieve a successful infection, R. solanacearum has evolved an intricate regulatory network to orchestrate the expression of an arsenal of virulence factors and fine-tune the allocation of energy. However, despite the wealth of knowledge gained in the past decades, many players and connections are still missing from the network. The importance of our study lies in the identification of PhcX, a novel conserved global regulator with critical roles in modulating the virulence and metabolism of R. solanacearum . PhcX affects many well-characterized regulators and exhibits contrasting modes of regulation from the central regulator PhcA on a variety of virulence-associated traits and genes. Our findings add a valuable piece to the puzzle of how the pathogen regulates its proliferation and infection, which is critical for understanding its pathogenesis and developing disease control strategies.

Protein elicitors can induce plant systemic resistance to pathogens. In an earlier study, we cloned an EsxA gene from the plant growth-promoting rhizobacterium Paenibacillus terrae NK3-4 and expressed it in Pichia pastoris. In addition to being important for the pathogenicity of animal pathogens, EsxA can also induce an immune response in animals. While, we found the exogenously expressed EsxA has the activity of elicitor, which can trigger hypersensitive response and reactive oxygen species burst in leaves as well as enhanced rice plant growth. The effects of EsxA on seedling blight (Fusarium oxysporum) resistance and gene transcription, including pathogenesis-related (PR) genes in rice were evaluated. The germination rate was 95.0% for seeds treated with EsxA and then inoculated with F. oxysporum, which was 2.8-times higher than that of F. oxysporum-infected control seeds that were not treated with EsxA (Con). The buds and roots of EsxA-treated seedlings were 2.4- and 15.9-times longer than those of Con seedlings. The plants and roots of seedlings dipped in an EsxA solution and then inoculated with F. oxysporum were longer than those of the Con seedlings. Theplant length, number of total roots, and number of white roots were respectively 23.2%, 1.74-times, and 7.42-times greater for the seedlings sprayed with EsxA and then inoculated with F. oxysporum than for the Con seedlings. The EsxA induction efficiency (spray treatment) on seedling blight resistance was 60.9%. The transcriptome analysis revealed 1137 and 239 rice genes with EsxA-induced up-regulated and down-regulated transcription levels, respectively. At 48 h after the EsxA treatment, the transcription of 611 and 160 genes was up-regulated and down-regulated, respectively, compared with the transcription levels for the untreated control at the same time-point. Many disease resistance-related PR genes had up-regulated transcription levels. The qPCR data were consistent with the transcriptome sequencing results. EsxA triggered rice ISR to seedling blight and gene differential transcription, including the up-regulated transcription of rice PR genes. These findings may be relevant for the use of EsxA as a protein elicitor to control plant diseases.