Ferulic Acid, But Not All Hydroxycinnamic Acids, Is a Novel T3SS Inducer of Ralstonia solanacearum and Promotes Its Infection Process in Host Plants under Hydroponic Condition

The growth-immunity tradeoff in Brassica oleracea-Xanthomonas campestris pv. campestris pathosystem

Plant responses against pathogens are influenced by growth immunity tradeoff, which ensure the best use of limited resources. We study how the immobilization of carbon resources and the induction of defensive responses (glucosinolates, phenolic compounds, stomatal closure) can influence the biomass of two Brassica oleracea lines, differing in their resistance, after infection with Xanthomonas campestris pv. campestris. Potentially, the growth immunity tradeoff can be influenced by the activation of all these processes. However, on the contrary of which is normally stated, our results suggest that the loss of biomass caused by pathogen infection is mainly due to the differential accumulation of starch and the immobilization of sugars rather than the reallocation of resources to synthesize secondary metabolites. Moreover, resistance may be related to the effectiveness of the tradeoff, since the resistant line immobilizes resources more efficiently than the susceptible one. Both inbred lines show a different phytohormones profile, which support the hypothesis that they are employing different strategies to defend themselves against the pathogen. This study emphasizes the key role of the primary metabolism in the defence strategies of plants against pathogens.

Identification of Genetic and Chemical Factors Affecting Type III Secretion System Expression in Pseudomonas syringae pv. actinidiae Biovar 3 Using a Luciferase Reporter Construct

The type III secretion system (T3SS) is a key factor in the pathogenesis of Pseudomonas syringae pv. actinidiae biovar 3 (Psa3), the causal agent of a global kiwifruit bacterial canker pandemic. To monitor the T3SS expression levels in Psa3, we constructed a luciferase reporter plasmid-expressing HrpA_Psa3-NLuc fusion protein. The expression of HrpA-NLuc was induced in hrp-inducing conditions whereas the level of luciferase activity correlated with the expression of hrp/hrc genes in Psa3 confirmed the reliability of the reporter construct. Based on the readout of the NLuc reporter construct, three small molecule compounds 4-methoxy-cinnamic acid, sulforaphane, and ferulic acid were determined as T3SS inhibitors in Psa3, whereas sodium acetate was determined to be a T3SS inducer. Moreover, the aqueous extract of fruit inhibited the accumulation of HrpA-NLuc in Psa3 in medium and in planta. Additionally, the T3SS inhibitors suppress Psa3 virulence, whereas the T3SS inducer promotes Psa3 virulence on kiwifruit. Thus, our findings may provide clues to why the fruit is not infected by Psa3, and the Psa3 T3SS inhibitors have potential as alternatives to current nonspecific antimicrobials for disease management.

The Role of Hydroxycinnamic Acid Amide Pathway in Plant Immunity

The compounds involved in the hydroxycinnamic acid amide (HCAA) pathway are an important class of metabolites in plants. Extensive studies have reported that a variety of plant hydroxycinnamamides exhibit pivotal roles in plant-pathogen interactions, such as p-coumaroylagmatine and ferulic acid. The aim of this review is to discuss the emerging findings on the functions of hydroxycinnamic acid amides (HCAAs) accumulation associated with plant defenses against plant pathologies, antimicrobial activity of HCAAs, and the mechanism of HCAAs involved in plant immune responses (such as reactive oxygen species (ROS), cell wall response, plant defense hormones, and stomatal immunity). However, these advances have also revealed the complexity of HCAAs participation in plant defense reactions, and many mysteries remain to be revealed. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future exploration of phytochemical defense metabolites.

Bacterial Tomato Pathogen Ralstonia solanacearum Invasion Modulates Rhizosphere Compounds and Facilitates the Cascade Effect of Fungal Pathogen Fusarium solani

Soil-borne pathogen invasions can significantly change the microbial communities of the host rhizosphere. However, whether bacterial Ralstonia solanacearum pathogen invasion influences the abundance of fungal pathogens remains unclear. In this study, we combined high-throughput sequencing, qPCR, liquid chromatography and soil culture experiments to analyze the rhizosphere fungal composition, co-occurrence of fungal communities, copy numbers of functional genes, contents of phenolic acids and their associations in healthy and bacterial wilt-diseased tomato plants. We found that R. solanacearum invasion increased the abundance of the soil-borne pathogen Fusarium solani. The concentrations of three phenolic acids in the rhizosphere soil of bacterial wilt-diseased tomato plants were significantly higher than those in the rhizosphere soil of healthy tomato plants. In addition, the increased concentrations of phenolic acids significantly stimulated F. solani growth in the soil. Furthermore, a simple fungal network with fewer links, nodes and hubs (highly connected nodes) was found in the diseased tomato plant rhizosphere. These results indicate that once the symptom of bacterial wilt disease is observed in tomato, the roots of the wilt-diseased tomato plants need to be removed in a timely manner to prevent the enrichment of other fungal soil-borne pathogens. These findings provide some ecological clues for the mixed co-occurrence of bacterial wilt disease and other fungal soil-borne diseases.

The Lifecycle of the Plant Immune System

Throughout their life span, plants confront an endless barrage of pathogens and pests. To successfully defend against biotic threats, plants have evolved a complex immune system responsible for surveillance, perception, and the activation of defense. Plant immunity requires multiple signaling processes, the outcome of which vary according to the lifestyle of the invading pathogen(s). In short, these processes require the activation of host perception, the regulation of numerous signaling cascades, and transcriptome reprograming, all of which are highly dynamic in terms of temporal and spatial scales. At the same time, the development of a single immune event is subjective to the development of plant immune system, which is co-regulated by numerous processes, including plant ontogenesis and the host microbiome. In total, insight into each of these processes provides a fuller understanding of the mechanisms that govern plant-pathogen interactions. In this review, we will discuss the "lifecycle" of plant immunity: the development of individual events of defense, including both local and distal processes, as well as the development and regulation of the overall immune system by ontogenesis regulatory genes and environmental microbiota. In total, we will integrate the output of recent discoveries and theories, together with several hypothetical models, to present a dynamic portrait of plant immunity.

Synthesis and biological evaluation of 1,3,4-thiadiazole derivatives as type III secretion system inhibitors against Xanthomonas oryzae

Xanthomonas oryzae pv. oryzae (Xoo) infection directly leads to a severe disease known as leaf blight, which is a major cause of yield loss of rice. Use of traditional bactericides has resulted in severe resistance in pathogenic bacteria. A new approach screening compounds that target the virulence factors rather than killing bacterial pathogens is imperative. In gram-negative bacteria, the type III secretion system (T3SS) is a conserved and significant virulence factor considered as a target for drug development. Therefore, we designed and synthesized a new series of 5-phenyl-2-furan carboxylic acid derivatives stitched with 2-mercapto-1,3,4-thiadiazole. Bioassays revealed that the title candidates attenuated the hypersensitive response through suppressing the promoter activity of a harpin gene hpa1 without affecting bacterial growth. Quantitative real time polymerase chain reaction (qRT-PCR) analysis demonstrated reduced the expression of several genes associated with T3SS, when title compounds were applied. Additionally, hrp gene cluster members, including hrpG and hrpX, had reduced mRNA levels. In vivo greenhouse tests showed that candidate compounds could alleviate the effects of Xoo infection in rice (Oryza sativa) and possess better protective activity against rice bacterial leaf blight than bismerthiazol and thiodiazole copper. All tested compounds were safe to rice. This work suggests there are new safe options for Xoo control in rice from these 1,3,4-thiadiazole derivatives.

A putative LysR-type transcriptional regulator PrhO positively regulates the type III secretion system and contributes to the virulence of Ralstonia solanacearum

LysR-type transcriptional regulators (LTTRs) are ubiquitous and abundant amongst bacteria and control a variety of cellular processes. Here, we investigated the effect of Rsc1880 (a putative LTTR, hereafter designated as PrhO) on the pathogenicity of Ralstonia solanacearum. Deletion of prhO substantially reduced the expression of the type III secretion system (T3SS) both in vitro and in planta, and resulted in significantly impaired virulence in tomato and tobacco plants. Complementary prhO completely restored the reduced virulence and T3SS expression to that of the wild-type. Moreover, PrhO-dependent T3SS and virulence were conserved amongst R. solanacearum species. However, deletion of prhO did not alter biofilm formation, swimming mobility and in planta growth. The expression of some type III effectors was significantly reduced in prhO mutants, but the hypersensitive response was not affected in tobacco leaves. Consistent with the key regulatory role of HrpB on T3SS, PrhO positively regulated the T3SS through HrpB. Furthermore, PrhO regulated hrpB expression via two close paralogues, HrpG and PrhG, which are two-component response regulators and positively regulate hrpB expression in a parallel manner. However, deletion of prhO did not alter the expression of phcA, prhJ and prhN, which are also involved in hrpB regulation. In addition, PrhO was expressed in a cell density-dependent manner, but negatively repressed by itself. No regulation was observed for HrpB, PhcA and PrhN on prhO expression. Taken together, we genetically demonstrated that PrhO is a novel virulence regulator of R. solanacearum, which positively regulates T3SS expression through HrpG, PrhG and HrpB and contributes to virulence.