Rice-Infecting Pseudomonas Genomes Are Highly Accessorized and Harbor Multiple Putative Virulence Mechanisms to Cause Sheath Brown Rot

The structure and diversity of microbial communities in Paederus fuscipes (Coleoptera: Staphylinidae): from ecological paradigm to pathobiome

BACKGROUND

Paederus fuscipes is medically the most famous rove beetle, which causes dermatitis or conjunctivitis in humans, as well as gastrointestinal toxicosis in livestock, via releasing toxic hemolymph containing pederin. Pedrin biosynthesis genes have been identified in uncultured Pseudomonas-like endosymbionts that are speculated to be acquired through a horizontal transfer. However, the composition of the P. fuscipes microbial community, especially of the gut and genital microbiome, remains unclear. This study was aimed to characterize the structure and diversity of P. fuscipes-associated bacterial communities in terms of gender, organ, and location using the Illumina HiSeq platform in the southern littorals of Caspian Sea.

RESULTS

The OTUs identified from P. fuscipes specimens were collapsed into 40 phyla, 112 classes, 249 orders, 365 families, 576 genera, and 106 species. The most abundant families were Pseudomonadaceae, Spiroplasmataceae, Weeksellaceae, Enterococcaceae, and Rhizobiaceae, respectively. Thirty top genera made up > 94% of the P. fuscipes microbiome, with predominating Pseudomonas, followed by the Spiroplasma, Apibacter, Enterococcus, Dysgonomonas, Sebaldella, Ruminococcus, and Wolbachia. Interesting dissimilarities were also discovered within and between the beetle microbiomes in terms of genders and organs. Analyses showed that Spiroplasma / Apibacter as well as Pseudomonas / Pseudomonas were the most abundant in the genitals / intestines of male and female beetles, respectively. Bacterial richness did not display any significant difference in the three provinces but was higher in male beetles than in females and more in the genitals than intestines.

CONCLUSIONS

The present study identified Pseudomonas-like endobacterium as a common symbiont of P. fuscipes beetles; this bacterium begins its journey from gut and genitalia of females to reach the male rove beetles. Additionally, male and female rove beetles were characterized by distinctive microbiota in different organs, likely reflecting different functions and/or adaptation processes. Evidence of the extension of P. fuscipes microbiome from the environmental paradigm to the pathobiome was also presented herein. A comprehensive survey of P. fuscipes microbiome components may eventually lead to ecological insights into the production and utilization of defensive compound of pederin and also the management of linear dermatitis with the use of available antibiotics against bacterial pathogens released by the beetles. Video Abstract.

Versatile role of Pseudomonas fuscovaginae cyclic lipopeptides in plant and microbial interactions

Pseudomonas fuscovaginae is the most prominent bacterial sheath rot pathogen, causing sheath brown rot disease in rice. This disease occurs worldwide and it is characterized by typical necrotic lesions on the sheath, as well as a reduction in the number of emitted panicles and filled grains. P. fuscovaginae has been shown to produce syringotoxin and fuscopeptin cyclic lipopeptides (CLPs), which have been linked to pathogenicity. In this study, we investigated the role of P. fuscovaginae UPB0736 CLPs in plant pathogenicity, antifungal activity and swarming motility. To do so, we sequenced the strain to obtain a single-contig genome and we constructed deletion mutants in the biosynthetic gene clusters responsible for the synthesis of CLPs. We show that UPB0736 produces a third CLP of 13 amino acids, now named asplenin, and we link this CLP with the swarming activity of the strain. We could then show that syringotoxin is particularly active against Rhizoctonia solani in vitro. By testing the mutants in planta we investigated the role of both fuscopeptin and syringotoxin in causing sheath rot lesions. We proved that the presence of these two CLPs considerably affected the number of emitted panicles, although their number was still significantly affected in the mutants deficient in both fuscopeptin and syringotoxin. These results reveal the importance of CLPs in P. fuscovaginae pathogenicity, but also suggest that other pathogenicity factors may be involved.

Pseudomonas Lipopeptide-Mediated Biocontrol: Chemotaxonomy and Biological Activity

Pseudomonas lipopeptides (Ps-LPs) play crucial roles in bacterial physiology, host-microbe interactions and plant disease control. Beneficial LP producers have mainly been isolated from the rhizosphere, phyllosphere and from bulk soils. Despite their wide geographic distribution and host range, emerging evidence suggests that LP-producing pseudomonads and their corresponding molecules display tight specificity and follow a phylogenetic distribution. About a decade ago, biocontrol LPs were mainly reported from the P. fluorescens group, but this has drastically advanced due to increased LP diversity research. On the one hand, the presence of a close-knit relationship between Pseudomonas taxonomy and the molecule produced may provide a startup toolbox for the delineation of unknown LPs into existing (or novel) LP groups. Furthermore, a taxonomy-molecule match may facilitate decisions regarding antimicrobial activity profiling and subsequent agricultural relevance of such LPs. In this review, we highlight and discuss the production of beneficial Ps-LPs by strains situated within unique taxonomic groups and the lineage-specificity and coevolution of this relationship. We also chronicle the antimicrobial activity demonstrated by these biomolecules in limited plant systems compared with multiple in vitro assays. Our review further stresses the need to systematically elucidate the roles of diverse Ps-LP groups in direct plant-pathogen interactions and in the enhancement of plant innate immunity.

Auxin, Abscisic Acid and Jasmonate Are the Central Players in Rice Sheath Rot Caused by Sarocladium oryzae and Pseudomonas fuscovaginae

Sheath rot is an emerging rice disease that causes severe yield losses worldwide. The main causal agents are the toxin producers Sarocladium oryzae and Pseudomonas fuscovaginae. The fungus S. oryzae produces helvolic acid and cerulenin and the bacterium P. fuscovaginae produces cyclic lipopeptides. Helvolic acid and the lipopeptide, fuscopeptin, inhibit membrane-bound H⁺-ATPase pumps in the rice plant. To manage rice sheath rot, a better understanding of the host response and virulence strategies of the pathogens is required. This study investigated the interaction of the sheath rot pathogens with their host and the role of their toxins herein. Japonica rice was inoculated with high- and low-helvolic acid-producing S. oryzae isolates or with P. fuscovaginae wild type and fuscopeptin mutant strains. During infection, cerulenin, helvolic acid and the phytohormones abscisic acid, jasmonate, auxin and salicylic acid were quantified in the sheath. In addition, disease severity and grain yield parameters were assessed. Rice plants responded to high-toxin-producing S. oryzae and P. fuscovaginae strains with an increase in abscisic acid, jasmonate and auxin levels. We conclude that, for both pathogens, toxins play a core role during sheath rot infection. S. oryzae and P. fuscovaginae interact with their host in a similar way. This may explain why both sheath rot pathogens cause very similar symptoms despite their different nature.

Syringopeptin Contributes to the Virulence of Pseudomonas fuscovaginae, Based on sypA Biosynthesis Mutant Analysis

Pseudomonas fuscovaginae, first reported from Japan in 1976, is now present in many agroecological regions around the world; it causes sheath brown rot of rice and is reported as a pathogen of a broad range of hosts. The pathogen can infect rice plants at all stages of growth and is known to cause significant losses due to grain discoloration, poor spike emergence and panicle sterility. Limited information is available on the virulence and mechanisms of pathogenicity for P. fuscovaginae. To address this, an analysis of genomes was conducted, which identified the presence of a gene showing homology to one of the genes contributing to syringopeptin synthetase (sypA) of P. syringae pv. syringae. To study the potential role of this gene in the virulence and pathogenicity of P. fuscovaginae, a site-specific mutation was created. Following inoculation of seeds and plantlets of rice and wheat with P. fuscovaginae wild types and their respective mutants, we demonstrated that the mutation significantly reduced virulence. This was evident on rice and wheat inoculated with mutants causing a significantly higher number of roots, length of roots and seedling height compared with their respective wild types. Characteristic disease symptoms of necrotic lesions were significantly less in rice seedlings infected with bacterial suspensions of mutants indicating a reduction in virulence. Chromatography analysis of bacterial exudates showed suppression of synthesis of metabolites analogous to syringopeptin in the mutants. These data demonstrate that the protein encoded by this sypA homolog gene is a major virulence determinant of P. fuscovaginae.

Convergent gain and loss of genomic islands drive lifestyle changes in plant-associated Pseudomonas

Host-associated bacteria can have both beneficial and detrimental effects on host health. While some of the molecular mechanisms that determine these outcomes are known, little is known about the evolutionary histories of pathogenic or mutualistic lifestyles. Using the model plant Arabidopsis, we found that closely related strains within the Pseudomonas fluorescens species complex promote plant growth and occasionally cause disease. To elucidate the genetic basis of the transition between commensalism and pathogenesis, we developed a computational pipeline and identified genomic islands that correlate with outcomes for plant health. One island containing genes for lipopeptide biosynthesis and quorum-sensing is required for pathogenesis. Conservation of the quorum-sensing machinery in this island allows pathogenic strains to eavesdrop on quorum signals in the environment and coordinate pathogenic behavior. We found that genomic loci associated with both pathogenic and commensal lifestyles were convergently gained and lost in multiple lineages through homologous recombination, possibly constituting an early step in the differentiation of pathogenic and commensal lifestyles. Collectively this work provides novel insights into the evolution of commensal and pathogenic lifestyles within a single clade of host-associated bacteria.

Rice Sheath Rot: An Emerging Ubiquitous Destructive Disease Complex

Around one century ago, a rice disease characterized mainly by rotting of sheaths was reported in Taiwan. The causal agent was identified as Acrocylindrium oryzae, later known as Sarocladium oryzae. Since then it has become clear that various other organisms can cause similar disease symptoms, including Fusarium sp. and fluorescent pseudomonads. These organisms have in common that they produce a range of phytotoxins that induce necrosis in plants. The same agents also cause grain discoloration, chaffiness, and sterility and are all seed-transmitted. Rice sheath rot disease symptoms are found in all rice-growing areas of the world. The disease is now getting momentum and is considered as an important emerging rice production threat. The disease can lead to variable yield losses, which can be as high as 85%. This review aims at improving our understanding of the disease etiology of rice sheath rot and mainly deals with the three most reported rice sheath rot pathogens: S. oryzae, the Fusarium fujikuroi complex, and Pseudomonas fuscovaginae. Causal agents, pathogenicity determinants, interactions among the various pathogens, epidemiology, geographical distribution, and control options will be discussed.