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Bacillus velezensis is considered as model species for plant-associated bacilli providing benefits to its host such as protection against phytopathogens. This is mainly due to the potential to secrete a wide range of secondary metabolites with specific and complementary bioactivities. This metabolite arsenal has been quite well defined genetically and chemically but much remains to be explored regarding how it is expressed under natural conditions and notably how it can be modulated upon interspecies interactions in the competitive rhizosphere niche. Here, we show that B. velezensis can mobilize a substantial part of its metabolome upon the perception of Pseudomonas, as a soil-dwelling competitor. This metabolite response reflects a multimodal defensive strategy as it includes polyketides and the bacteriocin amylocyclicin, with broad antibiotic activity, as well as surfactin lipopeptides, contributing to biofilm formation and enhanced motility. Furthermore, we identified the secondary Pseudomonas siderophore pyochelin as an info-chemical, which triggers this response via a mechanism independent of iron stress. We hypothesize that B. velezensis relies on such chelator sensing to accurately identify competitors, illustrating a new facet of siderophore-mediated interactions beyond the concept of competition for iron and siderophore piracy. This phenomenon may thus represent a new component of the microbial conversations driving the behavior of members of the rhizosphere community.

International audience; Cocoyam (Xanthosoma sagittifolium (L.)), an important tuber crop in the tropics, is severely affected by the cocoyam root rot disease (CRRD) caused by Pythium myriotylum. The white cocoyam genotype is very susceptible while the red cocoyam has some field tolerance to CRRD. Fluorescent Pseudomonas isolates obtained from the rhizosphere of healthy red and white cocoyams from three different fields in Cameroon were taxonomically characterized. The cocoyam rhizosphere was enriched with P. fluorescens complex and P. putida isolates independent of the plant genotype. LC–MS and NMR analyses revealed that 50% of the Pseudomonas isolates produced cyclic lipopeptides (CLPs) including entolysin, lokisin, WLIP, putisolvin and xantholysin together with eight novel CLPs. In general, CLP types were linked to specific taxonomic groups within the fluorescent pseudomonads. Representative CLP‐producing bacteria showed effective control against CRRD while purified CLPs caused hyphal branching or hyphal leakage in P. myriotylum. The structure of cocoyamide A, a CLP which is predominantly produced by P. koreensis group isolates within the P. fluorescens complex is described. Compared with the white cocoyam, the red cocoyam rhizosphere appeared to support a more diverse CLP spectrum. It remains to be investigated whether this contributes to the field tolerance displayed by the red cocoyam.

Aims: To determine the role of phenazines (PHZ) and lipopeptide surfactants (LPs) produced by Pseudomonas in suppression of stem rot disease of groundnut, caused by the fungal pathogen Sclerotium rolfsii. Methods and Results: In vitro assays showed that PHZ-producing Pseudomonas chlororaphis strain Phz24 significantly inhibited hyphal growth of S. rolfsii and suppressed stem rot disease of groundnut under field conditions. Biosynthesis and regulatory mutants of Phz24 deficient in PHZ production were less effective in pathogen suppression. Pseudomonas strains SS101, SBW25, and 267, producing viscosin or putisolvin-like LPs, only marginally inhibited hyphal growth of S. rolfsii and did not suppress stem rot disease. In contrast, Pseudomonas strain SH-C52, producing the chlorinated LP thanamycin, inhibited hyphal growth of S. rolfsii and significantly reduced stem rot disease of groundnut in nethouse and field experiments, whereas its thanamycin-deficient mutant was less effective. Conclusions: Phenazines and specific lipopeptides play an important role in suppression of stem rot disease of groundnut by root-colonizing Pseudomonas strains. Significance and Impact of the Study: Pseudomonas strains Phz24 and SH-C52 showed significant control of stem rot disease. Treatment of seeds or soil with these strains provides a promising supplementary strategy to control stem rot disease of groundnut

Cyclic lipopeptides (CLPs) are synthesized by nonribosomal peptide synthetases (NRPS), which are often flanked by LuxR‐type transcriptional regulators. Pseudomonas sp. CMR12a, an effective biocontrol strain, produces two different classes of CLPs namely sessilins and orfamides. The orfamide biosynthesis gene cluster is flanked up‐ and downstream by LuxR‐type regulatory genes designated ofaR1 and ofaR2, respectively, whereas the sessilin biosynthesis gene cluster has one LuxR‐type regulatory gene which is situated upstream of the cluster and is designated sesR. Our study investigated the role of these three regulators in the biosynthesis of orfamides and sessilins. Phylogenetic analyses positioned OfaR1 and OfaR2 with LuxR regulatory proteins of similar orfamide‐producing Pseudomonas strains and the SesR with that of the tolaasin producer, Pseudomonas tolaasii. LC‐ESI‐MS analyses revealed that sessilins and orfamides are coproduced and that production starts in the late exponential phase. However, sessilins are secreted earlier and in large amounts, while orfamides are predominantly retained in the cell. Deletion mutants in ofaR1 and ofaR2 lost the capacity to produce both orfamides and sessilins, whereas the sesR mutant showed no clear phenotype. Additionally, RT‐PCR analysis showed that in the sessilin cluster, a mutation in either ofaR1 or ofaR2 led to weaker transcripts of the biosynthesis genes, sesABC, and putative transporter genes, macA1B1. In the orfamide cluster, mainly the biosynthesis genes ofaBC were affected, while the first biosynthesis gene ofaA and putative macA2B2 transport genes were still transcribed. A mutation in either ofaR1, ofaR2, or sesR genes did not abolish the transcription of any of the other two.

Aims: To determine the role of phenazines (PHZ) and lipopeptide surfactants (LPs) produced by Pseudomonas in suppression of stem rot disease of groundnut, caused by the fungal pathogen Sclerotium rolfsii. Methods and Results: In vitro assays showed that PHZ-producing Pseudomonas chlororaphis strain Phz24 significantly inhibited hyphal growth of S. rolfsii and suppressed stem rot disease of groundnut under field conditions. Biosynthesis and regulatory mutants of Phz24 deficient in PHZ production were less effective in pathogen suppression. Pseudomonas strains SS101, SBW25, and 267, producing viscosin or putisolvin-like LPs, only marginally inhibited hyphal growth of S. rolfsii and did not suppress stem rot disease. In contrast, Pseudomonas strain SH-C52, producing the chlorinated LP thanamycin, inhibited hyphal growth of S. rolfsii and significantly reduced stem rot disease of groundnut in nethouse and field experiments, whereas its thanamycin-deficient mutant was less effective. Conclusions: Phenazines and specific lipopeptides play an important role in suppression of stem rot disease of groundnut by root-colonizing Pseudomonas strains. Significance and Impact of the Study: Pseudomonas strains Phz24 and SH-C52 showed significant control of stem rot disease. Treatment of seeds or soil with these strains provides a promising supplementary strategy to control stem rot disease of groundnut

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.