Seasonal Dynamics of Various Scab-Causing Streptomyces Genotypes Among Potato Fields on Prince Edward Island.

Potato common scab is an important bacterial plant disease caused by numerous Streptomyces species and strains. A better understanding of the genetic diversity and population dynamics of these microorganisms in the field is crucial to develop effective control methods. Our research group previously studied the genetic diversity of scab-causing Streptomyces spp. in Prince Edward Island, one of Canada's most important potato-growing provinces. Fourteen distinct Streptomyces genotypes were identified and displayed contrasting aggressiveness toward potato tubers. To better understand the distribution and occurrence of these genotypes over time under field conditions, the population dynamics were studied in nine commercial potato fields throughout a growing season. A comparative genomic-driven approach was used to design genotype-specific primers and probes, allowing us to quantify, using quantitative polymerase chain reaction, the abundance of each of the 14 genotypes in field soil. Thirteen of the previously identified genotypes were detected in at least one soil sample, with various frequencies and population sizes across the different fields under study. Interestingly, weakly virulent genotypes dominated, independent of time or location. Among them, three genotypes accounted for more than 80% of the genotypes' combined population. Although the highly virulent genotypes were detected in lower relative abundance than the weakly virulent ones, an increase in the highly virulent genotypes' population size was observed over the growing season in most fields. The results will ultimately be useful for the development of targeted common scab control strategies.

Pyoluteorin and 2,4-diacetylphloroglucinol are major contributors to Pseudomonas protegens Pf-5 biocontrol against Botrytis cinerea in cannabis.

Pseudomonas protegens Pf-5 is an effective biocontrol agent that protects many crops against pathogens, including the fungal pathogen Botrytis cinerea causing gray mold disease in Cannabis sativa crops. Previous studies have demonstrated the important role of antibiotics pyoluteorin (PLT) and 2,4-diacetylphloroglucinol (DAPG) in Pf-5-mediated biocontrol. To assess the potential involvement of PLT and DAPG in the biocontrol exerted by Pf-5 against B. cinerea in the phyllosphere of C. sativa, two knockout Pf-5 mutants were generated by in-frame deletion of genes pltD or phlA, required for the synthesis of PLT or DAPG respectively, using a two-step allelic exchange method. Additionally, two complemented mutants were constructed by introducing a multicopy plasmid carrying the deleted gene into each deletion mutant. In vitro confrontation assays revealed that deletion mutant ∆pltD inhibited B. cinerea growth significantly less than wild-type Pf-5, supporting antifungal activity of PLT. However, deletion mutant ∆phlA inhibited mycelial growth significantly more than the wild-type, hypothetically due to a co-regulation of PLT and DAPG biosynthesis pathways. Both complemented mutants recovered in vitro inhibition levels similar to that of the wild-type. In subsequent growth chamber inoculation trials, characterization of gray mold disease symptoms on infected cannabis plants revealed that both ∆pltD and ∆phlA significantly lost a part of their biocontrol capabilities, achieving only 10 and 19% disease reduction respectively, compared to 40% achieved by inoculation with the wild-type. Finally, both complemented mutants recovered biocontrol capabilities in planta similar to that of the wild-type. These results indicate that intact biosynthesis pathways for production of PLT and DAPG are required for the optimal antagonistic activity of P. protegens Pf-5 against B. cinerea in the cannabis phyllosphere.

In Tuber Biocontrol of Potato Late Blight by a Collection of Phenazine-1-Carboxylic Acid-Producing Pseudomonas spp.

Phenazine-1-carboxylic acid (PCA) produced by plant-beneficial Pseudomonas spp. is an antibiotic with antagonistic activities against Phytophthora infestans, the causal agent of potato late blight. In this study, a collection of 23 different PCA-producing Pseudomonas spp. was confronted with P. infestans in potato tuber bioassays to further understand the interaction existing between biocontrol activity and PCA production. Overall, the 23 strains exhibited different levels of biocontrol activity. In general, P. orientalis and P. yamanorum strains showed strong disease reduction, while P. synxantha strains could not effectively inhibit the pathogen's growth. No correlation was found between the quantities of PCA produced and biocontrol activity, suggesting that PCA cannot alone explain P. infestans' growth inhibition by phenazine-producing pseudomonads. Other genetic determinants potentially involved in the biocontrol of P. infestans were identified through genome mining in strains displaying strong biocontrol activity, including siderophores, cyclic lipopeptides and non-ribosomal peptide synthase and polyketide synthase hybrid clusters. This study represents a step forward towards better understanding the biocontrol mechanisms of phenazine-producing Pseudomonas spp. against potato late blight.

Rheological and water binding properties of xanthan, guar and ultra-finely milled oatmeal in white birch sap: Influence of sap minor constituents.

White Birch Sap (WBS) contains appreciable amounts of mineral ions and phenolic compounds and can be used as alternate solvent for food applications. In this study, the effect of the mineral and phenolic composition of WBS was evaluated on the physical properties of xanthan gum, guar gum, ultra-finely milled oatmeal and their combinations in solution. Solutions were formulated with WBS and with solvents mimicking WBS without phenolic compounds and WBS without phenolics nor mineral ions. The influence of solvent composition was evaluated on flow properties and water mobility of the solutions. From WBS without mineral ions nor phenolics, the addition of mineral ions led to increased pseudo-plasticity and decreased flow consistency, and decreased water mobility. Addition of phenolic compounds through WBS led to opposite effects possibly due to phenolic-driven aggregation of the hydrocolloids which also seemed to inhibit guar/xanthan interactions.

Inhibition of Three Potato Pathogens by Phenazine-Producing Pseudomonas spp. Is Associated with Multiple Biocontrol-Related Traits.

Phenazine-producing Pseudomonas spp. are effective biocontrol agents that aggressively colonize the rhizosphere and suppress numerous plant diseases. In this study, we compared the ability of 63 plant-beneficial phenazine-producing Pseudomonas strains representative of the worldwide diversity to inhibit the growth of three major potato pathogens: the oomycete Phytophthora infestans, the Gram-positive bacterium Streptomyces scabies, and the ascomycete Verticillium dahliae. The 63 Pseudomonas strains are distributed among four different subgroups within the P. fluorescens species complex and produce different phenazine compounds, namely, phenazine-1-carboxylic acid (PCA), phenazine-1-carboxamide (PCN), 2-hydroxyphenazine-1-carboxylic acid, and 2-hydroxphenazine. Overall, the 63 strains exhibited contrasted levels of pathogen inhibition. Strains from the P. chlororaphis subgroup inhibited the growth of P. infestans more effectively than strains from the P. fluorescens subgroup. Higher inhibition was not associated with differential levels of phenazine production nor with specific phenazine compounds. The presence of additional biocontrol-related traits found in P. chlororaphis was instead associated with higher P. infestans inhibition. Inhibition of S. scabies by the 63 strains was more variable, with no clear taxonomic segregation pattern. Inhibition values did not correlate with phenazine production nor with specific phenazine compounds. No additional synergistic biocontrol-related traits were found. Against V. dahliae, PCN producers from the P. chlororaphis subgroup and PCA producers from the P. fluorescens subgroup exhibited greater inhibition. Additional biocontrol-related traits potentially involved in V. dahliae inhibition were identified. This study represents a first step toward harnessing the vast genomic diversity of phenazine-producing Pseudomonas spp. to achieve better biological control of potato pathogens. IMPORTANCE Plant-beneficial phenazine-producing Pseudomonas spp. are effective biocontrol agents, thanks to the broad-spectrum antibiotic activity of the phenazine antibiotics they produce. These bacteria have received considerable attention over the last 20 years, but most studies have focused only on the ability of a few genotypes to inhibit the growth of a limited number of plant pathogens. In this study, we investigated the ability of 63 phenazine-producing strains, isolated from a wide diversity of host plants on four continents, to inhibit the growth of three major potato pathogens: Phytophthora infestans, Streptomyces scabies, and Verticillium dahliae. We found that the 63 strains differentially inhibited the three potato pathogens. These differences are in part associated with the nature and the quantity of the phenazine compounds being produced but also with the presence of additional biocontrol-related traits. These results will facilitate the selection of versatile biocontrol agents against pathogens.

Diversity of phytobeneficial traits revealed by whole-genome analysis of worldwide-isolated phenazine-producing Pseudomonas spp.

Plant-beneficial Pseudomonas spp. competitively colonize the rhizosphere and display plant-growth promotion and/or disease-suppression activities. Some strains within the P. fluorescens species complex produce phenazine derivatives, such as phenazine-1-carboxylic acid. These antimicrobial compounds are broadly inhibitory to numerous soil-dwelling plant pathogens and play a role in the ecological competence of phenazine-producing Pseudomonas spp. We assembled a collection encompassing 63 strains representative of the worldwide diversity of plant-beneficial phenazine-producing Pseudomonas spp. In this study, we report the sequencing of 58 complete genomes using PacBio RS II sequencing technology. Distributed among four subgroups within the P. fluorescens species complex, the diversity of our collection is reflected by the large pangenome which accounts for 25 413 protein-coding genes. We identified genes and clusters encoding for numerous phytobeneficial traits, including antibiotics, siderophores and cyclic lipopeptides biosynthesis, some of which were previously unknown in these microorganisms. Finally, we gained insight into the evolutionary history of the phenazine biosynthetic operon. Given its diverse genomic context, it is likely that this operon was relocated several times during Pseudomonas evolution. Our findings acknowledge the tremendous diversity of plant-beneficial phenazine-producing Pseudomonas spp., paving the way for comparative analyses to identify new genetic determinants involved in biocontrol, plant-growth promotion and rhizosphere competence.