Abstract
BackgroundThe biological control agent Pseudomonas chlororaphis PA23 is capable of protecting Brassica napus (canola) from the necrotrophic fungus Sclerotinia sclerotiorum via direct antagonism. While we have elucidated bacterial genes and gene products responsible biocontrol, little is known about how the host plant responds to bacterial priming on the leaf surface, including global changes in gene activity in the presence and absence of S. sclerotiorum.ResultsApplication of PA23 to the aerial surfaces of canola plants reduced the number of S. sclerotiorum lesion-forming petals by 91.1%. RNA sequencing of the host pathogen interface showed that pretreatment with PA23 reduced the number of genes upregulated in response to S. sclerotiorum by 16-fold. By itself, PA23 activated unique defense networks indicative of defense priming. Genes encoding MAMP-triggered immunity receptors detecting flagellin and peptidoglycan were downregulated in PA23 only-treated plants, consistent with post-stimulus desensitization. Downstream, we observed reactive oxygen species (ROS) production involving low levels of H2O2 and overexpression of genes associated with glycerol-3-phosphate (G3P)-mediated systemic acquired resistance (SAR). Leaf chloroplasts exhibited increased thylakoid membrane structures and chlorophyll content, while lipid metabolic processes were upregulated.ConclusionIn addition to directly antagonizing S. sclerotiorum, PA23 primes the plant defense response through induction of unique local and systemic defense networks. This study provides novel insight into the effects of biocontrol agents applied to the plant phyllosphere. Understanding these interactions will aid in the development of biocontrol systems as an alternative to chemical pesticides for protection of important crop systems.
Highlights
The biological control agent Pseudomonas chlororaphis PA23 is capable of protecting Brassica napus from the necrotrophic fungus Sclerotinia sclerotiorum via direct antagonism
P. chlororaphis PA23 reduces S. sclerotiorum only (Ss). sclerotiorum infection rates in B. napus To understand how B. napus responds to PA23 and how PA23 protects the host plant from S. sclerotiorum infection, we compared infection rates at the 30–50% flowering stage in the presence or absence of PA23
When comparing the rate of infection as the proportion of lesion-forming petals to total petals fallen onto the plant canopy, application of PA23 reduced the number of lesions by 91.1% (Fig. 1a) and sustained pathogen suppression for at least 7 days post treatment
Summary
The biological control agent Pseudomonas chlororaphis PA23 is capable of protecting Brassica napus (canola) from the necrotrophic fungus Sclerotinia sclerotiorum via direct antagonism. Morphological and structural barriers such as a waxy cuticle and tough cell wall are part of an innate defense mechanism against both living organisms and abiotic forces [1, 2] Bypassing these barriers causes activation of a defense response via detection of. Pseudomonas putida WCS358 suppresses soil-borne pathogens through siderophore-mediated competition for iron, but can induce ISR in Arabidopsis thaliana via host detection of flagellin, pseudobactin and lipopolysaccharides [19]. Such microorganisms are good candidates to replace chemical pesticides, and the number of commercially available BCAs is steadily increasing [18, 20, 21]. In order to successfully implement BCAs in the field, a complete understanding of biocontrol system interactions, including their impact on the host plant, is required
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