Abstract
Antibiosis is a key feature widely exploited to develop biofungicides based on the ability of biological control agents (BCAs) to produce fungitoxic compounds. A less recognised attribute of plant-associated beneficial microorganisms is their ability to stimulate the plant immune system, which may provide long-term, systemic self-protection against different types of pathogens. By using conventional antifungal in vitro screening coupled with in planta assays, we found antifungal and non-antifungal Bacillus strains that protected the ornamental plant Kalanchoe against the soil-borne pathogen Fusarium oxysporum in experimental and commercial production settings. Further examination of one antifungal and one non-antifungal strain indicated that high protection efficacy in planta did not correlate with antifungal activity in vitro. Whole-genome sequencing showed that the non-antifungal strain EC9 lacked the biosynthetic gene clusters associated with typical antimicrobial compounds. Instead, this bacterium triggers the expression of marker genes for the jasmonic and salicylic acid defence pathways, but only after pathogen challenge, indicating that this strain may protect Kalanchoe plants by priming immunity. We suggest that the stimulation of the plant immune system is a promising mode of action of BCAs for the development of novel biological crop protection products.
Highlights
The biological control of soil-borne plant diseases involving beneficial microorganisms has gained considerable attention as an alternative to synthetic fungicides that are environmentally problematic and difficult to use against these pathogens [1–3]
Two main and interrelated mechanisms are generally recognised as part of induced resistance (IR): (1) systemic acquired resistance (SAR), which is associated with salicylic acid (SA) signalling and accompanied by upregulation of genes encoding certain pathogenesis-related (PR) proteins; and (2) induced systemic resistance (ISR), which is associated with jasmonic acid (JA) signalling and upregulation of genes encoding, e.g., plant defensins and lipoxygenases (LOX) [21,22]
We tested endospore-forming bacteria sampled from waste material from Kalanchoe cultivation and, by combining conventional in vitro antifungal screening with in planta evaluation, we identified a number of Bacillus strains that provided protection to Kalanchoe against F. oxysporum, a recognised soil-borne pathogen, penetrating through the roots and invading the vascular tissue [43]
Summary
The biological control of soil-borne plant diseases involving beneficial microorganisms has gained considerable attention as an alternative to synthetic fungicides that are environmentally problematic and difficult to use against these pathogens [1–3]. These BCAs can positively influence fitness [4–8] and protect plants from pathogen infection by competition for space and nutrients, by parasitism and by antibiosis [9,10]. Plant-associated bacteria may activate defence priming [27–30], with no or low energy expenses in the absence of pathogen attack [31,32] This makes these microorganisms attractive BCAs that promote plant self-defence with low energy cost
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