Abstract
Being natural plant antimicrobials, saponins have potential for use as biopesticides. Nevertheless, their activity in plant–pathogen interaction is poorly understood. We performed a comparative study of saponins' antifungal activities on important crop pathogens based on their effective dose (EC50) values. Among those saponins tested, aescin showed itself to be the strongest antifungal agent. The antifungal effect of aescin could be reversed by ergosterol, thus suggesting that aescin interferes with fungal sterols. We tested the effect of aescin on plant–pathogen interaction in two different pathosystems: Brassica napus versus (fungus) Leptosphaeria maculans and Arabidopsis thaliana versus (bacterium) Pseudomonas syringae pv tomato DC3000 (Pst DC3000). We analyzed resistance assays, defense gene transcription, phytohormonal production, and reactive oxygen species production. Aescin activated B. napus defense through induction of the salicylic acid pathway and oxidative burst. This defense response led finally to highly efficient plant protection against L. maculans that was comparable to the effect of fungicides. Aescin also inhibited colonization of A. thaliana by Pst DC3000, the effect being based on active elicitation of salicylic acid (SA)-dependent immune mechanisms and without any direct antibacterial effect detected. Therefore, this study brings the first report on the ability of saponins to trigger plant immune responses. Taken together, aescin in addition to its antifungal properties activates plant immunity in two different plant species and provides SA-dependent resistance against both fungal and bacterial pathogens.
Highlights
Crop production is hampered by numerous plant diseases caused by diverse pathogenic microorganisms, such as fungi, bacteria or pests, affecting yield, harvest quality and safety
While aescin EC50 values for P. teres, M. nivale, and L. maculans isolates occurred in the range of 11–21 μg·ml−1, 7–29 μg·ml−1, and
Field crops are protected from fungal pathogens by such fungicide compounds as benzimidazoles, sterol biosynthesis inhibitors, strobilurins, or succinate dehydrogenase inhibitors
Summary
Crop production is hampered by numerous plant diseases caused by diverse pathogenic microorganisms, such as fungi, bacteria or pests, affecting yield, harvest quality and safety. Pesticides are currently employed to control crop pathogens and pests, growing problems of fungal resistance to fungicides appear to pose a serious future threat to agriculture (Fisher et al, 2018). Alternatives to fungicides are needed that are less harmful to health and the environment. These might include more intensive employment of biological control, greater crop diversity (Zhu et al, 2000), or developing safer compounds with new modes of action (Burketová et al, 2015). Higher plants could constitute a great source of such compounds. Most plants produce a wide variety of antimicrobial secondary metabolites, including alkaloids, flavonoids, terpenes, organic acids, essential oils, and saponins that are involved in plant defense responses essential for plant protection against microbial or pest attack (Osbourn, 1996; Field et al, 2006; da Cruz Cabral et al, 2013; Matušinský et al, 2015)
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