EFFECT OF EXOGENOUS BRASSINOSTEROIDS ON RESISTANCE OF ARABIDOPSIS THALIANA TO BACTERIAL INFECTION BY PSEUDOMONAS SYRINGAE
DOI:
https://doi.org/10.15407/dopovidi2026.04.012Keywords:
brassinosteroids, 24-epibrassinolide, BRI1 receptor, Arabidopsis thaliana, Pseudomonas syringae pv. tomato DC3000, pathogens, phytohormones, plant immunityAbstract
The effects of exogenous brassinosteroids—24-epibrassinolide and 24-epicastasterone—as well as a specific inhibitor of their biosynthesis, brassinazole, on the resistance of Arabidopsis thaliana plants to infection by the hemibiotrophic bacterial phytopathogen Pseudomonas syringae pv. tomato DC3000 (Pto DC3000) was comprehensively investigated. The study analyzed the progression of an intracellular bacterial infection in wild-type plants (ecotype Columbia-0) and in the bri1-6 mutant line, which is insensitive to brassinosteroids, using the flood inoculation method for in vitro infection of plants. The bacterial load within leaf tissues was quantified on the third day after inoculation by counting colony-forming units (CFU) per gram of fresh weight following surface sterilization. It was found that inhibition of endogenous brassinosteroids biosynthesis by brassinazole severely reduces the basal immunity of wild-type plants. The application of exogenous 24-epibrassinolide completely counteracts the effects of brassinazole that cause morphological dwarfism. When applied separately, exogenous 24-epibrassinolide resulted in a statistically significant reduction in pathogen accumulation only at the highest concentration tested — 100 nM. The protective effectiveness of 24-epicastasterone, a biosynthetic precursor with lower biological activity, strongly depended on the mode of application. Its addition to the growth medium alone conferred resistance only at higher concentrations of 100 nM and 1 μM, whereas a supplemental foliar spraying treatment performed two days prior to inoculation significantly enhanced plant resistance already at a lower concentration of 10 nM. Crucially, all protective and regulatory effects of exogenous brassinosteroids were observed only in the presence of a functional BRI1 receptor. This was confirmed by the complete resistance of bri1-6 plants to any chemical stress and their characteristically low level of constitutive immunity. These results highlight the important role of brassinosteroid homeostasis and BRI1-mediated signaling in balancing growth-defense trade-off.
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