Antifungal activity of the botanical compound rhein against Phytophthora capsici and the underlying mechanisms

Phytophthora capsici is an extremely destructive phytopathogenic oomycete that causes huge economic losses. However, due to the drug resistance risk and environmental threat of chemical fungicides, it is necessary to develop environmentally friendly biocontrol alternatives. Rhein is a major medicinal ingredient of traditional Chinese herbs, and it is widely used in the medical field. However, its inhibitory effect against phytopathogens is unknown. Herein, the antifungal spectrum of rhein and its possible action mechanism against P. capsici were investigated.Rhein possessed broad-spectrum antifungal activity against phytopathogens, particularly P. capsici, Phytophthora infestans, Helminthosporium maydis, and Rhizoctonia solani. Rhein inhibited the mycelial growth as well as the spore germination of P. capsici with mean 50% effective concentration (EC50 ) values of 4.68 μg mL-1 and 6.57 μg mL-1 against 117 P. capsici isolates, respectively. Rhein effectively suppressed the occurrence and spread of Phytophthora blight and significantly destroyed the cell membrane permeability and integrity of P. capsici, corroded its cell wall integrity, and damaged its morphology and ultrastructure. Moreover, rhein caused a considerable reduction in the phospholipid and cellulose contents. Genome-wide transcriptional profiling of P. capsici in response to rhein indicated significant reduction in the expression levels of genes participating in glycerolipid metabolism and starch and sucrose metabolism. Additionally, rhein strengthened the disease defense system of pepper by enhancing related enzyme activities.This study demonstrated that rhein could effectively inhibit P. capsici using multiple mechanisms of action. Rhein has the potential to be an efficient alternative to control diseases caused by P. capsici. © 2023 Society of Chemical Industry.