The litchi downy blight disease of litchi caused by Peronophythora litchii accounts for severe losses in the field and during storage. While ample quantitative studies have shown that 6-pentyl-2H-pyran-2-one (6PP) possesses antifungal activities against multiple plant pathogenic fungi, the regulatory mechanisms of 6PP-mediated inhibition of fungal pathogenesis and growth are still unknown. Here, we investigated the potential molecular targets of 6PP in the phytopathogenic oomycetes P. litchii through integrated deployment of RNA-sequencing, functional genetics, and biochemical techniques to investigate the regulatory effects of 6PP against P. litchii. Previously we demonstrated that 6PP exerted significant oomyticidal activities. Also, comparative transcriptomic evaluation of P. litchii strains treated with 6PP Revealed significant up-regulations in the expression profile of TOR pathway-related genes, including PlCytochrome C and the transcription factors PlYY1. We also noticed that 6PP treatment down-regulated putative negative regulatory genes of the TOR pathway, including PlSpm1 and PlrhoH12 in P. litchii. Protein-ligand binding analyses revealed stable affinities between PlYY1, PlCytochrome C, PlSpm1, PlrhoH12 proteins, and the 6PP ligand. Phenotypic characterization of PlYY1 targeted gene deletion strains generated in this study using CRISPR/Cas9 and homologous recombination strategies significantly reduced the vegetative growth, sporangium, encystment, zoospore release, and pathogenicity of P. litchii. These findings suggest that 6PP-mediated activation of PlYY1 expression positively regulates TOR-related responses and significantly influences vegetative growth and the virulence of P. litchii. The current investigations revealed novel targets for 6PP and underscored the potential of deploying 6PP in developing management strategies for controlling the litchi downy blight pathogen.

Litchi is a fruit of significant commercial value; however, its quality and yield are hindered by downy blight disease caused by Peronophythora litchii. In this study, volatile organic compounds (VOCs) from Streptomyces abikoensis TJGA-19 were investigated for their antifungal effects and studied in vitro and in planta for the suppression of litchi downy blight disease in litchi leaves and fruits. The growth of P. litchii was inhibited by VOCs produced by TJGA-19 cultivated on autoclaved wheat seeds for durations of 10, 20, or 30 days. Volatiles from 20-day-old cultures were more active in inhibition effect against P. litchii than those from 10- or 30-day-old cultures. These volatiles inhibit the growth of mycelia, sporulation, and oospore production, without any significant effect on sporangia germination. Additionally, the VOCs were effective in suppressing disease severity in detached litchi leaf and fruit infection assays. With the increase in the weight of the wheat seed culture of S.abikoensis TJGA-19, the diameters of disease spots on leaves, as well as the incidence rate and disease indices on fruits, decreased significantly. Microscopic results from SEM and TEM investigations showed abnormal morphology of sporangia, mycelia, and sporangiophores, as well as organelle damage in P. litchii caused by VOCs of TJGA-19. Spectroscopic analysis revealed the identification of 22 VOCs produced by TJGA-19, among which the most dominant compound was 2-Methyliborneol. These findings indicated the significant role of TJGA-19 compounds in the control of litchi downy blight disease and in improving fruit quality.

The cultivation and overall quality of Litchi, a fruit of significant commercial value in China, are hindered by the presence of the oomycetes pathogen Peronophythora litchii. This pathogen is responsible for the occurrence of litchi downy blight, resulting in substantial economic losses during the storage and transportation of the fruit, and affects nutritional growth. Effective and environmentally safe methods to control litchi downy blight are urgently needed. The application of biocontrol agents such as Streptomyces bacteria has proven effective for controlling plant diseases. Our present study isolated the Streptomyces strain TJGA-19, identified as S. abikoensis, with potent inhibitory activity against P. litchii. The antifungal active substances are mainly in the aqueous phase of TJGA-19 fermentation filtrate extraction. The fermentation filtrate of TJGA-19 not only suppressed the pathogen growth, sporulation, and sporangia germination, but also delayed the disease development of litchi downy blight. In addition, the stability of the TJGA-19 fermentation filtrate was not sensitive to the proteinase K, temperature, white-flourescence light, or ultraviolet treatment. Furthermore, the morphology and ultrastructure of P. litchii treated with fermentation filtrate was characterized by marked shrinking and deformation, with serious disruption of plasma membrane permeabilization and the organelles. Hence, S. abikoensis TJGA-19 and its metabolites demonstrated marked efficiency against the phytopathogenic pathogen P. litchii and provide a potential candidate for controlling litchi downy blight.