Control of citrus blue and green molds by Actinomycin X2 and its possible antifungal mechanism

Citrus blue and green molds caused by Penicillium digitatum , P. italicum , and P. polonicum , are the major postharvest diseases of citrus fruit. In the present study, Actinomycin X 2 (Act-X 2 ), a naturally occurring antibiotic produced by Streptomyces species, was found to show excellent antifungal effect against these three pathogens with a minimum inhibitory concentration (MIC) value of 62.5 μg/mL for them all, which was better than the positive control thiophanate-methyl. Act-X 2 significantly reduced the percentage of spore germination , and highly inhibited the mycelial growth of P. italicum , P. digitatum , and P. polonicum with EC 50 values being 34.34, 13.76, and 37.48 μg/mL, respectively. In addition, Act-X 2 greatly decreased the intracellular protein content while increasing the reactive oxygen species (ROS) level and superoxide anion (O 2 − ) content in the mycelia of pathogens. In vivo test indicated that Act-X 2 strongly inhibited the infection of navel oranges by these three Penicillium species, with an inhibition percentage of >50% for them all at the concentration of 10 MIC. Transcriptome analysis suggested that Act-X 2 might highly influence the ribosomal functions of P. polonicum , which was supported as well by the molecular docking analysis of Act-X 2 with some key functional proteins and RNAs of the ribosome. Furthermore, Act-X 2 significantly reduced the decay percentage and improved the firmness, color, and sugar-acid ratio of navel oranges spray-inoculated with P. polonicum during the postharvest storage at 4 °C for 60 d.