Biocontrol of postharvest citrus green mold by Meyerozyma caribbica: suppression of Penicillium digitatum development, enhanced ROS metabolism, induced resistance, and quality preservation.

Green mold, caused by Penicillium digitatum (Pd), is a major postharvest disease affecting citrus fruits. This study investigated the biocontrol efficacy and possible mechanisms of the antagonistic yeast Meyerozyma caribbica (Mc), isolated from orchard soil, against P. digitatum in postharvest mandarin fruit. M. caribbica reduced green mold incidence, lesion diameter, P. digitatum spore germination, and germ tube length in a concentration-dependent manner. Microscopic observations showed that M. caribbica cells adhered to P. digitatum hyphae. Population dynamics analysis further revealed that M. caribbica rapidly colonized citrus wounds and persisted on fruit surfaces during storage. In a 30-day natural storage trial, M. caribbica-treated fruit exhibited a lower decay rate than the control fruit (9.91 % vs. 29.48 %) and reduced weight loss (2.32 % vs. 3.64 %), without adversely affecting firmness, TSS, or titratable acidity. NBT staining and H2O2 content determination provided direct evidence that M. caribbica reduced excessive ROS accumulation induced by P. digitatum. In the Mc + Pd treatment, the activities of ROS-scavenging enzymes, including CAT, SOD, APX, and GR, were enhanced, and the levels of non-enzymatic antioxidants, including ascorbic acid and glutathione, were higher than those in the Pd treatment. Consistently, M. caribbica reduced MDA content and cell membrane permeability by 29.89 % and 15.46 %, respectively, on day 5 compared with the Pd treatment. In addition, defense-related enzyme activities, including PPO, POD, PAL, LOX, and GLU, as well as the total phenolic and flavonoid contents, were markedly increased in the Mc + Pd treatment group. Overall, M. caribbica suppresses citrus green mold through multiple possible modes of action, including the suppression of P. digitatum development, successful colonization, regulation of ROS homeostasis, and enhancement of tissue-associated antioxidant and defense responses. These findings suggest that M. caribbica Y1 has potential as an eco-friendly biocontrol agent for postharvest citrus disease management.