Inducing disease resistance in fruit is one of the mechanisms of antagonistic yeast. Antagonistic yeast induces disease resistance in postharvest fruit and delays fruit decay. In our study, tryptophan enhanced the ability to induce disease resistance in citrus fruit by Metschnikowia citriensis against blue mold, sour rot, and green mold. The disease incidence of blue mold on the 5th d decreased by 16.6% compared with the M. citriensis treatment group. Conversely, the disease incidence of sour rot on the 9th d decreased by 10.0% relative to the M. citriensis group, and the disease incidence of green mold on the 4th d decreased by 20.0% compared with the M. citriensis group. Further validation revealed that M. citriensis and tryptophan-treated M. citriensis significantly induced the upregulation of genes related to the biosynthesis of salicylic acid and its transduction pathways, sugar metabolism, the shikimic acid pathway, MYB transcription factors, the phenylpropane biosynthesis pathway, pathogenesis-related proteins, and activated defense pathways in citrus fruit. Moreover, the genes induced by the tryptophan-treated M. citriensis group showed significantly higher expression than those in the M. citriensis-treated group alone. These genes are linked to the biosynthesis of disease-resistant compounds in citrus. The upregulation of genes showed an enhancement in the defense mechanisms of citrus fruit against biotic stress, enabling them to allocate more resources to withstand unfavorable conditions. Further verification was conducted to establish the connection between the stimulation of disease resistance in citrus fruit by M. citriensis and the synthesis of pulcherriminic acid (PA). Results obtained from inoculation in citrus fruit by the PAdeficient mutant strain Delta M. citriensis showed a significant increase in disease incidence compared with M. citriensis; the disease incidence of blue mold was 16.6% higher than the M. citriensis-treated group alone on the 4th d; the disease incidence of sour rot was 19.4% higher on the 5th d; and the disease incidence of green mold was 16.7% higher on the 5th d. This suggests that PA impacts the effectiveness of M. citriensis in stimulating disease resistance in citrus fruits. Furthermore, the pulcherrimin secreted by M. citriensis was extracted, and it was discovered that PA extract significantly boosted the disease resistance of the fruit to blue and green molds. This suggests that PA produced by M. citriensis may play a role in yeast-induced disease resistance in citrus fruit.
