Abstract
Prior studies have shown that the secreted protein PgSCP from the biocontrol yeast Pichia galeiformis induces disease resistance responses in citrus fruit. However, the precise molecular basis of how PgSCP activates these disease resistance responses remains inadequately understood, particularly regarding direct interactions with host signalling components. Here, we demonstrate that PgSCP directly interacts with the citrus transcription factor CsMIKC through protein-protein interaction assays, including yeast two-hybrid, bimolecular flourescence complementation and pull-down experiments. Overexpression and gene-silencing assays revealed that CsMIKC functions as a positive regulator of resistance to citrus green mould. Based on the JASPAR database, we predicted that CsMIKC likely binds to the promoters of PR1-like and ATPase, two defence-associated genes in citrus fruits. This was confirmed via yeast one-hybrid and dual luciferase reporter assays, which showed that CsMIKC directly binds to and activates transcription of these genes. Importantly, co-expression of PgSCP and CsMIKC significantly enhanced the transcriptional activation of PR1-like and ATPase, suggesting a synergistic interaction between the microorganism-derived protein and the host transcription factor. This study identifies the molecular mechanism by which the yeast secreted protein PgSCP interacts with citrus transcription factor CsMIKC to activate defence gene expression, thereby enhancing citrus fruit resistance. The findings establish a new model of cross-kingdom signalling between biocontrol yeast and citrus fruit, offering a theoretical framework for engineering exogenous protein-based strategies to improve postharvest disease resistance.