Transcriptome sequencing reveals Vmplc1 involved in regulating the pathogenicity of Valsa Mali under low temperature...

Apple Valsa canker (AVC), caused by the fungal pathogen Valsa mali (Vm), results in significant economic losses in Chinese apple orchards. Although cold priming has been shown to enhance the pathogenicity of Vm, the underlying molecular mechanisms of this temperature-dependent virulence remain poorly understood.

RNA-seq analysis of low-temperature-primed Vm strain Vm263 (5 °C, 24 h) and infected apple tissues identified 962 differentially expressed genes (DEGs) in primed mycelia (LF), escalating to 3,977 DEGs during cold-primed infection (LL). Notably, 66% of differentially expressed carbohydrate-active enzymes (CAZymes) were upregulated during cold priming, with glycoside hydrolases (GHs) exhibiting predominant induction (66.15% upregulated). Through integrative analysis among LF and LL, 63 core cold-responsive virulence were prioritized, including nine secreted GHs and 12 short-chain dehydrogenases (SDRs). Venn analysis identified 229 co-expressed genes (136 upregulated, 93 downregulated) consistently altered across cold adaptation conditions (LF, LL, CKL). Among these, 44 key candidate DEGs were functionally categorized into five classes (Cold Response, Plant Cell Wall Degradation, Signal Transduction, Toxins/Secondary Metabolites, Transcription/Translation & Transport/Metabolism), collectively underpinning critical biological processes essential for cold-induced pathogenicity enhancement. In signal transduction functional class, Vmplc1 encodes canonical full-length thermosensory domains and implied as a signaling hub bridging low-temperature perception to virulence execution. Functional validation confirmed that Vmplc1 is essential for cold-primed pathogenicity. ΔVmplc1 mutants showed 58-62% reduced lesion formation despite unimpaired growth, with complementation restoring wild-type virulence. This phenotype correlated with 70-85% suppression of polygalacturonase/cellulase expression and enzymatic activity.

This study establishes that cold priming potentiates the pathogenicity of Valsa mali by activating a coordinated transcriptional program involving five functional classes of genes, with the Vmplc1-mediated signaling pathway serving as a central hub for upregulating virulence effectors. These findings elucidate a critical thermal adaptation mechanism in which Vmplc1 functions as a pivotal regulator of temperature-dependent virulence, reprogramming the pathogen for enhanced host colonization under low-temperature stress.