Apples are nutrient-rich, including various vitamins, flavonoids, and apple polyphenols [1,2]. However, apples are infected by plant pathogens during storage and transportation, causing postharvest disease in apples. Of all the dangerous pathogens, blue mold caused by Penicillium expansum is the most pernicious, as it causes blue mold disease in apples, drastically devaluing production and adversely affecting countries' economies [3]. Patulin (PAT), a mycotoxin produced by P. expansum, is a critical pollutant in moldy apples and apple products [4]. PAT can potentially cause a variety of hazards to the human body, and its presence in food is limited to the maximum value of 50 μg/Kg in solid foods [5]. To guarantee consumer health, it is essential to innovate with more practical and safer methods of combating postharvest diseases. Developing strategies based on in-depth molecular biology research can lead to sustainable postharvest prevention and limitation of blue mold disease in apples.

Interactions between plants and pathogens involve complex mechanisms and sophisticated molecular signal transduction processes. Specifically, plants use numerous cell surface receptors and intracellular immune effectors to identify various immune signals related to pathogen infection [6]. To perceive the pathogen signals of infection, plants rely on the perception of damaged pathogen-associated molecular patterns (PAMPs). PAMPs recognition will finally lead to PAMP-triggered immunity (PTI) [7]. To resist the invasion of pathogens, plants have evolved intracellular receptor proteins, nucleotide-binding sites, and leucine-rich repeat domain receptors (NLRs) to activate more robust effector-triggered immunity responses (ETI) [8]. PTI and ETI induce the plant to produce a panoply of peptides, proteins, and substances with defensive properties [9]. Existing development showed that disease-resistant genes and proteins are central in fruit response to pathogen infection. Likewise, the fungal pathogens that infect fruit actively express numerous genes and proteins designed to complete the infection process. Once the pathogen invades the fruit, it activates more production of a series of its defense genes that regulate the synthesis of downstream metabolites, reinforcing its ability to defend against plant defensive response, including PR protein. Finally, the dynamic equilibrium of the organism is maintained through the elimination and production of reactive oxygen species (ROS) to enhance disease resistance [10]. In the study of apple gray spot disease, CfSte12, the gene of a zinc-finger transcription factor, was highly expressed, and subsequent use of gene knockout technology revealed that CfSte12 regulates a wide range of important pathogenic cellular processes of gray leaf spot, including germination, and appressorium related mechanisms [11].

At present, proteomics technology has become one of the valuable methods to study the response mechanism of pathogenic bacteria to plant infection [12,13]. Proteomics is a powerful resource for understanding basic physiological and pathological processes, and elucidating possible relationships between protein abundance and biotic stress in plants. In the process of plant-pathogen interaction, proteins with important functions were identified by proteomics technology. For instance, Zhang et al. [14] exploited iTRAQ-based cotton proteomics to analyze the infection of cotton against Rhizoctonia solani, and established the involvement of proteins associated with ROS regulation, epigenetic modulation and biosynthesis of key phytochemicals in cotton. Liu et al. [15] adapted iTRAQ technology to discriminate proteins related to gray mold in kiwifruit. Bioinformatics analysis showed that DEPs were related to recognition at the point of penetration, cell wall disintegration, MAPK cascade, ROS signal, and PR protein, which led to a better grasp of kiwifruit and gray mold interaction. Our previous proteomics-related research highlighted the mobilization of defensive proteins, mainly PR proteins, when P. expansum infected apple fruit. In this previous study, Label-Free and Parallel Reaction Monitoring proved to be a reliable technique.

In this study, we studied the complex changes of proteins and their regulatory mechanisms after P. expansum infection of apples through proteomics, and proved the reliability of proteomics data by PRM technology. This study laid a foundation for further functional exploration and verification of related proteins.