Harnessing innate immunity: PR proteins expression in Malus domestica as an important defense mechanism against scab incited by Venturia inaequalis

Apple (Malus domestica) is a temperate fruit tree that can be affected by various plant pathogens throughout the year. Among all pathogens, scab disease incited by Venturia inaequalis Cooke (Wint.) is a serious problem in apple cultivation, as it decreases both the quality and quantity of fruits. Although there are many ways to manage plant diseases including cultural, chemical, and biological methods, managing through plant resistance is an affordable and long-term solution to biotic stress. Understanding innate immune responses in apples, such as the induction of pathogenesis-related (PR) proteins, can enhance efforts to improve disease resistance. The rapid induction of apoplastic PR proteins with antifungal activities, such as chitinases, glucanases, and proteases, can directly target the cell walls of the pathogen to restrict its growth. These mechanisms are supplemented by a durable, broad-spectrum defense response initiated by salicylic acid (SA) accumulation. This response involves the transcriptional activation of PR genes and the priming of uninfected tissues through regulatory proteins. The induction of PR proteins and systemic-acquired resistance mediated by SA confer multilayered innate immunity against Venturia invasion. Malus resistance alleles, such as HcrVf (homologue of the Cladosporium fulvum resistance genes of the Vf region) and Rvi (apple scab (V. inaequalis) resistance genes), are effective against Venturia. Overall, a deeper understanding of constitutive versus induced PR gene expression and SA-mediated immunomodulation pathways in apples provides a foundation to sustainably improve crop protection against this devastating disease. Both biotechnological and ecological approaches should be pursued to leverage ancient plant immune strategies for next-generation Venturia resistance. Therefore, this review focuses on synthesizing the current knowledge of localized and systemic defense mechanisms against Venturia infection in apples.