Rvi6-mediated defense mechanisms against apple scab

Apple scab, caused by the hemibiotrophic fungus Venturia inaequalis (cooke) Wint., is a globally prevalent disease that severely threatens apple yield and fruit quality. Although the key resistance gene Rvi6 (resistance to Venturia inaequalis 6) has been widely deployed in apple scab-resistant breeding programs, the molecular mechanisms underlying its resistance phenotype remain poorly characterized.

In this study, we generated transgenic apple calli overexpressing Rvi6 and systematically investigated both its resistance phenotype and underlying molecular mechanisms. The Rvi6 gene presented high expression levels in leaves and fruits throughout the growth cycle, and aligning with the infection window of V. inaequalis. Rvi6 overexpression significantly reduced the levels of IAA (indole-3-acetic acid), ABA (abscisic acid), and JA (jasmonic acid), and auxin signaling, as well as the callus growth, which directly evidencing the "growth-defense trade-off" hypothesis on Rvi6-mediated apple scab resistance. The marked inhibition of V. inaequalis infection in Rvi6-overexpressing calli was attributed to increased ROS (reactive oxygen species) scavenging capacity, increased osmolyte accumulation, and maintenance of plasma membrane integrity. Additionally, Rvi6 induction depressed apple growth by reducing auxin accumulation and attenuating auxin signaling. Transcriptome analysis revealed that multiple biological processes and signaling pathways are involved in Rvi6-mediated disease resistance. Pathways related to plant‒pathogen interactions, lipid and amino acid metabolism, and flavonoid biosynthesis were significantly enriched among the upregulated pathways. Conversely, plant hormone signal transduction, protein processing and modification, and carbohydrate metabolism were enriched predominantly in the downregulated pathways.

Rvi6 exhibits high expression in leaves and fruits across the growth cycle, aligning with the infection window of V. inaequalis. Rvi6 enhances ROS scavenging capacity, osmolyte accumulation, and plasma membrane integrity, as well as suppresses apple growth, thereby restricting V. inaequalis invasion. Plant immune responses mediated by Ca²⁺ or MAPK cascade reactions, plant hormone signaling and multiple secondary metabolic mechanisms, contribute to Rvi6-mediated resistance against apple scab. This study provides novel insights into the biological functions of Rvi6.