Non-climacteric but genotype-dependent ethylene response characteristics in highbush blueberry ( Vaccinium corymbosum ) fruit ripening

The ripening behavior of blueberries (Vaccinium corymbosum) remains debated, with classifications ranging from climacteric to non-climacteric or atypical climacteric. While the role of ethylene in blueberry ripening has been primarily studied through postharvest ethephon treatments, its endogenous function and the presence of system II ethylene biosynthesis remain unclear. This study aimed to evaluate the role of ethylene in blueberry fruit ripening and determine the presence or absence of system II ethylene biosynthesis for fruit ripening classification. Highbush blueberries exhibited low ethylene emissions, which increased at the fruit coloring and pink stages, coinciding with an increase in the ethylene precursors S-adenosylmethionine and 1-aminocyclopropane-1-carboxylic acid, increased respiration rate, acidity loss, sugar accumulation, anthocyanin synthesis, and fruit pigmentation. Genome-wide analysis identified two putative ethylene biosynthetic genes, VcACS3 and VcACO6, which were expressed during ripening and functionally validated via transient overexpression in 'O'Neal' fruit. However, in planta and postharvest propylene treatment, which mimics the effects of ethylene, failed to induce autocatalytic ethylene biosynthesis across cultivars. Additionally, in planta and postharvest 1-methylcyclopropene treatment in all genotypes, and transient overexpression of Atetr1-1 in 'O'Neal' fruit, inhibited ripening without reducing ethylene emission. These findings indicate that while ethylene modulates blueberry ripening, its biosynthesis is not autocatalytically controlled. Thus, blueberries are best classified as non-climacteric fruit with ethylene-dependent ripening characteristics.