How Does Abscisic Acid Control Fruit Quality as a Plant Bioregulator?

Various attempts have been made to use abscisic acid (ABA) as a plant bioregulator (PBR). Recently, a new ABA formulation, produced through microbial fermentation, has been commercialized in Japan as a PBR for improving grape peel color. Nonetheless, the horticultural application of ABA remains limited compared to other plant hormones. Generally, the vital functions of ABA include the regulation of abiotic stress tolerance and plant dormancy via metabolic changes associated with the antioxidant system and the modulation of sugar biosynthesis/translocation, respectively. These metabolic changes are crucial for the quality of fruit, suggesting the potential of ABA for promoting the production of high-quality fruit. In non-climacteric fruit, ABA plays a pivotal role in anthocyanin pigmentation, a significant component of the antioxidant machinery. Studies in grapes and strawberries have shown that the responses to ABA differ depending on the type of ABA agonist used and the specific organs to which it is applied. Herein, the influence of ABA on climacteric fruit exhibiting ethylene-regulated ripening is discussed. ABA-mediated ethylene induction followed by ripening suggests a shared regulatory mechanism that underlies ripening in both climacteric and non-climacteric fruits that involves crosstalk between ABA and ethylene. This review firstly summarizes the historical challenges with ABA and its agonist for application as PBRs and discusses the role of ABA in the ripening of non-climacteric fruit, particularly grapes. Subsequently, the molecular background of ABA in both climacteric and non-climacteric fruit ripening is explained, with a focus on peel coloration, sugar synthesis, and aroma volatile synthesis.