Tissue-Specific and Developmental Expression Pattern of Abscisic Acid Biosynthetic Genes in Peach Fruit: Possible Role of the Hormone in the Coordinated Growth of Seed and Mesocarp

Peach fruit tissues start differentiating into mesocarp, endocarp, and seed from the first phases of development. A seed is necessary for fruit set, and it may strongly regulate the growth of the whole fruit through metabolic or hormonal signals. Although the importance of sugar and hormone signaling in growing fruit, such as peach, has been analyzed extensively, no conclusions as to the cross-talk between these signal molecules and their role in seed and flesh development have previously been made. The present study provides insight into the regulatory steps of the carotenoid/ABA biosynthetic pathway to establish possible relationships between growth and accumulation of pigments, sugar, and hormone in the different tissues of peach fruit (Prunus persica L. Batsch, cv Redhaven). In the flesh, the transcriptional pattern of most of the genes involved in the ABA synthesis exhibits a good association with both color changes and hormone accumulation. In contrast, in the seed, along with other nongreen tissues, this association is not conceivable. The behavior of zeaxanthin epoxidase genes (Ppzep) well represents the presence of distinct regulatory mechanisms at various steps of the pathway and in a tissue-specific manner. Moreover, the key role of 9-cis-epoxycarotenoid dioxygenase (NCED) enzyme in regulating ABA synthesis appears to be substantiated by observed Ppnced expression profiles, both in the flesh and in the seed. Based on the results obtained in this study, a crucial connection between ABA biosynthesis, sugar content, and sucrose cleavage enzymes (sucrose synthase), at different stages of fruit development, is proposed.