Phytohormone Profiling and Transcriptional Regulation During Fruit Peel Development in Wax Apple

Wax apple (Syzygium samarangense) is valued for its appealing color and delicious taste and has gained increasing popularity, along with significant economic value. Previous research largely focused on orchard cultivation practices, fruit development stage management, and postharvest treatments aimed at improving yield, quality, and market profitability. Study of the peel color of the wax apple has been scarce, although the substance accumulations and pigment changes in the fruit peel are unclear. In this study, the maximum variation of fruit biological traits and peel color occurred between S2 and S3. A total of 946 metabolites were identified, of which 17 key phytohormone-related metabolites may be related to the change of fruit peel color, encompassing jasmonates (JAs), cytokinins (CTKs), gibberellins (GAs), and zeatin ribosides (ZRs). Using RNA sequencing, 212 expressed genes relating to plant hormone signal transduction were identified. Among these, 18, 39, and 11 differentially expressed genes (DEGs) were identified in S2-vs-S1, S3-vs-S2, and S4-vs-S3, respectively. Critical genes within the hormone signaling cascades—including ARF, GID1B, ARR, JAR, NCED1, and TGA—exhibited strong correlations with peel color. Furthermore, fifteen distinct expressed gene modules and six distinct compound modules were identified through weighted gene coexpression network analysis, of which the blue and green modules display a robust correlation between numerous distinctly expressed genes and phytohormone metabolites. Collectively, our findings offer novel insights into the combinatorial regulation of wax apple fruit development and peel color, serving as a valuable resource for functional genomics and providing a theoretical basis for the genetic improvement and cultivation management of the wax apple.