Abstract
BACKGROUND: Phalaenopsis orchids, belonging to the Orchidaceae family, one of the largest groups of angiosperms, possess significant commercial value due to their fascinating flowers. Petal size is a vital trait that directly determines flower size and shape of Phalaenopsis. However, the genetic and developmental regulation of petal size in Phalaenopsis remains unexplored. RESULTS: In this study, we tracked the petal growth pattern through five stages of flower opening, discovering that cell division stops at stage 2 and stages 3 to 5 are the critical periods of rapid petal expansion. RNA-seq was then conducted to further reveal the molecular mechanisms underlying petal size regulation. Gene ontology (GO) analysis indicated that the differentially expressed genes (DEGs) from the four comparable groups were both enriched in terms related to cell expansion. Endogenous hormone assays showed that auxin, cytokinin, and gibberellin were implicated in the petal growth of Phalaenopsis. Moreover, six auxin signaling pathway genes, 11 cell expansion-related genes, and 30 transcription factors (TFs) identified through trend analysis were abundantly expressed during the critical period of petal expansion, suggesting that they may influence petal size by regulating cell expansion. In contrast, 18 TFs exhibited the highest expression levels at the S1 stage, indicating their potential role in petal cell proliferation. Based on weighted gene co-expression network analysis (WGCNA), six hub genes (PaLOF2, PaSWEET11, PaVNI2, PaHDA3, PaPMEI3, PaXTH30) were screened from the green and yellow module which was highly associated with the petal size. CONCLUSION: Our results lay a foundation for further exploration of the molecular mechanisms regulating petal size development and are significant for molecular breeding programs aimed at generating novel Phalaenopsis with desirable traits.