Flower development and sexual dimorphism in Vernicia montana

The mu oil tree (Vernicia montana Lour.) is a dioecious species, but the genetic mechanisms underlying its phenotypic sexual dimorphism are unclear. In this study, we determined two pivotal phases of sex differentiation of mu oil tree via morphological and histological analyses of unisexual flowers: (Ⅰ) differentiation of male or female primordia to produce staminate flowers (SFs) or transient hermaphrodite flowers (HFs), and (Ⅱ) complete abortion of stamens in transient HFs to generate pistillate flowers (PFs). A total of 1621 sex-biased genes were identified by comparative transcriptome analysis which exhibited elevated rates of protein evolution than unbiased genes. The female-biased genes were enriched in the production of defense compounds while male-biased genes were focused on the production of viable pollens. Transcriptome-based analysis revealed that the differentially expressed genes (DEGs) between PFs and SFs in phase I involved in abscisic acid (ABA), auxin (AUX), cytokinin (CK), ethylene (ET), and gibberellin (GA) biosynthesis and signaling showed higher expression levels in males than in females in general, whereas the DEGs involved in jasmonic acid (JA) and salicylic acid (SA) pathways displayed opposite expression patterns. Moreover, differentially expressed endogenous ABA, AUX, GAs, JA, and SA exhibited consistent biased expression patterns with the DEGs by UPLC-MS-based analysis. Exogenous application of an anti-ethylene plant growth regulator could promote the development of stamens in PFs and generated HFs. Comparative transcriptomic and hormonal analyses of PFs and SFs in phase Ⅱ indicated an increase in ET concentration when abortion of stamens in PFs occurred. This study suggested that phytohormones play key roles in sex dimorphism and ET may determine the development of stamens in PFs of mu oil tree, which provides an insight into plant sex differentiation mechanisms.