Sexual dimorphic floral development in dioecious plants revealed by transcriptome, phytohormone, and DNA methylation analysis in Populus tomentosa

Dioecious plants have evolved sex-specific floral development mechanisms. However, the precise gene expression patterns in dioecious plant flower development remain unclear. Here, we used andromonoecious poplar, an exceptional model system, to eliminate the confounding effects of genetic background of dioecious plants. Comparative transcriptome and physiological analysis allowed us to characterize sex-specific development of female and male flowers. Transcriptome analysis identified genes significantly differentially expressed between the sexes, including genes related to floral development, phytohormone synthesis and metabolism, and DNA methylation. Correlation analysis revealed a significant correlation between phytohormone signaling and gene expression, identifying specific phytohormone-responsive genes and their cis-regulatory elements. Two genes related to DNA methylation, METHYLTRANSFERASE1 (MET1) and DECREASED DNA METHYLATION 1 (DDM1), which are located in the sex determination region of Chromosome XIX, have differential expression between female and male flowers. A time-course analysis revealed that MET1 and DDM1 expression may produce different DNA methylation levels in female and male flowers. Understanding the interactions of phytohormone signaling, DNA methylation and target gene expression should lead to a better understanding of sexual differences in floral development. Thus, this study identifies a set of candidate genes for further studies of poplar sexual dimorphism and relates sex-specific floral development to physiological and epigenetic changes.