Background: Sapium sebiferum, whose seeds contain high level of fatty acids, has been considered as one of the most important oil plants. However, the high male to female flower ratio limited the seed yield improvement and its industrial potentials. Thus, the study of the sex determination in S. sebiferum is of significant importance in increasing the seed yield. Results: In this study, we demonstrated that in S. sebiferum, cytokinin (CK) had strong feminization effects on the floral development. Exogenous application with 6-benzylaminopurine (6-BA) or thidiazuron (TDZ) significantly induced the development of female flowers and increased the fruit number. Interestingly, the feminization effects of cytokinin were also detected on the androecious genotype of S. sebiferum which only produce male flowers. To further investigate the mechanism underlying the role of cytokinin in the flower development and sex differentiation, we performed the comparative transcriptome analysis of the floral buds of the androecious plants subjected to 6-BA. The results showed that there were separately 129, 352 and 642 genes differentially expressed at 6 h, 12 h and 24 h after 6-BA treatment. Functional analysis of the differentially expressed genes (DEGs) showed that many genes are related to the hormonal biosynthesis and signaling, nutrients translocation and cell cycle. Moreover, there were twenty one flowering-related genes identified to be differentially regulated by 6-BA treatment. Specifically, the gynoecium development-related genes SPATULA (SPT), KANADI 2 (KAN2), JAGGED (JAG) and Cytochrome P450 78A9 (CYP79A9) were significantly up-regulated, whereas the expression of PISTILLATA (PI), TATA Box Associated Factor II 59 (TAFII59) and MYB Domain Protein 108 (MYB108) that were important for male organ development was down-regulated in response to 6-BA treatment, demonstrating that cytokinin could directly target the floral organ identity genes to regulate the flower sex. Conclusions: Our work demonstrated that cytokinin is a potential regulator in female flower development in S. sebiferum. The transcriptome analysis of the floral sex transition from androecious to monoecious in response to cytokinin treatment on the androecious S. sebiferum provided valuable information related to the mechanism of sex determination in the perennial woody plants.

6-BA; TDZ; Floral feminization; Bioenergy plant; Sapium sebiferum

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