Floral bud differentiation and mechanism underlying androdioecy of Osmanthus fragrans

Sweet osmanthus is an androdioecious plant; however, the mechanism underlying pistil sterility in male plants is still unclear. Scanning electron microscopy showed that the structure of pollen grains in the stamens does not differ between the sterile cultivar 'Chenghong Dangui' and the fertile cultivar 'Huangchuan Jingui'. Triphenyltetrazolium chloride and fluorescein diacetate staining as well as in vitro culture experiments revealed that pollen grains were active in both cultivars, indicating that the stamens in both 'Chenghong Dangui' and 'Huangchuan Jingui' could develop normally. When the pistils of the fertile cultivar 'Huangchuan Jingui' differentiated, two protrusions formed on the inner side of the stamen primordium, and these gradually developed and fused together to form the ovary, style, and stigma. The pistil of the sterile cultivar 'Chenghong Dangui' also formed two protrusions on the inner side of the stamen during differentiation; however, instead of fusing, two fronds were formed. These results suggest that male sweet osmanthus are formed due to the abortion of pistils during the development of floral organs. Transcriptome sequencing revealed that the expression levels of carpel development gene CRC, AG, and AGL11 were significantly lower in 'Chenghong Dangui' compared with 'Huangchuan Jingui' at different flowering stages, which provide new insight in the molecular mechanism of pistil abortion in 'Chenghong Dangui'. CRC and AG may regulate each other to promote carpel development.