Natural annual transcriptome dynamics of Eucalyptus reveal seasonal adaptation of tropical/sub-tropical trees.

Seasonal environment cues are primary factors that influence a plant's growth and adaptation. The molecular basis of seasonal phenology has been well studied in trees growing in boreal and temperate ecosystems. However, little is known about the molecular phenology of trees belonging to tropical/sub-tropical regions. Here, we characterize the annual transcriptome dynamics of Eucalyptus dunnii, one of the world's most widely planted tropical/sub-tropical hardwoods, in natural environments. Our transcriptome analysis combined with the geographical distribution, environmental cues, microscopic observations and heterologous transformation analyses, provides a molecular timetable of seasonal regulatory events of E. dunnii and its planting prospects in China. We further investigated the molecular mechanisms of the flowering phenology of E. dunnii. Our results suggest that low temperature is one of environment triggers for its seasonal flowering. In addition, a comparative transcriptome and cell ultrastructure analysis between Eucalyptus and Populus reveals molecular bases of different shoot apex growth habits of trees originating from tropical/sub-tropical and boreal/temperate regions. Our study will provide cues for further investigating the molecular mechanisms underlying the seasonal phenology of trees from tropical/sub-tropical regions.