A transcriptional roadmap of the yearly growth cycle in Populus trees.

Populus species have adapted to many different boreal environments, characterized by fluctuating seasons. The environmental shifts throughout the year trigger molecular responses in trees, regulating crucial developmental processes. To study these molecular responses, we performed RNA sequencing on 207 samples from European aspen (Populus tremula) trees grown outdoors during different stages of their annual growth cycle, together with samples from hybrid aspen (Populus tremula × tremuloides hybrid T89) trees grown in controlled conditions mimicking seasonal changes in day length and temperature. This created a complete transcriptional roadmap of the yearly growth cycle of Populus trees. Co-expression network analyses produced 46 modules, 36 of which show a seasonal expression profile where many aspects were mimicked by indoor samples. However, several modules differed between outdoor and indoor conditions, indicating that important aspects of growth regulation are missed in experiments conducted under controlled conditions. The module networks identify gene hubs involved in season-specific molecular processes of Populus trees during the year. To make the dataset easily accessible, we developed POPUL-R (https://lauragarciaromanach.shinyapps.io/popul_r_mini/), a Shiny app enabling users to visualize gene expression data and create interactive networks. POPUL-R will be a valuable tool for the scientific community to explore the role of specific genes in the annual growth cycle of trees.