Poplar wood rays are involved in seasonal remodeling of tree physiology

Understanding seasonality and longevity is a major challenge in tree biology. In woody species growth phases and dormancy follow one another consecutively. In the oldest living creatures the annual cycle may run for more than a thousand years. So far, however, not much is known about the processes triggering reactivation from dormancy. In this study we focused on wood rays, which are known to play an important role in tree development. The transition phase from dormancy to flowering in early spring was compared with the phase of active growth in summer. Rays from wood samples of poplar (Populus x canescens) were enriched by laser microdissection and transcripts were monitored by poplar whole genome microarrays. The resulting seasonally varying complex expression and metabolite patterns were subjected to pathway analyses. In February, the metabolic pathways related to flower induction were high, indicating that reactivation from dormancy was already taking place at this time of the year. In July, the pathways related to active growth like lignin biosynthesis, nitrogen assimilation and defense were enriched. Based on ‘marker’ genes identified in our pathway analyses we were able to validate periodical changes in wood samples by qPCR. These studies, and the resulting ray database, provide new insights into the steps underlying the seasonality of poplar trees.