Overwintering the boreal winter: From seasonal gene expression and photosynthetic responses to acclimation of conifers

The northern boreal forest is dominated by evergreen conifers. The needles of the most abundant species of the European boreal forest (Pinus sylvestrys and Picea abies) are able to survive the extremely cold (freezing) temperatures during the long dark winter and fully recover during the spring. Conifer needles possess potent regulatory processes that protect its photosynthetic apparatus during the harsh boreal winter (very low temperatures and very low light) and spring (low temperature and high light). However, the precise molecular and/or physiological mechanisms of this plasticity are not well understood. In this manuscript we discuss the seasonal adaptation of photosynthetic machineries of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) and suggest possible mechanisms allowing them to survive the harsh Scandinavian winter. First, we measure seasonal changes of gene expression (using RNA-Seq) of the major photosynthetic components in Norway spruce. Secondly, we use chlorophyll fluorescence to profile photosynthetic activities during two consecutive winters in needles of both Norway spruce and Scots pine. The functional performance of photosynthetic electron transport (Photosystem II and Photosystem I photochemistry) indicate the highly dynamic structural and functional seasonal rearrangement of the photosynthetic apparatus involved in balancing the electron flow through linear and PSI-dependent cyclic electron transfer pathways during the seasonal transitions. The involvement of a number of alternative electron transport pathways including the PTOX- and flv-dependent electron sinks during the overwintering period and recovery of photosynthesis in the spring is also established.