Candidate regulators and target genes of drought stress in needles and roots of Norway spruce

Drought stress impacts seedling establishment, survival and whole-plant productivity. Molecular responses to drought stress have been most extensively studied in herbaceous species, mostly considering only aboveground tissues. Coniferous tree species dominate boreal forests, which are predicted to be exposed to more frequent and acute drought as a result of ongoing climate change. The associated impact at all stages of the forest tree life cycle is expected to have large-scale ecological and economic impacts. However, the molecular response to drought has not been comprehensively profiled for coniferous species. We assayed the physiological and transcriptional response of Picea abies (L.) H. Karst seedling needles and roots after exposure to mild and severe drought. Shoots and needles showed extensive reversible plasticity for physiological measures indicative of drought response mechanisms, including changes in stomatal conductance (gs), shoot water potential and ABA (abscisic acid). In both tissues the most commonly observed expression profiles in response to drought were highly correlated with ABA levels. Still, root and needle transcriptional responses contrasted, with extensive root-specific downregulation of growth. Comparison between previously characterized A. thaliana drought-response genes and P. abies revealed both conservation and divergence of transcriptional response to drought. In P. abies, transcription factors belonging to the bZIP AREB/ABF (ABA Response Element Binding/ABRE Binding Factors) ABA-dependent pathway had a more limited role. These results highlight the importance of profiling both above- and below-ground tissues and provide a comprehensive framework to advance understanding of the drought response of P. abies. The results demonstrate that short term, severe drought induces severe physiological responses coupled to extensive transcriptome modulation and highlight the susceptibility of Norway spruce seedlings to such drought events.