Drought-associated genes exhibit high constitutive expression in Quercus douglasii, a drought-tolerant California oak

Drought stress is a strong selective pressure for all plant species. Plants respond to water shortage through various strategies that confer drought tolerance. These strategies may be plastic responses that occur with the onset of stress or may comprise continuously expressed (constitutive) traits regardless of water availability. Here, we used RNA-seq to characterize transcriptional responses to dehydration in seedlings of a drought-tolerant oak, Quercus douglasii, from a local population in the Sierra Nevada Foothills in California. In the greenhouse, we subjected 24 seedlings from 6 maternal families to dry-down or well-watered treatments and prepared RNA libraries from tissue collected before and after each treatment (48 libraries). Our goals were to characterize the pattern of up- and downregulated genes in response to dehydration and to assess the extent to which this drought-tolerant species shows differential versus constitutive expression as a drought response strategy. We identified few differentially expressed genes in response to dehydration. Upregulated genes were related to known drought response functions, while downregulated genes were enriched for gene ontology terms related to growth and carbohydrate metabolism. We discovered high constitutive expression of many putatively drought-responsive genes that had been found to exhibit gene expression plasticity in a different oak species, which is drought-sensitive. This novel finding demonstrates the potential for constitutive expression of genes involved in drought stress to provide an additional mechanism of drought tolerance for some tree species, such as Q. douglasii.