Effects of combined drought and pathogen stress on growth, resistance, and gene expression in young Norway spruce trees.

Abstract

Drought-induced mortality is a major direct effect of climate change on tree health, but drought may also affect trees indirectly by altering their susceptibility to pathogens. Here we report how a combination of mild or severe drought and pathogen infection affected growth, pathogen resistance, and gene expression in potted 5-year-old Norway spruce trees (Picea abies). After 5weeks of drought, trees were inoculated with the fungal pathogen Endoconidiophora polonica. Combined drought-pathogen stress over the next 8weeks led to significant reductions in growth of drought-treated trees relative to well-watered trees, and more so in trees subjected to severe drought. Belowground, growth of the smallest fine roots was most affected. Aboveground, shoot diameter change was most sensitive to the combined stress, followed by shoot length growth and twig biomass. Both drought-related and some resistance-related genes were upregulated in bark samples collected after five weeks of drought (but before pathogen infection), and gene expression levels scaled with the intensity of drought stress. Trees subjected to severe drought were much more susceptible to pathogen infection than well-watered trees or trees subjected to mild drought. Overall, our results show that mild drought stress may increase tree resistance to pathogen infection by upregulating resistance-related genes. Severe drought stress, on the other hand, decreased tree resistance. Because drought episodes are expected to become more frequent with climate change, combined effects of drought and pathogen stress should be studied in more detail to understand how these stressors interactively influence tree susceptibility to pests and pathogens.