Tree mortality and susceptibility to pathogens often increases under drought conditions. The interactive nature of these stressors is thought to be related to the balance between sinks and sources of non-structural carbon (NSC), such as defence and osmoregulation versus photosynthesis, respectively. Here, we studied the transcriptional response of Norway spruce subjected to concurrent drought stress and H. annosum s.s. infection. Infection caused an up-regulation of defence-response genes as well as a down-regulation of photosynthesis-related genes under both drought and well-watered conditions. Genes involved in osmoregulation were highly expressed during concurrent drought and infection. The carbon (C) metabolism of well-watered inoculated (W-I) and drought-stressed inoculated (D-I) saplings was strikingly different. In W-I saplings, the up-regulated C-metabolism associated genes are considered to have a role in cell wall formation or modification, suggesting that C was involved in cell wall reinforcement as part of a defence response. By contrast, in D-I saplings, the up-regulation of C-metabolism associated genes was related to the degradation of starch and sucrose into glucose. Transcriptional data suggest that trees cope with drought and pathogen infection at the expense of depleting NSC reserves. Thus, exposure to both drought and infection is likely to exhaust C reserves and decrease the defence capacity of trees in the long run, increasing the risk of tree mortality.
Caballol et al. (Sat,) studied this question.