Molecular mechanisms underlying tree host-pathogen interactions under drought stress and subsequent rewatering in Eucalyptus grandis

• Abiotic stresses could impact plant diseases by affecting both hosts and pathogens. • We assess pathogen infection in Eucalyptus under drought stress and rewatering. • We investigate transcriptomic changes in both the host and the pathogen. • Mild drought stress enhanced disease progression due to fungal pathogen infection. • Transcriptomic analysis showed the complexity of tree response to combined stress. Abiotic stresses such as drought change plant-pathogen interactions by affecting both hosts and pathogens. Here, we aimed to unravel the molecular mechanisms underlying forest tree-pathogen interactions under drought stress and subsequent rewatering. We conducted glasshouse experiments involving infection by the stem canker-causing fungal pathogen Chrysoporthe austroafricana under drought stress and rewatering in Eucalyptus grandis and investigated host and pathogen transcriptomic changes using RNA-seq data from our current combined stress experiment as well as previous single stress studies. We found that mild drought stress enhances disease progression while, upon rewatering, pathogen infection delays recovery of leaf stomatal conductance. Transcriptomic changes in the host support increased susceptibility to the pathogen while the in planta fungal transcriptome suggests prioritization of survival in the drought-stressed host. Upon rewatering, changes in the host transcriptome suggest allocation of resources to stress responses at the expense of growth and carbohydrate storage while that of the pathogen indicate downregulation of some fungal metabolic pathways potentially because the pathogen takes advantage of changes in the host. Our study identified key molecular processes and genes that provide mechanistic insights into tree-pathogen interactions under abiotic stresses. This enables prediction of tree resilience under a changing climate and contributes towards future tree health improvement endeavours.