(CS-13-2) Drought induced changes in defence related phytohormone signalling pathways contribute to increased susceptibility to fungal pathogen infection in Eucalyptus

Body of Abstract: Plants are vulnerable to a combination of multiple biotic and abiotic stresses. In trees, this happens repeatedly over their long lifetime. The response of plants to combined stress conditions is often different from their responses to individual stresses. Thus, extrapolations from single stress studies may not fully predict responses to multiple stressors. Here, we aimed to unravel the molecular mechanisms underlying the response of forest trees to combined drought stress and pathogen infection. Our greenhouse experiment, which involved infection of two-year-old ramets of a moderately resistant E. grandis clone with the fungal pathogen Chrysoporthe austroafricana, indicated that mild drought stress increases susceptibility to the pathogen. To understand the molecular basis of such increased susceptibility, we conducted co-expression network analyses using RNA-seq data from the current experiment as well as a previous study involving C. austroafricana infection in a susceptible clone and the moderately resistant clone we used in the present study. We identified key modules which included genes in the salicylic acid, gibberellic acid, jasmonic acid, and ethylene signalling pathways. Changes in the expression of genes related to these pathways were previously found to be among the main differences defining susceptibility/resistance to C. austroafricana. In the current study, we identified key genes including known markers of these signalling pathways as well as genes not previously associated with susceptibility/resistance to this pathogen. The expression patterns of some of these genes were altered in the moderately resistant clone under combined stress conditions and resembled that of the susceptible clone exposed to pathogen infection. This suggests that these genes could be key players governing drought induced susceptibility. Overall, our study gives an insight into the molecular mechanisms of tree response to combined biotic and abiotic stresses. This enables prediction of tree resilience under a changing climate and contributes to future genetic improvement.