The response of Eucalyptus grandis to pathogen infection under drought stress and subsequent rewatering.

Teshome, D. T.*¹, Zharare, G. E.², Ployet, R.³, Naidoo, S.¹

¹ Department of Biochemistry, Genetics and Microbiology, Forestry and Agricultural
Biotechnology Institute, University of Pretoria, Lynwood Road, Pretoria, 0028, South Africa
² Department of Agriculture, University of Zululand, 1 Main Road Vulindlela,
KwaDlangezwa, 3886, South Africa
³ Biosciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37831, USA

Abiotic stresses such as drought could change plant-pathogen interactions by affecting both plant hosts and pathogens. In plants, the molecular changes due to stress combinations could be different from those due to any of the individual stresses. Hence, extrapolations from single stress studies may not fully predict responses to multiple stressors. Here, we aimed to unravel the molecular mechanisms underlying forest tree-pathogen interactions under drought stress and subsequent rewatering. We conducted greenhouse experiments involving infection by the stem canker-causing fungal pathogen Chrysoporthe austroafricana under drought stress and rewatering in Eucalyptus grandis. We found that mild drought stress enhances disease progression, while transcriptomic changes in the host suggest increased susceptibility to the pathogen. Using co-expression network analysis integrating RNA-seq data from the current combined stress experiment and previous single stress studies, we identified key genes, including the potential orthologs of WRKY41, SCL13, and ERF4, which could potentially regulate combined stress response in E. grandis. Upon rewatering, we found no detectable difference in disease progression between the well-watered-inoculated control and recovery from combined stress (RCS) treatment. RNA-seq analysis using stem samples collected after 1 day of rewatering suggested that trees could have allocated resources to stress responses at the expense of growth and carbohydrate storage. We found transcription factors (TFs) such as EgrNAC123, EgrMYB46, and a potential ortholog of WRKY22 among the genes specifically regulated during RCS. These TFs have been shown to be related to carbon starvation, cell wall formation, and defense responses in model plants and Eucalyptus, suggesting their importance in regulating the trade-off between growth and stress response during RCS. Our study identified key molecular processes and genes that provide mechanistic insight into tree-pathogen interactions under cycles of abiotic stress and recovery. This enables prediction of tree resilience under a changing climate and contributes towards future genetic improvement.

Keywords: combined stress, drought, Eucalyptus, forest tree, recovery, transcriptome