Transcriptome and Hormone Regulations Shape Drought Stress-Dependent Fusarium Head Blight Susceptibility in Different Barley Genotypes

Little is known about regulatory mechanisms that crop plants use to respond to combinations of abiotic and biotic stress. We analysed four barley genotypes under simultaneous Fusarium culmorum infection and drought stress by phenotyping for Fusarium Head Blight (FHB) disease, drought stress responses, hormone profiling and transcriptome analysis. FHB severity was host genotype-dependent, with moderately resistant cultivars Avalon and Barke showing increased FHB under drought, while drought did not further increase FHB severity in highly susceptible Morex and Palmella Blue. Transcriptome analysis revealed largely additive effects of single stresses, with drought-dominated regulation with increasing drought severity. Co-expression analysis connected abscisic acid and auxin to gene expression modules functionally enriched with stress-specific physiological responses. Stress-response genes, uniformly expressed across genotypes, were linked to pathogen defence, detoxification and drought adaptation, whereas a cluster of hundreds of moderately Fusarium-responsive genes was limited in up-regulation under combined stress, possibly explaining enhanced FHB severity under drought. A multiple linear regression model accurately predicted combined stress expression from single stress responses, demonstrating that gene regulation under combined drought and Fusarium stress is largely driven by additive effects of individual stresses.