Combined Abiotic Stresses Repress Defense and Cell Wall Metabolic Genes and Render Plants More Susceptible to Pathogen Infection

Plants are frequently exposed to simultaneous abiotic and biotic stresses, a condition that induces complex responses, negatively affects crop productivity and is becoming more exacerbated with current climate change. In this study, we investigated the effects of individual and combined heat and osmotic stresses on Arabidopsis susceptibility to the biotrophic pathogen <i>Pseudomonas syringae</i> pv. <i>tomato</i> (<i>Pst</i>) and the necrotrophic pathogen <i>B</i><i>otrytis</i><i>cinerea</i> (<i>Bc</i>). Our data showed that combined abiotic and biotic stresses caused an enhanced negative impact on plant disease resistance in comparison with individual <i>Pst</i> and <i>Bc</i> infections. Pretreating plants with individual heat or combined osmotic-heat stress strongly reduced the expression of many defense genes including pathogenesis-related proteins (<i>PR-1</i> and <i>PR-5</i>) and the <i>TN-13</i> gene encoding the TIR-NBS protein, which are involved in disease resistance towards <i>Pst</i>. We also found that combined osmotic-heat stress caused high plant susceptibility to <i>Bc</i> infection and reduced expression of a number of defense genes, including <i>PLANT DEFENSIN 1.3</i> (<i>PDF1.3</i>), <i>BOTRYTIS SUSCEPTIBLE 1</i> (<i>BOS1</i>) and <i>THIONIN 2.2</i> (<i>THI2.2</i>) genes, which are important for disease resistance towards <i>Bc</i>. The impaired disease resistance against both <i>Pst</i> and <i>Bc</i> under combined abiotic stress is associated with reduced expression of cell wall-related genes. Taken together, our data emphasize that the combination of global warming-associated abiotic stresses such as heat and osmotic stresses makes plants more susceptible to pathogen infection, thus threatening future global food security.