Effect of elevated ammonium on biotic and abiotic stress defense responses and expression of related genes in cucumber (Cucumis sativus L.) plants

Ammonium (NH4+) enhances plant defense mechanisms but can be phytotoxic as the sole nitrogen source. To investigate the impact of a balanced NH4+ and NO3- ratio on plant defense parameters without adverse effects, cucumber plants (Cucumis sativus L.) were grown under control (14 mM NO3- + 2 mM NH4+) and elevated level of NH4+ (eNH4+, 8 mM NO3- + 8 mM NH4+). Plants subjected to eNH4+ showed significantly increased shoot and root biomass by about 41% and 47%, respectively. Among the antioxidant enzymes studied, ascorbate peroxidase (EC 1.11.1.11) activity was increased up to 3.3 fold in eNH4+ compared with control plants, which was associated with enhanced resistance to paraquat. Upregulation of PATHOGENESIS RELATED PROTEIN 4 (PR4) and LIPOXYGENASE 1 (LOX1), accompanied by increased concentrations of salicylic acid and nitric oxide, conferred more excellent resistance of eNH4+ plants to powdery mildew infection. However, the expression levels of ACC OXIDASE 1 (ACO1) and RESPIRATORY BURST OXIDASE HOMOLOGS B (RBOHB) were lower in eNH4+ plants, which was consistent with decreased NADPH oxidase activity and lower leaf H2O2 levels. The biosynthesis of phenolics was enhanced, whereas the activities of polymerizing enzymes and lignin deposition were reduced by half in eNH4+ plants. Besides, a significant effect on plant biomass under salt or drought stress has not been observed between control and eNH4+ plants. These results showed that different defense pathways are distinctively affected by eNH4+ treatment, and the NH4+ to NO3-ratio may play a role in fine-tuning the plant defense response.