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

Ammonium (NH) enhances plant defense mechanisms but can be phytotoxic as the sole nitrogen source. To investigate the impact of a balanced NH and NO ratio on plant defense parameters without adverse effects, cucumber plants (Cucumis sativus L.) were grown under control (14 mM NO + 2 mM NH) and elevated level of NH (eNH, 8 mM NO+ 8 mM NH). Plants subjected to eNH 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 eNH 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 eNH plants to powdery mildew infection. However, the expression levels of ACC OXIDASE 1 (ACO1) and RESPIRATORY BURST OXIDASE HOMOLOGS B (RBOHB) were lower in eNH plants, which was consistent with decreased NADPH oxidase activity and lower leaf HO levels. The biosynthesis of phenolics was enhanced, whereas the activities of polymerizing enzymes and lignin deposition were reduced by half in eNH plants. Besides, a significant effect on plant biomass under salt or drought stress has not been observed between control and eNH plants. These results showed that different defense pathways are distinctively affected by eNH treatment, and the NH to NO ratio may play a role in fine-tuning the plant defense response.