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

Abstract

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.