Regulation of NO-Generating System Activity in Cucumber Root Response to Cold

Nitric oxide (NO) functions as a signaling molecule in plant adaptation to changing environmental conditions. NO levels were found to increase in plants in response to low temperatures (LTs). However, knowledge of the pathways involved in enhanced NO production under cold stress is still limited. For this reason, we aimed to determine the role of different NO sources in NO generation in cucumber roots exposed to 10 °C for short (1 d) and long (6 d) periods. The short-term treatment of seedlings with LT markedly increased plasma membrane-bound nitrate reductase (PM-NR) activity and induced the expression of three genes encoding NR in cucumber (CsNR1-3). On the other hand, long-term exposure was related to both increased cytoplasmic NR (cNR) activity and induced expression of the CsARC gene, encoding the amidoxime-reducing component (ARC) protein. The decrease in nitrite reductase (NiR) activity and the higher NO₂^-/NO₃^- ratio in the roots of plants exposed to LTs for 1 d suggest that tissue conditions may favor NR-dependent NO production. Regardless of NR stimulation, a significant increase in NOS-like activity was observed in the roots, especially during the long-term treatment of plants with LT. These results indicate that diverse NO-producing routes, both reductive and oxidative, are activated in cucumber tissues at different stages of cold stress.