Proline Accumulation in Barley Under Salinity Is ABA-Independent, but Relies on the Level of Oxidative Stress When Modulated by Mo and W Ions

The accumulation of proline, an important osmoprotective and antioxidant compound, is a key defense mechanism induced in plants in response to stress factors, including salinity, and is likely dependent on abscisic acid (ABA). However, in barley grown for 8 days under salinity conditions (125 mM NaCl), proline accumulation was not accompanied by changes in ABA content. Co-application of 0.5 mM molybdenum (Mo) significantly reduced NaCl-induced oxidative stress, as measured by H2O2, O2−, MDA, and chlorophyll content, and increased the activity of Mo-containing aldehyde oxidase (AO), an enzyme involved in de novo ABA synthesis. As a result, elevated ABA levels were observed, but proline content under salinity conditions was similar in Mo-treated and non-Mo-treated plants. In contrast, exposing plants to 0.5 mM tungsten (W), an antagonist of Mo, inhibited AO activity without significantly altering ABA content, while proline and oxidative stress marker levels increased dramatically under both non-saline and saline conditions. The observed changes in proline content are mainly due to modulation of the rate of synthesis and, to a lesser extent, the rate of degradation, as revealed by transcript abundance of P5CS1 and PDH, which encode D1-pyrroline-5-carboxylate synthetase and proline dehydrogenase, respectively. The results indicate that in barley grown under salinity conditions, proline accumulation is ABA-independent but depends on the level of oxidative stress modulated by Mo and W ions.