NH4+ induces antioxidant cellular machinery and provides resistance to salt stress in citrus plants

Abstract

NH4 + nutrition in Carrizo citrange (Citrus sinensis L. Osbeck × Poncirus trifoliata L) plants acts as an inducer of resistance against salinity conditions. NH4 + treatment triggers mild chronic stress that primes plant defence responses by stress imprinting and confers protection against subsequent salt stress. In this work, we studied the influence of NH4 + nutrition on antioxidant enzymatic activities and metabolites involved in detoxification of reactive oxygen species (ROS) to clarify their involvement in NH4 +-mediated salt resistance. Our results showed that NH4 + nutrition induces in citrus plants high levels of H2O2, strongly inhibits superoxide dismutase (SOD) and glutathione reductase (GR) activities, and leads to higher content of oxidised glutathione (GSSG) than in control plants in the absence of salt, thus providing evidence to confirm mild stress induced by NH4 + nutrition. However, upon salinity, plants grown with NH4 + (N-NH4 + plants) showed a reduction of H2O2 levels in parallel to an increase of catalase (CAT), SOD, and GR activities compared with the control plants. Moreover, N-NH4 + plants were able to keep high levels of reduced glutathione (GSH) upon salinity and were able to induce glutathione-S-transferase (GST) and phospholipid hydroperoxide glutathione peroxidise (PHGPx) mRNA accumulation. Based on this evidence, we confirm that sublethal concentrations of NH4 + might act as a mild oxidative stressor, which triggers antioxidant cellular machinery that can provide resistance to subsequent salt stress.