Sodium hydrosulfide-mediated upregulation of nitrogen metabolism improves drought stress tolerance in pepper plants

Abstract

Although hydrogen sulfide (H2S) has been proven to promote plant tolerance to abiotic stress (including drought), the specific mechanism by which H2S mediates this process remains uncertain. The aim of the study was to fill this knowledge gap by investigating the potential of sodium hydrosulfide (NaHS) to improve drought tolerance in pepper plants through upregulating nitrogen metabolism-related enzymes and to gain insights into plant responses to drought stress. Pepper (Capsicum annuum) plants were grown at 80% and 40% of full field capacity. Reduced water availability led to a drastic decline in plant dry weight, leaf water potential (ΨI), relative water content (RWC), PSII operation efficiency (chlorophyll fluorescence Fv/Fm ratio), and chlorophyll content. Soluble protein and leaf and root nitrogen (N) levels were also lowered. Simultaneously, drought has significantly increased the levels of hydrogen peroxide (H2O2) and malondialdehyde (MDA), and resulted in increased content of nitrate (NO3-) and ammonium (NH4+). Also increased were the amount of free amino acid, soluble sugars, glycine betaine and free proline. The activities of peroxidase (POD), glutathione S-transferase (GST), and catalase (CAT) were elevated, as were those of endogenous hydrogen sulfide (H2S), L-cysteine desulfhydrase (L-DES), and methylglyoxal (MG). When 0.2 mM sodium dihydrosulfide (NaHS, a H2S donor) was sprayed on drought-affected plants, oxidative stress markers were reduced, as well as NO3-, NH4+ and free amino acid levels. Furthermore, NaHS improved endogenous H2S without changing L-DES activity. At the same time, NaHS activated nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthetase (GOGAT) and glutamine dehydrogenase (GDH) activities, as well as enzymes related to the glyoxalase system (Gly I and Gly II) thus improving nitrogen metabolism. This resulted in an increased plant growth, photosynthetic attributes, and N content. To demonstrate the causal role of H2S in drought tolerance, plants were administered an H2S scavenger, hypotaurine (HT), along with NaHS. HT treatment negated beneficial effects of NaHS by lowering H2S content without changing L-DES activity. Thus, exogenous application of NaHS could offer a simple and economical alternative for crop productivity in environments with restricted water supply.