New insights into molecular targets of salt tolerance in sorghum leaves elicited by ammonium nutrition

Abstract

This study investigated the proteome modulation and physiological responses of Sorghum bicolor plants grown in nutrient solutions containing nitrate (NO₃^-) or ammonium (NH₄⁺) at 5.0 mM, and subjected to salinity with 75 mM NaCl for ten days. Salinity promoted significant reductions in leaf area, root and shoot dry mass of sorghum plants, regardless of nitrogen source; however, higher growth was observed in ammonium-grown plants. The better performance of ammonium-fed stressed plants was associated with low hydrogen peroxide accumulation, and improved CO₂ assimilation and K⁺/Na⁺ homeostasis under salinity. Proteomic study revealed a nitrogen source-induced differential modulation in proteins related to photosynthesis/carbon metabolism, energy metabolism, response to stress and other cellular processes. Nitrate-fed plants induced thylakoidal electron transport chain proteins and structural and carbon assimilation enzymes, but these mechanisms seemed to be insufficient to mitigate salt damage in photosynthetic performance. In contrast, the greater tolerance to salinity of ammonium-grown plants may have arisen from: i.) de novo synthesis or upregulation of enzymes from photosynthetic/carbon metabolism, which resulted in better CO₂ assimilation rates under NaCl-stress; ii.) activation of proteins involved in energy metabolism which made available energy for salt responses, most likely by proton pumps and Na⁺/H⁺ antiporters; and iii.) reprogramming of proteins involved in response to stress and other metabolic processes, constituting intricate pathways of salt responses. Overall, our findings not only provide new insights of molecular basis of salt tolerance in sorghum plants induced by ammonium nutrition, but also give new perspectives to develop biotechnological strategies to generate more salt-tolerant crops.

Keywords: Nitrogen sources; Proteomics; Salt stress; Salt tolerance; Sorghum bicolor.