Barley (Hordeum vulgare) maintains tricarboxylic acid cycle activity without invoking the GABA shunt under salt stress

Plants maintain energy balance under salinity stress through increased respiration and energy use, processes also associated with reactive oxygen species generation. Although respiration imposes a high energy cost, mitochondrial respiration and the tricarboxylic acid (TCA) cycle activity are vital for ATP production and providing electron donors that drive ion exclusion and ROS detoxification. This study examined the molecular basis of respiratory responses to salinity in barley using physiological, biochemical, metabolomic, and proteomic analyses. Salt exposure resulted in sodium accumulation, decreased photosynthesis and biomass, and increased respiration. Metabolite profiling indicated activation of the TCA cycle, while proteomics showed increased abundance of all targeted TCA enzymes, including phosphoenolpyruvate carboxylase isoforms and succinate dehydrogenase. Enhanced pyruvate oxidation and accumulation of downstream metabolites suggested that the classical TCA cycle underpins barley's salinity tolerance. Conversely, reduced levels of 2-oxoglutarate and succinate implied limited activity of the GABA shunt. The absence of detectable arginine and ornithine, unlike their salt-induced increase in wheat, further indicated that the GABA shunt contributes minimally to barley's salinity tolerance. Overall, barley primarily relies on enhanced mitochondrial respiration and the canonical TCA cycle, rather than GABA shunt metabolism, to cope with salinity stress.