Genotype-dependent sulfur-related metabolic responses under salinity in rice

Soil salinity severely limits rice productivity by disrupting cellular homeostasis and inducing oxidative stress. Increasing evidence indicates that sulfur metabolism plays a central role in plant adaptation to salinity by sustaining glutathione biosynthesis and redox regulation. However, how rice genotypes differing in salinity tolerance adjust sulfur metabolism when exposed to a saline environment remains poorly understood. Here, we investigated genotype-dependent sulfur metabolic responses under a sulfur-containing saline condition designed to mimic ionic characteristics of saline environments, including the elevated sulfur concentrations typically associated with seawater infiltration. The interplay between sulfur metabolism and redox homeostasis was analyzed in four Oryza sativa ssp. japonica cultivars displaying contrasting salinity tolerance. Seedlings were exposed to a saline solution reproducing the ionic composition of salt-affected rice-growing soils, and early responses were characterized through integrated physiological, biochemical, and transcriptional analyses. Salinity induced a rapid accumulation of ROS in all cultivars after 4 h; however, cultivars showing higher tolerance under the experimental conditions exhibited a marked attenuation of ROS accumulation within 24 h, whereas cultivars showing lower tolerance under the experimental conditions, maintained ROS accumulation up to ∼170% above the control level. This response was associated with an approximately fourfold greater increase in total antioxidant capacity in tolerant than in sensitive cultivars at 24 h, accompanied by marked glutathione accumulation, reaching up to 2.3-fold over control levels in tolerant cultivars, and induction of the glutathione biosynthetic genes (γ-ECS and GS). Salinity also triggered a genotype-dependent remodeling of sulfur metabolism. Tolerant cultivars exhibited enhanced expression of sulfur assimilation genes (ATPS and APR), tissue-specific regulation of SULTR2;1, differential modulation of the miR395-SLIM1 regulatory module, and distinct sulfur metabolite profiles. Collectively, these findings suggest that, when exposed to a sulfur-containing saline environment, rice cultivars adopt distinct strategies for managing the increased sulfate availability. Here salinity tolerance was associated with enhanced utilization of sulfur for glutathione-associated responses and redox buffering as a genotype-dependent strategy during early acclimation to salinity.