Role of sulfur metabolism in acquiring of boron tolerance in Arabidopsis thaliana

The purpose of this study was to assess whether sulfate applications (pre- and combined) caused the changes of tolerance against boron (B) toxicity in Arabidopsis thaliana at physiological, biochemical and molecular levels. Plants were germinated and grown on MS medium for 12 days, and then, sulfate pre-treatment groups (PS-B) were transferred to sulfate-containing media for 3 days. Subsequently, PS-B, combined (S+B) and un-treated application groups were transferred to medium containing 3 and 5 mM B for 10 days. Leaf area, plant biomass and total chlorophyll content decreased significantly depending on the accumulation of B. Toxic levels of B adversely affected the structure and functionality of photosynthetic apparatus. It caused the changes in the specific and phenomenological energy fluxes, quantum yields, and the efficiencies of PSII's donor and PSI's acceptor sides. These changes let to decreases in the photosynthetic performance of plants. Although B stress adversely affects the integrity of the membrane and the amount of photosynthetic pigment in the antenna and reaction centers, it has been determined that this effect is not at a level to completely block the functionality of the photosystems, and this negative effect can be alleviated by sulfate applications. The increases in the antioxidant enzyme activities (especially POD) and, phenolic compounds with sulfate treatments may have provided tolerance against toxicity and oxidative damage by regulating the defense systems. Also, B toxicity affected the relative expression changes of genes related to sulfate uptake, transport and sulfur metabolism. In fact, SULTR 3;1, SULTR 3;3, SULTR 3,5 were more expressed in PS-B condition. The relative expressions of GSH1 and GSH2 genes were increased in treatment groups, indicating induced glutathione metabolism. In conclusion, plants may have acquired tolerance against B toxicity by assimilating the sulfate taken by especially provided by sulfate pre-treatment.