Limited sulfur resource forces Arabidopsis thaliana to shift towards non-sulfur tolerance under cadmium stress

Soils in some geographical regions suffer with combined stress of sulfur (S) deficiency and cadmium (Cd) contamination. Although the independent impacts of Cd and S-deficiency on plants are well studied but there are rare reports on synergistic effects of S-deficiency and Cd stress. Thus, this study focuses to investigate the response of Arabidopsis thaliana in terms of defense and growth as influenced by Cd under limited S regime. A. thaliana (Col-0) was grown on S-sufficient MS media for 2 weeks and then subjected to S-deficiency for 15 days. Control (+S/−Cd) and S-starved (−S/−Cd) plants were exposed to Cd (50 μM CdCl₂) for 3–5 days. Results show that S-deficiency (−S/−Cd) induces oxidative stress which was much lesser than Cd (+S/+Cd) but highest in combined stress of S-deficiency along with Cd (−S/+Cd). Interestingly, plant was found to elevate glutathione (GSH) biosynthetic pathway and also improved growth and antioxidative status when sulfur was present during Cd stress (+S/+Cd). Important studies in terms of photosynthetic parameters also support limited loss in +S plants as S-assimilation pathway was up-regulated. Proline accumulation was not influenced much by S-deficiency but stimulated with Cd stress strongly suggesting defense shift towards non-sulfur tolerance mechanism. Levels of glutathione and H₂O₂ removing catalase were also modulated to cope with oxidative stress in a better manner during S-sufficient conditions. Chloroplast ultrastructure showed loss of grana under S-deficiency, however, −S/+Cd resulted in severe disintegration of thylakoids too. Biomass accumulation was also most adversely affected with −S/+Cd followed by Cd stress alone (+S/+Cd) and S-deficiency (−S/−Cd). In conclusion, Arabidopsis maintains equilibrium between defense and growth and thus survive under limited S resource. Also S-assimilation is modulated by Cd stress and Cd-induced stress is prevented by S-nutrition.