The functional interplay between upstream and downstream thiol cascades was investigated in a cysteine-deficient mutant (cysLc1), a catalase-deficient (catLc2) mutant, a double mutant (showing deficiency of both cysteine (Cys) (upstream) and catalase (downstream antioxidant defense) (cysLc1/catLc2)), and the VL125 genotype of lentil under the following treatments: a) 50 µM sodium arsenate (As); and b) As + 1 mM L-buthionine-sulfoximine (BSO). Both cysLc1 and the double mutant experienced As-induced oxidative stress due to the lack of responsiveness of the entire thiol cascade and antioxidant defense. Contrastingly, the catLc2 mutant and VL125 exhibited As tolerance. Under the As + BSO treatment, glutathione (GSH) synthesis was inhibited, but sulfate transport and Cys synthesis were differentially regulated in the four genotypes. The cysLc1 and catLc2 mutants induced Cys desulfuration pathways and generated huge endogenous hydrogen sulfide, stimulating ascorbate-mediated antioxidant defense and catalases as an alternative mechanism of As tolerance under low GSH redox. The nonresponsiveness of this alternate route, coupled with a crippled ascorbate-mediated antioxidant defense, led to huge Cys build-up and ROS overaccumulation in VL125 and the double mutant, which consequently experienced Asinduced growth inhibition. The study indicated that a metabolic diversion in an upstream thiol cascade through Cys desulfuration is imminent for Cys homeostasis and modulation of the downstream antioxidant defense against As toxicity when the Cys consumption route towards GSH is blocked. © TÜBİTAK.