Figure 1.
Schematic representation of the synthesis of H2S and ethylene and its association with the S-assimilation pathway. In plants, absorption of S occurs through uptake of SO42− from the soil by roots. The assimilatory pathway is activated to produce APS from SO42− under catalysis by ATP-S, which is in turn reduced to SO32− via APR. Afterward, SO32− is reduced to S2−, which is used to produce H2S via catalysis by chloroplast-localized sulfite reductase. The subsequent catalyzation of S2− by OASTL yields Cys, which is the first stable compound in the S-assimilation pathway and the precursor for GSH and Met. Met is converted to SAM by SAM synthetase, from which ACC is synthesized by ACS, degradation of which by the ACO enzyme yields ethylene. Ethylene-induced H2S in turn regulates ethylene biosynthesis via the persulfidation of ACO. H2S can also be generated through degradation of Cys or through biosynthesis in mitochondria and cytosol by the enzymes CS, CAS, LCD, and DCD. Activity of LCD and DCD in the cytosol is accompanied by formation of pyruvate and NH3. ACC, 1-aminocyclopropane-1-carboxylic acid; ACO, ACC oxidase; ACS, ACC synthase; APR, APS reductase; APS, adenosine 5-phosphosulfate; ATP-S, ATP-sulfurylase; CAS, β-cyanoalanine synthase; Cys, cysteine; CS, cysteine synthase; DCD, D-cysteine desulfhydrase; GSH, glutathione reductase; LCD, L-cysteine desulfhydrase; NH3, ammonia; Met, methionine; S, sulfur; SAM, S-adenosyl methionine; SO42−, sulfate; SO32−, sulfite; S2−, sulfide; SiR, sulfide reductase; OASTL, O-acetylserine (thiol)-lyase. Blue arrows indicate upregulation and downregulation.