of essential biomolecules, such as vitamins, cofactors, antioxidants and many defence compounds, that are formed in response to adverse environmental conditions in which the sulfur moiety is the functional group. Sulfate is transported inside the plant, reduced to sulfide and incorporated into O-acetylserine (OAS) to form cysteine. The enzymes used in the last step of cysteine biosynthesis, serine acetyltransferase (SAT, also called SERAT) and O-acetylserine(thiol)lyase (OASTL), form a complex that senses the sulfur nutrient status of the plant and modulates cysteine biosynthesis. Arabidopsis thaliana contains different OASTL and SAT enzymes in the different cellular compartments, resulting in different sub-cellular pools of cysteine. In recent years, significant progress has been made in determining the specific roles of the different sub-cellular pools of cysteine, as well as its associated metabolites. In chloroplasts, OAS can serve either as the substrate of the OASTL isoform OAS-B in the stroma to produce cysteine, or as the substrate of the OASTL isoform CS26 in the thylakoid lumen, which uses thiosulfate instead of sulfide to produce S-sulfocysteine. S-sulfocysteine is a signalling molecule that is involved in the regulation and protection of the photosystems of the plant. In the mitochondria, cysteine is essential for detoxifying cyanide by a loop that is established by the activities of the OASTL isoforms CYS-C1 and OAS-C. Mitochondrial cyanide regulates the root hair development and modulates the plant immune response. The main OASTL isoform, OAS-A1, and DES1, an OASTL isoform that acts as a cytosolic desulfhydrase and degrades cysteine, contribute to the homeostasis of the cytosolic cysteine and sulfur. Cytosolic sulfide acts as a signalling molecule, regulating the process of autophagy, while cytosolic cysteine is essential for inducing the hypersensitive response to pathogens. Thus, cytosolic cysteine homeostasis contributes to orchestrating the plant response to pathogens

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