Dynamic association of the plastid localized cysteine synthase complex is vital for efficient cysteine production, photosynthesis, and granal thylakoid formation in transgenic tobacco.

This article investigates the effects of expressing active or inactive serine acetyltransferase (SAT) enzymes in plastids and cytosol of transgenic tobacco (Nicotiana tabacum) on cysteine biosynthesis and plant physiology. The cysteine synthase complex (CSC), formed by SAT and O-acetylserine-thiol-lyase (OAS-TL), regulates cysteine production, a key entry point for reduced sulfur into plant metabolism. Tobacco lines expressing active SAT in plastids (PSA) showed increased plastid cysteine but exhibited impaired granal thylakoid formation, reduced photosynthetic capacity, and fewer chloroplasts, whereas cytosolic expression of active SAT (CSA) did not cause these phenotypes despite similar thiol accumulation. Expression of enzymatically inactive SAT capable of forming the CSC in plastids (PSI) induced similar phenotypes to PSA lines without increasing SAT activity, indicating that CSC formation itself, rather than SAT enzymatic activity or altered redox state, triggers signaling that affects sulfur assimilation and photosynthesis. These findings suggest a compartment-specific regulatory role of the plastid CSC in controlling sulfur metabolism and chloroplast development in tobacco.