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Cysteine biosynthesis is essential for translation and represents the entry point of reduced sulfur into plant metabolism. The two consecutively acting enzymes serine acetyltransferase (SAT) and O -acetylserine-thiol-lyase catalyse cysteine production and form the cysteine synthase complex, in which SAT is activated. Here we show that tobacco (Nicotiana tabacum) expressing active SAT in plastids (referred to as PSA lines) shows substantial cysteine accumulation in plastids. Remarkably, enhanced cysteine production in plastids entirely abolished granal stack formation, impaired photosynthesis capacity, and decreased the number of chloroplasts in mesophyll cells of the PSA lines. A transgenic tobacco line expressing active SAT in the cytosol accumulated comparable amounts of thiols but displayed no phenotype. To dissect the consequences of cysteine synthase complex formation from enhanced SAT activity in tobacco plastids, we expressed an enzymatically inactive SAT that can still form the cysteine synthase complex in tobacco plastids (PSI lines). The PSI lines were indistinguishable from the PSA lines, although the PSI lines displayed no increase in plastid-localized SAT activity. Neither PSA lines nor PSI lines suffered from an oxidized redox environment in plastids that could have been causative for the disturbed photosynthesis. From these findings, we infer that the association of the plastid cysteine synthase complex itself triggers a signaling cascade controlling sulfur assimilation and photosynthetic capacity in leaves. [ABSTRACT FROM AUTHOR]