Arabidopsis (Arabidopsis thaliana) chloroplasts contain two O-acetyl-serine(thiol)lyase (OASTL) homologs, OAS-B, which is an authentic OASTL, and CS26, which has S-sulfocysteine synthase activity. In contrast with OAS-B, the loss of CS26 function resulted in dramatic phenotypic changes, which were dependent on the light treatment. We have performed a detailed characterization of the photosynthetic and chlorophyll fluorescence parameters in cs26 plants compared with those of wild-type plants under short-day growth conditions (SD) and long-day growth conditions (LD). Under LD, the photosynthetic characterization, which was based on substomatal CO2 concentrations and CO2 concentration in the chloroplast curves, revealed significant reductions in most of the photosynthetic parameters for cs26, which were unchanged under SD. These parameters included net CO2 assimilation rate, mesophyll conductance, and mitochondrial respiration at darkness. The analysis also showed that cs26 under LD required more absorbed quanta per driven electron flux and fixed CO2. The nonphotochemical quenching values suggested that in cs26 plants, the excess electrons that are not used in photochemical reactions may form reactive oxygen species. A photoinhibitory effect was confirmed by the background fluorescence signal values under LD and SD, which were higher in young leaves compared with mature ones under SD. To hypothesize the role of CS26 in relation to the photosynthetic machinery, we addressed its location inside of the chloroplast. The activity determination and localization analyses that were performed using immunoblotting indicated the presence of an active CS26 enzyme exclusively in the thylakoid lumen. This finding was reinforced by the observation of marked alterations in many lumenal proteins in the cs26 mutant compared with the wild type.

We thank Dr. Marika Lindahl and Dr. María Cruz González for providing us with the anti-PsbO and anti-Rubisco activase antibodies and Rocío Rodríguez from the Instituto de Bioquímica Vegetal y Fotosíntesis Protein Analysis Service for protein identification by mass spectrometry. This work was supported by the European Regional Development Fund through the Ministerio de Ciencia e Innovación (grant nos. BIO2010–15201, CSD2007\x{2013}00057, and AGL2009–07999) and the Junta de Andalucía (grant nos. P06–CVI–01737 and BIO–273). M.Á.B. was supported by fellowships from the Junta de Andalucía and the European Molecular Biology Organization.

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19 pages