PHOTOSYNTHESIS AND FLUORESCENCE QUENCHING, AND THE MESSENGER-RNA LEVELS OF PLASTIDIC GLUTAMINE-SYNTHETASE OR OF MITOCHONDRIAL SERINE HYDROXYMETHYLTRANSFERASE (SHMT) IN THE LEAVES OF THE WILD-TYPE AND OF THE SHMT-DEFICIENT STM MUTANT OF ARABIDOPSIS-THALIANA IN RELATION TO THE RATE OF PHOTORESPIRATION

The regulation by photorespiration of the transcript level correspondi ng to plastidic glutamine synthetase (GS-2) was investigated in the le aves of Arabidopsis thaliana (L.) Heynh.. Photorespiration was suppres sed by growing the plants in an atmosphere containing 300 Pa CO2. Supp ression of photorespiration was demonstrated by the ability of the con ditionally lethal serine hydroxymethyltransferase (SHMT)-deficient stm mutant of A. thaliana to grow normally under these conditions. In con trast to previous studies with bean or pea that were performed at very high CO2 partial pressure (2-4 kPa; Edwards and Coruzzi, 1989, Plant Cell 1: 241-248; Cock et al., 1991, Plant Mol Biol 17: 761-771), suppr ession of photorespiration during growth of A. thaliana in an atmosphe re with 300 Pa CO2 had no effect on the leaf GS-2 transcript level. In the short term, neither suppression of photorespiration induced by th e transfer of air-grown A. thaliana plants into a CO2-enriched atmosph ere, nor an increase in the rate of photorespiration achieved by the t ransfer of high-CO2-grown A. thaliana plants into air resulted in a ch ange in the GS-2 mRNA level. The absence of photorespiratory ammonium release in leaves of the stm mutant had no effect on the GS-2 transcri pt level. Overall, our data argue against a control by photorespiratio n of the A. thaliana leaf GS-2 mRNA pool. In contrast, regulation of t he leaf SHMT mRNA level may involve a negative feedback effect of at l east one metabolite derived from the glycine/serine conversion during photorespiration, as indicated by the overexpression of SHMT transcrip ts in the leaves of the stm mutant.