Therefore, the observed growth defects ofPGDH1-silenced plants appear to originate from Ser deficiency in specific cells, which are insufficiently supplied with Ser synthesized by photorespiration. When plants were grown at high CO2levels to inhibit photorespiratory Ser biosynthesis, leaf initiation was completely abolished in seedlings ofPGDH1-silenced plants (Physique 7).PGDH1and other genes of the PS pathway are predominantly expressed in cells of the SAM and RAM (Figure 4; seeSupplemental Physique 10online). inhibited in growth. Metabolic analyses ofPGDH1-silenced lines produced under ambient and high CO2conditions indicate a direct link between PS biosynthesis and ammonium assimilation. In addition, we obtained several lines of evidence for an interconnection between PS and tryptophan biosynthesis, because the expression ofPGDH1andPHOSPHOSERINE AMINOTRANSFERASE1is usually regulated by MYB51 and MYB34, two activators of tryptophan biosynthesis. Moreover, the concentration of tryptophan-derived glucosinolates and auxin were reduced inPGDH1-silenced plants. In essence, our results provide evidence for a vital function of PS biosynthesis for herb development and metabolism. == INTRODUCTION == Ser is an important amino acid that is an essential constituent of proteins, and is also a substrate for the biosynthesis of phosphatidylserine (Vance and Steenbergen, 2005;Nerlich et al., 2007), Trp (Tzin and Galili, 2010), and Trp-derived secondary metabolites such as auxin, indolic glucosinolates, and camalexin (Glawischnig et al., 2000;Gigolashvili et al., 2007;Sazuka et al., 2009;Vernoux et al., 2010). Furthermore, Ser functions as an important single-carbon (C1) donor in tetrahydrofolate (THF) metabolism in plants (Hanson and Gregory, 2002;Rbeill et al., 2007). Therefore, appropriate levels of Ser for each of these metabolic pathways have to be managed in almost all tissues to ensure proper plant development. In plants, photorespiration is DKFZp781H0392 usually thought to be the main route of Ser biosynthesis (Bauwe et al., 2010;Maurino and Peterhansel, 2010). Photorespiration, as a consequence of the oxygenase reaction of ribulose-1,5-bisphosphate carboxylase/oxygenase, results in one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). Via multiple reactions residing in chloroplasts and peroxisomes, 2-PG RA190 is usually converted to Gly, which is usually then metabolized to Ser in mitochondria by a glycine decarboxylase complex (GDC) and a serine hydroxymethyltransferase (SHMT), using THF as a coenzyme (Physique 1). Photorespiratory Ser biosynthesis requires photosynthesis and is thus restricted to photosynthetic tissues. Although plants are able to reallocate amino acids from source to sink tissue via the phloem (Winter et al., 1992), it remains unclear whether nonphotosynthetic cells can be sufficiently supplied with amino acids by phloem-mediated transport. == Physique 1. == Ser Biosynthesis Pathways in Plants. Ser can be synthesized by two major pathways: the PS pathway and the photorespiration pathway. The PS pathway starts with the oxidation of 3-PGA catalyzed by PGDH, yielding 3-PHP and reduced NADH. Phosphoserine (PSer) is usually synthesized by the transfer of an amino group from Glu to 3-PHP by PSAT. Finally, PS is usually dephosphorylated to Ser by PSP. Photorespiratory Ser biosynthesis starts with the synthesis of 2-PG by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. 2-PG is RA190 usually converted to Gly by several enzymatic reactions occurring in chloroplasts and peroxisomes. In mitochondria, Ser is usually synthesized from two molecules of Gly catalyzed by the GDC and SHMT. GDC catalyzes the decarboxylation of Gly by releasing CO2and NH3, yielding one molecule of 5,10-CH2-THF. Finally, Ser is usually synthesized by the transfer of the methyl group of 5,10-CH2-THF to an additional molecule of Gly, catalyzed by the SHMT enzyme. The genomes of several plant species encode enzymes for an alternative pathway of Ser biosynthesis, the glycolytic or phosphoserine (PS) pathway (Slaughter and Davies, 1968;Larsson and Albertsson, 1979;Ho and Saito, 2001). In prokaryotes and mammals, Ser is usually synthesized predominantly via the PS pathway using 3-PGA (Pizer, 1963;McKitrick and Pizer, 1980;Snell, 1984;Achouri et al., 1997;Dey et al., 2005), whereas in yeast (Saccharomyces cerevisiae), Ser can be produced either by the PS or the gluconeogenic pathway, depending on the available carbon source (Melcher and Entian, 1992;Sinclair and Dawes, 1995). The PS pathway consists of three enzymatic reactions (Physique 1). In the beginning, 3-PGA is usually reversibly oxidized to 3-phosphohydroxypyruvic acid (3-PHP) by phosphoglycerate dehydrogenase (PGDH). Subsequently, 3-PHP is usually converted by phosphoserine aminotransferase (PSAT) to PS and 2-oxoglutarate, using Glu as an amino group donor, and finally to Ser by a phosphoserine phosphatase (PSP). InArabidopsis thaliana,PGDH activity is usually encoded by three loci (PGDH1[At4g34200],PGDH2[At1g17745], andPGDH3[At3g19480]) and PSAT activity by two loci (PSAT1[At4g35630] andPSAT2[At2g17630]), whereas PSP is present as a single copy gene (PSP[At1g18640]). RNA gel blot analysis and in situ hybridization of some of the PS pathway genes inArabidopsisindicate a function of the pathway in both heterotrophic and autotrophic tissues (Ho et RA190 al., 1998,1999a,1999b;Ho. RA190