Study of the role of phosphorylated pathway of serine biosynthesis in arabidopsis development and its connections with nitrogen and carbon metabolism

Resumen

In plants three different serine biosynthesis pathways have been described: the glycolate pathway associated with photorespiration, and two nonphotorespiratory pathways, the glycerate pathway and the phosphorylated pathway of serine biosynthesis (PPSB). As the glycolate pathway is the most important, at least in quantitative terms, the role of the PPSB has been neglected until quite recently. This thesis provides insights into the role of the PPSB in Arabidopsis development and metabolism.. The PPSB uses 3-phosphoglycerate (3-PGA) as a precursor which can be supplied through the plastidial or the cytosolic glycolysis. It is assumed that the activity of these two glycolytic pathways is integrated through transport systems, which exchange glycolytic intermediates across plastidial membranes. It was unknown whether plastidial and cytosolic pools of 3-PGA can equilibrate in nonphotosynthetic cells. To solve this matter, we employed mutants of the plastidial glycolytic isoforms of glyceraldehyde-3-phosphate dehydrogenase (GAPCp), which express the Triose Phosphate Translocator (TPT) under the control of different promoters. TPT expression under the control of the 35S promoter complemented the vegetative developmental defects and metabolic disorders of GAPCp double mutants (gapcp1gapcp2). However, TPT expression under the control of the 35S, which is poorly expressed in the tapetum, hardly complemented gapcp1gapcp2 male sterility. Full vegetative and reproductive complementation of gapcp1gapcp2 was rescued only by transforming this mutant with a construct that carried the TPT under the control of the GAPCp1 native promoter. A negative anatomical correlation was found between TPT and GAPCp expression, which suggests that the translocator is inactive in heterotrophic cells where GAPCp is functionally significant. Our results indicated that the main function of GAPCp is to supply 3-PGA for anabolic pathways in heterotrophic cells. They also suggest a 3-PGA deficiency in the plastids of gapcp1gapcp2 and that 3-PGA pools between cytosol and plastid do not equilibrate in heterotrophic cells. In order to study the PPSB in Arabidopsis in depth, PSP1, the last enzyme of the pathway, was targeted. Lack of PSP1 activity delayed embryo development and led to aborted embryos, classified as early curled cotyledons. On the other hand, psp1.1psp1.1 mutants expressing PSP1 under the control of the 35S promoter (psp1.1psp1.1 35S:PSP1), which is poorly expressed in the anther tapetum, displayed a male sterile phenotype. Microspore development in psp1.1psp1.1 35S:PSP1 was arrested in the polarized stage. The tapetum of psp1.1psp1.1 35S:PSP1 also displayed delayed and irregular development. The PSP1 expression in the tapetum in critical microspore development stages suggests that PSP1 activity in this cell layer is essential for pollen development. PPSB also affects primary root development, by reducing cell division in the meristem and cell length in the elongation zone. A transcriptomics and metabolomics study showed that the PPSB plays a crucial role in plant metabolism by acting as a metabolic switch between carbon and nitrogen metabolism, and connecting glycolysis and the Krebs cycle with the biosynthesis of amino acids, and with sulfate and ammonium assimilation pathways