Abstract
Serine metabolism in plants has been studied mostly in relation to photorespiration where serine is formed from two molecules of glycine. However, two other pathways of serine formation operate in plants and represent the branches of glycolysis diverging at the level of 3-phosphoglyceric acid. One branch (the glycerate – serine pathway) is initiated in the cytosol and involves glycerate formation from 3-phosphoglycerate, while the other (the phosphorylated serine pathway) operates in plastids and forms phosphohydroxypyruvate as an intermediate. Serine formed in these pathways becomes a precursor of glycine, formate and glycolate accumulating in stress conditions. The pathways can be linked to GABA shunt via transamination reactions and via participation of the same reductase for both glyoxylate and succinic semialdehyde. In this review paper we present a hypothesis of the regulation of redox balance in stressed plant cells via participation of the reactions associated with glycerate and phosphorylated serine pathways. We consider these pathways as important processes linking carbon and nitrogen metabolism and maintaining cellular redox and energy levels in stress conditions.
Highlights
The most studied in plants is the pathway of serine synthesis related to photorespiration
In Arabidopsis genome serine hydroxymethyltransferase (SHMT) is encoded by seven genes, the products of two of which are targeted to mitochondria with SHM1 being the main mitochondrial isozyme (Voll et al, 2006), while SHM2 cannot replace it during photorespiration, being targeted to the vascular tissue (Engel et al, 2011), SHM3 is a plastidial form (Zhang et al, 2010), SHM4 and SHM5 are cytosolic, and SHM6 and SHM7 are located in the nucleus and their function is unknown (Bauwe and Kolukisaoglu, 2003)
As it can be seen from the scheme of serine glycolysis, the pathway from 3-PGA to glycolate and formate results in the formation of NAD(P)H in PGA dehydrogenase (PGDH) reaction, and in methyleneTHF dehydrogenase reaction, while NAD(P)H is scavenged in the glyoxylate reductases (GRs) reaction, and ATP is synthesized in formyl-THF synthetase reaction
Summary
The most studied in plants is the pathway of serine synthesis related to photorespiration. It leads to the formation of bulk amounts of serine reaching in photosynthetic cells of C3 plants concentrations of the order of tens millimolar, e.g., in cytosol of illuminated spinach leaf the reported concentration is 7.5 mM and in stroma – 4.3 mM (Winter et al, 1994), in cytosol of illuminated barley leaf – 50 mM, in stroma – 20 mM decreasing by more than twofold in darkness (Winter et al, 1993), and in phloem sap of barley – 25 mM in the light and 20 mM in darkness (Winter et al, 1992), which is the highest concentration of amino acid except glutamate and aspartate. The phosphorespiratory pathway of serine formation can be active only in the tissues exhibiting high photorespiration, i.e., in photosynthetic cells of C3 plants
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