Metabolic engineering of the serine/glycine network as a means to improve the nitrogen content of crops

In plants, L-serine (Ser) biosynthesis occurs through various pathways and is highly dependent on the atmospheric CO₂ concentration, especially in C₃ species, due to the association of the Glycolate Pathway of Ser Biosynthesis (GPSB) with photorespiration. Characterization of a second plant Ser pathway, the Phosphorylated Pathway of Ser Biosynthesis (PPSB), revealed that it is at the crossroads of carbon, nitrogen, and sulphur metabolism. The PPSB comprises three sequential reactions catalysed by 3-phosphoglycerate dehydrogenase (PGDH), 3-phosphoSer aminotransferase (PSAT) and 3-phosphoSer phosphatase (PSP). PPSB was overexpressed in plants exhibiting two different modes of photosynthesis: Arabidopsis (C₃ metabolism), and maize (C₄ metabolism), under ambient (aCO₂) and elevated (eCO₂) CO₂ growth conditions. Overexpression in Arabidopsis of the PGDH1 gene alone or PGDH1, PSAT1 and PSP1 in combination increased the Ser levels but also the essential amino acids threonine (aCO₂), isoleucine, leucine, lysine, phenylalanine, threonine and methionine (eCO₂) compared to the wild-type. These increases translated into higher protein levels. Likewise, starch levels were also increased in the PPSB-overexpressing lines. In maize, PPSB-deficient lines were obtained by targeting PSP1 using Cas9 endonuclease. We concluded that the expression of PPSB in maize male gametophyte is required for viable pollen development. Maize lines overexpressing the AtPGDH1 gene only displayed higher protein levels but not starch at both aCO₂ and eCO₂ conditions, this translated into a significant rise in the nitrogen/carbon ratio. These results suggest that metabolic engineering of PPSB in crops could enhance nitrogen content, particularly under upcoming eCO₂ conditions where the activity of GPSB is limited.