Heterologous Expression of Key C and N Metabolic Enzymes Improves Re-assimilation of Photorespired CO2 and NH3, and Growth.

We investigated the effect of the heterologous expression of phosphoenolpyruvate carboxylase (ZmPepcase), aspartate aminotransferase (GmAspAT), and glutamine synthetase (NtGS) on carbon (C) and nitrogen (N) metabolism in Arabidopsis (Arabidopsis thaliana). These transgenes were expressed either separately or in different combinations. The highest gains in shoot dry weight were observed in transgenic lines coexpressing all three genes. Tracer experiments using NaH14CO3 suggested that the coexpression of ZmPepcase, GmAspAT, and NtGS resulted in a higher flux of assimilated CO2 toward sugars and amino acids. Upon feeding the leaf discs with glycine-1-14C, transgenic lines evolved significantly lower 14CO2 levels than the wild type, suggesting that a higher reassimilation of CO2 evolved during photorespiration. Leaves of transgenic plants accumulated significantly lower ammonium without any significant difference in the levels of photorespiratory ammonium relative to the wild type, suggesting a higher reassimilation of photorespired NH3 Transgenic lines also showed improved photosynthetic rates, higher shoot biomass accumulation, and improved seed yield in comparison with wild-type plants under both optimum and limiting N conditions. This work demonstrates that the heterologous coexpression of ZmPepcase, GmAspAT, and NtGS reduced the photorespiratory loss of C and N with concomitant enhancements in shoot biomass and seed yield.