Figure 1.

Metabolic Role of PEP in Plastids of Heterotrophic or Mixotrophic Tissues (i.e., Developing Seeds).

In wild-type plants (A), PEP can be imported from the cytosol by PPT, or it may be produced from 3-PGA by the glycolytic sequence involving PGyM and ENO. Both enzymes exist as plastidic and cytosolic forms. In the stroma, PEP together with erythrose 4-phosphate (E-4-P) can enter the shikimate pathway for the biosynthesis of aromatic amino acids and derived compounds, or after conversion to pyruvate by PK, it can be fed into the biosynthesis of fatty acids, isoprenoids, or branched-chain amino acids. Pyruvate may also be imported by a pyruvate transporter (PyT). Other transporters of the phosphate translocator family, such as GPT or the triose phosphate/PT (TPT), may import Glc6P or 3-PGA, respectively. Glc6P can be fed into OPPP and starch biosynthesis. Note that TPT is not likely to be expressed in heterotrophic tissues. The OPPP produces reducing equivalents in the form of NADPH required for anabolic reactions and metabolic intermediates, such as E-4-P. In mixotrophic plastids, 3-PGA and reducing equivalents can be produced by the Calvin cycle (reductive pentose phosphate pathway [RPPP]). By cytosolic glycolysis, imported sucrose can be metabolized to pyruvate, which is subjected to respiration in the mitochondria. In (B), the consequences of a deficiency in both PPT1 and ENO1 are shown. Most likely all metabolic pathways shaded in light gray within the plastids would be negatively affected, which would also feed back on processes taking place in the cytosol.