Anabolic reaction sequences in chloroplasts (A,B) and root plastids (C,D) of wild-type (A,C) and cue1 mutant plants (B,D). In chloroplasts (A,B) CO2 is assimilated in the reductive pentose phosphate pathway (RPPP; Calvin-Benson cycle) via ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO;1). Its product, 3-PGA, is then converted to triose phosphates (TP) via the subsequent action of phosphoglycerate kinase and NADP glyceraldehydes 3-phosphate dehydrogenase (PGK/NADP-GAPDH; 2). TPs are exported via the TPT in counter exchange with inorganic phosphate (Pi) and subjected to Suc biosynthesis in the cytosol. Suc is the main transport sugar that is exported to the sinks via the phloem. In chloroplast glycolysis, 3-PGA can only be metabolized to 2-PGA via phosphoglycerate mutase (PGyM; 3), but not further to PEP. Under defined conditions, it can also enter the phospho-serine pathway of Ser biosynthesis. In leaves Ser is, however, mainly produced by photorespiration (PR) or is imported from non-green tissues, such as roots. TPs in the cytosol can undergo the glycolytic conversion to 3-PGA, involving PGK/NAD-GAPDH (5), and further to 2-PGA and PEP by the subsequent action of cytosolic PGyM (6) and enolase (ENO2; 7). PEP has to be imported to the stroma via PPT1 or PPT2 in counter exchange with either 2-PGA or Pi. The latter derive, for instance, from the shikimate pathway, where PEP and erythrose 4-phosphate (E-4-P) serve as precursors. End products of the shikimate pathway are the aromatic amino acids (AAA) Phe and Tyr, which are synthesized from the intermediate chorismate, and Trp, which is synthesized from anthranilate as well as phosphoribosyl pyrophosphate (PRPP) and Ser. AAA are exported via amino acid transporters (AAT). PEP can be further metabolized by plastidial (4) or cytosolic (8) pyruvate kinase (PK) yielding pyruvate. Outside the chloroplasts pyruvate is subjected to mitochondrial respiration. In the chloroplast stroma pyruvate can enter de novo fatty acid biosynthesis, the production of the branched-chain amino acids Leu and Val as well as the MEP pathway of isoprenoid biosynthesis. The third branched-chain amino acid Ile uses Thr as a precursor. In contrast to wild-type chloroplasts (A), chloroplasts from cue1 (B) suffer from a limitation in PEP provision and processes like the production of AAA via the shikimate pathway are impaired (green background) and thus rely on PEP supply by PPT2. Anabolic sequences with pyruvate as precursor would probably be less affected (light blue-green background) as pyruvate might also be supplied by pyruvate transporters (PyrT). In roots (C,D) reducing power and metabolic intermediates are provided by the oxidative pentose phosphate pathway (OPPP) with Glc6P as precursor and TP as end products. Cytosolic Glc6P deriving from the degradation of imported sucrose is provided to the plastid by the GPT in counter exchange with either TP or Pi. Glc6P can be converted to Fru6P by plastidial or cytosolic phosphoglucose isomerise (PGI; 9, 14), activated to Fru1,6P2 by phosphofructokinase (PFK; 10, 15), and cleaved by aldolase (11, 16) to TP, which are subsequently converted to 3-PGA, 2-PGA, and PEP by plastidial or cytosolic PGK/NAD-GAPDH (5, 12), PGyM (3, 6), and ENO (7, 13). Note that in root plastids ENO1 is present. The fate of PEP in plastidial and cytosolic metabolism of roots is similar as in leaves. However, a blocked PEP transport across the envelope of root plastids due to a knockout of PPT1 cannot be compensated by PPT2 (D), and would lead to an increased production rather than a depletion of products deriving from PEP and pyruvate (light red background). It is likely that processes like the OPPP or the phospho-serine pathway are also increased by feedback regulatory mechanisms.
Reticulate leaves and stunted roots are independent phenotypes pointing at opposite roles of the phosphoenolpyruvate/phosphate translocator defective in cue1 in the plastids of both organs. (2014)
Pia Staehr, et al. Front Plant Sci. 2014;5:126. Figure: F13.
