Figure 9.

Suggested Schemes for PGI1-Mediated Biosynthesis of MEP Pathway Isoprenoid-Derived CKs and GAs in Cells of Vascular Tissues, and FAs and Proteins in Mixotrophic Maturing Embryos of Arabidopsis.

In both cases, cytosolic G6P can be glycolytically converted to PEP and pyruvate, which can enter plastids via the PPT and BASS2 transporters, respectively. According to the scheme in (A), some of the cytosolic G6P can be incorporated into plastids through the hexose-P transporter (GPT) and subsequently glycolytically converted to GAP and pyruvate. Plastidial GAP and pyruvate are then metabolized by 1-deoxy-d-xylulose 5-phosphate (DXP) synthase (DXS) to products that enter the MEP pathway and fuel synthesis of isoprenoid hormones (Pulido et al., 2012; Pokhilko et al., 2015). DMAPP can be converted to ent-kaurene in a three-step process catalyzed by geranylgeranyl diphosphate (GGPP) synthase (GGPPS), ent-copalyl diphosphate synthase (CPS), and ent-kaurene synthase (KS). ent-Kaurene can be then transported to other parts of the plant or oxidized to GA₁₂, which is converted to bioactive GA in the cytosol by GA20ox and GA3ox. DMAPP can also be converted to CKs (Spíchal, 2012). In heterotrophic cells lacking plastid-localized phosphoglyceromutase (PGAM), enolase (ENO), and the triose-P translocator (TPT), 3PGA produced in the plastid via PGI1 can be exported to the cytosol via GPT or other nonspecific transporters then glycolytically converted into PEP and pyruvate, which can then enter plastids through the PPT and the BASS2 pyruvate transporter, respectively. According to the scheme in (B), some of the cytosolic G6P can be incorporated into plastids through GPT and converted to 3PGA by the Rubisco shunt, which involves reactions of the early steps of glycolysis (PFK and aldolase), reactions of the nonoxidative pentose phosphate pathway, phosphoribulokinase (PRK), and Rubisco. In plants lacking plastidial PFK, part of the 3PGA generated can be converted to GAP necessary for Ru5P production by means of plastidial GAPDH and PGK (highlighted in blue). 3PGA can leave plastids via the TPT, the GPT, or other nonspecific transporters and glycolytically converted to PEP and pyruvate in the cytosol, which can enter plastids, as indicated above. Alternatively, and/or additionally, plastidial 3PGA can be converted to pyruvate in plastids. The ATP required for FA synthesis can be generated from photosynthesis and the PK reaction in the plastidial compartment or obtained from the cytosol via the NTT transporter (Reiser et al., 2004). NADPH and NADH can be generated in plastids from the light photosynthetic and pyruvate dehydrogenase reactions, respectively. ALD, aldolase; TA, transaldolase; TK, transketolase; TPI, triose-phosphate isomerase; PDH, pyruvate dehydrogenase; HMBDP, (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate; IPP, isopentenyl diphosphate; CDP, ent-copalyl diphosphate. Enzymes are highlighted in red.