Introductions
C3 transferase (C3) is a promising treatment for glaucoma. In addition to reducing intraocular pressure (IOP), C3 can protect and regenerate retinal ganglion cells (RGC) after injury. However, these effects were limited by the short duration and uneven distribution of C3 effects.
PEPC Transformation of C3 Plants
Compared to Rubisco in C3 plants, PEPC in C4 plants has a higher affinity for airborne CO2 and is not inhibited by O2 competition. In early genetic transformation studies of C3 plants, the focus was on PEPC, a key enzyme for C4 photosynthesis. Twenty-fold in plants, overexpression of PEPC induced a 4- to 6-fold increase in cytoplasmic NADP-ME activity, thus promoting the release of CO from PEP carboxylation products in chloroplasts.
PEPCK Transformation of C3 Plants
Expression of the C4-type PEPC enzyme gene in C3 plants is only the first step in establishing an intracellular CO2 pump in C3 plants. Malate or aspartate formed under the action of PEPC moves via intercellular filaments from the chloroplast into the chloroplast of vascular sheath cells, and the release of CO2 from the chloroplast depends on the type of decarboxylase, either catalyzed by phosphoenolpyruvate carboxykinase (PEPCK) or by NADP malic enzyme, with PEPCK catalyzing the decarboxylation of OAA as the simplest way. Since PEPCK itself is a cytoplasmic enzyme, a segment of chloroplast transit peptide is also required to guide its expression in chloroplasts.
Transformation of C3 Plants by NADP-ME
NADP-ME is another key enzyme in the NADP-ME type C4 photosynthetic pathway. NADP-ME, located in the chloroplasts of vascular sheath cells, catalyzes the decarboxylation of malate or aspartate. NADP-ME activity is negatively correlated with chlorophyll content and photorespiratory activity.
PPT Transformation of C3 Plants
The efficient operation of the C4 photosynthetic pathway requires not only a series of C4 photosynthetic enzymes to catalyze the reaction, but also a variety of transmembrane transport processes, including the entry of pyruvate into the chloroplast via the pyruvate/H+ symporter in the chloroplast, and the entry of PEP produced in the chloroplast via the phosphoenolpyruvate/phosphate translocator (PPT). PPT is the only essential transporter protein for C metabolism and has been cloned from a variety of non-C4 tissues and analyzed for its low activity in the chloroplast endosomes and non-green plastids of C3 plants, which may be a factor limiting the tetracarbon dicarboxylic acid cycle.
Combinatorial Expression of C4 Photosynthetic Key Enzyme Genes in C3 Plants
Overexpression of individual C4 cycle enzyme genes in C3 plants is an attempt to genetically modify C3 plants for high light efficiency. Since the C4 cycle is accomplished by the synergistic action of multiple enzymes and transporter proteins, and there are endogenous C cycle enzymes and transporter proteins in C3 plants, only they are less active and expressed at different sites, thus increasing the expression of individual genes does not establish a complete C4 cycle in C3plants, and there is a risk of disturbing the normal metabolism of C3 plants. Since all C4 enzymes have different functions in C3 plants, the analysis of transbis(gene) and poly(gene) plants might identify an effective way to establish a single-cell C4 cycle in C:crops.
