Question

Engineering an entire photosynthetic pathway is more complicated than simply inserting a gene for, say, an insecticidal protein into the genome of a crop. How does the fact that C4 photosynthesis has evolved naturally many times in the grass family (which includes rice) suggest that it may be possible to introduce this trait to rice

Answer 1

Photorespiration is the phenomenon which occurs in C₃ plants during low CO₂ concentration and high O₂ concentration. The mesophyll cells contain chloroplast where light reaction as well as fixation of CO₂ and  dark reaction occur inside the mesophyll cells. During optimum photosynthesis when inside the leaf, the O₂ conc. becomes high and CO₂ conc. becomes low, Rubisco acts as oxygenase.

In the past, due to low CO₂  conc. in the atmosphere, the C₄ plants have evolved in which the bundle sheath cells also contain chloroplast along with the mesophyll cells. They have evolved a mechanism of concentrating CO₂ , thereby maintaining a high concentration of CO₂  inside the leaf. Due to this, Rubisco can not become Rubp - oxygenase. In this way they escape photorespiration. Light reaction, evolution of O₂ and initial fixation of CO₂  takes place inside the mesophyll cells. Here PEPco( Phosphoenol pyruvate carboxylase ) is the enzyme for CO₂ fixation. Oxaloacetate produced is converted to malate which is transported to bundle sheath cells. Here malate dissociate evolving CO_{2 .} Final fixation of CO2 i.e Calvin cycle takes place inside the bundle sheath cells. Because of high conc. of CO₂ in Bundle Sheath cells Rubisco can not become oxygenase.

So, it is evident that in C₄ plants, there is a cell specific gene expression which can be exploited to make genetically engineered C₄ rice.

C₄ photosynthesis have evolved independently more than 60 times in different distantly related plant families. This capacity to mutate and evolve shows the possibility of improving the productivity of rice by making genetically engineered C₄ rice which has a C₃ pathway originally.

To make genetically engineered C₄  rice, the Rubisco should be confined to bundle sheath chloroplast and its activity in the cytosol of mesophyll cell should be reduced.

Enzymes which code for PEPco and   carbonic anhydrase in the mesophyll cells of C₄ plants should be expressed in the mesophyll of rice.

To transport malate to bundle sheath cells, some enzymes are responsible in C₄ plants. The genes coding those enzymes also have to be inserted for efficient transport mechanism.

Gene manipulation of bundle sheath cell is also necessary to make it specialized for performing Calvin cycle. This will be done for increasing the number of chloroplast and mitochondria of the bundle sheath cells.

Kranz anatomy is the special feature of C₄ plants which shows a close association of mesophyll cells and bundle sheath cells . This anatomy also can be created in rice plants  by introducing appropriate genes.