How can we convert a C3 cycle to C4 cycle in plants by using Bioinformatics approaches?

Question

Please send me the whole phenomena of the conversion of C3 cycle to C4 cycle in plants in which Bioinformatics approaches are involved.

Answer 1

Salam! hope its late reply.

Most of our conventional crops, including rice and wheat, assimilate atmospheric CO2 by the C3 pathway of photosynthesis, which takes place in the mesophyll cells of leaves. Photosynthetically, these plants are underachievers because, on the one hand, they assimilate atmospheric CO2 into sugars but, on the other hand, part of the potential for sugar production is lost by respiration in daylight, releasing CO2 into the atmosphere, a wasteful process termed photorespiration. This is due to the dual function of the key photosynthetic enzyme, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). High CO2 favors the carboxylase reaction and thus net photosynthesis; whereas high O2 promotes the oxygenase reaction leading to photorespiration. When plants first evolved, photorespiration was not a problem because the atmosphere then was high in CO2 and low in O2. As a byproduct of photosynthesis, O2 accumulated in the atmosphere and reached the present level a million years ago. Current atmospheric CO2 levels limit photosynthesis in C3 plants. Furthermore, photorespiration reduces net carbon gain and productivity of C3 plants by as much as 40%. This renders C3 plants less competitive in certain environments. In contrast, with some modifications in leaf anatomy, some tropical species (e.g., maize and sugarcane) have evolved a biochemical "CO2 pump," the C4 pathway of photosynthesis, to concentrate atmospheric CO2 in the leaf and thus overcome photorespiration. Therefore, C4 plants exhibit many desirable agronomic traits: high rate of photosynthesis, fast growth, and high efficiency in water and mineral use.

Unfortunately, there are no closely related C3 and C4 crops that we can use to transfer the C4 traits to C3 crops by a traditional breeding approach. Thus, our research team has been interested in engineering the C4 traits in rice to enhance its productivity. In engineering C4 photosynthesis, there are two important components to be considered: the biochemical pathway (enzymes) and the specialized leaf structure. The coordination of two specialized leaf cells in C4 leaves, namely mesophyll and bundle sheath cells (together termed Kranz leaf anatomy), is important for pathway function. The enzymes and their corresponding genes involved in the C4 pathway of photosynthesis have been characterized. However, very little is known about the molecular mechanisms controlling the differentiation of Kranz leaf anatomy in C4 plants. Therefore, our first goal was to engineer the key enzymes involved in C4 photosynthesis in rice without Kranz leaf anatomy. At first thought, one may argue that rice plants thus engineered may not be very efficient in concentrating CO2 in the leaf, as Rubisco is located in the chloroplasts of the inner bundle sheath cells in C4 leaves. The cell wall of these well-differentiated inner cells has special constituents that prevent CO2 from leaking out of the leaf. However, in nature, a primitive aquatic plant, Hydrilla verticillata, is known to be able to use a simplified version of the C4 pathway (without Kranz leaf anatomy) to concentrate CO2 and eliminate the wasteful photorespiration process(1).

Using an Agrobacterium-mediated transformation system, we have independently introduced into rice three maize genes encoding the C4 photosynthetic pathway enzymes: phosphoenolpyruvate carboxylase (PEPC); pyruvate, orthophosphate dikinase (PPDK); and NADP-malic enzyme (ME)(2). The transgenic rice plants express high levels of these genes and the maize enzymes remain active. Most importantly, PEPC and PPDK transgenic rice plants exhibit higher photosynthetic capacity than untransformed plants, mainly due to an increased stomatal conductance (i.e., more atmospheric CO2 becomes available for fixation)(3). Preliminary field trials conducted in China and Korea also show 10-30% and 30-35% increases in grain yield for PEPC and PPDK transgenic rice plants, respectively. These results were totally unexpected since only one of the maize C4 pathway enzymes is being elevated in the transgenic rice plants and one would not expect this would be sufficient to concentrate CO2 as in a typical C4 plant. Indeed, direct fixation of atmospheric CO2 via these individual enzymes remains low in the transgenic rice plants. However, we believe increased synthesis of organic solutes (e.g., malate) by the enzymes in the guard cells may be responsible for the enhanced conductance of CO2 by the stomates since stomates open by pumping up their levels of solutes. In this regard, it is interesting to note that increased yields in new wheat cultivars, developed by CIMMYT in the past 30 years, are attributed to increased photosynthetic capacity, which is associated with an elevated stomatal conductance to CO2 diffusion.

A further enhancement of the photosynthetic capacity of rice will require engineering a limited C4 pathway of photosynthesis by simultaneously expressing the three previously mentioned key enzymes in proper cellular compartments. Using a conventional breeding approach, we have produced hybrids among the three transgenic lines. Tests for their photosynthetic and growth performance are underway. Ultimately, for most efficient operation of the pathway to concentrate CO2 around Rubisco in the leaf, the concomitant installation of Kranz leaf anatomy will be essential. More work is needed in order to convert the less efficient C3 rice to a more efficient "C4 rice."