Evolution isn’t perfect. It is not a controlled or directed system, but instead just a result of adaptation to one’s environment through trial and error. Over a long period of time and generations, this trends toward organisms with better traits suited for their environment, because those with less suited traits from random mutations when born are less likely to survive. Many, many organisms die in the process.
The Random Variables of Natural Selection
Similarly, evolution doesn’t reach for perfection. The random variances of it are often curtailed by the fact that more efficiency may often require less productivity in the interim time period. Due to this, certain more efficient traits may be less likely or even just plain impossible to develop naturally, because of the reduced fitness in the meantime.
Without an actual controlled direction, the traits in a population usually just settle for “good enough” when it comes to matching their environment. If the developed abilities give an organism’s species a slight edge over the competition, there isn’t a need for an even stronger edge, especially if it costs more physical energy to do so. Just a slight advantage to be able to increase one’s population while exploiting a niche in the environment is often the result of such adaptations, with little further changes so long as the environment stays the same.
Because of this, there are a large number of biochemical process throughout the kingdoms of life that are fairly inefficient, but work well enough for the organisms themselves. But for our usage of them, even more importantly in agriculture, we want to maximize their potential efficiency.
What has this long spiel been about? Plants, more or less. Specifically, the process of photorespiration, a side mechanism to photosynthesis.
The primary purpose of this system is to recycle Ribulose-1,5-bisphosphate (RuBP) from the photosynthesis cycle. The enzyme that moderates this reaction, RuBisCO, adds in carbon dioxide to the RuBP and this then takes it through the rest of the Calvin Cycle. This results in additional energy byproducts and a chemical used in photosynthesis, all of which can be put back into the original photosynthetic cycle.
But, unfortunately, RuBisCO doesn’t always function properly. Depending on the oxygen to carbon dioxide ratio of the surrounding environment and the temperature (high temperatures are bad), the enzyme may instead add oxygen to the RuBP rather than carbon dioxide. The resulting oxygenation cycle is not efficient and actually costs a fair amount of energy products to complete. As a whole, it is believed that it reduces the efficiency of photosynthesis by 25% or more.
As noted before, even this setback is still good enough for most plants. They don’t have a need to correct this inefficiency, as photosynthesis still provides them with more than enough energy. The plants that live with this are known as type C3 carbon fixation plants, which details what type of photosynthetic process they use.
Improving Bad Systems
This isn’t the case for all plants, however. Some plants grow in environments where they needed to squeeze out as much energy as possible from photosynthesis due to the stressful, often arid, environments they grow in. These plants are known as C4 carbon fixation plants, and they add some extra steps to their RuBP processing.
Instead of just letting RuBisCO choose which element to add, these plants convert their uptaken carbon dioxide into another form, malate, and transport it to the region of the plants where RuBisCO functions. These regions are purposefully kept low in oxygen and the carbon dioxide is then removed from its altered form and given to the enzyme.
Such a system forces the RuBisCO to use carbon dioxide in its cycling and completely removes, for the most part, the inefficiency of photorespiration. This results in plants that are hardier and more capable of growing in extreme environments. Some plants we’ve grown to work like this include the most widespread, like sugar cane, corn, and sorghum.
So, now you know the basic biochemistry of the carbon fixation cycles. Let’s talk some new science.
Researchers at the University of Illinois have been looking into the inefficient parts of C3 plants. There are several important crops, like rice and soybeans, that are still in this category and that could use some sort of genetic modification to improve their photosynthetic efficiency.
As for the process of photorespiration itself, while we know about most of the enzymes involved in facilitating the cycle, there is very little known about the membrane transporters that move important chemicals used in the cycle in and out of the cell. The research group was able to identify a transport protein, however, called bile acid sodium symporter BASS6.
When using the model plant organism Arabidopsis thaliana, they were able to use a gene knockout method to stop the function of this transporter. In response, the plants showed a significant drop in photosynthetic capability and how much they grew. Further experimentation revealed that this transporter is involved in moving around the chemical glycolate, or glycolic acid, in particular.
Trading Up Efficiency
Glycolate is the acid form of the amino acid glycine and is involved in the photorespiration cycle. The scientists were able to confirm this response by not only knocking out the bile acid sodium transporter, but also another transporter involved in glycolate. Such a double knockout mutant has even worse growth and photosynthesis, along with having a noticeable buildup in cellular concentrations of glycolate.
This is bad for the cell, as glycolate is a toxic byproduct of photorespiration that needs to be transported out of the chloroplast cells. It is then taken to another part of the plant where it recycles into glycerate. The whole process isn’t very energy efficient, however.
With the identification of this transporter, in addition to the one already known one, the researchers are now working on building a more efficient system that is able to directly convert glycolate into glycerate without any of the other inefficient byproducts resulting. This includes ammonia which later has to be re-fixed by other processes, an especially inefficient segment of the system.
This won’t fix all the problems with the photorespiration cycle, but it does improve one of the significant drains on energy production. Even a small increase in efficiency will result in major yield increases worldwide with C3 crops.
Photo CCs: Jong plantje bij kunstlicht – Young plant in artificial light (5679580299) from Wikimedia Commons