Plant-based oils make up an incredibly important part of agricultural research on both a food and fuel level. While we use many different kinds of oils from plants, they each have a meaningful niche in our food supply and, for some, as bioenergy fuel sources that are far cleaner to produce than other electricity generating methods. But one of the drawbacks of this system is that a large amount of plants are usually required to order to make the necessary amount of oils. So it is therefore understandable that research into how to increase oil production in key plants remains a major source of study.
Researchers from the Brookhaven National Laboratory in New York have been looking into how the class of molecules that oils are included within, known as lipids, are metabolically produced in plants as compared to yeast and animal cells. The process of breaking down lipids into fatty acids for energy is a part of the autophagy system and is unsurprisingly known as lipophagy. The entirety of autophagy is important for proper cell and whole organism growth and development, as tests in plants involving knockouts of the relevant genes has resulted in shorter life spans, reduced fertility, and greater susceptibility to stresses.
Of the two forms of autophagy in cells, macro and micro, only macro has seen any real scientific investigation due to its very direct and obvious impact on the target molecules. They are essentially wrapped up in a container called an autophagosome and moved to vacuoles for chemical degradation. This is often done on an organelle level for recycling of damaged or unneeded components.
Microautophagy, on the other hand, is on a smaller scale. It results in the direct engulfment of molecules into the lysosome for breakdown through the membrane of the lysosome invaginating or turning inside out. How this process is done on a genetic level is still obscure and what is known for plants is that microautophagy is used to degrade anthocyanins and starches, along with chloroplast components that aren’t working appropriately due to stress from oxidation.
On Lipid Metabolism
The past few years has seen new research suggesting the involvement of lipid metabolism with autophagy and storage mechanisms. In short, the given process is that energy is made from the beta-oxidation of fatty acids and that to have said molecules ready for such use, plants keep them as lipid droplets of triacylglycerol (TAG). If needed, these are hydrolyzed through a mechanism called lipolysis, creating fatty acids as desired. Mammals have similar ways of doing this, but they seem fundamentally different from how plant cells conduct the lipid breakdown.
Because lipid metabolism uses lipids to help sustain and build new organelle membranes, it is unneeded in the growing leaf cells and so there is a very limited amount of TAG in these regions. This is unfortunate since those very tissues is where oil buildup is desired for crop growers. However, it is known that disrupting either of the pathways used by this metabolism creates a buildup of TAG to some extent.
To better visualize and understand this, the researchers created a line of transgenic plants that were double knockouts for the background lipid pathways. Alongside this, a green fluorescent protein (GFP) marker protein fused to another that is targeted to lipid droplets was added, as were markers to detect the formation of autophagy structures. Then they set to work watching how lipids were stored and utilized in the different mutant plants.
On The Role of Autophagy
One of the first things discovered is that autophagy only becomes involved in TAG synthesis in mature tissues and not in young seedlings. This makes sense, as noted prior, since the tissues that would be broken down by autophagy would have to be older as it is for such organelle membrane-targeted breakdown to occur. It also appears to take place in the endoplasmic reticulum (ER), possibly because autosome production also occurs here.
Furthermore, disrupting autophagy itself only affected TAG synthesis that occurs in the light and not in the dark, as the latter is conducted with lipids in the thylakoids, a part of the chloroplast, instead. Overall, disrupting this process showed that normal light-based TAG creation and turnover of membranes was severely reduced when autophagy is taken out of the picture. In turn, this disruption also caused lower fatty acid creation for energy usage. It is unknown precisely how autophagy is involved in this later system, however.
Secondly, they tested these results in pure darkness, with the lipid droplets fluorescently marked. What they observed is that normally the droplets are taken up by the vacuole in order to be broken down through microautophagy. But when the autophagy pathway is blocked, these droplets instead accumulate in the cytosol, as they have no way to enter into the vacuoles through invagination without the autophagy proteins.
An Ongoing Area of Research
This buildup of lipid droplets would eventually be extremely negative for the health of the cell, but extremely beneficial for the production of oil in the plant. So long as the plant is harvested before the buildup kills it, then you have effectively made a boosted oil producing plant. Exactly as ordered.
While no actual testing has yet been done, hopefully the increased oil proves to be significant enough that the change can be generalized to all the different bioenergy crops and food oil crops that are grown. A single knockout of a gene in the right pathway can turn out to have an important effect and one that is extremely beneficial for farmers and the general public alike. Further research is needed to find out the rest of the system at play, as there are still a few dark areas whose mechanisms continue to be obscure. But this study is a big step forward toward that knowledge.
Photo CCs: Mouse Intestine Tissue Autophagy.jpg from Wikimedia Commons