The tannosome produces a bitter taste in plants.
There are some scientific papers that get massive media attention and spark heated debates, becoming the topic of conversation for weeks. However, there are also papers of enormous scientific value that do not get the attention they deserve. While the majority of people were buzzing about cancer research, zebrafish, and Alzheimer’s disease, most have missed a wonderful event—the discovery of a new plant organelle. It is always exciting when such discoveries happen in established and seemingly ‘researched enough’ fields. So, what is the new organelle, who discovered it, and why do we care?
The new organelle, named tannosome, is responsible for the production of condensed tannins in vascular plants. Condensed tannins, also called proanthrocyanidins, are present in the majority of vascular plants and serve multiple functions. They are important for protection against herbivores due to their bitter taste, and they provide defence against UV radiation and pathogens. For many years, researchers had proposed that tannins originate in the endoplasmic reticulum of plant cells. However, no ultrastructural studies had been performed to investigate this question. This is why a French research team decided to investigate the problem.
Scientists had already known that structurally condensed tannins are polymers of catechins. Catechins themselves are a broad family of aromatic compounds, meaning that they have ring structures that resemble benzene. Since all aromatic compounds in plants are produced in chloroplasts through the shikimate pathway, the research team decided to examine chloroplasts of many different plants for a possible site of tannin production. They found that not all chloroplasts were the same. Some were classic ‘textbook’ chloroplasts, while others appeared to be more swollen and circular in appearance. Not only did those chloroplasts look different, they also released vesicular structures in cytosol (intracellular space).
Further examination revealed that those vesicles were sectioned at regular intervals into small spheres filled with material that turned black upon staining from osmium tetroxide. Those were the tannosomes and these vesicles packed with the tannosomes ended their journey by penetrating into the vacuole.
Why do we care? First, this finding demonstrates the fact that even some of the most studied areas still retain a lot of surprises. Second, tannins are major components of wines and teas, adding bitterness in taste and contributing to viscosity. Perhaps in the future, we could be enjoying a glass of wine or a cup of tea made from plants with modified tannins. This has the potential to make those drinks taste better, add more health benefits, and possibly lead to developing new varieties.
What is next? Now that we know where the tannins are made, we can start learning more about the way they are polymerized. This can lead to a number of applications, from smoother wines to the invention of novel biodegradable materials. I am sure that the French research team is ecstatic about the former and hopeful about the latter. Their work has just started, but the possibilities are only limited by imagination.