Discovery of a new organelle: Introducing the tannosome

With all the advances in scientific technology over the last 50 years, sometimes it’s hard to believe there are still discoveries to be made. Particularly in areas that have already received a lot of study, like the components that make up a cell. Most eleven-year-olds should be able to list at least a few different organelles that are found in cells (the cell membrane, nucleus, and cytoplasm spring to mind immediately), even if those organelle aren’t necessarily part of the same type of cell. We have had our known list of cell parts for decades; it’s printed in elementary textbooks around the world. It’s done and dusted. Nothing more to discover here, folks, move along. Right?

Wrong.

As Marcel Proust once said, “The voyage of discovery is not in seeking new landscapes but in having new eyes.” And that is exactly the approach taken by a group of European scientists which led to the discovery of a new plant organelle: the tannosome.

Grapes on the vine. Sonoma, California

You may have heard the word “tannin” used to describe the aftertaste of tea or wine, or to explain the dark, tea-colored water of streams that flow through areas dominated with conifers or mangroves. Tannins are a type of polyphenol that serve many purposes for plants, including protection from predators (because they taste very bitter), regulating plant growth, and protecting plants from ultraviolet radiation. They are what give things like tea, pomegranates, and under-ripe raspberries their mouth-puckering, “dry” effect.

Tannins will form bonds with proteins, cellulose, starches, and even minerals to create substances that resist decomposition. In fact, tannins from trees like oak, maple, and mangrove were used by ancient and indigenous cultures to preserve animal skins for clothing and blankets. This is where the term “tanning” (as in tan a deer hide) originates. But, until this past year, it hasn’t been clear just where tannins themselves come from.

In an article published in the Annals of Botany (September 2013), researchers explain how they discovered the new organelle. They began by using the parts of a grape plant where tannins are at their highest concentration: the pistils, immature small fruits, leaflets, and pedicels. These are also the organs containing the highest chlorophyll content. These plant organs were then examined by transmission electron microscopy (TEM), where they observed small, dark circular and granular objects within the cell’s vacuole. After treating cells with a stain, they noticed that the vacuoles also contained numerous small bodies that had the same fluorescence as chlorophyll (though were structurally much smaller and differently shaped).

These two bodies – the dark, circular/granular and chlorophyllous objects – interested the researchers, so they separated them out from several plant organs, and from a variety of other tannin-containing plants, with a high speed centrifuge for additional study. Through chemical testing, they discovered the dark bodies have a double autofluorescence spectrum – meaning they have a natural emission of light in the range of two different types of compounds. The emission range was in line with both condensed tannins and chlorophyll.  So the researchers turned their attention to the tannin-containing cells as a whole. And what they found was even more interesting.

Within the chloroplast, the grana inside the thylakoid membranes unstacked and began “whirling…upon themselves,” as the paper states. As the whirling continued, “the periphery of these chloroplasts showed budding, where part of the inner plastidial content was encapsulated in an emerging structure projected into the cytosol.” In other words, they were observing the thylakoid within a chloroplast creating the tannosomes. As the process continued, the tannosomes were encapsulated in a membrane to be shuttled from the chloroplast to the vacuole for storage.

For some time, we’ve known the function of tannins in plants – now, we can add to our body of knowledge with where they originate from. While the full applications for such a discovery are yet to be unearthed, it isn’t hard to imagine that bioengineering might lead us to growing teas with modified tannins, or altering the length of time required to age wine – or even the creation of biofuels.

To Learn More:

To learn more about the role of thylakoids and chlorophyl in photosynthesis, visit our Photosynthesis I module.

To learn more about cellular organelles, visit our Cellular Organelles I module.

Written by Heather Falconer

Heather Falconer holds undergraduate degrees in Graphic Arts and Environmental Science, as well as an MFA in Writing and an MLitt in Literature. She is currently completing her PhD in Rhetoric and Composition, with an emphasis on rhetoric in/and/of science. Heather has worked internationally in academic publishing as both an author and editor, and has taught a wide range of topics – from research writing to marine biology – in the public and private educational sectors.