If you spend any time around wine snobs, you will likely hear the beverage described with a bewildering array of adjectives, from “flamboyant” to “unctuous” to “velvety.” Some connoisseurs lend a scientific air to their critiques through referencing the presence of tannins, a class of molecules that give red wine its astringency. Yet before last week, even a wine lover with a degree in enology (the study of wine, in which several US schools offer graduate programs) would be unable to say where exactly tannins arise in the grape cell. A recent study by Jean-Marc Brillouet and his colleagues at the French National Institute for Agricultural Research has revealed the existence of a previously unknown cellular structure, the tannosome, which is responsible for the production of these compounds.
Although humans have learned to enjoy the bitterness of tannins, their primary role in nature is that of predator deterrents. When an herbivore bites into a tannin-containing plant, its cells and cellular compartments are broken open, releasing a wide range of chemicals. After tannins are released, they tend to attach themselves to available proteins and enzymes, and in the case of herbivory, the digestive enzymes of the predator are the most likely targets. By binding to molecules like trypsin and chymotrypsin, tannins inhibit digestion and can even kill insects that ingest enough of them. The association of bitter tastes with tannins has evolved to encourage us to seek less well-defended plant foods.
Yet tannins can also bind to enzymes in the plant, which would disrupt its regular functions; to avoid harming itself, the plant must store tannins in a safe place. The most common device for isolating potentially dangerous compounds in a plant cell is the vacuole, a compartment (or organelle) surrounded by a membrane that the plant can regulate to determine what passes through. This membrane, called the tonoplast, had been previously suggested as the location of tannins in the cell, but Brillouet and colleagues were uncertain about this hypothesis. The researchers closely examined a wide range of tannin-containing plants under different settings with a transmission electron microscope, which allowed them to study the structure of the cells at a greater resolution than had been done before. The data indicated that tannins were not located in the tonoplast, but instead in an entirely separate part of the cell they called the tannosome, which both synthesizes and stores the compounds.
As explained by Ian Burbulis, a biochemist at the University of Virginia who was not involved in the study, the discovery of the tannosome is a big deal because “[p]eople have been trying to figure this out since the 1960s.” This step towards understanding the molecular mechanisms of tannins could allow researchers to manipulate the contents and taste of wine. For example, co-author Charles Romieu conjectured that his team could eventually “play with [tannins] perhaps to make wine feel more smooth in the mouth.” Although this level of precise engineering is still theoretical, it’s certainly a possibility to which it is worth raising a glass.