When you take a sip of red wine or black tea, you're swallowing a stiff swig of tannins. These astringent plant chemicals give the beverages their characteristic pucker. Now, the part of plant cells that makes and transports tannins -- long overlooked by botanists -- has at last been discovered, hiding right under our noses.
Tannins are a major way plants have of telling herbivores to graze elsewhere. They are deterrents because they denature -- that is, deactivate -- proteins. Humans have long taken advantage of this denaturing ability to "tan" animal skins with tannins, producing leather. The denaturation of the hide proteins by tannins renders them impervious to bacterial attack -- otherwise known as rotting. In plants, tannins may also dissuade microbes and fungi from attacking and help protect plants from damaging UV.
As such powerful chemicals, tannins are also dangerous to the plant itself and must be handled with kid gloves. Scientists have long known finished tannins are stored in special bags called vacuoles to prevent the cell from accidentally killing itself with its own bioweapons.
Woody plants most commonly make tannins, but they are found in everything from ferns to persimmons. Their astringency is off-putting to virtually all plant-eaters, from insects to birds to reptiles to humans, though in the right concentrations, they lend pleasant complexity to tea and wine. I love this description of the sensation of drinking of a tannin-laced liquid by scientist Geneviève Conéjéro: "They give feeling of pungency in the mouth, the feel of a cat's tongue licking your hand."
Too much tannin, however, and wine becomes spit-worthy -- for now. The preservative powers of tannins are one of the reasons wines destined for long aging start out tannin-rich and virtually undrinkable. But with time and the slow aggregation of tannins and other compounds in the wine, these harsh components slowly settle. Once this sediment is separated by decanting, the taste of the remaining elixir is otherworldly.
Tannins aren't just found outside plants inside bottles and cups. One of my enduring memories of a flight in a small airplane around Mount Katahdin in Maine, the northern terminus of the Appalachian Trail, is the rich tea tint of the quiet lakes and ponds surrounding the lonely mountain, stained by the needles and twigs to which the water was exposed. Though the waters were chestnut brown, they were clear. I could easily see the logs and rocks on the bottoms of the ponds.
Here's an image of a similarly tannin-stained stream in New Zealand.
For a class of chemicals so important to two of humanity's most beloved beverages -- not to mention to life on Earth in general -- you'd think that scientists would long ago have figured out how and where plants make them. You'd be wrong.
Until now, though scientists could see finished tannins in their biocontainment vacuoles inside plant cells, how those tannins got there was murky. They had had a vague idea that tannins might be made in big enzyme complexes embedded in the endoplasmic reticulum, the part of plant cells that facilitates protein and lipid synthesis and helps route these molecules to where they're needed.
On closer inspection, a team of French and Hungarian scientists realized they had something very different on their hands. After studying specimens from all the major groups of land plants, purifying cell components, studying their constituent tannins, making lots of microscope slides, and taking lots more pictures, they decided that the story of tannin is the story of an entirely new organelle: the tannosome.
New organelles -- the organs of a cell -- are not discovered every day. The ones high school biology students study -- nucleus, endoplasmic reticulum, Golgi apparatus, etc. -- were discovered many decades, if not more than a century ago. So the discovery of an entirely new organelle is an unusual find indeed.
Tannosomes may have eluded detection so long because they're made in an unexpected place: inside chloroplasts, the light-harvesting structures of plants. They bud, or pearl, from interior membranes called thylakoids. If that name is ringing a bell, it's because these are the stacked, folded membranes found inside the green plant organelles called chloroplasts where the famous light-to-sugar magic of photosynthesis happens.
Here is the model of tannin synthesis this team has pieced together based on their slides and other data. The structure in the upper left corner bounded by red and blue membranes is a good old-fashioned chloroplast. Inside are the stacks of thylakoid membranes called grana.
The model goes roughly like this: when it's time to make tannins, the thylakoids loosen and inflate. Small blobs bud into tannosomes. The tannosomes gather into groups bounded by a new membrane called shuttles and are shipped off to a storage vacuole. Somewhere along the way, tannins begin to be assembled from their component parts inside the tannosome. The tannosomes slowly fill until their receiving vacuole is packed.
Consistent with the idea that tannins must be handled with care lest the cell poison itself, they are segregated from the rest of the cell from their moment of birth, surrounded first by one tannosome membrane made from thylakoid, then by two membranes inside the shuttle, and finally by three membranes in the storage vacuole. The plant triple shrink-wraps their tannins to preclude destructive jail breaks. Indeed, if the tannins were formed directly inside their storage vacuoles -- bounded by just one membrane -- they'd wreck those vacuoles by denaturing the functional proteins called enzymes floating inside as well as the transport proteins embedded in their membranes.
In spite of what you might expect, though, tannosomes aren't actually tannin-colored -- at least not early in their lives. They're green. Like Splenda, tannosomes are made from chloroplasts, so they look like chloroplasts -- which is what scientists mistakenly thought they had the first few times they isolated them. On their way to storage, they're still studded with the marks of their birthplace, the same light-harvesting chlorophylls that give chloroplasts -- and almost every plant on Earth -- their characteristic hue.
Brillouet J.M., Romieu C., Schoefs B., Solymosi K., Cheynier V., Fulcrand H., Verdeil J.L. & Conejero G. (2013). The tannosome is an organelle forming condensed tannins in the chlorophyllous organs of Tracheophyta, Annals of Botany, 112 (6) 1003-1014. DOI: 10.1093/aob/mct168