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Love Wine and Tea? Scientists Discover Plant Part Whence Their Pucker Springs


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Teas made from variously fermented leaves. Left to right: Green, yellow, oolong, and black. Public Domain, via Wikimedia Commons. Click for source.

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.

Tannic acid, one of many tannins. By Michal Sobkowski (CC-By-3.0), via Wikimedia Commons. Click for license and source.

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.

The tannin-stained Oparara River in New Zealans. Public Domain, via Wikimedia Commons. Click image for source and higher-res version.

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.

Fig. 7 from Brillouet et al. 2013. Click image for source.

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.

Tannosomes inside shuttles in various stages of development and filling. Tannosomes are indicated by the red arrowheads. The pearling thylakoid is shown by a blue arrowhead. The small black dots are the forming clumps of tannin. Photo at lower right shows tannosomes nearing full. v = storage vacuole. t = vacuole membrane (tonoplast). cy = cytoplasm cw = cell wall. Fig. 5 (excerpt) from Brillouet et al. 2013. Click image for source.

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.

Reference

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:

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
Nature Blog Network
Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. BethJH 2:59 am 01/13/2014

    Oh marvelous! What a fun article. As a physical chemist, I really appreciate it when someone can clearly convey advances in biology. I never took bio, but I still followed the entire story with ease, and yes, I do love both my wine and my tea.

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  2. 2. Jennifer Frazer in reply to Jennifer Frazer 11:58 am 01/13/2014

    Thank you, BethJH!

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  3. 3. Looie 1:01 pm 01/15/2014

    Very interesting. Given that chloroplasts (like mitochondria) have their own genome, I wonder whether the genes responsible for making tannins belong to the chloroplast genome or the nuclear genome. Do they say anything about that.

    Regards, Bill Skaggs

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  4. 4. Jennifer Frazer in reply to Jennifer Frazer 2:33 pm 01/15/2014

    That’s a very interesting and plausible idea, Bill! Unfortunately, they don’t say anything about that in the article, other than to note in the press release that “The most intriguing finding from this study is the very close proximity of the tannin polymerization system and the photosynthesis supercomplexes.” Intriguing indeed!

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