How an autumnal alchemy transforms the omega-3s in leaves into seed-borne omega-6s
by Susan Allport
If omega-3s originate in plants (as we know they do), and the green leaves of plants are the most abundant source of these fast-moving fats (in the form of alpha linolenic acid, the parent omega-3), then what happens in autumn when leaves turn color and fall to the ground?
I've been asking myself this question as I've been raking leaves this November.
Do these omega-3 fats, the predominant fat in the membranes of the chloroplasts of green leaves, fall to the ground with those leaves?
Or do plants have some strategy for salvaging their omega-3s?
They must, I think. Otherwise, dead leaves would be much more nutritious than they obviously are. And animals wouldn't leave them for humans like me to rake up.
But how does this salvage operation take place? Are the omega-3s in chloroplasts turned into omega-6s, a second family of polyunsaturated fats, and then shuttled into a plant's developing seeds?
About Susan Allport
She is the author of two other highly praised books—The Primal Feast: Food, Sex, Foraging, and Love, and A Natural History of Parenting —and has appeared on Oprah & Friends Radio and NPR's "Science Friday" and "The Splendid Table".
We're sure you'll find her contributions enlightening pleasures to read!
Omega-6s are slightly less fast-moving than omega-3s, but they are far less susceptible to attack by oxygen molecules, or oxidation. It makes sense that the stored fats in seeds contain many more omega-6s than 3s.
I was able to find the answers to some of my questions about leaves in an article in Plant Physiology, appropriately titled, “Making Sense of Leaf Senescence.” There, I learned that leaf senescence, that which gives us our incredible fall colors, is an orderly process and under strict genetic control. It is not simply a degenerative process but is, indeed, a complex system for recycling fats, as well as nitrogen and other useful building materials.
Proof of this can be seen with the naked eye, I was surprised to learn. In most species of plants, such as the birch whose leaves are shown here, aging begins at the edges of the leaf and proceeds inwards. The last areas of the leaf to senesce are those around the veins of the leaf: the leaf's vascular system. Since the veins are necessary for exporting everything that can be exported out of the leaf, they are the last thing to go.
The leaf responds to changes in the environment
—be it light, drought, nutrient deficiency, infection, or wounding
—by expressing special senescing genes, genes that encode for enzymes that break down and rearrange all the useful, recyclable parts. Complex molecules such as chlorophyll and proteins are turned into smaller and more transportable molecules, which are then moved into a plant's roots, seeds, stems and bark, and reassembled, or stored, as necessary.
But what exactly happens to the alpha linolenic acid in a chloroplasts' membrane? That's what I really wanted to know. For land animals, there is a shift in the availability of the two families of essential fats, the fats that plants can make but not animals, with the changing seasons. This shift has important health and metabolic consequences, as I explained in a previous article: “Seasons of Fat”, and I wanted to understand how it takes place.
Is the alpha linolenic acid turned into linoleic acid (the parent omega-6 fatty acid) in the leaf? Or are the omega-3s stored somewhere in the plant for use the next spring? I couldn't find the answer to these questions in the article in Plant Physiology and so I emailed John Ohlrogge at the Plant Lipid Metabolism Lab at Michigan State University.
Nothing so simple, Dr. Ohlrogge, emailed me back, since plants transport very little lipid or fat. Fats are sticky substances, and plants don't have the lipoproteins (HDL and LDL) that animals use to transport fats. So plants turn fats into sugars, which are highly soluble, before moving them around.
The alpha linolenic acid in the chloroplasts, as Dr. Ohlrogge explained, is broken down by a series of enzymes into two-carbon fragments
—a process called beta oxidation.
These fragments are turned into sugars, and the sugars transported to the seed (or any another tissue that needs them). In the seeds, they are turned back into fat (primarily omega-6s, as I've said, as well as saturated and monounsaturated fats) or into starch – for storage. Come spring, as seeds germinate, the process is reversed. Stored starch and fats are mobilized and turned into sugars, and sugars are transported into the germinating leaf, before being reassembled as fats.
But this time, of course, the sugar is turned primarily into alpha linolenic acid because this faster-moving fat is what plants use to surround their fast-acting photosynthetic machinery. It is the lubricant that makes that machinery run; the medium in which the proteins necessary for photosynthesis (some 75 of them) can twist and turn. (Similarly, the longer omega-3s, DHA and EPA, which humans and other animals make out of alpha linolenic acid, are necessary for an animal's speediest functions: nerve transmission and vision.)
So here's the rub, or the kernel of the thing, as I see it. Since leaves have more omega-3s than 6s and seeds have more omega-6s than 3s, each falling leaf represents a slight loss of 3s and a slight gain of 6s on planet earth. Each germinating seed represents a slight loss of omega-6s and a slight gain of omega-3s.
These slight changes add up and represent a shift—for animals out foraging in the wild—in the dietary supply of these two families of essential fats. This shift then induces many physiological changes in those animals, including changes in their metabolic rate.
(Omega-3s affect metabolic rate, as the Australian scientist Tony Hulbert has shown, by increasing the looseness, or leakiness, of cell membranes and forcing membrane pumps, like the sodium and proton pump, to work harder at maintaining gradients across those membranes.
Omega-3s and omega-6s induce many other physiological changes, including changes in blood pressure, inflammation, cell growth and cell death, through potent cell messengers or eicosanoids made from these fats.)
The lives of plants and animals are tied together in ways we're just beginning to understand. And the sound of a leaf falling in the woods, if you will forgive the poetic license, is nothing less than the sound of the earth's dietary wheels shifting gears. This shift is a benefit to animals in the wild, by enabling their bodies to slow down in the fall and speed up in the spring. But it is not, necessarily, a benefit to humans, who wear clothing and live in heated houses and whose diet is already awash in omega-6s (from all the seed oils in the food supply).
We, who want our bodies to run optimally throughout the year, need to keep our menus green throughout the year (with greens and animals, like fish, that eat greens). But first we need to understand the important dietary implications of the changing seasons—and listen closely to the falling leaves.