What a Plant Knows Read online

  Tobacco (Nicotiana tabacum)

  In 1918, Wightman W. Garner and Harry A. Allard, two scientists at the U.S. Department of Agriculture, set out to determine why Maryland Mammoth didn’t know when to stop making leaves and start making flowers and seeds instead. They planted the Maryland Mammoth in pots and left one group outside in the field. The other group was put in the field during the day but moved to a dark shed every afternoon. Simply limiting the amount of light the plants saw was enough to cause Maryland Mammoth to stop growing and start flowering. In other words, if Maryland Mammoth was exposed to the long days of summer, it would keep growing leaves. But if it experienced artificially shorter days, then it would flower.

  This phenomenon, called photoperiodism, gave us the first strong evidence that plants measure how much light they take in. Other experiments over the years have revealed that many plants, just like the Mammoth, flower only if the day is short; they are referred to as “short-day” plants. Such short-day plants include chrysanthemums and soybeans. Some plants need a long day to flower; irises and barley are considered “long-day” plants. This discovery meant that farmers could now manipulate flowering to fit their schedules by controlling the light that a plant sees. It’s not surprising that farmers in Florida soon figured out that they could grow Maryland Mammoth for many months (without the effects of frost encountered in Maryland) and that the plants would eventually flower in the fields in midwinter when the days were shortest.

  What a Difference a (Short) Day Makes

  The concept of photoperiodism sparked a rush of activity among scientists who were brimming with follow-up questions: Do plants measure the length of the day or the night? And what color of light are plants seeing?

  Around the time of World War II, scientists discovered that they could manipulate when plants flowered simply by quickly turning the lights on and off in the middle of the night. They could take a short-day plant like the soybean and keep it from making flowers in short days if they turned on the lights for only a few minutes in the middle of the night. On the other hand, the scientists could cause a long-day plant like the iris to make flowers even in the middle of the winter (during short days, when it shouldn’t normally flower), if in the middle of the night they turned on the lights for just a few moments. These experiments proved that what a plant measures is not the length of the day but the length of the continuous period of darkness.

  Using this technique, flower farmers can keep chrysanthemums from flowering until just before Mother’s Day, which is the optimal time to have them burst onto the spring flower scene. Chrysanthemum farmers have a problem since Mother’s Day comes in the spring but the flowers normally blossom in the fall as the days get shorter. Fortunately, chrysanthemums grown in greenhouses can be kept from flowering by turning on the lights for a few minutes at night throughout the fall and winter. Then … boom … two weeks before Mother’s Day, the farmers stop turning on the lights at night, and all the plants start to flower at once, ready for harvest and shipping.

  These scientists were curious about the color of light that the plants saw. What they discovered was surprising: the plants, and it didn’t matter which ones were tested, only responded to a flash of red during the night. Blue or green flashes during the night wouldn’t influence when the plant flowered, but only a few seconds of red would. Plants were differentiating between colors: they were using blue light to know which direction to bend in and red light to measure the length of the night.

  Then, in the early 1950s, Harry Borthwick and his colleagues in the USDA lab where Maryland Mammoth was first studied made the amazing discovery that far-red light—light that has wavelengths that are a bit longer than bright red and is most often seen, just barely, at dusk—could cancel the effect of the red light on plants. Let me spell this out more clearly: if you take irises, which normally don’t flower in long nights, and give them a shot of red light in the middle of the night, they’ll make flowers as bright and as beautiful as any iris in a nature preserve. But if you shine far-red light on them right after the pulse of red, it’s as if they never saw the red light to begin with. They won’t flower. If you then shine red light on them after the far-red, they will. Hit them again with far-red light, and they won’t. And so on. We’re also not talking about lots of light; a few seconds of either color is enough. It’s like a light-activated switch: The red light turns on flowering; the far-red light turns it off. If you flip the switch back and forth fast enough, nothing happens. On a more philosophical level, we can say that the plant remembers the last color it saw.

  By the time John F. Kennedy was elected president, Warren L. Butler and his colleagues had demonstrated that a single photoreceptor in plants was responsible for both the red and the far-red effects. They called this receptor “phytochrome,” meaning “plant color.” In its simplest model, phytochrome is the light-activated switch. Red light activates phytochrome, turning it into a form primed to receive far-red light. Far-red light inactivates phytochrome, turning it into a form primed to receive red light. Ecologically, this makes a lot of sense. In nature, the last light any plant sees at the end of the day is far-red, and this signifies to the plant that it should “turn off.” In the morning, it sees red light and it wakes up. In this way, a plant measures how long ago it last saw red light and adjusts its growth accordingly. Exactly which part of the plant sees the red and far-red light to regulate flowering?

  We know from Darwin’s studies of phototropism that the “eye” of a plant is in its tip while the response to the light occurs in the stem. So we might conclude, then, that the “eye” for photoperiodism is also in the tip of the plant. Surprisingly, this isn’t the case. If in the middle of the night you shine a beam of light on different parts of the plant, you discover that it’s sufficient to illuminate any single leaf in order to regulate flowering in the entire plant. On the other hand, if all the leaves are pruned, leaving only the stem and the apex, the plant is blind to any flashes of light, even if the entire plant is illuminated. If the phytochrome in a single leaf sees red light in the middle of the night, it’s as if the entire plant were illuminated. Phytochrome in the leaves receives the light cues and initiates a mobile signal that propagates throughout the plant and induces flowering.

  Blind Plants in the Age of Genetics

  We have four different types of photoreceptors in our eyes: rhodopsin for light and shadows, and three photopsins for red, blue, and green. We also have a fifth light receptor called cryptochrome that regulates our internal clocks. So far we’ve seen that plants also have multiple photoreceptors: they see directional blue light, which means they must have at last one blue-light photoreceptor, now known as phototropin, and they see red and far-red light for flowering, which points to at least one phytochrome photoreceptor. But in order to determine just how many photoreceptors plants possess, scientists had to wait for the era of molecular genetics, which began several decades after the discovery of phytochrome.

  The approach spearheaded in the early 1980s by Maarten Koornneef at Wageningen University in Holland, and repeated and refined in numerous labs, used genetics to understand plant sight. Koornneef asked a simple question: What would a “blind” plant look like? Plants grown in darkness or dim light are taller than those grown in bright light. If you ever took care of bean sprouts for a sixth-grade science experiment, you’d know that the plants in the hall closet grew up tall, spindly, and yellow, but the ones out on the playground were short, vigorous, and green. This makes sense because plants normally elongate in darkness, when they’re trying to get out of the soil into the light or when they’re in the shade and need to make their way to the unobstructed light. If Koornneef could find a blind mutant plant, perhaps it would be tall in bright light as well. If he could identify and grow blind mutant plants, he would be able to use genetics to figure out what was wrong with them.

  He carried out his experiments on Arabidopsis thaliana, a small laboratory plant similar to wild mustard. He treated a batch of
arabidopsis seeds with chemicals known to induce mutations in DNA (and also cause cancer in laboratory rats) and then grew the seedlings under various colors of light and looked for mutant seedlings that were taller than the others. He found many of them. Some of the mutant plants grew taller under blue light, but were of normal height when grown under red light. Some were taller under red light but normal under blue. Some were taller under UV light but normal under all other kinds, and some were taller under red and blue lights. A few were taller only under dim light, while others were taller only under bright-light conditions.

  Many of these mutants that were blind to specific colors of light were defective in the particular photoreceptors that absorb the light. A plant that had no phytochrome grew in red light as if it were in the dark. Surprisingly, a few of the photoreceptors came in pairs, with one being specific for dim light and the other specific for bright light. To make a long and complex story short, we now know that arabidopsis has at least eleven different photoreceptors: some tell a plant when to germinate, some tell it when to bend to the light, some tell it when to flower, and some let it know when it’s nighttime. Some let the plant know that there’s a lot of light hitting it, some let it know that the light is dim, and some help it keep time.*

  Arabidopsis (Arabidopsis thaliana)

  So plant vision is much more complex than human sight at the level of perception. Indeed, light for a plant is much more than a signal; plants need light to eat. Plants use light to turn water and carbon dioxide into sugars that in turn provide food for all animals. But plants are sessile, unmoving organisms as well. They are literally rooted in one place, unable to migrate in search of food. To compensate for this sessile life, plants must have the ability to seek out and capture light. That means they need to know where the light is, and rather than moving toward the food, as an animal would, a plant grows toward its source of energy.

  A plant needs to know if another plant has grown above it, filtering out the light for photosynthesis. If a plant senses that it is in the shade, it will start growing faster to get out. And plants need to survive, which means they need to know when to “hatch” out of their seeds and when to reproduce. Many types of plants start growing in the spring, just as many mammals give birth then. How do plants know when the spring has started? Phytochrome tells them that the days are getting progressively longer. Plants also flower and set seed in the fall before the snow comes. How do they know it’s autumn? Phytochrome tells them that the nights are getting longer.

  What Plants and Humans See

  Plants must be aware of the dynamic visual environment around them in order to survive. They need to know the direction, amount, duration, and color of light to do so. Plants undoubtedly detect visible (and invisible) electromagnetic waves. While we can detect electromagnetic waves in a relatively tight spectrum, plants detect ones that are both shorter and longer than those we can detect. Although plants see a much larger spectrum than we do, they don’t see in pictures. Plants don’t have a nervous system that translates light signals into pictures. Instead, they translate light signals into different cues for growth. Plants don’t have eyes, just as we don’t have leaves.*

  But we can both detect light.

  Sight is the ability not only to detect electromagnetic waves but also the ability to respond to these waves. The rods and cones in our retinas detect the light signal and transfer this information to the brain, and we respond to the information. Plants are also able to translate the visual signal into a physiologically recognizable instruction. It wasn’t enough that Darwin’s plants saw the light in their tips; they had to absorb this light and then somehow translate it into an instruction that told the plant to bend. They needed to respond to the light. The complex signals arising from multiple photoreceptors allow a plant to optimally modulate its growth in changing environments, just as our four photoreceptors allow our brains to make pictures that enable us to interpret and respond to our changing environments.

  To put things in a broader perspective: plant phytochrome and human red photopsin are not the same photoreceptor; while they both absorb red light, they are different proteins with different chemistries. What we see is mediated through photoreceptors found only in other animals. What a daffodil sees is mediated through photoreceptors found only in plants. But the plant and human photoreceptors are similar in that they all consist of a protein connected to a chemical dye that absorbs the light; these are the physical limitations required for a photoreceptor to work.

  Yet there are exceptions to every rule, and despite billions of years of independent evolution plant and animal visual systems do have some things in common. Both animals and plants contain blue-light receptors called cryptochromes.* Cryptochrome has no effect on phototropism in plants, but it plays several other roles in regulating plant growth, one of which is its control over a plant’s internal clock. Plants, like animals, have an internal clock called the circadian clock that is in tune with normal day-night cycles. In our case, this internal clock regulates all parts of our life, from when we’re hungry, when we have to go to the bathroom, and when we’re tired, to when we feel energetic. These daily changes in our body’s behavior are called circadian rhythms, because they continue on a roughly twenty-four-hour cycle even if we keep ourselves in a closed room that never gets sunlight. Flying halfway around the world puts our circadian clock out of sync with the day-night signals, a phenomenon we call jet lag. The circadian clock can be reset by light, but this takes a few days. This is also why spending time outside in the light helps us recover from jet lag faster than spending time in a dark hotel room.

  Cryptochrome is the blue-light receptor primarily responsible for the resetting of our circadian clocks by light. Cryptochrome absorbs blue light and then signals the cell that it’s daytime. Plants also have internal circadian clocks that regulate many plant processes, including leaf movements and photosynthesis. If we artificially change a plant’s day-night cycle, it also goes through jet lag (but doesn’t get grumpy), and it takes a few days for it to readjust. For example, if a plant’s leaves normally close in the late afternoon and open in the morning, reversing its light-dark cycle will initially lead to its leaves opening in the dark (at the time that used to be day) and closing in the light (at the time that used to be night). This opening and closing of leaves will readjust to the new light-dark patterns within a few days.

  The plant cryptochrome, just like the cryptochrome in fruit flies and mice, has a major role in coordinating external light signals with the internal clock. At this basic level of blue-light control of circadian rhythms, plants and humans “see” in essentially the same way. From an evolutionary perspective, this amazing form of conservation of cryptochrome function is actually not so surprising. Circadian clocks developed early in evolution in single-celled organisms, before the animal and the plant kingdoms split off. These original clocks probably functioned to protect the cells from damage induced by high UV radiation. In this early clock, an ancestral cryptochrome monitored the light environment and relegated cell division to the night. Relatively simple clocks are even found today in most single-celled organisms, including bacteria and fungi. The evolution of light perception continued from this one common photoreceptor in all organisms and diverged into the two distinct visual systems that distinguish plants from animals. What may be more surprising, though, is that plants also smell …

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  What a Plant Smells

  Stones have been known to move and trees to speak.

  —Shakespeare, Macbeth

  Plants smell. Plants obviously emit odors that animals and human beings are attracted to, but they also sense their own odors and those of neighboring plants. Plants know when their fruit is ripe, when their neighbor has been cut by a gardener’s shears, or when their neighbor is being eaten by a ravenous bug; they smell it. Some plants can even differentiate the smell of a tomato from the smell of wheat. Unlike the large spectrum of visual input that a plant experiences, a plant’s
range for smell is limited, but it is highly sensitive and communicates a great deal of information to the living organism.

  If you look up the word “smell” in a standard dictionary today, you’ll see that it is defined as the ability “to perceive odor or scent through stimuli affecting the olfactory nerves.” Olfactory nerves can easily be understood as the nerves that connect the smell receptors in the nose to the brain. In olfaction, the stimuli are small molecules dissolved in the air. Human olfaction involves the cells in our nose that receive airborne chemicals, and it involves our brain, which processes this information so that we can respond to various smells. If you open a bottle of Chanel No5 on one side of a room, for example, you smell it on the other because certain chemicals evaporate from the perfume and disperse across the room. The molecules are present in very dilute quantities, but our noses are filled with thousands of receptors that react specifically with different chemicals. It only takes one molecule to connect with a receptor to sense the new smell.

  Our body’s mechanism for the perception of smells is different from the mechanism involved in the perception of light. As we saw in the previous chapter, we only need four classes of photoreceptors that differentiate between red, green, blue, and white to see the colors of a complete palette. When it comes to olfaction, however, we have hundreds of different receptor types, each specifically designated for a unique volatile chemical.