by Steve Jones
Dozens of plant hormones are known. The chemicals resound through their tissues in response to light, heat, damage, the passage of time and more. Some emerge in unexpected places. Human urine applied to a decapitated shoot alters its growth because a plant’s auxin passes unchanged through the body of those who eat it (and the substance was in fact itself first purified from that invaluable fluid). Now the messengers are studied not just with chemistry but with mutant plants whose altered growth is due to an aberrant response to hormones.
Most plant hormones are simpler and smaller than our own. Some have a chemical structure based on closed carbon rings, but many are small proteins. A few even look rather like the steroids that control human sexual attributes. Like mammalian chemical messengers, they are often arranged in pairs, with some that promote an action and others that oppose it. Each has a receptor on the target tissue to which it binds, and each acts - as do those of animals - to stimulate or repress the action of particular genes. Some cause individual cells to expand or to contract, while others change the rate of cell division - should, for example, cells divide faster on the dark rather than the light side of a shoot, then the whole structure bends towards the source of illumination.
Such molecules determine when their masters will ripen, lose their leaves, move towards or away from light and gravity, fight infection, and more. Those concentrated in the tip of a shoot act to suppress the activity of sections of the plant that lie below them. Auxin diffuses downwards and prevents the growth of buds that might compete with the tip itself for light. Cut off the tip and those segments burst into life - which is why gardeners prune their fruit trees to get a dense bush.
The locks into which the auxin keys will fit have also been discovered. Many of the inherited changes in shape treasured by gardeners result from errors in the hormone genes or their receptors. The auxins persuade the shoot to grow up or the root down, the flower to bloom and the fruit to swell under the influence of auxin from its seeds. The sinister movements of the sundew as it rolls its leaves over a trapped insect are due to another member of the same chemical family. In some ways the auxins resemble the substances involved in nerve transmission more than they do animal hormones. Indeed, some of our own nerve transmitters are found in plants but quite what they do is not yet clear.
The auxins and their relatives have often been turned to useful ends. Gardeners and farmers use artificial versions to help cuttings to take root. Agent Orange - so named after the colour code on its barrels - was an artificial auxin that caused vegetation to grow itself to death. It was used for a decade at the time of the Vietnam War. Over seventy-five million litres were used in an attempt to destroy the guerrillas’ crops and to open up the forest to expose the enemy. Its military value was never proved and the chemical was abandoned when found to be contaminated by the poison dioxin. Even so, synthetic auxins are still used as herbicides and appear to be safe.
The leaves that hid the Viet-Cong from aircraft formed a dense screen as the trees that bore them struggled for life - and for light. All plants need sunlight and will fight for that precious resource, often to the death. Every forest is the result of a silent battle between the leaves far above, as each jostles to get a view of the sun. Together, they block its rays. Some bathe in its beams, but others fail, pale and die. To survive they need to pick up the solar radiation, measure its intensity and move, or grow, in response.
The Darwins found that shoots can pick up light and pass on information but they had no idea of how they noticed the solar presence or of their exquisite sensitivity to wavelength, intensity and direction. They do the job in several ways, with a variety of special molecules, some of which have equivalents in the animal kingdom.
One group of receptors known as phototropins picks up blue light and plays a large part in the growth of shoots towards a source of illumination. Another group, the phytochromes, is sensitive to the longer waves, the red and infra-red. Phytochromes have a protein skeleton matched with a second chemical structure based on linked rings of carbons bent into a molecular knot. The molecule is poised like a set mousetrap and when light strikes it it flips from one shape to another. In the dark or shade the change is reversed. The balance between the alternative forms tells the plant how much light is in the sky. The light-sensitive part looks rather like the chlorophyll found in leaves, and - less predictably - resembles the breakdown products of our own red blood pigment (which is why jaundiced babies can be helped with a dose of intense light).
In a dense forest the proportion of infra-red that hits the ground is no more than a twentieth of that experienced by a plant exposed to full sun, because most of the energetic radiation is soaked up by the leaves above. Because the pigment measures not just light intensity (which changes as the day wears on), but also the ratio of red to infra-red, it can distinguish a shortage of light in the evening or on a cloudy day (when the proportion of the two wavelengths does not change) from an attack of gloom that arises because other leaves have shaded out the solar disc and have stolen the most valuable part of its spectrum. Once the infra-red alarm has sounded, those in the shade must respond to the emergency before the source of energy is blocked altogether.
A plant in this predicament shifts its whole pattern of life. It grows faster and the stalk stretches higher. Each leaf moves to present a flatter surface to what light is available. If the shortage continues, the leaves become thinner and more transparent and their parent becomes less branched as it reaches for the sky. If all this fails, and the light still stays red, the unpalatable truth becomes clear. The victim flowers as soon as it can to give at least some hope that its genes will be passed on before it dies in darkness.
The phytochromes are smart, but other pigments are even smarter. A third set of sensors, the cryptochromes, respond not to red, but to blue light. In a further parallel to our own eyes, with their three distinct receptors, a green-sensitive pigment has also been found. Cryptochromes have a structure rather like that of the enzymes that cut and splice DNA. They measure the intensity, rather than the wavelength, of light. Their main job is to sense how long each day is - an important factor when deciding whether to make flowers or fruit as the seasons move on. They are also hard at work in the newborn seedling as it wriggles its way towards adulthood, for they shift the whole biochemical economy of a young shoot from a life based on the gloomy world of the soil to a career bathed in sunshine.
Many of the sensor molecules have relatives in our own eyes. They too help to work out the length of each day and to assess wavelength (or colour, as we call it). Some of the magic proteins are the very stuff as dreams are made on for not only do they cause the sensitive plant to droop but they control human sleep rhythms. A long trip in a jet plane leads to unpleasant side-effects - and the cryptochromes help to put them right, which is why a global traveller finds it harder to adjust to local time in gloomy London than in sunny Sydney. Mice in which the blue-light gene has been damaged by mutation sleep more and have less active brains as they snooze - and, for reasons unknown, also show shifts in their response to anti-cancer drugs. The levels of such chemicals in our own eyes are also tied to the annual swings of mood familiar to those with seasonal affective disorder, the Black Dog of winter (although no fit has yet been found between genetic variation in the human genes and liability to that unpleasant illness). The sensitive plants droop in gloomy weather with the help of cryptochromes and so, it seems, do we.
Roots, in contrast, prefer darkness and make a real effort to avoid daylight. Once again, blue light does the job, with its own special receptor molecule in the tip. The root has another talent which helps it delve into the soil, for roots can sense the force of gravity. For climbers, too, the Earth’s attraction is important, although they prefer to move in the opposite direction. Darwin found that the crucial sense organ for gravity resided in the tip of the root and the shoot and that to cut off that tip much confused the growing plant. Now it has been tracked down - and, once again, it has some uncan
ny similarities with the human system that does the same job.
Men and women maintain their equilibrium with a set of liquid-filled tubes in the inner ear, arranged in three dimensions, left and right, forward and back or up and down. They contain a liquid that washes back and forth as we stand, sit or move about. Tiny grains of calcium carbonate rest on special cells on the inner surface of each tube and shift as gravity or acceleration directs them. The movements of the fine hairs on each cell are translated into electrical messages to the brain to give us a sense of where we stand.
Plants do the same with special cells in roots and shoots. Each contains small grains of starch which, like the minute particles within our ears, shift as their owner moves. Mutants unable to make starch lose both their sense of gravity and the ability to circle. Darwin speculated that the questing movements he found in hops and the like depend on the Earth’s attraction, but he was not altogether right, for, in an experiment that would have flabbergasted him, the tips of plants held in weightless conditions on the Space Station continue to make their measured rounds.
A closer look at both people and plants shows further parallels in the gravity sensor. In the ear, a molecular rack and pinion uses a pair of proteins that play a part in muscle to pick up the movement of the small grains as they are washed back and forth. In the plant a pair of almost identical molecules does the same job.
Poets, mystics and romantics often imagine the vibrations of a sixth, seventh or eighth sense (although Shelley had more sense than to do so). One popular candidate is magnetism - a topic tarnished from its earliest days when the German mountebank Franz Anton Mesmer claimed that ‘animal magnetism’ - the supposed ability of some people to open blocked bodily channels in the afflicted - could cure blindness and more. The idea was used by Mozart in Così fan tutte but blown out of the water by a French governmental commission headed by Benjamin Franklin. There are still plenty of magnetic therapists, who sell hundreds of millions of dollars’ worth of magic bracelets, insoles and blankets in the United States each year.
Biology has gained a renewed interest in our interactions with the Earth’s magnetic field. The subject still attracts odd claims: some say that blindfold students can find their way home thanks to a supposed internal compass, while aerial shots hint that cattle tend to line themselves up to face north or south. A strong magnetic field does spark off brain activity, but what relevance that has to daily life is not clear.
Many creatures do have a strong and unexpected ability to sense direction, with the help of a magnetic compass. Migratory birds, for example, have iron-rich cells in their brains, and use the Earth’s field to find their way back and forth across the globe as the seasons change. To do so, they use the products of a gene remarkably similar to that of a plant’s blue-light sensor. Its molecule is an essential part of an internal timer employed by the migrants as they measure the angle made by the sun as it sweeps across the sky and use the information to orient themselves north or south. In addition, it helps the birds to navigate by the Earth’s lines of magnetic force.
They can pick up the field only in daylight, and the blue sensor is what allows them to do so. It works at the atomic level. Electrons come in pairs that spin around the nucleus in opposite directions. As a result they cancel out each other’s ability to act as magnets. When energy - from light, heat or chemical reactions - enters the system, some particles are knocked off balance and are left with just a single spinning electron. Such ‘free radicals’ need to marry another partner as soon as they can. To find a match they must change their direction of rotation. A magnetic field at right angles to the spin makes that task harder. As a result, a bird can use unpaired electrons as a compass needle that allows them to sense the Earth’s magnetism. The cryptochromes generate lots of free radicals when they pick up the energy of blue light and transmit it into the cell and with their help the happy migrant gains an improved sense of direction.
Magnetism also has an effect on plant growth. The starch grains in the tip of a shoot can be moved with a magnet and the field then helps tell a plant where it stands (an attempt to test the results of that experiment without gravity perished in the Challenger disaster). A magnetic field also slows the growth of shoots - but only in blue light. Mutants that lack the relevant sensor are quite indifferent to its presence. Perhaps all this hints at a forgotten shared sixth sense, in both animals and plants. Darwin often spoke of ‘fool’s experiments’, ideas that were most unlikely to work but were worth a try. He speculated on the role of magnetism in animal navigation but he would be amazed to find that it applied to the movements in the other kingdom of life. The experiment was too foolish even for him.
In another instance of what his son called his ‘wish to test the most improbable ideas’, he persuaded Francis to play the bassoon to a mimosa to test whether it would respond. Sweet music, like kind words, had no effect; and a later experiment in which pop music was blasted at them for hours also left them unmoved. Even so, plants do hold conversations. They use not sound to do the job, but scent. A series of chemical messengers have evolved to help, and together they provide a sense of smell to add to those of sight, gravity and the rest.
Many pump out a simple gas called ethylene, which causes them to grow faster and also helps fruit to ripen. In dense vegetation, the gas persuades leaves to struggle harder towards the light, for it reveals the presence of competitors. The ability to smell can be used for more sinister ends. Dodder, also known as devil’s guts, witch’s shoelaces, hellbine and the like, is a parasite related to the morning glory. It is a pest of carrots, potatoes, clover and garden flowers. Its leaves are tiny and contain almost no chlorophyll. The seeds can survive for ten years. Once they germinate, the shoot pokes above the soil and searches for a potential victim. It circles round until it succeeds. Then it inserts fine needles whose cells fuse to those of its quarry. They suck out vital fluids and the plant loses its own root, to live as a parasite. It has a strong preference for juicy species like tomatoes over tougher kinds such as wheat.
The dodder, like a bloodhound, sniffs out its prey. When allowed to germinate in a closed container with two tunnels, one that leads to a chamber with healthy tomatoes and the other to a similar space with wheat, it directs its growth towards its preferred host - the soft and juicy tomato - and is repelled by its alternative.
A plant can also use its sense of smell for protection. Some species talk to themselves, for a damaged leaf causes others nearby to get ready for attack - but not when the leaf is sealed in a plastic bag, as proof that an airborne signal is involved. Leaves chewed by insects pour out signals that are picked up by their neighbours, who prepare their own defences. Other species listen in to foreign messages. Tobacco seedlings grown close to a sagebrush bush switch on their own anti-insect chemicals when the bush is pruned. A few can even call for help. A beetle that chews a lima bean takes a real risk, for its victim sends out an aerial message that persuades leaves nearby to make a sugary secretion that attracts ants and wasps. The visitors then attack the beetle.
Plants can taste chemicals in solution, as well as smell them in the air. They extract information from the liquids that bathe their roots and flow across their leaves. Roots and shoots sense the presence of enemies and grow away from them. They hunt for food, too, for when a root hits a rich spot, it stops, sprouts and sucks up what is on offer. The substances that they pump into the soil may attract friends such as helpful fungi, but are also hijacked by enemies. Witchweeds are pests of tropical crops such as sugarcane. They grow on the roots of their host and - like the dodder - drain its vitality. They, too, pick up the taste of a dissolved substance used by the host to attract fungi. In the same way corn seedlings whose roots are chewed by grubs pump out a chemical that attracts predatory worms. The American black walnut scares away competitors with its own secretions and leaves a dead zone beneath its shade - and it is no coincidence that our own tongues are titillated by the poisons found in pepper, coffee, lettuce and more
, which evolved not to satisfy the gourmet but to fight off an enemy.
Darwin himself saw that plants must have a sense of touch, for the climbers themselves, as soon as they contact a vertical object, change their behaviour, give up their wide sweeps and begin to twine. Roots, too, probe the soil and grow their way around a stone too large to move - although in this case they avoid, rather than embrace, the object. He found that the senses of touch and of direction interact, for a root held vertical will grow away from an object that blocks its path - but the same structure kept horizontal will always try to extend downwards, whatever obstacle is placed in the way.
Tree-huggers carry out what seems the entirely witless experiment of embracing a trunk to exchange energies with it; to inject their own vitality into the plant and to obtain some as yet undiscovered botanical spirit in return. In fact, to touch - or to hug - a plant has another unexpected effect, for it inhibits its growth. The pines of Highland forests are small, twisted and bent because they have been caressed - or battered - by the winds. Their equivalents around Down House live in calmer air and soar upwards. Identical seedlings grown in calm and windy places always end up with quite a different appearance. The plants are sensitive indeed for to bend a young tomato plant for half a minute stops its growth for a whole hour. That is why stormy places make for stunted trees.