Common senses

Plants and humans might share a surprising amount of DNA, but that doesn’t mean plants like our music.

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Plants seem about as unrelated to us humans as it’s possible to get, but we’re closer to trees on the family tree than you might think. ‘MADS-box’ genes control many critical functions. In plants, they help control the development of male and female gametophytes, and roots, flowers, fruit and seeds. In animals, they preside over muscle development and cell differentiation, among other things.

When genetic researchers traced the genes’ lineage, they found strong evidence of MADS-box gene duplication before plants and animals diverged. So it turns out, for instance, that humans and plants both have the same set of genes that govern responses to light.

Which shouldn’t surprise us. Plants face the same problems and trials as we do: they need sustenance, they have to cope with pred­ators, withering heat and numbing cold, they have to find a way to pass on their DNA. All these tasks demand senses to detect stimuli, and networks to communicate a response to them. But plants have to do all this without the benefit of complex tissues or neurons, while rooted to the spot. Instead, they use chemical and hormonal pathways to ‘see’, ‘smell’, ‘feel’, ‘deduce’ and even to ‘remember’.

This is how they know when you’re standing in their light, or whether you’re dressed in blue or red, or when you’re pruning their neighbour.

Plants can’t construct images, as we do, but they can ‘see’ a much wider range of the spectrum, including ultraviolet light, and they know, relatively speaking, how much of it there is. They know which direction it’s coming from (that’s how they know when you move their pot), and when a neighbour is competing for that light.

They know, too, how long the sun has been shining, but more importantly, how long it hasn’t. It turns out they’re most sensitive not to daylight length, but to continuous dark­ness, and it’s this response, called photoperi­odism, that regulates their flowering.

Short-day plants flower during winter, when the day lengths are less than their critical photoperiod. However, they can also be stimulated to flower during short days with bursts of light during the night. (Short-day plants cannot be stimulated to flower out of season in this way.) Long-day plants flower when the day length exceeds their critical photoperiod. These plants typically flower during late spring or early summer as days are getting longer.

Hothouse growers have learned that they can control the timing of flowering to commer­cial advantage. They prevent popular blooms from flowering by flashing them with short bursts of light in the middle of the night, which zeroes the plants’ internal darkness meters. As Mothers’ Day approaches, the growers stop flashing the lights, dial in the requisite night-time length, and the florists’ shops are full of chrysanthemums, right on time.

Plants are also sensitive to light wave­length. Experiments have shown that they bend towards blue light, and are encouraged to bloom by red light. This information is perceived and processed by light-sensitive proteins called photoreceptors. Arabidopsis, a humble flowering rockcress, has been more thoroughly mapped than practically any other plant, which is how we know that it’s directed by at least 11 photoreceptors that variously tell it when to flower, when to bend to the light (and which way), when to germinate, when it’s dark, and how much time has elapsed.

Plants can ‘smell’ when their neighbours are being devoured by insects, and again, they get that information from chemical messengers. Your grandmother knew that she could get a hard pear to ripen by putting it in a bag with a banana. She may not have known why, though: it’s because ripening fruit give off ethylene, which stimu­lates surrounding fruit to ripen too. All fruiting plants are sensitive to ethylene—lemons can detect one part in 100 million—and it ensures that all the fruit on the same tree ripen simul­taneously, and those of their neighbour’s too, maximising their chances of being noticed. Ripe fruit shifts from inconspicuous green to glaring red or orange to catch the eye of birds and other consumers, thereby getting the plants’ seeds out there as far and wide as possible.

Airborne chemicals alert plants to danger as well as food. In a US study, researchers put poplar and sugar-maple seedlings in an airtight herbarium, then tore the leaves of half their study group. After two days, even the unharmed plants were producing more toxic phenols and tannins—both chemical defences against caterpillars. This proved the distressed seed­lings were sending out gaseous signals that the healthy trees received and responded to.

This is not, however, arboreal altruism: it’s more likely that plants may have developed chemical listening devices specifically to eaves­drop for the sound of suffering neighbours.

When Mimosa pudica is disturbed, specific regions on the stems are stimulated to release chemicals which force water out of the cell vacuoles and the water diffuses out of the cells, producing a loss of cell pressure; this differential turgidity between different regions of cells results in the closing of the leaflets and the collapse of the leaf petiole.

Plants can feel. We know this because climbing vines put on a growth spurt when they contact a suitable substrate, and the Venus fly trap snaps shut the instant it feels a fly’s tickly legs. It also knows when the prey is big enough to warrant the energy expense­ literally, when the juice is worth the squeeze. If just one sensory hair is touched, the plant doesn’t close. At least two hairs have to be touched before the trap is sprung.

Neither is it fooled by false alarms, such as a shower of rain. Charles Darwin blew droplets of water at a Venus fly trap, trying to trick it into closing. It didn’t. It was John Burdon-Sanderson who realised that the trap’s trigger was electro-mechanical: when he placed an electrode across two hairs, it caused a reaction very similar to muscular activa­tion in animals—when an insect touches the hairs, it produces a faint current that the plant’s lobes detect, snapping them shut.

And here’s an irony: trees don’t actually like being hugged. Experi­ments have shown that repeated contact inhibits growth.

Scientists making regular measurements of cocklebur, a North American weed, were dismayed to find that the leaves they measured soon yellowed, and eventually withered. Persisting, they eventu­ally killed the plants, simply through contact of ruler on leaf. Just stroking an Arabidopsis plant in the lab three times a day arrested its development.

The burr cucumber, a climber, has been shown to be 10 times more sensitive to touch than the average human­ the faint stroke of a string just 0.25 grams in weight was enough to make it start writhing about in search of an anchor.

Plants have a memory—of sorts. Many flower and fruit only after they’ve expe­rienced extended cold—a process called vernalisation. It’s too risky to rely solely on daylight length as a flowering cue: equal daylight lengths happen twice a year, so a plant might be fooled into flowering in the autumn, exposing its fruit and seeds to the deathly chill of winter, instead of in the spring. So it needs to be able to recognise cold temperatures, and ‘remember’ them, to get its temporal bearings. In short, it knows when spring has sprung, because it recalls the winter that just passed.

Inside the plant, a gene impedes flowering until vernalisation is over—an epigenetic programme change that’s handed down from parent to daughter cells through the generations. In other words, seedlings come pre-loaded with the ‘memory’ of their parents’ seasonal stresses.

Research has shown that plants can remember all manner of trauma—from the UV bombardment of solar storms to pathogen attacks—and pass on an account of it, almost by way of a warning, down the generations. This is plants’ way of adapting; changes in the plant genome lead to recom­binations of DNA that offer flexibility to cope with a shifting environment. When you can’t simply move or migrate, you need a grab-bag of genetic responses to hand.

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