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Fighting back: How plants respond to wounds

Fighting back: How plants respond to wounds Gilroy et al
25 Sep
New videos reveal how plants respond to wounds, sending forth a wave of calcium to signal the need for defence

Spare a thought for that pot plant, neglected in a corner. Turns out it’s working harder than you might think. The humble plant is more aware of its environment and more active in response than the human eye could ever spot – until now.

Simon Gilroy, a professor of botany at the University of Wisconsin-Madison, knows that plants are dynamic and has spent much of his career trying to understand how they work. Now, his laboratory has produced a series of videos that reveals how plants respond to wounds (caterpillar bites, scissor cuts and crushing blows), utilising the visual power of a naturally occurring fluorescent-green protein.

The videos show that when one part of a plant is attacked or damaged, a wave of calcium (highlighted green) spreads throughout the rest of the plant. The calcium acts as a warning signal, alerting the plant to danger and the need to deploy defence tactics.

While scientists already knew that plants reacted to danger via an electrical charge that moves across the plant, they didn’t know exactly how it happened. Gilroy and one of his post-doctoral researchers, Masatsugu Toyota (who led the study), suspected it had something to do with calcium and have been able to confirm their suspicions. They were also able to reveal how glutamate – an abundant neurotransmitter in animals – activates this wave of calcium.

The find was fortuitous given that Gilroy hadn’t intended to study wounding at all. His real passion is understanding how plants sense gravity and seem to know which way is up – something that’s proving extremely hard to work out. It was during the early stages of an experiment into gravity that Toyota came across the wounding response.

‘We work very intensely on the calcium signal, because it’s a ubiquitous signal. Biology uses it absolutely everywhere,’ explains Gilroy. ‘It makes your heart beat, it makes your muscles contract. Plants use it for a lot of their signalling machinery. We had some hints that the gravity-sensing system is based around the calcium signal and so we were developing the technology to image calcium cells in real-time.’ It was during this process that Gilroy and Toyota realised they’d captured something never usually visible to humans.

The experiment was carried out on plants built by Toyota in the laboratory (it has subsequently been tested on larger tobacco plants and rice with identical results), constructed so that the fluorescent protein would bind to the calcium, making its path visible. The team then gave the plants a number of unpleasant tests and watched the results under the microscope. They found that the calcium travels at one millimetre per second, quick enough to spread to other leaves in just a couple of minutes. From the data collected so far it appears that the distance the calcium travels depends on the extent of the wound, or, as Gilroy puts it: ‘The more you hurt it, the louder it screams.’

That ‘scream’ can result in a range of responses designed to combat the threat. ‘Plants are masters of chemistry,’ says Gilroy. ‘We deal with the world by running away from it, plants deal with the wold by growing in response to it, or by making a tonne of stuff.’ That ‘tonne of stuff’ could be chemicals that poison a would-be snacker, or that make the plant unattractive, tough or unpalatable to its prey. Some plants even make proteins that block the ability of a caterpillar’s gut to digest the plant material. This is dinner that fights back.

The next step for Gilroy and his team is to delve deeper into the signalling process on a cellular level, dissecting the genes and proteins responsible. In comparison to our understanding of human nerve cells, Gilroy admits that the equivalent responses in plants are still barely understood. He also intends to widen the scope of the study and look at other signals that plants send out – signals regarding temperature and changes in light and touch.

Though Gilroy is happy enough to study plant mechanisms simply for the joy of understanding (‘I’m a university professor!’), he explains that there may be a wider use for the research, albeit a long way in the future. Once scientists have managed to identify the specific genes that make the signalling process work and can understand what happens when you switch those genes on and off, it’s not hard to think about the potential.

‘You can imagine that we should be able to take a crop plant and switch on its defences on-call,’ says Gilroy. ‘We’re nowhere near that point yet but once we get there – say you’re in a field and you predict there’s going to be an outbreak of some pest, you could go in and pre-defend all of the plants in the field, but you do it on-call so the plants aren’t wasting their resources defending themselves the whole time.’

For now though, Gilroy is happy to simply increase understanding of plants. He is energetic in his insistence that they are much more active than they let on. For that reason he’s as enthusiastic about the way the videos bring the response process to life as he is about the future potential of the research.

‘When you look at a plant, just because it doesn’t do what we do, and it doesn’t move, that doesn’t mean it isn’t doing anything. They’re hugely dynamic organisms,’ he says. ‘They are running as fast as a human being does in order to deal with the world around them. If you are stroking your plant, it knows you are there.’

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