Unit 3. Fungi

Script 5. Plant communications

Beans' talk

Vegetables employ fungi to carry messages between them.

The idea that plants have developed a subterranean internet, which they use to raise the alarm when danger threatens, sounds more like the science-fiction of James Cameron's film "Avatar" than any sort of science fact. But fact it seems to be, if work by David Johnson of the University of Aberdeen is anything to go by. For Dr Johnson believes he has shown that just such an internet, with fungal hyphae standing in for local Wi-Fi, alerts bean stalks to danger if one of their neighbours is attacked by aphids.

The experiment which suggests this was following up the discovery, made in 2010 by a Chinese team, that when a tomato plant gets infected with leaf blight, nearby plants start activating genes that help ward the infection off - even if all airflow between the plants in question has been eliminated. The researchers who conducted this study knew that soil fungi whose hyphae are symbiotic with tomatoes (providing them with minerals in exchange for food) also form a network connecting one plant to another. They speculated, though they could not prove, that molecules signaling danger were passing through this fungal network.

Dr Johnson knew from his own past work that when broad-bean plants are attacked by aphids they respond with volatile chemicals that both irritate the parasites and attract aphid-hunting wasps. He did not know, though, whether the message could spread, tomato-like, from plant to plant. So he set out to find out - and to do so in a way which would show if fungi were the messengers.

As they report in Ecology Letters, he and his colleagues set up eight "mesocosms", each containing five beanstalks. The plants were allowed to grow for four months, and during this time every plant could interact with symbiotic fungi in the soil.

Not all of the beanstalks, though, had the same relationship with the fungi. In each mesocosm, one plant was surrounded by a mesh penetrated by holes half a micron across. Gaps that size are too small for either roots or hyphae to penetrate, but they do permit the passage of water and dissolved chemicals. Two plants were surrounded with a 40-micron mesh. This can be penetrated by hyphae but not by roots. The two remaining plants, one of which was at the centre of the array, were left to grow unimpeded.

Five weeks after the experiment began, all the plants were covered by bags that allowed carbon dioxide, oxygen and water vapour in and out, but stopped the passage of larger molecules, of the sort a beanstalk might use for signalling. Then, four days from the end, one of the 40-micron meshes in each mesocosm was rotated to sever any hyphae that had penetrated it, and the central plant was then infested with aphids.

At the end of the experiment Dr Johnson and his team collected the air inside the bags, extracted any volatile chemicals in it by absorbing them into a special porous polymer, and tested those chemicals on both aphids (using the winged, rather than the wingless morphs) and wasps. Each insect was placed for five minutes in an apparatus that had two chambers, one of which contained a sample of the volatiles and the other an odourless control.

The researchers found, as they expected from their previous work, that when the volatiles came from an infested plant, wasps spent an average of 3 1/2 minutes in the chamber containing them and 1 1/2 in the other chamber. Aphids, conversely, spent 1 3/4 minutes in the volatiles' chamber and 3 1/4 in the control. In other words, the volatiles from an infested plant attract wasps and repel aphids.

Crucially, the team got the same result in the case of uninfested plants that had been in uninterrupted hyphal contact with the infested one, but had had root contact blocked. If both hyphae and roots had been blocked throughout the experiment, though, the volatiles from uninfested plants actually attracted aphids (they spent 3 1/2 minutes in the volatiles' chamber), while the wasps were indifferent. The same pertained for the odour of uninfested plants whose hyphal connections had been allowed to develop, and then severed by the rotation of the mesh.

Broad beans, then, really do seem to be using their fungal symbionts as a communications network, warning their neighbours to take evasive action. Such a general response no doubt helps the plant first attacked by attracting yet more wasps to the area, and it helps the fungal messengers by preserving their leguminous hosts.

Plant-fungus symbiosis is a surprisingly underexplored area of biology. The limited data available suggest most plants go in for it in one form or another, but its role is only slowly being illuminated. Work like Dr Johnson's suggests this is a serious omission, not least for the understanding of how crops like beans actually grow. The underground world, though invisible to the human eye, should not for that reason be ignored or underestimated. (From The Economist, July 06, 2013)

Script 5.1 Plant communications. Part I.

Beans' talk

Vegetables employ fungi to carry messages between them.

The idea that plants have developed a subterranean internet, which they use to raise the alarm when danger threatens, sounds more like the science-fiction of James Cameron's film "Avatar" than any sort of science fact. But fact it seems to be, if work by David Johnson of the University of Aberdeen is anything to go by. For Dr Johnson believes he has shown that just such an internet, with fungal hyphae standing in for local Wi-Fi, alerts bean stalks to danger if one of their neighbours is attacked by aphids.

The experiment which suggests this was following up the discovery, made in 2010 by a Chinese team, that when a tomato plant gets infected with leaf blight, nearby plants start activating genes that help ward the infection off - even if all airflow between the plants in question has been eliminated. The researchers who conducted this study knew that soil fungi whose hyphae are symbiotic with tomatoes (providing them with minerals in exchange for food) also form a network connecting one plant to another. They speculated, though they could not prove, that molecules signaling danger were passing through this fungal network.

Dr Johnson knew from his own past work that when broad-bean plants are attacked by aphids they respond with volatile chemicals that both irritate the parasites and attract aphid-hunting wasps. He did not know, though, whether the message could spread, tomato-like, from plant to plant. So he set out to find out - and to do so in a way which would show if fungi were the messengers.

Script 5.2 Plant communications. Part II.

As they report in Ecology Letters, he and his colleagues set up eight "mesocosms", each containing five beanstalks. The plants were allowed to grow for four months, and during this time every plant could interact with symbiotic fungi in the soil.

Not all of the beanstalks, though, had the same relationship with the fungi. In each mesocosm, one plant was surrounded by a mesh penetrated by holes half a micron across. Gaps that size are too small for either roots or hyphae to penetrate, but they do permit the passage of water and dissolved chemicals. Two plants were surrounded with a 40-micron mesh. This can be penetrated by hyphae but not by roots. The two remaining plants, one of which was at the centre of the array, were left to grow unimpeded.

Five weeks after the experiment began, all the plants were covered by bags that allowed carbon dioxide, oxygen and water vapour in and out, but stopped the passage of larger molecules, of the sort a beanstalk might use for signalling. Then, four days from the end, one of the 40-micron meshes in each mesocosm was rotated to sever any hyphae that had penetrated it, and the central plant was then infested with aphids.

At the end of the experiment Dr Johnson and his team collected the air inside the bags, extracted any volatile chemicals in it by absorbing them into a special porous polymer, and tested those chemicals on both aphids (using the winged, rather than the wingless morphs) and wasps. Each insect was placed for five minutes in an apparatus that had two chambers, one of which contained a sample of the volatiles and the other an odourless control.

The researchers found, as they expected from their previous work, that when the volatiles came from an infested plant, wasps spent an average of 312 minutes in the chamber containing them and 112 in the other chamber. Aphids, conversely, spent 114 minutes in the volatiles' chamber and 314 in the control. In other words, the volatiles from an infested plant attract wasps and repel aphids.

Crucially, the team got the same result in the case of uninfested plants that had been in uninterrupted hyphal contact with the infested one, but had had root contact blocked. If both hyphae and roots had been blocked throughout the experiment, though, the volatiles from uninfested plants actually attracted aphids (they spent 312 minutes in the volatiles' chamber), while the wasps were indifferent. The same pertained for the odour of uninfested plants whose hypha! connections had been allowed to develop, and then severed by the rotation of the mesh. Broad beans, then, really do seem to be using their fungal symbionts as a communications network, warning their neighbours to take evasive action. Such a general response no doubt helps the plant first attacked by attracting yet more wasps to the area, and it helps the fungal messengers by preserving their leguminous hosts.

Plant-fungus symbiosis is a surprisingly underexplored area of biology. The limited data available suggest most plants go in for it in one form or another, but its role is only slowly being illuminated. Work like Dr Johnson's suggests this is a serious omission, not least for the understanding of how crops like beans actually grow. The underground world, though invisible to the human eye, should not for that reason be ignored or underestimated.

Script 6. Violin-making

Magic mushrooms

Violins constructed from infected wood sound like those of Stradivari.

A few years ago Francis Schwarze noticed something unusual. Dr Schwarze, who works at the Swiss Federal Laboratories for Materials Science and Technology, in St Gallen, knew that sound travels faster through healthy wood, which is stiff and dense, than it does through the soft stuff left by a fungal attack. But some fungi, he found, do not slow sound. Moreover, the acoustic properties of wood so affected seem to be just what violin-makers desire. So Dr Schwarze had some violins made from the infected wood and discovered that they sounded like a Stradivarius.

Dr Schwarze is now trying to standardize this fungal treatment in order to make what he calls "mycowood". His hope is that it will endow modern instruments with the warm and mellow tones found in those made during the late 17th and early 18th centuries by Antonio Stradivari.

Exactly what makes a Strad so magical is contentious. Besides excellent craftsmanship, the master and members of his workshop in Cremona used different types of wood and, possibly, different chemical treatments. The period when they were active does, though, coincide with a cold spell in Europe's climate that occurred between 1645 and 1715. In the long winters and cool summers wood would have grown slowly and evenly, creating a lot of stiffness. Which is exactly what a good violin needs.

Treating wood with certain fungi endows it with similar properties. The species Dr Schwarze lit upon are Physisporinus vitreus, a type of white rot, and Xylaria longipes, commonly known as Dead Moll's Fingers. He applies them to Norway spruce (used for an instrument's body) and sycamore (for the back, ribs and neck).

What is unusual about Physisporinus and Xylaria is that they gradually degrade the cell walls of the wood they infect - thinning them rather than destroying them completely. That leaves a stiff scaffolding through which sound waves can readily pass, without compromising the wood's elasticity. When the fungi have done their work, Dr Schwarze treats the planks with a gas that kills the infection. He then hands the result over to Martin Schleske and Michael Rhonheimer, two master violin-makers, for conversion into instruments.

And it works. A blind trial conducted in 2009 by Matthew Trusler, a British violinist, for example, compared modem violins made with treated and untreated wood from the same trees with a Stradivarius made in 1711. A jury of experts, and also most of the audience, thought that the mycowood violin was the Strad. (From The Economist, September 22, 2012)