How the humble potato could feed the world – Part Two

In other crops that have more than two pairs of chromosomes breeders have found ways around the problem. Most wheat has six copies, but wheat breeders start with plants that are already inbred so that for most genes, all six copies are identical. That way they can predict the outcome of crosses. Attempts to do this with potatoes, and also to engineer potato plants with only two-copy genomes, have been disappointing, says Shelley Jansky of the US Department of Agriculture’s potato lab in Madison, Wisconsin. The genetically impoverished potatoes are spindly and weak. “Potatoes just need all that internal genetic diversity to thrive,” she says.

That means potato breeders are forced to take a broad approach when looking for useful new varieties. First, they cross genetically diverse parent plants to create up to 100,000 genetically different progeny. Then, they “walk across the field and choose the potatoes they think look promising, and get it down to a manageable number, say a thousand”, says Jansky, and examine those plants for the qualities they want.

This kind of classical breeding has given us all the potato varieties we have today, but it is very difficult to use this method to breed a single desired trait into an existing commercial potato variety. Recent efforts to cross commercial varieties with Solanum bulbocastanum, a wild Mexican potato which has two genes for resistance to all known strains of blight, did indeed result in blight-resistant potatoes – but they had other, unwanted wild genes as well, and lower yields.

Breeding these hybrids back with the original commercial potato will produce tubers more similar to the original, but they will never be quite the same. This is a problem for the potato industry, says Jansky. Processing companies take a third of the crop in rich countries, and the machines and processes are designed for potatoes of particular shapes, sizes and chemical properties. They know their King Edwards and their Russet Burbanks and they want nothing else – and because potatoes are propagated vegetatively by tuber, they can have exactly the same potato again and again, says Jansky.

Genetic engineering could be the answer to this problem, says Anton Haverkort of Wageningen University in the Netherlands. He is running a 10-year programme to find more genes for resistance to late blight in several wild potato species – and then put them, and nothing else, into three popular varieties of eating potato. Haverkort uses a relatively new method of genetic engineering that doesn’t require an antibiotic resistance marker gene – a common tool in creating engineered plants – to be introduced along with the desired genes. So far he has isolated eight genes and the first of his genetically modified plants are now in field trials.

“We call them cisgenic, instead of transgenic,” he says. “They contain no genes except what they could have acquired naturally by breeding with other potatoes – except it hasn’t taken decades.” He hopes EU law will take account of the development and lighten restrictions on such plants, and that Europe’s anti-GM public will accept them. “The only genes in there are from potatoes,” he says. Whether consumers accept cisgenic potatoes remains to be seen. Meanwhile, genetically engineered blight-resistant potatoes created by the German chemical giant BASF are already in their third year of field trials. The company has put the two resistance genes from Solanum bulbocastanum into commercial potato varieties along with an antibiotic resistance marker. BASF says the plants seem to have durable resistance to blight strains circulating in Europe, and it is hoping to start selling them by the middle of next decade.

The antibiotic resistance gene could be a problem, however. Its presence is central to objections to GM food; opponents say the gene could be taken up by bacteria in the environment, creating superbugs. BASF has another genetically engineered potato that yields more uniform starch for the paper and fabrics industries, which the European Commission declared safe last year, but as countries such as Austria harden their resistance to GM crops, it is holding back on the go-ahead for release. The same fate may await the company’s GM food potatoes. Developing countries, having had the potato for less time, seem to be more open to non-traditional varieties, and in some places GM food is less unpopular. China, for example, is rumoured to have developed varieties similar to BASF’s.

In Peru, CIP plans to keep studying how potatoes resist blight, and using its potato gene bank – the world’s largest – to find genes that confer resistance. CIP is using GM to develop late-blight-resistant strains for Asia and is also breeding potatoes conventionally. This is partly because CIP has imposed a moratorium on releasing GM potatoes in South America, where most governments are opposed to GM and where most of the potato’s wild relatives exist, until more is known about whether introduced genes might escape into wild potatoes. But it is also, she says, because “GM is one tool, it doesn’t do everything.” Resistance to blight, for instance, might be achievable by implanting one or two genes at a time, but eventually, the blight will adapt to those few genes. And other, more complex traits like nutritional quality and yield depend on many genes, few of which are known, and can only be bred into farmed varieties the old-fashioned way, says Anderson.

However we come by new varieties, as the humble potato spreads around the world, and more and more people depend on it for sustenance, the need to win the battle against disease becomes more urgent. Blight is a disaster waiting to happen, and this time we have no alternative but to fight back.

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