Peanuts get the Generation treatment

	credit:D Bertioli

From its origins in central South America, the peanut has spread far and wide to become one of the most extensively grown grain legumes in the world. The nutritious qualities first recognised by Brazilian Indians thousands of years ago guaranteed its passage to Asia and Africa, where it is now a key crop for smallscale farmers. But alongside the benefits that make growing peanuts so appealing, farmers have had to cope with significant drawbacks resulting from its narrow genetic base. However, new techniques are opening up a wealth of genetic possibilities for peanuts, by allowing access to the genetic diversity left behind in South America when peanut separated from its wild relatives.

Arachis hypogaea - peanut, or groundnut - is one of about 80 known species of the Arachis genus. FAO statistics reveal its global value: it is the 13th most important food crop, the 4th most important source of edible oil, and the 3rd most important source of vegetable protein. More than 90 per cent of the total annual production of 36 million tonnes comes from developing countries; yet diseases and drought mean that farmers in sub-Saharan Africa, for example, struggle to obtain yields of around 0.8 tonnes/ha, compared with yields of around 3.25 tonnes/ha in the irrigated and pesticide-treated fields of the USA. Leaf spot and rust fungi are the main culprits, as well as viral diseases and nematode pests.

Birth of the peanut

The story of the first peanut plant explains its isolated genome. Scientists are now convinced that all Arachis hypogaea grown today originate from just one, or very few, plants that resulted from a serendipitous cross thousands of years ago. Although this produced the peanut as we know it, it also isolated the species genetically because of its rare tetraploidy (that is, having four chromosome sets rather than the usual two found in diploid species). Resulting from a cross between two diploid species, with distinct genomes (A and B), the resulting AB species would have been infertile but for a very rare spontaneous duplication event. This created a fertile plant, with a tetraploid AABB genome which was then unable to cross back with its diploid relatives. Useful disease and drought resistance were some of the genetic traits 'left behind' in the wild relatives. Poor farmers would undoubtedly benefit, if these useful traits could be made available.

And that is the aim of a worldwide team of scientists, led by a group at the Brazilian Agricultural Research Corporation (EMBRAPA) and the Catholic University of Brasília. The approach involves creating 'AB hybrids' from wild relatives, and artificially re-creating the duplication event, to produce new tetraploid hybrids that are fertile and can cross with A. hypogaea.

"We've created two synthetic tetraploid hybrids so far," says David Bertioli, leader of the project. "They show good resistance to leaf spot and rust, and we've successfully crossed them with peanut to introduce these traits. Although, we still have quite a few undesirable wild genes as well, we're gradually eliminating them."

A wealth of genetic promise

Peanut (A. hypogaea) crossed with the synthetic hybrid (A. ipaensis x A. duranensis) (left) shows resistance to combined attack by the leaf spot and rust fungal pathogens, compared with normal peanut on the right.
credit: D Bertioli

The first hybrid was a simulation of the original ancestral pairing, but was delayed while researchers tried to agree on the two Arachis species that had created the peanut. Advances in a molecular detection technique called fluorescent in-situ hybridisation pointed them towards the 'A-genome' species A. duranensis and the 'B-genome' bearing A. ipaensis, and the genetic similarity to peanut of the resulting hybrid satisfied its creators. Their success was crowned when the plants bred to produce fertile hybrids with peanut. A second AABB hybrid from different species has since been produced, and more are in development. Each new sexually compatible hybrid opens up a wealth of genetic promise for peanut farmers.

Central to the success of this work has been the creation of a 'road map' that ranks wild species in similarity to the AA and BB genomes of peanut and which indicates the wild species most likely to produce fertile synthetic hybrids. At the same time, the search for useful genes within these species is under way, using genetic maps and new DNA markers. Drought resistance is at the top of the list.

The project resides under the global network of the Generation Challenge Program, and exemplifies the aims of the programme, to deliver the benefits of plant genetic diversity and genomics tools to poor farmers. Generation has helped Bertioli build crucial links with African and Asian partners, so that the results of his team's work should, within a few years, reach far and wide, to the fields where they are most needed.

For more information, contact David Bertioli
Article by Anne Moorhead