Cracking crop genomes for food security

French researchers took the first step last week in mapping the wheat genome, the genetic instruction manual for the crop.

By publishing a physical map of the largest wheat chromosome in the Oct. 3 issue of the journal Science, Blaise Pascal University researcher Etienne Paux and his colleagues shattered the long-held belief that the grain’s genome was too complex to sequence.

"Our work may pave the way for a major change in how the next genomes for de novo sequencing are selected, thereby accelerating improvements in economically important crop species," Paux and his colleagues wrote in their paper.

The work also provides researchers with opportunities to improve wheat’s yield and disease-resistance abilities, as well as possibly developing crop varieties that can tolerate a variety of climate conditions.

"Along with rice, wheat is the premier grain that feeds the world," Emanuel Epstein, a plant physiologist and professor emeritus of land, air and water resources at the University of California at Davis, told the Cleantech Group.

"Rice is simple, so the mapping of wheat is exceedingly important because it is a grain that has already spread to areas, from very cold and short summers to very warm climates with long summers."

Wheat is a staple food for 35 percent of the world’s population, but the 17-gigabase genome was widely considered near-impossible to sequence. One reason was that more than 80 percent of the genome consists of repeated sequences of DNA bases. Another problem was the size: The single wheat chromosome sequenced thus far is a whopping 1 gigabase in size. In comparison, the entire rice genome is a mere 400 megabases. The entire maize genome is 2.6 gigabases, nearly three times the size of the single wheat chromosome sequenced thus far. And the human genome is 3.2 gigabases, five times smaller than the entire wheat genome.

Paux and his colleagues demonstrated that the wheat genome could be sequenced in bite-sized pieces. If the genome is a full instruction manual, a chromosome is a chapter, containing information for several genes. The wheat genome has 21 chromosomes, and the other 20 are still being sequenced.

The chromosome-by-chromosome approach also means that sequencing other complex plant genomes is possible.

"It is a tremendous amount of work since a single wheat chromosome has more DNA than the complete rice genome, but that goal of sequencing the wheat genome is still very far," UC Davis plant scientist and wheat researcher Jorge Dubcovsky said.

"We still need to complete the physical map of that chromosome, sequence all the chromosome, annotate that huge amount of sequence and then repeat that painful process 20 more times to complete the job."

Sequencing the wheat genome could one day lead to increased global food security. In a report released Oct. 7 by the UN Food and Agriculture Organization, FAO Director-General Jacques Diouf raised concerns about the worldwide effects of promoting food crops for biofuel use.

Diouf said that biofuels have not lived up to their promise of reducing greenhouse gas emissions while promoting energy security. The issue was raised earlier this year in several scientific studies (see Biofuel industries call studies "naïve").

Biofuels have also raised concerns about global food security as well, Diouf said (see Sowing a solution for sustainable biofuel production).

Diouf blamed the global price increase in food crops on farmers' decisions to shifting food crops to energy use to benefit from government subsidies and other incentives offered to encourage production.

According to FAO estimates, 75 million people worldwide were malnourished in 2007 because they could not afford to buy enough food.

Diouf also noted that, although biofuels are unlikely to become a significant global energy supplier, demand for feedstocks such as sugar and maize will continue to rise over the next decade, further increasing food prices.

"Decisions about biofuels should take into consideration the food security situation but also the availability of land and water," Diouf said during the conference. "All efforts should aim at preserving the utmost goal of freeing humanity from the scourge of hunger."

Both Dubcovsky and Epstein said that using the wheat genome sequence to produce crops that might have higher yields or might be more resistant to conditions such as drought is dependent on how much time and work will be invested in crop research.

"Will it happen? Yes. Must it happen? Yes," Epstein said. "But the time element would be pretty presumptuous for anyone to predict."

Both scientists called for increased investment in crop productivity research to improve food security.

"Plant science is faring poorly because people are more concerned with being sick," Epstein said. "But in our country at least, being able to get enough to eat is taken for granted by the great majority. So the science of plant nutrition deserves far more support than it is getting."

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