Tag Archives: Ug99

A juicy candidate: GM oranges take on citrus greening

The story of GM oranges begins in pretty much the same way that every other GM story begins nowadays: an unstoppable disease. The fastest developments all start with a fungus or virus or bacteria that is wiping out a major crop. This time, it’s Huanglongbing or citrus greening as caused by Candidatus Liberibacter, a bacterium carried by psyllids (like aphids, but… not).

In 2005 this long-dreaded disease reached the citrus orchards of Florida, sparking a state-wide campaign of insecticide spraying and preventative chopping down of trees. The problem is that, being spread by a parasite, keeping infected trees away from each other isn’t enough. Just like how people can’t be protected from malaria just by keeping them away from malarial patients, as long as psyllids can make it from infected trees to uninfected trees the disease continues to spread.

Whenever a devestating disease appears, the first port of call is to find a source of immunity. Ug99 is a name known and feared by people in my line of business: a variety of wheat stem rust (originating in Uganda in 1999 – go figure) that wiped out huge parts of the African wheat harvest. When it began rampaging across Africa and Europe in 2006 the first port of call was to find a naturally resistant variety of wheat that could be used to breed with other elite cultivars. A variety of einkorn wheat (a diploid wheat) from Turkey was found to contain a gene Sr35 that conferred resistance. (Well, sort of. Technically they found a QTL. And then the gene earlier this year. I’ll talk about that another time.)

Only there was no resistant variety of citrus tree to be found anywhere. The only option it seemed was to search further afield: Early contenders included one from a bacteriophage (i.e. a virus that kills bacteria), but concerns that people would react more strongly to an organism modified with genes from a virus put that one to bed. Similar fears about a well-performing tree with a gene from pig put that one on the back burner too. An alternative was a gene from spinach, and in 2010 this was finally trialled.

Sunday’s New York Times carried this lovely piece about the fight to win both regulatory approval and consumer acceptance of the new GM orange trees, which will hopefully be available for juice production in the next 5 years.

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C Saintenac, W Zhang, A Salcedo et al (2013) Science
“Identification of Wheat Gene Sr35That Confers Resistance to Ug99 Stem Rust Race Group”
Science DOI: 10.1126/science.1239022

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