Researchers chart papaya gene map

Researchers chart papaya gene map

Not many papaya trees in the fields around Champaign-Urbana to be sure, but a University of Illinois professor was in the middle of the event when scientists unveiled a landmark draft of the papaya's gene map this week.

The papaya genome already is revealing new information about angiosperms – or flowering plants, a category that goes far beyond the melonlike tropical fruit rich in vitamins A and C – and the gene map should be a font of knowledge particularly as it relates to sex determination in those plants, UI Professor Ray Ming said.

The findings could be useful in studying plants ranging from roses and poplar trees to grapes and cabbage, radishes and rice.

The work – the cover article of the prestigious journal Nature – also represents the first complete gene sequencing of a transgenic, or genetically modified, crop, said Ming, a UI plant biology professor and papaya expert.

Papaya was a crop in trouble before researchers at Cornell and the University of Hawaii in the early 1990s found a way to make it resistant to the devastating papaya ringspot virus.

Ming and colleagues have identified exactly where the genetic insertions to yield that resistance took hold in the papaya's gene structure, which surprisingly occurred in just three places in the plant's genome and did little to disrupt the natural state of its genes.

Co-lead author of the study, Ming said the finding could help clear domestic papayas, a big crop in Hawaii, for sale in countries such as Japan, which have thus far resisted permitting products using the genetically modified version.

The researchers also found that the papaya has fewer functional genes than any other flowering plant sequenced so far, and they were able to trace its evolutionary divergence from Arabidopsis, a mustard family member whose cousins include cabbage and radishes.

The unveiling of the papaya's gene map would have been news elsewhere, if not in East Central Illinois, without the UI connection.

"It's a major crop ... worldwide in the tropics," Ming said. That includes Hawaii, where Ming worked before coming to the UI in 2005.

He's affiliated with the UI's Institute for Genomic Biology and was enticed from the Pacific paradise to the Midwest by the opportunity the institute affords for collaboration with other UI researchers and access to "the most advanced genomic tools available."

The papaya genome project involved scientists from 22 institutions, including Ming, who remains an affiliate of the U.S. Department of Agriculture's Hawaii Agriculture Research Center, led by Maqsudul Alam at the University of Hawaii. It was largely funded by the university and the agriculture department, along with the Defense Department and Nakai University in China.

Ming said the papaya is a good model for studying sex determination in flowering plants at a molecular level.

Papaya trees can be male, female or hermaphrodite. Growers prefer the latter, Ming said, because in a field mixing males and females, the males don't bear fruit, whereas with hermaphrodites, which handle both ends of the business, every tree is fruit-bearing. The fruit from a hermaphrodite also tends to be compact and easier to ship.

One goal of Ming and colleagues is developing a true hermaphrodite cultivar, one that always yields hermaphrodite plants.

At the same time, the knowledge gained also may help yield ways to further improve the papaya, to make it more resistant to insects, for example, reducing the need for and cost of pesticide applications.

Scientific breakthroughs at a basic level, like the molecular mechanics of pest or disease resistance, could "have implications for all crops, not just the papaya," Ming said. That's true for flowering plants in particular, which he noted are a large factor in the biodiversity of plant life worldwide.

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