'Doubled Haploids' Speed Breeding

LSU AgCenter rice breeder Qi Ren Chu grows the equivalent of thousands of acres of rice in his lab – a room the size of your kitchen.

“Instead of growing plants in a field, we grow five million pollen (grains) in a Petri dish,” explains Chu, straining to help a layman understand how he coaxes green rice plants to sprout in what starts out as a dish full of pollen swimming in a special chemical soup.

“The process has been established. It is legitimate. It is not a question of whether we can do it. It is a question of what scale is possible and what is the marketability of what you create,” says Chu, a native of China recognized as one of his country’s top rice breeders.

Chu cut his teeth as a research assistant and associate plant breeder at a top rice lab in Shanghai, China, starting in the 1970s. He earned a Ph.D. in genetics from LSU in 1988 and later rose to full professor and director of the Agro Biotechnology Center at the Shanghai Academy of Agricultural Sciences back home.

Chu signed on as a full-time rice breeder at the LSU AgCenter’s Rice Research Station in Crowley, La., in 1995. His specialty is anther culture, the process of growing complete rice plants from just the male half of the rice plant’s internal reproductive hardware.

The technique, first perfected in corn decades ago, requires a researcher to extract anthers from the head of the rice plant and make them grow in a Petri dish. Each anther contains 1,000 or more grains of pollen, the male part of a rice plant. The pollen is considered a “haploid,” meaning it has 12 chromosomes or exactly half of a rice plant’s normal 24-chromosome make-up.

When growing in soil, a rice plant needs both pollen and its ovule or egg (the plant’s female half, also with 12 chromosomes) to create the next generation of seed. The chromosomes are the seat of the rice plant’s genes, which govern everything from how quickly a rice plant will mature, how hearty it is against disease and how tall it will grow.

But Chu bypasses the normal fertilization method and instead uses a method known as a “doubled haploid” to grow a complete but uniform series of plants – all (or at least most) of which have the same characteristics.

The idea is to speed up efforts to breed new types of rice that mesh the attributes of one variety of rice with the favored aspects of another. For instance, the goal might be to create a new rice variety that matures early (like Rice “A”) but also one that has strong resistance to disease (like Rice “B”).

Using the doubled haploid method lets scientists breed rice more quickly, bypassing at least some of the confusion that comes when trying to select plants with a nearly uniform set of characteristics in the field.

In doubled haploids, the rice pollen is placed in a Petri dish suspended in a mixture of sucrose and other micronutrients that enhance the chances of the 12 male chromosomes spontaneously doubling to give you a complete plant.

In conventional rice breeding, it can take up to 10 generations of growing plants in the field after an artificial hybridization or “cross” is made to perfect a 99.99 percent pure variety. Crossing “Rice A” with “Rice B” in the field isn’t an exact science, especially when juggling the thousands of genes in a rice plant.

Plants in the field might appear identical at first, but neighboring plants from the same cross can have different degrees of disease resistance, for example, depending on whether Plant A’s traits or Plant B’s end up as dominant.

“In conventional breeding, you make the initial cross, grow out the F1 plants, grow the F2 (second generation), select a large number of plants, grow the F3 (third generation), look to see which plants are uniform,” Chu said. “With doubled haploids, we go from anther (or pollen) in a Petri dish to plants in the greenhouse within two and a half months. The pollen regenerates into entire plants.”

Chu says an 8 percent success rate (eight anthers growing up to be plants out of 100 anthers in a dish) when breeding U.S. long-grain rice varieties is acceptable and gives researchers enough healthy plants for further testing.

Steve Linscombe, director of the Southwest Region and a rice breeder, said the doubled haploid method is another tool in the rice breeder’s arsenal.

Linscombe said he expects one or more of Chu’s creations to be released by the LSU AgCenter in the next two to five years.

At this point, conventional rice breeding methods are still the research station’s No. 1 focus. And breeders use a winter nursery in Puerto Rico, which allows for multiple rice crops every year, to speed the development of new varieties. Use of the winter nursery has cut conventional rice breeding time frames to roughly eight years – down from about 12 years not so long ago.

But Linscombe said Chu’s approach has a definite place and could result in new varieties being released in as few as six years from Petri dish to farm.

Chu said he’s optimistic about several varieties in the doubled haploid pipeline.

“We have dozens of lines with excellent yield potential. Sooner or later we will release one,” Chu predicts.

Linscombe cautions that it still takes a lot of field work to fully test any new rice variety.

“You can’t fully analyze yields on an individual plant basis in the greenhouse,” Linscombe said. “Field conditions, with all their variables, are difficult to mimic in the greenhouse. Disease in a real field is a whole new ballgame.”

That’s why even varieties that start in Chu’s lab still have to weather several years of analysis outdoors, including progeny row testing, preliminary yield tests, advanced yield trials and three years of multi-state testing. “We not only work in the lab, we put a lot of time and effort in the field as well,” Chu said.

Chu’s research project, like many others at the Rice Station, is supported by Louisiana rice farmers through check-off contributions under the direction of the Louisiana Rice Research Board.


(This article appeared in the fall 2003 issue of Louisiana Agriculture.)