Pathogens push poinsettias to branch out.

Like a lot of other people, Ing-Ming Lee still has a Christmas poinsettia hanging around. But the U.S. Department of Agriculture researcher in Beltsville, Md., is less interested in the plant for interior decoration than for its inner workings.

Inside the plant's tissue lives a pathogen-no one has been sure what kind-that turns the otherwise tall, straight tropical plant into a bushy, branching, seasonal showpiece. In the February Nature Biotechnology, Lee and his colleagues at USDA and Ball FloraPlant in West Chicago, Ill., report that the branching pathogen is not a virus, as had been suspected, but a phytoplasma, the tiniest kind of bacterial cell known.

Phytoplasma is a relatively new term for the plant-associated equivalents of mycoplasmas, an amorphous group of bacteria that infect animal cells. Several hundred different phytoplasmas are known to target plants, causing such diseases as witches' broom and aster yellows.

Like mycoplasmas, phytoplasmas lack cell walls and cannot be grown in the lab. By screening commercial poinsettias with a set of DNA segments, Lee's team could pick up evidence of the pathogen's genetic sequence and thus identify it as a new type of phytoplasma.

The researchers then performed a series of experiments to demonstrate that the newly detected phytoplasma, not the poinsettia virus, causes the plant's branching. First, they connected poinsettia and periwinkle plants, which do not support the poinsettia virus, with a parasitic plant called dodder. Three to 6 months later, the experimental periwinkles were bushier than control plants.

Then the researchers used dodder as a bridge between the well-branched periwinkles and nonbranching poinsettias. Three to 4 months later, the poinsettias began to turn bushy, and the DNA test showed evidence of the phytoplasma-but not of virus.

"It's extremely well done," says plant microbiologist Robert Goodman of the University of Wisconsin-Madison. Lee's group did not address how the phytoplasma forces the poinsettia to branch out, but Goodman says it fits with a general belief that these agents operate by subverting a plant's growth hormones.

Of the poinsettia phytoplasma, Lee says that "technically, it's a disease. But there's not too much harm for the poinsettia." If anything, the pathogen has improved the plant's survival: 60 million pots of bushy poinsettias are sold each Christmas. Moreover, like the virus responsible for streaked tulips, the phytoplasma is beneficial to growers, generating $325 million annually.