Hidden Bacterium Plays Key Role in Poinsettia Height, Branching

Eight-foot-tall poinsettia plants might be the norm for Christmas decorating were it not for a microscopic tenant called a phytoplasma.

The bacteria-like organism serves as an important dwarfing agent that keeps the tropical plant roughly 18 inches tall instead of 8 feet--the norm in its native Mexico. Agricultural Research Service plant pathologist Ing-Ming Lee and former Ball FloraPlant colleagues in Chicago came to this conclusion through a series of plant-grafting experiments and genetic fingerprinting of the phytoplasma’s DNA signature.

The finding gives commercial poinsettia growers a first-time opportunity to breed wholesale stock plants without another microtenant: the poinsettia mosaic virus. Lee has identified 20 commercial cultivars that harbor it. Under certain conditions, this normally benign virus causes an unsightly leaf disease. The disease is more common in Europe than America, where wholesale poinsettia value is over $200 million annually.

The phytoplasma triggers a hormonal imbalance that instructs the plant to grow outward, rather than up like a tree. This “free-branching” phenomenon also produces more of the brilliant red, leaf-like bracts that American consumers find so appealing. For decades, credit for free-branching went to the virus, because heat treatments to kill it also stopped the poinsettia’s free-branching growth. But Lee and his Chicago colleagues exposed the virus for what it truly is: a nuisance with no role in free-branching.

Lee is now trying to determine how the phytoplasma affects cytokinin hormones that stimulate cell division in poinsettia plants. He's based at ARS’ Molecular Plant Pathology Laboratory, in Beltsville, Md. A story about the research appears in December's Agricultural Research magazine and on the web at:

http://www.ars.usda.gov/is/AR/archive/dec98/poin1298.htm

ARS is the U.S. Department of Agriculture’s principal research arm.

Scientific contact: Ing-Ming Lee, ARS Molecular Plant Pathology Laboratory, Beltsville, Md., phone (301) 504-6024, fax (301) 504-5449, imlee@asrr.arsusda.gov