One example of a useful microorganism is a Phytoplasma that infects the phloem of poinsettias. Phloem is like a blood vessel for plants: it takes the nutrients and water from the roots to the leaves. Poinsettias that are fuller and more highly branched are more marketable. To get these fuller plants, nurseries propagate them by grafting branches of Phytoplasma -infected poinsettias onto young poinsettias. The Phytoplasmas are infected in the wild by insects called leaf hoppers. When the leaf hoppers bite into the plant they leave behind the Phytoplasma. When infected, the poinsettia is shorter and has many more branches. How the microorganisms make the poinsettia shorter and fuller is not presently known.
Ing-Ming Lee and co-workers at the U.S. Department of Agriculture's Agricultural Research Service in Beltsville, Md., recently published an article identifying and describing this Phytoplasma (The previous link will take you to a search engine called PubMed. If you place "Lee IM" in the search box and only look for the past years articles then Lee's article and its title will show up as the first item found. If you select the abstract then you will get a short description of the article. Nature, Biotechnology Volume 15, pages 178-182, 1997).
Several hundred other Phytoplasma have been described. Most cause plant diseases like Yellow's disease, witches' brooms, pear decline, and peach X-disease. If you want more information on these plant diseases go to The Forest Pathology Course at Southern University of New York (SUNY). Researchers have yet to grow Phytoplasma in a test tube. If they could then research would progress much faster on how these organisms cause changes in the plants they infect. These organisms are bacteria that lack a cell wall. The Phytoplasma are transmitted from plant to plant by insects called leaf hoppers. These insects can be found in your garden and in the forests. How the organisms cause poinsettias to be more highly branched is not presently known.
If you want more information on Phytoplasmas go to The Forest Pathology Course at SUNY. Suite 101's own Diana Pederson has a wonderful article on poinsettias (Euphorbia pulcherrima) as well as a wealth of information on indoor gardens. Her article is titled The Fascinating World of Euphorbias.
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