Poinsettias are a very interesting plant. Originally, this plant grows as either a small shrub or a tree in its native range (Mexico). However, the little potted poinsettias that can be bought at the store never grow close to that size. There is a good reason for this: the potted poinsettias are infected with a pathogen known as a phytoplasma. Phytoplasmas are small bacteria that lack a cell wall and are limited to the phloem tissue within plants. They are spread by vegetative propagation, grafting and by sap-sucking insects that feed in the phloem. Normally phytoplasmas are associated with severe disease in plants by altering the structure of plants, such as causing the petals of flowers to develop into leaves instead of petals. One such disease is aster yellows which infects over 300 different plant species in 38 families (see picture below).
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| Aster yellows on the Purple coneflower (Echinacea purpura). Via wikipedia. |
But what does this have to do with poinsettias? It turns out that the phytoplasma that infects the potted poinsettias also causes stunting, but doesn't cause severe distortion of the leaves. Not only is the plant stunted, but it produces additional branches where normally the plant has a single branch. The trait has been used since the 1920's to sell poinsettias during November/December in North America. This trait was found to be transferable by grafting but could be lost when the plant was subjected to heat treatment or tissue culture techniques. It was thought that another pathogen that is common in poinsettias was the cause of the stunting, Poinsettia mosaic virus, but it wasn't until the 1990's that a phytoplasma was shown to be the causal agent of the stunting. Previous work found that the virus wasn't completely associated with this trait as plants without the virus developed stunting and free branching. This article contains further information on the history of poinsettias and the work to determine why commercial poinsettias are stunted and free branching. This article discusses how to care for poinsettias.
The phytoplasma infecting poinsettias is a perfect example of how not all pathogenic organisms are bad and that they might be of benefit. This is certainly the case in the ornamental industry as it is the basis of the entire ornamental poinsettia industry.
