Spare a thought for Poinsettias this Christmas

 
Poinsettias are a common part of Christmas floral displays but their economic success is partially due to infection by a pathogen.

Originating in Mexico, poinsettias were introduced to the United States in 1825 by the first U.S. Ambassador to Mexico. In the wild, they actually look like this:

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Wild Poinsettia

A little bit different to the poinsettias we are used to seeing.

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Commercial Poinsettia

This difference between the wild and the cultivated form is due to cultivation of the plant over many years-changing it from a tall, straight tree to a small bushy plant-and infection by a bacterial pathogen, phytoplasmas. I previously talked about phytoplasmas here.

There are two types of commercially grown poinsettia, which can be seen in the image below. Poinsettias which have a restricted branching morphology produce fewer ‘flowers’ (actually modified leaves) than the free-branching morphotype. The latter is more important economically because they produce more flowers (to sell on) and this morpohology is more desirable in a potted plant.

On the left- restricted-branching poinsettia morphotype. Right- free-branching morphotype

In 1997 it was shown that this free-branching morphology was due to infection by a phytoplasma. Phytoplasmas are able to interfere with stem development to create a smaller bushier plant, which happen to be desire by the growers.
In conclusion, a lot of research goes into controlling plant pathogens because of the loss of yield that they cause but there are cases where their intervention is desired- for example, Botrytis in wine and the colour-breaking in tulip petals caused by a virus.
TBVtulip.jpg

Colour break in tulips.

 So, spare a thought for poinsettias this Christmas, those characteristics are actually symptoms of disease.
References:
Lee, I-M et al. (1997) Phytoplasma induced free-branching in commercial poinsettia cultivarsNature Biotechnology, 15, 178-182.
I recommend the review below for a more detailed history of poinsettias:
Photos:
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