Restricted- and free-branching poinsettia plants. Cultivar V-17 Angelika Red (right), derived from a vegetative cutting of a stock plant, exhibits the free-branching morphotype; the same cultivar (left), derived from somatic embryo exhibits the restricted-branching morphotype.
For more than a decade, a biological agent has been suspected to cause free-branching in poinsettias. Attempts to identify the branching agent have failed.
In 1996, we proved that phytoplasma is the biological agent causing free-branching. This is the first reported example of a pathogenic phytoplasma as the causal agent of a desirable and economically important trait, free-branching in poinsettia.
For details see Nature Biotechnology 15:178-182., 1997.