On occasion you might come across an odd-looking plant where the flower organ has been replaced by a leafy structure. To the amateur gardener this will seem a remarkable curiosity, to the professional plant grower it will signify substantial economic losses. To Johann Wolfgang von Goethe such sights meant the plant had suffered a change in its "plan of construction." He referred to this abnormal growth as "metamorphosis." Based on these and other observations Goethe developed the concept of homology in his 1790 essay Versuch die Metamorphose der Pflanzen zu erklären. Today we know the abnormal development of flower organs into leaf-looking structures as phyllody. What might be the cause of such a developmental aberration?
Often phyllody is the result of a bacterial infection. And therein lies yet another notable evolutionary tale, the story of the Phytoplasma. These yet-to-be-cultivated bacteria, members of the wall-less Tenericutes phylum, are obligate parasites that also require an insect vector for their transmission from plant to plant. Although Phytoplasma all have very small genomes, they preserve a functional protein secretion system and encode numerous secreted effectors that affect their host's physiology. One particularly interesting family of effectors, termed the phyllogens, render some plant transcription factors unstable. The result is a developmental pathway gone awry, with leaves appearing where flowers should be. Why should this be selected for in evolution? It turns out, the insects that serve as vectors are particularly attracted to the phyllody features of the infected plant! A most clever strategy indeed. In an engaging review by Melissa Tomkins et al. from the John Innes Centre, Norwich, UK, the authors make the analogy between Phytoplasma and malware: "phytoplasma effectors appear to reprogram plant development and defence to lure insect vectors, similarly to social engineering malware, which employs tricks to lure people to infected computers and webpages." Wonderful analogy for wonderful biology!