Keep feeling fasciation

Apologies for the terrible 1980s music pun - if you don't know what I'm talking about, Wikipedia will explain... Anyhow, today's topic is fasciation - the flattened expansion of plant parts into flattened bands or ribbons, sometimes with multiplication of flower heads (the phenomenon is also known as 'cresting'). Although generally considered rare as a whole, fasciation has been documented in well over 100 plant species in 107 families (Presland et al., 2009), including some very common ones such as the dandelion - often called Taraxacum officinale agg., but in the UK this is incorrect as T. officinale does not occur and there are around 235 species with intermediates and much variability. If you want to investigate this tricky taxonomic area in more detail, see Dudman & Richards (1997).

A common species of dandelion (Taraxacum sp., probably T. subhamatum) showing a fasciated stem with normal stems in the background.

Fasciation is not well understood in all cases, and can be caused by a number of factors - mutation of meristematic cells (i.e. abnormality of the growing tip), bacterial infection (particularly by Rhodococcus fascians), attack by mites or insect attack, or chemical, frost and/or mechanical damage - and in some cases can be inherited. In most cases, a single stem is affected (as in the photo above) and does not recur the following year.

Dandelion showing a fasciated flower-head.

One area where active research has explained fasciation well is the action of the bacterium R. fascians. Infection leads to hyperdosage of plant hormones, especially auxins and cytokinins which it may induce or may produce itself (Vandeputte et al., 2005). Induced overexpression of plant hormones can be complex, involving biochemical/transcription pathways which include genes and their homologues, hormone-inactivating compounds, hormone precursors, cofactors in various aspects of mineral metabolism and so on (Simon-Mateo et al., 2006). The visible effect of this occur by apical dominance being broken and secondary or auxiliary meristems being activated (hence the proliferation of flowers in the second photo).

Other causes are less well understood and may overlap with gall-causing agents in the case of mites and insects, while R. fascians is itself known as a causer of 'leafy gall' as well as fasciation (Redfern & Shirley, 2011). So, a feature to look out for, and one with plenty of opportunities for research - now, where's that funding application..?

References

Dudman, A.A. & Richards, A.J. (1997). Dandelions of Great Britain and Ireland. BSBI, London.
Presland, J., Oliver, J. & Barber, M. (2009). Abnormalities in Plants. Wiltshire Botanical Society.
Redfern, M. & Shirley, P. (2011). British Plant Galls (2nd ed.). FSC, Shrewsbury.
Simon-Mateo, C., Depuydt, S., de Oliveira Manes, C.L., Cnudde, F., Holsters, M., Goethals, K. & Vereeke, D. (2006). The phytopathogen Rhodococcus fascians breaks apical dominance and activate auxiliary meristems by inducing plant genes involved in hormones metabolism. Molecular Plant Pathology 7(2): 103–12.
Vandeputte, O., Oden, S. & Mol, A. (2005). Biosynthesis of auxin by the gram-positive phytopathogen Rhodococcus fascians is controlled by compounds specific to infected plant tissues. Appl. Environ. Microbiol. 71(3): 1169–77.