Two genes in search of a character
Seminal work with floral mutants in the 1980s and 1990s led to an understanding of the interplay of three different classes of genes (A, B and C) in the formation of the whorls of organs in typical dicot flowers. Examples of these are the Bclass genes, APETALA3 (AP3) and ISTILLATA (PI), first identified in the model species, Arabidopsis thaliana and Antirrhinum majus, as being essential for specifying stamen and petal identity. In these species, expression of AP3 and PI occurs during the development of petals and stamens and does not occur in any nonfloral tissue. However, a much more extensive pattern of expression of these genes is described by Skipper (University of Copenhagen, pp. 83-88). He worked on Eranthis hyemalis (winter aconite), a member of the Ranunculaceae, a dicot family classified as 'lower' or 'primitive'. The key part of this work was a series of very elegant in situ RTPCR experiments. This technique amplifies the mRNA present in tissue sections allowing a detailed analysis of gene transcription on a cell by cell basis. The results clearly show expression of the two genes in the developing vascular bundles of the tuberous rhizome, in the flowering stem and in flower primordia in addition to the expected locations in the floral organs. As we so often find, there is more to life than model species! It is suggested that the different expression patterns in different dicots of these floral MADS box genes implies multiple evolutionary origins of petals. A role in floral organ identity may thus occur instead of (as in arabidopis and antirrhinum) or alongside (as described here) more primitive roles. What those roles are is unknown but the tissue-specific over-expression and suppression experiments suggested by Skipper will help to identify the nonfloral character(s) specified by these genes.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
