Flower development of Lilium longiflorum - The Lilium information ...

Floral homeotic mutants of Lilium
Regulatory elements of transcription and the ABCDE model for flower
development.
Transcription of developmental genes are under the regulation of trans and cis-
regulatory elements that dictate the exact moment the genes are turned on and off in
order to trigger a proper and regular developmental pattern for each species.
Trans-acting elements are protein factors operating in gene promoter regions
(i.e. cis-regulatory regions) in order to be associated with other factors in the RNA
polymerase complex and activate transcription or, alternatively, to inhibit the
transcription of a given gene. Transcription factors, such as the MADS proteins, act as
trans-acting elements in eukaryotic organisms.
Cis-regulatory regions are boxes present in gene promoters and other
regulatory regions that modulate the spatial and temporal transcription pattern of a
given gene by allowing selective interaction with transcription factors (i.e. trans-acting
elements) and RNA polymerase. The CArG box is a known cis-regulatory element
present in promoters of genes that interact with the MADS domain in order to assist
their transcription (Pollock and Treisman, 1991).
The ABCDE model is mostly composed of MADS-box transcription factors
ruling as key elements to activate (or repress) specific genes in flower development
that will ultimately result in formation of floral organs. Changes in the sequence of
their cis-regulatory regions or trans-acting elements may result in genetic misfunction,
leading to homeotic changes in the flower pattern.
Lily flowers with their homeotic identity partially lost in the third whorl, as
shown in Figure 1b, can be easily found. This change can be triggered by
environmental conditions, suggesting a form of epigenetic regulation of homeotic
genes involved in flower organ development. In the festiva phenotype, however, the
homeotic change of the third whorl organs is regular and complete, suggesting a more
severe cause than those triggered by stress conditions.
In this way, if a specific element regulating the expression of a C functional
gene is affected, in cis or in trans, its transcription may be suppressed specifically in
the third whorl and not in the fourth whorl, which would lead to specific homeotic
changes of stamens into petals (or tepals, in the case of Liliaceae). Keeping in mind
that A and C genes have antagonistic functions, alternatively, an extension of A-type
gene domain that allows its transcription in the third whorl would repress the C
function and raise specific homeotic changes of stamens in petals.
Important cis-regulatory regions of AG that control its spatial expression
pattern were found in the second large intron of the gene (Sieburth and Meyerowitz,
86