Download My favourite flowering image: floral organs with trichomes SUPPLEMENT PAPER

Journal of Experimental Botany, Vol. 66, No. 23, pp. 9–10, 2015
doi:10.1093/jxb/erv072 SUPPLEMENT PAPER
My favourite flowering image: floral organs with trichomes
Frank Wellmer*, Darragh Stewart and Diarmuid S. O’Maoileidigh
Smurfit Institute of Genetics, Trinity College Dublin, Dublin 2, Ireland
* To whom correspondence should be addressed. E-mail: [email protected]
Received 20 January 2015; Accepted 28 January 2015
Abstract
There is now ample genetic evidence that floral organs are in essence modified leaves, and that floral organ identity factors are instrumental in the conversion of leaves into floral organs. This idea is exemplified by flowers of
Arabidopsis thaliana that form trichomes on reproductive floral organs as a consequence of a perturbation of the
activity of floral organ identity factors.
In 1790, Johann Wolfgang von Goethe published his seminal essay Metamorphosis of Plants in which he proposed that
the above-ground organs of a plant are derived (‘metamorphosed’) from leaves (von Goethe, 1790). Over the years,
much experimental support for this idea has been garnered,
at least for flowers. When the floral organ identity genes were
discovered and studied through genetic analysis, it was found
that the combined loss of several of these genes led to flowers in which floral organs were replaced by leaf-like structures
(Bowman et al., 1991). Also, it has been reported that the
ectopic expression of specific combinations of floral organ
identity genes leads to the transformation of rosette or cauline leaves into specific floral organ types (Honma and Goto,
2001; Pelaz et al., 2001). Thus, these genes are clearly necessary and sufficient for the conversion of leaves into floral
organs.
The molecular mechanisms underlying the activity of the
transcription factors, which are encoded by the floral organ
identity genes, have long remained enigmatic. However, in
recent years, the use of transcriptomics experiments and
genome-wide localization studies led to first detailed and
global insights into their functions (Gomez-Mena et al., 2005;
Ito et al., 2004; Kaufmann et al., 2009; Kaufmann et al., 2010;
O Maoileidigh et al., 2013; Pajoro et al., 2014; Wellmer et al.,
2006; Wuest et al., 2012; Yant et al., 2010). One of the key
findings of this work was that target genes of the floral organ
identity factors are highly enriched for genes with regulatory
functions. These include genes that code for microRNAs, transcription factors and for other regulatory proteins. Many of
these regulatory genes had been previously identified through
genetic screens to play important roles during flower formation. However, there are notable exceptions. When we studied
the activities of the floral organ identity factors APETALA3,
PISTILLATA and AGAMOUS (AG), we found that several
known regulators of leaf development were among the direct
targets (O Maoileidigh et al., 2013; Wuest et al., 2012). These
included genes involved in the formation of trichomes, which
in Arabidopsis are mainly present on leaves and on the inflorescence stem. We then asked whether the floral organ identity
factors are involved in directly repressing trichome formation
on floral organs, which – with the exception of sepals - are trichomeless. Indeed, when we perturbed the activity of AG using
an artificial microRNA approach, we found that trichomes
started to form on the valves of carpels (O Maoileidigh et al.,
2013) . This effect was exacerbated when we conducted the
knockdown of AG in a background in which the repressor of
trichome formation, TRIPTYCHON, is mutated (Figure 1).
These results show that one key feature of leaf development
is directly repressed by a floral organ identity factor. Whether
other aspects of leaf morphogenesis are also directly modified or suppressed by these transcription factors is currently
unknown, but the number of regulators of leaf development
that act downstream of the floral organ identity factors make
this rather likely in our opinion.
© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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10 | Wellmer et al.
Fig. 1. A flower of a triptychon mutant, in which AG activity has been perturbed through an artificial microRNA at intermediate stages of development.
Trichomes are forming on carpel valves. Sepals and petals have been removed for better visibility of the reproductive floral organs.
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