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

Floral homeotic mutants of Lilium
to dicot model species). A carpel is regarded as the innermost whorl of lily flowers.
In our efforts to study lily floral development at the molecular level, we also
attempted to isolate a gene with an A function. In order to generate a homologous
probe for cDNA library screening, we tried amplifying a homologous fragment using
degenerated primers conceived from AP1 monocot sequences (Theissen et al., 2000).
Contrary to our successes in amplifying a putative B (PI-like, data not shown) and of
E-type gene fragment (SEP3-like), we were unable to amplify a putative A-type
fragment. Therefore, this function still remains a mystery in lily species, since no
orthologues with an A function have been reported by others as well.
The presence of tepals in the first whorl of lily flowers can be explained by the
extension of the B and E functional domains towards the outermost whorl. If so, the
loss-of-function of a B functional gene would lead to homeotic conversions of tepals in
both outer whorls into sepal-like organs. This phenomenon was indeed observed in
tulips, which belongs to the Liliaceae family, corroborating the modified ABCDE
model in this plant family (van Tunen and Angenent, 1993).
In this way, the B function presents a special character in Liliaceae flower
development and its candidate genes have already been object to scientific study. The
first gene belonging to the ABCDE model from Lilium to be functionally investigated
was a B-type orthologue of APETALA3 (AP3) designated LMADS1 (Tzeng and Yang,
2001). Strikingly, instead of showing a similar expression level of this gene in tepals
from both whorls, LMADS1 showed a stronger transcription activity in the second
whorl organs and no protein was detected in the first whorl. This result raised
uncertainties on the developmental identity of organs from the first and second whorls.
Another deviating feature found in B functional genetics of Liliaceae species is
the facultative homodimerization of PISTILLATA (PI) orthologues from Lilium regale
LRGLOA and LRGLOB whereas B type proteins from dicot species show obligate
heterodimerization with other B type proteins (Winter et al., 2002). The same
phenomenon was observed in Tulipa gesneriana, with the obligate heterodimerization
of the B types TGDEFA and TGDEFB, but with the facultative homodimerization of
the B type TGGLO (Kanno et al., 2003). These results support the dislocated B
function in Liliaceae species, since the absence of AP3 orthologues in the first whorl
organs may be functionally circumvented by the alternative PI-like protein
homodimerization.
We isolated and studied the molecular and functional characteristics of a
putative C functional gene from Lilium longiflorum, LLAG1, described in the second
chapter of this thesis. Functional analysis was carried out in the heterologous species
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