15th International Conference on Arabidopsis Research - TAIR

T01-005
PATTERN OF FUNCTIONAL EVOLUTION OF THE
FLORAL MERISTEM IDENTITY PROTEIN LEAFY
Alexis Maizel(1, 2), Detlef Weigel(1, 2)
1-Departement of Molecular Biology, Max Planck Institute for Developmental Biology, D-72076
Tübingen, Germany
2-Salk Institute, La Jolla CA 92037, USA
Two genes, LEAFY (LFY) and APETALA1 (AP1), are required to specify floral
identity in Arabidopsis. Expression of LFY in otherwise non-reproductive
meristems can cause their conversion into floral meristems in a variety of
species, demonstrating that it is a master regulator of floral development.
AP1 is a direct target of LFY, and both act in a partially redundant manner to
specify floral meristem identity. At least some of this function is due to activation
and regulation of various homeotic genes, which control the identity of
different floral organs. Biochemically, LFY is a sequence specific DNA-binding
protein with no similarity to any other plant or animal protein.
Homologs of LFY have been cloned from many seed plants (angiosperms and
gymnosperms), as well as from the more distant non-flowering Pteridophytes
and Bryophytes (ferns and mosses). All share two highly conserved domains.
Despite their similarity to their seed plant counterparts, the function of the
LFY homologs in species that arose prior to evolution of floral structure is
unclear.
We have decided to take advantage of the molecular diversity generated
during evolution to obtain insights into the structure-function relationships of
LFY. We sampled a set of homologs derived from all major clades of extant
plants and tested the functionality in several ways. By combining classical
phenotypic analysis with genome-wide molecular profiling, we assayed the
potency of the different homologs to rescue a strong null allele of LFY. We
also assayed in yeast models their transcriptional activity and DNA binding
activity. Several conclusions arose from this study. First, flowers and activity
of LFY homologs in Arabidopsis date to the same evolutionary time point.
Almost all angiosperms homologs are functionally interchangeable. However,
homologs from the non-flowering clades can only partially complement a
lfy null allele, their potency being inversely proportional to their evolutionary
distance. Second, among the molecular targets of LFY, AP1 is the main
output. AP1 is the only target upregulated by the homologs from non-flowering
plants. Third, the different homologs vary in their DNA binding affinity.
By building chimera between reference homologs, we have established that
the conserved domains contribute more than the non-conserved domains
to diversification of LFY activity. Finally the role of criticals amino-acids for
evolution of LFY function is examined.
15 th <strong>International</strong> <strong>Conference</strong> on Arabidopsis Research 2004 · Berlin
T01-006
A genetic model for floral meristem development
Hao Yu(1, 2), Toshiro Ito(1), Frank Wellmer(1), Elliot M Meyerowitz(1)
1-Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, USA
2-Department of Biological Sciences, Faculty of Science, National University of Singapore, 10
Science Drive 4, Singapore117543
The transition from vegetative to reproductive growth is a dramatic developmental
change in the life of plants, in which newly formed meristems acquire
floral rather than inflorescence identity. Floral meristem identity genes LEAFY
(LFY) and APETALA1 (AP1) promote establishment and maintenance of floral
identity in newly formed floral primordia. Without their activity, the floral
primordia develop with inflorescence characteristics. The underlying molecular-genetic
mechanism remains unknown. Our studies show that these phenotypes
are due in large part to the ectopic expression of AGAMOUS-LIKE 24
(AGL24), a central regulator of floral meristem identity. We present evidence
that AGL24 is an early target of transcriptional repression by LFY and AP1.
Without such repression, continued AGL24 expression in floral meristems is
sufficient to cause floral reversion regardless of the activation of floral organ
identity genes. This reveals that LFY and AP1 promote floral development not
only by positively regulating genes activated in flower development, but also
by repressing AGL24, a promoter of inflorescence fate.
Yu, H., Ito, T., Wellmer, F., Meyerowitz, E.M. (2004) Nature Genetics 36, 157-161.
T01 Development 1 (Flower, Fertilization, Fruit, Seed)