75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

145 Cryptochrome 2 (cry2) in Vascular Bundles Regulates Flowering in Arabidopsis
Motomu Endo, Nobuyoshi Mochizuki, Tomomi Suzuki, Akira Nagatani
Laboratory of Plant Physiology, Graduate School of Science, Kyoto University, Kyoto, Japan
Light is one of the most important stimuli to trigger the transition from vegetative phase to reproductive phase,
namely flowering. In Arabidopsis, phytochrome B (phyB) and cryptochrome 2 (cry2) are major photoreceptors to regulate
flowering time. Both are known to be expressed in almost all organs/tissues. It has been well-known since 1937 that
plants perceive the light stimuli only in leaves to regulate flowering. Hence, photoreceptors in restricted parts can regulate
flowering. We have investigated this issue and demonstrated that phyB regulates flowering time only in mesophyll cells
(Endo et al., 2005). However it had remained unclear where cry2 regulates flowering.
Here, we expressed a cry2-GFP fusion protein in the cry2 mutant background in organ/tissue specific manners by using
specific promoters. We confirmed that cry2-GFP was expressed in expected places for all the promoters. Immunoblot,
GFP fluorescence and real time PCR analyses demonstrated that the expression levels in respective organs/tissues in
these lines were comparable to those of the authentic cry2. Transgenic lines expressing cry2-GFP in vascular bundles
exhibited full complementation of the late flowering phenotype. By contrast, the other lines that expressed cry2-GFP in
mesophyll, epidermis, SAM or root flowered late. FT is a key positive regulator of flowering. It is expressed in vascular
bundles. We examined FT regulation by cry2 at the tissue level. In the lines expressing cry2-GFP in vascular bundles, FT
mRNA was increased in vascular bundles. As expected, no induction of the FT expression was observed in other lines.
Hence, only cry2 in vascular bundle regulates FT expression, which takes place in vascular bundles.
146 Analysis ff Natural Genetic Variation in Photoperiodic Regulation of Flowering Time in
Arabidopsis Thaliana
Antonis Giakountis, Frederic Cremer, George Coupland
Max Planck Institute For Plant Breeding Research, Carl-von-Linne weg 10, D-50829 Cologne, Germany
Plants utilize a variety of endogenous signals and external stimuli to regulate the transition from vegetative growth
to flowering. In Arabidopsis, GIGANTEA (GI) a clock output gene expressed in the evening, is considered to mediate
between the circadian clock and transcription of CONSTANS (CO), another major gene of the photoperiod pathway.
Upon exposure to inductive long-day conditions CO expression is up-regulated and in the presence of blue and far-red
light, the stabilized CO protein activates the expression of the downstream floral integrator FT.
The effect of photoperiod on flowering-time control of Arabidopsis has been studied by analyzing flowering under
extreme photoperiods of 8-10 h and 16 h, however under natural conditions photoperiod varies continuously with the
changing seasons. We have described the flowering-time behaviour of model accessions such as Columbia and Ler at 6
different daylengths. Natural variation between accessions has been widely used as a tool for functional genetics, allowing
the identification of genetic and/or allelic variation for a trait of interest. We have used two collections of Arabidopsis
accessions in order to identify natural variation in their flowering time responses under a wide range of photoperiods. In
addition forty transgenic lines of flowering time genes have been included in the analysis and their photoperiod responses
are compared with those of the accessions. In order to evaluate the contribution of the circadian clock in photoperiod
discrimination, the same accessions were transformed with a promoter fusion of GI to the firefly luciferase gene.
A two way ANOVA approach revealed the presence of extensive natural variation in the flowering time responses of
accessions under different photoperiods. By performing a real time monitoring of the bioluminescence under the same
range of photoperiods as for flowering time, we were able to identify additional natural variation in the peak time of GI
expression under different daylengths. A selection of interesting accessions was made and expression analysis of several
genes of the photoperiod pathway together with QTL mapping are in progress in an attempt to identify the sources of
the observed natural variation at the molecular level.