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Arabidopsis, LFY and AP1 activate each other's expression constituting a feedback
loop [12,58]. Moreover, AP1 is activated by FT-FD heterodimer shortly after
flowering induction [10]. Thus, the expression patterns of AP1and LFY in the
meristems of EB genotype suggest that flowering induction in these plants occurs
before two leaf stage in LD conditions. Consistent with this conclusion, flower
initials were clearly visible by stereomicroscope in the meristems at three or four
leaf stage, and plants flowered after producing on average 4.7 leaves in the main
crown. Based on our results, AP1 can be used as a marker for floral initiation in
wild strawberry. However, functional studies are needed to confirm the role of
AP1, LFY and SOC1 as floral integrator and identity genes, and this approach is
currently going on.
Conclusion
We have explored putative components for the genetic flowering pathways in
perennial SD plant wild strawberry by identifying 66 homologs of Arabidopsis
flowering time genes. Although few central genes are lacking, these data indicate
that all known genetic flowering pathways may be present in Fragaria. This is con-
sistent with the finding that EB genotypes, 'Hawaii-4' and 'Baron Solemacher',
show similar environmental regulation of flowering than Arabidopsis summer-
annuals. We also studied the expression of selected candidate genes and found
that few genes were co-regulated in the shoot apex of the EB genotype during
early floral development. Most strikingly, the mRNA of AP1 specifically accumu-
lated in EB genotype, but was absent in SD genotype, showing its usefulness as a
marker of floral initiation. Finally, identification of putative flowering time genes
reported here enables their transcriptional and functional characterization, as well
as genetic mapping, which may give answers for the relative importance of each
genetic flowering pathway and lead to cloning of the central repressor gene, SFL.
Ultimately, these genetic resources could be utilized in cultivar breeding of various
species of Rosaceae family through genetic transformation and marker assisted
selection breeding.
Methods
Plant Materials, Growing Conditions and Sampling
Seeds of SD and EB ('Baron Solemacher') genotypes of the wild strawberry
(NCGR accession numbers [PI551792] and [PI551507], respectively) were sown
on potting soil mixture (Kekkilä, Tuusula, Finland) and grown in a greenhouse
under LD conditions (day length min. 18 h), provided by 400 W SON-T lamps
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