Genetics of flowering time control in Aquilegia
Researcher: Anji Ballerini
The commitment to flowering is a critical decision in plant development with significant implications for individual fitness. In order to optimally regulate flowering time, plants integrate signals from multiple pathways, including hormone signaling, day length, developmental phase, ambient temperature, light quality, and seasonal cues. The core eudicot model species Arabidopsis has proven to be particularly well suited for the genetic dissection of these pathways, leading to a detailed understanding of the photoperiod, autonomous and vernalization mechanisms, as well as the components that integrate these signals. As shown above, the interactions among the loci involved in flowering time control are very complex. These pathways involve photoreceptors (PHYA, PHYB, CRY2), a diverse array of transcription factors (CO, FLC, SOC1, AGL24, LFY, AP1), regulators of chromatin structure (VRN2), proteins controlling RNA processing (FCA), at least one putative kinase inhibitor (FT), and novel loci (FRI), as well as many other genes (reviewed Simpson and Dean, 2002).
As might be expected, many aspects of this pathway are important targets of evolutionary adaptation. Within Arabidopsis, components of the vernalization and photoperiod pathways have been shown to map to major flowering time QTLs (Johanson et al., 2000; El-Assal et al., 2001; Gazzani et al., 2003). Across more distantly related taxa, the CO pathway appears to be largely conserved with critical modifications producing shifts from longday to shortday flowering in Oryza (rice) and Pharbitis (Yano et al., 2000; Liu et al., 2001; Hayama et al., 2003). It remains to be seen how conserved other aspects of flowering time control are across the angiosperms. Apparent homologs of many of the loci have been characterized in Oryza genome (Izawa et al., 2003), as well as other taxa (Litt and Irish, 2003; Maizel et al., 2005). Somewhat curiously, clear orthologs of the critical FLC locus have not been identified outside the core eudicots (Reeves et al., 2007). In particular, little comparative work has been done on the vernalization pathway and it has even been suggested that these genes are absent from the rice genome, correlating with a lack of vernalization response in rice (Izawa et al., 2003). What is currently lacking in the species used to study flowering time, therefore, is a phylogenetically intermediate model that exhibits vernalization responses, as well as other aspects of flowering time response. Aquilegia has the strong potential to meet this need.
Given the recent radiation of Aquilegia into an array of environments that vary widely in altitude and latitude, it seems likely that diversification has occurred in the genetic pathways controlling flowering time. Consistent with this, variability is observed in several aspects of flowering time response, most notably vernalization requirements, both between accessions of A. formosa and among species of Aquilegia (S. Hodges, pers. comm.). Previous studies have shown that flowering time in Aquilegia is influenced by plant age, photoperiod, light quality, vernalization, and GA signaling. These findings must be interpreted with caution, however, since many of the analyses were performed using hybrid lines derived for horticulture. Rapid cycling due to a reduced requirement for vernalization is observed in many hybrid lines, particularly the “Origami” hybrid series. This phenotype may be due to spontaneous mutation or the effects of transgressive segregation. These studies indicate that Aquilegia integrates an array of flowering time signals that are very similar to those characterized in Arabidopsis, thereby presenting an opportunity to test the broader applicability of the model species findings in a taxon that is phylogenetically intermediate between Arabidopsis and rice. Furthermore, the flowering time pathway is an obvious candidate for molecular evolutionary studies in Aquilegia. These analyses will have the added benefit of facilitating the development of optimized growth protocols and more rapid cycling lines.
We are taking two main strategies to characterize the genetics of flowering time in Aquilegia. The first is a candidate gene-based approach that is focused on the photoperiod pathway and the floral pathway integrator genes. Homologs of PHYA, PHYB, CRY2, CO, FT, SOC1, AGL24, AP1 and LFY have been identified in Aquilegia formosa and are being studied from the standpoint of molecular evolution as well as spatial and temporal aspects of transcriptional regulation. The second is a genomic approach aimed at identifying possible players in the vernalization pathway of Aquilegia. High-density oligonucleotide arrays have been developed for Aquilegia. These arrays will be used to compare the transcriptome of pre- and post-vernalization tissue in order to identify genes that are differentially regulated by cold-treatment. Loci that are transcriptionally repressed by vernalization will represent potential candidates for functional analogs of flowering repressors such as FLC in Arabidopsis.
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