Regulation of flowering time in other plants

Is all the detailed knowledge about regulation of flowering time in Arabidopsis applicable to other plant species? Similarities are found in the Brassicaceae, the plant family to which Arabidopsis belongs (Kole et al. 2001). In Brassica rapa several flowering-time QTLs were identified and highresolution mapping showed that one of them, VFR2, was homologous to Arabidopsis FLC. It seemed to have the same phenotypic effect: late flowering, downregulated by vernalization in the biennual genotype of B. rapa. Kim et al. (2003) identified genes sharing strong homology with AGAMOUS-LIKE 20, a MADS-box transcription factor downstream of FLC in B. rapa as well as in two other Brassicaceae, Cardamine flexuosa and Draba nemorosa. The expression pattern of AGL20 in C. flexuosa during the floral transition was similar to Arabidopsis, which together with the data from Kole et al. (2001) suggests that at least some aspects of the regulation of flowering time are conserved between Arabidopsis and other Brassicaeae.

It is likely that most of the flowering-time genes identified in Arabidopsis are also present in plants outside the Brassicaceae, but they do not necessarily have the same function. For example, Petersen et al. (2004) recovered MADS-box genes of perennial ryegrass, Lolium perenne, which were differentially expressed during transition from vegetative to reproductive growth induced by vernalization. They used a differential-display technique (see Section 2.1, Fig. 2.3) with one PCR primer targetting a monocotyledon-specific conservative region in the MADS-box gene. After sequencing nine ryegrass MADS-box genes and studying their expression with quantitative realtime PCR the authors noted both similarities and differences with the Arabidopsis system. Genes with sequence homology to the Arabidopsis AP1 subfamily appeared to have a different expression pattern and possibly a different function in vernalization, compared to Arabidopsis.

A more detailed comparison of flowering-time regulation is possible between Arabidopsis and rice (Hayama et al. 2003; Griffiths et al. 2003; Hayama and Coupland 2004; Putterill et al. 2004). Like many economically important crops Oryza sativa is a short-day plant; it promotes flowering when daylength falls below a critical threshold. Would this suggest that the mechanisms for regulation of flowering are entirely different from the long-day plant Arabidopsis? Recent genetic studies have elucidated part of the photoperiodic control of flowering time in rice. Several rice genes have been shown to be orthologues of Arabidopsis genes: a gene named Hd1 is homologous to CO, and Hd3a is an orthologue of FT. However, CO promotes expression of FT in Arabidopsis whereas Hd1 suppresses expression of Hd3a in rice (Hayama et al. 2003). So the photoperiodic response is reversed by using the same set of regulatory genes, regulated differently. Fig. 5.18 provides an overview of the similarities in the regulation of flowering time between the two plant species.

Obviously, the usefulness of A. thaliana as a model for life-history investigation is limited by the fact that it represents only one type—a winter annual—out of the large variety of plant life histories. Biennuals, perennials, shrubs, and trees may have completely different ways of dealing with the problem of optimal timing of reproduction. Examination of the gene content of the recently completed Populus trichocarpa genome shows that most of the Arabidopsis flowering-time genes have counterparts in the poplar genome; however, a significant exception is the central floral repressor FLC which seems to lack an orthologue in poplar (Brunner and Nilsson 2004). The FLC subgroup of MADS-box genes seems to be specific to the Brassicaceae lineage. This raises

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