Fig. 6. Models for the role of ESD4 in the regulation of flowering time. (A) ESD4 acts within the autonomous pathway to repress flowering and ESD4 activity is repressed by the activity of FCA/FVE. (B) ESD4 acts to repress flowering, in part by promoting high levels of the floral repressor FLC. Genes repressed by FLC include FT and SOC1, but it is likely that other genes (X) are also regulated by FLC. Both the autonomous and vernalization pathways act to regulate FLC mRNA levels (Sheldon et al., 1999; Sheldon et al., 2000; Michaels and Amasino, 1999; Michaels and Amasino, 2001), and ESD4 may interact with genes in these pathways. Genetic and molecular evidence suggests that ESD4 is likely to repress flowering independently of FLC (represented by dotted line), as does the promotion of flowering by vernalization (Michaels and Amasino, 2001). The genes regulated in an FLC-independent manner by ESD4 are represented by Y and probably include FT and SOC1, as the level of expression of FT and SOC1 is similar in wild type and esd4 fca-1 mutants, despite higher levels of FLC mRNA in esd4 fca-1 plants. Mutations in the long day and gibberellin pathways delay the flowering of esd4 mutants by affecting the activity of genes that are common targets of several floral promotion pathways.

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