Fig. 2. CO somatic sectors created by Cre/lox-mediated excision of 35S::GUS from the CO intron. (A) Excision of 35S::GUS from CO restores early flowering on co-2 mutants. (Left) Wild-type plant. (Middle) Plant homozygous for co-2 and CO(35S::GUS) exposed to heat shock flowers late. (Right) Plant homozygous for co-2, HS::CRE and CO(35S::GUS) exposed to heat shock flowers early. This correlates with excision of the 35S::GUS gene as detected by PCR (data not shown). (B) Comparison of flowering times of progenitor co-2 CO(35S::GUS) lines with those of wild-type and co-2 mutant plants. The transgenic progenitor lines do not carry HS::CRE and flower at the same time as co-2 mutants. Heat shock does not affect their flowering time. In the absence of exposure to heat shock, plants homozygous for co-2 CO (35S::GUS) HS::CRE, such as line 42(8)9, also flower at the same time as co-2 mutants. (C) GUS staining illustrates patterns of GUS-negative sectors obtained in plants homozygous for co-2 HS::CRE and heterozygous for CO(35S::GUS) heat shocked 7 days after pollination. Two cotyledons and five first leaves are stained from 18 different plants exposed to heat shock. Different sector patterns indicate excision of the 35S::GUS marker at different times during shoot development. (D) Effects on flowering time of heat shocking co-2 CO(35S::GUS) HS:CRE plants 7 days after pollination. The flowering time of heat-shocked plants is intermediate between that of wild type and co-2 mutants. Flowering time is measured as leaf number. Rosette and cauline leaf number is shown separately. (E) Flowering times of the progeny of two co-2 CO(35S::GUS) HS:CRE plants that were heat shocked either as imbibed seeds (Plant 1) or 10 days after germination (Plant 2). The progeny were harvested from individual inflorescences, and scored as early (similar to wild type) or late (similar to the co-2 mutant). Plants (n=25-40) were scored from each inflorescence, and the proportion of plants scored as early flowering from each inflorescence is shown. Branch 1 of Plant 1, and the main shoot and branch 2 of Plant 2, showed no early flowering plants.