Analysis of flowering time control in Arabidopsis by comparison of double and triple mutants

Abstract

Three genetic pathways promote flowering of Arabidopsis under long photoperiods. These pathways are represented by the genes CO, FCA, and GA1, which act in the long-day, autonomous, and gibberellin pathways, respectively. To test whether these are the only pathways that promote flowering under long photoperiods, the co-2 fca-1 ga1-3 triple mutant was constructed. These plants never flowered under long- or short-day conditions, indicating that the three pathways impaired by these mutations are absolutely required for flowering under these conditions. The triple mutant background represents a "vegetative ground state" enabling the roles of single pathways to be described in the corresponding double mutants. The phenotypes of plants carrying all eight combinations of wild-type and mutant alleles at the three loci were compared under long- and short-day conditions. This analysis demonstrated that under long photoperiods the long-day pathway promoted flowering most effectively, whereas under short photoperiods the gibberellin pathway had the strongest effect. The autonomous pathway had a weak effect when acting alone under either photoperiod but appeared to play an important role in facilitating the promotion of flowering by the other two pathways. The vegetative phenotype of the triple mutant could be overcome by vernalization, suggesting that a fourth pathway promoted flowering under these conditions. These observations are discussed in light of current models describing the regulation of flowering time in Arabidopsis.

Figure 1

Photographs illustrating the phenotypes of plants carrying all eight combinations of the co-2, fca-1, and ga1-3 mutations grown under long days. A, Six-week-old wild-type (left), co-2 (middle), and fca-1 (right) plants. Only the wild type is flowering. B, Six-week-old ga1-3 plant. The plant is not flowering. C, Eight-week-old co-2 (left), fca-1 (middle), and co-2 fca-1 (right) mutant plants. All plants are flowering, but the double mutant is delayed compared to the others. D, Eight-week-old ga1-3 mutant plant. The arrow indicates the position of floral buds. E, Nine-week-old fca-1 ga1-3 (left), co-2 ga1-3 (middle), and co-2 fca-1 ga1-3 (right) mutant plants. Only the fca-1 ga1-3 plant is flowering, and the arrow indicates the position of floral buds. F, Twelve-week-old co-2 ga1-3 (left) and co-2 fca-1 ga1-3 (right) plants. Older leaves have senesced. The co-2 ga1-3 plant is flowering, whereas the fca-1 co-2 ga1-3 plant remains vegetative. The arrow indicates the position of floral buds.

Figure 2

Schematic illustration of Arabidopsis flowering time pathways. The long-day, autonomous, and GA pathways promote flowering of Arabidopsis under standard long-day conditions, and are represented respectively by the CO, FCA, and GA1 genes that are discussed in the text. The autonomous pathway acts by repressing expression of the floral inhibitor FLC. The genetic data presented here indicate that these three pathways are required for flowering to occur, and suggests that the autonomous pathway accelerates flowering by facilitating the activity of the long-day and GA pathways. Vernalization promotes flowering by repressing expression of the floral inhibitor FLC, but detailed comparison of the flowering times of vernalized co-2 fca-1 ga1-3 triple mutants and co-2 ga1-3 double mutants suggests that vernalization may also promote flowering independently of FLC, as represented by the dotted line.