A New Function for BRC1 in Axillary Buds: Suppression of Floral Identity

In many plants, the transition to flowering is characterized by a large change in plant architecture, including development of axillary buds. Although floral induction in the apical meristem is well studied (reviewed in Fornara et al., 2010), less is known regarding axillary bud differentiation. In Arabidopsis thaliana, BRANCHED1/TEOSINTE BRANCHED1-LIKE 1 (BRC1), a TCP transcription factor, integrates branching signals and negatively regulates axillary bud development (Aguilar-Martínez et al., 2007). FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF), known for their roles as floral pathway integrators, were recently reported to influence bud outgrowth (Hiraoka et al., 2013). Now, Niwa et al. (pages 1228–1242) report that BRC1 interacts with FT to suppress the floral transition in axillary buds.

FT interacts with the bZIP transcription factor FD to induce the floral transition in the apical meristem (reviewed in Fornara et al., 2010). In a screen for other transcription factors that interact with FT, Niwa and co-authors identified the branching regulator BRC1. They found that BRC1 also interacted with TSF (which is redundant with FT in the same clade), but not with TERMINAL FLOWER1 (TFL1; a homolog from a different clade that acts antagonistically to FT). Several lines of evidence indicated that FT and BRC1 can interact directly, in contrast to the interaction between FT and FD, which is likely mediated by 14-3-3 proteins in Arabidopsis, by analogy to the situation in Solanum lycopersicum and Oryza sativa.

Axillary shoots and the subtending leaf removed from the main shoot of wild type (Col) and brc1-2 mutant plants, showing increased outgrowth of flowering shoots in the mutant. Arrowheads: vegetative nodes; arrows: first flowers. Numbers indicate cauline-leaf axils and rosette-leaf axils as illustrated in the diagram (right). In the latter, circles, arrows and bars represent flowers, shoots and leaves, respectively. [reprinted from Niwa et al. (2013), Figure 4A.]

BRC1 is expressed in axillary buds, but FT is not. FT protein is produced in the vasculature and moves into the shoot apical meristem for its function in floral induction. Niwa et al. show that FT also can move into axillary buds, confirming that BRC1 and FT could be present together in the plant.

Niwa et al. found that the brc1 mutant displays earlier/increased floral transition of axillary shoots, suggesting that BRC1 suppresses floral induction in axillary meristems. When the authors examined the ft mutant for axillary meristem phenotypes, they found delayed floral transition. The ft brc1 double mutant was similar to ft, indicating that the accelerated floral transition in brc1 requires FT. Moreover, a line in which BRC1 expression was driven in the shoot apical meristem displayed delayed flowering, consistent with BRC1 inhibiting FT function when the two are present in the same tissue.

This work enhances our understanding of flowering control in Arabidopsis by establishing that FT promotes the floral transition in axillary buds in addition to its role in the apical meristem, and showing that BRC1, previously known as a regulator of branching, negatively regulates the floral transition in axillary meristems.