Terminal flower2, an Arabidopsis homolog of heterochromatin protein1, counteracts the activation of flowering locus T by constans in the vascular tissues of leaves to regulate flowering time

Abstract

The flowering time of plants is tightly regulated by both promotive and repressive factors. Molecular genetic studies using Arabidopsis have identified several epigenetic repressors that regulate flowering time. Terminal flower2, (TFL2), which encodes a homolog of heterochromatin protein1 represses flowering locus T (FT) expression, which is induced by the activator constans (CO) in response to the long-day signal. Here, we show that TFL2, CO, and FT are expressed together in leaf vascular tissues and that TFL2 represses FT expression continuously throughout development. Mutations in TFL2 derepress FT expression within the vascular tissues of leaves, resulting in daylength-independent early flowering. TFL2 can reduce FT expression even when CO is overexpressed. However, FT expression reaches a level sufficient for floral induction even in the presence of TFL2, suggesting that TFL2 does not maintain FT in a silent state or inhibit it completely; rather, it counteracts the effect of CO on FT activation.

Figure 1.

Whole-Mount Expression Analyses of CO, FT, and TFL2. (A) Constructs used in expression analyses (see Methods). Yellow boxes indicate open reading frames. Blue boxes indicate the uidA open reading frame (GUS gene). ATG and STOP indicate what actually function, so that only gTFL2:GUS encodes a fusion protein. nost, the nopaline synthase terminator. (B) to (Q) GUS expression patterns of gCO:GUS ([B], [F], [J], and [N]), gTFL2:GUS ([C], [G], [K], and [O]), and pFT:GUS ([D], [H], [L], and [P]) in ecotype Columbia (Col) and pFT:GUS in tfl2-2 ([E], [I], [M], and [Q]) in whole-mount staining of 6-day-old seedlings ([B] to [E]), 12-day-old seedlings ([F] to [I]), the first true leaves of 6-day-old seedlings ([J] to [M]), and the first true leaves of 8-day-old seedlings ([N] to [Q]). (R) to (T) GUS expression in short-day conditions of 12-day-old gCO:GUS (R), 6-day-old pFT:GUS (S), and 12-day-old pFT:GUS (T) plants. The inset in (S) shows a higher magnification of the first true leaf. (U) In situ hybridization against FT mRNA in the cotyledon of tfl2-2 (longitudinal section). The arrowhead indicates GUS expression in the primary vein in (M). Bars = 1 mm in (B) for (B) to (E), 1 mm in (F) for (F) to (H), 1 mm in (I), 0.1 mm in (J) for (J) to (M), 0.1 mm in (N) for (N) to (Q), 1 mm in (R) to (T), and 0.1 mm in (U).

Figure 2.

Histological Analyses of GUS Expression Patterns. (A) to (D) Longitudinal sections through 8-day-old seedlings of gCO:GUS (A), gTFL2:GUS (B), and pFT:GUS (C) in Col and pFT:GUS in tfl2-2 (D). (E) to (H) Transverse sections through leaves of gCO:GUS (E), gTFL2:GUS (F), and pFT:GUS (G) in Col and pFT:GUS in tfl2-2 (H). Asterisks indicates the SAM. P, phloem; X, xylem. Bars = 0.1 mm in (A) for (A) to (D), 10 μm in (E) for (E) to (H), and 10 μm for the inset in (G).

Figure 3.

tfl2 Affects Both the CO-Dependent and CO-Independent Expression of FT. (A) Quantitative GUS expression analysis of pFT:GUS in Col (solid line) and tfl2-2 (dashed line). GUS activity is shown as the mean ± se of 4-methylumbelliferyl glucuronide·min⁻¹·μg⁻¹ protein of three independent experiments. (B) to (D) Real-time quantitative RT-PCR analysis of FT ([B] and [C]) and CO (D) expression. The same RNA extract was used for the experiments shown in (B) and (D). These data are normalized to the amount of ACT2 (set as 100%) and are means ± se of three independent experiments. (B) FT expression levels in 6-day-old Col, tfl2-2, co-101, and co-101 tfl2-2 seedlings. (C) FT expression over time in Col, tfl2-2, co-101, and co-101 tfl2-2 seedlings. (D) CO expression levels in 6-day-old Col and tfl2-2 seedlings. (E) Flowering time measured as the mean number (±se) of rosette leaves at flowering in Col, co-101, tfl2-2, and co-101 tfl2-2 (n = 27, 7, 27, and 9, respectively). All plants were grown under long-day conditions. Asterisks indicate data that do not show statistically significant differences.

Figure 4.

pFT:GUS Expression Requires CO Activity. pFT:GUS expression is shown in 12-day-old seedlings of wild-type Col (A), co-101 (B), and co-101 tfl2-2 (C) (roots were cut off for genotyping) and in leaves of wild-type Col (D), co-101 (E), and co-101 tfl2-2 (F). Bars = 1 mm.

Figure 5.

Roles of TFL2 during FT Repression in the CO Overexpressor. (A) Real-time quantitative RT-PCR analysis of FT (closed bars) and CO (open bars) expression in 12-day-old seedlings of Col, tfl2-2, 35S:CO, and 35S:CO tfl2-2. The data are normalized to the amount of ACT2 (set as 100%) and are means ± se of three independent experiments. There was no statistically significant difference among genotypes marked with the same symbols. (B) Flowering time measured as the mean number (±se) of rosette leaves at flowering in Col, tfl2-2, 35S:CO, and 35S:CO tfl2-2 (n = 27, 27, 43, and 22, respectively). (C) and (D) Whole-mount analysis of pFT:GUS expression in 35S:CO (C) and 35S:CO tfl2-2 (D). Bars = 1 mm.