The FLOWERING LOCUS T-like gene family in barley (Hordeum vulgare)

Abstract

The FLOWERING LOCUS T (FT) gene plays a central role in integrating flowering signals in Arabidopsis because its expression is regulated antagonistically by the photoperiod and vernalization pathways. FT belongs to a family of six genes characterized by a phosphatidylethanolamine-binding protein (PEBP) domain. In rice (Oryza sativa), 19 PEBP genes were previously described, 13 of which are FT-like genes. Five FT-like genes were found in barley (Hordeum vulgare). HvFT1, HvFT2, HvFT3, and HvFT4 were highly homologous to OsFTL2 (the Hd3a QTL), OsFTL1, OsFTL10, and OsFTL12, respectively, and this relationship was supported by comparative mapping. No rice equivalent was found for HvFT5. HvFT1 was highly expressed under long-day (inductive) conditions at the time of the morphological switch of the shoot apex from vegetative to reproductive growth. HvFT2 and HvFT4 were expressed later in development. HvFT1 was therefore identified as the main barley FT-like gene involved in the switch to flowering. Mapping of HvFT genes suggests that they provide important sources of flowering-time variation in barley. HvFTI was a candidate for VRN-H3, a dominant mutation giving precocious flowering, while HvFT3 was a candidate for Ppd-H2, a major QTL affecting flowering time in short days.

Figure 1.—

ClustalW multiple alignment of the complete protein sequences of Arabidopsis, rice, and barley FT family proteins. OsMFTL1, used for construction of the phylogenetic tree (Figure 2A), is also included. The PEBP domain boundaries are marked by horizontal arrows. The residue distinguishing FT- and TFL1-type functions (Hanzawa et al. 2005) is marked by a vertical arrow. All the FT-like proteins have a tyrosine at this position. The 14-amino-acid stretch and the LYN triad identified by Ahn et al. (2006) as diagnostic of true FT genes are boxed. The indica OsFTL9 is predicted to be functional but the japonica sequence should be nonfunctional because of a stop codon at position 95.

Figure 2.—

Phylogenetic relationships of FT-like proteins from Arabidopsis, rice, and barley. (A) Phylogenetic tree of PEBP domain protein sequences. Major groups are marked 1–3. Bootstrap support values are shown at each node. (B) Exon/intron structure of Arabidopsis FT, rice Hd3a, and the five barley FT family genes. Exon and intron sizes are in base pairs.

Figure 3.—

Expression of HvFT genes under short-day and long-day conditions. (a) Levels of gene expression in arbitrary units normalized against 18s rRNA. (b) Timing of the transition of the developing apex from vegetative to reproductive growth (extension of the apical dome and appearance of primordia with a double-ridge structure). The transition was observed between weeks 1 and 2 in LD conditions and between weeks 3 and 4 in SD conditions.

Figure 4.—

Genetic map positions of barley HvFT genes and their relationships to rice. Solid lines show the mapped segments in relation to the approximate size of complete barley linkage groups (H). Barley BACs are shown in boldface type. Shaded lines show homologous sequences from rice (R) physical maps and individual BAC clones. Barley clones on chromosomes 7H, 3H, 2H, and 4H were mapped using an Igri × Dairokkaku (Vrn-H3) population and clones on 1H using an Igri × Triumph population.

Figure 5.—

Flowering times (days to awn emergence from the flag leaf) of DH lines from an Igri × Triumph population showing the association of early flowering with the Triumph allele of HvFT3. The effect of additional QTL is shown in supplemental Figure S4 at http://www.genetics.org/supplemental/.