The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals

Abstract

Background: In arabidopsis (Arabidopsis thaliana), FLOWERING LOCUS T (FT) and FLOWERING LOCUS C (FLC) play key roles in regulating seasonal flowering-responses to synchronize flowering with optimal conditions. FT is a promoter of flowering activated by long days and by warm conditions. FLC represses FT to delay flowering until plants experience winter.

Scope: The identification of genes controlling flowering in cereals allows comparison of the molecular pathways controlling seasonal flowering-responses in cereals with those of arabidopsis. The role of FT has been conserved between arabidopsis and cereals; FT-like genes trigger flowering in response to short days in rice or long days in temperate cereals, such as wheat (Triticum aestivum) and barley (Hordeum vulgare). Many varieties of wheat and barley require vernalization to flower but FLC-like genes have not been identified in cereals. Instead, VERNALIZATION2 (VRN2) inhibits long-day induction of FT-like1 (FT1) prior to winter. VERNALIZATION1 (VRN1) is activated by low-temperatures during winter to repress VRN2 and to allow the long-day response to occur in spring. In rice (Oryza sativa) a VRN2-like gene Ghd7, which influences grain number, plant height and heading date, represses the FT-like gene Heading date 3a (Hd3a) in long days, suggesting a broader role for VRN2-like genes in regulating day-length responses in cereals. Other genes, including Early heading date (Ehd1), Oryza sativa MADS51 (OsMADS51) and INDETERMINATE1 (OsID1) up-regulate Hd3a in short days. These genes might account for the different day-length response of rice compared with the temperate cereals. No genes homologous to VRN2, Ehd1, Ehd2 or OsMADS51 occur in arabidopsis.

Conclusions: It seems that different genes regulate FT orthologues to elicit seasonal flowering-responses in arabidopsis and the cereals. This highlights the need for more detailed study into the molecular basis of seasonal flowering-responses in cereal crops or in closely related model plants such as Brachypodium distachyon.

Fig. 1.

A comparison of the molecular pathways regulating flowering time in (A) arabidopsis, (B) rice and (C) temperate cereals. Vernalization and long days promote flowering in arabidopsis and temperate cereals wheat and barley (A, C), whereas short days promote flowering in rice (B). The role of FT in mediating the day-length response is conserved. The role of the CO protein might also be conserved. In rice a pathway involving OsMADS51 and Ehd1 promotes expression of Hd3a (FT-like) in short days, while Hd1 weakly activates Hd3a (dotted line). In long days Ghd7 represses Hd3a. In arabidopsis and the temperate cereals vernalization is required for the long-day flowering response, but this response has evolved independently in these plants. In arabidopsis, FLC represses FT to block the day-length response until FLC is itself repressed by vernalization. In temperate cereals, VRN2, which is distantly related to Ghd7, blocks expression of FT1 in long days. VRN1 is induced by vernalization to repress VRN2 and to allow long-day induction of FT1. PPD1 is required for long-day induction of FT1. Warm conditions also promote expression of FT in arabidopsis, while SVP represses FT at lower temperatures. FT-like genes might have similar roles in regulating thermo-sensitive flowering responses in cereals. SVP-like genes are induced by low-temperatures in barley, but it is not known whether these genes inhibit expression of FT1 at low temperatures in cereals.