The Promoter of the Cereal VERNALIZATION1 Gene Is Sufficient for Transcriptional Induction by Prolonged Cold

Maria M Alonso-Peral,M Cristina Casao,Sandra N Oliver,Ben Trevaskis,Aaron A Greenup,Edward Newbigin

doi:10.1371/journal.pone.0029456

Abstract

The VERNALIZATION1 (VRN1) gene of temperate cereals is transcriptionally activated by prolonged cold during winter (vernalization) to promote flowering. To investigate the mechanisms controlling induction of VRN1 by prolonged cold, different regions of the VRN1 gene were fused to the GREEN FLUORESCENT PROTEIN (GFP) reporter and expression of the resulting gene constructs was assayed in transgenic barley (Hordeum vulgare). A 2 kb segment of the promoter of VRN1 was sufficient for GFP expression in the leaves and shoot apex of transgenic barley plants. Fluorescence increased at the shoot apex prior to inflorescence initiation and was subsequently maintained in the developing inflorescence. The promoter was also sufficient for low-temperature induction of GFP expression. A naturally occurring insertion in the proximal promoter, which is associated with elevated VRN1 expression and early flowering in some spring wheats, did not abolish induction of VRN1 transcription by prolonged cold, however. A translational fusion of the promoter and transcribed regions of VRN1 to GFP, VRN1::GFP, was localised to nuclei of cells at the shoot apex of transgenic barley plants. The distribution of VRN1::GFP at the shoot apex was similar to the expression pattern of the VRN1 promoter-GFP reporter gene. Fluorescence from the VRN1::GFP fusion protein increased in the developing leaves after prolonged cold treatment. These observations suggest that the promoter of VRN1 is targeted by mechanisms that trigger vernalization-induced flowering in economically important temperate cereal crops.

Highlights

Plants growing in temperate regions time flowering to coincide with favourable seasonal conditions
GREEN FLUORESCENT PROTEIN (GFP) fluorescence increased at the shoot apex at the late vegetative stage (Figure 2B) and remained active throughout the shoot apex during reproductive development, expression was lower in the developing florets than in other parts of the inflorescence (Figure 2D)
At later stages of development, the VERNALIZATION1 gene (VRN1)::GFP signal was detected throughout the shoot apex, expression was lower in the developing florets than in other parts of the inflorescence, similar to the PVRN1:GFP construct (Figure 4H)

Summary

Introduction

Plants growing in temperate regions time flowering to coincide with favourable seasonal conditions. One cue that promotes spring flowering is prolonged exposure to winter cold vernalization [1]. Vernalization-induced flowering is a feature of many plants, including Arabidopsis and economically important temperate cereal crops, such as wheat (Triticum spp.) and barley (Hordeum vulgare), different genes mediate this seasonal flowering response in these distantly related angiosperms (see [2]). The response of VRN1 expression to cold is quantitative, with longer cold treatments inducing higher transcript levels [6], [7], [10], [11]. This parallels the degree to which flowering is accelerated [11]. VRN1-like genes are likely to play similar roles in other temperate grasses, including economically important species such as Lolium perenne and Phleum pratense [14], [15]

Methods

Results

Conclusion