Department of Plant Sciences, University of California Davis, Davis, California, United States of America.
PLoS Genet. 2012;8(12):e1003134. doi: 10.1371/journal.pgen.1003134. Epub 2012 Dec 13.
Most of the natural variation in wheat vernalization response is determined by allelic differences in the MADS-box transcription factor VERNALIZATION1 (VRN1). Extended exposures to low temperatures during the winter (vernalization) induce VRN1 expression and promote the transition of the apical meristem to the reproductive phase. In contrast to its Arabidopsis homolog (APETALA1), which is mainly expressed in the apical meristem, VRN1 is also expressed at high levels in the leaves, but its function in this tissue is not well understood. Using tetraploid wheat lines with truncation mutations in the two homoeologous copies of VRN1 (henceforth vrn1-null mutants), we demonstrate that a central role of VRN1 in the leaves is to maintain low transcript levels of the VRN2 flowering repressor after vernalization. Transcript levels of VRN2 were gradually down-regulated during vernalization in both mutant and wild-type genotypes, but were up-regulated after vernalization only in the vrn1-null mutants. The up-regulation of VRN2 delayed flowering by repressing the transcription of FT, a flowering-integrator gene that encodes a mobile protein that is transported from the leaves to the apical meristem to induce flowering. The role of VRN2 in the delayed flowering of the vrn1-null mutant was confirmed using double vrn1-vrn2-null mutants, which flowered two months earlier than the vrn1-null mutants. Both mutants produced normal flowers and seeds demonstrating that VRN1 is not essential for wheat flowering, which contradicts current flowering models. This result does not diminish the importance of VRN1 in the seasonal regulation of wheat flowering. The up-regulation of VRN1 during winter is required to maintain low transcript levels of VRN2, accelerate the induction of FT in the leaves, and regulate a timely flowering in the spring. Our results also demonstrate the existence of redundant wheat flowering genes that may provide new targets for engineering wheat varieties better adapted to changing environments.
大多数小麦春化反应的自然变异是由 MADS 框转录因子 VERNALIZATION1（VRN1）等位基因差异决定的。冬季（春化作用）长时间暴露在低温下会诱导 VRN1 表达，并促进顶端分生组织向生殖阶段过渡。与主要在顶端分生组织中表达的拟南芥同源物（APETALA1）不同，VRN1 在叶片中也高水平表达，但在该组织中的功能尚不清楚。我们使用在 VRN1 的两个同源拷贝中具有截短突变的四倍体小麦系（以下简称 vrn1-缺失突变体），证明 VRN1 在叶片中的一个主要作用是在春化后维持 VRN2 开花抑制剂的低转录水平。在春化过程中，vrn1-缺失突变体和野生型基因型的 VRN2 转录本水平逐渐下调，但仅在 vrn1-缺失突变体中春化后上调。VRN2 的上调通过抑制 FT 的转录来延迟开花，FT 是一个开花整合基因，它编码一种可移动的蛋白质，从叶片运输到顶端分生组织以诱导开花。使用双 vrn1-vrn2-缺失突变体证实了 VRN2 在 vrn1-缺失突变体中延迟开花的作用，该突变体比 vrn1-缺失突变体早开花两个月。这两个突变体都产生了正常的花和种子，表明 VRN1 对于小麦开花并不是必需的，这与当前的开花模型相矛盾。这一结果并没有降低 VRN1 在小麦季节性开花调控中的重要性。冬季 VRN1 的上调是维持 VRN2 低转录水平、加速叶片中 FT 的诱导以及调节春季适时开花所必需的。我们的结果还表明，冗余的小麦开花基因的存在可能为工程小麦品种提供新的目标，以更好地适应不断变化的环境。
