Mathieu Johannes, Warthmann Norman, Küttner Frank, Schmid Markus
Max Planck Institute for Developmental Biology, Department of Molecular Biology, Spemannstrasse 37 - 39, D - 72076 Tübingen, Germany.
Curr Biol. 2007 Jun 19;17(12):1055-60. doi: 10.1016/j.cub.2007.05.009. Epub 2007 May 31.
Several endogenous and environmental factors need to be integrated to time the onset of flowering. Genetic and molecular analyses, primarily in Arabidopsis thaliana and rice, have shown that CONSTANS (CO) and FLOWERING LOCUS T (FT) play central roles in photoperiod-dependent flowering. The overall picture is that CO acts in the phloem companion cells of leaves and that its main effect is to induce FT mRNA in these cells. Surprisingly, FT, a small globular protein of 20 kDa, interacts at the shoot apex with the bZIP transcription factor FLOWERING LOCUS D (FD) to induce downstream targets. Given that green fluorescent protein (GFP), which as a monomer is 27 kDa, can be easily exported to sink tissue including flowers when expressed in phloem companion cells, the latter finding strongly implied that FT protein is the mobile floral-inductive signal. In agreement with this hypothesis, an FT-GFP fusion, just like GFP, can be exported from the phloem of both rice and Arabidopsis. It has been unknown, however, whether mobile FT protein is sufficient for transmitting the flowering signal. Here we show that FT mRNA is required in phloem companion cells where it acts partially redundant with its paralog TWIN SISTER OF FT (TSF) to induce flowering. Furthermore, we have devised a method that uncouples FT mRNA and protein effects in vivo. We demonstrate that export of FT protein from phloem companion cells is sufficient to induce flowering.
需要整合多种内源性和环境因素来确定开花时间。主要在拟南芥和水稻中进行的遗传和分子分析表明，CONSTANS（CO）和开花位点T（FT）在光周期依赖性开花中起核心作用。总体情况是，CO在叶片的韧皮部伴胞中起作用，其主要作用是在这些细胞中诱导FT mRNA。令人惊讶的是，FT是一种20 kDa的小球形蛋白，它在茎尖与bZIP转录因子开花位点D（FD）相互作用以诱导下游靶标。鉴于绿色荧光蛋白（GFP）作为单体为27 kDa，当在韧皮部伴胞中表达时可以很容易地输出到包括花在内的库组织中，后一发现强烈暗示FT蛋白是移动的开花诱导信号。与该假设一致，FT-GFP融合蛋白与GFP一样，可以从水稻和拟南芥的韧皮部输出。然而，尚不清楚移动的FT蛋白是否足以传递开花信号。在这里，我们表明FT mRNA在韧皮部伴胞中是必需的，在那里它与其旁系同源物FT的孪生姐妹（TSF）部分冗余以诱导开花。此外，我们设计了一种在体内分离FT mRNA和蛋白作用的方法。我们证明，FT蛋白从韧皮部伴胞中的输出足以诱导开花。
