Emborg Thomas J, Walker Joseph M, Noh Bosl, Vierstra Richard D
Department of Genetics, University of Wisconsin, Madison, 53706, USA.
Plant Physiol. 2006 Mar;140(3):856-68. doi: 10.1104/pp.105.074211. Epub 2006 Jan 20.
The oxidative cleavage of heme by heme oxygenases (HOs) to form biliverdin IXalpha (BV) is the committed step in the biosynthesis of the phytochrome (phy) chromophore and thus essential for proper photomorphogenesis in plants. Arabidopsis (Arabidopsis thaliana) contains four possible HO genes (HY1, HO2-4). Genetic analysis of the HY1 locus showed previously that it is the major source of BV with hy1 mutant plants displaying long hypocotyls and decreased chlorophyll accumulation consistent with a substantial deficiency in photochemically active phys. More recent analysis of HO2 suggested that it also plays a role in phy assembly and photomorphogenesis but the ho2 mutant phenotype is more subtle than that of hy1 mutants. Here, we define the functions of HO3 and HO4 in Arabidopsis. Like HY1, the HO3 and HO4 proteins have the capacity to synthesize BV from heme. Through a phenotypic analysis of T-DNA insertion mutants affecting HO3 and HO4 in combination with mutants affecting HY1 or HO2, we demonstrate that both of the encoded proteins also have roles in photomorphogenesis, especially in the absence of HY1. Disruption of HO3 and HO4 in the hy1 background further desensitizes seedlings to red and far-red light and accelerates flowering time, with the triple mutant strongly resembling seedlings deficient in the synthesis of multiple phy apoproteins. The hy1/ho3/ho4 mutant can be rescued phenotypically and for the accumulation of holo-phy by feeding seedlings BV. Taken together, we conclude that multiple members of the Arabidopsis HO family are important for synthesizing the bilin chromophore used to assemble photochemically active phys.
血红素加氧酶（HOs）将血红素氧化裂解形成胆绿素IXα（BV）是植物光敏色素（phy）生色团生物合成中的关键步骤，因此对植物正常的光形态建成至关重要。拟南芥（Arabidopsis thaliana）含有四个可能的HO基因（HY1、HO2 - 4）。先前对HY1位点的遗传分析表明，它是BV的主要来源，hy1突变体植株表现出下胚轴长和叶绿素积累减少，这与光化学活性phy的大量缺乏一致。最近对HO2的分析表明，它也在phy组装和光形态建成中发挥作用，但ho2突变体表型比hy1突变体更不明显。在这里，我们确定了拟南芥中HO3和HO4的功能。与HY1一样，HO3和HO4蛋白具有从血红素合成BV的能力。通过对影响HO3和HO4的T-DNA插入突变体与影响HY1或HO2的突变体进行组合的表型分析，我们证明这两种编码蛋白在光形态建成中也发挥作用，特别是在没有HY1的情况下。在hy1背景中破坏HO3和HO4会使幼苗对红光和远红光进一步脱敏并加速开花时间，三重突变体与缺乏多种phy脱辅基蛋白合成的幼苗非常相似。通过给幼苗喂食BV，hy1/ho3/ho4突变体的表型和全phy积累可以得到挽救。综上所述，我们得出结论，拟南芥HO家族的多个成员对于合成用于组装光化学活性phy的胆色素生色团很重要。
