Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) OT Gatersleben, Corrensstr 3, 06466 Seeland, Germany.
Leibniz-Institut für Analytische Wissenschaften (ISAS) e.V., Bunsen-Kirchhoff-Str 11, 44139 Dortmund, Germany.
Plant Cell. 2024 Feb 26;36(3):642-664. doi: 10.1093/plcell/koad279.
Many non-graminaceous species release various coumarins in response to iron (Fe) deficiency. However, the physiological relevance of these coumarins remains poorly understood. Here, we show that the three enzymes leading to sideretin biosynthesis co-exist in Arabidopsis (Arabidopsis thaliana) epidermal and cortical cells and that the shift to fraxetin at alkaline pH depends on MYB72-mediated repression of CYTOCHROME P450, FAMILY 82, SUBFAMILY C, POLYPEPTIDE 4 (CYP82C4). In vitro, only fraxetin and sideretin can reduce part of the Fe(III) that they mobilize. We demonstrate that coumarin-mediated Fe(III) reduction is critical under acidic conditions, as fraxetin and sideretin can complement the Fe(III)-chelate reductase mutant ferric reduction oxidase 2 (fro2), and disruption of coumarin biosynthesis in fro2 plants impairs Fe acquisition similar to in the Fe(II) uptake-deficient mutant iron-regulated transporter 1 (irt1). Disruption of sideretin biosynthesis in a fro2 cyp82C4-1 double mutant revealed that sideretin is the dominant chemical reductant that functions with FRO2 to mediate Fe(II) formation for root uptake. At alkaline pH, Fe(III) reduction by coumarins becomes almost negligible but fraxetin still sustains high Fe(III) mobilization, suggesting that its main function is to provide chelated Fe(III) for FRO2. Our study indicates that strategy-I plants link sideretin and fraxetin biosynthesis and secretion to external pH to recruit distinct coumarin chemical activities to maximize Fe acquisition according to prevailing soil pH conditions.
许多非禾本科植物在缺铁时会释放各种香豆素。然而，这些香豆素的生理相关性仍知之甚少。在这里，我们表明，导致 sideretin 生物合成的三种酶共同存在于拟南芥（Arabidopsis thaliana）表皮和皮层细胞中，而在碱性 pH 下向 fraxetin 的转变依赖于 MYB72 介导的对细胞色素 P450、家族 82、亚家族 C、多肽 4（CYP82C4）的抑制。在体外，只有 fraxetin 和 sideretin 可以还原它们动员的部分 Fe(III)。我们证明，香豆素介导的 Fe(III)还原在酸性条件下至关重要，因为 fraxetin 和 sideretin 可以补充 Fe(III)-螯合还原酶突变体 ferric reduction oxidase 2（fro2），并且在 fro2 植物中破坏香豆素生物合成会损害铁的获取，这与铁摄取缺陷突变体 iron-regulated transporter 1（irt1）相似。在 fro2 cyp82C4-1 双突变体中破坏 sideretin 生物合成表明，sideretin 是主要的化学还原剂，与 FRO2 一起介导 Fe(II)的形成，用于根部吸收。在碱性 pH 下，香豆素对 Fe(III)的还原作用几乎可以忽略不计，但 fraxetin 仍能维持高 Fe(III)动员，这表明其主要功能是为 FRO2 提供螯合 Fe(III)。我们的研究表明，策略-I 植物将 sideretin 和 fraxetin 的生物合成和分泌与外部 pH 联系起来，根据当前土壤 pH 条件，利用不同的香豆素化学活性来最大限度地获取铁。
