蘚苔植物鐵吸收策略與演化之研究

Abstract

鐵是植物生長所不可或缺的元素，自然環境下以氧化鐵的形式存在並附著於土壤粒子中。為了吸收鐵，植物發展出不同的吸收策略。被子植物中非禾本科植物使用鐵還原吸收策略一， 而禾本科植物則使用鐵螯合吸收策略二。目前開花植物和藻類鐵吸收的機制所知甚詳，但對於蘚苔植物的認識卻很匱乏。蘚苔植物是陸生植物最早期的分支類群，在植物演化過程中具有其舉足輕重之地位。本研究以蘚苔植物中的模式植物-地錢（Marchantia polymorpha）和小立碗蘚（Physcomitrella patens）為研究對象，發現地錢使用鐵還原吸收策略一、而小立碗蘚卻使用鐵螯合吸收策略二。缺鐵逆境下，地錢體內FRO和H+-ATPase的活性會提高。此外，地錢缺乏鐵螯合吸收策略中最重要的化合物-phytosiderphores、nicotianamine（NA）和 NA合成酶，這意味著地錢不使用此策略。地錢基因組內有五個和鐵鋅運輸相關之基因，缺乏鐵鋅運輸基因（MpZIP3）的地錢突變株在缺鐵逆境下，體內鐵含量下降；反之若大量表現此基因，則轉殖株體內鐵含量增加。由此可知MpZIP3基因參與鐵的吸收機制。因此我們可以得到結論，地錢和大多數的陸生植物相同，使用還原策略吸收鐵元素。另一方面，鐵同位素分析得知小立碗蘚使用鐵螯合吸收策略二。小立碗蘚體內NA的含量會隨著缺鐵天數的增加而增加，但卻沒有發現phytosiderphores的存在。NA在維管束植物體內扮演鐵、鋅等重金屬的螯合劑與長距離運輸之角色，這意味著小立碗蘚可能直接利用NA做為三價鐵的螯合物。有趣的是，小立碗蘚基因組裡可以同時找到鐵還原及螯合吸收策略相關的基因群；暗示著小立碗蘚可能同時使用此兩種策略。綜合本研究之成果，我們得知鐵還原吸收策略普遍存在於陸生植物當中，而鐵螯合吸收策略則是植物為了適應環境所另行演化出來。本研究對於陸生植物鐵吸收機制之演化過程，提供了一個重要的連結。

To acquire appropriate Fe, flowering plants have developed two unique strategies, the reduction-based Strategy I of non-graminaceous plants for Fe2+ and the chelation-based Strategy II of graminaceous plants for Fe3+. However, the mechanism of Fe uptake in bryophytes, the earliest diverging branch of land plants and dominant in gametophyte generation is less clear. Fe isotope fractionation analysis demonstrated that the liverwort Marchantia polymorpha and moss Physcomitrella patens use reduction-based and chelation-based Fe acquisition, respectively. In M. polymorpha, enhanced activities of ferric chelate reductase and proton ATPase were detected under Fe-deficient conditions. However, M. polymorpha did not show mugineic acid family phytosiderophores, the key components of Strategy II, or the precursor nicotianamine. Five ZIP (ZRT/IRT-like protein) homologs were identified and speculated to be involved in Fe uptake in M. polymorpha. MpZIP3 knockdown conferred reduced growth under Fe-deficient conditions, and MpZIP3 overexpression increased Fe content under excess Fe. Thus, a nonvascular liverwort, M. polymorpha, uses Strategy I for Fe acquisition. Interestingly, Fe uptake related genes involved in both reduction- and chelation- based strategies were identified in the moss Physcomitrella patens transcriptome database. Although the relative abundance of nicotianamine was increased under Fe deficiency, none of known phytosiderophores were found in the moss. NA plays important roles of Fe/Zn chelation and transport in plants. It implied that P. paten might use NA directly for Fe chelation. These findings also suggested that the reduction system was acquired in the green lineage, and unique strategy for Fe uptake was developed in P. paten. This system may have been acquired in the common ancestor of land plants and co-opted from the gametophyte to sporophyte generation in the evolution of land plants.