植物铁吸收、转运和调控的分子机制研究进展

铁是植物正常生命活动所必需的微量矿质元素, 铁离子的吸收、转运和利用是一个复杂的过程, 很多基因参与了这一过程。本文对近10年来发现和分离的参与植物铁吸收、转运及调控的基因研究进展进行了综述。根据最近的研究结果, 提出了植物控制铁吸收的分子调控模式(机理I)。     

铁转运机制与相关基因的研究进展

铁素营养分子生物学方面的研究有了很大进展。人们利用各种特异突变株和差异筛选已克隆到部分与铁转运有关的基因。本文主要在分子生物学水平概括了酵母铁吸收转运机制和植物缺铁胁迫相关基因及其基因表达的研究进展。     

铁转运机制与相关基因的研究进展

铁素营养分子生物学方面的研究有了很大进展。人们利用各种特异突变株和差异筛选已克隆到部分与铁转运有关的基因。本文主要在分子生物学水平概括了酵母铁吸收转运机制和植物缺铁胁迫相关基因及其基因表达的研究进展。     

高等植物中铁的代谢机制

铁在高等植物的生长发育中发挥着重要作用,但随着人类的耕作及土壤的盐碱化,缺铁已成为一个世界性植物营养问题。高等植物在长期的进化过程中,形成了完善的对环境铁信号响应的体系。本文围绕植物与环境的相互作用,综述了近年来植物铁营养的吸收、转运、分配和储存的研究进展,并总结了植物中铁营养代谢调控的相关机理。     

膜铁转运蛋白1，铁调素的靶分子？

膜铁转运蛋白1是重要的跨膜铁输出分子,主要分布于十二指肠和单核巨噬系统的细胞膜上,参与机体的肠铁吸收和巨噬细胞对铁的再循环等过程。铁调素是调节机体铁代谢平衡的激素,机体通过肝脏分泌的铁调素对铁转运相关蛋白的表达进行调控,从而实现机体自身的铁稳态。最新研究显示,铁调素的靶分子可能是膜铁转运蛋白1,它通过直接的作用引起膜铁转运蛋白1的内化(internalization)、降解,从而调节其在细胞膜上的表达量,进而控制肠铁吸收和巨噬细胞对铁的再循环过程,以维持机体的铁稳态。     

高等植物硝酸盐转运蛋白的功能及其调控机制

硝酸盐是植物从土壤中吸收的重要无机氮素形态。植物为适应含有不同浓度NO3-的土壤环境,进化出了高亲和硝酸盐转运系统(HATS)和低亲和硝酸盐转运系统(LATS),两个基因家族NRT1和NRT2家族分别参与了LATS和HATS的NO3-的吸收和转运。近年来,随着分子生物学技术和植物基因组学的快速发展,研究人员克隆出了大量参与硝酸盐吸收和转运的基因,并对这些基因的功能进行了深入研究,逐渐形成了复杂的硝酸盐调控网络。综述了植物中硝酸盐转运蛋白基因的克隆、表达及调控,并对进一步的研究作了展望,这些结果对于理解植物硝酸盐吸收的调控机制具有重要作用。     

与植物镉吸收转运相关的主要基因家族

镉(cadmium)是一种对植物毒性极强的非必需微量元素,影响植物生长发育,甚至死亡,并可在植物体内积累而威胁食物链顶端生物的生命健康。目前已发现有多类基因家族的成员参与了植物中镉的吸收转运过程,包括P型ATP酶、ABC、MATE、NRAMP、CE、CAX、ZIP、OPT等。这些基因家族主要是在吸收转运铁、锌、镁等植物必需微量元素的同时,也具有吸收转运镉等有毒重金属的功能。     

小肠铁释放机制及相关疾病研究进展

铁是生物体必需的微量元素。铁缺乏和铁过载均会导致铁代谢紊乱相关疾病,因此有关机体铁水平稳态的调节机制已成为了目前铁代谢领域的研究热点。小肠吸收细胞是调节肠铁吸收、肠铁释放,以及维持机体铁稳态的重要部位。最新的研究表明,铁从小肠吸收细胞基底端释放入血液循环,主要是由膜铁转运蛋白（ferroportin1,Fp1）介导,并在膜铁转运辅助蛋白（haphaestin,Hp）和铜蓝蛋白（ceruloplasmin,Cp）的参与下完成。其中Fp1在小肠铁释放过程中起着至关重要的作用。本文重点阐述铁释放相关蛋白Fp1的作用机制及其调节机制,并详细介绍Fp1基因突变导致的铁代谢相关疾病方面的最新研究讲展。     

ZIP蛋白在植物中的功能分析

锌和铁是植物生长发育所必需的微量营养元素,在植物的光合作用、呼吸作用以及许多生化反应中起着非常重要的作用。植物体内锌铁处于平衡状态才能保证其正常的生长发育,而锌铁调控转运体ZIP对于锌铁吸收、转运及体内平衡的调节有重要作用。目前,对于植物中ZIP家族基因的研究有一定进展。对植物ZIP基因的表达、蛋白定位、酵母互补实验、过表达及基因敲除等研究结果进行综述,揭示了ZIP蛋白在植物发育过程中的作用。了解ZIP对于锌铁吸收、转运及体内平衡中的作用有助于通过转基因改良及常规育种将ZIP蛋白应用于农业生产上。     

Structural biology of bacterial iron uptake

To fulfill their nutritional requirement for iron, bacteria utilize various iron sources which include the host proteins transferrin and lactoferrin, heme, and low molecular weight iron chelators termed siderophores. The iron sources are transported into the Gram-negative bacterial cell via specific uptake pathways which include an outer membrane receptor, a periplasmic binding protein (PBP), and an inner membrane ATP-binding cassette (ABC) transporter. Over the past two decades, structures for the proteins involved in bacterial iron uptake have not only been solved, but their functions have begun to be understood at the molecular level. However, the elucidation of the three dimensional structures of all components of the iron uptake pathways is currently limited. Despite the low sequence homology between different bacterial species, the available three-dimensional structures of homologous proteins are strikingly similar. Examination of the current three-dimensional structures of the outer membrane receptors, PBPs, and ABC transporters provides an overview of the structural biology of iron uptake in bacteria.     

微生物嗜铁素介导的铁摄取

嗜铁素是好氧菌和兼性厌氧菌的一种产物,它是微生物在低铁条件下产生的小分子的、特异性的Fe^3＋螯合因子。大多数的好氧和兼性厌氧微生物至少合成一种嗜铁素,由嗜铁素介导的铁摄取可能是细菌最普遍的一种获取铁元素的方式。     

Two pathways of iron uptake in bovine spleen apoferritin dependent on iron concentration

Iron incorporation by bovine spleen apoferritin either with ferrous ammonium sulfate in different buffers or with ferrous ammonium sulfate and phosphate was studied. Iron uptake and iron autoxidation were recorded spectrophotomerically. The buffers [4-(2-hydroxyethyl)-1-piperazinyl]ethanesulphonic acid (Hepes) and tris(hydroxymethyl)aminoethane (Tris) exhibited pH-dependent iron autoxidation, with Tris showing less iron autoxidation than Hepes. An Eadie-Scatchard plot (v/[s] versus v) of the iron uptake rate in Hepes was a curved rather than a straight line, suggesting that there are two iron uptake pathways. On the other hand, the Eadie-Scatchard plots of Tris and of Hepes after the addition of phosphate showed a straight line. Phosphate accelerated the iron uptake rate. The iron loading kinetics of apoferritin in Hepes was dependent on apoferritin concentration. The K_m value obtained from iron uptake kinetics was 4.5 M, corresponding to the physiological iron concentration. These results demonstrate that iron loading of apoferritin was accomplished at physiological iron concentrations, which is essential for iron uptake, via two uptake pathways of dependent on iron concentration.     

Kinetic analysis of bovine spleen apoferritin and recombinant H and L chain homopolymers: Iron uptake suggests early stage H chain ferroxidase activity and second stage L chain cooperation

Ferritin utilizes ferroxidase activity to incorporate iron. Iron uptake kinetics of bovine spleen apoferritin (H: L = 1 : 1.1) were compared with those of recombinant H chain ferritin and L chain ferritin homopolymers. H chain ferritin homopolymer showed an iron uptake rate identical to bovine spleen apoferritin (0.19 and 0.21 mmol/min/micromol of protein, respectively), and both showed iron concentration-dependent uptake. In contrast, the L chain homopolymer, which lacks ferroxidase, did not incorporate iron and showed the same level of iron autoxidation in the absence of ferritin. Bovine spleen apoferritin was shown to have two iron concentration-dependent uptake pathways over a range of 0.02-0.25 mM ferrous ammonium sulfate (FAS) by an Eadie-Scatchard plot (v/[FAS] versus v), whereas the H chain ferritin homopolymer was found to have only one pathway. Of the two Km values found in bovine spleen apoferritin, the lower mean Km value was 9.0 microM, while that of the H chain homopolymer was 11.0 microM. H chain ferritin homopolymer reached a saturating iron uptake rate at 0.1 mM FAS, while bovine spleen apoferritin incorporated more iron even at 0.25 mM FAS. These results suggest that the intrinsic ferroxidase of ferritin plays a significant role in iron uptake, and the L chain cooperates with the H chain to increase iron uptake.     

水稻铁吸收、转运及调控的分子机制研究进展

铁是水稻生长和发育所必需的营养元素之一。研究表明,水稻既可以以螯合物的形式从土壤中吸收Fe³⁺、Fe²⁺,又可以直接转运根际土壤中游离的Fe²⁺。科研人员已经鉴定了很多参与铁离子吸收和转运的重要分子元件,包括转运蛋白、酶、螯合物等,同时也挖掘了部分调控这些分子元件表达的上游基因。碱性土壤的高pH值影响水稻对铁离子的吸收和利用,因此,科研人员通过改良碱性土壤中铁离子的利用效率来改良水稻的耐碱性,并取得了一定的成效。本文主要对上述内容进行了综述,并对该领域未来的研究方向进行了展望。     

SIDEROPHORE‐INDEPENDENT IRON UPTAKE BY IRON‐LIMITED CELLS OF THE CYANOBACTERIUM ANABAENA FLOS‐AQUAE1

Iron acquisition by iron‐limited cyanobacteria is typically considered to be mediated mainly by siderophores, iron‐chelating molecules released by iron‐limited cyanobacteria into the environment. In this set of experiments, iron uptake by iron‐limited cells of the cyanobacterium Anabaena flos‐aquae (L.) Bory was investigated in cells resuspended in siderophore‐free medium. Removal of siderophores decreased iron‐uptake rates by ～60% compared to siderophore‐replete conditions; however, substantial rates of iron uptake remained. In the absence of siderophores, Fe(III) uptake was much more rapid from a weaker synthetic chelator [N‐(2‐hydroxyethyl)ethylenediamine‐N,N′,N′‐triacetic acid (HEDTA); log K_cond = 28.64 for Fe(III)HEDTA(OH)^?] than from a very strong chelator [N,N′‐bis(2‐hydroxybenzyl)‐ethylenediamine‐N,N′‐diacetic acid (HBED); log K_cond = 31.40 for Fe(III)HBED^?], and increasing chelator:Fe(III) ratios decreased the Fe(III)‐uptake rate; these results were evident in both short‐term (4 h; absence of siderophores) and long‐term (116 h; presence of siderophores) experiments. However, free (nonchelated) Fe(III) provided the most rapid iron uptake in siderophore‐free conditions. The results of the short‐term experiments are consistent with an Fe(III)‐binding/uptake mechanism associated with the cyanobacterial outer membrane that operates independently of extracellular siderophores. Iron uptake was inhibited by temperature‐shock treatments of the cells and by metabolically compromising the cells with diphenyleneiodonium; this finding indicates that the process is dependent on active metabolism to operate and is not simply a passive Fe(III)‐binding mechanism. Overall, these results point to an important, siderophore‐independent iron‐acquisition mechanism by iron‐limited cyanobacterial cells.     

Ferrous iron is transported across the peribacteroid membrane of soybean nodules

Symbiosomes and bacteroids isolated from soybean (Glycine max Merr.) nodules are able to take up ferrous iron. This uptake activity was completely abolished in the presence of ferrous-iron chelators. The kinetics of uptake were characterized by initially high rates of iron internalization, but no saturation was observed with increasing iron concentration. This process does not appear to involve the ferric reductase of the peribacteroid membrane. The transport of ferrous iron was inhibited by other transition metals, particularly copper. Ferrous iron was taken up by symbiosomes more efficiently than the ferric form. This indicates that the iron transport from the plant host cell to the microsymbiont in vivo may occur mainly as the ferrous form. Received: 11 February 1998 / Accepted: 29 May 1998     

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.     

Iron uptake by the ichthyotoxic Chattonella marina (Raphidophyceae): impact of superoxide generation1

Significant production of superoxide, a known reductant of both inorganic and organically complexed iron(III), occurs in natural systems by both biotic and abiotic pathways. We have investigated the generation of superoxide by Chattonella marina (Subrahman.) Y. Hara et Chihara, a phytoplankton taxon known to produce high levels of this reactive oxygen species, and examined the role of superoxide in the acquisition of iron by this organism. Additionally, a generalized model for iron acquisition by C. marina has been developed, which includes three pathways of iron acquisition from organically complexed iron(III): nondissociative reductive uptake, dissociative reductive uptake, and nonreductive dissociative uptake. The model is shown to be particularly useful in ascertaining the relative importance of these various iron‐uptake pathways as a function of solution parameters including concentration and iron‐binding strength of the organic ligand and superoxide concentration. Our results suggest that superoxide can participate in the C. marina iron‐uptake process when iron is complexed to weak ligands, such as citrate, but plays only a minor role when iron is bound to a strong ligand. It thus appears that facilitation of iron acquisition is not the sole purpose of superoxide production by these organisms.     

Effect of ammonium and nitrate on ferric chelate reductase and nitrate reductase in Vaccinium species

BACKGROUND AND AIMS: Most Vaccinium species have strict soil requirements for optimal growth, requiring low pH, high iron availability and nitrogen primarily in the ammonium form. These soils are limited and are often located near wetlands. Vaccinium arboreum is a wild species adapted to a wide range of soils, including high pH, low iron, and nitrate-containing soils. This broader soil adaptation in V. arboreum may be related to increased efficiency of iron or nitrate uptake compared with the cultivated Vaccinium species. METHODS: Nitrate, ammonium and iron uptake, and nitrate reductase (NR) and ferric chelate reductase (FCR) activities were compared in two Vaccinium species grown hydroponically in either nitrate or ammonia, with or without iron. The species studied were the wild V. arboreum and the cultivated V. corymbosum interspecific hybrid, which exhibits the strict soil requirements of most Vaccinium species. RESULTS: Ammonium uptake was significantly greater than nitrate uptake in both species, while nitrate uptake was greater in the wild species, V. arboreum, compared with the cultivated species, V. corymbosum. The increased nitrate uptake in V. arboreum was correlated with increased root NR activity compared with V. corymbosum. The lower nitrate uptake in V. corymbosum was reflected in decreased plant dry weight in this species compared with V. arboreum. Root FCR activity increased significantly in V. corymbosum grown under iron-deficient conditions, compared with the same species grown under iron-sufficient conditions or with V. arboreum grown under either iron condition. CONCLUSIONS: V. arboreum appears to be more efficient in acquiring nitrate compared with V. corymbosum, possibly due to increased NR activity and this may partially explain the wider soil adaptation of V. arboreum.