根分泌作用与植物对金属毒害的抗性

在金属污染进入体内之前将其有效性和毒性降低是植物的主要抗金属机制之一,根系是金属等土壤污染物进入植物的门户,它能分泌有机酸、氨基酸,糖、生长物质等根分泌物与根际环境,根分泌物在植物吸收金属的过程中影响很大,它们可以通过改变根球环境的ＰＨ、Ｅｈ等物理、化学性质而影响根系对金属的吸收;通过螯合、络合,沉淀等作用将金属污染物滞留于根外;通过改变根际微生的组织,活性和分泌作用而改变根际环境中金属的数量和活     

甜菜夜蛾的抗性及抗性机理研究进展

甜菜夜蛾Spodoptera exigua(Hübner)是世界性农业害虫,近年来在我国由次要害虫上升为主要害虫。甜菜夜蛾对很多种化学杀虫剂和生物杀虫剂产生了抗性。本文分别阐述了甜菜夜蛾对常用杀虫剂如有机磷类、拟除虫菊酯类、氨基甲酸酯类、生长调节类杀虫剂、Bt杀虫剂等的抗性发展现状,并且对抗药性机理进行了总结,提出了抗性治理措施。     

蒙古柳金属硫蛋白的拟南芥遗传转化及抗性分析

本文研究在NaC l胁迫下筛选抗性pY ES2-蒙古柳c DNA-酵母菌,通过克隆得到蒙古柳c DNA序列,生物信息学分析该序列编码区为240 bp,编码79个氨基酸,在NCBI数据库中BLASTp分析表明和旱柳和毛果杨金属硫蛋白MT2序列有较高相似性。把克隆得到的cD NA与pB I121质粒通过Bam HI和XhoI进行酶切连接,通过农杆菌的介导法转入拟南芥。用Kana筛选的抗性苗提取基因组DNA,PCR鉴定得到6株转基因株系。将转基因拟南芥和野生型拟南芥在不同逆境下进行抗性分析,结果表明转基因拟南芥在NaC l、NaH CO3、H2O2、CuC l2、ZnC l2、CdC l2和Sorbitol胁迫培养基中表现出比对照强的抗性,其中在CdC l2培养基中比野生型植株表现出强的抗性。     

植物四倍体抗性及其机理研究进展

植物四倍体是重要的育种资源,其创制和抗性研究受到越来越多的关注。针对植物四倍体的抗性研究,国内外学者开展了大量的工作。本文对近年来四倍体与二倍体植物的抗性（抗寒、抗旱、抗热、抗盐、抗病等）差异进行了总结,并从植物组织细胞结构、生理、分子应答和表观遗传组修饰等方面对其抗性机理进行了探讨。     

PR—蛋白与高等植物的诱导抗性

一某些过敏性寄主植物在感染病毒或其它病原物后,以牺牲局部叶片(在接种叶片上形成局部枯斑或坏死)而使全侏免遭罹难,当相关性或非相关性病原物再次侵染时,植株即表面出抗性。这种抗性不同于植物本身所固有的先天抗性,由于它是在受病原物侵染(或化学药物处理)后诱导产生的,故称之为诱导抗性(或获得抗性)。诱导抗性的共同特征是作用的非持异性,即由病毒诱导的抗性不仅可以抵抗非相关病毒的侵染,还可以抵抗     

铝胁迫下植物根系的有机酸分泌及其解毒机理

酸性土壤中的铝毒害问题,已成为限制植物生长发育的主要因素之一.耐铝植物通过根系分泌有机酸来解除或减轻铝的毒害是外部解铝毒的重要机制.文章对铝胁迫下植物根系分泌有机酸的种类,有机酸解铝毒机理、解铝毒能力,有机酸分泌方式及调控其分泌的主要因素等相关研究进行综述.     

有机酸在疫霉营养中作用的进一步研究

利用放射性同位素标记的~(14)C-琥珀酸跟踪,进一步证明有机酸在晚疫病菌的营养中作为碳架与铵盐结合而合成菌体的氨基酸,同时也进入三羧酸循环参加呼吸。所以其作用不仅仅是缓冲作用。所试6个生理小种19株国内外晚疫病菌都对有机酸有迫切需要而旺盛生长,只有两株国外菌种长势太弱。其他测试过的疫霉P.boehmeriae,P.capsici,P.cinnamomi,P.citrophthora,P.colocasiae,P.erythroseptica,P.fragariae,P.megasperma f.sp.glycinea,P.palmivora,P.parasitica,P.sinensis和P.syringae 12种包括A_1与A_2交配型共18株菌都对有机酸有需要,只在迫切程度上不同。但有机酸在疫霉属营养上有普遍意义则是显而易见的。     

紫羊茅重金属抗性和敏感品种中类金属硫蛋白基因的鉴定及表达初探

重金属对生命机体的作用具有双重性。一方面,作为多数辅酶的辅助因子对细胞的正常代谢必不可少;另一方面,当重金属超过一定的浓度时对细胞有较大的毒性。在长期的进化过程中,生物可能形成一种调节细胞内重金属浓度的机制。这种机制在动物和真菌中被认为同金属硫蛋白(metallothionein,MT)的作用密切相关。植物中也存在类似的与重金属结合的低分子量蛋白(heavy metal binding pep-tide)。最近对拟南芥菜和水稻中类金属硫蛋白(MT-like)基因的研究证实其作用与动物MT相似。紫羊茅品种“Merlin”是一种从锌铅矿区的重金属污染地采集的单子叶草种,对镉和铜的抗性都较高,分别达到50mg/L和30mg/L,而5mg/L Cd~(2 )或2mg/L Cu~(2 )便可抑制敏感品种“S59”的正常生长。目前     

Protecting Cell Walls from Binding Aluminum by Organic Acids Contributes to Aluminum Resistance

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major AI resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify AI in the growth medium is frequently questioned. The genotypes of Al-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate, respectively. In the present study we found that at a 35% inhibition of root elongation, the AI activities in the solution were 10, 20, and 50 μM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol/cm2 per 12 h, respectively, for the above three plant species. When exogenous organic acids were added to ameliorate Al toxicity, twofold and eightfold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of AI adsorbed to the cell walls all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect the cell walls from binding AI. These findings suggest that protection of cell walls from binding Al by organic acids may contribute significantly to AI resistance.     

Protecting Cell Walls from Binding Aluminum by Organic Acids Contributes to Aluminum Resistance

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major AI resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify AI in the growth medium is frequently questioned. The genotypes of AI-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate, respectively. In the present study we found that at a 35% inhibition of root elongation, the AI activities in the solution were 10, 20, and 50 μM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol/cm2 per 12 h, respectively, for the above three plant species. When exogenous organic acids were added to ameliorate AI toxicity, twofold and eightfold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of AI adsorbed to the cell walls all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect the cell walls from binding AI. These findings suggest that protection of cell walls from binding AI by organic acids may contribute significantly to AI resistance.     

Resolution of an ATP-metal chelate from metal-free ATP by reverse-phase high-performance liquid chromatography

The modulation of many enzymatic reactions involved in the metabolism of nucleotide phosphates such as ATP often require divalent metal ions. In the present study reverse-phase high-performance liquid chromatography (HPLC) was used to study the chelation of divalent metal ions, such as Mn²⁺, Mg²⁺, and Ca²⁺, by ATP. The results of our study using radiolabeled [⁴⁵Ca] showed that the metal-ATP chelate formed in solution was retained longer than the metal-free ATP due to the nonpolar groups on the column packing. Recovery of the two forms of ATP showed that the [⁴⁵Ca] coeluted exclusively with the ATP-metal chelate. Other experiments showed that the retention time of the chelated form of the ATP was unaffected by eluent flow rate, but was affected by eluant pH and methanol concentration. The amount of ATP in the chelated form was found to be dependent on the amount of the metal in solution and that under appropriate conditions, i.e., with 0.1 m CaCl₂ in the mobile phase, on the divalent cation as well. Thus, we found that in terms of effectiveness in chelate formation, the metal ions were Ca²⁺ > Mg²⁺ > Mn²⁺. Recovery of the chelate and its reanalysis by HPLC revealed that the complex had dissociated. The chelate could be reformed by restoring the metal concentration to its original value and dissociated again by the addition of EDTA. The resolution of the ATP in a metal chelated form from the ATP in an unchelated form is discussed in terms of the stability of these chelates and the role of the hydrophobic groups of the column packing used in the reverse-phase HPLC in enhancement of this stability.     

Mechanisms of metal resistance in plants: aluminum and heavy metals

Plants have evolved sophisticated mechanisms to deal with toxic levels of metals in the soil. In this paper, an overview of recent progress with regards to understanding fundamental molecular and physiological mechanisms underlying plant resistance to both aluminum (Al) and heavy metals is presented. The discussion of plant Al resistance will focus on recent advances in our understanding of a mechanism based on Al exclusion from the root apex, which is facilitated by Al-activated exudation of organic acid anions. The consideration of heavy metal resistance will focus on research into a metal hyperaccumulating plant species, the Zn/Cd hyperaccumulator, Thlaspi caerulescens, as an example for plant heavy metal research. Based on the specific cases considered in this paper, it appears that quite different strategies are used for Al and heavy metal resistance. For Al, our current understanding of a resistance mechanism based on excluding soil-borne Al from the root apex is presented. For heavy metals, a totally different strategy based on extreme tolerance and metal hyperaccumulation is described for a hyperaccumulator plant species that has evolved on naturally metalliferous soils. The reason these two strategies are the focus of this paper is that, currently, they are the best understood mechanisms of metal resistance in terrestrial plants. However, it is likely that other mechanisms of Al and/or heavy metal resistance are also operating in certain plant species, and there may be common features shared for dealing with Al and heavy resistance. Future research may uncover a number of novel metal resistance mechanisms in plants. Certainly the complex genetics of Al resistance in some crop plant species, such as rice and maize, suggests that a number of presently unidentified mechanisms are part of an overall strategy of metal resistance in crop plants.     

Extracellular and cellular mechanisms sustaining metal tolerance in ectomycorrhizal fungi

This review focuses on recent evidence that identifies potential extracellular and cellular mechanisms that may be involved in the tolerance of ectomycorrhizal fungi to excess metals in their environment. It appears likely that mechanisms described in the nonmycorrhizal fungal species are used in the ectomycorrhizal fungi as well. These include mechanisms that reduce uptake of metals into the cytosol by extracellular chelation through extruded ligands and binding onto cell-wall components. Intracellular chelation of metals in the cytosol by a range of ligands (glutathione, metallothioneins), or increased efflux from the cytosol out of the cell or into sequestering compartments are also key mechanisms conferring tolerance. Free-radical scavenging capacities through the activity of superoxide dismutase or production of glutathione add another line of defence against the toxic effect of metals.     

The determination of the stability constant for the iron(II) complex of the biochelator pyridine-2,6-bis(monothiocarboxylic acid)

Pyridine-2,6-bis(monothiocarboxylic acid), also known as pyridine-2,6-dithiocarboxylic acid (pdtc), is a unique and powerful metal chelator produced by Pseudomonas stutzeri and Pseudomonas putida. The actual physiological roles of pdtc in these pseudomonads are not known with certainty, though it is likely that the compound acts as a siderophore, an antibiotic, or both. The stability constant of Fe^III(pdtc)₂ ^2- was determined in previous work to be 10^33.36. Here we determined that the stability constant of Fe^II(pdtc)₂ ^2- is 10¹². We determined this stability constant through potentiometric and spectrophotometric measurements of a ligand-ligand competition study using 2,6-pyridine dicarboxylic acid as the competitor for iron. Comparing the stability constant for Fe^II(pdtc)₂ ^2- to the constant for Fe^III(pdtc)₂ ^2- shows that the stability constant of Fe^II(pdtc)₂ ^2- is approximately 21 orders of magnitude smaller. This represents a very significant decrease in the binding strength of pdtc toward iron. Thus, if the host cell produces pdtc as a siderophore for sequestering Fe(III), it is likely that a second metabolite or a membrane protein of the host cell is used for reduction of the chelated iron at or near the cell membrane in order to facilitate its release from pdtc for cellular use.     

Comparative microarray analysis of Arabidopsis thaliana and Arabidopsis halleri roots identifies nicotianamine synthase, a ZIP transporter and other genes as potential metal hyperaccumulation factors

The hyperaccumulation of zinc (Zn) and cadmium (Cd) is a constitutive property of the metallophyte Arabidopsis halleri. We therefore used Arabidopsis GeneChips to identify genes more active in roots of A. halleri as compared to A. thaliana under control conditions. The two genes showing highest expression in A. halleri roots relative to A. thaliana roots out of more than 8000 genes present on the chip encode a nicotianamine (NA) synthase and a putative Zn2+ uptake system. The significantly higher activity of these and other genes involved in metal homeostasis under various growth conditions was confirmed by Northern and RT-PCR analyses. A. halleri roots also show higher NA synthase protein levels. Furthermore, we developed a capillary liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry (CapLC-ESI-QTOF-MS)-based NA analysis procedure and consistently found higher NA levels in roots of A. halleri. Expression of a NA synthase in Zn2+-hypersensitive Schizosaccharomyces pombe cells demonstrated that formation of NA can confer Zn2+ tolerance. Taken together, these observations implicate NA in plant Zn homeostasis and NA synthase in the hyperaccumulation of Zn by A. halleri. Furthermore, the results show that comparative microarray analysis of closely related species can be a valuable tool for the elucidation of phenotypic differences between such species.     

Fusaric acid contributes to virulence of Fusarium oxysporum on plant and mammalian hosts

Fusaric acid (FA) is amongst the oldest identified secondary metabolites produced by Fusarium species, known for a long time to display strong phytotoxicity and moderate toxicity to animal cells; however, the cellular targets of FA and its function in fungal pathogenicity remain unknown. Here, we investigated the role of FA in Fusarium oxysporum, a soil‐borne cross‐kingdom pathogen that causes vascular wilt on more than 100 plant species and opportunistic infections in humans. Targeted deletion of fub1, encoding a predicted orthologue of the polyketide synthase involved in FA biosynthesis in F. verticillioides and F. fujikuroi, abolished the production of FA and its derivatives in F. oxysporum. We further showed that the expression of fub1 was positively controlled by the master regulator of secondary metabolism LaeA and the alkaline pH regulator PacC through the modulation of chromatin accessibility at the fub1 locus. FA exhibited strong phytotoxicity on tomato plants, which was rescued by the exogenous supply of copper, iron or zinc, suggesting a possible function of FA as a chelating agent of these metal ions. Importantly, the severity of vascular wilt symptoms on tomato plants and the mortality of immunosuppressed mice were significantly reduced in fub1Δ mutants and fully restored in the complemented strains. Collectively, these results provide new insights into the regulation and mode of action of FA, as well as on the function of this phytotoxin during the infection process of F. oxysporum.     

Plant Root Responses to Three Abundant Soil Minerals: Silicon,Aluminum and Iron

Silicon (Si), aluminum (Al), and iron (Fe) are the three most abundant minerals in soil; however, their effects on plants differ because they are beneficial, toxic, and essential to plant growth, respectively. High accumulation of silicon in the shoots helps some plants to overcome a range of biotic and abiotic stresses. However, plants vary in their ability to take up Si from the soil and load it into the xylem and so the accumulation of silicon varies greatly between plant species. Aluminum toxicity is characterized by a rapid inhibition of root elongation but some species and even genotypes within species can tolerate Al toxicity better than others. While the mechanisms controlling this tolerance in most of the more resistant species are poorly understood, some plants are able to detoxify Al externally and/or internally by complexation with ligands or by pH changes in the rhizosphere. Iron is taken up from the soil by two efficient mechanisms called Strategy I and Strategy II, which operate in distinct phylogenic groups. Strategy I plants increase soil Fe solubility by releasing protons and reductants/chelators, such as organic acids and phenolics, into the rhizosphere, while Strategy II plants are characterized by the secretion of ferric chelating substances (phytosiderophores) coupled with a specific Fe³⁺: chelate uptake system. In this review, the molecular mechanisms underlying root response to Si, Al, and Fe are described.

     

The nicotianamine molecule is made-to-measure for complexation of metal micronutrients in plants

The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous among plants. In meristematic tissues it reaches concentrations of about 400mol (g fresh weight)^–1. NA forms complexes, among others, with the metal micronutrients (MN) copper, zinc, iron and manganese (logK _MeNA 18.6-8.8). Calculations of the dissociation curves of the metal-NA complexes based on the complex formation constants and on the acid dissociation constants of NA revealed their stability at the neutral or weak alkaline pH of cytoplasm and sieve tube sap. For the Mn-NA complex, dissociation begins at about pH 6.5, for all others dissociation occurs at more acid pHs. Thus, metal-NA complexes could theoretically persist also in the apoplasm and in xylem sap. The octanol water partition coefficient of NA is about 1 and those of its metal complexes are in the range of 0.3–0.4. The reason for this shift is perhaps the negative charge of the complexes. The higher lipophilicity of the free NA indicates that the NA supply to sites of requirement is faster than the removal of the complexes as long as membranes are an integral part of the transport paths. Changing phloem transport rates of MN-NA complexes by manipulation of the cotyledon apoplasm of Ricinus commuais L. suggest a competition of MN for NA at the site(s) of phloem loading. Thus, NA could control MN transport via phloem including recirculation.