植物病原物无毒基因及其功能

植物抗病基因与病原物无毒基因产物间直接或间接相互作用导致产生的基因对基因抗性是植物抗病性的重要形式。无毒基因已在多种植物病原物 ,包括真菌、细菌、病毒和卵菌等中得到克隆。绝大多数已克隆无毒基因之间 ,及其与已知蛋白之间 ,均无显著序列同源性。然而 ,多数已克隆植物抗病基因有较高序列一致性 ,产物往往具有相似的结构域。由序列一致性很高的抗病基因产物与没有明显序列同源性的无毒基因产物相互作用 ,介导产生的过敏性细胞坏死和抗病性 ,在产生速度、强度和组织特异性等方面均可能有显著差异。无毒基因具有双重功能 :在含互补抗病基因植物中表现无毒效应 ,而在不含互补抗病基因植物中显示小种、菌株、致病型、或种特异性毒性效应     

植物抗病基因(R)与病原物无毒基因(Avr)相互作用机制的研究进展

PTI和ETI是植物在长期进化过程中形成的两类抵抗病原物的机制。基因对基因假说的抗病方式属于ETI抗性机制的一种,该假说认为具有保守NB-LRR结构域的R蛋白识别病原物非保守的无毒蛋白效应子(Avr),激活防卫反应信号途径,导致过敏性坏死。植物抗病基因(R)与病原菌无毒基因(Avr)产物间的直接或间接相互作用而产生的基因对基因抗性是植物抗病性的重要形式,该文对植物抗病蛋白与无毒蛋白相互作用机制进行了综述。其中,间接相互作用模式是主要方式。     

多基因抗性的QTL作图及其在作物持久性抗病育种上的应用

QTL作图已成为解析生物复杂性状遗传基础和基因座之间互作机制的一种有效的研究工具。多基因抗性没有明显的生理小种特异性,一般表现为数量性状。多基因抗性的QTL作图在植物持久性抗病育种中有重要的应用价值,有助于分离到广谱性抗病基因。从作图群体(F1、F2、DH、RIL、BIL和NIL)构建、抗性表型测定和标记辅助育种等方面论述了多基因抗性QTL作图的最新研究进展。     

植物NBS类抗病基因的进化

NBS(Nucleotide-binding site)类抗病基因是植物中最重要的一类抗病基因, 其进化模式、结构特点和功能调控一直是抗病基因研究领域的热点。这类基因具有保守的结构域, 广泛存在于植物基因组中, 在不同植物基因组中数目差异较大且具有较低的表达量。此外, 同源NBS类抗病基因之间通过频繁的序列交换产生广泛的序列多样性, 且抗病基因位点具有较差的线性。依据基因之间序列交换的频率, 抗病基因可分为TypeⅠ和TypeⅡ两类。文章从抗病基因的结构、数量、分布、序列多样性、进化模式以及表达调控等方面进行了综述, 旨在为后续NBS类抗病基因的相关研究提供参考。     

玉米与其他六种植物NBS类抗病基因的进化比较分析

具有核苷酸结合位点(nucleotide binding site,NBS)的抗病基因在植物抵抗各种病原菌侵染中起关键作用。对玉米全基因组中具有NBS结构的基因进行鉴定和分析,并结合水稻、高粱、拟南芥、百脉根、苜蓿和杨树的NBS类基因比较其在数量、复制、染色体定位和亲缘关系上的进化差异。发现玉米NBS类基因数量、复制数和成簇基因数均明显少于其他植物。低复制频率可能导致玉米NBS类基因较少,并推测可能导致其功能具有多样性。在基因染色体定位上,除高梁外,玉米与其他五种植物相似,呈不均衡分布。此外,进化树分析表明玉米NBS类基因与高粱的亲缘关系最近,与拟南芥的最远,在物种间表现出较高的保守性。结果对掲示玉米NBS基因的进化特点与发掘有益的NBS类抗病基因提供了重要的理论依据。     

植物抗病分子机制研究进展

植物抗病机制是目前研究的热点。在长期的进化过程中,植物形成了一系列复杂有效的防御机制来抵御、破坏病原物的侵染。植物抗病基因在植物抗性反应中起着重要的作用,植物一旦监测到病原物马上起始防御反应,并伴随着植物体内一系列细胞和生理生化的变化。近年来,基因沉默作为一个重要的细胞内防御外源核酸的机制,越来越受到科学家重视。综述了植物抗病基因和基因沉默机制在植物抗病反应中的重要作用,并对研究植物抗病机制的前景做了展望。     

植物抗病基因结构、功能及其进化机制研究进展

植物与病原菌在长期的共进化和相互选择的过程中,逐渐形成了组织障碍、非寄主抗性和小种专化抗性等有效的防御机制。小种专化抗性(基因对基因抗性)主要是由植物抗病基因识别相应的病原菌无毒基因并激活植物体内抗病信号进而抵御病原菌的侵染。从目前已克隆的 70 多个抗病基因来看,它们在结构上具有高度保守性,主要包括核苷酸结合位点(NBS),亮氨酸重复结构(LRR), 蛋白激酶结构域(PK), 果蝇蛋白 Toll 和哺乳动物蛋白质白细胞介素 1 受体[interleukin(IL)-1 receptor]类似结构域(TIR), 双螺旋结构(CC)或亮氨酸拉链(LZ)和跨膜结构域(TM)等,其在抗病基因与病原菌无毒(效应)蛋白互作以及植物内部免疫信号传导中起着重要的作用。同时,抗病基因又通过基因复制、遗传重组等进化机制形成多基因家族,为植物抗病的专化性和多样性提供了重要的遗传基础。本文主要讨论了近来已克隆抗病基因的结构特征、功能以及抗病基因进化机制研究的进展。     

植物抗病基因的分子生物学研究进展

植物在长期的进化过程中,需要不断地抵抗病原微生物、昆虫甚至食草动物的侵害。在这种长期相互作用、相互影响的共进化过程中,植物逐渐获得了一系列复杂的防御机制来保护自己。这里既有专一性的抗性,也有非专一性抗性,有的表达是组成型的而有的是诱导型的。在植物抵抗病原侵染的过程中,关键环节是病原与寄主植物间的相互识别及其随后诱发的一系列反应。人们虽然对植物抗病机理及植物抗病性在生产应用等方面做了大量的工作,积累了相当多的资料,但由于一直未能克隆到植物的抗病基因,所以在90年代初期以前有关植物抗病性的分子生物学研究进展一直都很缓慢。而最近的几年里,人们克隆到了一些植物的抗病基因,使得对寄主-病原相互作用分子机理的研究得到迅速的发展,并为抗病基因在生产中的应用带来了新的前景。     

植物抗病基因克隆的研究进展

本从抗病基因克隆技术,抗病基因的产物和结构,抗病基因介导的抗病信号传导以及抗病基因进化等方面入手,综述了近几年来植物抗病基因克隆的研究进展,并对转座子标签法和定位克隆法做了较为详细的阐述。     

植物抗病基因同源序列及其在抗病基因克隆与定位中的应用

近10年来已有20多个植物抗病基因被克隆,测序,这些抗病基因所编码的蛋白中大多含有核苷酸结合位点,富含亮氨酸重复序列,蛋白激酶,亮氨酸拉链结构,跨膜结构域,Toll白介素－1区域等保守结构域。利用这些保守结构域合成PCR引物,已扩增出大量的植物抗病基因同源序列（RGA）。对RGA与抗病基因的关系进行了分析,讨论了RGA在研究抗病基因进化中的作用,指出RGA在抗病基因定位和转基因中具有重要意义。     

Mode of amplification and reorganization of resistance genes during recent Arabidopsis thaliana evolution.

The NBS-LRR (nucleotide-binding site plus leucine-rich repeat) genes represent the major class of disease resistance genes in flowering plants and comprise 166 genes in the ecotype Col-0 of Arabidopsis thaliana. NBS-LRR genes are organized in single-gene loci, clusters, and superclusters. Phylogenetic analysis reveals nine monophyletic clades and a few phylogenetic orphans. Most clusters contain only genes from the same phylogenetic lineage, reflecting their origin from the exchange of sequence blocks as a result of intralocus recombination. Multiple duplications increased the number of NBS-LRR genes in the progenitors of Arabidopsis, suggesting that the present complexity in Col-0 may derive from as few as 17 progenitors. The combination of physical and phylogenetic analyses of the NBS-LRR genes makes it possible to detect relatively recent gene rearrangements, which increased the number of NBS-LRR genes by about 50, but which are almost never associated with large segmental duplications. The identification of 10 heterogeneous clusters containing members from different clades demonstrates that sequence sampling between different resistance gene loci and clades has occurred. Such events may have taken place early during flowering plant evolution, but they generated modules that have been duplicated and remobilized also more recently.     

The evolution of disease resistance genes

Several common themes have shaped the evolution of plant disease resistance genes. These include duplication events of progenitor resistance genes and further expansion to create clustered gene families. Variation can arise from both intragenic and intergenic recombination and gene conversion. Recombination has also been implicated in the generation of novel resistance specificities. Resistance gene clusters appear to evolve more rapidly than other regions of the genome. In addition, domains believed to be involved in recognitional specificity, such as the leucine-rich repeat (LRR), are subject to adaptive selection. Transposable elements have been associated with some resistance gene clusters, and may generate further variation at these complexes.     

Evolution of plant resistance at the molecular level: ecological context of species interactions

Molecular data regarding the diversity of plant loci involved in resistance to herbivores or pathogens are becoming increasingly available. These genes demonstrate variable patterns of diversity, suggesting that they differ in their evolutionary history. In parallel, the study of natural variation for resistance, generally conducted at the phenotypic level, has shown that resistance does not evolve solely under selection pressures exerted by enemies. Metapopulation dynamics and other ecological characteristics of interacting species also appear to have a large impact on resistance evolution. Until now, studies of resistance at the molecular level have been conducted separately from ecological studies in extant populations. Future progress requires an evolutionary approach integrating both molecular and ecological aspects of resistance evolution. Such an approach will contribute greatly to our understanding of the evolution of molecular diversity at loci involved in biotic stress.     

Cloning, characterization, and evolution of the NBS-LRR-encoding resistance gene analogue family in polyploid cotton (Gossypium hirsutum L.)

The nucleotide-binding site-leucine-rich repeat (NBS-LRR)-encoding gene family has attracted much research interest because approximately 75% of the plant disease resistance genes that have been cloned to date are from this gene family. We cloned the NBS-LRR-encoding genes from polyploid cotton by a polymerase chain reaction-based approach. A sample of 150 clones was selected from the NBS-LRR gene sequence library and was sequenced, and 61 resistance gene analogs (RGA) were identified. Sequence analysis revealed that RGA are abundant and highly diverged in the cotton genome and could be categorized into 10 distinct subfamilies based on the similarities of their nucleotide sequences. The numbers of members vary many fold among different subfamilies, and gene index analysis showed that each of the subfamilies is at a different stage of RGA family evolution. Genetic mapping of a selection of RGA indicates that the RGA reside on a limited number of the cotton chromosomes, with those from a single subfamily tending to cluster and two of the RGA loci being colocalized with the cotton bacterial blight resistance genes. The distribution of RGA between the two subgenomes A and D of cotton is uneven, with RGA being more abundant in the A subgenome than in the D subgenome. The data provide new insights into the organization and evolution of the NBS-LRR-encoding RGA family in polyploid plants.     

Identification and genetic regulation of the chalcone synthase multigene family in pea.

Chalcone synthase (CHS) is a key enzyme in the biosynthesis of diverse flavonoids involved in disease resistance, nodulation, and pigmentation in pea. We describe a multigene family encoding CHS and the effects of two regulatory loci, a and a2, on the pattern of expression of three of its member genes. Two of the genes, CHS1 and CHS3, are expressed in both petal and root tissue, whereas expression of a third gene, CHS2, is detected only in roots. The products encoded by the a and a2 loci are required for the expression of the CHS1 gene and for wild-type levels of expression of the CHS3 gene in petal tissue. In root tissue, all three CHS genes are expressed and induced by CuCl2 regardless of the genotype at the a and a2 loci. These results show that the various members of the CHS multigene family interact in diverse ways with multiple genetic signals in the plant, providing a basis for the differential expression of these genes. Spatially specific genetic regulation of distinct members of a multigene family has been clearly demonstrated.     

Intergeneric transfer and functional expression of the tomato disease resistance gene Pto.

Plant disease resistance loci have been used successfully in breeding programs to transfer traits from resistant germplasm to susceptible plant cultivars. The molecular cloning of plant disease resistance genes now permits the transfer of such traits across species boundaries by genetic transformation of recipient hosts. The tomato disease resistance gene Pto confers resistance to strains of the bacterial pathogen Pseudomonas syringae pv tomato expressing the avirulence gene avrPto. Transformation of Nicotiana benthamiana with Pto results in specific resistance to P. s. pv tabaci strains carrying avrPto. The resistant phenotype is manifested by a strong inhibition of bacterial growth and the ability to exhibit a hypersensitive response. Resistance cosegregates with the Pto gene in transgene selfings and testcrosses. Our results demonstrate the conservation of disease resistance functions across genus boundaries and suggest that the utility of host-specific resistance genes can be extended by intergeneric transfer.     

Evolving disease resistance genes

Defenses against most specialized plant pathogens are often initiated by a plant disease resistance gene. Plant genomes encode several classes of genes that can function as resistance genes. Many of the mechanisms that drive the molecular evolution of these genes are now becoming clear. The processes that contribute to the diversity of R genes include tandem and segmental gene duplications, recombination, unequal crossing-over, point mutations, and diversifying selection. Diversity within populations is maintained by balancing selection. Analyses of whole-genome sequences have and will continue to provide new insight into the dynamics of resistance gene evolution.