Structure and role in symbiosis of the exoB gene of Rhizobium leguminosarum bv trifolii

The Rhizobium leguminosarum bv trifolii exoB gene has been isolated by heterologous complementation of an exoB mutant of R. meliloti. We have cloned a chromosomal DNA fragment from the R. leguminosarum bv trifolii genome that contains an open reading frame of 981 bp showing 80% identity at the amino acid level to the UDP-glucose 4-epimerase of R. meliloti. This enzyme produces UDP-galactose, the donor of galactosyl residues for the lipid-linked oligosaccharide repeat units of various heteropolysaccharides of rhizobia. An R. leguminosarum bv trifoliiexoB disruption mutant differed from the wild type in the structure of both the acidic exopolysaccharide and the lipopolysaccharide. The acidic exopolysaccharide made by our wild-type strain is similar to the Type 2 exopolysaccharide made by other R. leguminosarum bv trifolii wild types. The exopolysaccharide made by the exoB mutant lacked the galactose residue and the substitutions attached to it. The exoB mutant induced the development of abnormal root nodules and was almost completely unable to invade plant cells. Our results stress the importance of exoB in the Rhizobium-plant interaction. Received: 31 May 1996 / Accepted: 18 December 1996     

Molecular identification of Liriomyza trifolii (Burgess) (Dipt., Agromyzidae) based on real-time PCR

Abstract:  Rapidly identifying juvenile individuals of Liriomyza trifolii (Burgess) from Liriomyza sativae Blanchard is crucial in plant quarantine. We report a molecular method to identify L . trifolii based on real-time polymerase chain reaction (PCR). By comparing partial DNA sequences of mitochondrial COI genes of L . trifolii samples collected from Guangdong and Taiwan provinces in China, Japan, Philippine, Israel, Germany, the USA, Mexico and Honduras sequenced by authors, and those of related species recorded in GenBank, a L . trifolii -specific probe was developed. There was no difference in individuals of different stages tested by this probe. The total time for real-time PCR assay system was 2 h, and it would save 3–7 h compared with conventional PCR.     

Annual Medicago: from a model crop challenged by a spectrum of necrotrophic pathogens to a model plant to explore the nature of disease resistance

BACKGROUND: Annual Medicago spp., including M. truncatula, play an important agronomic role in dryland farming regions of the world where they are often an integral component of cropping systems, particularly in regions with a Mediterranean or Mediterranean-type climate where they grow as winter annuals that provide both nitrogen and disease breaks for rotational crops. Necrotrophic foliar and soil-borne pathogens dominate these regions and challenge the productivity of annual Medicago and crop legume species. SCOPE: This review outlines some of the major and/or widespread diseases these necrotrophic pathogens cause on Medicago spp. It then explores the potential for using the spectrum of necrotrophic pathogen-host interactions, with annual Medicago as the host plant, to better understand and model pathosystems within the diseases caused by nectrotrophic pathogens across forage and grain legume crops. CONCLUSIONS: Host resistance clearly offers the best strategy for cost-effective, long-term control of necrotrophic foliar and soil-borne pathogens, particularly as useful resistance to a number of these diseases has been identified. Recently and initially, the annual M. truncatula has emerged as a more appropriate and agronomically relevant substitute to Arabidopsis thaliana as a model plant for legumes, and is proving an excellent model to understand the mechanisms of resistance both to individual pathogens and more generally to most forage and grain legume necrotrophic pathogens.     

吉林省白粉茵属叉丝壳组新记录变种与新寄主

记录了采自长春地区的白粉菌属叉丝壳组（Erysiphe sect.Microsphaera）吉林省新记录变种2个:万布白粉菌原变种（E.vanbruntiana var.vanbruntiana）和车轴草白粉菌原变种（E.trifolii var.trifolii）,其寄主东北接骨木（Sambucus manshuri...     

温度对三叶草彩斑蚜种群参数的影响

为了探讨温度对三叶草彩斑蚜种群增长的影响,在实验室研究了15～35℃范围内共9个温度下三叶草彩斑蚜的发育、繁殖和生命表.结果表明:三叶草彩斑蚜种群在35℃下不能存活.若虫的发育历期随着温度的上升而显著缩短,在15 ～ 32℃内为18.33～4.02 d,存活率在40.0％～83.6％,25℃下若虫的存活率最高、32℃下存活率最低.成蚜的平均寿命在10.64 ～ 20.87 d,23℃下寿命最长、32℃下最短.产蚜高峰期随温度的上升而提前,除15℃为15 d以外,其余温度下均在3～6d.平均繁殖力和单头最高繁殖力以25℃下最高,分别为82.0和149.0头.平均世代周期随着温度的上升从15℃的31.17 d逐渐缩短到32℃的10.17 d.净增殖率在25℃下最大(68.62),32℃下最小(13.96).内禀增长率在0.10 ～0.30 d-1,其中28 ℃下最高、15℃下最低.若虫的发育起点温度和有效积温分别为9.35℃和97.83 d·℃.繁殖力、净增殖率和内禀增长率与温度的关系均可用一元二次方程描述.     

种间竞争梯度对白车轴草与其病原菌白车轴草单孢莠菌相互作用的影响

白车轴草（Trifolium repens)在与其病原菌白车轴草单孢锈菌（Uromyces trifolii-repentis)的长期相互作用中分化形成了抗病型（Resistance)无性系和易感型（Susceptibility)无性系。该研究工作旨在了解：1）在种间竞争不断增强的环境梯度中,抗型无性系和易感型无性系的生长表现有何区别？2）在同样的实验条件下,分别对抗病型无性系和易感型无性系进行接种感染后,两者的生长表现又有何区别？在一严重感病的白车轴草自然种群中,分别标定了17个抗病型无性系和14个易感型无性系,从各元性系的匍匐茎上分别剪取长5cm,具两个叶两个腋芽的小段作实验材料。接种孢子采自同一植物种群植株的病叶。实验在一个处在次生演替阶段的草本植物群落上进行,在该群落的种间竞争梯度上选定3个分别具有弱、中等和强种间竞争的生境（Habitat)建立实验站,每站设置等量的实验和对照组。结果显示：1）随着环境中种间竞争强度的增加,无论接种与否,抗病型和易感型白车轴草的叶生长量均凝减;2）在弱或强种间竞争环境条件下,无论接种与否,易感型无性系的生长均好于抗抗病型无性系;但在中等间竞争强度的环境条件下,对照实验中易感型无性系的生长显著好于抗病型无性系,而接种实验中的结果相反。这一研究结果表明,种间竞争强度可能是影响给定生境中寄主植物抗病型和易感型遗传型比率的重要因子之一,这一影响还将随着病原菌存在而否而发生改变。     

Elevated atmospheric CO2 alters microbial population structure in a pasture ecosystem

An increase in concentration of atmospheric CO₂ is one major factor influencing global climate change. Among the consequences of such an increase is the stimulation of plant growth and productivity. Below‐ground microbial processes are also likely to be affected indirectly by rising atmospheric CO₂ levels, through increased root growth and rhizodeposition rates. Because changes in microbial community composition might have an impact on symbiotic interactions with plants, the response of root nodule symbionts to elevated atmospheric CO₂ was investigated. In this study we determined the genetic structure of 120 Rhizobium leguminosarum bv. trifolii isolates from white clover plants exposed to ambient (350 μmol mol^?1) or elevated (600 μmol mol^?1) atmospheric CO₂ concentrations in the Swiss FACE (Free‐Air‐Carbon‐Dioxide‐Enrichment) facility. Polymerase Chain Reaction (PCR) fingerprinting of genomic DNA showed that the isolates from plants grown under elevated CO₂ were genetically different from those isolates obtained from plants grown under ambient conditions. Moreover, there was a 17% increase in nodule occupancy under conditions of elevated atmospheric CO₂ when strains of R. leguminosarum bv. trifolii isolated from plots exposed to CO₂ enrichment were evaluated for their ability to compete for nodulation with those strains isolated from ambient conditions. These results indicate that a shift in the community composition of R. leguminosarum bv. trifolii occurred as a result of an increased atmospheric CO₂ concentration, and that elevated atmospheric CO₂ affects the competitive ability of root nodule symbionts, most likely leading to a selection of these particular strains to nodulate white clover.     

江苏地区三叶斑潜蝇和美洲斑潜蝇的发生危害及种群动态

【目的】三叶斑潜蝇Liriomyza trifolii和美洲斑潜蝇Liriomyza sativae是一类危害蔬菜、花卉的世界性害虫,也是我国重要的外来入侵害虫。了解其发生危害特征,可为这两种斑潜蝇的防治提供科学依据。【方法】于2015年对江苏地区的两种斑潜蝇的发生分布、危害程度以及在扬州地区的发生动态进行了调查,采集含有潜道的叶片,待蛹羽化为成虫后鉴定斑潜蝇种类,同时统计叶片上的潜道数量划分危害等级。【结果】美洲斑潜蝇主要在苏北地区发生危害,三叶斑潜蝇主要在苏南、苏中地区发生危害;两种斑潜蝇在江苏地区的发生危害程度均未超过3级。在扬州地区的蔬菜上仅调查到三叶斑潜蝇的危害,其危害的寄主植物有6科、12种,主要包括茄科、豆科、葫芦科、十字花科的蔬菜。三叶斑潜蝇的发生危害从3月开始,11月结束,全年有3个发生危害高峰,其中在7月下旬至8月初的发生危害最重。【结论】两种斑潜蝇在江苏地区的种类分布呈现出明显的地理差异,同时扬州地区三叶斑潜蝇的虫口密度随寄主和时间不同呈现的明显,因此生产上需加强对斑潜蝇发生危害进行监测,并指导斑潜蝇防治。     

广东省春季三叶草斑潜蝇寄主种类

对广东省冬后蔬菜调查结果表明,三叶草斑潜蝇Liriomyzatri folii(Burgess)的寄主主要有菊科茼蒿、非洲菊,伞形科西芹、香芹,豆科豆角,葫芦科冬瓜,茄科番茄等;其中,茼蒿、芹菜、番茄等受害较重。     

旋幽夜蛾迁飞的雷达观测和虫源分析

旋幽夜蛾Scotogramma trifolii Rottemberg是我国北方农牧地区局部暴发为害的农业害虫,近年来危害逐渐呈上升趋势,但目前的研究对其大范围发生危害规律及是否具有迁飞性尚缺乏明确的认识。本文利用垂直监测昆虫雷达和相关辅助设备的长期自动观测和基于GIS的地理信息系统分析,研究了2005年吉林省白城地区通榆县和大安市旋幽夜蛾暴发的虫源性质及成虫空中飞行参数。结果表明：白城地区5月中旬20 cm的平均地温并不满足越冬代蛹羽化的最低温度;2005年5月下旬成虫盛发期东北、华北地区空中风场以南风和偏南风居多,为成虫迁入提供了有利的运载气流。6月下旬雷达观测点卵巢解剖显示,雌蛾卵巢发育进度整齐、级别较低,具有典型的迁出昆虫生理特征。雷达观测显示,成虫飞行最高可达距地面1 000 m, 主要选择在距地面300～500 m高度段,在500 m高度可以整夜飞行。初步判定旋幽夜蛾具有迁飞特性,这一发现为进一步认识其发生规律以及提高其预测预报水平提供了科学依据。