Note: Comparison of three laboratory methods to evaluate the pathogenicity and virulence of tenPseudomonas syringae pv.syringae strains on apple, pear, cherry and peach trees

The pathogenicity and virulence of ten GreekPseudomonas syringae pv.syringae strains from different hosts (citrus, pear, apple, peach and cherry) were evaluated using three different laboratory methods, which produced results in good agreement. All ten strains were virulent on apple, pear, cherry and peach trees. The extent of tissue colonized varied considerably among strains and cultivars. On excised shoots and twigs of apple and pear, strains BPI 176, BPI 203, PI 2 and PI 14 were the most virulent and strains BPI 689, BPI 992, BPI 4, BPI 20, PI 18 and PI 19 were the least virulent. On excised shoots and twigs of peach and cherry, strains BPI 176, BPI 203, PI 2, PI 14, PI 18 and PI 19 were the most virulent and strains BPI 4 and BPI 20 were the least virulent. Moderate virulence was evinced by strains BPI 689 and BPI 992. These pathogenicity assays are proposed as rapid and reproducible screening systems to evaluate the susceptibility of apple, pear, cherry and peach cultivars to this bacterial pathogen.     

Age-related resistance to Pseudomonas syringae pv. tomato is associated with the transition to flowering in Arabidopsis and is effective against Peronospora parasitica

As plants mature it has been observed that some become more resistant to normally virulent pathogens. The ability to manifest the Age-Related Resistance (ARR) response in Arabidopsis to Pseudomonas syringae pathovars tomato (Pst) coincided with the transition to flowering in plants both delayed and accelerated in the transition to flowering. ARR was also associated with a change in PR-1 gene expression, such that young plants expressed PR-1 abundantly at 3 days post inoculation (dpi) while mature plants expressed much less. The Arabidopsis ARR response requires SA accumulation via isochorismate synthase (ICS1) [24]. ICS1 was expressed one dpi with virulent and avirulent Pst in both young and mature plants. The ARR response was also effective versus avirulent Pst providing an additional 4-fold limitation in bacterial growth. Arabidopsis ARR was found to be ineffective against two necrotrophs, Erwinia carotovora subspecies carotovora (bacterium) and Botrytis cinerea (fungus) and one obligate biotroph, Erysiphe cichoracearum (fungus). However, mature wild type, SA-deficient sid2 and NahG plants supported little growth of the obligate biotrophic oomycete, Peronospora parasitica. Therefore ARR to P. parasitica appears to be SA-independent, however the level of ARR resistance was somewhat reduced in these mutants in some experiments. Thus, there may be numerous defence pathways that contribute to adult plant resistance in Arabidopsis.     

Apoplastic redox metabolism: Synergistic phenolic oxidation and a novel oxidative burst

The plant apoplast is an important mediator of communication between the cell cytoplasm and its surroundings. Plant cell suspensions offer a convenient model system to gain insight into apoplastic physiology. Here, we describe a novel phenomenon that took place when two naturally occurring phenolics were added together to either soybean or tobacco cell suspensions. Acetosyringone (AS) and/or hydroxyacetophenone (HAP), phenolics found in the extracellular/apoplast of tobacco cells, were added to soybean or tobacco cell suspensions undergoing an oxidative burst. Individually, AS appeared to be utilized as a typical peroxidase substrate to scavenge hydrogen peroxide, while HAP was utilized at a much lower rate. However, when added together the rate of utilization of both phenolics increased and surprisingly resulted in the production of hydrogen peroxide. We have further characterized this novel phenomenon in suspension cells. This study demonstrates that certain phenolics in plants can cause co-oxidation which, as in animals, could alter the structure and bioactivity of surrounding phenolics.     

Suppression of wheat-seedling diseases caused by Fusarium culmorum and Microdochium nivale using bacterial seed treatment

Snow mould, caused by Microdochium nivale , and seedling blight caused by members of the Fusarium complex, are cereal diseases of great economic importance in many temperate zones. In a glasshouse bioassay designed to enhance disease, about 600 plant-associated bacterial isolates obtained by different methods were screened for suppressive effects in wheat against infection caused by Fusarium culmorum . Although most of the isolates tested had a neutral effect on test plants and disease development, a few were synergistic to the pathogen and about one-fifth showed > 80% disease suppression. During five consecutive growing seasons, 164 bacterial isolates were tested in field experiments against both F. culmorum and M. nivale as causal agents of seedling blight. Tests for effects on yield in experiments with spring and winter wheat, performed in different climatic regions of Sweden, showed that disease-suppressive effects were repeatable. The most efficient isolates, three fluorescent pseudomonads and a species of Pantoea , suppressed disease equal to that of the fungicide guazatine, both with respect to crop stand and yield. Seed treatment with Pantoea sp. (isolate MF 626) increased yield by an average of more than 500 kg ha⁻¹ in six field experiments.     

Differential Responses to Pea Bacterial Blight in Stems, Leaves and Pods Under Glasshouse and Field Conditions

Resistance to pea bacterial blight (Pseudomonas syringae pv. pisi) in different plant parts was assessed in 19 Pisum sativum cultivars and landraces, carrying race-specific resistance genes (R-genes) and two Pisum abyssinicum accessions carrying race-nonspecific resistance. Stems, leaves and pods were inoculated with seven races of P. s. pv. pisi under glasshouse conditions. For both race-specific and nonspecific resistance, a resistant response in the stem was not always associated with resistance in leaf and pod. Race-specific genes conferred stem resistance consistently, however, there was variability in the responses of leaves and pods which depended on the matching R-gene and A-gene (avirulence gene in the pathogen) combination. R2 generally conferred resistance in all plant parts. R3 or R4 singly did not confer complete resistance in leaf and pod, however, R3 in combination with R2 or R4 enhanced leaf and pod resistance. Race-nonspecific resistance conferred stem resistance to all races, leaf and pod resistance to races 2, 5 and 7 and variable reactions in leaves and pods to races 1, 3, 4 and 6.Disease expression was also studied in the field under autumn/winter conditions. P. sativum cultivar, Kelvedon Wonder (with no R genes), and two P. abyssinicum accessions, were inoculated with the most frequent races in Europe under field conditions (2, 4 and 6). Kelvedon Wonder was very susceptible to all three races, whereas P. abyssinicum was much less affected. The combination of disease resistance with frost tolerance in P. abyssinicum enabled plants to survive through the winter. A breeding strategy combining race-nonspecific resistance derived from P. abyssinicum with race-specific R-genes should provide durable resistance under severe disease pressure.     

茴香轮作调控土壤细菌群落缓解三七连作障碍的效应及机制

三七是我国特有的中药材,但连作障碍严重制约了其产业发展,轮作是生产上最常用且有效的缓解措施.本研究在三七收获后的土壤上轮作茴香,通过测定轮作土壤水浸液对三七存苗的影响来评价轮作对三七连作障碍的缓解效果,并从轮作对土壤细菌群落的影响方面深入阐释轮作缓解连作障碍的机制.结果表明,茴香轮作后的土壤水浸液可以显著提高三七的存苗...     

黄瓜细菌性茎枯萎病病原鉴定及温度对其致病力的影响

自2003年以来, 在我国北京?辽宁和山东等地的黄瓜上发现了一种重要细菌病害—黄瓜细菌性茎枯萎病, 对黄瓜生产造成了严重危害?从发病的黄瓜叶片?茎部和果实上分离的菌株接种到黄瓜后, 症状与自然发病症状完全一致?经过多位点序列分型和BOX-PCR技术, 以及LOPAT?GATTa和Ayers碳源利用试验等, 确定该病害的病原菌为丁香假单胞流泪致病变种 Pseudomonas syringae pv. lachrymans?以黄瓜细菌性茎枯萎病菌A2为供试菌株, 黄瓜细菌性角斑病菌丁香假单胞菌流泪致病变种 P.syringae pv. lachrymans pslb8为对照菌株, 对其进行不同温度下致病力?体内和体外生长能力测定, 结果表明:20℃条件下, A2菌株的致病力?体内和体外生长能力均强于pslb8菌株, 相对低温的条件更有利于黄瓜细菌性茎枯萎病病原菌的侵染和病害的发生?     

烟草根际细菌铜绿假单胞菌swu31-2的定殖能力及其对烟草青枯病的防治作用

从重庆黔江烟草田间分离获得一株烟草青枯菌拮抗菌株Pseudomonas aeruginosa swu31 2（简称swu31 2）。采用逐步提高药物浓度的方法,筛选获得了抗链霉素300 μg/mL对烟草青枯病菌拮抗活性稳定的swu31 2突变菌株。采用灌根接种法,研究其在烟草根、茎和叶表面及内部的定殖能力及其对烟草青枯病的防治作用。结果表明,swu31 2能在烟草各组织的表面及内部定殖。该菌株在烟草各组织内部的数量均表现为“由增到减”的趋势。在接种后第8天定殖数量达到最高峰,随后有所下降;到第20天各组织内的数量仍然维持较高水平（105 cfu/g 以上）。同时,在接种20 d后,烟草的根、茎和叶的表面仍然可以检测到swu31 2的存在。盆栽试验结果表明,swu31 2的菌液和活性物质对烟草青枯病均有一定的防治效果。其中先施swu31 2菌液和活性物质粗提物的防效好于农用链霉素（51.25%）,分别为60.87%和 60.32%,而后施菌液和活性物质粗提物的防效也分别达39.50%和20.90%。     

A New Cell Wall Located N-rich Protein is Strongly Induced During the Hypersensitive Response in Glycine Max L.

Soybean (Glycine max (L.) Merill, cv. Williams 82) plants and cell cultures respond to avirulent pathogens with a hypersensitive reaction. After inoculation of soybean with Pseudomonas syringae pv. glycinea, carrying the avirulence gene avrA, or zoospores from the fungus Phytophthora sojae Race 1, a resistance-gene-dependent cell death programme is activated. A new gene was identified by differential display of mRNAs that is specifically activated during the early phase of incompatible pathogen-soybean interactions but does not respond to compatible pathogens. The gene is strongly induced within 2h after addition of P. sojae zoospores. A similar kinetic pattern was observed for P. syringae (avrA) inoculated soybean cell cultures. The gene encodes a deduced protein of 368 amino acids with a very high content of asparagine and was therefore termed N-rich protein (NRP). The protein is composed of two distinct domains, of which only the C-terminal domain has striking homology to proteins of unknown function from other plants. An antibody raised against the recombinant NRP recognizes a protein of 42kDa. The protein is located in the cell wall as indicated by cell fractionation studies. Comparison of the genomic DNA-sequence with the cDNA, identified two introns within the open reading frame. The NRP-gene is not directly induced by salicylic acid or hydrogen peroxide, indicating a distinct and specific signal transduction pathway which is only activated during programmed cell death. The NRP-gene appears to be a new marker in soybean activated early in plant disease resistance.     

Activation of systemic disease resistance in pea by an avirulent bacterium or a benzothiadiazole, but not by a fungal leaf spot pathogen

Inoculation of first expanded leaves of pea seedlings with an avirulent strain of Pseudomonas syringae pv. pisi , or treatment with sprays of a benzothiadiazole (20 or 100  μ g a.i. mL⁻¹), decreased the susceptibility of subsequent leaves 7 or 14 days later to challenge inoculation with Mycosphaerella pinodes . Inoculation of first leaves with a virulent strain of P. syringae pv. pisi or with M. pinodes did not decrease the susceptibility of plants to M. pinodes . Treatments effective in decreasing susceptibility to M. pinodes were similarly active against Uromyces viciae-fabae and virulent P. syringae pv. pisi . Effective treatments also enhanced the activities of the enzymes β-1,3-glucanase and chitinase in untreated upper leaves 6 days later. Ineffective treatments for decreased susceptibility had no effect on the activity of the enzymes. None of the treatments enhanced peroxidase activities. The results are discussed in relation to the reported signalling effects of the benzothiadiazole and in relation to a suggested high activity of the avirulent P. syringae pv. pisi strain and inactivity of M. pinodes in enhancing natural signalling.