Autoregulator-dependent Control of Extracellular Polysaccharide Production in Phytopathogenic Bacteria

Extracellular polysaccharides (EPSs) likely provide phytopathogenic bacteria a selective advantage both inside and outside plants. Despite the relatively scant knowledge about EPS biosynthesis in phytopathogenic bacteria, it clearly is a well controlled, complex, energy-intensive process. Unexpectedly, three phytopathogenic bacteria have been found to autoregulate EPS production in response to extracellular signal compounds (pheromones) that they produce. Like many bacteria, Pantoea stewartii subsp. stewartii produces a N-acyl-homoserine lactone (AHL) autoinducer. However, unlike most AHL-dependent autoinduction systems, that in P. stewartii subsp. stewartii somehow represses EPS production in the absence of autoinducer. Instead of an AHL-dependent system (which it also has), Ralstonia solanacearum uses a novel autoregulator identified as 3-hydroxypalmitic acid methyl ester to regulate EPS biosynthesis. A lack of this autoregulator in R. solanacearum results in repression of EPS biosynthesis by a complex two-component sensor/response regulator signal cascade. Xanthomonas campestris pv. campestris has two partially overlapping autoregulatory systems. The autoregulators are incompletely characterized, but one diffusible signal factor (DSF) is thought to be a fatty acid derivative and the other diffusible factor (DF) may be a butyrolactone. The autoregulation pathways in X. campestris pv. campestris are essentially unknown, but EPS production is controlled by both the DSF and DF systems, whereas production of extracellular enzymes and pigment production are regulated independently. In a confined micro-environment, population density and intercellular concentrations of an autoregulator will increase in parallel, so autoregulation is one way that bacteria can coordinate gene expression to synthesize EPS only at high cell density. However, because there is often limited evidence that it is actually cell density that is being detected, researchers should not assume a priori that autoregulation must function for quorum sensing. Some possible reasons for why phytopathogenic bacteria would benefit from delaying EPS production are discussed.     

Visualization of three-dimensional chaos

Most chaotic mixing experiments have been restricted to two-dimensional, time-periodic flows, and this has shaped advances in theory as well. A prototypical, bounded, three-dimensional flow with a moderate Reynolds number is presented; this system lends itself to detailed experimental observation and allows for high-precision computational inspection. The flow structure, captured by means of cuts with a laser sheet (experimental Poincare section), was visualized with the use of continuously injected fluorescent dye streams and revealed detailed chaotic structures with high-period islands.     

杏贮藏保鲜技术研究进展

概述了杏的生产现状,对影响杏贮藏效果的因素、贮期主要病害及防治方法、目前普遍采用的杏贮藏保鲜技术进行归纳,并展望今后杏贮藏保鲜技术的发展方向.     

Role of Extracellular Polysaccharide and Endoglucanase in Root Invasion and Colonization of Tomato Plants by Ralstonia solanacearum

ABSTRACT Ralstonia solanacearum is a soilborne plant pathogen that normally invades hosts through their roots and then systemically colonizes aerial tissues. Previous research using wounded stem infection found that the major factor in causing wilt symptoms was the high-molecular-mass acidic extracellular polysaccharide (EPS I), but the beta-1,4-endoglucanase (EG) also contributes to virulence. We investigated the importance of EPS I and EG for invasion and colonization of tomato by infesting soil of 4-week-old potted plants with either a wild-type derivative or genetically well-defined mutants lacking EPS I, EG, or EPS I and EG. Bacteria of all strains were recovered from surface-disinfested roots and hypocotyls as soon as 4 h after inoculation; that bacteria were present internally was confirmed using immunofluorescence microscopy. However, the EPS-minus mutants did not colonize stems as rapidly as the wild type and the EG-minus mutant. Inoculations of wounded petioles also showed that, even though the mutants multiplied as well as the wild type in planta, EPS-minus strains did not spread as well throughout the plant stem. We conclude that poor colonization of stems by EPS-minus strains after petiole inoculation or soil infestation is due to reduced bacterial movement within plant stem tissues.     

Reduced spray programs for Drosophila suzukii management in berry crops

Since the arrival of Drosophila suzukii M. (Diptera: Drosophilidae), field applications of broad-spectrum insecticides have increased in berry crop production to protect susceptible fruits from infestation. Field studies were conducted from 2011 to 2013 to determine whether alternate row middle or border spray programs could manage D. suzukii as well as complete sprays and have less of an impact on non-target arthropods. Four raspberry sites and one blueberry site were alternate row middle and border sprayed, respectively. Adult and larva counts of D. suzukii were compared to conventional complete spray programs. Non-target arthropods were evaluated 7 d post-harvest. In both reduced spray trials, no differences in mean adult numbers and larvae of D. suzukii were detected between treatments. Mean counts immediately after sprays (i.e. 3–12 d after treatment) were also similar in complete and alternate row sprays. Both reduced spray strategies had significantly more Stethorus spp.; additionally, alternate row sprays had significantly more Psyllobora spp. These reduced pesticide strategies are additional tools to consider in D. suzukii IPM programs that can reduce the area sprayed, application time, and input costs while conserving natural enemies.     

鱼类消化酶活性及其影响因素的研究进展

鱼类消化酶是鱼类消化生理研究的重要内容,随着水产养殖业的发展,鱼类消化生理及其影响因素日益受到人们的重视.综述了鱼类消化酶的分布,以及各种因素对消化酶活性的影响,并重点阐述了近年来广泛研究的各种营养因素与鱼类消化酶活性的关系.     

药物基因组学科研合作网络的统计学性质

药物基因组学是一门年轻的学科领域。该领域内相关科学研究工作者之间形成的科研合作关系网络,也具有类似许多大型合作关系网络数据库所具有的无尺度网络特性,其非连通合作网络内部具有较大连通组群的聚类特性和小世界特征等相关性质。