Introduced Plant Viruses and the Invasion of a Native Grass Flora

Weed and native grasses from the South Island of New Zealand were surveyed for virus infection. Cocksfoot mottle virus (CfMV) and Ryegrass mosaic virus (RgMV) were restricted to a few introduced species; however, Barley yellow dwarf viruses (BYDVs) have invaded native grasses in New Zealand. Virus incidence was significantly lower in the native species (2%) than in the introduced species (12%). Four different serotypes (RMV, RPV, PAV, MAV) were detected in the introduced grass flora but only two (RMV, PAV) were detected in native species. In experimental transmission tests the aphid vector Rhopalosiphum padi's survival was variable on the 20 native species tested but this was not due to the presence or absence of endophytic fungi as none were detected in the New Zealand species. Aphid numbers increased and plants were killed when R. padi fed on Agrostis muelleriana and Festuca multinodis. R. padi transmitted a PAV isolate to these and six other native species. BYDVs infected 4/5 of the subfamilies tested. Virus incidence in native Arundinoideae and Pooideae was significantly lower than in introduced Pooideae and Panicoideae. One species of Bambusoideae collected from the field was not infected but was found susceptible in glasshouse tests. Agrostis capillaris, Dactylis glomerata and Lolium perenne were identified as the most likely reservoirs of infection for the native flora. Anthoxanthum odoratum was not infected but if the SGV serotype and its vector Schizaphis graminum were ever introduced, A. odoratum could form an effective reservoir from near sea level into alpine areas. This revised version was published online in July 2006 with corrections to the Cover Date.     

Plant Community Responses to Simultaneous Changes in Temperature,Nitrogen Availability,and Invasion

Background

Increasing rates of change in climate have been observed across the planet and have contributed to the ongoing range shifts observed for many species. Although ecologists are now using a variety of approaches to study how much and through what mechanisms increasing temperature and nutrient pollution may influence the invasions inherent in range shifts, accurate predictions are still lacking.

Methods and Results

In this study, we conducted a factorial experiment, simultaneously manipulating warming, nitrogen addition and introduction of Pityopsis aspera, to determine how range-shifting species affect a plant community. We quantified the resident community using ordination scores, then used structural equation modeling to examine hypotheses related to how plants respond to a network of experimental treatments and environmental variables. Variation in soil pH explained plant community response to nitrogen addition in the absence of invasion. However, in the presence of invasion, the direct effect of nitrogen on the community was negligible and soil moisture was important for explaining nitrogen effects. We did not find effects of warming on the native plant community in the absence of invasion. In the presence of invasion, however, warming had negative effects on functional richness directly and invasion and herbivory explained the overall positive effect of warming on the plant community.

Conclusions and Significance

This work highlights the variation in the biotic and abiotic factors responsible for explaining independent and collective climate change effects over a short time scale. Future work should consider the complex and non-additive relationships among factors of climate change and invasion in order to capture more ecologically relevant features of our changing environment.     

Differences in Competitive Ability between Plants from Nonnative and Native Populations of a Tropical Invader Relates to Adaptive Responses in Abiotic and Biotic Environments

The evolution of competitive ability of invasive plant species is generally studied in the context of adaptive responses to novel biotic environments (enemy release) in introduced ranges. However, invasive plants may also respond to novel abiotic environments. Here we studied differences in competitive ability between Chromolaena odorata plants of populations from nonnative versus native ranges, considering biogeographical differences in both biotic and abiotic environments. An intraspecific competition experiment was conducted at two nutrient levels in a common garden. In both low and high nutrient treatments, C. odorata plants from nonnative ranges showed consistently lower root to shoot ratios than did plants from native ranges grown in both monoculture and competition. In the low nutrient treatment, C. odorata plants from nonnative ranges showed significantly lower competitive ability (competition-driven decreases in plant height and biomass were more), which was associated with their lower root to shoot ratios and higher total leaf phenolic content (defense trait). In the high nutrient treatment, C. odorata plants from nonnative ranges showed lower leaf toughness and cellulosic contents (defense traits) but similar competitive ability compared with plants from native ranges, which was also associated with their lower root to shoot ratios. Our results indicate that genetically based shifts in biomass allocation (responses to abiotic environments) also influence competitive abilities of invasive plants, and provide a first potential mechanism for the interaction between range and environment (environment-dependent difference between ranges).     

Genetic Diversity and Population Structure in Polygonum cespitosum: Insights to an Ongoing Plant Invasion

Molecular markers can help elucidate how neutral evolutionary forces and introduction history contribute to genetic variation in invaders. We examined genetic diversity, population structure and colonization patterns in the invasive Polygonum cespitosum, a highly selfing, tetraploid Asian annual introduced to North America. We used nine diploidized polymorphic microsatellite markers to study 16 populations in the introduced range (northeastern North America), via the analyses of 516 individuals, and asked the following questions: 1) Do populations have differing levels of within-population genetic diversity? 2) Do populations form distinct genetic clusters? 3) Does population structure reflect either geographic distances or habitat similarities? We found low heterozygosity in all populations, consistent with the selfing mating system of P. cespitosum. Despite the high selfing levels, we found substantial genetic variation within and among P. cespitosum populations, based on the percentage of polymorphic loci, allelic richness, and expected heterozygosity. Inferences from individual assignment tests (Bayesian clustering) and pairwise F _ST values indicated high among-population differentiation, which indicates that the effects of gene flow are limited relative to those of genetic drift, probably due to the high selfing rates and the limited seed dispersal ability of P. cespitosum. Population structure did not reflect a pattern of isolation by distance nor was it related to habitat similarities. Rather, population structure appears to be the result of the random movement of propagules across the introduced range, possibly associated with human dispersal. Furthermore, the high population differentiation, genetic diversity, and fine-scale genetic structure (populations founded by individuals from different genetic sources) in the introduced range suggest that multiple introductions to this region may have occurred. High genetic diversity may further contribute to the invasive success of P. cespitosum in its introduced range.     

Linking Improvement of Soil Structure to Soil Carbon Storage Following Invasion by a C4 Plant Spartina alterniflora

Ecosystems - Coastal wetlands are increasingly recognized as important ecosystems for long-term carbon (C) storage. However, how soil aggregation mediates C accumulation and sequestration in these...     

Near Surface Swimming of Salmonella Typhimurium Explains Target-Site Selection and Cooperative Invasion

Targeting of permissive entry sites is crucial for bacterial infection. The targeting mechanisms are incompletely understood. We have analyzed target-site selection by S. Typhimurium. This enteropathogenic bacterium employs adhesins (e.g. fim) and the type III secretion system 1 (TTSS-1) for host cell binding, the triggering of ruffles and invasion. Typically, S. Typhimurium invasion is focused on a subset of cells and multiple bacteria invade via the same ruffle. It has remained unclear how this is achieved. We have studied target-site selection in tissue culture by time lapse microscopy, movement pattern analysis and modeling. Flagellar motility (but not chemotaxis) was required for reaching the host cell surface in vitro. Subsequently, physical forces trapped the pathogen for ∼1.5–3 s in “near surface swimming”. This increased the local pathogen density and facilitated “scanning” of the host surface topology. We observed transient TTSS-1 and fim-independent “stopping” and irreversible TTSS-1-mediated docking, in particular at sites of prominent topology, i.e. the base of rounded-up cells and membrane ruffles. Our data indicate that target site selection and the cooperative infection of membrane ruffles are attributable to near surface swimming. This mechanism might be of general importance for understanding infection by flagellated bacteria.     

Plant Growth Analysis: Growth and Yield Component Responses to Population Density in Forage Maize

Forage maize (Zea mays L.) was grown in monocultures at populationdensities ranging from 4·9 to 11·1 plants m^–2.Data for plant growth analysis were obtained from six harvestscarried out from 21 to 115 d after planting. Conventional plantgrowth analysis indicated that improvements in forage productivityper unit land area by high population density resulted directlyfrom increased plant presence. Reduction in dry weight per shootat high population density was associated with reduced unitleaf rate. Leaf area ratio was little affected, which may implythat competition for soil nutrients or oxygen was the chiefcause of plant interference. Yield component analysis demonstratedthe increasing importance of population density treatments asa source of variation as growth progressed. Direct relationshipsbetween variation in yield per plant and variation in two yieldcomponents, stem diameter and the inverse of leaf area ratio,were demonstrated. Both conventional plant growth analysis andyield component analysis indicated complex physiological andmorphological adjustments to species population density. Plant growth analysis, yield component analysis, Zea mays L     

Soil Nematode Responses to Crop Management and Conversion to Native Grasses

Soil nematode community response to treatments of three, four-year crop rotations (spring wheat-pea-spring wheat-flax, spring wheat-green manure-spring wheat-flax, and spring wheat-alfalfa-alfalfa-flax) under conventional and organic management, and native tall grass restoration (restored prairie) were assessed in June 2003, and July and August 2004. The research site was the Glenlea Long-term Rotation and Crop Management Study, in the Red River Valley, Manitoba, established in 1992. The nematode community varied more with sample occasion than management and rotation. The restored prairie favored high colonizer-persister (c-p) value omnivores and carnivores, and fungivores but less bacterivores. The restored prairie soil food web was highly structured, mature and low-to-moderately enriched as indicated by structure (SI), maturity (MI) and enrichment (EI) index values, respectively. Higher abundance of fungivores and channel index (CI) values suggested fungal-dominated decomposition. Nematode diversity was low even after more than a decade of restoration. A longer time may be required to attain higher diversity for this restored fragmented prairie site distant from native prairies. No consistent differences were found between organic and conventional management for nematode trophic abundance, with the exception of enrichment opportunists of the c-p 1 group which were favored by conventional management. Although EI was lower and SI was higher for organic than conventional their absolute values suggested decomposition channels to be primarily bacterial, and fewer trophic links with both management scenarios. A high abundance of fungivores in the rotation including the green manure indicates greater fungal decomposition.     

Mapping QTL for Grain Yield and Plant Traits in a Tropical Maize Population

The vast majority of reported QTL mapping for maize (Zea mays L.) traits are from temperate germplasm and, also, QTL by environment interaction (QTL × E) has not been thoroughly evaluated and analyzed in most of these papers. The maize growing areas in tropical regions are more prone to environmental variability than in temperate areas, and, therefore, genotype by environment interaction is of great concern for maize breeders. The objectives of this study were to map QTL and to test their interaction with environments for several traits in a tropical maize population. Two-hundred and fifty-six F_2:3 families evaluated in five environments, a genetic map with 139 microsatellites markers, and the multiple-environment joint analysis (mCIM) were used to map QTL and to test QTL × E interaction. Sixteen, eight, six, six, nine, and two QTL were mapped for grain yield, ears per plant, plant lodging, plant height, ear height, and number of leaves, respectively. Most of these QTL interacted significantly with environments, most of them displayed overdominance for all traits, and genetic correlated traits had a low number of QTL mapped in the same genomic regions. Few of the QTL mapped had already been reported in both temperate and tropical germplasm. The low number of stable QTL across environments imposes additional challenges to design marker-assisted selection in tropical areas, unless the breeding programs could be directed towards specific target areas.     

Plant Responses to Water Stress

This Special Issue comprises a series of papers that developsthe theme of plant responses to water stress, encompassing recentdevelopments at the molecular level, through responses of photosynthesisand metabolism, to their application in crop selection and yield.The consideration of water deficits is particularly timely,given the huge developments in this area in the past decade.This issue specifically sets out to place molecular and physiologicalprocesses and their agronomic applications in an environmentalcontext.     

Positive Feedback between Mycorrhizal Fungi and Plants Influences Plant Invasion Success and Resistance to Invasion

Negative or positive feedback between arbuscular mycorrhizal fungi (AMF) and host plants can contribute to plant species interactions, but how this feedback affects plant invasion or resistance to invasion is not well known. Here we tested how alterations in AMF community induced by an invasive plant species generate feedback to the invasive plant itself and affect subsequent interactions between the invasive species and its native neighbors. We first examined the effects of the invasive forb Solidago canadensis L. on AMF communities comprising five different AMF species. We then examined the effects of the altered AMF community on mutualisms formed with the native legume forb species Kummerowia striata (Thunb.) Schindl. and on the interaction between the invasive and native plants. The host preferences of the five AMF were also assessed to test whether the AMF form preferred mutualistic relations with the invasive and/or the native species. We found that S. canadensis altered AMF spore composition by increasing one AMF species (Glomus geosporum) while reducing Glomus mosseae, which is the dominant species in the field. The host preference test showed that S. canadensis had promoted the abundance of AMF species (G. geosporum) that most promoted its own growth. As a consequence, the altered AMF community enhanced the competitiveness of invasive S. canadensis at the expense of K. striata. Our results demonstrate that the invasive S. canadensis alters soil AMF community composition because of fungal-host preference. This change in the composition of the AMF community generates positive feedback to the invasive S. canadensis itself and decreases AM associations with native K. striata, thereby making the native K. striata less dominant.     

Antipredator Behavioral Responses of Native and Exotic Tadpoles to Novel Predator

Factors related to the invasion process, such as high abundance of invaders, residence time, and functional distinctiveness, are well documented, but less attention has been given to the effects of antipredator strategy of invasive species during colonization. In this study, we explored the antipredator strategy of an introduced species by comparing the predator avoidance behaviors of two native anuran species and one introduced(exotic) species in the presence of different predators. The two native anuran species used in the study were Black-spotted Pond Frog Rana nigromaculata and Terrestrial Frog Rana limnocharis. The introduced(invasive) species used was American bullfrog Lithobates catesbeianus. Chinese pond turtle Chinemys reevesii, Red-backed rat snake Elaphe rufodorsata, and Big-headed turtle Platysternon megacephalum were used as predator species. Chinese pond turtles and Red-backed rat snakes are native predators of Black-spotted Pond Frogs and Terrestrial Frogs, while Big-headed turtles are novel(unfamiliar) to the two frogs. All three predator species are novel(unfamiliar) to the American bullfrog. The results show that tadpoles of the two native species displayed behaviors of recognizing the two native predators, but did not display the capability of identifying the novel predator. Results from our study also suggest that American bullfrog tadpoles exhibited strong antipredator behavioral responses by displaying the capability of identifying unfamiliar predators without cohabitation history and prior exposure to them. Such antipredator behavioral responses could have resulted in more favorable outcomes for an invading species during the invasive introductory process.     

植物对土壤紧实度的反应

概述了土壤紧实度对植物生长、水肥吸收、产量和水分利用效率的影响以及植物对紧实度反应机制的研究现状。     

The Myriad Plant Responses to Herbivores

Abstract Plant responses to herbivores are complex. Genes activated on herbivore attack are strongly correlated with the mode of herbivore feeding and the degree of tissue damage at the feeding site. Phloem-feeding whiteflies and aphids that produce little injury to plant foliage are perceived as pathogens and activate the salicylic acid (SA)-dependent and jasmonic acid (JA)/ethylene-dependent signaling pathways. Differential expression of plant genes in response to closely related insect species suggest that some elicitors generated by phloem-feeding insects are species-specific and are dependent on the herbivore's developmental stage. Other elicitors for defense-gene activation are likely to be more ubiquitous. Analogies to the pathogen-incompatible reactions are found. Chewing insects such as caterpillars and beetles and cell-content feeders such as mites and thrips cause more extensive tissue damage and activate wound-signaling pathways. Herbivore feeding is not equivalent to mechanical wounding. Wound responses are a part of the induced responses that accompany herbivore feeding. Herbivores induce direct defenses that interfere with herbivore feeding, growth and development, fecundity, and fertility. In addition, herbivores induce an array of volatiles that creates an indirect mechanism of defense. Volatile blends provide specific cues to attract herbivore parasites and predators to infested plants. The nature of the elicitors for volatile production is discussed.     

乌鲁木齐周边3种野生灌木对干旱胁迫的响应

木本猪毛菜（Salsola arbucula）、刺旋花（Convolvulos tragacanthoides）和刺木蓼（Atraphaxis spinosa）为乌鲁木齐周边植被组成的重要种,在维持荒漠生态系统的稳定中发挥着重要作用,是植被恢复过程中潜在的候选植物种。该文以盆栽控水法,分析比较了3种野生灌木在不同干旱胁迫下的生理生长特征的差异。结果表明,干旱胁迫下3种灌木均表现出良好的耐旱能力。在生长方面,刺旋花生长受到的影响最小,重度胁迫下其生物量与适宜水分处理相比仅减少9.7%,根系极发达,根冠比大。木本猪毛菜受抑制严重,重度干旱时株高和基茎增量为适宜水分处理的17.85%和21.63%。生理方面,刺旋花叶片叶绿素含量最高,膜系统受破坏较小,渗透调节可以通过可溶性糖的积累来实现。木本猪毛菜膜系统受破坏较小,脯氨酸的积累对渗透调节起重要作用。刺木蓼的脯氨酸和可溶性糖两种渗透调节物质均起重要作用。总之,木本猪毛菜和刺木蓼在轻度干旱下表现良好,刺旋花则能够在重度干旱下保持植物体正常的生理代谢功能,表现出对干旱环境较强的适应能力。     

Extrapolation from Individual-Level Responses to Population Growth Rate Using Population Modeling

Ecological risk assessments of chemicals are often based on simple measurements of toxicity in individuals. However, the protection goals are often set at the population and community levels. Population models may be a useful tool to extrapolate from individual-level measurements to population-level endpoints. In the present study, the population growth rate (λ) was calculated for three sets of full life-cycle data (Tetranychus urticae exposed to agrimek, and Daphnia pulex exposed to spinosad and diazinon). The results were compared to λ from population models, where survival and/or reproduction were adjusted according to 4 d of data from the same life-cycle data. This was done to determine whether truncated demographic data can give results similar to that obtained with full life-cycle data. The resulting correlations were strong when both effects on survival and reproduction were included in the model (p < .001, 0.93 < R₂ < 1.00). There were also strong correlations in several cases when only effects on survival or reproduction were considered, although the total risk to the population tended to be underestimated. The results of the present study show that population models can be useful to extrapolate truncated data on the individual level to more ecologically relevant population-level endpoints.     

转录后基因沉默--植物抵御外来病毒入侵的一种机制

基因沉默是近几年来在转基因植物中发现的一种后生遗传现象。基因沉默大体可以分为两类：位置效应引起的基因沉默和同源依赖的基因沉默。其中,同源依赖的基因沉默又可以分为转录水平的基因沉默和转录后水平的基因沉默。基因沉默的发现使得人们对植物和病毒的相互关系有了一个新的认识。基因沉默研究中所发现的转录后基因沉默现象是植物抵御病毒入侵、保持自身基因组完整性的一种防御机制,是植物与病毒共进化的结果。对于沉默产生的机理,尤其是转录后基因沉默,已经提出不少模型,但是都未能较全面地解释基因沉默中出现的各种实验现象。该文现就实验所取得的相关结果、转录后基因沉默机制和植物对病毒防御机制的相互关系,以及其研究进展进行综述。