Relatedness affects competitive performance of a parasitic plant (Cuscuta europaea) in multiple infections

Theoretical models predict that parasite relatedness affects the outcome of competition between parasites, and the evolution of parasite virulence. We examined whether parasite relatedness affects competition between parasitic plants (Cuscuta europaea) that share common host plants (Urtica dioica). We infected hosts with two parasitic plants that were either half-siblings or nonrelated. Relative size asymmetry between the competing parasites was significantly higher in the nonrelated infections compared to infections with siblings. This higher asymmetry was caused by the fact that the performance of some parasite genotypes decreased and that of others increased when grown in multiple infections with nonrelated parasites. This result agrees with the predictions of theories on the evolution of parasite virulence: to enhance parasite transmission, selection may favour reduced competition with genetically related parasites in hosts infected by several genotypes. However, in contrast to the most common predictions, nonrelated infections were not more virulent than the sibling infections.     

Epidermal cell-patterning genes of the stem parasitic plant Cuscuta campestris are involved in the development of holdfasts

Cuscuta campestris, a stem parasitic plant, commences its parasitic behavior by forming a specialized disk-like adhesive structure called a holdfast, which facilitates tight adhesion to the stem surface of the host plant. The morphology of epidermal cells in the holdfast is similar to that of the leaf trichome and root hairs of dicotyledonous plants. However, the regulatory network underlying the development of the holdfast has not been elucidated to date. In this study, we assessed the roles of epidermal cell-patterning genes in the development of a holdfast. Epidermal cell-patterning genes of C. campestris, including CcWER, CcGL3, CcTTG1, CcGL2, and CcJKD, were expressed slightly before the initiation of the outgrowth of stem epidermal cells. CcJKD-silencing repressed CcJKD, CcWER, CcGL3, CcTTG1, CcGL2; therefore, CcJKD is an upstream regulator of other epidermal cell-patterning genes. Unlike other genes, CcCPC was not upregulated after attachment to the host, and was not repressed by CcJKD-silencing. Protein interaction assays demonstrated that CcJKD interacted with CcTTG1 and CcCPC. Furthermore, CcJKD-silencing repressed the outgrowth of holdfast epidermal cells. Therefore, C. campestris invokes epidermal cell-patterning genes for the outgrowth of holdfast epidermal cells, and their regulatory mechanism is different from those for leaf trichome or root hairs.     

Anatomy and ultrastructure of epidermal cells in the haustorium of a parasitic flowering plant,Cuscuta japonica, during attachment to the host

Changes of epidermal cells in the haustorium of the parasiticCuscuta japonica during its attachment to the host plantimpatiens balsamina were studied with light and electron microscopy. In the transverse sections of dodder stems not in contact with the host, epidermal cells had rounded outlines. However, when haustorial initials developed in the cortex of the parasite stem at the contact site, the epidermal cells had more dense cytoplasm and conspicuous nuclei than before, and their outline was flat in the longitudinal section. As meristem cells developed from those initials, the epidermal cells became more elongated. When the haustorium was fully matured, the apical tips of the elongated epidermal cells at the contact site branched like toes, producing numerous projections via cell wall invaginations. This event caused spaces to form between the projections; coincidently, the surface area of the apical ends of the epidermal cells increased. The dense cytoplasm at those projections contained prominent nuclei and abundant other organelles, suggesting a active metabolism. Osmiophilic particles, releasing into the cell walls from the cytoplasm, were though to be associated with the loosening and elongating of the epidermal cell walls. Dense and homogeneous materials were secreted within the spaces between the projections. These materials could play an important role in cementing the haustorium onto the surface of the host organ.     

基质养分对寄生植物南方菟丝子生长的影响

群落中各营养级的相互作用在群落结构形成中起了重要作用.以南方菟丝子(Cuscuta australis R.Br.)和三叶鬼针草(Biden pilosa L.)为研究对象,采用完全随机区组实验设计方法,测定并分析基质养分(不施肥与施肥)对寄生植物生长的影响,探讨寄生植物生物量与寄主生长特性、生物量和光源捕获能力的相关性.结果表明,施肥显著增加寄生植物南方菟丝子的吸器数量、缠绕圈数、相对盖度、营养器官生物量、生殖器官生物量和总生物量,但对生殖器官的生物量比无显著影响.施肥显著增加寄主植物的根、茎、叶生物量和总生物量、叶生物量比、比叶面积和叶绿素含量,但显著降低根冠比与根生物量比.南方菟丝子生物量与三叶鬼针草生物量、叶生物量比、比叶面积以及相对叶绿素含量之间均存在显著正相关,与根生物量比和根冠比存在显著负相关.研究结果表明施肥可以提高寄主植物的光资源捕获能力,将更多地生物量分配至叶等光合机构上,从而促进寄主植物(生产者)的生长,并间接促进寄生植物(初级消费者)的生长.     

sRNASVM——基于SVM方法构建大肠杆菌sRNA预测模型

在理解细菌与环境的相互作用方面,细菌sRNA的识别发挥重要作用。文章介绍了一个通过增加训练集中实验证实的sRNA来构建细菌sRNA预测模型的策略,并以大肠杆菌K-12的sRNA预测为例来说明策略的可行性。结果表明,按此策略构建的模型sRNASVM的10倍交叉检验精度达到92.45%,高于目前文献中报道的精度。因此,构建的这一模型将为实验发现sRNA提供较好的生物信息学支持。有关模型和详细结果可以从网站http://ccb.bmi.ac.cn/srnasvm/下载。     

The function of small RNAs in plant biotic stress response

Small RNAs(s RNAs) play essential roles in plants upon biotic stress. Plants utilize RNA silencing machinery to facilitate pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity to defend against pathogen attack or to facilitate defense against insect herbivores. Pathogens, on the other hand, are also able to generate effectors and s RNAs to counter the host immune response. The arms race between plants and pathogens/insect herbivores has triggered the evolution of s RNAs,RNA silencing machinery and pathogen effectors. A great number of studies have been performed to investigate the roles of s RNAs in plant defense, bringing in the opportunity to utilize s RNAs in plant protection. Transgenic plants with pathogen-derived resistance ability or transgenerational defense have been generated, which show promising potential as solutions for pathogen/insect herbivore problems in the field. Here we summarize the recent progress on the function of s RNAs in response to biotic stress, mainly in plant-pathogen/insect herbivore interaction,and the application of s RNAs in disease and insect herbivore control.     

Role of plant parasitic nematodes and abiotic soil factors in growth heterogeneity of sugarcane on a sandy soil in South Africa

A feature of many sugarcane fields is the patchy growth. This is often thought to be due to physical or chemical differences in the soil. In this paper we investigate the causes of growth heterogeneity of sugarcane on a sandy soil in KwaZulu-Natal, South Africa. To identify the factors that were associated with the good and poor areas, soil texture, pH, organic matter content and a number of soil chemical elements and nematode community data were subjected to principal component analysis (PCA). The numbers of each of the nematode species (Meloidogyne sp., Pratylenchus zeae, Helicotylenchus dihystera, Xiphinema elongatum and Paratrichodorus sp.) were first converted to relative proportions of ectoparasites and endoparasites. The data were collected from the 2nd ratoon crop of a nematicide trial, where half of the plots had been treated with aldicarb in the preceding two crops. Yields of control plots varied from 34.7 to 126.8 t cane ha^–1 and from 85.4 to 138.7 for the treated plots. The yield data were centred and normalised separately for the treated and control plots and the values projected on the trial map to study spatial distribution. Plots with above-average yields, whether treated or untreated, occurred in the lower part of the trial site. The PCA factorial values were also projected onto the map of the trial. According to the first factor of the analysis of the abiotic soil characteristics in the 0–20 cm surface layer, the trial site could be divided into two areas, one on the left and one on the right. PCA of the soil data from the 0–20 and 20–40 cm layers showed that there was little or no difference between the two that might explain the two growth areas. However, analysis of the nematode community distinguished two main areas that largely corresponded to the distribution of the plots of low and high yielding cane. Correlation analysis confirmed the relationship between nematodes and yield. H. dihystera was positively correlated with yield of cane whereas the reverse was true for the Meloidogyne species.     

Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed

Background and Aims: While invasive species may escape from natural enemies in thenew range, the establishment of novel biotic interactions withspecies native to the invaded range can determine their success.Biological control of plant populations can be achieved by manipulationof a species' enemies in the invaded range. Interactions weretherefore investigated between a native parasitic plant andan invasive legume in Mediterranean-type woodlands of SouthAustralia. Methods: The effects of the native stem parasite, Cassytha pubescens,on the introduced host, Cytisus scoparius, and a co-occurringnative host, Leptospermum myrsinoides, were compared. The hypothesisthat the parasitic plant would have a greater impact on theintroduced host than the native host was tested. In a fieldstudy, photosynthesis, growth and survival of hosts and parasitewere examined. Key Results: As predicted, Cassytha had greater impacts on the introducedhost than the native host. Dead Cytisus were associated withdense Cassytha infections but mortality of Leptospermum wasnot correlated with parasite infection. Cassytha infection reducedthe photosynthetic rates of both hosts. Infected Cytisus showedslower recovery of photosystem II efficiency, lower transpirationrates and reduced photosynthetic biomass in comparison withuninfected plants. Parasite photosynthetic rates and growthrates were higher when growing on the introduced host Cytisus,than on Leptospermum. Conclusions: Infection by a native parasitic plant had strong negative effectson the physiology and above-ground biomass allocation of anintroduced species and was correlated with increased plant mortality.The greater impact of the parasite on the introduced host maybe due to either the greater resources that this host providesor increased resistance to infection by the native host. Thisdisparity of effects between introduced host and native hostindicates the potential for Cassytha to be exploited as a controltool.     

Comparative study on the host susceptibility of different banana cultivars to plant parasitic nematodes in Malappuram district of Kerala,India

Six cultivars of banana grown widely in Malappuram district of Kerala were evaluated for the diversity of plant parasitic nematodes in their respective rhizosphere. The genotypes evaluated include Nendran, Poovan (Mysore), Njali poovan, Mysore poovan, Chenkadali and Ponnan. The study aims on a comparative analysis of rhizosphere of different banana varieties for the occurrence of plant parasitic nematodes, diversity, frequency of occurrence and to determine the most vulnerable type of banana variety. Cobb’s decanting and sieving method was used to isolate nematodes from soil. Thirty three samples randomly collected from different blocks of Malappuram district are subjected to analysis and a ten species of nematodes were reported. Among the six varieties analysed both Nendran and Mysore Poovan variety seems to be more susceptible to phytoparasitic nematodes. With respect to eight blocks of Malappuram district, Helicotylenchus sp. shows highest prominence value followed by Radopholus similis, reniform nematode and Meloidogyne incognita.     

Host‐driven divergence in the parasitic plant Orobanche minor Sm. (Orobanchaceae)

Many parasitic angiosperms have a broad host range and are therefore considered to be host generalists. Orobanche minor is a nonphotosynthetic root parasite that attacks a range of hosts from taxonomically disparate families. In the present study, we show that O. minor sensu lato may comprise distinct, genetically divergent races isolated by the different ecologies of their hosts. Using a three‐pronged approach, we tested the hypothesis that intraspecific taxa O. minor var. minor and O. minor ssp. maritima parasitizing either clover (Trifolium pratense) or sea carrot (Daucus carota ssp. gummifer), respectively, are in allopatric isolation. Morphometric analysis revealed evidence of divergence but this was insufficient to define discrete, host‐specific taxa. Intersimple sequence repeat (ISSR) marker‐based data provided stronger evidence of divergence, suggesting that populations were isolated from gene flow. Phylogenetic analysis, using sequence‐characterized amplified region (SCAR) markers derived from ISSR loci, provided strong evidence for divergence by clearly differentiating sea carrot‐specific clades and mixed‐host clades. Low levels of intrapopulation SCAR marker sequence variation and floral morphology suggest that populations on different hosts are probably selfing and inbreeding. Morphologically cryptic Orobanche taxa may therefore be isolated from gene flow by host ecology. Together, these data suggest that host specificity may be an important driver of allopatric speciation in parasitic plants.     

Interaction of a host plant and its holoparasite: effects of previous selection by the parasite

If parasites decrease the fitness of their hosts one could expect selection for host traits (e.g. resistance and tolerance) that decrease the negative effects of parasitic infection. To study selection caused by parasitism, we used a novel study system: we grew host plants (Urtica dioica) that originated from previously parasitized and unparasitized natural populations (four of each) with or without a holoparasitic plant (Cuscuta europaea). Infectivity of the parasite (i.e. qualitative resistance of the host) did not differ between the two host types. Parasites grown with hosts from parasitized populations had lower performance than parasites grown with hosts from unparasitized populations, indicating host resistance in terms of parasite’s performance (i.e. quantitative resistance). However, our results suggest that the tolerance of parasitic infection was lower in hosts from parasitized populations compared with hosts from unparasitized populations as indicated by the lower above‐ground vegetative biomass of the infected host plants from previously parasitized populations.     

Assessment of correlational selection on tolerance and resistance traits in a host plant–parasitic plant interaction

Resistance and tolerance are considered to be different plant strategies against disease. While resistance traits prevent hosts becoming parasitized or reduce the extent of parasitism, tolerance traits reduce the fitness-impact of parasitism on infected hosts. Theoretical considerations predict that in some circumstances mutual redundancy will give hosts with either high resistance or high tolerance a fitness advantage over hosts that exhibit both of these traits together. However, empirical evidence has provided mixed results. In this paper, I describe the pattern of phenotypic selection imposed by the holoparasitic mistletoe Tristerix aphyllus upon resistance (spine length) and tolerance (branching) traits in the cactus Echinopsis chilensis. Results indicate that branching was an efficient compensatory mechanism, reducing 75.5% of the fitness-impact attributable to parasitism. Even though both traits showed a negative correlation, as expected from the presence of allocation costs between strategies, no correlational selection coefficient was significant indicating that selection did not favor alternative combinations of traits. Consequently, I did not find evidence for selection promoting mutually exclusive defense strategies against the mistletoe, which suggests that tolerance and resistance traits may coexist stably in populations of E. chilensis.