Forest habitat characteristics affect balance between sexual reproduction and clonal propagation of the ectomycorrhizal mushroom Hebeloma cylindrosporum

Endophytic fungi, especially asexual, systemic endophytes in grasses, are generally viewed as plant mutualists, mainly through the action of mycotoxins, such as alkaloids in infected grasses, which protect the host plant from herbivores. Most of the evidence for the defensive mutualism concept is derived from studies of agronomic grass cultivars, which may be atypical of many endophyte-host interactions. I argue that endophytes in native plants, even asexual, seed-borne ones, rarely act as defensive mutualists. In contrast to domesticated grasses where infection frequencies of highly toxic plants often approach 100%, natural grass populations are usually mosaics of uninfected and infected plants. The latter, however, usually vary enormously in alkaloid levels, from none to levels that may affect herbivores. This variation may result from diverse endophyte and host genotypic combinations that are maintained by changing selective pressures, such as competition, herbivory and abiotic factors. Other processes, such as spatial structuring of host populations and endophytes that act as reproductive parasites of their hosts, may maintain infection levels of seed-borne endophytes in natural populations, without the endophyte acting as a mutualist.     

禾草内生真菌的遗传多样性及其共生关系

 近百年的禾草内生真菌研究历经了由浅入深的过程,从最初的家畜中毒事件认识到是一种共生内生真菌存在的缘故,到如今利用分子生物学技术揭示其共生机制,人类发现这类植物内生真菌并非想象中的对生态系统无足轻重。Epichloë及其无性型Neotyphodium与禾本科植物是系统发生的互利共生关系,尤其是Neotyphodium可提高宿主抵抗环境胁迫的能力和抵御动物的取食,增强植物的竞争力。禾草内生真菌有3种生活史：有性生活史、无性生活史和兼性生活史,后者表明真菌在不同的宿主及环境下既能营有性生殖也可营无性繁殖,是一种更灵活而有效的生活史对策。对内生真菌分子系统学、生活史以及与宿主禾草协同进化的研究发现,Neotyphodium起源于禾草致病真菌Epichloë的某些种,或是Epichloë与Neotyphodium的种间杂交后代。植物和内生真菌各异的生活史策略,真菌的种间杂交,两者的协同进化亦或种群间基因流的差异,都促成了共生体多样化的基因组合（Genetic combination ）,也是其共生关系多样化的根源。内生真菌对宿主的有益作用只在特定基因型真菌、宿主和一定环境条件下才起作用,自然生态系统的共生关系要比农业系统复杂得多,是一个从互利共生至寄生关系的连续系统。未来对于更多共生体的遗传背景和基因与环境相互作用的阐明将有助于对禾草内生真菌共生关系本质更加深入的认识。     

Grass-herbivore interactions altered by strains of a native endophyte

Many plants support symbiotic microbes, such as endophytic fungi, that can alter interactions with herbivores. Most endophyte research has focused on agronomically important species, with less known about the ecological roles of native endophytes in native plants. In particular, whether genetic variation among endophyte symbionts affects herbivores of plant hosts remains unresolved for most native endophytes. Here, we investigate the importance of native isolates of the endophyte Epichlo? elymi in affecting herbivory of the native grass host, Elymus hystrix. Experimental fungal isolate-plant genotype combinations and endophyte-free control plants were grown in a common garden and exposed to natural arthropod herbivory. Fungal isolates differed in their effects on two types of herbivory, chewing and scraping. Isolates exhibiting greater sexual reproduction were associated with greater herbivore damage than primarily asexual isolates. Endophyte infection also altered patterns of herbivory within plants, with stroma-bearing tillers experiencing up to 30% greater damage than nonstroma-bearing tillers. Results suggest that intraspecific genetic variation in endophytes, like plant genetic variation, can have important 'bottom-up' effects on herbivores in native systems.     

Does An Asexual Endophyte Symbiont Alter Life Stage and Long-Term Survival in a Perennial Host Grass?

Asexual, seedborne endophytic fungi in perennial grasses are often viewed as strong mutualists because fitness of the symbiont and host grass are closely coupled. However, at least for some native grasses, the asexual endophyte, Neotyphodium, acts parasitically, yet remains at high frequencies in natural populations. Most previous studies of Neotyphodium effects on host survival have been short term relative to the long life span of the perennial grass host. We therefore tested the hypothesis that Neotyphodium alters the survival in various life stages and long-term survival of adult native Arizona fescue (Festuca arizonica). To test the former, we planted 40 infected (E+) and 40 uninfected (E−, endophyte removed) seeds from four different maternal plants in the field under ambient conditions. We followed survival of seeds, seedlings, and adult plants over a 5-year period. To test the latter, we determined the infection of 1633 adult plants and followed their survival over the next 5–7 years. E+ seeds did not differ from uninfected seeds in terms of overall survival from seed germination to seedling to adult. However, the shape of the survival curve differed, with E+ plants showing higher mortality in early life stages. E+ adult plants did not differ from E− plants in long-term survival. Survival was generally very high during the study, which included a severe and prolonged drought. Infection by asexual Neotyphodium does not increase survival in early life stages or that of adult plants. Because asexual, vertically transmitted symbionts are predicted by evolutionary theory to be strong mutualists, the persistence of high infection frequencies in natural populations without long-term benefits to the host remains enigmatic. One possible explanation is that the long life span of the perennial host and low seedling recruitment may obscure either the costs or benefits of endophyte infection.     

Fungal endophytes: common host plant symbionts but uncommon mutualists

Fungal endophytes are extremely common and highly diverse microorganismsthat live within plant tissues, but usually remain asymptomatic.Endophytes traditionally have been considered plant mutualists,mainly by reducing herbivory via production of mycotoxins, suchas alkaloids. However, the vast majority of endophytes, especiallyhorizontally-transmitted ones commonly found in woody plants,apparently have little or no effect on herbivores. For the systemic,vertically-transmitted endophytes of grasses, mutualistic interactionsvia increased resistance to herbivores and pathogens are morecommon, as predicted by evolutionary theory. However, even inthese obligate symbioses, endophytes are often neutral or evenpathogenic to the host grass, depending on endophyte and plantgenotype and environmental conditions. We present a graphical model based upon variation in nitrogenflux in the host plant. Nitrogen is a common currency in endophyte/hostand plant/herbivore interactions in terms of limitations tohost plant growth, enhanced uptake by endophytes, demand forsynthesis of nitrogen-rich alkaloids, and herbivore preferenceand performance. Our graphical model predicts that low alkaloid-producingendophytes should persist in populations when soil nutrientsand herbivory are low. Alternatively, high alkaloid endophytesare favored under increasing herbivory and increasing soil nitrogen,at least to some point. At very high soil nitrogen levels, uninfectedplants may be favored over either type of infected plants. Thesepredictions are supported by patterns of infection and alkaloidproduction in nature, as well by a manipulative field experiment.However, plant genotype and other environmental factors, suchas available water, interact with the presence of the endophyteto influence host plant performance.     

Plant population and genotype effects override the effects of Epichloë endophyte species on growth and drought stress response of Achnatherum robustum plants in two natural grass populations

Aims In cool-season grasses, systemic and vertically transmitted Epichlo? infections often provide a suite of benefits including increased growth, reproduction and competitive abilities. However, these effects of Epichlo? endophytes on their hosts often depend upon host and endophyte genotype and environmental factors.Methods Achnatherum robustum (sleepygrass) harbors at least two Epichlo? species within natural populations in the Southwest USA. We tested the effects of endophyte infection and species, host population and plant genotype (by experimentally removing the endophyte), and soil moisture (a key limiting factor) on growth and drought stress response of infected A. robustum plants from two populations (Weed and Cloudcroft) in the Sacremento Mountains of New Mexico, USA).Important findings Although the two populations harbor distinct Epichlo? species each with very different chemoprofiles, neither endophyte status (infected vs. uninfected) nor endophyte species affected most growth parameters at 8 or 25 weeks of the experiment, except for leaf length. In high water treatment, infected plants from the Weed population had longer leaf length compared with uninfected plants. In contrast, the population of origin affected all growth parameters, including plant height, leaf number, length and width, tiller number and shoot and root biomass, as well as wilting time. Grasses from the Cloudcroft population generally showed greater growth than grasses from the Weed population. Endophyte infection did affect wilting time, with infection in the Weed population generally reducing time to wilting under low and high water, whereas infection in the Cloudcroft population reduced time to wilting only under high water conditions. Our results suggest that plant population and their associated plant genotypes may play a much larger role in endophyte–host grass interactions in varying environments than previously thought. Asexual Epichlo? species may be compatible with only specific host genotypes within populations such that the phenotypic effects due to population may be greater than phenotypic changes influenced by variation in the endophyte.     

Vertically transmitted fungal endophytes: different responses of host-parasite systems to environmental conditions

The relationship between vertically transmitted asexual fungal grass endophytes and their hosts is considered to be mutualistic. Results from agronomic field support this line of reasoning but recent studies have shown more variable results in natural systems. We investigated how high and low nutrient and water treatments affected biomass allocation patterns of endophyte‐infected and uninfected Festuca pratensis and F. rubra in greenhouse experiments over two growing seasons. Irrespective of infection status, both grass species showed improved performance on highly fertilized and watered soils. However, infected F. pratensis plants produced larger tillers than endophyte‐free plants on soil low in nutrients and water in the first growing season, although they (E+) otherwise showed decreased performance on nutrient‐poor soil. In low nutrient and water conditions, endophyte‐infected plants produced less tillers and had lower total biomass compared to uninfected plants, and displayed a negative phenotypic correlation between seed production and vegetative growth. The latter indicates costs of reproduction when the plant shares common resources with the fungal endophyte. However, endophyte infection status (E+, E?) interacted significantly with the soil fertilisation in terms of plant growth, having a stronger positive effect on growth in infected F. pratensis plants. In F. rubra, endophyte‐infected plants showed higher vegetative growth in fertilized and watered soils compared to uninfected plants. However, infected plants tended to produce fewer inflorescences. This had no effect on seed production, perhaps because seed production was partly replaced by asexual pseudovivipary. Contrary to the general assumption in the literature that fungal endophytes are plant mutualists, these findings suggest that the costs of endophytes may outweigh their benefits in resource limited conditions. However, the costs of endophyte infections appear to differ among the grass species studied; costs of endophytes were mainly detected in F. pratensis under low nutrient conditions. We propose that differences in response to endophyte infection in these species may depend on the differences in life‐history strategies and environmental requirements of these two fescue and fungal species and may change during the life span of the plant.     

Asexual Neotyphodium endophytes in a native grass reduce competitive abilities

Asexual, vertically transmitted endophytes are well known for increasing competitive abilities of agronomic grasses, but little is known about endophyte–host interactions in native grasses. We tested whether the asexual Neotyphodium endophyte enhances competitive abilities in a native grass, Arizona fescue, in a field experiment pairing naturally infected (E+) and uninfected (E?) plants, and in a greenhouse experiment pairing E+ and E? (experimentally removed) plants, under varying levels of soil water and nutrients. In the field experiment, E? plants had greater vegetative, but not reproductive, growth than E+ plants. In the greenhouse experiment, where plant genotype was strictly controlled, E? plants consistently outperformed their E+ counterparts in terms of root and shoot biomass. Thus, Neotyphodium infection decreases host fitness via reduced competitive properties, at least in the short term. These findings contrast starkly with most endophyte studies involving introduced, agronomic grasses where infection increases competitive abilities, and the interaction is viewed as highly mutualistic.     

The effects of genetic and environmental factors on disease expression (stroma formation) and plant growth in Brachypodium sylvaticum infected by Epichloë sylvatica

Fungi in the genus Epichloë (Clavicipitaceae, Ascomycota) are endophytic and often mutualistic symbionts of many grasses in temperate areas. Species with a sexual cycle suppress host flowering and seed formation, whereas asexual fungi remain asymptomatic and transmit vertically by seed. Thus, the mode of reproduction may determine whether the symbiosis is mutualistic or parasitic. The level of sexual reproduction (disease expression) varies among different endophytes and on different grass hosts, but factors responsible for this variation, and evolutionary mechanisms leading to one or the other life strategy are not understood. As experimental system, we chose Brachypodium sylvaticum in which the endophyte E. sylvatica can express both reproduction modes. A field experiment was done in plots of a free air carbon dioxide enrichment (FACE) facility. We investigated the effects of three environmental factors (elevated CO2 concentration, shading and fertilisation) and one genetic factor (plant and fungal genotype combination), on plant growth and disease expression. Variation in plant growth was mainly dependant on the genotype and was increased by fertilisation. Elevated CO2 and shading slightly stimulated plant growth, but only in fertilised plants. Disease expression was overwhelmingly dependent on the genotype, hence genetic factors. Fertilisation slightly stimulated disease expression in some genotypes, while the effect of elevated CO2 was negligible, and by interaction with fertilisation inconsistent in the two years. Horizontal transmissions during the experiment, presumably mediated by ascospores, confounded the original infection status of the plants. Contagious infections occurred more frequently in the shade, and in endophyte‐free host plants. The latter suggests that pre‐existing infections render host plants less susceptible to superinfection by choke forming strains. Although our results clearly indicate that disease expression of E. sylvatica has a genetic basis, it is still unclear whether selection on the plants or the fungi is driving the evolution of this association.     

Evolutionary origins and ecological consequences of endophyte symbiosis with grasses

Over the past 20 yr much has been learned about a unique symbiotic interaction between fungal endophytes and grasses. The fungi (Clavicipitaceae, Ascomycota) grow intercellularly and systemically in aboveground plant parts. Vertically transmitted asexual endophytes forming asymptomatic infections of cool-season grasses have been repeatedly derived from sexual species that abort host inflorescences. The phylogenetic distribution of seed-transmitted endophytes is strongly suggestive of cocladogenesis with their hosts. Molecular evidence indicates that many seed-transmitted endophytes are interspecific hybrids. Superinfection may result in hyphal fusion and parasexual recombination. Most endophytes produce one or more alkaloid classes that likely play some role in defending the host plant against pests. Hybridization may have led to the proliferation of alkaloid-production genes among asexual endophytes, favoring hybrids. The ergot alkaloid ergovaline, lolitrems, and lolines are produced by only a single sexual species, Epichlo? festucae, but they are common in seed-transmitted endophytes, suggesting that E. festucae contributed genes for their synthesis. Asexual hybrids may also be favored by the counteracting of the accumulation of deleterious mutations (Muller's rachet). Endophyte infection can provide other benefits, such as enhanced drought tolerance, photosynthetic rate, and growth. Estimates of infection frequency have revealed variable levels of infection with especially high prevalence in the subfamily Pooideae. Longitudinal studies suggest that the prevalence of seed-transmitted endophytes can increase rapidly over time. In field experiments, infected tall fescue suppressed other grasses and forbs relative to uninfected fescue and supported lower consumer populations. Unlike other widespread plant/microbial symbioses based on the acquisition of mineral resources, grass/endophyte associations are based primarily on protection of the host from biotic and abiotic stresses.     

Distribution of fungal endophyte genotypes in doubly infected host grasses

Fungal endophytes of the genus Epichloë live intercellulary in above ground plant parts of many pooid ‘grasses of the temperate regions. The associations are characterized by single genotype entities since a given host individual normally contains a single endophyte genotype. They can persist over the life span of the hosts. This study examines whether two fungal genotypes can co-exist within a host plant, and how fungal genotypes are distributed within a host in the case of double infections. We selected four Epichloë bromicola strains that we identified as unique genotypes through RAPD‘ analysis. Young Bromus erectus plants, derived from callus cultures, were artificially inoculated with all possible double-strain mixtures of these fungal genotypes. For identification of fungal genotypes in planta, we designed genotype-specific primer pairs that flanked size-variable loci in the fungal genomes. Diagnostic PCR revealed that only one fungal genotype was present in most inoculated plants, but double infections were also observed with a frequency of 8% of all infected plants. Subsequent analyses of individual tillers of doubly infected plants revealed that, in a given tiller, both the leaf-blade and the leaf-sheath were colonized with only one endophyte genotype. Tillers without any detectable fungal DNA were also observed. Thus, co-existence of multiple endophyte genotypes within a single host plant is governed by mutual exclusion at the tiller level.     

Do the costs and benefits of fungal endophyte symbiosis vary with light availability?

? Here, we examined whether fungal endophytes modulated host plant responses to light availability. First, we conducted a literature review to evaluate whether natural frequencies of endophyte symbiosis in grasses from shaded habitats were higher than frequencies in grasses occupying more diverse light environments. Then, in a glasshouse experiment, we assessed how four levels of light and the presence of endophyte symbioses affected the growth of six grass species. ? In our literature survey, endophytes were more commonly present in grasses restricted to shaded habitats than in grasses from diverse light environments. ? In the glasshouse, endophyte symbioses did not mediate plant growth in response to light availability. However, in the host grass, Agrostis perennans, symbiotic plants produced 53% more inflorescences than nonsymbiotic plants at the highest level of shade. In addition, under high shade, symbiotic Poa autumnalis invested more in specific leaf area than symbiont-free plants. Finally, shade increased the density of the endophyte in leaf tissues across all six grass species. ? Our results highlight the potential for symbiosis to alter the plasticity of host physiological traits, demonstrate a novel benefit of endophyte symbiosis under shade stress for one host species, and show a positive association between shade-restricted grass species and fungal endophytes.     

Insect oviposition preference between Epichloë‐symbiotic and Epichloë‐free grasses does not necessarily reflect larval performance

Variation in plant communities is likely to modulate the feeding and oviposition behavior of herbivorous insects, and plant‐associated microbes are largely ignored in this context. Here, we take into account that insects feeding on grasses commonly encounter systemic and vertically transmitted (via seeds) fungal Epichloë endophytes, which are regarded as defensive grass mutualists. Defensive mutualism is primarily attributable to alkaloids of fungal origin. To study the effects of Epichloë on insect behavior and performance, we selected wild tall fescue (Festuca arundinacea) and red fescue (Festuca rubra) as grass–endophyte models. The plants used either harbored the systemic endophyte (E+) or were endophyte‐free (E?). As a model herbivore, we selected the Coenonympha hero butterfly feeding on grasses as larvae. We examined both oviposition and feeding preferences of the herbivore as well as larval performance in relation to the presence of Epichloë endophytes in the plants. Our findings did not clearly support the female's oviposition preference to reflect the performance of her offspring. First, the preference responses depended greatly on the grass–endophyte symbiotum. In F. arundinacea, C. hero females preferred E+ individuals in oviposition‐choice tests, whereas in F. rubra, the endophytes may decrease exploitation, as both C. hero adults and larvae preferred E? grasses. Second, the endophytes had no effect on larval performance. Overall, F. arundinacea was an inferior host for C. hero larvae. However, the attraction of C. hero females to E+ may not be maladaptive if these plants constitute a favorable oviposition substrate for reasons other than the plants' nutritional quality. For example, rougher surface of E+ plant may physically facilitate the attachment of eggs, or the plants offer greater protection from natural enemies. Our results highlight the importance of considering the preference of herbivorous insects in studies involving the endophyte‐symbiotic grasses as host plants.     

Endophyte-grass-herbivore interactions: the case of Neotyphodium endophytes in Arizona fescue populations

Neotyphodium endophytes in introduced agronomic grasses are well known to increase resistance to herbivores, but little is known of interactions between Neotyphodium endophytes and herbivores in native grass populations. We investigated whether endophytes mediate plant-herbivore interactions in a native grass species, Festuca arizonica in the southwestern United States, in two ways. First, to test the prediction that the presence and frequency of endophyte-infected (E+) plants should increase with increasing herbivory, we determined endophyte frequencies over a 4-year period in six natural Arizona fescue populations. We compared Neotyphodium frequency among plants growing inside and outside long-term vertebrate grazing exclosures. Second, we experimentally tested the effects of Neotyphodium infection, plant clone, and soil nutrients on plant resistance to the native grasshopper Xanthippus corallipes. Contrary to predictions based upon the hypothesis that endophytes increase herbivore resistance, levels of infection did not increase in plants subjected to grazing outside of exclosures relative to ungrazed plants within exclosures. Instead, endophyte frequencies tended to be greater inside the exclosures, where long-term vertebrate grazing was reduced. The grasshopper bioassay experiment corroborated these long-term patterns. Survival of grasshoppers did not differ between infected (E+) and uninfected (E–) plants. Instead, mean relative growth rate of grasshoppers was higher on E+ grasses than on E– ones. Growth performance of newly hatched grasshopper nymphs varied among host plant clones, although two of six clones accounted for most of this variation. Our results suggest that Neotyphodium-grass-herbivore interactions may be much more variable in natural communities than predicted by studies of agronomically important Neotyphodium-grass associations, and herbivory is not always the driving selective force in endophyte-grass ecology and evolution. Thus, alternative hypotheses are necessary to explain the wide distribution and variable frequencies of endophytes in natural plant populations. Received: 15 February 1999 / Accepted: 19 July 1999     

内生真菌对羽茅生长及光合特性的影响

通过温室栽培实验,以感染两种内生真菌(Neotyphodium sibiricum和Neotyphodium gansuence)和未感染内生真菌的羽茅(Achnatherum sibiricum)为实验材料,分析感染不同种内生真菌对宿主植物的生长及光合特性的影响。结果表明,感染两种内生真菌羽茅的株高和CO₂补偿点显著低于未染菌的羽茅,而染菌羽茅的蒸腾速率和气孔导度显著高于未染菌羽茅,但对于感染不同种内生真菌的羽茅,无论是分蘖数与生物量的积累还是光合生理值之间均无显著差异。     

Geographic distribution patterns of vertically transmitted endophytes in two native grasses in Argentina

The incidence of epichloid endophytes in populations of wild grasses is usually variable, and the knowledge about distribution patterns and how environmental factors affect such an incidence is limited. Here we performed a broad scale survey data to study whether the distribution patterns and the incidence of vertically-transmitted endophytes in populations of two native grasses from South-America, Poa lanuginosa Poir. and Poa bonariensis (Lam.) Kunth., are associated with environmental characteristics. We also characterized the endophytes from different populations to establish if the genotype of the endophytes is also correlated with environmental variables. The incidence of endophytes ranged from 0 to 100 % in both host species. In P. lanuginosa, endophytes were only found in populations on sandy coastal dunes and their incidence was positively associated with winter regime rainfall and soil water availability in the growing season. In P. bonariensis, endophytes were only found in populations in xerophytic forests and their incidence was highly associated with plant community. The distributions of infested populations suggested that the endophytes are not found in those areas with the most favorable or most stressing growth conditions accordingly to climatic or edaphical characteristics. Only the vertically transmitted hybrid endophyte species Neotyphodium tembladerae was detected in both host species. Under the hypothesis of vertical transmission, these results suggested that the endophyte should have been lost in endophyte free populations but is maintained in populations established in environments presenting moderate stress as salinity or short drought periods.     

Endophytic fungi in wild and cultivated grasses in Finland

We examined the occurrence of vertically via host seeds transmitted endophyte infections of 14 grass species in natural populations in Finland and totally 97 agricultural cultivars of 13 grass species. Although endophyte infections were widespread in native grass species, overall endophyte occurrence and frequencies were lower than published reports have suggested. In natural populations, 10 out of 14 grass species examined harbor fungal endophytes in their seeds. The highest species-specific mean incidences of endophyte infected plants in infected populations were found in Agrostis capillaris. Festuca arundinacea. F. ovina. F. Pratensis. F. rubra and Phleum pratense (67%, 98%, 29%., 42%. 32% and 33%, respectively). Mean incidences were < 20% in Dactylis glomerata. Deschampsia flexuosa. D. cespitosa and Elymus repens. and no infections were detected in Calamagrostis lapponica. C. epigejos. Alopecurus pratensis and Phalaris arundinacea. However, we detected a very high variation in infection incidences among natural populations and a large proportion of populations was, indeed, endophyte-free. This supports the ideas that 1) endophytic fungi provide selective advantage of infected grasses to their uninfected eonspecifics in some habitats, and/or 2) fungi are occasionally transmitted horizontally by spores. In grass cultivars, endophyte infected seeds were detected only in F. Pratensis and Lolium perenne. and endophyte frequencies were either very high or very low. Cultivars of 11 other grass species were endophyte-free.     

Systematics, distribution, and host specificity of grass endophytes.

Clavicipitaceous endophytes (Ascomycetes) are distributed worldwide in many grasses and sedges forming a perennial and often mutualistic association with their hosts. Most endophytes appear to produce alkaloid toxins in infected plants. The high frequency of infection in many grasses and in certain grassland communities may indicate a selective advantage of infected over non-infected host plants due to their toxic effects on grazing animals and insects. Field observations and artificial inoculations of seedlings have demonstrated a high degree of specificity of most endophytes to their host plant, particularly in asexual, seed-borne endophytes. Specific isozyme genotypes found on several host species suggest that host-specific physiological races may occur. Knowledge of host range and host specificity is vital for potential applications of endophytes in pest control.     

Variable effects of endophytic fungus on seedling establishment of fine fescues

Seedborne systemic endophytic fungi of grasses are thought to be plant mutualists, because they have been shown to improve their host’s resistance against biotic and abiotic stresses. The interactions in plant–endophyte associations vary from mutualistic to parasitic with environmental conditions and the genotypes of interacting species. The possible pros and cons of endophytic fungi are expected to be most evident during the seedling establishment, where host fitness is most directly affected. If this holds true, endophytes may play a focal role in local adaptation of hosts to different environments. We examined if endophyte-infected and uninfected seeds and seedlings of two native grass species, Festuca rubra and F. ovina, differ in seed germination and seedling growth rates under greenhouse conditions. The germination of F. rubra seeds was also studied in the field. This is the first time that the effects of Epichloë endophyte on seedling establishment of fine fescues from natural populations have been experimentally evaluated. Mother plant (seed family) had a marked effect on many response variables in both grass species. Length and mean biomass of tillers of endophyte-infected (E+) F. ovina seedlings were lower, but root:shoot ratios were higher than in endophyte-free (E?) seedlings. In F. rubra, the effects of the endophyte were dependent on the habitat where the seeds were collected. The E+ seeds from river banks germinated faster than E+ seeds from meadows, and E+ seedlings from the river banks produced fewer but taller and heavier tillers than the other seedlings. Our data suggest that the effects of the endophyte infection on the seedling stage of fine fescues are dependent the species of grass, host genetic background and mother plant habitat. The germination strategy and growth form of E+ red fescue seedlings from river banks may be beneficial to surviving in the harsh conditions of that habitat.     

禾草-内生真菌共生体在草地农业系统中的作用

综述了国内外近年对禾草 -内生真菌 (N eotyphodium)的研究进展。全世界现已报道 14种禾草内生真菌 ,与 2 3个属的禾草形成共生体 ,我国已在 13属 2 5种天然草地禾草中发现内生真菌 ,其中发草属 (Descampsia)、大麦属 (H ord eum)和赖草属(L eymus)以往在国际文献中未曾报道。内生真菌在禾草体内产生的生物碱 ,致使采食带菌禾草的马、牛、羊、鹿等家畜产生中毒症状 ,每年给美国、新西兰等国造成的经济损失达 6.4亿美元之多 ,对其毒理研究取得了进展。已发现我国醉马草 (Achnatheruminebrians)对家畜的毒性与内生真菌的侵染有关。内生真菌侵染增加可使禾草对 42种害虫的抗性 ,并可增加对某些线虫和病害的抗性。与不带菌禾草相比 ,带菌禾草的另一特点是抗逆性强 ,牧草产量高。国际在该领域的研究主要集中在多年生黑麦草(L olium p erenne)和高羊茅 (Festuca arundinacea)。我国的研究发现 ,带菌布顿大麦草 (H ordeum bodg anii)和圆柱披碱草(Elymus cylind ricus)的牧草产量分别增加 3 3 .3 %和 2 78.8% ,分孽数分别增加 13 6.8%和 84.5%。目前 ,国际研究的重点包括大规模开展内生真菌生物学与生态学特性的研究 ,创造不含对家畜有害毒素的有益禾草 -内生真菌共生体 ,培育带内生真菌的草坪草品种 ,培育抗毒