Infection by the systemic fungus Epichloë glyceriae alters clonal growth of its grass host,Glyceria striata

Parasites and pathogens are hypothesized to change host growth, reproduction and/or behaviour to increase their own transmission. However, studies which clearly demonstrate that parasites or pathogens are directly responsible for changes in hosts are lacking. We previously found that infection by the systemic fungus Epichloë glyceriae was associated with greater clonal growth by its host, Glyceria striata. Whether greater clonal growth resulted directly from pathogen infection or indirectly from increased likelihood of infection for host genotypes with greater clonal growth could not be determined because only naturally infected and uninfected plants were used. In this study, we decoupled infection and host genotype to evaluate the role of pathogen infection on host development and clonal growth. We found that total biomass production did not differ for clones of the same genotype, but infected clones allocated more biomass to clonal growth. Disinfected clones had more tillers and a greater proportion of their biomass in the mother ramet. Infected clones produced fewer tillers but significantly more and longer stolons than disinfected clones. These results support the hypothesis that pathogen infection directly alters host development. Parasite alteration of clonal growth patterns might be advantageous to the persistence and spread of host plants in some ecological conditions.     

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

Summary Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.     

Impact of the flower smut Ustilago cynodontis (Ustilaginaceae) on the performance of the clonal grass Cynodon dactylon (Gramineae)

Effects of the flower-infecting smut fungus Ustilago cynodontis on the growth, survival, and competitiveness of the clonal rhizomatous perennial grass Cynodon dactylon were explored through a series of glasshouse experiments. Ustilago cynodontis sterilized C. dactylon by replacing floral structures with a teliospore-producing fungal stroma. Infection caused a reduction in overall dry matter production and the growth rate of stolons, changed the allocation of resources between roots and shoots, and affected the survival of C. dactylon plants growing in pots under crowded conditions in a naturally lit glasshouse. In mixtures of healthy and infected plants, the growth of infected ones was disproportionally affected. Infection had no effect on the germination or emergence of seedlings. Transmission of the pathogen along stolons was incomplete, especially as the length of the stolon increased. The nutrient status of the soil had no significant effect on stolon growth. Results are discussed in terms of the differential effect of systemic floral smut diseases on tiller-forming and clonal rhizomatous perennial grasses.     

高寒退化与未退化草甸土壤下匍匐茎鹅绒萎陵菜的克隆生长特征的比较

通过鹅绒萎陵菜的野外移栽试验,分析了在矮嵩草草甸土壤和退化草甸土壤下该种植物的克隆繁殖特征．结果表明,在母株产生的匍匈茎数量、母株高度和叶片大小在两类不同土壤下没有明显差异．母株叶片数、匍匈茎的粗度、匍匈茎长度以及间隔子的长度有较明显的差异．在退化草甸土壤中母株叶片较多,匍匈茎长度和间隔子长度明显较长,匍匈茎直径也明显较大．在退化草甸环境中,由于土壤养分相对较缺乏,鹅绒萎陵菜可能通过增加母株叶片的数量,尽可能多地积累光合产物,来保证匍匈茎的生长,匍匈茎也以增加粗度和增加长度来尽量增强其觅食能力．鹅绒萎陵菜在生物量投资分配上也表现出一定的差异．与退化草甸土壤环境相比,在未退化草甸土壤中,鹅绒委陵菜克隆母株和分株的生物量均明显偏向地下部分(根系)的投资,以期从土壤中吸收更多的养分,从而最终提高子株的成活率．     

酸雨和采食模拟胁迫下克隆整合对空心莲子草生长的影响

研究表明克隆整合可以显著提升异质环境中克隆植物的生长,然而当克隆植物遭受均质环境压力时,整合对植物生长影响的研究相对较少。本文以典型入侵克隆植物空心莲子草（Alternanthera philoxeroides）为例,研究均质环境压力酸雨和采食模拟胁迫对空心莲子草生长的影响,以及克隆整合在空心莲子草适应不利环境过程中所起的作用。酸雨设3种浓度梯度：pH值3.5 、pH值4.5和 pH值6.5（对照）;采食设3种水平：不去叶、去叶50%和去叶90%;整合水平：匍匐茎切断和连接。结果表明：无论保持或切断匍匐茎的连接,酸雨处理都不影响空心莲子草生物量。当保持匍匐茎连接时,pH值4.5酸雨处理增加了空心莲子草匍匐茎长度和分株数目,因此,低度酸雨可能对空心莲子草生长有一定的促进作用。同样,无论匍匐茎是否被切断,采食处理都显著降低了空心莲子草克隆片段生物量,而显著增加了叶片数目。当切断匍匐茎连接时,采食处理使空心莲子草分株数目显著增加。本文得出的结论是：空心莲子草能较好地适应酸雨和采食的环境压力,当空心莲子草全部克隆分株遭受均质环境胁迫时,克隆整合并不能显著改善它的生长。     

Impact of the fungus Balansia henningsiana on Panicum agrostoides: frequency of infection,plant growth and reproduction,and resistance to pests

Summary The impact of the systemic fungus Balansia henningsiana (Clavicipitaceae) on the grass Panicum agrostoides was examined in field and greenhouse studies comparing infected and uninfected plants. Approximately one-half of all plants in three populations located in southern Indiana were infected. In field samples and greenhouse studies infected plants were significantly heavier than uninfected plants and produced significantly more tillers. Infection tended to suppress flowering but occasional asymptomatic tillers on infected plants produced healthy inflorescences. Although infected plants produced fewer inflorescences than uninfected plants as a proportion of total tillers, absolute numbers of inflorescences were similar in the two groups. Because other grasses infected by different species of Balansia and related fungi often are more resistant to insect damage, pest damage was quantified in one population. No differences between infected and uninfected plants were detected in levels of herbivory but infected plants had significantly less damage by the common leaf spot fungus Alternaria triticina. The results suggest that there is no selective disadvantage for plants infected by B. henningsiana.     

The effect of stolon fragmentation on the colonization of clonal invasive Carpobrotus edulis in a coastal dune system: a field test

Disturbances usually initiate processes of fragmentation in clonal plants, with the consequent division into portions of different size. The ability of these portions to survive and regrow after fragmentation plays an important role in the maintenance of populations and the colonization of new environments. In this field experiment we aim to determine the importance of stolons as reserve organs in the colonization of a coastal sand dune by a clonal invader. We simulated an event of fragmentation of clones of an aggressive invader into portions with short and long stolon sizes. Our results showed a reduction of biomass allocation to roots in the long stolon treatment that was balanced by an increase in the above‐ground growth; consequently, the area colonized by the invader was greater. We report evidence that stolons can contribute to buffering stressful conditions and allow expansion of the invader into a natural coastal sand dune.     

DENSITY-MEDIATED SEED AND STOLON PRODUCTION IN VIOLA (VIOLACEAE)

Seed and stolon production and spatial distribution were studied in two populations of Viola blanda and two populations of Viola rostrata in West Virginia. Mean individual plant biomass and the proportion of the population producing seeds and/or stolons both decreased with density. Both species possessed a characteristic “threshold” weight required before the onset of seed production; a similar minimum threshold weight was also reached before stolons were produced. It is suggested that competition at higher densities results in fewer individuals reaching the threshold weight for seed or stolon production. Thus, the density response appears as a reduction in the proportion of plants producing seeds or stolons.     

Host genotype overrides fungal endophyte infection in influencing tiller and spike production of Lolium perenne (Poaceae) in a common garden experiment

Leaves of many cool-season grasses are infected by endophytic fungi that can impact their populations. A common garden experiment with Lolium perenne was established in a lawn in New Jersey, USA, to investigate the impact of endophyte infection and host genotype on tiller and spike production over three years. Infected (E+) and uninfected (E-) plants of each genotype were monitored every 2-3 mo. Infection intensity within plants varied significantly among genotypes and years, but there was no evidence of directional change over time. Tiller production varied significantly among genotypes and was affected by endophytes: E+ plants of several genotypes produced more tillers than E- plants during the third year. E+ plants had greater aboveground biomass, but host genotype explained a far greater proportion of variation in tiller production, number, and biomass than infection. Plant survival, percentage flowering, flowering date, number of spikes, and mean tiller mass were unaffected by endophytes. However, the last three variables showed significant variation among host genotypes. Although studies have demonstrated a positive growth effect of endophytes on several grass hosts, in this experiment host genotype accounted for far more of the variation in tiller and spike production and in biomass of Lolium perenne than endophyte infection.     

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.     

Relative costs and benefits of clonal integration of Zoysia japonica under various N:P ratios

Clonal plants adaptively respond to abiotic stress, but to date little is known about under what circumstanses clonal integration is beneficial, or costly. To study the costs and benefits of clonal integration on clonal growth, we cultivated Zoysia japonica under three ratios of N:P (N:P ≈ 7, 14:1 and 21:1), and four types of stolon severing treatments (connected stolon CS, light severing LS, moderate severing MS, serious severing SS). The results showed that Z. japonica performed best at a low ratio of N:P (N:P ≈ 7:1). When the stolons were connected (CS), the growth of primary A‐ramets, multiple‐nodes and stolons benefited from clonal integration; however, the growth of primary B‐ramets on the primary stolons, A and B ramets was significantly reduced (p < 0.05). In the moderate stolon severing treatment MS_7:1 (N:P ≈ 7:1), clonal integration appeared more cost effective than in all other treatments. On the whole, with increasing stolon severing intensity, the cost of clonal integration increased and the clonal growth of Z. japonica declined significantly. The pattern of biomass allocation may be useful for Z. japonica to adapt to the various environments, and clonal integration plays a significant role under adverse environmental conditions.     

匍匐茎草本蛇莓对基质养分条件的克隆可塑性

A pot experiment with three levels of nutrient (N, P, K) supply was carried out to investigate clonal plasticity of stoloniferous herb Duchesnea indica Focke in response to nutrient availability. The plants had greater biomass at higher levels of nutrient availability. The root/shoot ratio of the plants changed with the nutrient availability in the following order: low level > high level > medium level. They had the largest biomass allocation to stolon at the medium level of nutrient availability. The biomass allocation to petiole did not respond to the treatments. The plants formed more stolons and ramets at the high and medium levels than the low level of nutrient availability. The petiole length, specific petiole weight（mg/cm）and stolon internode length of the plants did not respond to the treatments, while the specific stolon weight (mg/cm）of the plants was greater at the high and medium levels than the low level of nutrient availability. The results have been discussed in the context of adaptation of clonal plants to environmental heterogeneity.     

Effects of inoculation with native arbuscular mycorrhizal fungi on clonal growth of Potentilla reptans and Fragaria moschata (Rosaceae)

Many clonal plants live in symbiosis with ubiquitous arbuscular mycorrhizal (AM) fungi, however, little is known about their interaction with respect to clonal reproduction and resource acquisition. The effects of arbuscular mycorrhiza on the growth and intraclonal integration between ramets of two stoloniferous species were studied experimentally in a nutritionally homogenous soil environment. Two species coexisting at the same field site, Potentilla reptans and Fragaria moschata, were selected as model plants for the study. Pairs of their ramets were grown in neighbouring pots with each ramet rooted separately. Four inoculation treatments were established: (1) both mother and daughter ramets remained non-inoculated, (2) both ramets were inoculated with a mixture of three native AM fungi from the site of plant origin, (3) only mother or (4) daughter ramet was inoculated. The stolons connecting the ramets were either left intact or were disrupted. Despite the consistent increase in phosphorus concentrations in inoculated plants, a negative growth response of both plant species to inoculation with AM fungi was observed and inoculated ramets produced fewer stolons and fewer offspring ramets and had lower total shoot dry weights as compared to non-inoculated ones. A difference in the extent of the negative mycorrhizal growth response was recorded between mother and daughter ramets of P. reptans, with daughter ramets being more susceptible. Due to AM effect on ramet performance, and thereby on the source-sink relationship, inoculation also significantly influenced biomass allocation within clonal fragments. Physiological integration between mother and daughter ramets was observed when their root systems were heterogeneous in terms of AM colonization. These results hence indicate the potential of mycorrhizal fungi to impact clonal growth traits of stoloniferous plant species, with possible consequences for their population dynamics.     

Effects of orientation on survival and growth of small fragments of the invasive, clonal plant Alternanthera philoxeroides

Background

The ability of small clonal fragments to establish and grow after disturbance is an important ecological advantage of clonal growth in plants and a major factor in the invasiveness of some introduced, clonal species. We hypothesized that orientation in the horizontal position (typical for stoloniferous plants) can increase the survival and growth of dispersed clonal fragments, and that this effect of orientation can be stronger when fragments are smaller and thus have fewer reserves to support initial growth.

Methodology/Principal Findings

To test these hypotheses, we compared performance of single-node pieces of stolon fragments of Alternanthera philoxeroides planted at angles of 0, 45 or 90° away from the horizontal position, with either the distal or the proximal end of the fragment up and with either 1 or 3 cm of stolon left attached both distal and proximal to the ramet. As expected, survival and growth were greatest when fragments were positioned horizontally. Contrary to expectations, some of these effects of orientation were stronger when attached stolons were longer. Orientation had smaller effects than stolon length on the performance of fragments; survival of fragments was about 60% with shorter stolons and 90% with longer stolons.

Conclusions/Significance

Results supported the hypothesis that orientation can affect establishment of small clonal fragments, suggested that effects of orientation can be stronger in larger rather than smaller fragments, and indicated that orientation may have less effect on establishment than amount of stored resources.     

Fungal endophyte infection changes growth attributes in Lolium multiflorum Lam

Abstract Lolium multiflorum is a successful invader of postagricultural succession in the Inland Pampa grasslands in Argentina, becoming a dominant species in the plant community. Individual plants of this annual species are naturally highly infected with fungal endophytes (Neotyphodium sp.) from early successional stages. We assessed the effect of Neotyphodium infection on the biology of L. multiflorum. We evaluated growth attributes between endophyte infected (E+) and uninfected (E–) plants under non‐competitive conditions during the normal growing season. E+ plants produced significantly more vegetative tillers and allocated more biomass to roots and seeds. Although seed germination rates were greater in endophyte free plants, the rate of emergence and the final proportion of emerged seedlings were similar between the biotypes. The greater production of vegetative tillers, and the greater resource allocation to roots and seeds are likely to confer an ecological advantage to E+ plants, thus enabling their dominance over the E– individuals in natural grasslands.     

土壤养分水平影响绢毛匍匐委陵菜匍匐茎生物量投资

为研究匍匐茎草本植物对基质养分供应水平的生物量分配格局的可塑性,在一盆栽实验中对绢毛匍匐委陵菜(Potentilla reptans var. sericophylla)进行了8种不同的养分处理。绢毛匍匐委陵菜植株生物量、匍匐茎数、分株数以及匍匐茎节间长在中等养分条件下最大。随土壤养分的降低,绢毛匍匐委陵菜对叶片和叶柄的生物量投资减小,而对根系的生物量投资增加。在中等养分条件下,绢毛匍匐委陵菜对匍匐茎的生物量投资倾向于最大,而在更高或更低的养分条件下倾向于减少。此生物量分配格局与de Kroon和Schieving的模型模拟结果相符合,结果表明在中等资源水平下增加对匍匐茎的生物量投资是克隆植物增加资源获取的对策之一。     

Relationships between the genetic distance of Epichloë festucae isolates and the ergovaline and peramine contents of their Festuca rubra hosts

In semiarid grasslands of western Spain, plants of Festuca rubra are frequently infected by Epichloë festucae , an endophyte capable of producing the alkaloids ergovaline, toxic to mammals, and peramine, toxic to insects. The objectives of this paper were to estimate the variability in the content of ergovaline and peramine in plants of Festuca rubra from natural populations and to determine the relationship between the genetic distance among Epichloë strains and the alkaloid contents of plants infected by them. The results showed that a significant variation exits in the ergovaline and peramine contents of infected F. rubra plants, and that variation patterns are different for each alkaloid. Ergovaline content showed a significant variation between years. In contrast, most variation observed in the concentration of peramine occurred between populations. Mantel tests, constrained correspondence analysis (CCA) and generalised analysis of molecular variance (GAMOVA) all revealed a significant relationship between the genetic distances among 35 Epichloë isolates, and the differences in peramine content of their host plants. The relationship observed for ergovaline was more ambiguous. This indicates that plants infected by genetically close isolates of Epichloë festucae tend to be similar in terms of peramine content, while the ergovaline content of infected plants seems to be more dependent on external factors. The results of this study suggest that the selection of Epichloë festucae endophytes based on the alkaloid content of their host plants can be a successful strategy to select good or bad producers of peramine, but may not be a reliable method to detect fungal strains associated with particular ergovaline production profiles.     

Heavy metals modify costs of reproduction and clonal growth in the stoloniferous herb Potentilla anserina

We examined costs of sexual reproduction and clonal propagation, and their consequences for resource allocation in the clonal stoloniferous herb, Potentilla anserina, a typical pioneer species in disturbed areas. We used heavy-metal treatment in soil to create unfavourable growing conditions, because costs of reproduction are more likely to be expressed when resources are limited. We also studied whether heavy metals affect the plasticity of clonal growth form that enables the plants to avoid poor growing conditions. Ramets collected from field were grown in a greenhouse under the heavy-metal treatment consisting of a control and two levels of heavy-metals added in soil. Clonal propagation was costly in terms of total biomass of flowering ramets. Also the costs of sexual reproduction were detected in flowering ramets. Contrary to our predictions, the costs of flower production were visible in the control but not in the heavy-metal contaminated plants. Only the flowering ramets were able to produce longer stolons under heavy-metal stress, but the stolon biomass was not affected by heavy metals. Results indicate that clonal propagation and sexual reproduction may be costly for P. anserina. However, the costs are modified by heavy-metal contamination.Co-ordinating editor: J. Tuomi     

Ditylenchus dipsaci Infestation of Trifolium repens. II. Dynamics of Infestation Development

Trifolium repens (white clover) stolons were inoculated with Ditylenchus dipsaci (stem nematode), and the development of resulting infestations was monitored. Nematodes initially remained confined to superficial locations, concentrating in petiole axils near inoculation points. They were able to migrate slowly from the inidal inoculation points and infest adjacent axils, especially in regions near the stolon tip. As time progressed, in some axils, nematodes migrated through the stolon epidermis and colonized slowly expanding subepidermal pockets of host tissue (ca. 0.2-mm length of stolon/day). In these loci nematodes established exponentially increasing populations, but the rates of locus expansion remained constant, indicating that locus expansion was limited by unidentified host-dependent factors. As a result of increasing population pressure within subepidermal loci, J4 entered a "diapause" state and the rate of egg production by adults declined, thereby reducing rate of population growth to more sustainable levels. Typically, these populations peaked at ca. 10,000 individuals in ca. 160 days occupying 3-cm lengths of stolon. Thereafter, heavily infested regions of stolons started to die, leading to the formation of longitudinal splits in their epidermis. In other axils, nematodes did not migrate into the stolons but remained confined to axils. Some of these populations increased a hundred-fold in 95 days, with population growth ending when petioles started to die. Host plant stolon morphology was affected only when subepidermal stolon populations developed high population levels (>100 nematodes) within close proximity (<2 cm) to active terminal meristems. This occurred either when axillary buds became active on previously infested nodes or when nematodes established endoparasitic populations at locations near the stolon tip during winter and spring, when the rate of stolon extension was limited by low light intensity. Affected stolon tips could "escape" from the influence of such infestations when light intensity and temperature increased. Nematode activity was limited by low temperature rather than light intensity. Global warming is likely to lead to greater damage to infested plants during the winter and early spring because the predicted milder winter temperatures will enhance nematode activity but not necessarily promote stolon growth.     

Stroma-forming endophyte Epichloë glyceriae provides wound-inducible herbivore resistance to its grass host

Fungal endophyte-grass associations are diverse and complex. Some endophytes (e.g. Neotyphodium spp.) reproduce asexually by growing vegetatively into host seeds and many of these vertically-transmitted endophytes form mutualisms with their hosts by providing high levels of alkaloids, such as lolines, that reduce herbivore performance. Additionally, Neotyphodium coenophialum provides wound-inducible herbivore resistance through increased production of lolines. Neotyphodium likely evolved from Epichloë spp. which are sexually reproducing endophytes that are transmitted horizontally to the next host generation through production of stromata (fruiting bodies), which sterilize host grasses. We asked if wound-inducible resistance like that in N . coenophialum also occurs in the ancestral, sexually reproducing Epichloë glyceriae , which infects Glyceria striata . Host grasses were damaged by fall armyworm caterpillars, artificially cut, or left undamaged. An aphid bioassay tested the plant's toxicity to herbivores, expression of lolc (a gene in the loline biosynthesis pathway) was quantified using real-time RT-PCR, and loline concentration was quantified using gas chromatography and mass spectrometry. Artificially-damaged plants supported fewer live aphids, had greater lolc mRNA expression, and greater loline concentration than undamaged plants. Herbivore-damaged plants supported intermediate performance by aphids, low lolc mRNA expression, and minimal loline concentration. Our study is the first to demonstrate sexual endophytes can produce lolines following wounding. This suggests wound-induced responses are ancestral within the Epichloë / Neotyphodium clade and reveals a trait of grass endophytes that may have predisposed them for the evolution of defensive mutualisms with their hosts.