Piippa Wäli | University of Oulu (original) (raw)

Papers by Piippa Wäli

It is well recognized, that outcomes of mutualistic plant-microorganism interactions are often co... more It is well recognized, that outcomes of mutualistic plant-microorganism interactions are often context dependent and can range from mutualistic to antagonistic depending on conditions. Instead, seemingly pathogenic associations are generally considered only harmful to plants. The ergot fungus (Claviceps purpurea) is a common seed pathogen of grasses and cereals. Ergot sclerotia contain alkaloids which can cause severe toxicity in mammals when ingested, and thus the fungal infection might provide protection for the host plant against mammalian herbivores. Theoretically, the net effect of ergot infection would positively affect host seed set if the cost is not too high and the defensive effect is strong enough. According to our empirical data, this situation is plausible. First, we found no statistically significant seed loss in wild red fescue (Festuca rubra) inflorescences due to ergot infection, but the seed succession decreased along increasing number of sclerotia. Second, in a food choice experiment, sheep showed avoidance against forage containing ergot. Third, the frequency of ergot-infected inflorescences was higher in sheep pastures than surrounding ungrazed areas, indicating a protective effect against mammalian grazing. We conclude that, although ergot can primarily be categorized as a plant pathogen, ergot infection may sometimes represent indirect beneficial effects for the host plant. Ergot may thus serve as a conditional defensive mutualist for its host grass, and the pathogenic interaction may range from antagonistic to mutualistic depending on the situation.

PLoS ONE, 2010

Even highly mutually beneficial microbial-plant interactions, such as mycorrhizal-and rhizobial-p... more Even highly mutually beneficial microbial-plant interactions, such as mycorrhizal-and rhizobial-plant exchanges, involve selfishness, cheating and power-struggles between the partners, which depending on prevailing selective pressures, lead to a continuum of interactions from antagonistic to mutualistic. Using manipulated grass-endophyte combinations in a five year common garden experiment, we show that grass genotypes and genetic mismatches constrain genetic combinations between the vertically (via host seeds) transmitted endophytes and the out-crossing host, thereby reducing infections in established grass populations. Infections were lost in both grass tillers and seedlings in F 1 and F 2 generations, respectively. Experimental plants were collected as seeds from two different environments, i.e., meadows and nearby riverbanks. Endophyte-related benefits to the host included an increased number of inflorescences, but only in meadow plants and not until the last growing season of the experiment. Our results illustrate the importance of genetic host specificity and transgenerational maternal effects on the genetic structure of a host population, which act as destabilizing forces in endophytegrass symbioses. We propose that (1) genetic mismatches may act as a buffering mechanism against highly competitive endophyte-grass genotype combinations threatening the biodiversity of grassland communities and (2) these mismatches should be acknowledged, particularly in breeding programmes aimed at harnessing systemic and heritable endophytes to improve the agriculturally valuable characteristics of cultivars.

Vertically transmitted fungal endophytes: different responses of host-parasite systems to environ... more Vertically transmitted fungal endophytes: different responses of host-parasite systems to environmental conditions. -Oikos 99: 173-183.

Microbial Ecology, 2007

Epichloë species are systemic fungal endophytes that usually specialize in a certain group of rel... more Epichloë species are systemic fungal endophytes that usually specialize in a certain group of related grass species. We examined the infection frequency of Epichloë festucae in populations of two fine fescue species (Festuca rubra and F. ovina) in natural and seminatural habitats at 86 study sites (total = 2514 plants) across Finland and northern Norway. Infection incidence varied significantly among grass species and populations. A substantial number of the F. rubra and F. ovina populations (53 out of 77 and 25 out of 30, respectively) were either endophyte-free or had very low (G20%) infection frequencies. The highest infection frequencies were found in subarctic areas. Moreover, infection incidence differed between habitats. In the area with the highest infection frequencies, we used microsatellite markers to study genetic diversity and the rates of gene flow of E. festucae among 12 F. rubra populations. Twenty out of the 25 fungal genotypes detected with four microsatellite markers were carrying multiple alleles in at least one locus, indicating multiple infections or vegetative hybridization of the fungus. One dominant genotype occurred in all 12 populations, representing 63.5% of all isolates. We found a moderate level of average genotypic variation and a low level of genetic differentiation (F st = 0.0814). There was no correlation between infection frequency and genotypic diversity. Although the existence of a dominant genotype and the detected linkage disequilibrium suggest that the fungus is mainly asexual and vertically transmitted, the multiallelic loci and variation of genetic diversity among populations indicate occasional contagious spread and sexual or parasexual recombination of the fungus in some populations. Furthermore, the genotypes carrying multiallelic loci suggest the possibility of multiple infections or hybridization of the endophyte.

Canadian Journal of Forest Research, 2006

Fungal endophytes of birch trees are nonsystemic and transmitted horizontally by spores. Because ... more Fungal endophytes of birch trees are nonsystemic and transmitted horizontally by spores. Because the endophytes reinfect the leaves every growing season, the frequency of occurrence and species composition of the fungi depend on various abiotic and biotic environmental conditions. This was a study of possible effects of silviculture and local environmental variables on endophyte frequencies. Silver birch (Betula pendula Roth) leaves were sampled from seedling stands (five transects), managed mature forest (seven transects), and old natural forest (five transects) in the boreal vegetation zone in east-central Finland. The sapling stands had the highest endophyte frequency and the managed forest the lowest total infection frequency. The old natural forest tended to have the most diverse identified fungal species community, but the difference was not statistically significant. The most frequently isolated endophytic fungi were Fusicladium betulae (Rob. & Desm.) Aderh., consisting of 70% of the isolates from the sapling stands and 31% and 21% of the isolates from the managed forest and natural forest, respectively. It is probable that the sapling stands had plenty of available spores combined with an otherwise favourable microclimate. In the natural forests Gnomonia setacea (Pers.:Fr.) Ces. & de Not. was the most frequently isolated endophyte (30%), while in the sapling stands only 4% of the isolates belonged to G. setacea. In natural forest the frequency of G. setacea infections was positively correlated with stand age, indicating that G. setacea is favouring the old forest habitats.

Oecologia, 2009

Seedborne systemic endophytic fungi of grasses are thought to be plant mutualists, because they h... more Seedborne systemic endophytic fungi of grasses are thought to be plant mutualists, because they have been shown to improve their host&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;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.

Fungal Diversity, 2014

Fungal endophytes are suggested to manipulate host grass reproduction. Using different grass-endo... more Fungal endophytes are suggested to manipulate host grass reproduction. Using different grass-endophyte combinations in a four year common garden experiment, we show that endophyte can retain control over the reproductive functions of the host. Plants were collected as seeds from two different environments, meadows and riverbanks. Natural endophyte infection (E+) increased reproductive effort of plants; 13 and 15 % higher proportion of E+ plants produced panicles compared to naturally uninfected (E-) and manipulatively endophyte-infected (ME+) plants, respectively. Meadoworigin E+ plants produced also higher number of panicles compared to E-in the last two years, and both meadow-and riverbank-origin ME+ plants produced higher number of panicles compared to ME-plants in most of the years. Pseudovivipary was recorded in 5 % of the plants each year. Pollen limitation appears not to induce pseudovivipary. Both E+ and ME+ plants produced higher number of seminiferous panicles compared to E-and ME-plants. The higher proportion of pseudoviviparism in ME+ plants compared to E+, Eand ME-plants from meadows suggests specific genotypegenotype cross-talk between endophyte and grass. Pseudovivipary was affected by year and primarily explained by plant reproductive effort since most vigorous plants produced more pseudoviviparous panicles. We propose that endophytes can promote host grass reproduction in a habitatspecific manner and depending on genetic compatibility between partners. However, reproductive behavior depended on maternal habitat and the annual environmental conditions highlighting the importance of long-term experiments to estimate the effects of endophytes on host plant ecology.