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1. Cool-season grasses can be simultaneously infected by foliar fungal endophytes and colonised by mycorrhizal fungi, the integrated functions of which are strong predictors of plant fitness within grassland ecosystems. Evidence has been presented previously that infection of grass species with foliar endophytes can negatively affect mycorrhizal colonisation. Here, we tested the hypothesis that mycorrhizal colonisation in turn adversely affects Neotyphodium endophyte concentrations and that the competitive interaction between the two endosymbionts is affected by resource supply.
2. Specifically, we report how competition between Glomus (G. mosseae– GM, G. intraradices– GI) mycorrhizal fungi and N. lolii (common strain (CS) and AR1) foliar endophytic strains is affected by P supply and water-soluble carbohydrate (WSC) content in two Lolium perenne (perennial ryegrass) cultivars: a high sugar grass, AberDart, and a conventional (control) grass, Fennema.
3. The presence of Glomus mycorrhizae reduced the concentrations of endophytes and alkaloids in leaf blades and pseudostems. The reduction depended on P supply, ryegrass cultivar (notably WSC content) and endophyte strain. Conversely, foliar endophyte infection reduced mycorrhizal colonisation rates and concentrations in the roots of the control cultivar Fennema, although not in the high sugar cultivar, AberDart.
4. Neither GM nor N. lolii infection had an effect per se on the yield of root or blade compared with mycorrhiza-free (M-) and endophyte-free (E-) plants, respectively; though, yield of roots and blades was reduced by GI infection and at low P.
5. Competitive interactions between ecologically widespread foliar endophytes (valuable for plant protection) and mycorrhizal endosymbionts (valuable for P acquisition) as seen in this study are of critical importance especially in areas of high pest prevalence and low P availability. Our work stresses the need for elucidating the physiological/metabolic basis for such interactions between endosymbionts to understand how these processes contribute to plant performance and fitness in grassland ecosystems.
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- Materials and methods
Grass species belonging to the family Poaceae are often hosts of systemic symbiotic fungal endophytes that reside in aerial plant parts and are vertically transmitted through the seeds of infected grasses (Leuchtmann 1992; Clay & Schardl 2002). In agricultural systems, associations between (i) Lolium perenne L. (perennial ryegrass) and Neotyphodium lolii (Latch, Christensen & Samuels) Glenn, Bacon & Hanlin, and (ii) Schedonorus phoenix (Scop.) Holub (tall fescue) and N. coenophialum (Morgan-Jones & Gams) Glenn, Bacon & Hanlin predominate (Christensen et al. 1993). Endophyte infection is proposed to benefit agricultural grasses through increased tolerance to abiotic stress (e.g. drought), enhanced growth and competitive ability, and increased resistance to herbivory because of the production of endophyte-derived alkaloids (Clay & Schardl 2002). Major alkaloids produced by the common N. lolii strain (CS = Lp19) in ryegrass are peramine, lolitrem B and ergovaline, while N. coenophialum produces peramine, ergovaline and lolines in fescue (Powell & Petroski 1992). The neurotrophic alkaloids lolitrem B and ergovaline are thought to be toxic to mammalian herbivores and are the putative causes of ryegrass staggers in sheep and fescue toxicosis in cattle, respectively, while peramine and lolines are associated with resistance to invertebrates. For use in pasture grazing systems, novel associations between naturally occurring endophyte strains and high yielding grass cultivars have been developed to retain insect-deterrent properties while reducing the detrimental impact on grazing herbivores (Fletcher & Easton 1997). For example, the N. lolii strain ‘AR1’ contains peramine only and ‘AR37’ produces only janthitrems, while the N. coenophialum strain ‘AR542’ produces peramine and N-acetyl norloline only. Although these are widely commercially marketed, few studies have examined the broader ecological implications of such associations and how resource supply, plant and fungal genotypes and competitive interactions may alter the grass–endophyte relationship and the putative benefits derived by their use in agricultural systems. A recent study by Rudgers, Fischer & Clay (2010) found that both endophytic strain and host plant cultivar play an important role in plant species diversity and plant competitive interactions in tall fescue plots.
Grasses are also able to form associations with a second type of obligate symbiotic fungi belonging to the phylum Glomeromycota (Smith & Read 1997; Schüssler, Schwarzott & Walker 2001). Arbuscular mycorrhizal fungi such as Glomus mosseae (GM) and G. intraradices (GI) colonise host roots and form specific structures (vesicular and arbuscular) as well as extensive intra- and extra-radical hyphal nets, which facilitate the uptake of mineral elements, mainly phosphorus (P), from the soil (Smith & Gianinazzi-Pearson 1988; Marschner & Dell 1994; Bucher 2007). Mycorrhizae are thought to be widely beneficial in agricultural systems because of improvements in plant nutrition status and yield, resistance to environmental stresses, improved soil aggregate formation and stability against erosion (Smith & Read 1997).
Dual infection of host plants with both mycorrhizal and endophytic fungi occurs in both natural and agricultural ecosystems. Figure 1 depicts the anatomy of a dually infected grass plant. While studies on the interactions of host plants with either one of these fungal symbionts are numerous, studies considering tripartite interactions between endophytic and mycorrhizal fungi, and host plants, are rare. Foliar endophytic fungi can suppress mycorrhizal colonisation of roots in tall fescue (Chu-Chou et al. 1992; Mack & Rudgers 2008), perennial ryegrass (Müller 2003) and annual ryegrass (Omacini et al. 2006) but they can also enhance mycorrhizal colonisation in Bromus setifolius (Novas, Cabral & Godeas 2005). Effects of mycorrhizal colonisation on endophyte infection have been much less studied, although it has been reported that mycorrhizal infection reduced the level of resistance to Argentine stem weevil in L. perenne infected with N. lolii (Barker 1987) and that foliar N. lolii endophyte-conferred resistance in L. perenne against the moth Phlogophora meticulosa was reduced by mycorrhizal (GM) colonisation (Vicari, Hatcher & Ayres 2002).
Figure 1. Schematic diagram of the anatomy of a dually infected grass plant (a) showing endophytic Neotyphodium hyphae in leaf blades (b) and colonisation by mycorrhizal arbuscules in root tissues (c).
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While host plant associations with both foliar endophytes and mycorrhizal fungi are generally regarded as mutualistic, recent evidence suggests that these associations represent a mutualism–parasitism continuum. In mycorrhizal associations, host growth depression at high levels of soil P availability can occur (Hall, Johnstone & Dolby 1984; Peng et al.1993), while the relationship between foliar endophytes and their hosts depend on host genotype, endophytic strain and environmental conditions, notably those affecting resource availability (Müller & Krauss 2005; Saikkonen et al. 2006; Cheplick 2007). In a previous controlled environment study, we used quantitative polymerase chain reaction (qPCR) to estimate endophyte concentrations in N. lolii-infected ryegrass to test whether resource supply affected the interactions between the host plant and its endophyte (Rasmussen et al. 2007). We found that high N supply reduced both endophyte and alkaloid concentrations by 50% and that endophyte and alkaloid concentrations in a high sugar grass (HSG) cultivar, bred for elevated water-soluble carbohydrate (WSC) content (AberDove-HSG), were also reduced to 50% compared with a control cultivar (Fennema). This clearly shows that simple changes in nutrient supply and resource (energy)-related cultivar traits can affect the symbiotic association between ryegrass and its fungal endophyte, and it is likely that any association of plants with endosymbionts that would change the host nutrient status, e.g. mycorrhizal fungi, might also affect endophyte and alkaloid concentrations.
Here, we performed an environmental study to test the hypothesis that mycorrhizal fungi (GM, GI) and N. lolii strains (CS, AR1) interact antagonistically in simultaneously infected ryegrass, and that the outcomes of these interactions depend on resource supply (P) and WSC content of the L. perenne host.