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- Methods and Materials
Arbuscular mycorrhizal fungi (AMF) (Division Zygomycetes, Order Glomales) are involved in an important mutualism with most terrestrial plants. Differences in morphological and developmental traits of AMF have been used to classify them into seven genera and five families (Morton & Redecker, 2001). It is not yet clear whether existing taxonomic groups, which are based almost exclusively on morphological traits, are useful predictors of their ecology. There is some evidence for different ecological strategies among AMF, both in terms of their function in ecosystems (Newsham et al., 1995; van der Heijden et al., 1998) and on a smaller, physiological and morphological scale. For example, AMF isolates have recently been shown to differ with respect to phosphorus metabolism in extra-radicle hyphae (Boddington & Dodd, 1999) and also in their nutrient transfer efficiencies (Smith et al., 2000). However, studies that examine an array of AMF species from different taxonomic groups are lacking and life-history strategies for AMF have yet to be described.
A life-history strategy provides an ecological description of how an organism fulfills its life-cycle requirements. A life-history strategy can be described using a wide range of species traits but usually these include colonizing ability, dispersal ability, tolerance of stress and disturbance, investment into reproduction vs vegetative growth, and mode of reproduction (Grime, 1977; Pianka, 1970).
In this paper we focus on one aspect of AMF life-history; namely, colonizing ability. Our objectives are to describe the colonizing strategies of some AMF and to determine whether there is a taxonomic basis for variation in colonization ability among AMF. Colonization ability includes both the rate and extent of colonization. These are commonly used measures of AMF activity so they make a good starting point for a description of AMF life-history strategies. Because AMF colonize plant roots and soil, we studied the rate and extent of colonization of both plant roots and soil by 21 different AMF from three families (Acaulosporaceae, Glomaceae and Gigasporaceae). Specifically, we addressed the following four questions:
Do taxonomic groups differ in colonization rate?
It is generally believed that members of Glomaceae and Acaulosporaceae have a highly infective extra-radicle mycelium whereas members of Gigasporaceae regenerate most frequently from spores (Tommerup & Abbot, 1981; Biermann & Lindermann, 1983; Morton, 1993; J. N. Klironomos & M. M. Hart, unpublished). We predicted that members of the Gigasporaceae would colonize plant roots more slowly than members of either the Acaulosporaceae or the Glomaceae (Table 1). We suspected that a spore might require specific conditions or a dormancy period for regeneration. Therefore, AMF that depend on spores for colonization are likely to be at a disadvantage compared with AMF that can colonize immediately from hyphal fragments.
Table 1. Predicted differences in the rate, extent and location of colonization by three arbuscular mycorrhizal fungi (AMF) families
Do taxonomic groups differ in colonization extent?
Taxonomic differences among AMF in hyphal structure and mycelial architecture are well known. Members of the Glomaceae and Acaulosporaceae tend to have very delicate, diffuse hyphae whereas members of the Gigasporaceae tend to have robust, densely aggregated hyphae (Jakobsen et al., 1992; Smith et al., 2000). Such differences could be the result of differences in hyphal longevity. Delicate, diffuse hyphae may simply be shorter-lived than robust, densely aggregated forms. This is based on the premise that large-bodied organisms have slower rates of turn-over than small-bodied organisms. We predicted that AMF with delicate, diffuse hyphae (i.e. Glomaceae and Acaulosporaceae) would be limited in the extent of their colonization. Conversely, AMF with robust, densely aggregated hyphae (i.e. Gigasporaceae) would be better able to create a large mycelium (Table 1).
Do taxonomic groups differ in the extent of root vs soil colonization?
AMF may have two very different colonization patterns: mycelia primarily within roots vs mycelia primarily within soil. Based on our personal observations, we predicted that members of Glomaceae and Acaulosporaceae should colonize roots more profusely than members of Gigasporaceae. Conversely, members of the Gigasporaceae should colonize soil more than members of the Acaulosporaceae or Glomaceae (Table 1). This prediction is based on the assumption that AMF cannot invest heavily in both intra- and extra-radicle mycelia simultaneously. We reasoned that it is unlikely that a plant host could sustain a symbiosis involving extensive carbon drain.
Do taxonomic groups differ in both rate and extent of colonization?
Based on the arguments made above, we predicted that AMF differ in both their rate and extent of colonization. It is important to establish the relationship between colonization and extent for AMF because ecological theory dictates that there is a necessary trade off between the two. That is, an organism cannot be, at once, a quick and extensive colonizer (Grime, 1977; Pianka, 1970).
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- Methods and Materials
We found evidence for distinct AMF colonization strategies that are related to taxonomic differences at the family level. These different strategies were evident regardless of the host plant involved.
AMF isolates showed large differences in the rate at which they initially colonize plant roots. Some isolates in our study colonized roots within 1 wk while other isolates required an additional 7 wk to colonize roots. The biological basis for this variation requires further study but the pattern of colonization we observed was consistent with our prediction that AMF regenerating primarily from spores (i.e. members of the Gigasporaceae) would be the slowest colonizers. Spore dormancy and specific environmental requirements for spore germination presumably slow the rate at which spore-regenerating AMF can colonize plant roots. Our prediction that AMF regenerating primarily from hyphal fragments (i.e. members of the Glomaceae and Acaulosporaceae) would initially colonize roots faster than spore-regenerating AMF was only partly correct. As predicted, Glomaceae isolates were generally the fastest colonizers of plant roots. Members of the Acaulosporaceae are generally assumed to have equally infective mycelia as Glomaceae (Morton, 1993). However, we found that Acaulosporaceae isolates generally colonized roots more slowly, at a rate more similar to spore-regenerating Gigasporaceae. Our results are consistent with those of Tommerup & Abbott (1981) who found that an Acaulospora spp. was unable to form a new infection from a root fragment containing hyphae whereas a Glomus spp. successfully colonized from root fragments.
The differential rate at which AMF isolates intercept a root may help to explain why some AMF have ‘parasitic’ effects on host plants in pot experiments (Francis & Read, 1995; Klironomos, 2000). ‘Parasitic’ AMF may simply be slow-colonizers and thus more demanding of plant resources for a longer time than faster colonizers.
The differential rate of colonization by AMF also has implications for methods of isolating AMF in the field. Current practices involve successful colonization of ‘trap plants’ by indigenous AMF and their subsequent sporulation. Based on our results, trap cultures that run less than 8 wk may be biased in favour of fast colonizing AMF. To represent all AMF in a community, it is important to allow adequate time for colonization of slow and fast colonizing isolates.
AMF isolates also showed large differences in the extent to which they colonize plant roots and soil. While there is some variation within families in the amount of root colonization, overall there are strong, family based trends. Based on the sum of root plus soil fungal biomass (ergosterol), Gigasporaceae isolates had the highest mean (±1 SE) extent of colonization (2.62 ± 0.04 µg g−1 root plus soil d. wt), followed by Glomaceae isolates (0.58 ± 0.09 µg g−1 root plus soil d. wt) and Acaulosporaceae isolates (0.55 ± 0.09 µg g−1 root plus soil d. wt). This pattern is consistent with our prediction that Gigasporaceae isolates would have the most extensive mycelium based on its traits (i.e. robust, densely aggregated hyphae). AMF families with more delicate, diffuse hyphae were predicted to have less extensive colonization. Acaulosporaceae isolates were consistent with this prediction but a surprising number of Glomaceae isolates showed more extensive colonization than Acaulosporaceae isolates. The reason for this difference is not clear. Our results are consistent with a study that examined the growth of extra-radicle mycelia in three AMF (Glomus spp., Acaulospora laevis, Scutellospora calospora) (Jakobsen et al., 1992). They, too, found that Scutellospora had the most extra-radicle mycelium and that it was concentrated closer to the root system compared with the more diffuse mycelia of the Glomus spp. and A. laevis.
AMF isolates also differed substantially in their colonization of roots vs soil. Based on previous observations, we predicted that members of the Gigasporaceae would colonize soil more extensively than plant roots and, conversely, members of the Acaulosporaceae and Glomaceae would colonize roots more extensively than soil. Data for the 21 AMF isolates studied here are largely consistent with this prediction. Such large differences among isolates in root vs soil colonization have potentially important implications for their role as a plant mutualist. A high ratio of soil to root colonization may provide the plant host with the greatest benefit (increased potential for nutrient transfer from root to soil). Conversely, a low ratio of soil to root fungal biomass may provide the host plant with little net benefit because nutrient transfer may be limited by the extent of the absorptive hyphal network. In any case, our data confirm that AMF isolates exhibit a wide range of colonization strategies with respect to root vs soil colonization.
These results suggest that community level studies that use only root measurements of AMF to characterize the AMF community may be biased against AMF, which have less mycelium in roots than in soil (i.e. Gigasporaceae spp.). Both internal and external measures of colonization are important for an accurate representation of an AMF community.
By combining measures of AMF colonization ‘rate’ and ‘extent’, we found that AMF isolates from three taxonomic families could be reliably separated. In other words, AMF taxonomy has a functional basis as well as morphological and developmental basis. The strong relationship between AMF colonizing strategy and taxonomy suggests that the taxonomic status of an AMF may be a useful predictor of its ecology, at least in terms of colonizing ability. Members of the Glomaceae usually contact roots quickly and produce extensive mycelium in roots compared with in soil. Members of the Gigasporaceae typically contact roots more slowly and establish an extensive mycelium in soil rather than in roots. Members of the Acaulosporaceae also contact roots more slowly and establish a much less extensive mycelium in either roots or soil than members of the other two families.
It remains to be seen how these functional groups of AMF differ with respect to mycelial architecture. We documented major differences in the internal and external mycelium among different AMF. It is unclear, however, exactly how they are different in terms of mycelial architecture. For example, Friese & Allen (1991) characterized an assemblage of AMF in this way, differentiating absorptive and runner hyphae from hyphal bridges, germ tubes spores and root fragments. In our study, Gigaspora and Scutellospora isolates had extensive external mycelia. Because it has been shown that their mycelia are not infective (Biermann & Lindermann, 1983; Brundrett et al., 1999, J. N. Klironomos & M. M. Hart unpublished) it may be that these genera have mycelia that are lacking in infective, runner hyphae (Friese & Allen, 1991) and form many absorptive hyphae instead. Glomus isolates, conversely, have a highly infective mycelium and thus might have a mycelium comprised of runner hyphae with little absorptive hyphae. As such, Gigasporaceae may be more ‘mutualistic’ because they provide the most nutritive benefits for their hosts due to a heavy investment into primarily absorptive hyphae.
While it is clear that AMF have distinct functional groups, it is unclear whether these functional groups have any bearing on host benefit. That is, are certain functional groups of AMF better at nutrient acquisition than others? For instance, because Gigasporaceae species have a larger soil mycelium, does that increased surface area result in more nutrients for the host? Or are the AMF with high root colonization (Glomaceae) more effective at nutrient uptake due to the higher incidence of arbuscules? It is unclear at this point whether the arbuscule or the absorptive mycelium is a more limiting factor for host nutrient acquisition. Further, such large differences in mycelium sizes might mean that some AMF are more ‘costly’ to their hosts than others. That is, AMF with large mycelia may be better at nutrient uptake but may pose a larger carbon-sink than AMF with small mycelia (i.e. Acaulosporaceae). Because AMF and their host plants are so closely linked, it will be important for future studies to address the significance of AMF functional groups in host plant response.