Host-specificity and Functional Diversity of Mycorrhizal Fungi

Host-specificity and Functional Diversity of Mycorrhizal Fungi

Arbuscular mycorrhizal fungi play a crucial role in plant health and ecosystem dynamics. This study by John Klironomos investigates the host-specificity and functional diversity of these fungi, revealing that while they are not host-specific, they exhibit significant functional variability. The research highlights the importance of different fungal species in enhancing phosphorus uptake and protecting plants from pathogens. Conducted at the Long-Term Mycorrhiza Research Site, the findings underscore the potential impact of mycorrhizal diversity on plant community structure and productivity. This work is essential for ecologists and agricultural scientists interested in plant-fungal interactions and ecosystem management.

Key Points

  • Examines host-specificity among arbuscular mycorrhizal fungi in plant interactions
  • Highlights functional diversity of mycorrhizal fungi affecting plant health
  • Investigates phosphorus uptake and pathogen protection roles of fungi
  • Conducted at the Long-Term Mycorrhiza Research Site in Ontario
  • Demonstrates implications for plant community structure and productivity
,
  • newtopiccyclegrowin
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Plant-Microbe Interactions
Host-specificity and functional diversity among arbuscular
mycorrhizal fungi
Klironomos, John
Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1 Canada
ABSTRACT
Arbuscular mycorrhizal (AM) fungi are mutualistic symbiotic associations between 150
species of fungi and the roots of approximately 300 000 species of plants. As a result of
this low fungus:host species ratio, it has been assumed that the fungi are not host-specific
and that there is a high functional redundancy among fungal species. In this study, fungi and
plants were isolated from an old field meadow and used in a series of experiments to test for
host-specificity and functional redundancy. AM fungi were not host specific - 96% of
combinations yielded successful colonization. However, there was strong evidence for
functional variability among fungal species. Differential responses in P uptake, protection
against non-mycorrhizal fungal infection, and productivity was detected for almost every
plant-fungus combination tested. The nature of the responses was highly fungus- and plant-
species specific. These results suggest that changes in the species composition of AM
fungal communities can strongly alter plant community structure.
Introduction
Few studies have explored the role of soil organisms, particularly mycorrhizas, in regulating
the diversity of plant species or guilds. This is despite the fact that most significant
ecosystem functions (ie. primary productivity, nutrient cycling, trophic patterns) are based
on feedback mechanisms between above and belowground organisms, and that mycorrhizal
fungi are known to act as critical mediators in many of these processes (Brundrett 1991;
Francis and Read 1994).
Many plant and fungal ecologists have assumed that AM fungi are functionally redundant
(Allen et al. 1995), at least with regard to plant fitness and community structure, and have
ignored the possible role of AM fungal diversity in regulating floral diversity and
productivity. Despite the proliferation of research on AM fungi, most of it has focused on
only a handfull of plant and fungal taxa (Klironomos and Kendrick 1993) and very few
researchers have examined feedbacks between the diversity of plants and AM fungi. Two
studies have examined the influence of plants on AM populations and both found that
different plant species stimulated different levels of AM propagation (Johnson et al. 1992;
Sanders and Fitter 1992), while a unique microcosm study (Grime et al. 1987) found that
plant diversity increased in response to a single AM species inoculation. A more recent
study (van der Heijden et al. 1998a) showed that plant species differed in their dependancy
on AM fungi and that specific fungal species had significantly different effects on plant
growth. Together, these studies suggest that different AM fungi play different roles within
ecosystems.
In this study we tested the hypothesis that AM fungi within a single habitat are not host-
specific in their ability to successfully colonize plant roots, but that AM fungal diversity is
Plant-Microbe Interactions
positively correlated with functional diversity of the mycorrhizae and that there may be little
functional redundancy.
Methods
The study site was the Long-Term Mycorrhiza Research Site (LTMRS) established in 1996
at the University of Guelph (43°32'30"N, 80°13'00"W). The site was originally farmed but
has not been disturbed since at least 1967. It is now considered an old-field meadow,
dominated by perennial grasses, asters, and goldenrods. Seeds from the following plants
were collected from the site:
Agrostis gigantea
Roth.,
Bromus inermis
Leysser.,
Achillea
millefollium
L.,
Aster novae-angliae
L.,
Chrysanthemum leucanthemum
L,
Daucus carota
L.,
Fragaria virginiana
Duchesne,
Plantago lanceolata
L.,
Rudbeckia hirta
L. and
Solidago canadensis
L. The following AM fungi were isolated from the site using trap
cultures (Brundrett et al. 1994) and maintained in pot culture on
Allium porrum
:
Acaulospora denticulata
Sieverding & Toro,
Acaulospora spinosa
Walker & Trappe,
Entrophospora colombiana
Spain & Schenck,
Gigaspora gigantea
(Nicolson &
Gerdemann) Gerdemann & Trappe,
Gigaspora margarita
Becker & Hall,
Glomus
etunicatum
Becker & Gerdemann,
Glomus intraradices
Schenck & Smith,
Glomus mosseae
(Nicolson & Gerdemann) Gerdemann & Trappe,
Scutellospora calospora
(Nicolson &
Gerdemann) Walker & Sanders, and
Scutellospora pellucida
(Nicolson & Schenck) Walker
& Sanders.
Experiment 1
The ten plant species were crossed with the ten fungal species for a total of 100
combinations which were replicated 5 times. Each pot received 15 g of leek-root inoculum
and three, 1 week old seedlings. They were cultured in a completely randomized design for
4 months in the greenhouse and received a weekly fertilization of half-strength Hoagland’s
solution. Roots were harvested and stained using Chlorazol Black E (Brundrett et al. 1984).
Compatibility was deemed positive if arbuscules or vesicles were found in the roots in at
least one of the five replicates.
Experiment 2
Five plants (
B. inermis
,
A. millefollium
,
A. novea-angliae
,
P. lanceolata
and
R. hirta
) and
fungi (
A. denticulata
,
G. etunicatum
,
G. intraradices
,
G. mosseae
and
S. calospora
) were
crossed and set up as in the previous experiment with the addition of a non-mycorrhizal
control for each plant species. After three months, two leaves were collected from each
plant and analyzed for %P (Olsen and Sommers 1982). A root subsample was assessed for
% colonization by arbuscules, vesicles and hyphae (McGonigle et al. 1990). Five grams of
substrate was collected and extraradical hyphae were assessed (Miller et al. 1995). The
remainder of the plant was dried at 60
o
C for 48hrs to determine dry weight.
Experiment 3
This was set up exactly as the previous experiment except that all the pots were further
inoculated with 1 x 10
6
conidia of
Fusarium solani
(Mart.) Sacc. isolated from the LTMRS.
This fungus was found to be a weak parasite/saprobe when associated with the plants in this
study. After 3 months, roots were harvested, stained with Chlorazol Black E and %
infection by non-AM fungi was determined.
In experiments 2 and 3, differences between mycorrhizal treatments and non-mycorrhizal
Plant-Microbe Interactions
Figure 1.
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FAQs of Host-specificity and Functional Diversity of Mycorrhizal Fungi

What is the significance of arbuscular mycorrhizal fungi in ecosystems?
Arbuscular mycorrhizal fungi are essential for nutrient cycling and enhancing plant health. They form symbiotic relationships with plant roots, facilitating the uptake of nutrients, particularly phosphorus. This symbiosis can significantly influence plant community dynamics and ecosystem productivity. By improving nutrient availability, these fungi help plants thrive in various environments, making them critical for sustainable agriculture and natural ecosystems.
How does the study measure the functional diversity of mycorrhizal fungi?
The study measures functional diversity by assessing the responses of different plant species to various arbuscular mycorrhizal fungi. It evaluates parameters such as phosphorus uptake, plant growth, and resistance to pathogens. By crossing multiple plant and fungal species, the research identifies specific interactions that enhance plant health. This empirical approach provides insights into how different fungi contribute uniquely to plant nutrition and protection.
What were the main findings regarding host-specificity in the study?
The study found that arbuscular mycorrhizal fungi are largely not host-specific, with 96% of plant-fungal combinations resulting in successful colonization. However, it also revealed significant functional variability among fungal species, indicating that while they can colonize many plants, their effects on plant health and productivity can differ greatly. This suggests that the diversity of mycorrhizal fungi is crucial for maintaining healthy plant communities.
What impact do arbuscular mycorrhizal fungi have on plant community structure?
Arbuscular mycorrhizal fungi can significantly influence plant community structure by affecting which plants thrive in a given environment. Different plant species exhibit varying levels of dependency on these fungi for nutrient uptake and pathogen resistance. As a result, changes in mycorrhizal fungal diversity can lead to shifts in plant species composition and overall ecosystem health. This highlights the importance of preserving fungal diversity in natural and agricultural systems.
What methods were used to assess the interactions between plants and mycorrhizal fungi?
The study employed a series of controlled greenhouse experiments where ten plant species were crossed with ten different arbuscular mycorrhizal fungi. Each combination was replicated multiple times to ensure reliability. The researchers measured parameters such as root colonization rates, phosphorus content in plant leaves, and resistance to non-mycorrhizal pathogens. These methods provided a comprehensive understanding of the functional roles of different mycorrhizal fungi in plant health.

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