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CHOICE OF HOST PLANT SPECIES

Choice of host plant in the propagation of arbuscular fungi in pot cultures is an important decision. The obligate nature of the symbiosis prevents separation of host variables from those of soil and ambient environments as they impact interactively on growth and sporulation of colonizing fungi. A host species can switch from being compatible to incompatible with a change in only one environmental variable (e.g. phosphorus level, soil water content, pH, salinity, temperature, intensity and quality of light, etc.). As a result, a single host species universally compatible in all environments is not likely to be found. The first paper which directly addressed the methodology of open pot culture propagation of arbuscular fungi (Gilmore, 1968) recommended strawberry ( Fragaria species). This host had been used previously by Barbara Mosse at the same location (Rothamsted Experiment Station, England) and was shown to support growth of six fungal species (designated E2-E7). The range of plant species used since then are too numerous to list. Common temperate hosts included corn ( Zea mays), onion ( Allium species), and peanut ( Arachis hypogaea). Widely-used tropical hosts included Stylosanthes species, bahia grass ( Paspalum notatum) and kudzu ( Pueraria phaseoloides).

General properties of an optimum host species

  1. Mycotrophic (obviously), but with obligately mycotrophic hosts tending to support the widest range of fungal genotypes
  2. Compatible with greenhouse climate (growth in a potting medium, lack of UV radiation, constant watering, temperature fluctuations, etc.)
  3. Tolerant of a wide range of soils varying in nutrient composition and physical structure
  4. Rate of root growth is moderate so that complete infiltration of a pot culture takes at least 2-3 months
  5. Branching of roots is extensive and usually is colonized by mycorrhizal fungi
  6. High photosynthetic efficiency with a high P requirement (C4 metabolism)
  7. Resistant to a wide range of insect pests, pathogens including soil-borne nematodes.

The two largest collections of arbuscular fungi in the 1980’s (INVAM, directed by Norm Schenck and CIAT, directed by Ewald Sieverding) consisted of > 1000 isolates of a wide range of arbuscular fungal species. They were propagated on different standard hosts, although both were tropical species: bahiagrass and kudzu, respectively. Being tropical, they fluorish in greenhouses with high day and nightime temperatures and high light intensities with long daylengths. Despite their proven compatability with many arbuscular fungi, neither host could be used in West Virginia greenhouses because growth slowed or ceased altogether when air temperatures dropped below 20oC.

When cultures were started at WVU (pre-INVAM days), the first two hosts used were red clover ( Trifolium pratense) and meadow fescue ( Festuca arundinacea). The former proved suboptimal because it was too susceptible to common greenhouse insects and too sensitive to manganese (which solubilized in our acidic soils after steaming or autoclaving). Meadow fescue yielded highly variable results among fungal strains, even those collected from similar habitats. Then a switch was made to sudangrass ( Sorghum sudanense), based on a recommendation by Barbara Daniels Hetrick. She had successfully cultured Glomus epigaea (= Diversispora epigaea) and other fungi at Kansas State University. After almost 30 years of using sudangrass, we now know that in our greenhouse environment, this host plant is compatible with fungal species in all genera described to date and from a wide range of habitats.

Under conditions of low light intensity and cooler temperatures, more success was obtained with Plantago lanceolata in small pots either open or in closed Sunbags (Walker and Vestberg, 1994). Plantago appears to be well suited to conditions of high humidity and constrained space. However, this system is unworkable in greenhouses with high light and warm-hot temperatures.

In the examples above, obvious differences in environmental conditions were major determinants in host-fungus compatbility. However, what about situations where the differences are not so obvious? For example, Frank Pfleger and Jim Kurle at the University of Minnesota have greenhouse conditions resembling those at West Virginia, but sporulation on sudangrass was inconsistent and therefore unpredictable. In the winter season, supplemental lighting with HID lamps in the greenhouse did not improve matters. These researchers discovered that big bluestem ( Andropogon gerardii) was a more optimal host for their greenhouse conditions. Mycorrhiza formation and sporulation were more consistent throughout the year and in growth media with or without soil. This host has many of the same attributes of sudangrass: a C4 plant, high mycorrhizal dependency, prolific root system, a perennial habit (regrowth after cutting back of shoots), and drought tolerance. Specific reasons for the more consistent performance of this host relative to sudangrass have not been found. Big bluestem has a slightly greater tolerance to high ambient air temperatures (up to 40oC) and is resistance to all of the major greenhouse insect pests (especially aphids, whiteflies, mites). A methodological difference also may be important: big bluestem cultures were started from 10-day-old seedlings grown from surface-sterilized seed in moist vermiculite at 28oC and then transplanted. INVAM cultures are started from seed, often at a density higher than normal to keep plants short and to prevent splashing between pots.

Other situations exist where greenhouse conditions are very different from those at WVU, but sudangrass has proven to be a good host. For example, Jean Stutz routinely has used sudangrass to grow monospecific cultures as well as trap cultures of fungi from the Senoran and Chihuahuan deserts in her greenhouse in Phoenix, Arizona (Stutz and Morton, 1996). Some host plants are chosen because they have been used historically in experimentation of mycorrhizal phenomena. Onion was an early host partly for this reason (Mosse and Hayman, 1971; Mosse et al., 1969). Lynette Abbott and coworkers at Perth, Australia routinely used subterranean clover ( Trifolium subterraneum) as a culture host because of compatability with a diverse range of fungal strains from Australian soils.

It is important to keep in mind that choice of host also can differ with objective. In general, we use pot cultures for three purposes:

  1. trapping as many fungal organisms as possible that are indigenous to a field soil
  2. establishing each organism of a species as unique accession (or culture)
  3. increasing inoculum of each organism for use internally or for distribution to users.

In our greenhouses at WVU, sudangrass is optimal for (1) and (3), but not for (2). Our move to use sorghum as the “purification host” (carefully picked spores of one morphotype placed directly on roots of a seedling transplant) was serendipitous due to an exhausted supply of sudangrass. Our success rate jumped from an average of 60% to 95% (unpublished). We attribute the better performance of sorghum to greater root hardiness and hence less transplant shock and premature root senescence. Sorghum is just as optimal as sudangrass in (1) and (3), but it is more sensitive to overcrowding in pots.

Our experience suggests that corn is an optimal host for (2) and (3) under some greenhouse conditions, but it tends to limit the number of different species in a fungal community in trap cultures (at least in relation to sudangrass). We suspect that some causes are more rapid root growth rate and a lower overall root length in pots. These factors would tend to favor colonization and sporulation of those fungi in the mix with more infective propagules or more rapid growth rate after ingress. Similarly, onion has been shown to be a good host for culture of individual organisms at many locations, but it is less optimal as a trap culture host possibly because of both slow root development and lower root biomass and length than either corn or sudangrass. Roots also produce volatile compounds whose effects on fungal growth and development are not known.

Plantago lanceolata, while functioning well as a host of single organism in open or enclosed pots (see above), has not been tested for its ability to support sporulation by all fungal organisms in a species mixture. The small pots required for Sunbags almost assure a loss of diversity in trap cultures. We find that 3-5 fungal species from various habitats recovered in 15-cm diameter pots are reduced to 1-2 species when the same soil is placed in 150 cm3 conetainers, regardless of host. We hypothesize that, in this situation, the full complement of species diversity may not be lost, but the poor colonizers have just failed to sporulate. Similar considerations come into play for other hosts such as strawberry and subterranean clover. Regardless of host, we suggest that a pot volume of one liter or greater be used in any trapping regime.

Tests of a host’s trapping ability should involve either use of a preexisting trap culture (either established locally, by a colleague or from a collection) of known species diversity or a defined “cocktail” of known monospecific cultures. A field soil should not be used because the number of species present may not be accurate, depending on the number of nonsporulating fungi (which can be up to 80-100% of the species present in some environments).

We now are very cautious in making host recommendations, because our experiences and results are not applicable to all situations. In general, however, we suggest C4 over C3 grasses, legumes if they are resistant to insect pests, and perennial or long-lived annual plants.

References

Gilmore, A. E. 1968. Phycomycetous mycorrhizal organisms collected by open-pot culture methods. Hilgardia 39:87-105.

Morton, J. B., S. P. Bentivenga, and W. W. Wheeler. 1993. Germ plasm in the International Collection of Arbuscular and Vesicular-arbuscular Mycorrhizal Fungi (INVAM) and procedures for culture development, documentation and storage. Mycotaxon 48:491-528.

Mosse, B. and D. S. Hayman. 1971. Plant growth responses to vesicular-arbuscular mycorrhiza. II. In unsterilized field soils. New Phytologist 70:29-34.

Mosse, B. D. S. Hayman, and G. J. Ide. 1969. Growth responses of plants in unsterilized soil to inoculation with vesicular-arbuscular mycorrhiza. Nature 224:1031-1032.

Stutz, J. C. and J. B. Morton. 1996. Successive pot cultures reveal high species richness of arbuscular endomycorrhizal fungi in arid ecosystems. Can. J. Bot. 74:1883-1889.

Walker, C. and M. Vestberg. 1994. A simple and inexpensive method for producing and maintaining closed pot cultures of arbuscular mycorrhizal fungi. Agric. Sci. Finland 3:233-240.