Host Plant Choices

Sudangrass

(Preferred Host Plant)

Host plant identity can differentially impact AM fungal sporulation (Bever et al. 1996). Currently INVAM uses Sudangrass (Sorghum × drummondii) as a suitable host plant for our cultures. Some species of mycorrhizae may benefit from being cultured with ecosystem-specific host plants. The Bever-Schultz lab also use highly responsive perennial prairie plants (Koziol & Bever 2015) to grow native prairie-sourced mycorrhizal strains. 

Further Considerations

(of host choice, courtesy of Joe Morton)

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. 

General Properties

(of an optimum host species)

  1. Mycotrophic (obviously), but with obligately mycotrophic hosts tending to support the widest range of fungal genotypes

  1. Compatible with greenhouse climate (growth in a potting medium, lack of UV radiation, constant watering, temperature fluctuations, etc.)

  1. Tolerant of a wide range of soils varying in nutrient composition and physical structure

  1. Rate of root growth is moderate so that complete infiltration of a pot culture takes at least 2-3 months

  1. Branching of roots is extensive and usually is colonized by mycorrhizal fungi

  1. High photosynthetic efficiency with a high P requirement (C4 metabolism)

  1. Resistant to a wide range of insect pests, pathogens including soil-borne nematodes

 

After almost 30 years of using Sudangrass, we now know that in our greenhouse environments, this host plant is compatible with fungal species in all genera described to date and from a wide range of habitats. 

Plantago lanceolata in small pots with either open or closed Sun bags (Walker and Vestberg, 1994) proved more successful under conditions of low light intensity and cooler temperatures. Plantago also appears to be well suited to conditions of high humidity and constrained space. However, this system is unusable in greenhouses with high light intensity and warm to hot temperatures. 

Keep in Mind

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 indigenous fungal organisms from field soil as possible

  1. Establishing each organism of a species as unique accession (or culture)

  1. 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 a 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 organisms 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 Sun bags 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 cone-tainers, 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 the 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. 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

  • Bever, J. D., Morton, J. B., Antonovics, J., & Schultz, P. A. 1996. Host-dependent sporulation and species diversity of arbuscular mycorrhizal fungi in a mown grassland. Journal of Ecology. 84: 71-82.

  • Koziol, E & JD Bever.  2015.  Mycorrhizal response trades off with plant growth rate and increases with plant successional status.  Ecology.  96:1478–1484.

  • 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.