FUNGAL FORM AND FUNCTION
BY FELISA ROGERS
OVER 400 MILLION years of coevolution between fungi and plants has resulted in one of the most complex and important associations in the natural world, which fueled the rapid land colonization of photosynthetic plants. This association between vascular plants and fungi is the foundation of the earth’s forested ecosystems, from the temperate forests of North America to the subtropical and tropical forests of Southeast Asia. There are examples of mycorrhizal symbiosis in every ecosystem that is dominated by vascular plants.
The mycorrhizal association and morphology are highly diverse. There are examples of mycorrhizal fungi that only associate with genera from specific plant families; for example, the families Ericaceae and Orchidaceae host only a few species of mycorrhizae, and the infection morphology is specific to that association as well. Current estimations state that greater than 90% of vascular plants have a mycorrhizal association. One notable exception is the plant family Brassicaceae, which has not been observed forming a mycorrhizal association. Mycorrhizae have even been observed in association with non-vascular plants, such as bryophytes and hepatophytes (Smith S.E. et al.).
There are several types of mycorrhizae root morphology. The most commonly known in the industry are ectomycorrhizae and endomycorrhizae. Ectomycorrhizae typically associate with woody plants, more specifically conifers and hardwoods, and form a sheath or mantle of hyphae around root tips. The hyphae grow between the epidermal and cortical cells in a structure called the Hartig net. This is the nutrient/photosynthate interface for both
Arbuscular mycorrhizal culture plant and fungus. The Hartig net will form invagination within the epidermal tissue that will inhibit vesicular-arbuscular mycorrhizal (VAM) infection on the root tips (Smith, S.E. et. al.). Ectomycorrhizae are the fungi that one often observes fruiting in forest ecosystems and comprise many of the edible gourmet mushrooms. Ectomycorrhizae are typically used in the forestry industry to inoculate reforestation seedlings.
Endomycorrhizae primarily associate with herbaceous plants and grasses. Unlike ectomycorrhizae, their hyphae penetrates into the root cortex and individual root cells forming arbuscules. Arbuscules are the fungal organs that absorb photosynthates and deliver water and phosphorus into the root cell cytosol. Endomycorrhizae are typically used in the horticultural and agricultural industries as growth enhancers. We will primarily be focusing on vesicular-arbuscular mycorrhiza (VAM), which is the most common type of mycorrhiza being applied in cannabis horticulture and agricultural settings.
The role of VAMs in an ecosystem is to aide in the translocation and assimilation of macro and micronutrients and supply water to the photobiont root system from the soil. VAMs are typically found in ecosystems that have limited nutrition or water. Mycorrhizae also increase the range and surface area of the rhizosphere, which gives the photobiont a greater ability to acquire nutrients and facilitates water up-take that reaches far beyond the physical boundaries of the rhizosphere. VAMs also contribute to soil structure by synthesizing a compound called glomalin, which aids in the production of soil aggregates.
VAM inoculation on cannabis root systems has tremendous potential; however, there has been a long history of inaccurate information regarding mycorrhizae and their association with
a cannabis host. For example, VAM inoculation of the root zone takes several weeks, and if the horticulturist continues to fertilize with high amounts of phosphorus it will have a deleterious effect on spore germination and hyphal growth. This will obviously reduce VAM infection rates and the efficiency of symbiosis if infection occurs. With that being said, it is up to the horticulturalist to create the correct environment that will benefit the symbiosis between the photobiont and mycobiont. With the right media, fertilizer application, and, most importantly, a complex soil microorganism community, one will experience a positive effect when using VAMs in their system. VAMs should help to increase phosphorus assimilation, mitigate drought and transplant stress, and aid in the prevention of deleterious bacterial and fungal root pathogens. The mycorrhizae association will benefit most in an organic soil media that has either a coco base or peat base that is capable of hosting a diverse microorganism community. Hydroponic systems and inert medias can be inoculated with VAMs; however, there are some limited factors that will inhibit infection rates and the growth of mycorrhizae. Some of these issues are desiccation, continued exposure to an aqueous environment, and excessive exposure to high-phosphorus fertilizers. With that said, VAMs have great potential within the cannabis industry if the correct information is conveyed to horticulturists and an in-depth understanding of mycorrhizae function, physiology, and the limitations of the symbiosis is achieved.
We exist in an exciting and new age of cannabis-based academic and private research in regards to VAMs and understanding complex rhizosphere community ecology. The potential benefits VAMs hold for field industrial hemp and medicinal and recreational cannabis are endless.