Northern Suburbs Branch
Dr Mark Brundrett asked to delay his presentation until March so that he could utilise material from a very recent paper he co-authored entitled, Evolutionary history of mycorrhizal symbioses and global host plant diversity1.
Mark split his presentation into four parts, commencing with an overview of Australia’s climate, soils, landforms and plants. It was not surprising to learn that with less winter and more summer rainfall, our Southwest Mediterranean Climate Zone was shrinking. However climate is just one of the seven factors that Mark linked to plant diversity, the others being topography, soil complexity, adverse soil properties, landscape age, hydrology and fire.
In the second part we were introduced to mycorrhizal associations. In a mycorrhizal association the fungal hyphae of an underground mycelium are in contact with plant roots, but without the fungus parasitizing the plant. Based on extensive research, an estimated 92 percent of all plants have mycorrhizal association.
There are four main types of mycorrhizas based on the criteria of morphological differentiation of root tissues and host plant lineages, all of which Mark illustrated with superb high-magnification photos.
| arbuscular mycorrhizas (AM)
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|Fungal hyphae penetrate the cortical cells of the roots of a vascular plant to form arbuscules (“little trees”)
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| ectomycorrhizas (EcM)
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|The hyphae form a mantle around the root and also grow into the spaces between root cells – but do not penetrate the root cells. The hyphae form a net-like covering, called a Hartig net.
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| Ericoid mycorrhizas (ErM)
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|Only occurs in Ericaceae. Hyphal coils in cells, separately colonized from root surface. Formed in narrow “hair roots”
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| Orchid mycorrhizas (OrM)
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|Only occurs in Orchidaceae. Coils of hyphae within root cells. Old coils are digested by plant.
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The 8 percent of plants which are non-mycorrhizal are mainly nutritional specialists like halophytes, epiphytes or live in artic or alpine regions, or are habitat specialists such as carnivorous and parasitic plants, or plants with cluster roots like banksias.
Mark then led us through a set of case studies showing the variation in mycorrhizal plant diversity, from a worldwide to a Western Australian regional scale. Western Australia has the largest percentage of ectomycorrhizal plants of any region. A comparison of over 10,000 mycorrhizal and non-mycorrhizal plant species in similar sized areas in south-eastern Australia and south-western Australia, showed the plant diversity was greater in the south-west for all categories except for orchid mycorrhiza, which had similar diversity.
In the last part of his talk, Mark took us through his recently published work on the root evolution in plant families. There appear to have been three waves of mycorrhizal evolution, starting with AM in early land plants, probably in the Ordovician, at least 450 million years ago (>450Ma). The second wave was characterised by the emergence of multiple new NM or EcM linages, ericoid and orchid mycorrhizas but with root types usually being consistent within families. For most families this started in the Cretaceous (145 -66Ma). The third wave, which is recent and ongoing, has resulted in root complexity linked to rapid plant diversification in biodiversity hotspots, peaking in the past 30Ma.
So why is the Southwest Biodiversity Hotspot unique globally? Mark provided the following reasons:
- Most rapid diversification occurs in plants with complex roots.
- We have higher diversity of EcM and NM plants than anywhere else.
- We are a global diversity hotspot for carnivorous plants.
- Complexity exists within genera such as Melaleuca and Acacia here but is extremely rare elsewhere.
- We also are a global hotspot for highly specialised pollination.
- Our ancient landscapes provide a preview of soils for other regions.
Don Poynton
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Mark C. Brundrett and Leho Tedersoo (Natural History Museum, University of Tartu, Estonia), New Phytologist, (2018) (open access)