Mycorrhiza (ErM & OM)

145 - 66 mya Cretaceous

Paleosols Flowering Plants Leaves Roots Animals

Mycorrhiza

The mycorrhiza we first came across, over 200 million years earlier, endomycorrhiza, are also called Arbuscular mycorrhizas (AMF) and are the most widespread type . The later Ectomycorrhizas (ECM), a mere 50-100 my earlier, occur in certain families of gymnosperms and, dicotyledons and in one monocotyledon genus. The remaining types of mycorrhizas are restricted to specific plant families, about four of them, including heathers and orchids. Certain angiosperm families have plants with nonmycorrhizal roots. (Brundrett 2002)

Vesicular-arbuscular mycorrhiza (VAM) is formed by the symbiotic association between certain phycomycetous fungi and angiosperm roots. Two groups of new mycorrhiza appeared in this period.

Heathers

One group developed relating with heathers, and so are called Ericoid mycorrhizas (ErM) . Molecular clock estimates indicate the first of these to be around 140mya. (Cullings 2011) Ericaceae species occupy at least some habitats on all continents except Antarctica, indicating a later development. They have been shown to have enzymatic capabilities to break down complex organic molecules, which may allow some ErMs to consume dead matter.

These enzymes could access organic forms of nutrients where there is nitrogen, not otherwise available in its soluble mineral form, in habitats where heathers grow. This new ‘symbiosis’ would help adaptation to acidic and nutrient poor soils that heather species typically inhabit, like boreal forests, moors, bogs, and heathlands. This may reflect a widening ‘terrestrialisation’ of poorer terrain in this period.

Ericoid mycorrhiza

Orchids

The second new mycorrhizas are those associated with orchids. Orchids are considered by many to be the most beautiful of all plants. That is why we know more about their mycorrhizal relations than most others. The fungi are crucial to their growing. “Orchids are among the most endangered in the plant kingdom. Lack of endosperm in their seeds renders orchids to depend on nutrients provided by orchid mycorrhizal fungi (OM) for seed germination and seedling formation in the wild. OM that parasitize in germination seeds is an essential element for orchid seedling formation, which can also help orchid reintroduction."(Zhao et al 2021)

The key to this relationship may be that plants developed ‘polyploidization, where there are more than two sets of chromosomes. This could lead to an increased adaptive potential. Data from a large number of plant genomes suggest that a wave of whole‐genome duplications (WGDs) occurred in angiosperms close to the end of this period - 100-50mya. Changes such as these could have driven adaptive success in both plant and fungi.

Heather and orchid mycorrhizas, as well as non-mycorrhizal roots, evolved during the period of rapid angiosperm radiation between 150 – 50 mya (Cretaceous) . Roots may have gradually evolved from rhizomes to provide more suitable habitats for mycorrhizal fungi and provide plants with complex branching and leaves with water and nutrients. (Brundrett 2002)

Co-evolution with plants

The coevolution of plants and fungi has played a powerful role in plant and fungal evolution, although it seems that the plants diversified more than the fungi as a result of the relations. The relation between orchid–Cantharellales is evolutionarily significant for several reasons. The fungal component shifts from the fungial phylum Glomeromycota (asexual), responsible for endomycorrhizaz (AM) to Basidiomycota (filamentous sexual) occurred four times in vascular plants, including the heather and orchids. Among the Basidiomycota–vascular plant symbioses, only orchids form relations with three of the main Basidiomycota clades.

Evolution

Some orchids’ have an association with form of Basidiomycetes (rhizoctonias) that is probably the ancestral symbiosis that persists in most existing orchids. During orchid evolution numerous secondary transitions occurred to other fungal taxa. Both the rhizoctonia partners and the secondarily acquired ones are from fungal taxa that have broad endophytic ability.

Signalling

Based on mechanistic similarities between OM and AM, and recent findings on orchids' endophytes, orchids may have developed new signalling methods (Favre-Godal et al 2020). Orchids can excrete plant molecule signals such as strigolactones and flavonoids, and in response, OM fungi would secrete mycorrhizal factors (Myc factors) to activate the common symbiosis genes (CSGs). Fungi could also overcome the defense mechanism by secreting phytohormones to disrupt the ‘crosstalk’.

Orchid mycorrhizal fungi belonging to the Cantharellales obtain nutrients by consuming detritus or pathogenic parasites (Yukawa et al 2009) This opens up all sorts of new habitats, compared with other kinds of mycorrhizal relations which are ‘mutualistic symbioses’ instead of ‘obligate saprophytism’.

Endophytes

Endophytes are fungi, or bacteria, that live within (endo) healthy plant (phyte) tissue living above-ground, in leaves flowers and stems. You can imagine this would have been an ideal period for endophytes to diversify into all those new veined leaves, new stems, and now flowers.

They do not damage the plant, instead can improve plant nutrition uptake by several mechanisms including formation of extra-root hyphae for nutrient absorption, stimulating root growth, altering plant metabolism to promote nitrogen and phosphate uptake, nitrogen fixation, and modifying soil directly or altering root exudates. They can infect multiple host genera yet exhibit genotype specificity within a species. "Soil is considered the dominant environment from which bacterial endophytes originate, and soil-to-root is the best-studied horizontal transmission route". (Frank et al 2017)

The study of endophytes is increasing as we realise the roles microbes inside plants may have in defending plants in any crop management schemes. In particular there is a need to evaluate what are called ‘endophyte assembly rules’ and how this may help to fine-tune crop management strategies (Saunders et al 2010).

Transmission routes for bacterial endophytes across the life cycle of an apple tree. (A) Vertical transmission via seed; (B) Colonization of the spermosphere, depicted as the grey area surrounding the seed; (C) Colonization of developing reproductive organs via the shoot apical meristem as part of vertical transmission; (D) Colonization of root from soil; (E) Colonization of leaves though stomata after transmission via air; (F) Transmission via sap-feeders; (G) Transmission to flowers via pollinators. Not drawn to scale.

Waiting Room Hypothesis

There are also root forms. ‘Root endophytes were found wherever scientists searched' (Barberis et al 2020). They differ from mycorrhizal fungi in that they are easy to cultivate on a Petri plate, whereas mycorrhiza need more cultivated techniques. Endophytes also colonise in a diffuse way so that there no morphological changes, nor any contribution to plant nutrition.

"The orchid family offers one of the best documented examples of the ‘Waiting Room Hypothesis’: their mycorrhizal symbioses support the idea that extant mycorrhizal fungi have been recruited among endophytic fungi that colonized orchid ancestors". (Selosse et al 2022) This is an evolutionary scenario stating that mycorrhizal fungi of land flora were recruited among endophytic fungi that colonised orchid ancestors.

Summary of the evolution of mycorrhizal partners in orchids, emphasizing that they may have been recruited from fungi that were ancestrally endophytic...

Both the orchid and heather mycorrhiza seem to have different relations compared to the AMF and EcM we met earlier. The orchid relation is not so much ‘symbiotic’ but ‘essential, and both of the newly associated fungi are Basidiomycetes and capable of breaking down dead matter for the plants, thereby making new terrains viable.

Mycorrhiza (AMF) (late Devonian)

Mycorrhiza (EcM) (Jurassic)

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