Could Gleditsia have been made by megafungi as well as megaherbivores?

Among leguminous woody plants, mimosas (Mimosaceae) are far more likely than caesalps (see https://www.inaturalist.org/journal/milewski/59528-caesalps-on-southern-continents-part-1#new_comment) to be favourite foods of large herbivores. Accordingly, it is mimosas, not caesalps, that tend to possess large spines.

However, the genus Gleditsia (https://en.wikipedia.org/wiki/Gleditsia) is exceptional among caesalps (see https://www.inaturalist.org/journal/milewski/59528-caesalps-on-southern-continents-part-1#) in the degree to which it looks and acts like a member of the mimosas (http://www.nzdl.org/cgi-bin/library?e=d-00000-00---off-0hdl--00-0----0-10-0---0---0direct-10---4-------0-1l--11-en-50---20-help---00-0-1-00-0-0-11----0-0-&a=d&c=hdl&cl=CL1.12&d=HASH011ec19a37bb817d72319187.33).

Gleditsia has extremely large spines (https://en.wikipedia.org/wiki/Honey_locust and (https://www.britannica.com/plant/honey-locust-tree-genus and https://www.feedipedia.org/content/honey-locust-gleditsia-triacanthos-thorns-madrid-spain and https://canr.udel.edu/udbg/?plant=gleditsia-triacanthos)
and https://www.etsy.com/au/listing/1065477392/huge-honey-locust-thorns-real-acacia?ref=pla_sameshop_listing_top-1 and https://www.etsy.com/au/listing/777216641/huge-honey-locust-thorns-real-acacia?ref=pla_sameshop_listing_top-2&cns=1), as well as small leaflets, flowers lacking obvious petals, and fruits functionally similar to those of mimosas such as that widespread 'acacia', Vachellia nilotica (https://en.wikipedia.org/wiki/Vachellia_nilotica).

Implicit in this convergence with mimosas is that nutritional modes will have likewise converged: we can expect Gleditsia to be adapted to exploit nutrients recycled in urine and faeces, while at the same time fixing atmospheric nitrogen.

It comes as a surprise, therefore, that Gleditsia not only lacks nitrogen-fixing nodules but is listed as possessing ectomycorrhizae (see https://www.inaturalist.org/journal/milewski/59485-the-mystery-of-the-missing-mycorrhizae#). These macroscopic fungal root-extensions are rare among mimosas (https://mycorrhizas.info/ecm.html) and antithetical to the nutritional strategy typical of mimosas.

Because the largest mycorrhizal fungi tend to be associated with the roots of plants unattractive to large herbivores, and to promote nutritional regimes antagonistic to herbivory, Gleditsia presents a puzzle. Is its nutritional strategy really similar to those of caesalps which are neither attractive as food for herbivores, nor spinescent (https://mycorrhizas.info/ecm.html)?

If it really is ectomycorrhizal, Gleditsia is unique in combining some of the largest stem-protecting spines and some of the largest root-assisting fungi.

I cannot resolve this question because I have yet to find the original paper reporting ectomycorrhizae in Gleditsia. However, I can further explain spinescence in the context of ectomycorrhizal plants generally.

Spines, thorns and prickles vary greatly in their size, the part of the plant from which they are derived, and their adaptive functions. The large spines of many species of 'acacias' (now genus Vachellia) are derived from stipules (e.g. http://elephantseyegarden.blogspot.com/2010/06/fever-tree-umbrella-thorn-mimosa-port.html).

The largest spines derived from stipules and stems of dicotyledonous plants serve to control the foraging of the largest herbivores. This control should not be confused with deterrence, because plants with large spines are likely to be promoted by regimes of intense herbivory that recycle nutrients via faeces and urine while retarding competing plants. The function of these large spines is not to prevent loss of foliage as much as to protect the crucial regenerative organs/tissues, such as shoots and cambium, in the interests of plant and herbivore alike.

Spines have been established to occur in several genera of ectomycorrhizal plants, but they are of a different kind, compatible with a nutritional regime marginalising large herbivores.

The boreal conifer Picea (https://www.shutterstock.com/nb/video/clip-17336719-closer-look-picea-pungens-blue-spruce-thorny), the Australian pea Gastrolobium (https://www.ukwildflowers.com/Web_pages/gastrolobium_spinosum_prickly_poison.htm), and several Mediterranean and Californian species of oaks Quercus (https://naturenet.net/blogs/2009/01/15/crimson/) are ectomycorrhizal and possess leaf-spines similar to that familiar plant, holly (https://en.wikipedia.org/wiki/Ilex_aquifolium).

Leaf-spinescent plants are basically different from stem- or stipule-spinescent plants in that the leaves are too sclerophyllous (rigidly fibrous and lignified) to be particularly attractive to herbivores in the first place. They generally occur in ecosystems poor in large herbivores, where the main consumer of plant matter is episodic wildfires (https://www.academia.edu/7565234/Ecology_of_Australia_the_effects_of_nutrient_poor_soils_and_intense_fires). The spines can be thought of as an extension of the unpalatability of the leaves themselves, rather than a means of collaboration with herbivores as seem in spinescent 'acacias'.

Spinescence in ectomycorrhizal plants can therefore be summarised as follows:

In general, ectomycorrhizal plants lack spines, which is unsurprising because the nutritional regimes to which they are adapted are unsuitable for large herbivores. Those ectomycorrhizal species possessing spines tend to have a form of spinescence (i.e. leaf-spinescence) associated with limited herbivory. The stem-spinescence of Gleditsia is so unusual among ectomycorrhizal plants, and the association of Gleditsia with large herbivores is so obvious, that it calls into question the very claim that Gleditsia possesses ectomycorrhizae.

Posted by milewski milewski, November 17, 2021 00:47

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