Critter of the Week – Part 1

Written by Joe Cicchetto

Critter of the Week is a special series brought to you by Science at St. Andrews. Once a week, Associate Professor of Biology Dr. Tracy Feldman shares a critter – whether that is a plant, insect, mammal, fungus, or another living thing – that one can find on St. Andrews’ campus. Each installment will feature a photo of one or two critters found on St. Andrews’ campus, with a little bit of background text about what they are and what they do. Check out the installment below featuring endophytic fungi!

Organisms as hotels for microorganisms, part I.

Plants are a lot like old hotels—they are made of wood, they are tall, and it hurts a lot if they fall on you… Most importantly, plants (for example trees) also house myriad organisms in their many “rooms” made of roots (basement rooms), branches, and leaves (upper leaves might be like a penthouse?). And like many residence hotels, there are some tenants that no one even seems to notice—they are quiet and almost invisible. Amongst the mammals, birds, lizards, and insects that live on and in trees, and amongst the amazing variety of insects living on plants, including those like leaf/stem miners and gall insects living inside of plant tissues, there are many additional small, secret tenants: fungi, and sometimes bacteria, that live inside plant tissues of all known plants. For today, I’ll focus on the fungi, a group called endophytes, meaning “inside plants.”

Fungal endophytes (also called endophytic fungi) are different from plant diseases (although many disease-causing fungi are in the environment as well). Most of these symbiotic fungi seem to live among the plant’s cells with minimal negative effects on the plants they inhabit—fungal endophytes do not cause disease. But what are they doing there? Many biologists think they are like vultures, camping out and waiting for the plants to die. Endophytes aren’t always the fastest-growing fungi out there, so many decomposers can beat them to the dead plants in the environment—but if they are already there in a plant, waiting for the plant to die, then when the plant does die, these fungi will get a head start decomposing them before they are outcompeted by the faster-growing species (Cline et al. 2018). Some endophytic fungi may be waiting for the plant to get stressed out so they can emerge as diseases… yet most endophytes produce no symptoms at all. In fact, many are mutualistic (with benefits to both partners).

Inside any given plant, there may be one or many endophyte species. In plants that have only a single dominant endophyte, the fungus may grow through the entire plant (even sometimes into seeds), and may confer the plant with tolerance to drought, tolerance to other stresses like heat or salt, or even resistance to diseases (Rodriguez et al. 2008) or herbivores (Clay 1988). Fungal endophytes often produce chemicals that keep herbivores away (pesticides?)—this means that in some cases, the poisons in a poisonous plant are produced by its endophytes rather than by the plant itself! Native Americans in Argentina sometimes used endophytes to escape pursuers, leading them through grasslands with poisonous endophytes. The pursuers would stop to feed their horses on the grasses there, and the horses would get intoxicated and die (White et al. 2003).

Most plants have multiple endophyte species living in them, and the community of endophytes might change depending upon the season or the location (Carol 1995). Even diverse endophytes might help their plant host resist diseases or herbivore attack. They may resist diseases through chemical means or simply by occupying all the space in the hotel, preventing diseases from establishing (Arnold et al. 2003).

I have only begun to glimpse the endophyte communities of our campus. The endophytes are there inside their plant hotels and come out if you sterilize the outsides of roots, leaves, or stems (to get rid of contaminants) and put the surface-sterilized tissue on petri plates of food for fungi (like sugars soaked in a gel called agar). Some examples of endophytes that emerged from plants on campus are in the photos below (figure 1, attached). The possible benefits (or costs) of these endophytes in their plants are completely unknown. Another world worth exploring…

References:

Arnold A.E., Mejia L.C., Kyllo D., Rojas E.I., Maynard Z., Robbins N., and Herre E.A. 2003. Fungal endophytes limit pathogen damage in a tropical tree. Proceedings of the National Academy of Science 100(26): 15649-15654.
Carroll G. 1995. Forest endophytes: pattern and process. Canadian Journal of Botany 73(Supplement 1): S1316-S1324.
Clay K. 1988. Fungal endophytes of grasses: a defensive mutualism between plants and fungi. Ecology 69: 10–16.
Cline L.C., Schilling J.S., Menke J., Groenhof E., and Kennedy P.G. 2018. Ecological and functional effects of fungal endophytes on wood decomposition. Functional Ecology 32: 181-191.
Rodriguez R.J., Henson J., Van Volkenburgh E., Hoy M., Wright L., Beckwith F., Kim Y.O., and Redman R.S. 2008. Stress tolerance in plants via habitat-adapted symbiosis. International Society for Microbial Ecology Journal 2: 404-416.
White J.F. Jr., Bacon C.W., and Hywel-Jones N.L. 2003. Clavicitalean fungi: evolutionary biology, chemistry, biocontrol and cultural impacts (Mycology series). Taylor and Francis Group LLC, Boca Raton, FL.