Agriculture involves a series of process that humans stumbled upon nearly 10,000 years ago. Far before then, other eukaryotes have enhanced their fitness through agricultural practices. So far, I have talked about my experience with fungal farming ants as well as the ambrosia beetles that tend to fungal gardens within the dead trees they inhabit. One fungal farming insect I haven’t mentioned are the termite specialists that grow immense patches of edible fungal mycelia, similar to the way leafcutter ants do. I had once thought that major criterion for acquiring an agricultural ecology was to be eusocial, mobile, and to have a brain. I then learned about solitary beetles that grow fungal gardens for their own nourishment, so I had to discard the eusocial criteria I once held. Just recently, I had to rethink the entire agricultural concept, upon reading a 2013 paper by Martin Pion and his team. Together, they describe how a single species of fungus maintain and use bacterial cultivars. Although Morchella crassipes is mobile on a much slower scale, these agricultural fungi lack brains, and like the solitary beetle farmers, lack a hierarchical society.
Using an intimating number of laboratory studies (9), these researchers began to unfold this novel ecological interaction. Together, they grew Morchella crassipes with two strains of Pseudomonas putida. One bacterial strain was the flagellated wild type, which in nature utilizes mycelial networks to navigate its soil habitat. The other was a mutant strain without flagella, to test how important bacterial locomotion is in this agricultural system. Morchella crassipes was used in this study because it supposedly forms an intimate, mycorrhizal relationship with trees, so, a larger ecological picture could be more comprehensively understood. Also, M. crassipes forms sclerotia, or fungal storage tissue that become utilized upon fungal stress.
Fluorescent labeled flagellated Pseudomonas putida using hyphae as fungal highways. Pion et al. 2013.
With this, they discovered the wildtype P. putida easily dispersed throughout the media using the network of mycelia, kind of like a fungal highway. The non-flagellated mutant could not traverse anywhere near the same distance, and thus, wouldn’t be an appropriate cultivar in this system. Within the petri dishes, after seven days with the bacteria and fungi coexisting, sclerotia formed. Without bacteria, sclerotia formed uniformly across the nutrient agar. With bacteria however, the sclerotia became localized, growing in a position furthest from the colony of bacteria.
In a closely related species, Morchella esculenta forms its sclerotia in soil regions more depleted of nutrients. This helps explain what’s going on in these experimental petri dishes. The growing bacteria colony acts as a nutrient source, while the sclerotia act as a nutrient and energy sink, without physically interfering with the bioactive regions. Before sclerotia formation, authors here talk about the initial dispersal stage, in which the flagellated bacteria, facilitated by fungal hyphae colonize much of the petri dish. Experiments involving radioactive carbon labeling show that fungal sugars are sent to the bacteria to help them grow. After several days of bacterial cultivation, the harvest phase ensues, as the fungi absorbs much of the bacteria, including the nutrient rich media surrounding them. The harvest phase is easily recognized with the drastic reduction in bacterial colonies that once riddled the dish. Again, by radio labeling the carbon within the bacteria, they saw a spike in the radioactive element in the fungus during the harvest phase.
(a,c)Sclerotia growing throughout media in the absence of bacteria. (b,d,e) Sclerotia growing furthest away from where bacteria was inoculated (arrows). Pion et al. 2013.
Much like how a farmer stores a subset of the seeds produced from his/her crop to grow for the next season, some of the bacteria absorbed by the fungus became stored in the sclerotia. These bacteria remained viable, and when sclerotia was transferred to a new petri dish, hyphae propagated through the media, and bacteria again used these fungal highways where they began growing, breaking down the contents of the nutrient agar and taking up fungal sugars.
This might seem like a contradiction here, with the fungus sending sugars to the bacteria, only for the fungi to reabsorb the sugars. In this experiment, only carbon was analyzed. Surely the bacteria extract other nutrients like nitrogen and phosphorus that become reabsorbed with the carbon, but as always, we need sound evidence. We need more science! For now we can only predict what other benefits the fungi receive from this agricultural system. This research should definitely raise some eyebrows though.
Bacterial colonies are significantly reduced during the harvesting phase while fungal sclerotia form. Pion et al. 2013.
Many sclerotia forming morels are categorized as mycorrhizal, but are they truly forming a mutualistic relationship with trees? Just because they inhabit tree rhizospheres should not delineate their ecology. After reading this study, I think it’s a major possibility the photosynthate allocated to the rhizosphere may be feeding carbohydrates to a diverse array of bacteria. These plant fed bacteria can then be reared and translocated by species like Morchella crassipes. With their hyphae acting as soil highways, bacteria benefit by dispersing longer distances. Their persistence in soils is also benefited by the formation of sclerotia. When plants enter winter dormancy, sugars for the most part stop flowing, and bacteria can take refuge in sclerotia. Warmer temperatures and the arrival of soil sugars initiate hyphal growth and the dispersal of rhizosphere bacteria.
It’s just a theory, but many sclerotia forming morels may be associated with the rhizosphere not through a mycorrhizal ecology, but through an intimate relationship bacteria. What is not just a theory, is that beneath the forest floor, mushrooms farm bacteria in a complex agricultural system analogous to other eukaryotic farmers. Though, these organisms are solitary, lack brains and move super slow.