The Fungi Cometh

Once the chestnuts began to grow roots, they also sent out a chemical invitation that could be picked up only by mycorrhizae. Because the soil was harvested in the winter, the majority of the fungal material, or inoculum, consisted of what are called "resistant propagules" or, more simply, bits of fungus that can produce more fungus but also survive adverse conditions. The two kinds of resistant propagules in my soil were spores and sclerotia, which are tightly woven bundles of fungal cells that break off from the main body of the fungus. Both kinds of propagules would wake up and start growing in the warm, moist conditions of the growth chamber, but they would try to conserve energy as much as possible, because there wasn't much access to food until the chestnut trees germinated.

Once that signal went out, though, the fungi would have responded rapidly, extending new strands (called hyphae) toward the root tip. Once a mycorrhiza encountered a plant, it wrapped around an extension of the root and started secreting hormones into it, directing it to grow short and stubby instead of long and slender (have a look at this article to see the difference). The plant responded with hormones of its own, directing the fungus to send long filaments of hyphae out into the soil to find nutrients. 

And with that, a new partnership was established. By May, after the plants had been growing for three months, it was time to have a look to see just how many mycorrhizal connections formed. This required sacrificing a few trees (this lab note is dedicated to their memory) so that I could have a look at their root systems under the microscope.

The result? Tons of mycorrhizae, although restricted to a few types. One of the most common fungi (of the "I don't know" variety) formed a gauzy white sheath around its host roots.

 

The roots from the sample were stored in a DNA preservative to await genetic analysis this spring, when the fun of actually identifying these fungi begins.