Arthrobotrys -- a carnivorous fungus!

Source: Society for General Microbiology (www.sgm.ac.uk)

In the fungal lab last week we were introduced to Arthrobotrys oligospora, a nematophagous fungus. When we first looked at our agar-filled petri dishes inoculated with Arthrobotrys, we just saw pretty standard hyphae. But after introducing a population of nematodes into the dish the fungus went through some spectacular changes.

After a day we began to see specialized nematode-trapping structures:

A photo from an Arthrobotrys plate in our lab. This is looking across the surface of the agar, with the arrow pointing out the adhesive networks of hyphae the fungus uses to trap nematodes; they stick up above the surface to ensnare prey.

Source: http://www.uoguelph.ca/~gbarron/index.htm

The "adhesive" on these loops binds strongly to sugar compounds on the surface of the poor nematode who happens into their hyphal booby trap. Some fungi even secrete chemicals that attract nematodes to these snares.

Other nematophagous fungi utilize other specialized trapping structures like constricting rings (shown at right). These rings are made up of three connected cells that swell rapidly when a nematode passes through, squeezing the unsuspecting passerby to death.

So you've captured a nematode -- now you've got to digest it! I'll quote directly from the Society for General Microbiology to explain how the fungus goes about this:

Once ensnared, the fungus pierces the nematode’s cuticle using a narrow penetration peg which swells inside the host to form an infection bulb that the hyphae grow from. Fungal enzymes break down the contents of the nematode and the nutrients are transported elsewhere within the hyphal system for growth or spore production. Growth does not occur at the site of the hyphal trap. This phase usually takes 1–3 days, before hyphae grow out of the cadaver and sporulate.

Pretty wicked, huh? This is what the nematodes on our plates looked like a few days after capture:

The remains of three nematodes that met an unfortunate end in Botany Lab #8. But their lives weren't for naught -- check out the Arthrobotrys conidiophore at top center (marked by the red dot)!

Scanning electron micrograph of nematodes (tan tubes) trapped by the adhesive loops of A. oligospora (source: www.sgm.ac.uk)

Single conidiophore

Conidiophore forest!

All those nutritious nematodes fueled a large asexual reproductive spurt in our Arthrobotrys. At right (top) you see a single conidiophore (asexual reproductive structure), viewed from the side on one of our plates. Right (bottom) shows a group of them.


















And check out the conidiophore "forest" swaying in the breeze! (Sorry for the poor video quality.)

Mushrooms of the Midwest

Did the fungal lab make you want to go out and see some of these fascinating organisms in situ? Check out the UMN Mycology Club for foraging trips and other FUNgal events. For a great Minnesota mushroom guide, grab a copy of Mushrooms of the Midwest (not yet on the shelves at the UMN library, but available for a reasonable price on Amazon.)

Ocotillo, a drought deciduous species

Last week we looked at how auxin production can influence leaf abscission in Coleus. Here in Minnesota, most deciduous tree species drop their leaves in the fall, when temperatures are getting lower and the days are getting shorter. But in arid regions, many plants are drought deciduous. Below is an excerpt from an earlier Field Notes blog post, taking a look at one such species that I encountered in Big Bend National Park.

Ocotillo (Fouquieria splendens) is another well-adapted desert plant very common in the Chihuahuan Desert. Most of its life is spent as a rather dead looking, spiny, leafless stalk (left) that can reach heights of 30 feet (more commonly 10-15 ft). But look closely and you’ll see green stripes along the stalk — these are active photosynthetic areas, allowing the plant to continue to produce sugars while leafless. Leaves (center) are produced quickly in response to rain, and are kept until drought conditions return, when these water-costly structures are dropped and the plant returns to its spiny, barren state. Brilliant orange-red tubular flowers (right) are produced at the tips of the stems sporadically throughout the year, with pollination service provided by hummingbirds and honeybees.