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.

Pollination Videos

Here's the link to the BBC pollination video collection, and below is some spectacular footage from Louie Schwartzberg's film, "Wings of Life," which is now on Netflix!

Gibberellic acid and starch hydrolysis

So in lab two weeks ago we saw some unexpected results in our gibberellic acid (GA) experiments...to refresh your memory, let's look at slides from Lab 6.

We had plates with starch agar, and plates with starch agar + GA. Seed halves (with and without embryo) were assigned to quadrants in each plate. (It's important to note that when we split our barley seeds in half, the embryo half would still have some endosperm/starch, as seen above, and would also retain some of the aleurone layer -- we didn't perfectly isolate the embryo.) We then stained the plates with iodine to look for presence of starch hydrolysis -- which would show up as "halos" (see below) of light-colored agar where starch had been hydrolyzed into glucose molecules (which do not stain black with iodine like starch does).

 Starch + GA plate with "hydrolysis halos"

We also looked at the GA-->hydrolysis pathway from another angle -- accumulation of glucose (as opposed to absence of starch).

We did this with barley seed halves as before, but then we tested for presence of glucose using Clinistix test strips.

So what would be our expected results from these experiments? Something like this, right?

Without the embryo to release the signal molecule GA, embryo-less seed halves on the starch plate would have no "trigger" to initiate production of alpha-amylase, the enzyme that facilitates starch hydrolysis. But when we provide that hormone on the starch + GA plate, even embryo-less barley halves should show signs of hydrolysis. (We'll just worry about presence/absence of hydrolysis now, not relative intensities or halo size.)

Same idea here: without GA, the embryo-less seed halves won't hydrolyze any starch (thus, there shouldn't be any significant level of glucose when we test for it).

But this isn't what we saw in lab, was it? No! We saw (with a few "correct" exceptions) high glucose levels in all tubes, and hydrolysis halos in all treatments! What's up with that??

Of course we could have mixed up our seed halves, right? That hilum can be hard to distinguish sometimes. But many of the seed halves on the agar plates had begun to germinate, allowing us to double-check ourselves -- and all the ones I checked were correct (also, the Tuesday labs saw similar results). Perhaps some groups, when cutting their seeds, included a small portion of the scutellum (where the GA is released from), in their embryo-less halves. Contamination or mixing of plates/tubes could also be an issue, but likely it has more to do with processes going on in the seeds themselves. Most importantly, it's likely that some early germination processes had been initiated in many of the barley seeds, perhaps by storage in a humid environment, so some amount of GA had actually already been sent to the aleurone layer, promoting the production of alpha-amylase and beginning the starch hydrolysis process in the endosperm. Thus, even in our embryo-less / endosperm halves, we were seeing evidence of alpha-amylase activity through starch reduction and glucose accumulation.

Here we just looked at presence/absence of starch hydrolysis. What hypotheses would you make about relative intensities of this process among the different treatments (we talked about this a bit in class)? i.e., which halos would be bigger? where would we find more or less glucose?

Plants in Motion

Here's the link to Indiana University's Plants in Motion Theater 

You can find analogous videos on YouTube, but this is a good starting place to get an appreciation for how much these supposedly sessile organisms can move!

Photosynthesis resources

Let's face it. Photosynthesis is confusing. Here are some resources to help you figure it out.

Essentials of Photosynthesis

A handy 4-page PDF that outlines the major reactions, and includes some questions to test your understanding.

List of Web Resources

This is a massive list of online resources regarding photosynthesis. They're sorted by category (Overview, Light-Independent Reactions, etc.) and compiled by profs at Arizona State and University of Illinois.

If you find any of these particularly helpful, comment below!

-jb