Taraxacum

I've really enjoyed reading all of your natural history observations. One photo I found especially cool was this artsy shot of one of our most ubiquitous Composites here in Minnesota, Taraxacum officinale, the Common Dandelion. Look at all those stigmas arising from the disk florets! (Photo credit: Cory Hollinger)

Ever wondered why this species is called "dandelion"? It comes from the French, "dent-de-lion," which is a translation of the medieval Latin, "dens leonis," or "lion's tooth," which refers to the jagged-edged leaves!

Double fertilization -- WOAH

The angiosperm life cycle can be a bit tricky to learn, so let's break it down. Here's a pretty good diagram of the generalized process:

Following along by the numbers, let's start with the male anatomy:

1. So the anthers are where all the male material originates. Remember, sporangium is just tissue that holds spores (from Greek 'sporos' meaning ‘spore’ + 'angeion' meaning ‘vessel’), and when we see 'micro' it always refers to male reproductive cells (at least in the context of plant reproduction). So inside the anthers we find our microsporangium, which are at first holding diploid microsporocytes. These are our microspore mother (or father, to keep the male thing going) cells. 
2. These microsporocytes go through meiosis and we get 4 haploid microspores (i.e., male spores) from each microsporocyte. 
3. Each microspore develops (germinates) into a pollen grain (still haploid), each with a generative cell (where the sperm come from) and a tube cell. We can imagine a bee has now visited the flower and the pollen grains have stuck onto her legs, so let's leave them flying around for now, and move on to the 
4. Feminine side of things. Inside the ovary of the flower are the diploid ovules, which each have a megasporangium (sometimes called the nucellus) holding a megasporocyte, which is also diploid. 
5. Just like in the anthers with the microsporocytes, this megasporocyte (megaspore mother cell) goes through meiosis and we get 4 haploid megaspores (female spores). But, only one of these survives; the others are absorbed by the megasporangium. (Interestingly, it's the megaspore farthest from the micropyle that survives.)
6. This surviving megaspore germinates into the megagametophyte by dividing its nucleus mitotically, with the end result being 8 haploid nuclei. Cell walls form to make 7 different cells within the megagametophyte -- 
1. 3 antipodal cells that are at the chalazal end (opposite the micropyle) of the megagametophyte
2. 1 central cell containing 2 polar nuclei.
3. 2 synergids at the micropyle end of the megagametophyte, flanking the
4. 1 Egg cell.
7. Now let's go back to our pollen grains. The bee has now deposited the pollen grains on the stigma of another flower (let's pretend it's the flower our megagametophyte just formed in). The tube cell starts to digest its way down the style toward the ovary and ovules. It's basically digging a tunnel so that the sperm cells can enter the ovule.
8. The tube cell reaches the micropyle (opening) of an ovule and enters one of the two synergids. 
9. It then discharges two sperm cells (derived from the dividing of the pollen grain's generative cell) into the synergid.
10. One sperm cell fertilizes the egg cell to make a diploid zygote (which becomes the embryo), and the other fuses with the two polar nuclei in the central cell to form a triploid endosperm. (The actual ploidy of the endosperm varies widely among species, but we'll call it triploid here.) This is double fertilization -- one sperm cell fuses with the egg, another with the polar nuclei. This trait is a hallmark of the angiosperms!
11. The triploid endosperm serves as a food reserve for the developing diploid embryo. The integuments develop into the seed coat, and once the seed is mature (with cotyledons, radicle, etc.), it can
12. Germinate!

Fruits!

We got to look at, identify and EAT a bunch of fruit this week. Fruit classification can be tricky, especially when we don't get to see the preceding flower -- here's a more detailed list of fruit types (and examples of each) from the Northern Ontario Plant Database. Try to think about how floral structure translates into fruit structure next time you dine on some fibrous drupes or balaustas!

Mycorrhizae - a flora / fungi partnership

When we were looking at fungi we had a lot of ground to cover in just one lab -- Ascos, Basidios, Zygos -- we learned about an entire KINGDOM in three hours! So obviously there were fascinating aspects of fungi that we had to leave out. But one especially important interaction, the mycorrhizal symbiosis, is just too cool to go unexplored.

Certain species of fungi form symbioses with plants underground, setting up a "trading network" where plant photosynthate (carbon) is exchanged via roots and hyphae for soil nutrients obtained by the fungus. This is beneficial for both the plant and the fungus -- the fungus (a heterotroph) gets a reliable source of carbon while the plant can take advantage of the fungus' extensive mycelial network to gain access to nutrient resources (e.g., phosphorus, nitrogen) its own root system can't access. Fungal hyphae can extend much farther afield than most plant root systems, and can fit through very small gaps (pores) in the soil matrix to get at nutrients.

This ancient interaction, the mycorrhizal symbiosis, is found in over 80% of plant species worldwide, and is thought to have facilitated the rapid spread of land plants ~400 million years ago (fungi were on land before plants!). Though the symbiosis can at times be parasitic (especially in human-altered systems), mycorrhizas in natural systems are thought to be generally mutualistic (i.e., both partners benefit from the trading). But only some species of fungi engage in this symbiosis; they are called (not surprisingly), mycorrhizal fungi. There are two main groups of mycorrhizal fungi that we'll explore here.

Ectomycorrhizal fungi

The first are ectomycorrhizal fungi -- "ecto" meaning "outside," which is in reference to how the fungus interacts with its plant host. We'll see later that other mycorrhizal fungi actually penetrate plant cells with their hyphae, but ectos (as they're colloquially called) keep their hyphae outside the plant root cells. As you see in the illustration below, fungal hyphae grow between root epidermal and cortical cells to form what's known as a Hartig net; this network of hyphae is where nutrients are exchanged between the plant and fungal symbionts. Hyphae also often envelop root tips in what are called mantles, or fungal sheaths, seen at left below.

Diagram 1

But we see the real benefit of associating with mycorrhizal fungi when we look at the mycelial network of hyphae extending through the soil. In the photo below, you see a young pine seedling colonized by an ectomycorrhizal fungus -- the tree roots are brown, with thousands of white fungal hyphae extending into the soil around them. These mycelial networks can increase a plant's absorptive area by orders of magnitude.

Many of our most common forest mushrooms are the result of sexual reproduction in ectomycorrhizal fungal species, like chanterelles and many boletes (below). Though there are a few exceptions, most ectomycorrhizal fungi are Ascomycetes or Basidomycetes.

Cantharellus cibarius

Boletus reticulatus

Vesicular-arbuscular endomycorrhizal fungi

The other main class of mycorrhizal interactions involve fungi that actually penetrate their host plant's cell walls when forming symbioses. We call these vesicular-arbuscular endomycorrhizal fungi, and often abbreviate using VAM fungi (Vesicular Arbuscular Mycorrhizal) or AMF (Arbuscular Mycorrhizal Fungi). Endo refers to the fact that the fungi actually take up residence inside plant cell walls (as opposed to ectomycorrhizal fungi). We'll soon see where the rest of their (very long) name comes from.

Diagram 2 - Arbuscular mycorrhizal fungi hyphae, vesicles and spores

All AM fungi are included in the phylum Glomeromycota. Like ectomycorrhizal fungi, AMF have huge mycelial networks running throughout the soil matrix. But when AMF colonize plant root tissue, their hyphae actually go through the cell walls of root cortex cells and form specialized structures called arbuscules that exchange nutrients with the plant (see details below); note that though the hyphae penetrate the plant cell wall, they cannot get past the plasma membrane and do not invade the cytoplasm (that would be very messy indeed!).

Here's a photo of some heavily colonized Clarkia xantiana ssp. parviflora roots that I sampled out in Southern California, showing lots of AMF hyphae and arbuscules:

Many AMF also form vesicles (see Diagram 2 above) that act as storage organs for the fungus, somewhat analogous to vacuoles in plant cells. AMF have been shown to be important not only in supplying limiting nutrients like N and P to plants, but also protecting against pathogens and mediating water stress. There's a great Nature Review on AMF here if you'd like more info.

We just scratched the surface of the mycorrhizal symbiosis here (we didn't even get into ecology!); there are even other mycorrhizal fungal groups such as ericoid fungi and orchid mycorrhizal fungi, but I'll leave it up to you to explore further!

-j

Lichens

We talked a bit about lichens in both our algal and fungal labs...why? Because lichens are symbiotic organisms comprised of a mycobiont (the fungus) and a photobiont (green algae or cyanobacteria). Found in almost every habitat on earth, lichens exhibit a fascinating array of morphologies and ecological characteristics. Check out the portrait gallery in Lichens of North America to see a sampling of this diversity. Then go outside and find some on your own!

A stunning lichen illustration by 19th century German naturalist Ernst Haeckel

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.