Bacterial Root Nodules

These are bacterial root nodules on the root of runner bean Phaseolus coccineus. Each nodule contains a population of Rhizobium bacteria that are capable of converting atmospheric nitrogen into soluble forms of nitrogen that the plant can use for growth - which is what makes this symbiotic association between plant and bacterium so valuable for agriculture. In annual legume crops, once the bean crop has been harvested the root nodules decay and release nitrogen in the soil, where it can give a yield boost to following non-legume crops in the crop rotation - like wheat, for example.

In this image one of the nodules has been cut in transverse section and stained with the fluorochromes calcofluor and auramine O. The plant root, with its xylem vessels visible, is at the top. The bacteria filling the root nodule, encased in blue-stained plant cells, are stained yellow. The Rhizobium bacteria in the soil penetrate through a root hair, trigger proliferation of the host plant root cells to form a nodule and multiply within. Healthy root nodules are pink when you cut them open due to the presence of leghaemoglobin which, like haemoglobin in mammalian blood, absorbs oxygen. This is important because oxygen would otherwise inhibit the enzymes in the nodule that 'fix' nitrogen into soluble forms. Bacterial nodules that are not pink when you cut them open are likely to be parasitic on the host plant, rather than symbiotic.

This is the difference that nodulation makes. The plant on the right has effective nodules, the one on the left doesn't. The interaction between plant and bacterial strain is complex; for any give crop cultivar, different bacteral strains will show varying degrees of effectiveness in boosting crop yield and different crop varieties perform best with different bacterial strains. Deliberately inoculating seeds with effective Rhizobium strains can produce significant yield benefits, although there is no guarantee that any particular inoculum will persist in a soil type or location where it's not a naturally-occurring strain amongst the existing soil microbial community.



Out of Sight,Out of Mind...

Roots are the most neglected parts of plants, perhaps because they are out of sight and - superficially at least - lack the intrinsic aesthetic beauty of the above-ground parts. For most (although not all) plants they are vital structures and - when you look really closely - they have an intricate beauty of their own.

Root tips are sensitive gravity detectors, ensuring that the root always grows downwards into the soil. This root was held in the horizontal plane for less than an hour before it redirected its growth downwards. Behind the root tip you can see the point where the root hairs develop, with newly initiated root hairs just visible nearest the root tip but becoming longer as you move away from it. Further back still the root hairs die away continually and each has a life span of just a day or two, but they are continually replaced as the root penetrates further into the soil. The passage of the root through the soil is assisted by lubricating mucilage produced by the root tip, whose surface cells slough off. The mucilage also supports a bacterial microflora that helps the root acquire nutrients and may provide some protection from disease-producing organisms.

This is a root tip sectioned vertically and stained with a  fluorescent dye called DAPI. If you click on the image to enlarge it the details will be a little clearer. The brightly fluoresescing dots are the nuclei, one per cell, and you can see the files of cells produced by sequential cell division followed by cell elongation, which pushes the root ever-further into the soil.

This is a root in transverse section, further back from the tip than the previous image, in the middle of the root hair zone. It has been stained with a fluorescent dye called calcofluor, which makes the cellulose cell walls fluoresce blue in ultraviolet light. From the outside inwards, you can see the long root hairs, each a single cell that arises from the root epidermis (surface layer of cells). Next inwards lies the root cortex, which constitutes the vast bulk of the cells, then in the centre you can see the stele - the cylinder of vascular tissue that transports water upwards to the rest of the plant and carries sugars and amino acides downwards to support the continued growth of the root.

The arrangement of the various cells and structures is more clearly visible here, at higher magnification. The large circles in the stele, top left, are xylem vessels that conduct water away from the root.

The root hairs, which are in intimate contact with the soil particles, absorb water and soluble minerals that are transported through the root cortex, both from cell-to-cell within cell cytoplasm (the symplastic route) and through cell walls and the spaces between cells (the apoplastic route), to the stele in the centre of the root.

Once the water reaches the stele it encounters a single layer of cells called the endodermis, that sheaths the stele. The walls of the endodermal cells contain a substance called suberin which renders them impermeable, so water that arrived via the apoplastic route is forced into and through the cytoplasm of these cells, where dissolved minerals are selectively removed. You can see the suberin deposits, known as the Casparian strip, as the orange staining in the single ring of cells that lies between the blue and the yellow cells in the section of a stele above. Some water also passes unimpeded through specialised passage cells in the endodermis - if you follow the ring of cells with the orange stained Casparian strip in their cell walls around the stele in the image above, you'll notice a few passage cells with no orange-stained suberin deposit in their walls.

Almost all the water taken up and transmitted via both routes, via the cytoplasm of the endodemis cells or via their passage cells,  then enters the dead xylem cells that carry it aloft in the water column that is drawn upwards by transpiration from the leaves.

When gardeners buy plants in garden centres there's a great temptation to simply dig a hole and plant them, without teasing out the pot-bound roots or cultivating the soil around the planting hole, but a little tender, loving care for root systems pays great dividends: the vigour of the plant above the soil depends on the health of the roots, hidden below the surface.

Banana Stellate Parenchyma

These beautiful cells come from the midrib of a banana leaf. Each is shaped like a 6- or 7-armed star, with its arms joined to the arms of surrounding cells, forming a lattice of cells. This form of tissue is known as stellate parenchyma and you can find another example here. The image was produced using polarised light and the brightly coloured birefringent objects inside the cells are calcium oxalate crystals inside the cell vacuole. You can see further examples of calcium oxalate crystals, including a video of their Brownian motion inside a cell, if you click here.

To find these cells you need to look inside the midrib of a banana (Musa sp.) leaf .....

by cutting transversely across the midrib, which reveals this internal pattern of strenthening tissue filled with very delicate, transverse plates of glassy cells ....

... then dissect out one of these plates of cells and mount it on a microscope slide.

Life in 10 Drops of Water: Assorted Protists and rotifers

This is the fourth in the series of images of some of the organisms found in just a few drops of water collected from a pond in a disused quarry on the edge of the moors in Weardale.

An amoeba. I could spend a long time watching these - it's rather relaxing watching an organism whose motto for life must be 'go with the flow'

It seems to have ingested a wide varieties of objects.

This is one of the free-swimming ploimate rotifers, with tails that look like scissors - possibly Monommata caudata...? 

Vorticella - a ciliate protist on a stalk, that contracts like a spring when disturbed. The green object is a cyanobacterium - possibly Gloeocapsa.

A ciliate protist that creeps along using strange 'whiskers' - and also swims very actively using smaller cilia. You can see a contractile vacuole quite nicely here. I think this, and the three below, might all be Oxytricha.

All three of the above ciliate protists look rather well fed - full of undigested algae.

This beautiful object is the flask-shaped shell of the testate rhizopod Cyphoderia ampulla. The amoeba that lived inside has long-since died.

I thought this might be the shell of a testate rhizopod, but Natalia has kindly identified it as a tintinnid  ..........

... at higher magnification you can see that it's constructed of tiny translucent particles....

... that are especially fine and fit together beautifully around the orifice

.... and finally another heliozoan, that appears to be ingesting something

Life in a few Drops of Water:Dinoflagellates

This is the third in the series of images of some of the organisms found in just a few drops of water collected from a pond in a disused quarry on the edge of the moors in Weardale

This little object (which I think may be a species of Peridinium), a mere five one hundredths of a millimetre across, is a dinoflagellate - a single-celled, hardshelled organism that's powered by two rapidly undulating flagellae. One runs on the horizontal groove around the 'equator' of the example you can see here. The other runs in vertical groove, extending from the equator to the apex - it's out of sight on the distal side of this image, although you can just see the apex of the vertical groove at the top. It seems like an unlikely means of motive power but it works - this one whizzed all over the slide before it paused for long enough for me to get a photograph.

This is the empty shell of a dead dinoflagellate, which reveals the intricate pattern on the armoured surface composed of cellulose plates. Unfortunately....

....it wasn't intact - you can see where about a quarter of the sphere has broken away, but you can also see the equatorial flagellar groove rather nicely. The green object approrach from lower right is a diatom, which is just about to punt the diatom out of the way as it glides past.

For some fascinating recent research on dinoflageelates, take a look at this post about a predatory dinoflagellate at Jennifer Frazer's The Artful Amoeba web site, where you can read about their amazing biology and also find links to drawings and electron micrographs that show the organism rather more clearly than my photographs.

Coming next: An assortment of protists

Life in a few Drops of Water: Desmids

This is the second in the series of images of some of the organisms found in just a few drops of water collected from a pond in a disused quarry on the edge of the moors in Weardale.

This is  a desmid - probably a species of Cosmarium. Desmids are typically constricted in the centre of the cell to form two mirror-image halves.

These are single-celled, photosynthetic algae that often have a patterned cell wall that's ...

.... most clearly visible after the cell has died and lost its chlorophyll.

This appears to be one half of a desmid that has broken at the bridge joining the two halves (known as the isthmus), revealing the fractured hole.

Coming next: Dinoflagellates

Life in a Few Drops of Water: Diatoms

A few days ago, when we were out walking in Weardale, I collected a small sample of water from a pond in a disused quarry on the edge of the moors, then spent several hours that same evening exploring just a few drops of the pond water - maybe ten in total - under the microscope. The organisms pictures in this post are all diatoms - minute photosynthetic organisms encased in a shell of pure silica that often bears the most intricate pattern on its surface. This one, caught in the act of dividing, is Pinnularia.

 

Some are long and thin (this is Nitzchia sigmoidea), while.....

....others are joined together in chains.

Navicula? When diatoms die their silica shell remains, almost indestructible, and in that transparent state, with no contents, the full beauty of the sculpturing on their shells can be appreciated.

The pattern of fine ridges on Cymbella tests the ability of microscope lenses to resolve such fine detail.

Diatoms generally move in a gliding motion through the water but sometimes they can be attached to a substrate via mucilaginous stalks.....

... seen here at higher magnification.

Most of these organisms are less than one twentieth of a millimetre long.

Coming next: Desmids....mirror-images in miniature

Give Me Strength

This cross section of the stem of a soybean seedling shows the early stages in a developmental process that will produce a stem capable of supporting the mature plant. I stained the section with two fluorescent dyes - calcofluor, which binds to cellulose cell walls and fluorescences blue in ultraviolet light and auramine O, which binds to lignin and fluorescences yellow. It's the lignin laid down in cell walls that gives the stem the strength it will need to support the leaves and flower.

Working from the bottom left-hand corner towards top right, the core of the stem is filled with blue, thin-walled pith cells, which are simply packing tissue. Some of these have become slightly lignified and are fluorescing yellow and some, that are arranged in vertical rows of between two and five cells have distinctly thicker walls - these are xylem vessels, which are dead cells that form tubes that conduct water up the plant from the roots.

Above those lies a broad band of blue-fluorescing thin-walled cells that are very small and arranged like piles of bricks. This is the cambium - the plant's stem cells that divide continuously to produce new xylem on the inside and new phloem elements on their outer surface. The small, bright blue-fluorescing cells on the outside of the cambium are the phloem sieve tubes and associated companion cells, which conduct sugars produced by photosynthesis in the leaves to other parts of the plant.

The sinuous layer of yellow-fluorescing cells above the phloem are becoming lignified and these will contribute major structural rigidity to the stem as it grows, forming a continuous cylinder inside the stem. Outside of these lies the stem cortex, with blue cells becoming smaller in the layers just below the epidermis - and then the outer epidermis of the stem is covered in the yellow-fluorescing cuticle, which restricts water loss and defends that plant against pathogens.

At the stage when this section was taken the stem was about 3mm. in diameter and about 10 centimetres tall.

Swallowed by the Sun Animacule

I've long been fascinated by heliozoans aka 'sun animacules' and have posted on them before, but thought I would again because I've captured some pictures of one without squashing it under a coverslip - and also photographed it digesting its last meal. Briefly, heliozoans are single-celled amoebae which, instead of flowing around and engulfing their food with cytoplasmic arms called pseudopodia, radiate spines of cytoplasm supported by an internal scaffold of microtubules. They roll through the water like a one-tenth-of-a-millimetre-in-diameter sea mine, bringing dead to anything small that contacts those spines. This is a through-focus sequence of this microscopic protist menace, so in the image above we are looking down on the upper surface, onto what looks like a sphere of cytoplasm radiating needles in every dimension... 

.... and here the focus has moved down a bit, onto the surface of that cytoplasm, which is broadly divided up into a pattern of hexagonal blisters....

.... and now we are looking at the contents of its central food vacuole - and this heliozoan has somehow managed to ingest a testate rhizopod - a form of amoeba that lives in a shell shaped like a hot air balloon. Incidentally, all the pictures on this post will be clearer if you double-click on them to enlarge them in a separate window. Below is the same sequence, but this time at x400 magnification instead of x100......

..... so here are the surface hexagons, with the base of those lethal radiating spines....

..... and here is the blistered pattern - which looks like cells but this is a single-celled protist - so they must be formed with the aid of microtubules, I guess ......

.... and here's its food vacuole, with ingested testate rhizopod. It will digest the cytoplasm of its prey and spit out the empty shell.

It's easy to loose a sense of scale when peering down the microscope at small organisms - a sense of their place in the universe. So the picture above shows the place where this 100 micron predator came from - from the moss around the edge of this small pool on bleak moorland in Weardale, County Durham . The pool is about a metre long, so if you lined up 10,000 heliozoans spine-tip-to-spine-tip they'd stretch from one end to the other ....

..... and that tiny pool is somewhere in the middle distance in this landscape ......... and beyond that the solar system and beyond that the universe, home to an exquisite tiny life form that has been rolling through our planet's water, ingesting whatever it touches, for well over a billion years. Time travel is possible if you have a microscope....