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.