Root Anatomy: Cross Sectional View
From outermost layer to innermost:
- endodermis - selectively permeable layer; unique to roots
- pericycle - a secondary/lateral meristem that
gives rise only to side branch roots; unique to roots
- vascular cylinder (a.k.a. stele)
Root epidermis is the surface that meets the environment, and it is the first selectively permeable membrane the plant uses to filter uptake.
Surface area is increased by trichomes that form root hairs:
These are found primarily in the Region of Maturation, and die off once the cells age. Although the cell walls contain suberin, water and minerals can pass easily between the cells of the epidermis, so further filtration is needed down the line.
Exam I material ends here.
Exam II material begins here.
This is a symbiotic relationship between a fungus and a plant root. (What does each partner get out of the relationship?)
Some of the most valuable edible organisms in the world are TRUFFLES, various species of
mycorrhizal (ascomycete and basidiomycete) fungi that partner with plants.
- Vesicular Arbuscular Mycorrhizae (V.A.M.) - association between a zygomycete fungus ("Black Bread Mold") and a plant
- Ectomycorrhizae - association between ascomycete (Sac Fungus) or basidiomycete (Club Fungus) and a conifer
or flowering plant (usually large trees).
In mycorrhizal plants, root hair surface area is negligible compared to that provided by the interface of mycorrhiza, plant and fungus. Most absorption is done via the mycorrhizal hyphae.
Recent research suggests that mycorhizzal associations may be a symbiotic partnership between not two, but three species, including special bacteria that live inside the gungus and are essential to the establishment of the symbiosis.
Just internal to the epidermis lies the cortex, composed primarily of parenchyma.
Cortex plastids are primarily for storage (fats, carbs). Only in some species with photosynthetic roots (which types of plants would you expect these to be?) are there chloroplasts in these cells.
In woody plants, the cortex is shed off once woody growth begins. In herbaceous plants, the cortex is maintained throughout the life of the plant.
Most of the cortex is airy, with a lot of space (filled with fluid and or air) between the cells.
Fluids travel via:
(recall: the tonoplast is the continuous fluid pathway formed by the plasma membrane of the vacuoles)
- symplast - the connection formed by plasmodesmata
- apoplast - the continuous surface formed by adjoining cell walls
The innermost layer of the cortex is the endodermis, the main "filtration" surface of the root.
The Casparian strips banding each endodermal cell are made of suberin (sometimes lignin, as well), and prevent interstitial entry of water into the stele (central core of vascular tissue). Thus, water cannot travel via the apoplast, and must pass through the selectively permeable plasma membrane of the endodermal cells before it reaches the vascular system.
This is a layer of pluripotent parenchyma cells located just inside the endodermis. Pericycle gives rise to side branch roots.
Root morphology is fairly well conserved across plant taxa. Therefore, differences in the morphology of the stele can be an important tool for classifying plants and determining evolutionary relatedness.
The stele consists (from outermost to innermost layers):
- phloem from three to many bundles, alternating with xylem
- xylem - central core
The number of xylem "arms" in the stele determines its classification as a...
In young roots, the phloem and xylem are nested in a central core of parenchymal pith
- In monocot roots only the xylem core surrounds a central cylinder of parenchymal pith. This is the simplest way to tell the difference between a typical root and the highly derived monocot root.
The Variety of Roots
Roots of various plant species have evolved various specializations.
- food storage roots - used by the plant to store
starch for metabolic activities later in the season. Typical
examples: carrot, beet, sweet potato.
- water storage roots - found in arid regions,
these are roots that collect large amounts of water during rainy
season for the plant to use during dry season. These are most often found in xeriphytes (sometimes spelled xerophyte). Local examples include the East Indian Rosewood and the Starburst.
- propagative roots - have meristematic regions
from where new, genetically identical plantlets can grow. These regions are not the same as nodes: they do not contain a true apical meristem.
Local examples include the East Indian Rosewood and the Starburst.
- pneumatophores - gas exchange surfaces on root
tips protruding from water-logged soil. Certain species of mangrove have these. But, contrary to popular myth, cypress "knees" apparently have no gas exchange function. (Cypress trees with knees removed do not suffer from any apparent lack of oxygen.
- prop roots - These grow from the lower part of a stem or trunk down to the ground, and providing extra support for the plant. These tend to be more common in plants with a tall, soft stem structure, as well as in plants that live in softer soils.
Common examples include corn (Zea mays), Screw "Pine" (Pandanus tectorius), various species of palms, and red mangroves (Rhizophorus mangle.
- aerial roots - typical of epiphytes such as orchids
(in which these roots are called velamen, with a spongy outer
surface very good at absorbing and holding water) and bromeliads.
- buttress roots - wall-like extensions off the
base of the trunk which provide support against physical assault
from high winds.
Our local Ficus spp. and the Royal Poinciana (Delonix regia tend to develop these in certain environments.
- contractile roots - these specialized roots, usually found at the base of an underground organ (e.g., a bulb) actually contract to perform such functions as getting a bulb to its proper soil dept for growth
- haustoria - parasitic plant roots that invade
the tissues of a host plant and transfer nutrients from host to
Examples of plants that have haustoria are dodder and mistletoe
- adventitious roots - are roots that grow anywhere they are not "expected." Examples are the adventitious roots that grow so prodigiously from some of our native and introduced species of Ficus trees. Several of the root types listed above (e.g., prop roots, aerial roots) can also be considered adventitious.
Life-sustaining Root Symbiosis: Nitrogen Fixation
The Nitrogen Cycle is the pathway by which nitrogen moves through living and non-living components of the ecosystem.
Nitrogen is one of the four main elements most common in biological macromolecules, and yet no eukaryotes are capable of fixing atmospheric nitrogen , N2, into its usable forms, such as ammonium (NH4+) with other species changing it into nitrite (NO2-) and nitrate (NO3-).
Certain nitrogen-fixing bacteria, however, are capable of converting gaseous nitrogen into its biologically useful forms, and some of these have formed symbiotic relationships with plants, notably in the Fabaceae (Pea Family), commonly called legumes.
The roots of legumes are covered with swellings called nodules within which reside symbiotic bacteria that fix nitrogen. Various strains of a bacterial species named Rhizobium form this association.
Nitrogen fixation into ammonium requires an anaerobic environment such as that found in the root nodules. The root nodule surfaces are highly lignified, helping to prevent gas exchange. Also, root nodules often contain leghemoglobin, a hemoglobin-like molecule with high affinity for free oxygen. This protein provides a sort of "buffer" for oxygen, allowing the bacteria enough oxygen to produce ATP for the very energy-expensive reactions of nitrogen fixation without allowing too much oxygen to build up in the nodule tissues and interfere with nitrogen fixation itself.
The figure below shows the sequence of events leading to nodule formation.
How does this symbiosis develop? It's amazing...
- The plant root emits flavonoids into the soil.
- Certain species of Rhizobium take up these flavonoids (the strain of Rhizobium colonizing each plant species is different, and determined by the exact structure of the flavonoid messenger.)
- The flavonoid activates a transcription factor protein, the activity of which results in the activation of a bacterial operon known as nod (for "nodule").
- The genes in the nod group produce enzymes that catalyze Nod proteins, specific to the bacterial strain.
- The Rhizobium secrete the Nod molecules into the soil, and these signal to the plant root to elongate root hairs and form the infection thread that the bacteria will use to enter the root.
There is some evidence to suggest that early mycorrhizal fungus/plant communication pathways (which also employ flavonoids) led to the evolution of the bacteria/plant communications resulting in nitrogen fixation symbiosis.
Most agricultural crops severely deplete soil nitrogen. Hence, good farming technique usually includes crop rotation, in which the farmer will grow a non-legume crop in a field for one or more years, and then plants a legume crop for a year to help restore soil fixed nitrogen.