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The Root

This organ is the first to emerge from a seed.

Its functions are

Origin of the Root

The primary root is the first root that forms in and emerges from the seed.

In most other plants, the primary root develops into a taproot, a large, central root from which lateral roots emerge.

  • Root depth depends on environment
  • Porous soil - deep roots; rocky soil - shallower roots
  • Temperature and soil water content also affect root depth.
  • And of course, different species have different root characteristics
  • The main "feeder roots" spread in the topmost soil layers, usually no deeper than about a meter.
  • Roots tremendously increase plant surface area where the plant needs it: for water absorption.
  • Root surface area is much greater than shoot surface area.
  • Root and shoot are balanced: If the roots are damaged, the shoot may die back.
    If the shoot is damaged, there's less photosynthate available for new root growth. In the most derived plants (monocots), the taproot is replaced by a system of fibrous roots that all emerge in a bunch at the base of the stem.


    Root Growth and Development

    Let's have a look at where it all begins: The Root Tip.

    Again, note the relative locations of the apical and three primary meristems.


    Can you see the quiescent center? Labeled with a radioactive nucleoide, the tip shows up on an autoradiograph with actively dividing nuclei showing dark (taking up the marker), and relatively inactive cells with invisible nuclei (not taking up marker).

    The meristematic quiescent center

    Root Cap

  • The root cap is a protective cap of live parenchyma cells

    Root Meristems

    The apical meristem (from the Greek merismos, which means "division") is found at the very tip of the root, just behind the root cap.
    As in all meristems, the apical meristem contains some cells that will always remain meristematic: one daughter cell remains in the meristem (the initial) and continues to divide, whereas its sister cell (the derivative) stays behind as the meristem grows out. The derivative differentiates into some type of cell, depending on its gene expression. This is known as primary growth.

    The very end of the root tip contains the initials and the immediate derivatives, and is known as the promeristem. Note the relative locations of the apical and three primary meristems.

    (What's different about this quiescent center? Why?)

  • Region of Cell Division - the apical meristem
  • Region of Elongation - growing cells elongating (primary meristems)
  • Region of Maturation - cells mature, no longer elongating (mature tissues)


    Root Primary Structure

    Recall that any plant organ has three main layers:
  • epidermis (and derivatives)
  • vascular tissue
  • ground tissue


    Root Anatomy: Cross Sectional View

    From outermost layer to innermost:


    Root Epidermis

    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.


    Mycorrhizae

    This is a symbiotic relationship between a fungus and a plant root. (What does each partner get out of the relationship?)
    • 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).
    Some of the most valuable edible organisms in the world are
    TRUFFLES, various species of mycorrhizal (ascomycete and basidiomycete) fungi that partner with plants.

    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.


    The Cortex

    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)

    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.


    Pericycle

    This is a layer of pluripotent parenchyma cells located just inside the endodermis. Pericycle gives rise to side branch roots.


    The Stele

    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):


    The Variety of Roots

    Roots of various plant species have evolved various specializations.


    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...

    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.

    Crop Rotation

    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.