Download Roots - Botany and Plant Pathology

Roots
Botany 313
Plant Structure
Tamra Prior
Overview:
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Evolutionary Origins
Root Growth & Development
Structures & Tissues
Monocots vs. Dicots
Root Functions
Apoplastic and Symplastic
Transport
Types of Roots
Nitrogen Fixation
Mycorrhizae
Weird Plants that are exceptions
Root Origins
• The first vascular plants had rhizoids that
absorbed water and nutrients.
• Rhizoids are delicate uni- or multicellular root- and hair-like structures that
function in anchorage and absorption but
lack xylem and phloem.
• Roots evolved in response to the
pressures of a land environment and
increasing plant size.
• Roots have changed relatively little over
time, likely because of their protective
subterranean environment, they were not
exposed to the same selective pressures
as the stem.
Root Origins
• Seed plant roots may be considered to be:
1.) A modified stem or
2.) An entirely new organ that evolved
independently of the stem.
• The first view is accepted now because
the roots of extant plants are very similar
to stems of ancestral fossil plants.
• Roots of most plants are protostelic (like
the stems of very primitive plants); roots
with a central pith have an alternate
arrangement of xylem and phloem that
may reflect a protostelic origin.
Radicle To Root
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Radicle becomes the Root
What about here?
Root Tip
Pericycle and Lateral Root Growth
• Here you can see two root apices
initiating from the pericycle.
• What is the pericycle doing?
Growing out through cortex
Root cap
Zone of cell division
• In monocot plants, there are few lateral
root formations. Monocots rely on many
new fibrous roots
• Notice their connection to the ridges of
xylem
Maturation of Dicot Root
Dicot Vascular Cambium Maturation Shape
Monocot Root
Root Vascular Cylinder and Cortex
Ranunculus acris - buttercup
Cortex
Endodermis
Phloem
Meta-xylem
Pericycle
Endarch: protoxylem is inside
the metaxylem
protoxylem is outside
√Exarch:
the metaxylem
Proto-xylem
Monocot vs. Dicot
Root Functions
1. Roots anchor plants to soil; provide stability to above ground parts of the
plant.
2. Roots absorb water and dissolved mineral nutrients from soil and transport
them to the stem.
cortex cell
epidermal cell
root hair penetrates soil
spaces
©1996 Norton Presentation Maker, W. W. Norton & Company
Root Hairs
intercellular
gas space
+
to
vascular
cylinder
voids
with air
space
water
Root Functions
3. Roots can be used for storage of food
reserves.
4. Roots can determine the longevity of the
plant – annual, biennial or perennial. In
biennial plants, large food reserves
accumulate during the first year of vegetative
growth. Then they are used to produce
flowers, fruits and seeds the second year. In
perennial plants, the food reserves allow the
root to survive adverse environmental
conditions and then produce new vegetative
growth in the spring. Do roots of annuals
store a lot of food? Why or why not?
5. Roots are sites of hormone production –
especially cytokinins and gibberellins, which
are then transported upward in xylem to stems
and leaves.
Endodermis
Xylem inside
• The endodermis is thus responsible
for selective mineral uptake.
Suberin- waxy barrier to
apoplastic movement
Cortex outside
• minerals cannot go
between cells
• minerals must go
through cells
• Cell membrane proteins (active
transporters) determine which
minerals may be taken up
Close-up of Transport through Endodermis
Root Types
• Tap Root- The primary root of a plant
formed from the radicle of the embryo;
a stout tapering main root from which
arise smaller lateral roots or branches.
• If the radicle or taproot is damaged, a
lateral root emerges and assumes the
dominating role. Growth of the taproot
continues throughout the life of the
plant. This is common in
gymnosperms and dicots.
• Lateral or Branch Roots- A root that
arises from another older root, arises
from the pericycle.
Root Types
• Fibrous Roots – A root system in which no
one root is more prominent than the others –
all roots are more or less the same size (at
least in diameter – length may vary),
typically the plant has a mass of these.
• Common in monocots
• Arise because the radicle dies during or soon
after germination; usually adventitious in
origin.
• Plants may be perennial or annual. While
individual roots are small, the mass of them
have enough stored food reserves to allow
the plant to survive more than one growing
season.
Root Types
• Adventitious Roots- Roots that develop from organs
other than pre-existing roots (so leaves and stems!)
• The root system of grasses is all or mostly adventitious.
Also found in ferns, clubmosses and horsetails.
• Contractile Roots- Roots that contract to pull the shoot,
corm, or bulb down deeper into the soil. Roots extend
through the soil and become firmly anchored. The
uppermost parts begin to contract and the stem, etc. is
pulled downward so it buries deeper.
• This is caused by changes in the shape of cortical cellsthey expand radially and shorten, losing ½ - ⅔ of their
height; vascular tissue buckles but does not lose its
function.
Narcissus
Root Types
• Aerial Roots- Are adventitious roots produced from
above ground structures; in ivy (Hedera), the roots
cling to the surface of objects (trees or walls) to
support the climbing stem.
• Velamen- A multiple layered epidermis that surrounds
the aerial roots of some epiphytic orchids and
provides mechanical protection for cortex and
reduction of water loss.
There are many specialized types of aerial roots:
• Prop Roots- Roots that serve for support, as in corn.
They can branch down from lower nodes of the stem
or drop down from branches as in some trees. They
can enter the soil and absorb water and nutrients.
Examples: Mangroves, Banyan tree, palms.
Root Types
• Pneumatophores- Roots that grow upward from
submerged roots in mud/water; found in trees
that grow in swamps, such as mangroves
• Do help to anchor the plant, but main function is
to aerate the plant; allows air to travel through
the root system and provide O2 for respiration;
roots have aerenchyma and lenticels.
• Buttress Roots- In shallowly rooted trees at the
base of the trunks.
• They may be up to 4 meters tall; and are
specialized for support with large amounts of
fibers and small amounts of xylem.
• Where else have you heard the word buttress
before?
Root Types
• Storage Roots- most roots store food
reserves, some plants have roots
specialized for this function.
• They are usually fleshy because of
abundance of storage parenchyma
permeated by vascular tissue; the
upper portion of most fleshy roots
actually develop from the hypocotyl.
• Examples: sweet potato, sugar beet;
roots of some desert plants (they can
store more than 70 kg of water!)
Haustoria (Haustorium)
• Haustoria- specialized roots on plants that are
parasitic; they are modified structures that penetrate
the host tissue to transfer nutrients from the host to the
parasite
• A primary haustorium is a direct outgrowth of the
radicle
• A secondary haustorium is a lateral organ that
develops from a modified adventitious root or from
outgrowths of roots or stems
• Examples: mistletoe (Viscum), dodder (Cuscuta),
broomrapes (Orobanche)
• In some species, the haustorium penetrates the host
tissue to the xylem and forms a continuous xylem
bridge.
• In others, the haustorium forms a mantle of
parenchymatous tissue around the host organ. This
induces the host to form a placenta-like outgrowth of
vascular tissue to supply nutrients to the parasite.
Rhizosphere
Region of space around the
whole root system including
the soil between roots of the
same plant.
Nitrogen Fixation
• Root Nodules- enlargements found on the roots of
many plants inhabited by nitrogen-fixing bacteria.
• Plants cannot use atmospheric N2, but some bacteria
can convert it to a form plants can use.
• The bacteria provide the enzymes needed for nitrogen
fixation; the host plant provides the sugar to run the
operation.
• Examples” Fabaceae (pea family) and the bacteria
Rhizobium; Alnus (alder), Ceanothus (tea bush),
Myrica (sweet gale) and Elaeagnus (oleaster) are
usually infected by an actinomycete Frankia. Cycads
have root nodules containing cyanobacteria – Nostoc
or Anabaena; the water fern Azolla may also be
infected with Aanabaena.
• Infestation of the host root
occurs through root hairs.
• The plant starts forming
nodules in response and
bacteria aggregate within cells
in the center of the nodule.
• Bacteria are surrounded by
parenchyma tissue with
vascular bundles and an
endodermis.
• The endodermis retards entry of
O2 to the nodule, which would
adversely affect N2 fixation.
• Plasmodesmata connect the
bacteria-containing cells with
surrounding parenchyma cells.
Mycorrhizae
• The surface area of root tips and root hairs
are relatively small.
• In times of drought, the volume of soil that
provides water and minerals can be
depleted.
• Mycorrhizae greatly enhance the roots’
absorptive capacity.
• This is a symbiotic relationship between a
plant and a fungus.
• The fungus can be either an ascomycete,
an basidiomycete or an zygomycete.
• 90% of all terrestrial plants are
mycorrhizal.
• Mycorrhizae have NOT been found in
Brassicaceae, Cyperaceae, and
Cucurbitaceae.
• Ectomycorrhizae- the fungus forms a
dense sheath of hyphae around the
root tip.
• Hyphae extend from the sheath into
the soil and also penetrate the
epidermis and cortex of the root.
• The Hartig net is a much branched
network of hyphae in the
intercellular channels of the
parenchyma cells. These cells
function as transfer cells,
transporting photosynthates to the
fungus and water and minerals from
the fungus to the plant.
• Endomycorrhizae- the most common type (80% of vascular plants)
• The fungal hyphae do not form a sheath around the root; the hyphae penetrates the roots
and grows within the cortex. Hyphae penetrate cells walls but not the plasma membranes.
• Complex much-branched structures called arbuscules are formed by the hyphae that are
enclosed by plasma membranes (but not within the protoplast).
• This is an efficient region of transfer of photosynthates to the fungus, as well as water and
minerals to the plant. This is also called vesicular-arbuscular mycorrhize.
Cool Exceptions to the Rules!
This is Dendrophylax lindenii , and the green things are
photosynthetic roots (chlorophyll in its root cortex). The leaves are
microscopic or non-existent.
• A unique (and really cool) aerial
root is found in the epiphyte
Dischidia rafflesiana
(Asclepiadaceae), a.k.a. the flower
pot plant.
• Some leaves form hollow
containers that collect debris and
rainwater. Ant colonies live in the
“pots” and add N2.
• Roots form at the node above the
leaf and grow downward into the
pot to absorb water and nutrients.
Look a monocot with a tap root!
Cordyline australis
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