Roots and Root-Soil Relations
& Nitrogen Assimilation
Chp. 13 & 8
Roots, Soils, and Nutrient Uptake
With the exception of carbon and oxygen, which are supplied as
carbon dioxide from the air, terrestrial plants generally take up nutrient
elements from the soil solution through the root system.
The root systems of most plants are surprisingly extensive.
Through a combination of primary roots, secondary and tertiary
branches, and root hairs, root systems penetrate massive volumes of soil in
order to mine the soil for required nutrients and water.
Soil is a complex
medium.
It consists of a solid phase that includes mineral particles
derived from parent rock plus organic material in various stages of
decomposition, a liquid phase that includes water or the soil solution, gases
in equilibrium with the atmosphere, and a variety of microorganisms.
The solid phase, in
particular the mineral particles, is the primary source of nutrient elements.
In the process of
weathering, various elements are released into the soil solution, which then
becomes the immediate source of nutrients for uptake by the plant.
THE SOIL AS A NUTRIENT RESERVOIR
Soils vary widely with respect to composition, structure, and
nutrient supply.
Especially important
from the nutritional perspective are inorganic and organic soil particles
called colloids.
Soil colloids retain
nutrients for release into the soil solution where they are available for
uptake by roots.
Thus, the soil colloids serve to maintain a reservoir of
soluble nutrients in the soil.
In this chapter we will examine the availability of nutrients
in the soil and their uptake by roots. This will include
the soil as a source of nutrient elements, the colloidal
nature of soil, and ion exchange properties that determine the availability of
nutrient elements in a form that can be taken up by roots
mechanisms of solute transport across membranes, including
simple and facilitated diffusion and active transport, the function of membrane
proteins as ion channels and carriers, and the role of electrochemical
gradients
ion traffic into
and through the root tissues and the concept of apparent free space and
the beneficial
role of microorganisms, especially fungi, with respect to nutrient uptake by
roots.
Nutrient Uptake & Sustainable
Agriculture
A
deficiency of an element makes it difficult or impossible for the plant to
complete a vegetative or reproductive stage of development
A
deficiency can be prevented or corrected by supplying the element
Nutrient Uptake from soil to plant via root
Movement to the roots:
1)
Root extension - exposure to soil and new supplies of nutrients - roots could
contact 3% of the soil or nutrients in the soil.
2) Mass Flow
Ψ
water absorbed by the root creates a
water deficit near the root,
Ψ
more water moves to the root carrying
nutrients with the water.
Ψ
Important for nutrients in large
quantities in the soil solution - N, K & Ca
NUTRIENT UPTAKE BY PLANTS
REQUIRES TRANSPORT OF THE NUTRIENT ACROSS ROOT CELL MEMBRANES
In order for mineral nutrients to be taken up by a plant, they
must enter the root by crossing the plasma membranes of root cells.
From there they can be
transported through the symplast to the interior of the root and eventually
find their way into the rest of the plant.
Nutrient
uptake by roots is therefore fundamentally a cellular problem, governed by the
rules of membrane, transport.
There
are, however, three fundamental concepts--simple diffusion, facilitated
diffusion, and active transport-that have persevered, largely because they have
proven particularly useful in categorizing and interpreting experimental
observations.
Nutrient Uptake
3) Diffusion - movement of nutrients due to an imbalance of
concentration ( diffusion gradient)
Requirements for nutrient uptake by plants
Actively growing plants - anything that
affects the metabolism of the plant will affect nutrient uptake
Metabolic energy is required. Plant roots must be able to respire. Soils
must have oxygen
Conditions required for Nutrient Uptake by plants
Nutrient Uptake
Nutrient Uptake
Functions of Roots
Absorption
Anchorage
Storage
Root Systems
Tap
vs. fibrous root systems
Root Systems
Roots
may occupy about 1% of total soil volume (with actively growing crops)
In
wildlands (e.g. forest, grassland), roots may represent up to 50% of total
plant mass (roots + shoots)
In
agriculture, roots may represent about 25% of total plant mass
Depth of Root Systems
Lettuce 30
cm
Cotton 120-150
cm
Alfalfa 250-300
cm
Turf 60-90
cm
Mesquite 50 m ?????
Regardless of root system depth, the most
important roots for nutrient and water uptake are usually found in the top
15-20 cm of soil.
Roots and Plant Survival Strategies
Desert Perennials (other than cacti)
Much
of their biomass is below ground
Desert Annualssmall root systems
Phreatophytes (e.g. mesquite)
Dual
Root system
Shallow
for capturing rainfall
Deep
for tapping into the water table
Many plant species, when confronted with
drought or nutrient deficiency, will increase root growth at the expense of
shoots.
Root Morphology
Longitudinal
Meristematic
zone
Elongation
zone
Maturation
zone
Mature
zone
Cross-sectional
Epidermis
Cortex
Endodermis
Stele
Root Morphology
Functions
Root
cap
Protection
of meristem
Secretion
of mucigel
Initiation
of symbiotic relationships
Elongation
Zone
Elongation
of cells forces root through the soil
Maturation
zone
Root
Hairs
Major
zone of water and nutrient uptake
Root Hairs
Single-cell
extensions of epidermal cells
Tremendously
important for providing surface area for water and nutrient uptake
Produced
in the maturation zone
Sites
of infection by pathogens, N fixing bacteria, and mycorrhizae
Root Hairs
Root hairs can account for 2/3
of total root surface area.
Root hairs are fragile and susceptible to
breakage as soils dry.
Size of Roots
Fine lateral roots are 0.1 to 0.2 mm in
diameter.
Root hairs are 0.01 to 0.05 mm in
diameter.
Soil micropores are considered to be
those <0.08 mm in diameter.
Therefore, root hairs are important for
accessing water and nutrients in micropores.
Mature Roots
The root epidermis and endodermis become
covered with a waxy substance known as suberin.
Function:
Water and nutrient uptake rates are lower in mature root zones than in
immature:
Lower permeability
Formation of aerenchyma (air pockets)
Aerenchyma
Implications of Root Morphology
The youngest part of the root is more
permeable to water and nutrients than is the older part (behind the maturation
zone).
Most nutrients are taken up in the younger root zone (feeder roots) with
abundant root hairs.
Excessive soil drying and mechanical
disturbance will damage young roots, reduce nutrient uptake.
The Rhizosphere
Definition: a zone of soil where microbial activity is influenced by roots
Why?
Border cells, root leakage and secretion
of organic compounds
May extend to about 2 mm from root
surface.
The Rhizosphere
Properties:
Higher available C for microbial growth
Higher microbial population and activity
Lower pH than bulk soil
Lower
O2 than bulk soil
Altered nutrient availability for plants
Border Cells in the Rhizosphere
Organic C in the Rhizosphere (Rhizodeposition)
Plants release simple and complex
carbohydrates, nucleic acids, enzymes into the rhizosphere
These compounds are used by
microorganisms as sources of C and energy.
Up to 30% of plant C fixed in
photosynthesis may be released from roots.
Importance of the Rhizosphere
A
healthy rhizosphere will help plants by:
Increasing
nutrient availability
Suppressing
pathogens
Increasing
water availability
However,
the effects of agricultural management on the rhizosphere are still largely
unknown.
Mycorrhizae (my·cor·rhi·za \ˌmī-kə-ˈrī-zə) were originally
discovered by the nineteenth-century German botanist A. B. Frank, who
concluded, on the basis of experiments conducted with beech seedlings, that
mycorrhizal inoculation stimulated seedling growth.
Although not universally accepted in the beginning, these
results have been amply confirmed by more modern studies.
Numerous studies with pine and other tree seedlings in the United States, Australia,
and the former Soviet Union have demonstrated
30 to 150 percent increases in dry weight of tree seedlings infected with
mycorrhizae when compared with noninfected controls.
. The primary cause of mycorrhizal-enhanced growth appears to
be enhanced uptake of nutrients, especially phosphorous.
In a classic experiment, Hatch demonstrated in 1937 that
infected pine seedlings absorbed two to three times more nitrogen, potassium,
and phosphorous.
Coupled with enhanced
nutrient uptake is the observation that mycorrhiza-induced growth responses are
more pronounced in nutrient-deficient soils.
Mycorrhizae
Two
kinds:
Ectomycorrhizae
Endomycorrhizae
MYCORRHIZAE ARE FUNGI THAT INCREASE THE VOLUME OF THE NUTRIENT
DEPLETION ZONE AROUND ROOTS
Perhaps
the most widespread-and from the nutritional perspective, more
significant-associations between plants and microorganisms are those formed
between roots and a wide variety of soil fungi.
A
root infected with a fungus is called a mycorrhiza (literally, fungus root).
Mycorrhizae
are a form of mutualism, an association in which both partners derive benefit.
The
significance of mycorrhizae is reflected in the observation that more than 80
percent of plants studied, including virtually all plant species of economic
importance, form mycorrhizal associations.
Two major forms of mycorrhizae are known: ectotrophic and
endotrophic
The
ectotrophic form, also known as ectomycorrhizae, is restricted to a few
families consisting largely of temperate trees and shrubs,the intercellular or
apoplastic space of the root cortex,such as pines (Pinaceae) and beech
(Fagaceae).
Ecto-forming an intercellular network called a Hartig net-mycorrhizae are
typically short, highly branched.
Endotrophic
mycorrhizae, or endomyensheathed by a tightly interwoven mantle of fungal
mycorrhizae, are found in some species of angiosperm family and most
gymnosperms as well (except the Pinaceae).
Unlike
the ectomycorrhizae, the hyphae of endomycorrhizae develop extensively within
cortical cells of the host roots.
Plants can also increase effective root
length/surface area by investing in symbioses
with soil fungi, mycorrhizae
Mycorrhizae, by increasing effective root
length, increase the volume of soil
exploited by roots
Most plants, (around 75%), are
mycorrhizalIn mutualistic mode, the plant provides photosynthate (fixed C) to
the fungus, whereas the fungus provides nutrients and water to the plant
The mycorrhiza (fungus-root) is the
interface where fungal hyphae actually penetrate the root, forming an interface
where nutrients and carbon are exchanged
The structure and nature of this
interface varies somewhat among mycorrhizal types
There are
four major types of mycorrhizae:
1. ectomycorrhizae: temperate, boreal, some tropical forests
2. arbuscular mycorrhizae (AM, used to be called VAM): herbaceous communities
and tropical forests
3. ericoid mycorrhizae: heathlands, tundra
4. orchid mycorrhizae
ecto- and arbuscular- mycorrhizae are the most widespread
Arbuscular Mycorrhizae
fungal hyphae penetrate walls of cortical cells (but not
plasma membrane)
produce highly branched "arbuscules" in close
association with plant cell plasma membrane, forming the point of transfer of
nutrients and carbon
especially important for P uptake
also important in water stress
Ectomycorrhizae
common in woody plants
Mantle or sheath of hyphae surrounds the root
Hyphae penetrate intercellular spaces of
root cortex to form Hartig Net
(point of material exchange between plant and fungus), but do not penetrate cortical cell walls
plants respond to infection by growing
short branched "club" roots
important in acquiring P and N;
fungus produces proteases that
cleave proteins into amino acids
may be important in water stress
Roots and Mycorrhiza an old symbiosis
Mutual benefit
Carbohydrates for the fungus
P,
Zn, Cu, water, N for plant
Different types
1. Vesicular-arbuscular mycorrhiza
VA-mycorrhiza
2. Ectomycorrhiza
Other types
ericoid
orchid
endomycorrhiza
1.
Vesicular arbuscular mycorrhiza (AM)
Glomales (130 species infects 300.000 plant
species)
Found on roots of herbaceous angiosperms, most
trees, mosses, ferns
not present on Cruciferae, Chenopodiaceae,
Proteaceae
small biomass compared to roots
Vesicular
Arbuscular Mycorrhiza
Inside root
Intercellular mycelium
Intracellular arbuscule
tree-like haustorium
Vesicle with reserves
Outside root
Spores (multinucleate)
Hyphae
thick runners
filamentous hyphae
Form extensive network of hyphae
even connecting different plants
Summary on mycorrhizae
Symbiosis with mycorrhiza allows greater soil
exploration,
and increases uptake of nutrients (P, Zn, Cu,
N, water)
Great SA per mass for hyphae vs. roots
Mycorrhiza gets carbon from plant
Two main groups of mycorrhiza Ectomycorrhiza
and VA-mycorrhiza
Ectomycorrhizae
common in woody plants
Mantle or sheath of hyphae surrounds the
root
Hyphae penetrate intercellular spaces of
root cortex to form Hartig Net (point of material exchange between plant
and fungus), but do not penetrate
cortical cell walls
plants respond to infection by growing
short branched "club" roots
important in acquiring P and N;
fungus produces proteases
that cleave proteins into amino acids
may be important in water stress
Two Other Important Types
of Mycorrhizae
Ericoid mycorrhizae (ericoid fungus
associations)
like EM fungi, ericoid fungi produce
proteases (enzymes that hydrolyze organic forms of N); amino acids can then be
transferred to the host plant
thus, important in plant N nutrition
ericoid fungi also produce phenol
oxidases, capable of humus degradation
ericoid hyphae do not extend as far
beyond the root as EM fungi
Orchid mycorrhizae (orchid fungus
associations)
in some cases, fungus provides carbon
as well as nutrients!)
Mycorrhizae
Mycorrhizae
Mycorrhizae
A fungal-root symbiosis
increase
root length and surface area
help
plants to take up nutrients that are immobile in soil (e.g. P)
inhibit
heavy metal uptake
increase
water uptake by roots
are
vital for the growth and survival of plants where nutrients are likely to be
limiting, and/or where heavy metals are problems
Managing Mycorrhizae
About 80% of all plants have mycorrhizael
associations.
Innoculation (addition) with mycorrhizae
is often used for nursery and forestry plantings.
Addition of mycorrhizae to agricultural
soils has shown few benefits so far:
Difficulty of establishing Endo fungi
Nitrogen Fixation
Definition: The conversion of atmospheric N2
to forms usable by plants.
Certain bacteria and actinomycetes can
carry out N fixation.
The most important N fixation occurs
through a symbiotic relationship between legume roots and bacteria.
Legume Root Nodules
Nitrogen Fixation
Legumes
commonly form N-fixation symbioses
N
fixation is not free
may
cost the plant up to 30% of the C it captures from the atmosphere in
photosynthesis
Rate
of N fixation is inversely proportional to available N in the soil.
Importance of N Fixation
Legume crops can derive virtually all
their N requirement through N fixation.
In crop rotations, legume crop residues
can be incorporated into the soil. Upon decomposition, N is released.
Legumes and non-legumes can be grown
togetherin this case 20-50% of the N fixed in the legume may become available
to the non-legume.
Final Thoughts on Roots
Root system is responsible for water and
nutrient uptake.
The youngest part of the root system is
responsible for much of this uptake
Root hairs are critical, fragile and can
be easily damaged by compaction, tillage, etc.
The root system is dynamic and will
respond to changes in soil conditions.
Final Thoughts (2)
The root is part of a microbial community
known as the rhizosphere
A healthy rhizosphere is important for
plant health
Theres a lot about it we dont know yet
Mycorrhizae - water, nutrients, pathogen
protection
Rhizobia - N fixation
Assimilation and fixation of nitrogen
Remember, Plants are:
Capable
of making all necessary organic compounds from inorganic compounds and elements
in the environment (autotrophic)
Required
to compete with other organisms for these nutrients
Required
to employ complex energetic pathways to convert macronutrients to useable forms
Nitrogen in the environment
Many biochemical compounds present in
plant cells contain nitrogen
Nucleoside phosphates
Amino acids
These form the building blocks of nucleic
acids and protein respectively
Only carbon, hydrogen, and oxygen are nor
abundant in plants than nitrogen
Nitrogen Assimilation
On a dry-weight basis, nitrogen is the
fourth most abundant nutrient element in plants.
It
is an essential constituent of proteins, nucleic acids, hormones, chlorophyll,
and a variety of other important primary and secondary plant constituents.
Most plants obtain the bulk of their nitrogen
from the soil in the form of either nitrate (NO3) or ammonium (NH4), but the
supply of nitrogen in the soil pool is limited and plants must compete with a
variety of soil microorganisms for what nitrogen is available.
As
a result, nitrogen is often a limiting nutrient for plants, in both natural and
agricultural ecosystems.
Nitrogen in the environment
Present in many forms
78% of atmosphere is N2
Most of this is NOT available to living
organisms
Getting N2 for the atmosphere requires
breaking the triple bond between N2 gas to produce:
Ammonia (NH3)
Nitrate (NO3-)
So, N2 has to be fixed from
the atmosphere so plants can use it
Nitrogen in the environment
This
occurs naturally by:-Lightning:
8%:
splits H2O: the free O and H attack N2 forms HNO3
(nitric acid) which fall to ground with rain
Photochemical
reactions:
2%:
photochemical reactions between NO gas and O3 to give HNO3
Nitrogen
fixation:
90%:
biological bacteria fix N2 to ammonium (NH4+)
Nitrogen in the environment
Once fixed in ammonium or nitrate :-
N2 enters biochemical cycle
Passes through several organic or
inorganic forms before it returns to molecular nitrogen
The ammonium (NH4+)
and nitrate (NO3-) ions generated via fixation are the
object of fierce competition between plants and microorganisms
Plants have developed ways to get these
from the soil as fast as possible
Root uptake soon depletes nutrients near the roots
Formation of a nutrient depletion zone in
the region of the soil near the plant root
Forms when rate of nutrient uptake
exceeds rate of replacement in soil by diffusion in the water column
Root associations with Mycorrhizal fungi
help the plant overcome this problem
Mycorrhizal associations
Not unusual
83%
of dicots, 79% of monocots and all gymnosperms
Ectotrophic
Mycorrhizal fungi
Form a thick sheath around root. Some mycelium
penetrates the cortex cells of the root
Root
cortex cells are not penetrated, surrounded by a zone of hyphae called Hartig
net
The
capacity of the root system to absorb nutrients improved by this association the
fungal hyphae are finer than root hairs and can reach beyond nutrient-depleted zones
in the soil near the root
Mycorrhizal associations
Nutrients move from fungi to root cells
Ectotrophic
Mycorrhizal
Occurs by simple diffusion from the
hyphae in the hartig net to the root cells
Vesicular
arbuscular mycorrhizal fungi
Occurs
by simple diffusion from the arbuscules to the root cells
Also,
as arbuscules are degenerating as new ones are forming, the nutrients may be
released directly into the host cell
Stored ammonium can be toxic
Plants can store high levels of nitrate
or translocate it via the phloem without any effect.
However, high levels of ammonium are
toxic
Dissipates
transmembrane proton gradients required for both photosynthetic
and respiratory electron transport
AND movement
of metabolites to vacuoles.
Stored ammonium can be toxic
At high pH in stroma,
matrix or cytoplasm:
Ammonium reacts with OH- to produce NH3.
NH3 is
membrane permeable and diffuses freely across a membrane
down a concentration gradient
At
low pH in intermembrane space, lumen, or vacuole:
NH3 reacts with H+
to form ammonium
Remember: Nitrogen the most important mineral nutrient in
the soil
Nitrogen
is frequently limiting in in terrestrial systems terrestrial systems
Microbial
activity is continually converting N to lower energy forms
Conversion
to organic form requires raising N to higher energy levels
Nitrate Assimilation
Nitrogen
assimilation
NO3 NO2
NH4+ amino acids
nitrate nitrite ammonium
Requires
large input of energy
Forms
toxic intermediates
Mediated
by specialized enzymes that are closely regulated are closely regulated
Doesnt
have to start at the beginning
Nitrogen assimilation
Plants assimilate most of the nitrate
absorbed by their roots into organic nitrogen compounds.
The first step of this process is the
reduction of nitrate to nitrite in the cytosol by the enzyme nitrate
reductase.
Nitrogen assimilation
NAD(P)H induces NADH or NADPH
The most common form of nitrate reductase
uses only NADH as an electron donor
The nitrate reductases of higher plants
are composed of two identical sub-units, each containing three prosthetic
groups
FADflavin
adenine dinucleotide
Heme
Molybdenumorganic
molecule called pterin
Nitrate Assimilation
Nitrate reductase
is the main molybdenum containing protein in vegetative tissues
Nitrate levels, light intensity, and
concentration of carbohydrates all influence the activity of nitrate
reductases at the transcription and translation levels
These factors stimulate a protein, phosphatase,
that dephosphorylates several serine residues on the nitrate reductase protein
thereby activating the enzyme
This dephosphorylation/phosphorylation
cycle provides more rapid control over this enzyme than degredation/synthesis
of new enzyme would achieve ( minutes versus hours)
Nitrite Reductase Converts Nitrite to Ammonium
Nitrite (NO2-)is
highly reactive
Plant cells immediately transport the
nitrite generated by nitrite reduction from the cytosol into chloroplasts in
leaves and plastids in roots
In
these organelles, nitrite reductase reduces nitrite to ammonium
Nitrite Reductase Converts Nitrite to Ammonium
Chloroplast and root plastids contain
different forms of the enzyme, but both forms consist of a single polypeptide
containing an iron sulfur cluster and a specialized heme group
The
heme does redox reactions and electron flow, just like the reaction sites of
chlorophyll
Nitrite Reductase Converts Nitrite to Ammonium
Nitrite is highly reactive
Plant cells immediately transport the
nitrite generated by nitrite reduction from the cytosol into chloroplasts in
leaves and plastids in roots
In
these organelles, nitrite reductase reduces nitrite to ammonium
Chloroplast and root plastids contain
different forms of the enzyme, but both forms consist of a single polypeptide
containing an iron sulfur cluster and a specialized heme group
The
heme does redox reactions and electron flow, just like the reaction sites of
chlorophyll
Plants assimilate nitrate in both roots and shoots
In many plants, when the roots receive
small amounts of nitrate, this nitrate is reduced primarily in the roots
As nitrate supply increases, a greater
proportion of the absorbed nitrate is translocated to the shoot and assimilated
there
Generally, species native to temperate
rely more heavily on nitrate assimilation by the roots than do species of
tropical or subtropical origins
Ammonium Assimilation
Plants cells avoid ammonium toxicity by
rapidly converting the ammonium generated from nitrate assimilation or
photorespiration into amino acids
This requires the action of two enzymes
Glutamine synthetase combines
ammonium with glutamate to form glutamine
Glutamate synthase stimulated by
elevated levels of glutamine synthetase
Transfers the amino group
of glutamine to an intermediate yielding two molecules of glutamate
This accounts for most of the fixation of
atmospheric N2 into ammonium
Represents
the key entry point of molecular nitrogen into the biogeochemical cycle of
nitrogen
Free living and symbiotic bacteria are
responsible for converting atmospheric nitrogen into ammonium
Most of these are free living in the
soil, a few form symbiotic associations with higher plants
The prokaryote directly provides the host
plant with nitrogen in exchange for other nutrients and carbohydrates
The
most common association is between members of the plant family leguminosae
and bacteria of the genera Azorhizobium
Nitrogen Fixation Requires Anaerobic Conditions
As
oxygen irreversibly inactivates the nitrogenase enzymes involved in nitrogen
fixation, nitrogen must be fixed under anaerobic conditions
Therefore
each of the nitrogen-fixing organisms either functions under natural anaerobic
conditions or can create an internal anaerobic environment in the presence of
oxygen
Nitrogen Fixation Requires Anaerobic Conditions
In
cyanobacteria, anaerobic conditions are created in specialized cells called heterocysts
These
are thick-walled cells which lack photosystem IIthe oxygen producing
photosystem of chloroplasts
Cyanobacteria can fix nitrogen under
anarobic conditions such as those that occur in flooded fields
In Asian countries, nitrogen fixing
cyanobacteria of both the heterocyst and non-heterocyst types are the major
means of maintaining an adequate nitrogen supply in rice fields
They
fix nitrogen when the fields are flooded, and die as the fields dry, releasing
the fixed nitrogen into the soil
Symbiotic Nitrogen Fixation Occurs in Specialized Structures
Symbiotic nitrogen-fixing prokaryotes
dwell within nodules
Special
organs of the plant host that enclose the nitrogen-fixing bacteria
Grasses can also develop symbiotic
relationships with nitrogen-fixing organisms, but these associations do not
lead to the formation of root nodules
Nitrogen-fixing bacteria seem to colonize
plant tissues or anchor to the root surface, mainly around the elongation zone
and the root hairs
Known
as actinorhizal plants
Symbiotic Nitrogen Fixation Occurs in Specialized Structures
Both legumes and actinorhizal plants
regulated gas permeability in their root nodules
Maintaining a level of oxygen within the
nodule that can support cellular respiration for the bacteria, but still
sufficiently low to avoid inactivation of the nitrogenase
Nodule formation involves several phytohormones
During root nodule formation, two process
occur simultaneously
Infection
and Nodule Organogenesis
(A)
Rhizobia attach to the root hairs and release nod factors that produce a
pronounced curling of the root hair cell
(B)
Rhizobia get caught and curl, degrade the root hair cell wall allowing the
bacterial cells direct access to the outer surface of the plant plasma membrane
Nodule formation involves several phytohormones
(C)
Then the infection thread forms
Formed from Golgi depositing material at
the tip at the site of infection. Local
degradation of root hair cell wall also occurs
(D)
Infection thread reaches the end of the cell, and thread plasma membrane fuses
with plasma membrane of root hair cell
Then bacterial cells are released into
the fused plasma membranes
Nodule formation involves several phytohormones
(E)
Rhizobia are released into the apoplast and enter the middle lamella,
This leads to the formation of a new
infection thread, which forms an open channel with the first
(F)
Infection thread expands and branches until it reaches target cells
Vesicles composed of plant membrane
enclose bacterial cells and they are released into the cytoplasm
Nodule formation involves several phytohormones
At first bacteria continue to grow with
vesicles expanding by fusing with smaller vesicles
Following an as yet determined chemical
signal from the plant, bacteria stop dividing and differentiate
Forms nitrogen-fixing
organelles called bacteroids
The nodule itself develops a vascular
system
To
exchange fixed nitrogen for nutrients from the plant
And a layer of cells to exclude O2
from the rood nodule interior
The nitrogenase enzyme complex fixes N2
Biological nitrogen fixation produces ammonium
(NH3) from molecular nitrogen.
N2 + 8e-
+ 8H+ + 16 ATP 2NH3
+ H2+ 16 ADP + 16 Pi
Note that the reduction
of N2 to 2NH3 is a six-electron transfer, and is coupled
to the reduction of two protons to evolve H2
This
reaction is catalyzed by nitrogenase enzyme complex
The nitrogenase enzyme complex fixes N2
Can be separated into two components
The
Fe protein
The
MoFe protein
Neither of which has catalytic activity
by itself
The nitrogenase enzyme complex fixes N2
Ferredoxin
reduces the Fe protein
Binding
and hydrolysis of ATP to the Fe protein is thought to cause a conformational
change of the Fe protein that facilitates the REDOX reactions
The Fe
protein reduces the MoFe protein, and the MoFe
protein reduces the N2
The MoFe protein can reduce many substances
The MoFe protein can reduce many
substrates
Although under natural conditions the
MoFe only reacts with N2 and H+.
Summary
Nutrient assimilation is the process by
which nutrients acquired by plants are incorporated into the carbon
constituents necessary for growth and development.
For
Nitrogen:
Assimilation is but one in a series of
steps that constitute the nitrogen cycle.
The principal sources of nitrogen
available to plants are nitrate (NO3-) and ammonia (NH4+).
Nitrate absorbed by roots is assimilated
in either shoots or roots
depending on nitrate
availability and plant species
In nitrate assimilation, nitrate (NO3-)
is reduced to nitrite (NO2-) in the cytosol via the
enzyme nitrate reductase.
Then nitrite is reduced to ammonium (NH4+)
in roots by nitrite reductase.
Ammonium (NH4+)
from either root absorption or generated through nitrate assimilation or photorespiration is converted glutamine or
glutamate through the sequential actions of glutamine synthase and glutamate
synthase.
Once assimilated into either glutamine or
glutamate, nitrogen mat be transferred to many other organic compounds
Via
transaminatation reactions
Summary
Many plants form a symbiotic relationship
with nitrogen fixing bacteria that contain an enzyme complex, nitrogenase,
that can reduce atmospheric nitrogen to ammonia.
Legumes and actinorhizal plants form
associations with rhizobia.
These associations result from a finely
tuned interaction between the bacteria and the host plant
Involves
the recognition of specific signals between the symbiotic bacteria and the host
plant
Any Questions?
Nitrogen - the most limiting soil
nutrient
Evidence - factorial fertilization
experiments (N, P, K, etc.)show largest growth response to N.
Required in greatest amount of all soil
nutrients
2. A component of proteins (enzymes,
structural proteins,
chlorophyll, nucleic acids)
3. The primary photosynthetic enzyme,
Rubisco, accounts for a 25 to 50% of leaf N.
Photosynthetic
capacity is strongly correlated with
leaf N concentration.
4. Availability in most soils is low
5. Plants spend a lot of energy on N
acquisition growing roots, supporting symbionts, uptake into roots,
biochemical assimilation into amino acids, etc.
The inorganic forms of nitrogen in soils.
NH4+, ammonium ion. A cation that is
bound to clays.
NO3-, nitrate ion. An anion that is not
bound to clays.
Nutrient mobility in soils refers to
the rate of diffusion, which is influenced by nutrient ion interactions with
soil particles.
Would you expect NH4+ or
NO3- to diffuse more rapidly?
Would you expect a more
pronounced depletion zone for NH4+ or NO3-?
The beneficial role of mycorrhizae, particularly with respect
to the uptake of phosphorous, appears to be related to the nutrient depletion
zone that surrounds the root .
This zone defines the limits of the soil from which the root
is able to readily extract nutrient elements.
Additional nutrients can be made available only by extension
of the root into new regions of the soil or by diffusion of nutrients from the
bulk soil into the depletion zone.
The extent of the depletion zone varies from one nutrient
element to another, depending on the solubility and mobility of the element in
the soil solution.
The depletion zone for nitrogen, for example, extends some
distance from the root because nitrate is readily soluble and highly mobile.
Phosphorous, on the other hand, is less soluble and relatively immobile in
soils and, consequently, the depletion zone for phosphorous is correspondingly
smaller.
Mycorrhizal fungi assist in the uptake of phosphorous by
extending their mycelia beyond the phosphorous depletion zone.
Apparently, mycorrhizal plants find it advantageous to expend
their carbon resources supporting mycorrhizal growth as opposed to more
extensive growth of the root system itself.
As we continue to learn about mycorrhizae, their nutritional
role becomes increasingly evident.
Many mycorrhizae are host species-specific.
Attempts to establish a plant species in a new environment may
be unsuccessful if the appropriate mycorrhizal fungus is not present.
Inoculation of fields with mycorrhizal fungi is now an additional
factor taken into account by the forest and agricultural industries when
attempting to resolve problems of soil infertility.
CHAPTER REVIEW
1. Distinguish between simple diffusion, facilitated
diffusion, and active transport. Which of these three mechanisms would most
probably account for:
(a) entry of a small lipid-soluble solute;
(b) extrusion of sodium ions leaked into a cell; (c) rapid
entry of a neutral hydrophilic sugar; (d) accumulation of potassium ions?
2. The Casparian band was encountered earlier with regard to
root pressure and again in this chapter with regard to ion uptake. What is the
Casparian band and how does it produce these effects?
3. Trace the pathway taken by a potassium ion from the point
where it enters the root to a leaf epidermal cell.
4. What are channel proteins and what role do they play in
nutrient uptake?
5. Describe the concept of apparent free space. What role does
apparent free space play in the uptake of nutrient ions?
6. Why is an accumulation ratio greater than 1.0 not
necessarily an indication that active transport is involved?