37.3 Plants Nutrition Often Involves
Other Relationship with Other Organisms
Farrah Younes
Period 7 and 8
AP Biology
What is going to be covered:
A.
B.
C.
D.
E.
Rhizobacteria
Nitrogen Cycle/ Root Nodules
Detail of Figure 37.12
Crop Rotation
Mycorrhizae
Background Information
Plants and soil have a two-way relationship; Dead plants provide much
of the energy needed by soil dwelling microorganisms, while secretion
from living roots support a wide variety of microbes in the near-root
environment. Mutualistic relationships are evident between plants and
soil bacteria or fungi. There are also nonmutualistic relationships with
other plants or sometimes with animals.
Rhizobacteria
• Rhizobacteria are soil bacteria with especially large populations in the
rhizosphere.
• Different types of Soil vary greatly in the types and numbers of
Rhizzobacteria they have.
• There is 10 to 100 times higher microbial activity within the plants
rizosphere than in nearby soil. This happens because the roots
secrete nutrients such as sugars, amino acids, and organic acids.
Rhizobacteria (Continued)
• Rhizobacteria that enhances plant growth by a variety of mechanisms
are plant promoting Rhizobacteria.
-Some produce chemicals that stimulate plant growth
-Others produce antibiotics that protect the root from diseases
-Others absorb toxic metals or make nutrients more available to roots
Bacteria benefits by interacting with plants by:
-roots secretions supply most of the energy in the rhizosphere, so bacterial
adaptations that help a plant thrive and secrete nutrients also help the
bacteria.
Nitrogen Cycle/ Root Nodules
• Plants have mutualistic relationships with several different groups of
bacteria which help makes nitrogen more available. No mineral
nutrient is more limiting to plant growth then Nitrogen. Nitrogen is
needed in large amounts for synthesizing proteins and nucleic acids
Nitrogen Cycle/Root Nodules (Continued)
• The nitrogen cycle transforms nitrogen and nitrogen containing
compounds. This happens because plants cannot use nitrogen in the
form of N2.
• The forms of nitrogen plants use are ammonium ions (NH4+) and
nitrite ions (NO3-).
• The main source of nitrogen is the decomposition of humus by
microbes, including amonifying bacteria
• http://www.youtube.com/watch?v=DP24BceOwt8
Atmosphere
N2
Figure 37.10- The roles of Soil Bacteria in
the nitrogen nutrition of plants
N2
Soil
H2
Nitrogen- Fixing Bacteria
Denitrifying Bacteria
Nitrite and
nitrogenous
organic
compounds
exported in xylem
to shoot system
H+
(from Soil)
NH4+
Soil
NH3
Ammonifying Bacteria
Organic Material Humus
NO3
NH4+
Nitrifying Bacteria
Root
Nitrogen Cycle/Root Nodules (Continued)
•The conversion to NH4
• Ammonifying bacteria breaks down organic
compounds and release ammonia (NH3)
• Nitrogen fixing bacteria breaks down converts
the N2 to NH3
Nitrogen Cycle/Root Nodules (Continued)
• Conversion to NO3-
• Nitrifying bacteria oxidizes NH3 to nitrite (N02- )
then nitrite to nitrate NO3• Nitrogen is lost in the atmosphere when converting
NO3- to N2
Nitrogen Cycle/Root Nodules (Continued)
• Nitrogen fixation is the conversion from nitrogen from N2 to NH3
• Reactants and Products:
N2+ 8e- + 8 H+ 16 ATP
2 NH3 + NH2 + 16 ADP + 16 Pi
The enzyme complex nitrogenase catalyzes the entire reaction sequence, which
reduces N2 to NH3 by adding electrons and H+
Since Nitrogen fixation need 8 ATP molecules for every NH3 synthesized,
nitrogen – fixing bacteria requires a rich supply of carbohydrates from decaying
materal, root secretions, or Rhizobium (the vascular tissue of roots)
Nitrogen Cycle/Root Nodules (Continued)
• Nodules are roots swelling along legume’s roots. They have been
“infected” by Rhizobium (root living) bacteria.
• Inside each nodule Rhizobium bacteria assume a form called
bacteroids, which are contained in vesicles formed in the root cells.
• Legume-Rhizobium relationships make more usable nitrogen for
plants than all industrial fertilizers. The mutualism provides the
correct amount of nitrogen at the right time without the cost of the
farmer. This is a form of nitrogen fixation which significantly reduces
spending on fertilizers
Nodules
Roots
Bacteroids within
the vesicle
Nitrogen Cycle/Root Nodules (Continued)
• Bacteroids live inside nonphotosynthetic cells is conductive for
nitrogen fixation, which requires an anaerobic environment.
• Some root nodules appear red due to a molecule called
leghemolobin. That is an iron containing protein that binds reversibly
to oxygen.
• This protein is an oxygen “buffer” by reducing the concentration of
free oxygen and thereby provides an anaerobic environment for
nitrogen fixation while regulating the oxygen supply for cellular
respiration required to produce ATP for nitrogen fixation.
Figure 37.12
• This figure describes how a root nodules develops after bacteria
enters through an “infection thread”
• There is a symbiotic relationship between a legume and nitrogen
fixing bacteria that is mutualistic. What happens is that the bacteria is
the supply the host plant with fixed nitrogen while the plants provide
the bacteria with carbohydrates and other organic compounds
• The root nodules take what is given to them and make amino acids
which are then transported up the shoot through xylem
Figure 37.12
1. Roots emit chemical signals that attract
Rhizobium bacteria which then emits
chemical signals that stimulate root hairs
to elongate and to for an infection thread
by an imagination of the plasma
membrane
5. The mature nodule grows to be many times the
diameter of the root. A layer of lignin-rich sclerenchyma
cells forms. This reduces absorption of oxygen and
thereby helping maintain the anaerobic environment
needed for nitrogen fixation
2. The infection thread containing the
bacteria penetrates the root cortex.
Cells of the rootcells and pericyle begin
dividing ad vesicles containing the
bacteria bud into cortical cells from
branching infection thread. Bacteria
within the vesicles develop into
nitrogen fixing bacteroids.
3. Growth continues in the
affected regions of the cortex
and pericyle, and these two
masses of dividing cells fuse,
forming the nodules.
4. The nodules develops vascular tissue
that supplies nutrients to the nodule and
carries nitrogenous compounds into the
vascular cylinder for distribution
throughout the plants
Crop Rotation
• Crop rotation takes advantage of the agricultural benefits of the
mutualistic relationship nitrogen fixation has.
• A nonlegume such as maize is planted one year, then the following
year alfalfa or some other legume is planted to restore concentration
of fixed nitrogen in the soil. The seeds are exposed to bacteria before
sowing to ensure that the legume encounters its specific Rhizobium
strain.
• The legume crop is not harvested, often times it is plowed under so
that it will decompose as “green manure”, which reduces the needs
for manufactured fertilizers.
Mycorrhizae
• Mycorrhizae (“fungus roots”) are mutulastic associations of roots and
fungi in the soil.
• The host plant provides the fungus with a steady supply of sugar. The
fungus then increases the surface area for water uptake and also
supplies the pant with phosphate and other minerals absorbed from
the soil. The fungi also secrete growth factors that stimulate roots to
grow and branch, as well as antibiotics that help protect the plant
from pathogens in the soil
Mycorrhizae
• There are 2 main types of mycorrhizae
• Ectomycorrhizae- the mycelium forms a dense sheath, or mantle, over the
surface of the root. Fungal hyphae extend from the mantle into the soil which
greatly increases the surface area for water and mineral absorption. The
hyphae does not penetrate the root cells but form a network in the apoplast
(extra cellular space) that facilitates nutrients exchange between the fungus
and the plants. Happens in 10% of plant families
• Arbuscular mycorhizzae- this varies from ectomycorrhizae in that it does not
have a dense mantle ensheathing the root. The mycorrhizae penetrates the
cell wall but not the plasma membrane to form branched arbuscules within
the root cells. The hyphae that form arbuscules within the cells are important
sites of nutrient transder. Happens in 85% of plant species.
Cortical Cortex
Ectomycorrhizae. The mantle
of the fungal mycelium
ensheathes the root. Fungal
hyphae extend from the
mantle into the soil,
absorbing water and
minerals, especially
phosphate. Hyphae also
extend into the extracellular
spaces of the root cortex,
providing extensive surface
area for nutrient exchange
between the fungus and its
host plant.
Mantle (Fungal Sheath)
Arbuscular
mycorrhizae. No
mantle forms around
the root, but
microscopic fungal
hyphae extends into
the root. Withint the
root cortex, the fungus
makes extensive
contact with the plant
through branching of
hyphaarbuscles e that
form which provides a
larger surface area for
nutrient exchange.
Arbuscles
Symbiotic Relationships that are not
mutualistic:
• Parasitic plants- such as mistletoe, are not photosynthetic and rely on
other plants for their nutrients. To get the nutrients they need, they
tap into the host plant’s vascular system
• Epiphytes- they are not parasitic but just grow on the surfaces of
other plants instead of the soil. Many orchids grow this way
• Carnivorous plants- are photosynthetic, but they get some nitrogen
and other minerals by digesting small animals. They are commonly
found in nitrogen- poor soil, such as bogs.