37.3 Plants Nutrition Often Involves Other Relationship with

Other OrganismsFarrah YounesPeriod 7 and 8AP Biology

What is going to be covered:A. RhizobacteriaB. Nitrogen Cycle/ Root NodulesC. Detail of Figure 37.12D. Crop RotationE. Mycorrhizae

Background InformationPlants 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 thebacteria.

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

H2

Soil

N2 N2

Soil

Nitrogen- Fixing Bacteria

Ammonifying Bacteria

NH3 NH4+

H+

(from Soil)

Nitrifying Bacteria

Denitrifying Bacteria

NO3

RootOrganic Material Humus

NH4+

Nitrite and nitrogenous organic compounds exported in xylem to shoot system

Figure 37.10- The roles of Soil Bacteria in the nitrogen nutrition of plants

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 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)

2 NH3 + NH2 + 16 ADP + 16 Pi

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 37.121. 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

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

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

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.

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.

Cortical Cortex

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.