Soil and Plant Nutrition

Page
6

Chemical signals

attract bacteria

1

1

2

2

3

3

4

4

Infected root hair

Bacteroid

Bacteroids form

Dividing cells

in root cortex

Dividing cells in

pericycle

Developing

root nodule

Bacteroid

Nodule

forms

Nodule

vascular

tissue

Bacteroid

Nodule develops

vascular tissue

Slide 44

Nitrogen Fixation and Agriculture

Crop rotation takes advantage of the agricultural benefits of symbiotic nitrogen fixation.

A non-legume such as maize is planted one year, and the next year a legume is planted to restore the concentration of fixed nitrogen in the soil.

Instead of being harvested, the legume crop is often plowed under to decompose as “green manure” and reduce the need for manufactured fertilizer.

Non-legumes such as alder trees, certain tropical grasses, and rice benefit either directly or indirectly from nitrogen-fixing bacteria.

Slide 45

Fungi and Plant Nutrition

Mycorrhizae are mutualistic associations of fungi and roots ++.

The fungus benefits from a steady supply of sugar from the host plant.

The host plant benefits because the fungus increases the surface area for water uptake and mineral absorption.

Mycorrizal relationships are common and might have helped plants to first colonize land.

Slide 46

The Two Main Types of Mycorrhizae

In ectomycorrhizae, the mycelium of the fungus forms a dense sheath over the surface of the root.

These hyphae form a network in the apoplast, but do not penetrate the root cells.

In arbuscular endomycorrhizae, microscopic fungal hyphae extend into the root.

These mycorrhizae penetrate the cell wall but not the plasma membrane to form branched arbuscules within root cells.

Slide 47

Epidermis

Mantle - fungal sheath

(a) Ectomycorrhizae

Cortex

Mantle

(fungal

sheath)

Endodermis

Fungal

hyphae

between

cortical

cells

(colorized SEM)

100 µm

10 µm

Cortical cells

Endodermis

Fungal

vesicle

Casparian

strip

Arbuscules

Plasma

membrane

(LM, stained specimen)

Cortex

Epidermis

Fungal

hyphae

Root

hair

(b) Endomycorrhizae Arbuscular mycorrhizae

Slide 48

In endomycorrhizae, microscopic fungal hyphae extend into the root

Epidermis

Fungal

hyphae

Cortex

Endodermis

Fungal

vesicle

Casparian

strip

Arbuscules

Plasma

membrane

Root

hair