1. Soil Bacteria and Mycorrhizal Fungi and Unusual Plants
Chapter 29
Soil Bacteria and Mycorrhizal Fungi and Unusual Plants

2. Concept 29.3: Plants roots absorb essential elements from the soil
Water, air, and soil minerals contribute to plant growth.
80–90% of a plant’s fresh mass is water.
96% of plant’s dry mass consists of carbohydrates from the CO2 assimilated during photosynthesis.
4% of a plant’s dry mass is inorganic substances from soil.
What is it made of? 2

3. Plants and soil microbes have a mutualistic relationship.
Microbial activity within a plant’s rhizosphere is 10 to 100 times higher than in nearby soil.
Dead plants provide energy needed by soil-dwelling microorganisms.
Secretions from living roots support a wide variety of microbes in the near-root environment. Soil bacteria exchange chemicals with plant roots, enhance decomposition, and increase nutrient availability. The soil layer surrounding the plant’s roots is the rhizosphere.
Rhizobacteria thrive in the rhizosphere, and some can enter roots.
The rhizosphere has high microbial activity because of sugars, amino acids, and organic acids secreted by roots.
Up to 20% of the plant’s photosynthetic output is secreted into the soil. 3

4. Rhizobacteria promote plant growth in several ways.
Produce hormones that stimulate plant growth
Produce antibiotics that protect roots from disease
Absorb toxic metals or make nutrients more available to roots
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5. Bacteria in the Nitrogen Cycle
Nitrogen can be an important limiting nutrient for plant growth.
The nitrogen cycle transforms atmospheric nitrogen and nitrogen-containing compounds.
Most usable soil nitrogen comes from actions of soil bacteria.
Nitrogen is abundant in the atmosphere but unavailable to plants due to the triple bond between atoms in N2.
Nitrogen fixation is the conversion of nitrogen from N2 to NH3:
Some nitrogen-fixing bacteria are free-living; others form intimate associations with plant roots.
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7. Plants can only absorb nitrogen as either nitrate (NO3–) or ammonium (NH4)
ATMOSPHERE N2
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
SOIL
Proteins from humus
(dead organic material)
Nitrogen-fixing
bacteria
Microbial
decomposition
Amino acids
Denitrifying
bacteria
NH3
(ammonia)
Conversion to NH4
Ammonifying bacteria break down organic compounds and release ammonium (NH4)
Nitrogen-fixing bacteria convert N2 gas into NH3
NH3 is converted to NH4
Conversion to NO3–
Nitrifying bacteria oxidize NH4 to nitrite (NO2–) then nitrite to nitrate (NO3–)
Different nitrifying bacteria mediate each step
Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3– to N2.
Ammonifying
bacteria
NH4+ H+
(from soil)
NH4+
(ammonium)
NO2−
(nitrite)
NO3−
(nitrate)
Nitrifying
bacteria
Nitrifying
bacteria
Root 7

8. Symbiotic relationships with nitrogen-fixing Rhizobium bacteria provide some legumes with a source of fixed nitrogen.
Nodules
Along a legume’s roots are .swellings called nodules, composed of plant cells “infected” by nitrogen-fixing Rhizobium bacteria.
Inside the root nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell. The plant obtains fixed nitrogen from Rhizobium, and Rhizobium obtains sugar and an anaerobic environment.
Each legume species is associated with a particular strain of Rhizobium.
Roots 8

9. Fungi and Plant Nutrition
Mycorrhizae are mutualistic associations of fungi and roots.
Mantle
(fungal sheath)
1.5 mm
(Colorized SEM)
Epidermal
cell
(LM)
50 m
Fungal
hyphae
between
cortical
cells
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.
Mycorrhizal fungi also secrete growth factors that stimulate root growth and branching.
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10. Figure Inquiry: Does the invasive weed garlic mustard disrupt mutualistic associations between native tree seedlings and arbuscular mycorrhizal fungi?
(See page 585 for more detail.)
Seeds can be inoculated with fungal spores to promote formation of mycorrhizae.
Some invasive exotic plants disrupt interactions between native plants and their mycorrhizal fungi.
Garlic mustard slows growth of other plants by preventing the growth of mycorrhizal fungi. 10

11. Epiphytes, Parasitic Plants, and Carnivorous Plants
Some plants have nutritional adaptations that use other organisms in nonmutualistic ways.
Three unusual adaptations are
Epiphytes
Parasitic plants
Carnivorous plants
Indian pipe, a nonphoto-
synthetic parasite of
mycorrhizae
Pitcher plants
Epiphytes grow on other plants and obtain water and minerals from rain, rather than tapping their hosts for sustenance.
Parasitic plants absorb water, sugars, and minerals from their living host plant.
Some species also photosynthesize, but others rely entirely on the host plant for sustenance.
Some species parasitize the mycorrhizal hyphae of other plants. Carnivorous plants are photosynthetic but obtain nitrogen by killing and digesting mostly insects.
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