1. Chapter 29
Part 2

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Concept 29.4: Plant nutrition often involves relationships with other organisms
Plants and soil microbes have a mutualistic relationship
Dead plants provide energy needed by soil-dwelling microorganisms
Secretions from living roots support a wide variety of microbes in the near- root environment
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3. Many mutually beneficial relationships occur between species from different kingdoms or domains
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Figure 29.10
MAKE CONNECTIONS: Mutualism Across Kingdoms and Domains
Animal–Fungus
Animal–Bacterium
Plant–Fungus
Leaf-cutter ants tending
a fungal garden
Puffer fish (fugu)
Fungus on a
sorghum root (SEM)
Fungus–Bacterium
Plant–Bacterium
Plant–Animal
Figure Make connections: mutualism across kingdoms and domains
The lichen Peltigera
The floating
fern Azolla
Ants on an Acacia plant
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5. Bacteria and Plant Nutrition
Soil bacteria can benefit plants by
Exchanging beneficial chemicals with plant roots
Increasing nutrient availability by decomposing dead organic materials
Converting nitrogen
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Rhizobacteria
The soil layer surrounding the plant’s roots is the rhizosphere
Nonpathogenic bacteria (or fungi) called endophytes live between cells within the plant; free-living rhizobacteria inhabit the rhizosphere
Rhizobacteria and endophytes depend on sugars, amino acids, and organic acids secreted by roots
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7. Rhizobacteria and endophytes enhance plant growth by
Producing growth-stimulating chemicals
Producing antibiotics that protect roots from disease
Absorbing toxic metals
Making nutrients more available to roots
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8. 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
Plants can only absorb nitrogen as either NO3– or NH4+
Most usable soil nitrogen comes from actions of soil bacteria
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ATMOSPHERE
Figure 29.12 N2
SOIL N2
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
Figure The roles of soil bacteria in the nitrogen nutrition of plants
Amino acids
Denitrifying
bacteria
NH3
(ammonia)
Ammonifying
bacteria
Weathering
of rock
NH4+ H+
(from soil)
NH4+
(ammonium)
NO2-
(nitrite)
NO3-
(nitrate)
Nitrifying
bacteria
Nitrifying
bacteria
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Root

10. © 2016 Pearson Education, Inc.
Figure N2
ATMOSPHERE
Proteins from humus
(dead organic material)
SOIL
Nitrogen-fixing
bacteria
Microbial
decomposition
Amino acids
NH3
(ammonia)
Ammonifying
bacteria H+
(from soil)
Figure The roles of soil bacteria in the nitrogen nutrition of plants (part 1)
NH4+
(ammonium)
NO2-
(nitrite)
Nitrifying
bacteria
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11. © 2016 Pearson Education, Inc.
Figure N2
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
Denitrifying
bacteria
Weathering
of rock
NH4+
Figure The roles of soil bacteria in the nitrogen nutrition of plants (part 2)
NO2-
(nitrite)
NO3-
(nitrate)
Nitrifying
bacteria
NH4+
Root
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12. Conversion to NH4+ Conversion to NO3–
Ammonifying bacteria break down organic compounds and release ammonium (NH4+)
Nitrogen-fixing bacteria convert N2 gas into NH3
NH3 combines with H+ from soil to form NH4+
Conversion to NO3–
Nitrifying bacteria oxidize NH4+ to nitrite (NO2–) then nitrite to nitrate (NO3–)
Different nitrifying bacteria mediate each step
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13. Nitrogen is lost to the atmosphere when denitrifying bacteria convert NO3– to N2
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14. Nitrogen-Fixing Bacteria: A Closer Look
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
N2 + 8 e– + 8 H ATP → 2 NH3 + H ADP + 16
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15. Symbiotic relationships with nitrogen-fixing Rhizobium bacteria provide some legumes with a source of fixed nitrogen
Along a legume’s roots are swellings called nodules, composed of plant cells “infected” by nitrogen-fixing Rhizobium bacteria
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Figure 29.13
Nodules
Figure Soybean root nodules
Roots
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Crop Rotation
Alternate crops planted in the same field every other year; one depletes nitrogen, the other fixes nitrogen
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18. 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
Mycorrhizal fungi also secrete growth factors that stimulate root growth and branching
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19. Mycorrhizae and Plant Evolution
Neither early land plants nor fungi were fully equipped to exploit the terrestrial environment
Fossil evidence indicates that mycorrhizae were an early adaptation that helped plants and fungi colonize the land
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20. Agricultural and Ecological Importance of Mycorrhizae
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
For example, garlic mustard slows growth of other plants by preventing the growth of mycorrhizal fungi
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21. 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
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22. Epiphytes grow on other plants and obtain water and minerals from rain, rather than tapping their hosts for sustenance
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Figure
Epiphytes
Figure Exploring unusual nutritional adaptations in plants (part 1: epiphytes)
Staghorn fern, an epiphyte
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Figure
Epiphytes
Figure Exploring unusual nutritional adaptations in plants (part 1: epiphytes)
Drynaria fern, an epiphyte
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Figure
Epiphytes
Figure Exploring unusual nutritional adaptations in plants (part 1: epiphytes)
Orchid, an epiphyte
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26. Some species parasitize the mycorrhizal hyphae of other plants
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
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