soil formation is
accomplished by a combination of physical (including geologic),
chemical and biological processes by which parent material is
fragmented into successively smaller particles
plant contributions - erosive action of root growth
and accumulation of organic debris (plant litter)
insect and other animal contributions -
decomposer-reducer organisms decrease the physical size of
plant litter particles (making them more accessible to
microbial decomposers)
microbial contributions - decomposition of organic
materials is dependent upon conditions
tropical - temperature greater than 25C plus
abundant moisture causes rapid plant litter decomposition
(soil low in organic matter)
temperate - varying temperature and water
availability slows down decomposition, resulting in
accumulation of plant litter on soil surface (eventually
results in richer soil)
cool coniferous forests - lower temperatures and
seasonal variation of water availability result in excessive
accumulation of plant litter and production of organic acids
during cool, moist periods (burns are the major nutrient
recycling mechanism here)
bogs - waterlogged soil is essentially anaerobic,
causing slow decomposition of plant litter (peat (lignin,
cellulose) accumulation)
soil characteristics
dry - but may contain water (many bacteria
dormant)
aerobic - but has anaerobic microsites if moisture
content is high
oligotrophic - very low in available nutrients
particulate - provides surfaces influencing nutrient
availability and interactions of organisms
pH influences ion attraction and binding due to
charge on particles
mixture of microorganisms present - generally
10^8-10^9 bacteria (coryneforms, nocardioforms, actinomycetes)
and hundreds of meters of hyphae per gram of soil) which are
autochthonous (utilize exclusively native organic
matter), oligotrophic (maintained in low nutrient
medium, e.g., less than 15 mg/L organic matter), and may be
zymogenous (utilize additional, non-native
substrates)
Plant-Microbe
Interactions
rhizosphere - region in the
soil immediately surrounding plant roots; ecause nutrients
(sugars, amino acids, vitamins, etc.) that leach into the soil
from roots act as growth factors for microbes, microbial growth is
most dense near (or on) roots, especially when water is plentiful.
bacterial
associations with plant roots are important, especially for
nitrogen-fixation
Rhizobium or
Bradyrhizobium species establish symbioses through
which nitrogen-fixing
nodules form in roots (and stems) of legumes
symbiosis
is established as follows:
recognition/attachment (to root hairs)
bacteria are attracted to plant rhizosphere by
flavinoids produced by plant cells
attachment is mediated by plant lectin(s) that
bind polysaccharide of the bacteria
root tip "curling" caused by differential
growth induced by auxin stimulation
invasion/travel (within roots)
cellulosic mucopolysaccharide "infection
thread" is formed by plant cells
bacteria proliferate along it, thus traveling
to main root
bacteroid/nodule formation (inside root
cells)
bacteria enter tetraploid root cells (kill
diploid cells)
both bacteria and plant cells proliferate
rapidly
bacteria transform into bacteroids
(swollen, misshapen, branched forms surrounded by
peribacteroid) that fix nitrogen and produce
cytokinins (which induce plant cell growth to
maintain nodule formation)
basis of
symbiosis
plant supplies:
TCA cycle intermediates (succinate, malate,
fumarate) for bacterial energy and reducing power
needed for nitrogen-fixation
globin (protein) portion of leghemoglobin -
binds oxygen so the anaerobic bacterial nitrogenase
can function in nitrogen-fixation
nodulins - lipopolysaccharides that support
nitrogen-fixation, carbon or nitrogen metabolism in
nodules
bacteria supply:
nod and other (exo, lps, ndv, etc.) genes
(nod, sym, nif genes are on
plasmids in Rhizobium, but are chromosomal
in Bradyrhizobium)
cytokinins
heme (porphyrin) portion of leghemoglobin
ammonia which the plant converts into amino
acids, etc.
nitrogen-fixing nodules
are also produced by Frankia (streptomycete) when
it interacts with Alnus glutinosa (alder tree) and
other (mostly small) woody plants in forests, etc.
free-living
nitrogen-fixing bacteria interact with non-leguminous
plants
Anabena (cyanobacterium) interacts with
Azolla (fern) to produce nitrogen-fixing nodules,
thus providing nitrogen fertilization of fern-enriched
rice paddies
Azospirilum lipoferum lives in casual
association with the roots of tropical grasses; it may
also penetrate roots, but grows between cells
(intercellular) and does not form nodules, but still
fixes nitrogen
fungal associations
with plant roots result in formation of mycorrhizae
(much like the relationship seen in lichens - the fungi supply
the plant with moisture and some growth factors while they
depend upon plant roots for their nutrient supply)
ectomycorrhizal - hyphae only outside the plant
root
endomycorrhizal - hyphae primarily inside the
plant root
ectendomycorrhizal - hyphae both inside and
outside plant root
tripartite
associations - simultaneous interactions of plants, fungi
and bacteria
phyllosphere - surface of
plant leaf; plant leaves secrete carbohydrates as well as
"catching" water and minerals, thus promoting microbial
colonization (via glycocalyx-mediated attachment), especially when
moisture is plentiful
ice-nucleating
activity (INA)
Pseudomonas syringae (also other bacteria and
fungi such as Fusarium) produce ice-nucleating
factors which promote formation of ice crystals
crystals counteract the freezing-point lowering effect
of minerals on the leaf surface (minerals normally cause
supercooling)
ice-nucleating active (INA) microbes cause ice crystals
to form at higher-than-normal temperatures, causing frost
damage to plants
this effect can also be useful--e.g., killed INA
bacteria mixed with water assists artificial snow on
commercial ski slopes
induced tissue death - hypersensitivity response (HR)
water loss from tissue surrounding site of
infection kills cells and decreases turgor, which
flattens plant cells
microbes trapped in dead tissue - blockaded
crown gall disease
Agrobacterium tumefaciens (Gram-negative
microaerophilic rods) which carries a tumor induction
(Ti) plasmid that causes uncontrolled growth (tumor)
of plant callus
bacteria bind to plant pectin at wound sites via
bacterial LPS and/or beta-glucans, then synthesize
cellulose fibrils which anchor them
bacteria invade plant tissue using their flagella
for motility
Ti plasmid is transferred into plant cells
plasmid transfer DNA (T-DNA) integrates into plant
DNA
T-DNA genes cause uncontrolled growth of plant
callus (tumor) due to production of auxins and
cytokinins
growth of bacteria is promoted due production of
opines, modified amino acids that are chemoattractants
as well as carbon and nitrogen sources for
Agrobacterium
Ti plasmid can also be used to introduce helpful
genes into plants - increased resistance to
herbicides, drought, salt
