Download MMG 301 Dr. Frank Dazzo Microbial Ecology: Plant

MMG 301 Dr. Frank Dazzo
Microbial Ecology: Plant-Microbe Interactions
Associations of Soil Microorganisms with Vascular Plants
Topic areas:
General colonization: phyllosphere, rhizosphere/rhizoplane
Specific beneficial associations: root nodulation, mycorrhizae
Detrimental pathogenic associations: crown gall tumorigenesis
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Plants secrete various organic compounds resulting in a
nutritionally enriched environment favorable for microbial growth
As a result, plants are heavily colonized with a diversity of
microorganisms whose reservoir is primarily the soil.
Microbes that colonize plants are called either epiphytes (colonize
plant surface) or endophytes (colonize plant interior)
Microbial communities influence plants in direct and indirect
ways: commensalisms, mutualisms, amensalism, and pathogenic
consequences
Phyllosphere: aerial leaf surface of plants
• Communities of microorganisms that develop on the phyllosphere
are adapted to tolerate high irradiation and low humidity stresses
• Many phyllosphere microorganisms antagonize airborne
pathogens thereby protecting the plant.
SEM of phyllosphere bacteria and fungi colonized on corn leaf surface.
Rhizosphere and rhizoplane colonization by microorganisms
• Plant roots secrete various nutrient-rich compounds (e.g., sugars,
amino acids, vitamins, organic acids) into the surrounding soil.
This process, called “rhizodeposition,” can amount up to 25% of
newly fixed photosynthates.
• This nutritional enrichment around roots creates unique
environments for soil microorganisms, including the rhizosphere
(that volume of soil around roots influenced by root exudation)
and the rhizoplane (the immediate root epidermal surface that
interfaces the rhizosphere soil)
Epifluorescence micrographs of bacteria colonized on the white clover rhizoplane
developing in soil. Acridine orange, laser scanning confocal microscopy.
Microbial communities that develop in the rhizosphere/ rhizoplane
differ from microbial communities in bulk non-rhizosphere soil:
1. Population sizes are higher in the rhizosphere
2. Dominant species: rhizosphere dominated by fast-growing,
predominantly culturable, amino-acid requiring, microaerophilic
Gram negative rods, e. g., Pseudomonas. Bulk soil dominated
by Gram positive rods / dwarf cocci, predominantly nonculturable or grow slowly with complex nutritional requirements
satisfied by soil organic matter, e.g., Arthrobacter.
N2-fixing Rhizobium-legume root-nodule symbiosis
• Several genera of soil bacteria form a symbiotic relationship with
specific legumes (pod-bearing angiosperms) that develop N2fixing root nodules
• Groups of rhizobial species (or biovars) that specifically nodulate
the same legume host are called cross-inoculation groups
Legume host
many clovers
peas, vetch
common bean
Rhizobial cross-inoculation group
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Rhizobium leguminosarum biovar trifolii
R. leguminosarum biovar viciae
R. leguminosarum biovar phaseoli,
R. etli, R. tropici
soybean
Bradyrhizobium japonicum, B. elkanii, R. fredii
alfalfa
Sinorhizobium meliloti
lotus
Mesorhizobium loti
sesbania
Azorhizobium caulinodans
neptunia (aquatic) Allorhizobium undicola
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• This N2-fixing symbiosis is of major importance to agriculture
because N is the nutrient most commonly limiting plant
productivity, and legume crops can offset that limitation by
forming an efficient N2-fixing symbiosis with Rhizobium.
• In Nature, legumes are nodulated by both effective and ineffective
strains of rhizobia. Effective rhizobial strains symbiotically fix N2,
whereas ineffective strains don’t.
• Legume crops inoculated with selected
strains of rhizobia in a commercial
inoculant help to ensure that an effective,
efficient N2-fixing root nodule symbiosis
results, reducing the crop’s dependence on
chemical N-fertilizer to achieve high yields.
By proper placement and timing of
inoculation on seed just before planting,
the rhizobial inoculant gains a preemptive
colonization of the root and successful
competition for nodule occupancy.
Symbiotic root-nodule development involves various complex cellcell interactions defined at cellular and molecular levels
• Molecular communication between the rhizobial and legume
symbionts is mediated by signal molecules that activate
expression of genes required for the symbiotic pathway:
Rhizobia → Plant
Plant → Rhizobia
The host legume root secretes phenolic compounds called
flavonoids. These are taken up by the rhizobial symbiont, where they
activate expression of various symbiotic plasmid-encoded nod
(nodulation) genes. Some of these nod genes encode enzymes to
synthesize a special class of glycolipids (chitolipooligosaccharides).
These signal molecules vary somewhat in structure, but their nonreducing end containing a N-acyl long-chain fatty acid is bioactive in
the plant host, triggering root hair deformations and cortical cell
divisions within the root leading to nodule formation.
• Within the root nodule, the bacteria are released from infection
threads into the host cell while still enclosed within a hostderived membrane called the peribacteroid membrane. They then
divide and transform into enlarged pleomorphic bacteroids and
make the enzymatic machinery which carry out N2-fixation. The
entire endosymbiotic structure is called a symbiosome.
Root nodules contain the
red O2-binding pigment,
leghemoglobin (Lb)
Metabolic reactions involved in N2-fixation by
rhizobia within legume root nodules.
Some rhizosphere bacteria promote plant growth (plant growthpromoting rhizobacteria = PGPR) by various mechanisms
independent of root nodulation:
a. fixation and solubilization of nutrients
so they can be utilized by plants, e.g.,
N2 → NH3; insoluble P → soluble P
b. production of bioactive growthstimulating hormones (e.g, auxins,
gibberellins) that expand root
architecture (see example) so it is
more efficient in uptake of plant
nutrients from the soil reservoir
c. antagonism of soil-borne rootinfecting plant pathogens resulting in
suppression of plant pathogenesis
Most PGPR only colonize the rhizosphere/rhizoplane
["associative" interaction]; others are more invasive and
establish an intimate "endophytic" interaction.
Examples: Azospirillum brasiliense and wheat, Acetobacter
diazotrophicus and sugarcane, Azoarcus and kallar grass.
Also Rhizobium and cereals (e.g., rice) rotated with legumes:
See Brock 10th edition, p. 691.
Mycorrhiza: Fungus-plant root symbiosis
• Very common - nearly universal; roots of ∼ 95% of vascular plants
are normally involved in mycorrhizal symbiotic associations.
Several different types, most common are:
• Ectomycorrhiza -- form a sheath around the root without penetration
into plant cells; normal case for many gymnosperms (e.g., pine)
• Vesicular-Arbuscular endomycorrhiza – invade plant root cells;
associated with many angiosperms (e.g., many agricultural crops).
• Distinguishing morphological features:
Ectomycorrhizae on
pine rootlets
• The plant provides a steady supply of photosynthetic organic
nutrients to feed the mycorrhizal fungus
• The fungus provides increased surface area for absorption of
plant nutrients (e.g., phosphate) and water from the surrounding
soil and provides them to the plant.
• The mycorrhizal fungus also protects the plant root from
invasion by soil-borne root-infecting pathogens.
Major Plant Diseases Caused by Bacteria and Fungi
Symptoms
Host & Disease
Pathogen
Bacteria:
Spots & blights
Vascular wilts
Bean haloblight
Pseudomonas syringae
Apple wilt
Erwinia amylovora
Banana wilt
Burkholdaria solanacearum
Soft rots
Potato black rot Erwinia caratovora
Onion skin rot
Pseudomonas cepacia
Canker
Citrus canker
Xanthomonas campestris
Crown Gall Tumor (numerous)
Agrobacterium tumefaciens
Fungi:
Rusts
Wheat rust
Puccinia graminis
Necrotic rot
Potato famine
Phytophthora infestans
(several other plant diseases are caused by viruses)
Crown gall tumors on
tobacco made in
response to infection
by Agrobacterium
tumefaciens
Main events of crown gall disease following infection of a
susceptible plant by the bacterial pathogen, Agrobacterium
tumefaciens.
• Only the T-DNA (red) portion
of the bacterial Ti-plasmid is
transferred to the plant host
• This T-DNA portion encodes
the synthesis of auxin and
cytokinin hormones to
sustain tumor production,
and the synthesis of opines
that provides the C+N
nutrition for the bacterium
• Represents inter-kingdom
transfer of DNA. Bacterium
is a useful vector for genetic
engineering of plants; major
applications in plant
biotechnology industry.