MMG 301 Dr. Frank DazzoMicrobial Ecology: Plant-Microbe Interactions

Associations of Soil Microorganisms with Vascular PlantsTopic areas:

General colonization: phyllosphere, rhizosphere/rhizoplaneSpecific beneficial associations: root nodulation, mycorrhizaeDetrimental pathogenic associations: crown gall tumorigenesis

• Plants secrete various organic compounds resulting in anutritionally enriched environment favorable for microbial growth

• As a result, plants are heavily colonized with a diversity ofmicroorganisms whose reservoir is primarily the soil.

• Microbes that colonize plants are called either epiphytes (colonizeplant surface) or endophytes (colonize plant interior)

• Microbial communities influence plants in direct and indirectways: commensalisms, mutualisms, amensalism, and pathogenicconsequences

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% ofnewly fixed photosynthates.

• This nutritional enrichment around roots creates uniqueenvironments for soil microorganisms, including the rhizosphere(that volume of soil around roots influenced by root exudation)and the rhizoplane (the immediate root epidermal surface thatinterfaces the rhizosphere soil)

Microbial communities that develop in the rhizosphere/ rhizoplanediffer from microbial communities in bulk non-rhizosphere soil:

1. Population sizes are higher in the rhizosphere2. Dominant species: rhizosphere dominated by fast-growing,

predominantly culturable, amino-acid requiring, microaerophilicGram negative rods, e. g., Pseudomonas. Bulk soil dominatedby Gram positive rods / dwarf cocci, predominantly non-culturable or grow slowly with complex nutritional requirementssatisfied by soil organic matter, e.g., Arthrobacter.

Epifluorescence micrographs of bacteria colonized on the white clover rhizoplanedeveloping in soil. Acridine orange, laser scanning confocal microscopy.

N2-fixing Rhizobium-legume root-nodule symbiosis• Several genera of soil bacteria form a symbiotic relationship with

specific legumes (pod-bearing angiosperms) that develop N2-fixing root nodules

• Groups of rhizobial species (or biovars) that specifically nodulatethe same legume host are called cross-inoculation groups

Legume host Rhizobial cross-inoculation group _______many clovers Rhizobium leguminosarum biovar trifoliipeas, vetch R. leguminosarum biovar viciaecommon bean R. leguminosarum biovar phaseoli,

R. etli, R. tropicisoybean Bradyrhizobium japonicum, B. elkanii, R. frediialfalfa Sinorhizobium melilotilotus Mesorhizobium lotisesbania Azorhizobium caulinodansneptunia (aquatic) Allorhizobium undicola__________________________________________________________• This N2-fixing symbiosis is of major importance to agriculture

because N is the nutrient most commonly limiting plantproductivity, and legume crops can offset that limitation byforming an efficient N2-fixing symbiosis with Rhizobium.

• In Nature, legumes are nodulated by both effective and ineffectivestrains of rhizobia. Effective rhizobial strains symbiotically fix N2,whereas ineffective strains don’t.

• Legume crops inoculated with selectedstrains of rhizobia in a commercialinoculant help to ensure that an effective,efficient N2-fixing root nodule symbiosisresults, reducing the crop’s dependence onchemical N-fertilizer to achieve high yields.By proper placement and timing ofinoculation on seed just before planting,the rhizobial inoculant gains a preemptivecolonization of the root and successfulcompetition for nodule occupancy.

Symbiotic root-nodule development involves various complex cell-cell interactions defined at cellular and molecular levels

• Molecular communication between the rhizobial and legumesymbionts is mediated by signal molecules that activateexpression of genes required for the symbiotic pathway:

The host legume root secretes phenolic compounds calledflavonoids. These are taken up by the rhizobial symbiont, where theyactivate expression of various symbiotic plasmid-encoded nod(nodulation) genes. Some of these nod genes encode enzymes tosynthesize a special class of glycolipids (chitolipooligosaccharides).These signal molecules vary somewhat in structure, but their non-reducing end containing a N-acyl long-chain fatty acid is bioactive inthe plant host, triggering root hair deformations and cortical celldivisions within the root leading to nodule formation.

Plant →→→→ Rhizobia

Rhizobia →→→→ Plant

• Within the root nodule, the bacteria are released from infectionthreads into the host cell while still enclosed within a host-derived membrane called the peribacteroid membrane. They thendivide and transform into enlarged pleomorphic bacteroids andmake the enzymatic machinery which carry out N2-fixation. Theentire endosymbiotic structure is called a symbiosome.

Root nodules contain thered O2-binding pigment,leghemoglobin (Lb)

Metabolic reactions involved in N2-fixation byrhizobia within legume root nodules.

Some rhizosphere bacteria promote plant growth (plant growth-promoting rhizobacteria = PGPR) by various mechanismsindependent of root nodulation:

Most PGPR only colonize the rhizosphere/rhizoplane["associative" interaction]; others are more invasive andestablish an intimate "endophytic" interaction.Examples: Azospirillum brasiliense and wheat, Acetobacterdiazotrophicus and sugarcane, Azoarcus and kallar grass.Also Rhizobium and cereals (e.g., rice) rotated with legumes:

a. fixation and solubilization of nutrientsso they can be utilized by plants, e.g.,N2 →→→→ NH3; insoluble P →→→→ soluble P

b. production of bioactive growth-stimulating hormones (e.g, auxins,gibberellins) that expand rootarchitecture (see example) so it ismore efficient in uptake of plantnutrients from the soil reservoir

c. antagonism of soil-borne root-infecting plant pathogens resulting insuppression of plant pathogenesis

See Brock 10th edition, p. 691.

Mycorrhiza: Fungus-plant root symbiosis

• Very common - nearly universal; roots of ∼∼∼∼ 95% of vascular plantsare 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:

• The plant provides a steady supply of photosynthetic organicnutrients to feed the mycorrhizal fungus

• The fungus provides increased surface area for absorption ofplant nutrients (e.g., phosphate) and water from the surroundingsoil and provides them to the plant.

• The mycorrhizal fungus also protects the plant root frominvasion by soil-borne root-infecting pathogens.

Ectomycorrhizae onpine rootlets

Major Plant Diseases Caused by Bacteria and Fungi

Symptoms Host & Disease PathogenBacteria:

Spots & blights Bean haloblight Pseudomonas syringaeVascular wilts Apple wilt Erwinia amylovora

Banana wilt Burkholdaria solanacearumSoft rots Potato black rot Erwinia caratovora

Onion skin rot Pseudomonas cepaciaCanker Citrus canker Xanthomonas campestrisCrown Gall Tumor (numerous) Agrobacterium tumefaciens

Fungi:Rusts Wheat rust Puccinia graminisNecrotic rot Potato famine Phytophthora infestans

(several other plant diseases are caused by viruses)

Crown gall tumors ontobacco made inresponse to infectionby Agrobacteriumtumefaciens

Main events of crown gall disease following infection of asusceptible plant by the bacterial pathogen, Agrobacteriumtumefaciens.

• Only the T-DNA (red) portionof the bacterial Ti-plasmid istransferred to the plant host

• This T-DNA portion encodesthe synthesis of auxin andcytokinin hormones tosustain tumor production,and the synthesis of opinesthat provides the C+Nnutrition for the bacterium

• Represents inter-kingdomtransfer of DNA. Bacteriumis a useful vector for geneticengineering of plants; majorapplications in plantbiotechnology industry.