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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 • • • • 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 _______ 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 __________________________________________________________ • 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.