Microbe-Plant-Interactions-Full-Save.docx

Microbe-Plant Interactions Are all Microbes Harmful? When we think of microbes, what comes to mind is that they are detrimental to plants. I.e.: verticillium wilt to plants, bacterial soft rot caused by Pectobacterium spp. in cabbage. Botrytis rot. Some Microbes are Beneficial to plants. Pine seedli...

Microbe-Plant Interactions Are all Microbes Harmful? When we think of microbes, what comes to mind is that they are detrimental to plants. I.e.: verticillium wilt to plants, bacterial soft rot caused by Pectobacterium spp. in cabbage. Botrytis rot. Some Microbes are Beneficial to plants. Pine seedlings with fungal network attached to roots tap deeper into the soil layers. Some plants grow better when microbes are added compared to those without microbes. Benefits: Maintains the health of garden without polluting the environment or food. Decreases the water costs and protects plants from drought. Doubles plant ability for nutrient uptake. Plant-Microbe Systems Mutualism Fungus – root system. Bacterium – root nodule system. Plants produce a carbon source to the microbial community. Microbial community remove iron and reduce toxicity, he iron is metabolized and used in plant growth. Commensalism Bacteria and fungi grow in close proximity to roots but providing no apparent benefit to plants. Because the plant releases some exudates that benefits the bacterial and fungal growth. Chemicals released by plants stimulate the growth of bacteria. Plants that do not synthesize vitamin B12 bacteria that require this vitamin will not grow. Plants limit microbial penetration by their thick cell wall. Secretion of gums and chemicals limit invasion of bacteria and fungi. Symbiotic Interactions with Cyanobacteria Azolla aquatic fern. Used as green manure to enrich nitrogen levels of rice fields. Azolla provides carbohydrates to the cyanobacteria, in return, cyanobacteria fix nitrogen for the plant. Cyanobacteria are usually found in the leaf cavity of the fern. Green manure are organic nutrient sources, where leguminous plants are grown, slashed, and left to the soil surface, due their ability in nitrogen fixation. Nitrogen is used by another plant grown in the area. It also reduces soil erosion and conserves soil moisture. Gymnosperm Zamia pseudoparasitica growing at nearly 10m above ground level. Growing on another tree. Transverse fresh section of the coralloid root shows the abundance of inner cyanobionts. Fluorescence microscopy of the small coralloid roots showing the cyanobacterial zone (in red). Fluorescence image of the filamentous heterocysts forming cyanobacteria culture from Zamia; note the abundant cyanphycin granuels (black dots) in many cells. Angiosperm Gunnera produces cuo-like glands that house cyanobacteria (C. Nostoc). The glands are filled with a special mucilage that not only attracts cyanobacteria but also stimulates their growth. Liverwort Blasia pusilla L. in its natural habitat. Symbiotic cavities, auricles, housing Nostoc are encircled. Moss Epi-fluorescence microscopic images of cyanobacteria cells on moss leaves at 100x magnification. Stigonema branching cells on leaves of Racomitrium elongatum (a) and Pleurozium schreberi (d) Chains of Nostoc on R. elongatum (b) and Rhytidiadelphus triquetrus. Interactions in the Rhizosphere Rhizosphere – soil and all the biological agents present in soil within a few mm from the root system. Coined by Lorenz Hiltner in 1904. Rhizospheric microflora – collective term for organisms of the rhizosphere (bacteria, fungi, algae, protozoans, and soil animals). Characteristic of a specific cultivar gene content selects for specific bacteria and fungi in the rhizosphere. Greatest activities are observed when plants are flowering. Rhizodeposition – release of organic acids and secretion of mucilage from plant roots. Rhizosphere pH is generally acidic due to proton secretion which enhances solubilization of minerals. Rhizospheric microflora consume O2 and lower redox potential of rhizosphere thus enhancing Nitrogen fixation. If you uproot the plant and shake of the remaining soil in the root, that is the rhizosphere. Mycorrhizae Stable fungus-root relationships. It is coined by Albert Bernard Frank. ‘ 2 Major Types of Mycorrhizal Fungi Endomycorrhizae – far more common type, hyphae penetrate root cells; intracellular (within cells). Form symbiotic with approximately 85% of plant families. Pair with the most commercially produced plants, including green, leafy, and fruiting or flowering plants. Penetrate into root cortext and form nutrient exchange structures within the root cells (arbuscules, vesicles, etc). Ectomycorrhizae – Hyphae do not penetrate root cells; intercellular (between cells). Form symbiotic relationships with about 10% of plant families. Mainly pair with conifers and many American hardwoods. Do not penetrate into the root cell walls, but form a sheath around the root, and nutrient exchange structures known as “Hartig net”. Ericoid mycorrhizae: involve partnerships between ascomycetes and members of Ericaceae, Epacridaceae, and Empetraceae families. Epidermal cells of small-diameter roots lack root hair and instead are frequently filled with fungal hyphae. Orchid mycorrhizae: Fungi forming mycorrhizas with orchids typically live as saprotrophs in the soil or dorm endophytic/ectophytic mycorrhizae associations with neighboring trees. Orchid seedlings lack chlorophyll and rely on nutrients and C that they obtain these fungi. Others Brassica Family is non-mycorrhizal. Ericaceae and Orchids have species of mycorrhizal fungi (less commercially available). Key Benefits of Mycorrhizal Fungi Root System Growth Mycorrhizal fungi support faster plat establishment. Mycorrhizal hyphae access water and nutrients beyond the root zone and deliver them to the plant’s vascular network. Increases absorption area by as much as 50 times. Increases overall root biomass. Nutrient Efficiency Mycorrhizal hyphae absorb and actively deliver nutrients directly to the roots. Improves utilization of soil nutrients including: Nutrogen, Phosphorus, Potassium, Micronutrients. Water Absorption Mycorrhizal hyphae absorb and transport soil moisture from beyond the root zone to the plant’s roots. The mycorrhizal symbiosis increases the plant’s effective water utilization capability: Improved tolerance to stress greater resistance to drought. Nitrogen-fixing bacteria and higher plants Enzyme system for N fixation is found only in prokaryotes. Specificity between the legume symbiont and bacteria. Plant: provides carbon and energy source for bacteria. Bacteria: fix nitrogen with the production of amino acids for plant growth. Root Associations Legume nodules Rhizobia: bacteria that grow symbiotically with the roots of legumes. (A) Host plant releases flavonoids into the rhizosphere that are perceived by the specific rhizobia. The flavonoids induce transcription of the genes for biosynthesis of the rhizobial Nod factors, which the plant perceives to allow symbiotic infection of the root. After transcription, the activated NodD binds to the nod box promoter (B), inducing the transcription and synthesis of Nod factors (C). The Nod factors induce the development of the infection thread that traps rhizobia within the curled surfaces (D). The infection thread grows through epidermal cells into the cortical cell, where rhizobia are released and internalized by the cortical cells (E). Further proliferation and differentiation of both bacteria (bacteroid: differentiated rhizobia for nitrogen fixation) and infected cortical cells results in nodule formation (F). Internal features of nodules Central infected tissue uniform. Central tissues contain a mixture of infected and uninfected cells. Bacteria located in symbiosomes (temporary plant organelle within the plant cell housing the endosymbiont). Bacteria retained within fixation threads, which are modified infection threads which bacteria are not released into symbiosomes. Bacteroides are not terminally differentiated. They can be seen as both longitudinal and transverse section; rod shaped. Terminally differentiated bacteroids, greatly enlarged and pleomorphic (exists in different forms). Actinorhizal Nodules Actinorhizal plants interact with Frankies to produce fibrous root nodules. Several bacterial species exist among Frankia species older process than rhizobia – legume system because of wide diversity of plants and microorganisms in actinorhizal symbiosis. Stem Associations Aquatic legume species have N-fixing nodules on their stems. Nodules are produced on submerged stems or on stems at air-water interface, Examples: Sesbania rostrata (legume) and Azorhizobium caulinodan. Aeschynomene indica and Bradyrhizobium strain BTAil (Bchl a limited anoxygenic photosynthesis). Bacteria supporting plant growth Production of plant-like hormones. Plant growth promoting rhizobacteria (PGPR) Metabolic products of fluorescent Pseudomonas stimulate mycorrhizae formation. Soil bacteria metabolizing glycine, serine, methionine, or threonine produce cyanide PGPR pseudomonas produce Fe-chelating siderophores cyanide-producing bacteria do not grow in iron-deficient soils. Bacillus species are known to stimulate phosphorus uptake from organophosphate compounds by Pinus caribea containing ectomycorrhiza Posolithus tinctorius. Bacteria in one environment promote plant growth but in another deleterious for plant growth. Examples: Strains of bacteria isolates are as important as the particular bacterial species. Leaf Surfaces and Microorganisms Phyllosphere – area of the leaf where various forms of life may be found. Pseudomonas syringae can turn water into an ice crystal. It has killed many plants by freezing them. The process is called ice nu