plant/microbe interactions - beneficial interactions involving plant roots interactions involving...
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Plant/Microbe Interactions - Beneficial
• Interactions involving plant roots
– Rhizoplane - the surface of the plant root, root hairs present large surface area (> 6 m2 for an average wheat plant). Only 4 - 10% of the rhizoplane is in direct contact with soil microbes.
– Rhizosphere - the area of the soil directly influenced by plant roots (extremely variable). Soil that remains after shaking off roots.
Plant/Microbe Interactions - Beneficial
• Interactions involving plant roots
– Rhizosheath - some plants excrete a mucous-like material that cement sand grains together around the root. Most common in dry soils.
Plant/Microbe Interactions - Beneficial
• R/S ratio - indicates the importance of the root system to the microbial community.
– R the number or biomass of microbes in the rhizoshphere.
– S the number or biomass of microbes in root-free soil.
– R/S typically between 5 and 20, can be >100
Plant/Microbe Interactions - Beneficial
• In the rhizosphere (relative to root-free soil)
– abundance of Gram-negative rods is higher
– abundance of Gram-positive rods and cocci is lower
• Reflects the influence of plant root exudates and the selection of organisms with high growth rates
Plant/Microbe Interactions - Beneficial
• Root exudates
– amino acids (proteins)
– keto acids (TCA cycle)
– vitamins (enzyme co-factors)
– sugars (C and energy)
Plant/Microbe Interactions - Beneficial
• Roots surrounded by active microbes produce more exudates than roots in sterile soil.
• The roots are not just leaky, there is an interaction with the microbial community.
• As a plant grows the community in the rhizosphere changes to fast-growing, growth factor-requiring organisms.
Plant/Microbe Interactions - Beneficial
• Microbial populations are clearly benefited by the interaction with roots . . . but what does the plant get?
• One major plant benefit is nutrient uptake . . .
Plant/Microbe Interactions - Beneficial
• Mycorrhizae
– mutualistic associations between fungi and plant roots
– fungi become integrated into the root structure
– both partners benefit (not a disease)
Mycorrhizal symbioses
Advantages:• Enhancing plant nutrient adsorption• Reducing soil born diseases• Improving plant water resistant
Mycorrhizal Fungi
Mycorrhizae
Tree root
Mycorrhizal structure
Fungal hyphae
Plant/Microbe Interactions - Beneficial
• Ectomycorrhizae
– fungi form an external sheath on the root and extends into intercellular spaces (not inside individual cells)
– approximately 40 mm thick
– the root association can be up to 40% fungi by dry weight
Ectomycorrhizae
Plant/Microbe Interactions - Beneficial
• Ectomycorrhizae
– found in most trees in temperate forests
– benefits to the tree include:– drought resistance– pathogen resistance– enhanced nutrient uptake (PO4 and K)
– increased tolerance to pH changes– increased root growth
Plant/Microbe Interactions - Beneficial
• Ectomycorrhizae
– benefits to the fungus includes:
• first access to plant exudates
• direct benefit from trees photosynthetic activity
Plant/Microbe Interactions - Beneficial
• Endomycorrhizae
– fungal mycelia penetrate both between cells and inside individual cells
– heath, rododendrons, laurels, orchids
– the fungal partner does not fix nitrogen, but does seem to enhance the uptake of combined nitrogen
Plant/Microbe Interactions - Beneficial
• Endomycorrhizae
– orchids are pollinated at night and some mycorrhizal fungi are bioluminescent (insect attraction?)
– rRNA sequence data place the origin of the endomycorrhizal fungi at or near the origin of land plants . . . may indicate a long term co-evolution.
Plant/Microbe Interactions - Beneficial
• Vesicular-Arbuscular (VA) Endomycorrhizae– the most common of all mycorrhizal
associations– Phytobionts : 80% of plant species– wheat, corn, potatoes, beans, soybeans,
tomatoes, strawberries, apples, oranges, grapes, cotton, tobacco, tea, coffee, cocoa, sugar cane, sugar maple, rubber . . .
– Phylum : Glomeromycota– Genera: Glomus, Paraglomus, Sclerocystis,
Acaulospora, Entrophospora, Gigaspora, Scutellospora, Diversispora, Geosiphon, and Archaeospora
Plant/Microbe Interactions - Beneficial
• Vesicular-Arbuscular (VA) Endomycorrhizae
– extensive network of mycelia that extends well out into the soil surrounding the root hair (vesicle and tree-like shapes)
– arbuscules = tree-like – vesicles = intracellular fungal storage
structures which are lipid containing bodies
Plant/Microbe Interactions - Beneficial
Colonization of a root by an endomycorrhizal fungus(Brundrett et al. 1985 Can. J. Bot. 63: 184).
Plant/Microbe Interactions - Beneficial
Plant/Microbe Interactions - Beneficial
• Vesicular-Arbuscular (VA) Endomycorrhizae
– benefits to the tree include:
• drought resistance
• pathogen resistance
• enhanced nutrient uptake (combined
nitrogen, P, Zn, & Cu)
• increased tolerance to pH changes
• increased root growth
Interaction of AM & Agricultural Practices
• Fertilizer Application : High P• Crop Rotation : non-host plant
species• Tillage : reduced / no tillage practices• Liming
Mycorrhizal DependencyPlant name RFMD (%)
Cabbage (Brassicaceae) 0
Carrot 99.2
Chicory (witloof) 82.4
Faba bean 93.5
Garden beet (Chenopodiaceae) 0
Garden pea 96.7
Kentucky blue grass 72.4
Kidney bean 94.7
Leek 95.7
Pepper 66.1
Potato 41.9
Tomato (according cultivars) 59.2 - 78.0
Sweet corn 72.7
Wheat (according cultivars) 44.5 - 56.8
Propagation cycle of AMF
a. Spores of (i) Gigaspora, (ii) Glomus, (iii) Entrophospora, and (iv) Acaulospora; b. germinating spore; c. hyphal network and spores; d. hypha and spores around root; e. hyphal penetration inside root; f. intracellular arbuscules; g. intraradical vesicles; h. colonized plant
Inoculum Propagation
• Pot-culture propagation- Isolation of AMF pure culture strain : single spore- Choice of a host plant : Allium porrum, Sorgum bicolor, Zea Mays, Paspalum otatum
• In vitro propagation on root-organ culture
In Vitro Propagation
a. Isolated spores; b. germinating colonized root segment; c. carrot root in culture; d. AMF root-organ culture; e. closer view of an AMF root-organ culture
a. Seeding mycorrhizal substrates; b. mycorrhizal seedling production; c. growth chamber inoculum propagation; d. root growth and colonization; e. colonized seedlings; f. field inoculum propagation
In Vivo Propagation
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