Plant/Microbe Interactions - Beneficial

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 Fungal hyphae Mycorrhizal structure

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 Dependency

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

In Vivo Propagation 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

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Plant/Microbe Interactions - Beneficial