Plant Microbe Interaction

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.


• 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.


• 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


• 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


• Root exudates

– amino acids (proteins)

– keto acids (TCA cycle)

– vitamins (enzyme co-factors)

– sugars (C and energy)


• 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.


• Microbial populations are clearly benefited by the interaction with roots . . . but what does the plant get?

• One major plant benefit is nutrient uptake . . .


• 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


• Ectomycorrhizae

– fungi form an external sheath on the root and extends into intercellular spaces (not inside individual cells)

– approximately 40 m thick

– the root association can be up to 40% fungi by dry weight

Ectomycorrhizae


• 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


• Ectomycorrhizae

– benefits to the fungus includes:

• first access to plant exudates

• direct benefit from trees photosynthetic activity


• 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


• 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.


• 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


• 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


Colonization of a root by an endomycorrhizal fungus(Brundrett et al. 1985 Can. J. Bot. 63: 184).



• 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 Plant name RFMD (%) RFMD (%)

Cabbage (Brassicaceae)Cabbage (Brassicaceae) 0 0

Carrot Carrot 99.2 99.2

Chicory (witloof) Chicory (witloof) 82.4 82.4

Faba bean Faba bean 93.5 93.5

Garden beet (Chenopodiaceae)Garden beet (Chenopodiaceae) 0 0

Garden pea Garden pea 96.7 96.7

Kentucky blue grassKentucky blue grass 72.472.4

Kidney beanKidney bean 94.794.7

LeekLeek 95.795.7

PepperPepper 66.166.1

PotatoPotato 41.941.9

Tomato (according cultivars)Tomato (according cultivars) 59.2 - 78.059.2 - 78.0

Sweet cornSweet corn 72.772.7

Wheat (according cultivars)Wheat (according cultivars) 44.5 - 56.844.5 - 56.8

Propagation cycle of AMF

a. Spores of (i) a. Spores of (i) GigasporaGigaspora, (ii) , (ii) GlomusGlomus, (iii) , (iii) EntrophosporaEntrophospora, , and (iv) and (iv) AcaulosporaAcaulospora; b. germinating spore; c. hyphal ; b. germinating spore; c. hyphal network and spores; d. hypha and spores around root; e. network and spores; d. hypha and spores around root; e. hyphal penetration inside root; f. intracellular arbuscules; hyphal penetration inside root; f. intracellular arbuscules; g. intraradical vesicles; h. colonized plant 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; a. Isolated spores; b. germinating colonized root segment; c. carrot root in culture; d. AMF root-organ culture; e. c. carrot root in culture; d. AMF root-organ culture; e. closer view of an AMF root-organ culture closer view of an AMF root-organ culture

a. Seeding mycorrhizal substrates; b. mycorrhizal a. Seeding mycorrhizal substrates; b. mycorrhizal seedling production; c. growth chamber inoculum seedling production; c. growth chamber inoculum propagation; d. root growth and colonization; e. propagation; d. root growth and colonization; e. colonized seedlings; f. field inoculum propagation colonized seedlings; f. field inoculum propagation

In Vivo Propagation

End

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Plant Microbe Interaction