BIOL0129_Lecture 9

BIOL0129_Lecture 9 - Nature of interactions Introductory...

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1 Introductory Microbiology BIOL0129 Lecture 9: Microbial interactions Nature of interactions • Microbes exist in many associations with other organisms as: Parasites Pathogens Saprobes Symbionts Syntrophs • We will focus on symbiotic relationships in this lecture Microbe-microbe symbioses • Symbioses between different microorganisms is rare since they are usually competitors for nutrients and space, during rapid colonization of substrates • A notable exception are lichens Lichen on a dead tree branch Lichens • Lichens are the result of symbiosis between a fungal and algal partner – The alga usually occurs as discreet clumps of cells within the fungal colony • The relationship has little speciFcity – Several different algal and fungal species can be involved with each other – The algal symbiont can also sometimes be a nitrogen-Fxing cyanobacterium • The alga is a photoautotroph – It produces organic matter via photosynthesis – The fungus utilizes part of this organic matter
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2 • So what’s in it for the alga? – The fungus provides a structure in which the alga is embedded • Protection from wind and rain – Uptake and translocation of water and micronutrients from the substrate • The fungus produces strong acids to dissolve nutrients from the substrate, often this is rock! Plant-microbe symbioses • Plant leaves are exposed to a desiccating environment (the air) and so most have developed waxy coatings to retain moisture – Such coatings also act as a barrier to microbial ingress • Conversely, the roots of plants in soil are: – subject to less moisture stress than leaves – in a comparatively nutrient-rich environment • So it is not surprising that most plant- microbe interactions occur in the rhizosphere Mycorrhizae • The name ‘mycorrhiza’ literally means ‘root fungus’ – It is a symbiosis between plant roots and fungi – The roots of most terrestrial plants are thought to be mycorrhizal • This is a non-speciFc relationship – Individuals from one species of pine can have mycorrhiza from up to 40 different fungal species • There are two classes of mycorrhizae: – Ectomycorrhizae – fungal cells form an extensive sheath around the outside of roots, with little penetration into root tissue – Endomycorrhizae – fungal mycelium embedded in root tissue Ectomycorrhizal association of Suillus bovinus with lodgepole pine ( Pinus rigida )
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3 • Ectomycorrhizae are found in angiosperm and gymnosperm trees – Notably pine, beech and oak – Most common in boreal and temperate forests • Endomycorrhizae are found in ~80% of all terrestrial plants • Few are encountered without a root symbiosis – They are probably obligate symbionts • Most mycorrhizae do not attack cellulose, hemicellulose or lignin (plant cell wall materials) – They obtain carbon from carbohydrate root secretions – Nutrients are obtained from soil • The fungus produces plant growth substances
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This note was uploaded on 10/28/2010 for the course BIO microbiolo taught by Professor Point during the Spring '09 term at HKU.

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BIOL0129_Lecture 9 - Nature of interactions Introductory...

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