Lecture 42: Microbial Symbionts of Animals and PlantsAnimal Microbe SymbiosesMMG301 Fall 2002 Page 1 Lecture 42: Microbial Symbionts of Animals and Plants (unless noted otherwise, all figures & tables are from: Microbial Life by Perry, Staley & Lory (2002) Sinauer Associates, Inc.) Symbiosis (de Bary, 1879): The living together of two or more dissimilar organisms, on a permanent or semi-permanent basis. “C. aggregatum” Mycorrhizae (left) v. control citrus seedlings. So-oxidizing autotrophic bacterial endosymbionts of deep sea tube worms (Fig. 25.14)MMG301 Fall 2002 Page 2 The important experimental issues/questions: • Size & position of partners. • Duration & stability of the interaction. • Mechanism(s) of perpetuation. • Specificity & recognition of partners. • Modes of interaction of partners. • Significance of the interaction to the partners. The Rhizobium-Legume Symbiosis: an agriculturally important, N2-fixing, microbe-plant symbiosis. (Rhizobium, Bradyrhizobium, Azorhizobium, Sinorhizobium, Mesorhizobium, Allorhizobium) Bradyrhizobium-induced root nodules on peanut (Fig. 25.10a). Important Factors: - flavonoids (“substance” 1) - polyaromatic chemicals. (Madigan et al.) - rhicadhesin + plant root lectins for recognition (specificity) & attachment. - Nod factors (Rhizobium-produced polysaccharides) → root hair curling & infection thread formation. - leghemoglobin (pink): O2-delivery to bacteroids w/o poisoning nitrogenase.MMG301 Fall 2002 Page 3 Bacteroid in root nodule (Madigan et al.) Luminescent Vibrio on plate↓ & in Light Organ↓ (K. Nealson) Vibrio in light organ tubules (K. Nealson). Vibrio cell↓ (K. Nealson) Animal Microbe Symbioses Bioluminescent Bacteria (certain sp. of Vibrio and Photobacterium) and Fish & Squid. “Icthylicht” (K. Nealson)MMG301 Fall 2002 Page 4 Benefits to: animal bacteria Relevant factors: - specificity - “autoinducer” (N-acyl homoserine lactone) accumulation → expression of bioluminescence genes. luciferase - luciferase: FMNH2 + O2 + RCHO ---------------.> FMN + RCOOH + H2O + light Buchnera: intracellular symbionts of aphids. (P. Baumann) Establishment of the symbiosis: ca. 150 million years ago. Buchnera within “symbiosome” within “bacteriocyte” within “bacteriome”. Buchnera genome = 641 kb vs E. coli genome (5, 400 kb). Aphid food: plant sap. Rich in sugars; poor in N. Nature of the symbiosis: provision of amino acids for insect by overexpression of aa-synthesizing
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