17 Notes - Topic 17 Fundamentals of Microbiology...

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Unformatted text preview: Topic 17 Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: To appreciate the interkingdom interactions that occur between specific Alphaproteobacteria and plants. The members of the Rhizobiaceae are defined by their interaction with plant hosts. The best known examples are Agrobacterium, which form tumours and hairy roots with dicotyledons (dicot) plants, and the root nodule bacteria, which form N2 fixing symbioses with legume plants. While they obtain benefit from their intimate interactions with the plant host, they are also well adapted to life in the soil. The Rhizobiaceae are Alphaproteobacteria. Agrobacterium general concepts • Trans ­kingdom genetic transfer (Bacteria to Plant). • Gene regulation in response to environmental signals. Expression of bacterial virulence genes induced by signals from wounded plant.  ­ genes only expressed when susceptible host plant is available. • Design and creation of habitat in which it survives best. Not only does Agrobacterium colonize a specific habitat, but it actually creates (or enhances) that habitat. This sophisticated relationship invoves three players: a) Oncogenic plasmid b) Agrobacterium cell c) Plant • Agrobacterium is used for genetic engineering of plants. INFECTION PROCESS 1. Wounding required. 2a. Attachment of Agrobacterium to plant cells. 2b. vir gene induction in response to signals from plant (phenolics, monosaccharides, acidic pH) 3. T ­DNA processing. 4. T ­DNA transfer into plant cell. 5. Integration of T ­DNA into plant nuclear genome and expression of genes. 6. Tumour formation, opine synthesis. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Root Nodule Bacteria 1. Legume Root Nodules There are over 15,000 species in the Leguminosae family, ranging from forage legumes to grain legumes to trees. Many of these plants have organs on their roots called nodules. These nodules are packed full of bacteria called “rhizobia”. The rhizobia can live either within the nodule or in the soil, but they can only fix N2 while they are inside the nodule. Inside the nodule, the rhizobia are provided with carbon and energy in the form of photosynthate, and the oxygen ­sensitive nitrogenase enzyme is protected. 2. What are Rhizobia? The rhizobia are all members of the same branch of the Gram ­negative phylogenetic tree. While all legume root ­nodulating bacteria were originally placed in the same genus, Rhizobium, there are currently several recognized genera : Rhizobium, Bradyrhizobium, Azorhizobium, Photorhizobium, Sinorhizobium, Mesorhizobium. The best known rhizobia are those that form root nodules on agriculturally important crop plants such as pea, bean, soybean, alfalfa and clover. There are certainly many other types of rhizobia waiting to be discovered. 3. How is a Root Nodule Formed? Root nodule formation is actually induced by the rhizobia which attach to root hairs by means of glucans for adherence (encoded by ndv genes; nodule development genes). The colonization of a legume root by the appropriate rhizobia results in controlled meristematic activity within the root tissue. As the nodule is developing, the rhizobia infect through plant cell wall invaginations, which form tubes called infection threads. The rhizobia are then released into the plant cells and differentiate into N2 ­fixing cells called bacteroids. 4. How is Nodulation Studied? The most dramatic advances in the understanding of nodule formation and function have been made in the last fifteen years. Genetic studies have indicated that specific bacterial genes (nod genes) are required for nodule formation to take place. Signal molecules are involved in the communication between the bacterium and the plant. The nod genes are only expressed in the presence of flavonoid compounds that indicate the presence of a suitable plant host. The products of the nod genes are enzymes that carry out the formation of lipo ­chito ­oligosaccharides, which then signal nodule induction in the plant. Genetic and biochemical studies have also revealed that C4 ­dicarboxylic acids such as malate are used by the bacteroid to generate energy and reducing power. Many other aspects of nodulation are being studied using bacterial mutants, and, more recently, plant mutants. ...
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This note was uploaded on 12/23/2011 for the course BIOL 140 taught by Professor Dr.joshneufeld during the Fall '10 term at Waterloo.

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