Transcript Rhizobium
Archaea, Bacteria, and Viruses Chapter 19 Prokaryotes and Eukaryotes • Terms introduced by Edouard Chatton in 1920s • Based on microscopic observations Prokaryotes Eukaryotes “primitive nucleus” “true nucleus” Lack clear Clear nucleus nucleus and other and other inclusions inclusions All organisms with cells that lack a nucleus All organisms with cells that have a nucleus Prokaryotes • Carl Woese – Late 1970s – Proposed using the rRNA gene to create universal tree of life • rRNA critical for proteins synthesis • Useful in determining evolutionary relationships Three Domains Domain Cell Type Description Eukaryotic Membrane bounded organelles, linear chromosomes Archaea Prokaryotic Found in extreme environments, cell structure differs from members of Domain Bacteria Bacteria Prokaryotic Ordinary bacteria, found in every habitat on earth, play major role as decomposers Eukarya Why Should A Botanist Study Prokaryotes? • Reasons for studying prokaryotes – Many of the biochemical compounds, enzymes, and metabolic pathways of plants also are found in prokaryotes. – The evolutionary ancestors of plants were prokaryotes. – Plants form ecological associations with prokaryotes. Viruses • Consist of – Either DNA or RNA – Protein coat • Not prokaryotes • Noncellular cannot live independently • Discoveries obtained studying viruses can be used to guide plant research Prokaryotic Cell Structure • Lacks internal membrane-enclosed organelles • Surrounded by plasma membrane – Bacteria plasma membrane lipids similar to those of eukaryotes – Archaeal plasma membrane lipids very different • Held together by stronger bonds Categories of Bacterial Cells • Divided on basis of differential staining technique developed by Gram – Gram-negative cells – Gram-positive cells Gram-positive Cells • Capsule – Waxy polysaccharide – Protects some human pathogens from being engulfed by immune system cells • Penicillin very effective against Grampositive cells – Inhibits formation of cell wall – Causes lysis of cells in hypotonic solutions Comparison of Gram-positive and Gram-negative Cells Gram-positive Gram-negative Thick peptidoglycan cell wall Thin peptidoglycan cell wall No second membrane – some have waxy polysaccharide capsule Second membrane outside cell wall – lipopolysaccharide layer composed of phospholipids, polysaccharide, and protein No periplasmic space Periplasmic space between cell wall and lipopolysaccharide layer Shapes of Bacteria • Determined by cell wall – Also functions to keep cell from bursting in hypotonic solution – Composed of peptidoglycan • Shapes – – – – – Cocci small, round cells Bacilli rods Vibrios bent or hooked rods Spirilla helical forms Stalked forms Archaea • Most have paracrystalline surface layer (S layer) – Composed of protein or glycoprotein – Sensitive to proteases and surfactants • Some have outer covering of pseudopeptidoglycan • Some have thick walls of polysaccharide • Typically lack an outer membrane Bacterial DNA • Typically a single, circular chromosome – Size in Escherichia coli • 1.4 mm in length • Contains 4.6 million nucleotide pairs • Not surrounded by nuclear envelope • Complexed with specific structural proteins that organize it into loops • Localized in area called nucleoid Archaeal DNA • Chromosome complexed with histone proteins, similar to chromosomes of eukaryotes Plasmids • Accessory genes • Small circles of DNA approximately 2,000 to 200,000 nucleotide pairs in length • Can replicate independently of main chromosome Plasmids • Examples of information carried by plasmid genes – Antibiotic resistance – Enzymes and structural proteins that transfer copies of plasmid to Bacteria that do not have any Plasmids – F plasmid • Can incorporate itself into main chromosome • Contains genes for making tube called F pilus – Connects its cell with another that lacks F plasmid – Transfers plasmid or chromosomal DNA from donor (cell with pilus) to receiver cell – Transfer of DNA is called conjugation Prokaryotic Ribosomes • General composition and structure similar to those of eukaryotes – Two subunits made of RNA and protein • Smaller than eukaryotic ribosomes • rRNAs of Archaea – More similar to those of eukaryotes than Bacteria Binary Fission • Method of reproduction in prokaryotic cells • Differs from mitosis – Prokaryotes lack microtubules therefore do not have spindle apparatus Flagella • Used by many Bacterial and some Archaeal cells for swimming • Formed of subunits of protein flagellin • Parts of flagellum – Filament – Hook – Basal body • Powered by basal body • In some instances can reverse swimming direction Pilus • Extracellular organelle • Thin, hollow, nonmotile projection from cell • Proteins at ends of structure attach cell to solid surfaces or to receptors on other cells Cisternae • Cisternae or thylakoid membranes • Found in some prokaryotic cells • Consist of flattened bladders than enclose separate compartments within cytoplasm • Function – in light reactions of photosynthesis in photosynthetic prokaryotes – Energy storage Endospores • Means of survival for some prokaryotic cells • Small, desiccated cells in condition of suspended animation • Contain complete genome and needed chemicals for germination and growth when conditions improve Endospores • Resistant to many things such as boiling, oxidizing agents, antibiotic compounds • Formation involves activation of special genes – In Bacteria such as Clostridium tetani • Nucleoid and ribosomes surrounded by spore wall • Rest of cell degenerates Endospores – Actinobacteria • Form spores on vertical stalk • Spores blown to new sites by air currents – Myxobacteria • Form sacs of endospores • Spores released when sac is hydrated Nutritional Requirements of Prokaryotes • Methods of obtaining carbon – Autotroph (“self-feeding) incorporate carbon into organic molecules from inorganic sources – Heterotroph (“other feeding”) derive carbon from breakdown of organic compounds Nutritional Requirements of Prokaryotes • Methods of deriving energy – Chemotroph (“chemical feeding”) obtain energy from catalyzing inorganic reactions – Phototroph (“light feeding”) obtain energy by absorbing light photons Nutritional Requirements of Prokaryotes Carbon source Energy source Chemoautotroph Carbon from inorganic source incorporated into organic molecules Catalyze inorganic reactions Chemoheterotroph Carbon from breakdown of organic compounds Catalyze inorganic reactions Photoautotroph Carbon from inorganic source incorporated into organic molecules Derive energy by absorbing light photons Photoheterotroph Carbon from breakdown of organic compounds Derive energy by absorbing light photons Groups of Archaea • Methanogens • Halophiles • Thermoacidophiles Groups of Archaea • Methanogens – Chemoautotrophs – Require anoxic environment to obtain energy – Produce methane Reaction used by methanogens to derive energy: CO2 + 4H2 CH4 + 2H2O Groups of Archaea • Halophiles – Live in saturated salt solutions – Some have little or no cell wall • Will burst if moved from its normal environment – Example: Halobacterium halobium • • • • • Unique type of photosynthesis Photoreceptor – bacteriorhodopsin No electron transport chain Cannot make carbohydrates by reducing CO2 Photoheterotroph Groups of Archaea • Thermoacidophiles – Live in hot, acidic environments – Optimum temperature is 70 to 75ºC with maximum of 88ºC – Optimum pH is 2-3 (minimum pH 0.9) Chemoheterotrophs • Live on organic compounds of living or dead tissue or on excretions of other organisms • Roles – May be harmful parasites Chemoheterotrophs – Can be beneficial • Compete for niches with potential pathogens • Gut chemoheterotrophs provide humans with vitamin K • In dead tissue and on excretions, play role of recycling carbon, nitrogen, and other elements Chemoheterotrophs • Some undergo fermentation – Lactobacillus • Extensively studied bacterium, E. coli, is chemoheterotroph – Group Proteobacteria – Family Enterobacteriaceae • Members live in soil and in intestines of animals • Often called enteric or coliform Bacteria Chemoheterotrophs – Presence of coliform Bacteria in water supplies indicates contamination with sewage • Humans sewage carries pathogenic Bacteria and viruses – Some strains of E. coli are not harmful, others produce toxins that cause severe infections Examples of Chemoheterotrophs Genus Phylum Comment Rhizobium Proteobacteria N2-fixing plant symbionts Frankia Actinobacteria N2-fixing plant symbionts Erwinia, Agrobacterium, Pseudomonas syringae Proteobacteria Plant pathogens Desulfovibrio, Desulfomonas Proteobacteria Sulfate-reducing bacteria Stigmatella, Chondromyces Proteobacteria Myxobacteria; colonial spore formers Streptomyces Actinobacteria Antibiotic producers Spirochaeta, Treponema Spirochaetes Long, thin, spiral-shaped; some pathogenic Bacillus Firmicutes Aerobic endospore-formers Escherichia Proteobacteria Enteric; model organism **Rhodopseudomonas, Chromatium Proteobacteria Anaerobic phototrophs; purple nonsulfur and purple sulfur groups All of the above examples are in the Domain Bacteria. **Can be photoautotroph or chemoheterotroph Chemoautotrophs • Examples – Lithotrophs • Specialize in oxidation of inorganic compounds • Recycle nitrogen and sulfur – Nitrogen and methane oxidizers – Methanogenic Archaea – Thermophilic and thermoacidophilic Archaea Examples of Chemoautotrophs Domain Genus Phylum Comment Archaea Methanococcus, Methanospirillum Euryarchaeota Methanogenic CO2 + 4H2 CH4 + 2H2O Archaea Archaea Bacteria Thermoplasma Euryarchaeota Thermoacidophile; grows at pH 1-4 and 33-67ºC Pyrolobus Crenarchaeota Extreme thermophile; grows up113ºC Nitrosomonas, Nitrobacter, Methylomonas Proteobacteria Nitrogen, methane oxidizers Photoautotrophs • Includes – Green sulfur Bacteria – Purple nonsulfur Bacteria – Cyanobacteria • Light absorbing pigments – Bacteriochlorophyll • Anaerobic phototrophs – Chlorophyll • Cyanobacteria • All reduce carbon to CO2 Examples of Photoautotrophs Domain Genus Phylum Comment Bacteria **Rhodopseudomonas, Chromatium Anaerobic phototrophs; Proteobacteria purple nonsulfur and purple sulfur groups Bacteria Chlorobium Chlorobi Bacteria Anabaena, Nostoc, Prochloron Cyanobacteria Oxygen producers Anaerobic phototrophs; green sulfur group **Can be either photoautotroph or chemoheterotroph Photoautotrophs and Endosymbiosis • Primitive cyanobacteria and chloroxybacteria thought to be evolutionary precursors of plastids of photosynthetic eukaryotes • Strong evidence on similarities between light-harvesting complexes of – Cyanobacteria and red algae – Chloroxybacteria and green algae Symbiotic Relationships Between Prokaryotes and Plants • Rhizobium lives in soil – Synthesizes enzyme nitrogenase • Converts N2 to ammonium (NH4+) • Forms close mutualistic relationship with legumes – Plant contributes high energy carbohydrates and a protected environment – Bacterium contributes nitrogenase and other enzymes – Both partners benefit from supply of fixed nitrogen Symbiotic Relationships Between Prokaryotes and Plants • Association occurs in special organs called root nodules • Sequence of events in establishment of relationship – Root secretes attractive chemical – Chemical induces Rhizobium in vicinity to swim toward root and begins induction of nitrogen fixation genes in Rhizobium Symbiotic Relationships Between Prokaryotes and Plants – Rhizobium enters at a root hair and moves inward through infection thread – Rhizobium loses its cell wall and begins synthesizing nitrogen-fixing enzymes as it moves inward – Bacteria reach root cortex – Bacteria are released from infection thread into several cells – Bacteria without cell walls are now called bacteroids Symbiotic Relationships Between Prokaryotes and Plants – Bacteroids become surrounded by special membrane called the peribacteroid membrane – Chemicals secreted by Bacteria (or bacteroids) during formation of infection thread • Induce cell division in root cortex and pericycle, forming nodule • Induce synthesis of nodule proteins including leghemoglobin that buffers oxygen concentration in part of nodule where nitrogen is fixed Symbiotic Relationships Between Prokaryotes and Plants • Other examples of symbiotic nitrogen fixing Bacteria – Frankia – lives within cells of root nodules of alder trees and other plants – Anabaena – association with water fern, Azolla – Nostoc – invades cavities in gametophytes of hornworts and specialized cells of cycads Bacterial Parasites • Parasitism – Symbiotic relationship in which one organism benefits at the expense of the other • Plant pathogens divided into subgroups called pathovars according to plants they infect Bacterial Parasites • Pathovars of Pseudomonas syringae cause – Wildfire disease of tobacco – Blights of beans, peas, and soybeans • Pathovars of Erwinia amylovora cause – Fire blight of apple and pear Bacterial Parasites • Vectors for carrying Bacteria to uninfected plants include water, insects, humans, or other animals • Bacteria enter plants through natural openings – Stomata – Lenticels – Hydathodes – Nectarthodes Bacterial Parasites • Some Bacterial pathogens can overwinter in dead tissue – Return to infect new plant tissue during next growing season • Plant defenses against infecting bacteria – Hypersensitivity response • Produce antibiotic compounds – Phytoalexins directly kill some pathogenic cells – Hydrogen peroxide may restrict spread of infection by causing necroses of adjacent plant cells Viruses • Subcellular parasites – Lack internal structures found in prokaryotic and eukaryotic cells – Cannot reproduce on their own • Invade host cells and use host’s metabolism to reproduce themselves • Simple structure – Either DNA or RNA surrounded by protein coat Viruses • Shape varies • Well-studied plant virus – tobacco mosaic virus • Plant viruses – Too large to pass through cell wall – In nature, almost always spread by insects that pick up viral particles as they chew or suck on infected plants and then transmit them to uninfected plants – Mites and fungi can also infect plants with viruses when they enter plant cells Viruses • Viral infections usually do not kill plants • Infected plants usually stunted compared to uninfected plants • Infection often causes changes in color or shape of foliage