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Plant-associated Proteobacteria (and a few outsiders): the good and the bad

nitrogenase. N 2. NH 3. Plant-associated Proteobacteria (and a few outsiders): the good and the bad. Today’s Topics: Rhizobeacae and other nitrogen-fixing genera Nitrogen fixation and why we need it Examples of nitrogen-fixing symbioses in plants Processes of nodulation

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Plant-associated Proteobacteria (and a few outsiders): the good and the bad

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  1. nitrogenase N2 NH3 Plant-associated Proteobacteria (and a few outsiders): the good and the bad

  2. Today’s Topics: • Rhizobeacae and other nitrogen-fixing genera • Nitrogen fixation and why we need it • Examples of nitrogen-fixing symbioses in plants • Processes of nodulation • Non-symbiotic nitrogen-fixing Proteobacteria • Cyanobacterial associations • Crown gall: the selfish doings of Agrobacterium tumefaciens

  3. Greensulfur Prokaryotes Bacteroides Spirochetes Deinococci Green, nonsulfur Chlamydiae Thermotoga Gram positive Cyano bacteria Rhizobium Bradyrhizobium Sinorhizobium Agrobacterium Azospirillum a b d Herbaspirillum g Desulfoivbrio E. coli Klebsiella Azotobacter Proteobacteria

  4. Ecology of nitrogen-fixing bacteria

  5. Biological nitrogen fixation: nitrogenase N2 + 8 flavodoxin- + 8H+ + 16 MgATP2- + 18 H2O 2NH4+ + 2OH- + 8 flavodoxin + 16 MgADP- + 16H2PO4- + H2 • Rare, extremely energy consuming conversion because of stability of triply bonded N2 • Produces fixed N which can be directly assimilated into N containing biomolecules

  6. GOGAT glutamate -ketoglutarate glutamine + Ammonia assimilatory cycle: How nitrogen enters biological pathways Amino acids proteins purines pyrimidines Pathway 1 GS NH4+ glutamate glutamine + + ATP + ADP + Pi Pathway 2 GDH NH4+ -ketoglutarate glutamate + Amino acids proteins

  7. N2 dinitrogen gas (78% of air) Nitrogen fixation the Haber Process and lightning Denitrification N2O nitrous oxide The Nitrogen Cycle NH4+ ammonium BIOSPHERE Denitrification NO3- nitrate nitrification NO2- nitrite nitrification

  8. Plants Prokaryotes Animals The Nitrogen Cycle N2 dinitrogen gas (78% of air) Biological nitrogen fixation N2O nitrous oxide nitrification assimilation NH4+ ammonium consumption uptake NO3- nitrate NO2- nitrite

  9. A growing population must eat! • Combined nitrogen is the most common limiting nutrient in agriculture • Estimated that 90% of population will live in tropical and subtropical areas where (protein-rich) plant sources contribute 80% of total caloric intake. • In 1910 humans consumed 10% of total carbon fixed by photosynthesis, by 2030 it is predicted that 80% will be used by humans.

  10. Consumes 1.4% of total fossil fuels annually Why chemical fertilizers aren’t the answer • Production of nitrogenous fertilizers has “plateaued” in recent years because of high costs and pollution • Estimated 90% of applied fertilizers never reach roots and contaminategroundwater

  11. Rhizobium-legume symbioses Host plantBacterial symbiont Alfalfa Rhizobium meliloti Clover Rhizobium trifolii Soybean Bradyrhizobium japonicum Beans Rhizobium phaseoli Pea Rhizobium leguminosarum Sesbania Azorhizobium caulinodans Complete listing can be found at at: http://cmgm.stanford.edu/~mbarnett/rhiz.htm Both plant and bacterial factors determine specificity

  12. legume Fixed nitrogen (ammonia) Fixed carbon (malate, sucrose) rhizobia

  13. Obvious signs of nodulation by common rhizobial species MEDICAGO (alfalfa) LOTUS (birdsfoot trefoil)

  14. Pea Plant R. leguminosarum nodules Pink color is leghaemoglobin a protein that carries oxygen to the bacteroids

  15. Physiology of a legume nodule

  16. Very early events in the Rhizobium-legume symbiosis Flavonoids nod-gene inducers rhizosphere Nod-factor

  17. nod genes Sinorhizobium meliloti chromosome NodD nod-gene inducers from alfalfa roots (specificity) plasmid pSym activated NodD positively regulates nod genes

  18. Nod factor biosynthesis NodM NodC Nod factor R-group “decorations” determine host specificity NodB Nod Factor: a lipooligosaccharide

  19. Attachment and infection Rhizobium Nod factor (specificity) Invasion through infection tube Flavonoids (specificity) Nitrogen fixation Bacteroid differentiation Formation of nodule primordia From Hirsch, 1992. New Phyto. 122, 211-237

  20. Rhizobium encoding GFP from jellyfish as a marker Infection thread (From Quaedvlieg et al. Plant Mol. Biol. 37: 715-727, 1998)

  21. Bacteria divide as they traverse infection thread

  22. Nodule development Enlargement of the nodule, nitrogen fixation and exchange of nutrients

  23. The Nodulation Process • Chemical recognition of roots and Rhizobium • Root hair curling • Formation of infection thread • Invasion of roots by Rhizobia • Cortical cell divisions and formation of nodule tissue • Bacteria fix nitrogen which is transferred to plant cells in exchange for fixed carbon

  24. Inoculation of a mutated Sinorhizobium strain does not transfer fixed N to the plant wild-type mutant Genes & Development 11:1194, 1997

  25. wt glnB10 6 days 7 days wt glnBP5 Genes & Development 11:1194, 1997

  26. Azorhizobium caulinodans on Sesbania

  27. Non-symbiotic nitrogen fixation Aquatic: Cyanobacteria Anabaena Nostoc Terrestrial and rhizosphere-associated: Azospirillum Azotobacter Acetobacter Klebsiella Clostridium

  28. Plant-associated nitrogen fixation: the endophytes and epiphytes Acetobacter diazotropicus Lives as an endophyte of sugarcane and various other monocots and some dicots On sugarcane

  29. A nitrogen-fixing fern -Co +Co The aquatic fern Azolla is the only fern that can fix nitrogen. It does so by virtue of a symbiotic association with a cyanobacterium (Anabaena azollae).

  30. Another cyanobacterium on the palm Welfia regia in an epiphyllic relationship It is believed that these bacteria transfer some % of fixed N to the plants through the leaf surfaces

  31. Symptoms of crown gall http://ohioline.osu.edu/hyg-fact/3000/3054.html

  32. Agrobacterium tumefaciens “transforms” plant cells Transgenes produce OPINES, unique amino acid-like molecules, as well as plant hormones

  33. The End

  34. Current approaches to improving biological nitrogen fixation • Enhancing survival of nodule forming bacterium by improving competitiveness of inoculant strains • Extend host range of crops, which can benefit from biological nitrogen fixation • Engineer microbes with high nitrogen fixing capacity What experiments would you propose if you were to follow each of these approaches?

  35. Rhizobium’s bad brother: Agrobacterium tumefaciens Crown gall on rose and on grapevine Opines are an Agrobacterium-specific C- source to feed future generations

  36. Rhizobium’s bad brother: Agrobacterium tumefaciens Crown gall on rose and on grapevine Opines are an Agrobacterium-specific C- source to feed future generations

  37. N2 NH4+ Exchange of nutrients during Rhizobium-legume symbiosis Malate to bacteria nitrogen- fixing bacteroid containing Rhizobium TCA NH4+ to plant ATP ADP+Pi

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