Plant Promoting Microbes

• Plant Growth Promoting Microbes


  Beneficial microbes that may act by providing a plant growth enhancement product, facilitating the uptake of certain nutrients, or inhibiting the growth of soil phytopathogens
• Factors for successful plant development and high crop yields under natural environmental conditions

  • The genetic constitution of crop species
  • The availability of nutrients
  • The presence of beneficial microorganisms
  • Absence of phytopathogens
• Plant growth promoting mechanism

  1. Fixing atmospheric nitrogen and supplying it to the plant

  2. Increasing the availability of or sequestering of iron from the soil by the synthesis of siderophores

  3. Synthesizing phytohormones that trigger plant cell proliferation

  4. Solubilizing minerals such as phosphorus

  5. Synthesizing enzymes that can modulate the level of the plant hormone ethylene

  6. Antibiosis
• Current research focuses on plant growth promoting bacteria and not fungi
• Phytohormones


  Auxins, cytokinins, gibberellins
• ACC deaminase


  An enzyme produced by plant growth promoting bacteria that hydrolyses ACC, the biosynthetic precursor of ethylene in plants, thereby lowering the level of ethylene in the developing or stressed plant
• Ethylene can be stimulatory or inhibitory


  Ethylene can result in fruit ripening and degreening, flower initiation, flower opening or senescence, and responses of plants to biotic and abiotic stress
• Stress ethylene refers to the increase in ethylene biosynthesis associated with biological and environmental stresses and pathogen attack
• Chemicals have been used to counteract the effect of ethylene on the plants but this is hazardous to the environment
• ACC-deaminase plant growth producing bacteria are being used as an environmentally safe method to reduce the levels of ethylene
• Role of ACC deaminase bacteria in decreasing plant stress

  1. Plants respond to flooding by activating transcription in root cells of some of the genes that code for enzymes of ACC synthase, which converts S-adenosyl-methione to ACC

  2. However under anaerobic conditions (flooding) ethylene cannot be formed from ACC, so the ACC is transported to the shoots

  3. Ethylene in the shoots causes plants to wilt and to lose biomass and the plant may eventually die if the condition is prolonged

  4. Treatment of tomato plants with ACC deaminase containing plant growth promoting bacteria significantly decreases the damage suffered by these plants due to ethylene stress brought about by flooding

  5. These plant growth promoting bacteria act as a sink for ACC, lowering the levels of ethylene that can be formed in the shoots, protecting the tomato from damage caused by flooding
• Biocontrol of phytopathogens by plant growth promoting bacteria

  1. Production of siderophores that bind Fe3+ and prevent its availability to phytopathogens

  2. Production of antibiotics that limit the damage to plants by phytopathogens

  3. Production of enzymes that can degrade fungal cell walls and cause fungal cells to lyse
• Siderophores


  Iron-binding molecules synthesized and secreted by microorganisms to transport Fe3+ into the cell
• Plant growth promoting bacteria can prevent the proliferation of fungal phytopathogens by producing siderophores that bind most of the Fe3+ in the rhizosphere
• Siderophore biosynthesis is a complex process, and genetically engineering bacteria to produce modified siderophores is not a simple matter
• Extending the range of iron-siderophore complexes that one bacterial strain can utilize can give it a competitive advantage
• Antibiotics produced by Pseudomonads

  • Agrocin 84
  • Agrocin 434
  • 2-4 diacetylphloroglucinol
  • Herbcolin, oomycin
  • Phenazines
• Genetic manipulation to increase antibiotic production

  1. Modifying the global transcription regulator that controls antibiotic production

  2. Chromosomal insertion of antibiotic operons
• A modified version of Agrobacterium radiobacter K84 has been marketed in Australia since 1989 to control crown gall disease caused by Agrobacterium tumefaciens
• Enzymes produced by plant growth promoting bacteria

  • Chitinase
  • β-1-3-glucanase
  • Proteases
  • Lipases
• Many bacterial enzymes that lyse fungal cells are carried on a single gene, and these genes can be isolated and transferred into other bacteria
• Pseudomonas syringae damages plants by synthesizing ice nucleation proteins that cause ice crystal formation and cell lysis
• Preventing freezing damage caused by P. syringae

  Spraying the plant with a mutant form of the ice nucleating bacterium constructed by recombinant DNA manipulation or conventional mutagenesis and selection
• Plant growth promoting bacteria were recently discovered that could grow at 5°C and secrete antifreeze proteins at low temperature
• After the genes for bacterial antifreeze proteins have been isolated, it should be possible to transfer them to certain strains of plant growth promoting bacteria and create strains that can persist at low temperature
• Read up on the construction of a tra-derivative of plasmid pAgK84 from A. radiobacter and chromosomal insertion of antibiotic phz operon in P. fluorescens