Molecular Biotechnology: Principles and Applications - Glick B., Pasternak J. 2002

Molecular Biotechnology of Microbial Systems
Plant Growth-Promoting Bacteria
Conclusion

Many soil microorganisms possess The ability to promote plant growth. The molecular mechanisms underlying this stimulation have been investigated to determine whether beneficial soil Bacteria can be used as alternatives to chemical fertilizers. Beneficial bacteria can exert their influence directly by supplying plants with fixed nitrogen, chelated iron, and phytohormones, or by facilitating phosphorus uptake. Alternatively, this influence can be indirect, achieved through the suppression of phytopathogenic microorganisms.

Among all plant growth-promoting bacteria already used in agriculture, members of the genera Rhizobium and Bradyrhizobium have been studied in the greatest detail. These microorganisms enter into complex, obligate symbiotic relationships with highly specific host plants.

The Molecular Basis of Nitrogen Fixation has been extensively studied in K. pneumoniae, which serves as a model system for investigating symbiotic bacteria of the genera Rhizobium and Bradyrhizobium. Nitrogenase, the nitrogen-fixing enzyme, has been characterized in detail. Molecular genetic studies have shown that bacterial Nitrogen fixation is a complex process involving seven coordinately regulated operons that encode a total of 20 different Proteins. This currently makes it impossible to genetically engineer plants capable of fixing nitrogen on their own, or to develop other nitrogen-fixing bacteria.

The nitrogen-fixing enzyme nitrogenase, utilizing the energy of ATP Hydrolysis, catalyzes The formation of hydrogen gas (H₂). Some Rhizobium strains synthesize the enzyme Hydrogenase. It catalyzes the in vivo conversion of H₂ to H⁺, which increases the efficiency of nitrogen fixation. If a strain contains an inactive hydrogenase, its ability to fix nitrogen and stimulate plant growth is reduced. Based on the above, attempts have been made to introduce cloned hydrogenase genes into Rhizobium strains that form symbiotic relationships with crops. Preliminary data suggest that Introduction/32.html">Genetic Engineering OF hydrogenase genes can yield Rhizobium strains with an enhanced capacity to promote plant growth.

By entering into symbiotic relationships with plants, Rhizobium strains stimulate the formation of ROOT nodules, where these bacteria multiply and fix nitrogen. It was reasonable to assume that if genetic engineering could produce bacteria that promote the formation of more nodules, the competitiveness of inoculant Rhizobium strains for colonization sites on the roots of symbiotic plants would increase compared to wild-type strains. Unfortunately, it was discovered that nodule formation involves many different genes, and this complexity complicates the design of relevant molecular genetic experiments.

Indirect Plant Growth Promotion by bacteria involves protecting plants from Damage caused by phytopathogenic Fungi or bacteria. This protection is mediated by specific compounds synthesized by plant growth-promoting bacteria, such as siderophores, Antibiotics, other small molecules, and various Enzymes. Certain other synthetic products, particularly phytohormones and ACC deaminase, directly affect plant growth. It is hoped that eventually, the biosynthetic genes for all these compounds can be utilized to engineer bacteria that are more effective plant growth promoters.

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STUDY QUESTIONS

1. Suppose you have a strain of Rhizobium japonicum capable of fixing nitrogen and entering into a symbiotic relationship with soybean plants. What approach would you use to identify the gene cluster encoding nodulation, assuming you do not have a Hybridization probe for nod genes?

2. What is hydrogenase? How can it be used to increase the yield of alfalfa?

3. Propose a strategy to identify all Azotobacter vinelandii genes involved in nitrogen fixation, keeping in mind that you do not have the w z/¹ genes of other microorganisms that could be used as hybridization probes.

4. How, in your opinion, would introducing Mutations into the nifA or nifL genes affect The amount of nitrogen fixed by this Organism?

5. Discuss the feasibility of developing recombinant plants capable of fixing nitrogen.

6. Propose a scheme for isolating hydrogenase genes.

7. What are siderophores? How can modifying siderophore genes enhance the ability of bacteria to promote plant growth?

8. Propose a scheme for identifying siderophore biosynthesis genes.

9. What are the advantages of microbial fertilizers over chemical ones?

10. How can genetic engineering techniques be used to enhance the efficacy of Agrobacterium radiobacter as a biocontrol agent?

11. Which enzymes secreted by plant growth-promoting bacteria are responsible for their "biocontrol" properties? What is the MECHANISM OF ACTION of these enzymes?