Genetic Manipulation

Rhizosphere and the Phyllosphere
Rhizosphere Introduction
Rhizospher Effect
Nitrogen Fixation in the Rhizosphere
Alteration of Rhizosphere Microflora
Associative and Antagonistic Activities in the Rhizosphere
Root Exudates
Organic Compounds Detected in Plant Root Exudates
Fungistasis
Techniques of Rhizosphere
The Soil Dilution and Plate Count Method
The Soil Plate
The Soil Box
The Contact Slide Technique
Artificial Rhizosphere
Model System

Genetic Manipulation

Genetic Manipulation

The approach that has received much attention has been the transfer of genetic loci encoding the pathways for the synthesis of antifungal metabolites such as phenazines, phloroglucinols, oomycin(A), pyoluteorin, pyrrolinitrin and HCN. For example a strain of P.

fluorescens Hv37a was constructed by placing the biosynthetic genes for the production of oomycin A under the control of the constitutive tac promoter from Escherichia coli; this recombinant strain suppressed Pythium damping-off cotton to a greater extent than the parent strain.

Another example is the overproduction of phenazine-1-carboxylase or phloroglucinol by the introduction of plasmids possessing the respective biosynthetic loci into a number of different strains of fluorescent Pseudomonas spp; the recombinant strains had better ability to suppress the take-all disease of wheat than the parent strain. One more example is the use of Tn5 mutagenesis approach to generate mutants of P. fluorescens CN 12 with enhanced level of in vitro antibiosis towards the take-all disease organisms of wheat; in four years field trials, the mutant strain provided significantly greater control of the take-all disease than the parent strain.

Another strategy in genetic manipulation has been the introduction of the ability to catabolize a novel substrate so that the recombinant strain, when added to soil with the novel substrate, gets the ability to colonize the root region preferentially over indigenous microflora.
The influence of Pseudomonas sp. (strain BI0) or its siderophore with or without FeEDTA on the intensity of wilt of flax incited by Fusarium oxysporum f. sp. lini or take-all of barley incited by Gaeumannomyces graminis in disease suppressive or conducive soils of California (summary of data from Leong., J., Ann. Rev. Phytopathol., 24, 187-209)

Soil	Pathogen Present/ Absent	Treatment	Per cent of seedling survival
			Flax	Barley
Disease suppressive soil	Present Present Absent	H₂O 50 μm FeEDTA 50 μm FeEDTA	82 47 90		83 38 85
Disease Conducive Soil	Present Present Absent Present Present Present Present	H₂O 50 μm FeEDTA 50 μm FeEDTA Strain B10 Strain B10 + 50 μm FeEDTA 50 μm Pseudobactin (Siderophore) 50 μm Ferric Pseudobactin	48 52 92 87 48 90 50		27 25 87 88 25 73 20

strain of Pseudomonas R20 was genetically engineered to possess the plasmid NAH 7, which encodes the enzymes for salicylate degradation; when this recombinant strain [R 20 (pnAH 7)] was added to soil with salicylate, its population was two fold in sugarbeet soil when compared to the parent strain, thus affording better chance of operating in the root region to bring about reduction in the severity of DRMOs.

Integrated Approach
Many reports have come on the dual application of fluorescent pseudomonads with species of Trichoderma or other fungi possessing disease controlling abilities that are mutually compatible. Combinations of biocontrol agents and chemical control agents (fungicides) provide yet another approach to minimize fungal infections of seedlings caused by Fusarium, Pythium, Rhizoctonia, Cylindrocarpon and Cylindrocladium.

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