Siderophores

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

Siderophores

	Siderophores Siderophores are low molecular weight, high affinity iron chelators that transport iron into bacterial cells. Fluorescent pseudomonads produce yellow-green, fluorescent siderophores which specifically recognize and sequester the limited supply of iron in the rhizosphere and thereby reduce the availability of this trace element for the growth of the pathogen. The availability of iron in soil decreases with increase in pH and therefore PGPRs function better in neutral and alkaline soils than in acid soils.
The demonstration that siderophore minus mutants are less suppressive to pathogens in the rhizosphere than parental strains appear to convince the role of siderophores in pathogen suppression. As stated earlier, an example of the involvement of antibiotics is the role of Agrocin-84 produced by Agrobacterium radiobacter in controlling the crown-gall symptoms of plants caused by Agrobacterium tumefaciens. The phenazin-type antibiotic produced by Pseudomonas fluorescence in the control of take-all disease of wheat has also been cited as another example.
Several investigations have shown that fluorescent pseudomonads in the rhizosphere produce yellow-green fluorescent pigment. Some strains, particularly B10 inhibit the growth of Erwinia caratovora which causes the soft rot disease of potato. In the presence of iron, no beneficial effect of Pseudomonas inoculation was observed when the soil was amended with Fe in the form of FeEDTA (Ethylenediaminetetraacetatoferrate) in spite of effective colonization in the rhizosphere.
A yellow-green pigment called 'pseudobactin' isolated from this fluorescent pseudomonad also did not exhibit beneficial effect when iron was sequestered (bound) in the form of 'red-brown ferric pseudobactin' whereas pseudobactin by itself was effective. These results imply that the siderophore pseudobactin deprived E. caratovora of Fe, by scavenging the element available in the vicinity and thus reduce disease severity by minimizing the virulence of the pathogen. In California, when FeEDTA was added to a Fusarium suppressive soil where no inoculation was done to flax seedlings with its pathogen Fusarium oxysporum f. sp. lini 90% of the seedlings survived. On the other hand, in the same suppressive soil with the pathogen in the presence of FeEDTA, only 47% of the seedlings survived. However, the presence of the pathogen alone in the same soil had adverse reaction resulting in the survival of only few seedlings.
	These results reflect the possibility that microorganisms present in the soil produce siderophores which have the affinity for the limited available iron in the root milieu and thus depriving the pathogen of this vital element. A similar experiment carried out in pathogen conducive soil also proved that sequestering the limited Fe was the reason behind the reduction in the severity of the disease. The experiment involved the addition of strain B10 of Pseudomonas sp. or siderophore pseudobactin to a pathogen conducive soil, infested with Fusarium oxysporum F.

lini and growing flax seedlings. Pseudobactin is a linear hexapeptide requiring at least five gene clusters with a minimum of five genes for its biosynthesis. The addition of PGPR or its siderophore pseudobactin increased the survival of flax seedlings to 87 to 90 per cent when compared to other treatments where ferric pseudobactin or Fe EDTA plus Pseudomonas sp. strain BI0 were added bringing down seedling survival to 48 to 50 per cent. Similar results were also obtained with the take-all disease of wheat caused by Gaeumannomyces graminis var. tritici by the addition Pseudomonas sp. strain B10. Some of these results obtained either with Pseudomonas or siderophores with and without EDT A have been summarised.

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