plant surface microbiology.pdf

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Ramesh Chander Kuhad et al.
in solubility and transport of iron, hydroxamate siderophores are also
involved in iron storage.
Based on the chemical nature of their coordination sites, microbial
siderophores are classified as hydroxamates, catecholates, carboxylates and
mixed type. Hydroxamates are produced both by bacteria and fungi. In most
fungi, a mixture of siderophores is produced which varies depending on cultivation
conditions. Aspergilli produce ferricrocin accompanied by fusarinines,
while certain penicillia produce ferrichrome accompanied by coprogen.
Similar observations have been made with Neurospora, Gliocladium,
Trichoderma and Agaricus bisporus (Neilands and Leong 1986). Fusarinines
(fusigens) produced by species of Fusarium and Penicillium are linear and
cyclic hydroxamic acids joined by ester bonds. Ericoid mycorrhizal fungi produce
ferrichrome and fusarinines.
Varieties of bacterial hydroxamates are known. Ferrioxamine, produced by
actinomycetes, Nocardia and Pseudomonas stutzeri, is a cyclic trihydroxamate.
Citrate hydroxamates are characterized by the presence of two hydroxamates
and one citrate group as ligand, it is a linear citratehydroxamic acid
obtained from Klebsiella pneumonia and several enteric bacteria. Catecholate
siderophores are generally less diverse than the hydroxamates, and are conjugated
to amino acids or polyamine backbones. Species of Bacillus, Aeromaonas
and Erwinia are known to produce catecholate siderophores. Carboxylate
(complexone) siderophores are produced by Rhizopus microsporus,
Rhizobium meliloti and Staphylococcus hycius. Pyoverdines, the mixed types
form a wide class of mixed siderophores showing a great variety of structures.
Some strains of fluorescent pseudomonads produce hydroxamate siderophores
(ferribactin) in addition to pyoverdine siderophores.
The plant growth-promoting rhizobacteria (PGPR) owe their plant growth
promoting activity to their stronger siderophores with higher stability constants
that outgrow the other bacterial population in competition for iron and
finally displace them from the root surface. The siderophore-producing PGPR
have become important in the biological control of plant pathogens (Glick
and Bashan 1997).
5 Conclusions
Microorganisms play an essential role in the functioning and sustainability of
soil ecosystems including biogeochemical cycling of nutrients and biodegradation.
Recent advances in soil community analysis using molecular and biochemical
approaches have helped us understand the enormous microbial
diversity and their functional significance in nutrient recycling in soil and
plant development. Soil diversity exceeds that of aquatic environments and
provides a great resource for the biological exploitation of novel organisms,
processes and products. Microbes isolated from soil and developed as biofer-