School of Engineering and Science - Jacobs University

Introduction
An impact of iron uptake systems on biocontrol was proven for many different
pathosystems including the control of Gaeumannomyces graminis var. tritici by
fluorescent strains of Pseudomonas or the control of Pythium-induced
damping-off of tomato by Pseudomonas aeruginosa 7NSK2 (Buysens et al.,
1996; Hamdan et al., 1991).
As a result of their effective iron uptake systems many screenings for new
biocontrol organisms are limited to the group of fluorescent Pseudomonads
(Haas et al., 2000; Haas & Defago, 2005). Another recently discovered
biocontrol mechanisms is quorum sensing silencing (Dong et al., 2004). Many
virulence determinants are regulated in a cell-density dependent manner. In
Gram-negative bacteria the quorum sensing signals are N-acyl homoserine
lactones (AHLs). The gram-positive bacterium, Bacillus thuringiensis, a
common biological control agent, produces an AHL-lactonase that breaks down
AHL and thus silences virulence gene expression of gram-negative pathogens
(Dong et al., 2004).
The interaction between pathogen and control agent is not unidirectional. Plant
pathogens have evolved their own defense mechanisms against antagonizing
microbes, thus newly identified antagonists should always be tested for their
efficiency against several different strains of the pathogen (Duffy et al., 2003).
2.2 Iron and its implications for disease and disease control
2.2.1 The biology of iron
Iron is the fourth most abundant element in the outer crust of earth. Despite
this, the low water solubility of Fe 3+ at neutral pH limits the availability of free
iron in most habitats (Loper & Buyer, 1991). The importance of iron as cofactor
for enzymes involved in electron-transfer makes it an essential nutrient for
microbial growth. In contrast, high internal iron concentrations are deleterious
for the cell as it catalyses the formation of active oxygen species such as
hydrogen peroxide (H 2 O 2 ) and the hydroxyl radical (˙OH) by the Fenton
reaction:
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