3.8.4S
THE POTENTIAL ROLE OF SIDEROPHORES IN DISEASE SUPPRESSION IN CLOSED HYDROPONIC SYSTEMS

PAHM BAKKER

Dept. of Plant Ecology and Environmental Biology, Section of Plant Pathology, Utrecht University, PO Box 800,84,3508 TB Utrecht, The Netherlands.

Under conditions of low iron availability most microorganisms excrete low molecular weight compounds with a high specific affinity for iron, the so called siderophores. The siderophores sequester ferric iron in the environment and the resulting iron-siderophore complex is recognised by a receptor, resulting in the uptake of iron into the cell.

For several fluorescent Pseudomonas spp., strains with biocontrol properties, it has been suggested that siderophore mediated competition for iron with soilborne pathogens is an important mechanisms for biological control. In hydroponic systems the iron availability for micro-organisms will greatly depend on the affinity for iron of the synthetic chelate that is used for iron supply to the plant. If the chelate used has a low affinity, iron will be readily available for both the biocontrol agent and the pathogen, resulting in a lack of competition for iron between the groups of microorganisms. Moreover, the choice of synthetic chelate itself may result in effective control of the pathogen, simply by low iron availability to the pathogen.

Another mode of action of siderophores in biological control of plant pathogens appears to be induced systemic resistance. For several strains it has been established that their ability to induce systemic resistance depends on iron regulated metabolites. Moreover, purified siderophores have been demonstrated to induce systemic resistance in different species.

Thus the role of siderophores in suppression of plant disease in hydroponic systems may be highly effective. First the pathogen is weakened in the rhizosphere by a lack of iron and subsequently it encounters a host plant that has an enhanced defensive capacity.