natural-products-in-plant-pest-management

Suppressive Effects of Compost Tea on Phytopathogens 247below 10 –18 M in soil solutions at neutral pH (Handelsman and Stabb, 1996).Iron salts that have low solubility such as iron oxides, carbonates, phosphates,hydroxides and some forms of insoluble chelates are created in certain soiltypes that make iron not readily available to plants. To circumvent the solubilityproblem, many microorganisms synthesize and utilize very specificlow molecular weight (500–1000 Da) iron chelators called siderophores (thatcan bind iron and are sent out by the cells to absorb iron, as well as being usedinside the cell to store it). When grown under iron-deficient conditions, manymicrobes will synthesize and excrete siderophores in excess of their own drycell weight to kidnap and solubilize iron and made it unavailable to the pathogens.The typical system involves a siderophore, which is an iron-bindingligand, and an uptake protein, which transports the siderophore into the cell.The fluorescent pseudomonads produce a class of siderophore known as thepseudobactins, which are structurally complex iron-binding molecules. Analysesof mutants lacking the ability to produce siderophores suggest that theycontribute to the suppression of certain fungal and oomycete diseases (Duijffet al., 1994; Buysens et al., 1996). An interesting aspect of siderophore biologyis that diverse organisms can use the same type of siderophore. Microorganismsmay use each other’s siderophores if they contain the appropriate uptakeprotein (Koster et al., 1993; Raaijmakers et al., 1995), and plants can evenacquire iron from certain pseudobactins (Duijff et al., 1994).Microorganisms in compost produce siderophores that keep iron in anavailable form for plants in the soil, even at high pH. Compost also produceswater-soluble humic substances, including fulvic acids, which keep iron, zinc,manganese and other trace elements in solution (Chen and Inbar, 1993).AntibiosisAntibiosis is defined as antagonism mediated by specific or non-specificmetabolites of microbial origin, by lytic agents, enzymes, volatile compoundsor other toxic substances (Jackson, 1965; Fravel, 1988). Antibiotic productionappears to be important to the survival of microorganisms through the eliminationof microbial competition for food sources, which are usually verylimited in soil (Ellis et al., 2000; Slattery et al., 2001). Antibiotic production isvery common among soil-dwelling bacteria and fungi. Inhibition in the Petridish may be the result of antibiosis, but it is not easy to show that this antibiosisis actually responsible for disease suppression. First, the antibioticmust be extracted, purified and identified chemically. Then it is necessary toshow that the microorganism grows in the microhabitat of the pathogen, andthat the antibiotic is produced in the right place, at the right time, and in sufficientamounts to control disease. It is also necessary to demonstrate that thepathogen is sensitive to the antibiotic. Genetic analyses have been particularlyinformative in determining the role of antibiotics in biocontrol, in partbecause mutants can be screened easily in vitro for changes in antibioticaccumulation, providing the means to conduct thorough genetic analyses(Handelsman and Stabb, 1996).