EVIDENCE IMPLICATING THE LIPOXYGENASE PATHWAY IN PROVIDING RESISTANCE TO SOYBEANS AGAINST ASPERGILLUS-FLAVUS

The fungus, Aspergillus flavus, secretes lipases while parasitizing oilseed hosts and may contaminate the seed with aflatoxin. We found that exogenous lipase applied to soybean cotyledons results in the generation of volatile aldehydes by the lipoxygenase pathway that halts the growth of fungi. Volatiles generated from untreated homogenized soybeans did not affect the germination of A. flavus spores in a closed petri plate assay in which spores were streaked on corn-meal agar and soybean homogenate was placed on the lid of the inverted plate. When lipase (500 units/ml) was added to the soybean homogenate, all fungal spore germination was inhibited. However, the addition of nordihydroguaiaretic acid (a lipoxygenase inhibitor) to the soybean homogenate/lipase mixture partially reversed the inhibition of spore germination. The extent of spore germination was dependent on both the number of soybean cotyledons in the homogenate and the activity of the added lipase. Addition of linoleic or linolenic acids to the homogenates also inhibited spore germination, whereas palmitic, stearic, or oleic acid had no effect, compared to untreated homogenates. Headspace analysis indicated that hexanal, a product of the lipoxygenase pathway with known antifungal activity, was the major volatile generated from lipase-treated homogenates. Soybeans are exceptional among oil-rich seed crops in that they are resistant to A. flavus infection and aflatoxin contamination. Our evidence strongly suggests that the lipoxygenase pathway may contribute to this resistance.