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

The fungus, Aspergillus flavus, secretes lipases while parasitizing oi lseed hosts and may contaminate the seed with aflatoxin. We found that exogenous lipase applied to soybean cotyledons results in the generat ion of volatile aldehydes by the lipoxygenase pathway that halts the g rowth of fungi. Volatiles generated from untreated homogenized soybean s 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 soybe an 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 nordihydroguaiaret ic acid (a lipoxygenase inhibitor) to the soybean homogenate/lipase mi xture partially reversed the inhibition of spore germination. The exte nt of spore germination was dependent on both the number of soybean co tyledons in the homogenate and the activity of the added lipase. Addit ion of linoleic or linolenic acids to the homogenates also inhibited s pore germination, whereas palmitic, stearic, or oleic acid had no effe ct, compared to untreated homogenates. Headspace analysis indicated th at hexanal, a product of the lipoxygenase pathway with known antifunga l activity, was the major volatile generated from lipase-treated homog enates. Soybeans are exceptional among oil-rich seed crops in that the y are resistant to A. flavus infection and aflatoxin contamination. Ou r evidence strongly suggests that the lipoxygenase pathway may contrib ute to this resistance.