Possible involvement of lipoxygenase in a defense response in aflatoxigenic Aspergillus – cotton plant interactions

Abstract

Damage to the carpel surface of the developing cotton boll such as that caused by infection with aflatoxigenic Aspergillus spp. results in activation of the lipoxygenase pathway. Lipoxygenase pathway activation generates a series of diverse, antifungal, volatile aldehydes that affect the growth of Aspergillus flavus and indirectly has an effect on aflatoxin production. Analysis of the head space volatile aldehydes by gas chromatography – mass spectrometry trapped during a 1-h sampling period of carpel surfaces of 23- to 35-day developing cotton bolls treated with exogenous lipase resulted in the identifications of C5–C9 alkanals and alkenals including hexanal and trans-2-hexenal. Hexanal concentrations generated in lipase-treated bolls resulted in a two-fold increase of hexanal production. The effects of trans-2-hexenal on aflatoxin production were determined on intact 30-day-old developing cotton bolls treated with 2.2–8.6 μmol amounts of trans-2-hexenal and inoculated with aflatoxigenic Aspergillus in three different treatment systems. Trans-2-hexenal was placed on the developing cotton boll (i) 24 h before, (ii) 24 h after, or (iii) simultaneously with fungal inoculation. After a 3-week incubation on the intact cotton plants, bolls were harvested and aflatoxin determinations were conducted on cottonseeds from the bolls. The results suggest that pretreatment of the boll with trans-2-hexenal initiates a strong antagonism to aflatoxin formation due to the antifungal effects on the growth of the fungus in developing cotton bolls. These results also demonstrate a series of host-plant defense responses that suggest the possible involvement of the lipoxygenase pathway in interactions between aflatoxigenic Aspergillus and the cotton plant. Keywords: aflatoxin, Aspergillus flavus, cotton (Gossypium hirsutum), lipoxygenase (LOX) pathway, lipids.