Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/16/2006
Publication Date: 5/17/2007
Citation: Camas, A., Magbanua, Z., Demoraes, C., Wilkinson, J., Windham, G.L., Williams, W.P., Luthe, D.S. 2007. Jasmonic acid biosynthesis pathway confers resistance to maize embryos against Aspergillus flavus [abstract]. In: Proceedings of the 2006 Multicrop Aflatoxin/Fumonisin Elimination & Fungal Workshop, October 16-19, 2006, Ft. Worth, Texas. p. 80.
Technical Abstract: Much evidence suggests that lipid-derived secondary metabolites (oxylipins) produced by plants are mediating plant host-pathogen interactions. It has been hypothesized that several host plant and fungal genes are involved in determining the degree of aflatoxin contamination of maize kernels. For example, it has been reported that corn oxylipins could have a sporogenic effect on Aspergillus species. Jasmonic acid (JA) is a naturally occurring oxylipin in higher plants and it has been proposed a as potent inhibitor of aflatoxin biosynthesis. JA is a cyclopentanone derivative synthesized from linolenic acid via the octadecanoic branch of the lipoxygenase (LOX) pathway. Lipoxygenases initiate the octadecanoic branch in response to fungal attack. JA synthesis begins with the conversion of '-linolenic acid to HPOTE (13-hydroperoxy octadecatrienoic acid) by a 13-LOX. HPOTE is further metabolized by Allene oxide Synthase (AOS) and cyclized by Allene oxide cyclase (AOC) to produce 12-oxo-pytodienoic acid (12-OPDA), the natural precursor for JA biosynthesis. OPDA is reduced by Oxophytodienoic acid reductases (OPR) enzymes. However, it has been shown that OPDA by itself is an oxylipin involved in plant defense response in the absence of JA. Another branch of the LOX pathway involved in the inhibition of the sporogenesis and aflatoxin synthesis is the HPL (hydroperoxide lyase) branch. HPL branch produces volatile aldehydes and alcohols that also inhibit fungal growth and aflatoxin synthesis. In this work, we investigated the gene expression of two branches of the LOX pathway, Hidroxyperoxide Lyase (aldehydes synthesis pathway) and the Octadecanoic branch (jasmonic acid synthesis pathway) in relation to corn embryos resistance to A. flavus infection in the field. For this purpose, specific primers were designed to detect the expression pattern of several LOX pathway genes such as two 9-lipoxygenases genes (ZmLOX1 and ZmLOX3), one Allene oxide synthase (AOS), two Oxophytodioenoic acid reductases (OPR7 and OPR-2), and one Hydroperoxide Lyase (HPL), as well. We compared the gene expression patterns between the embryos of one resistant (Mp313E) and one susceptible (Sc212M) maize genotypes to aflatoxin contamination. We sampled the embryos 6 days after ears inoculation with A. flavus spores. The ears were inoculated 14 days after pollination. ZmLOX1, ZmLOX3, OPR7, OPR2, and HPL transcript levels were up-regulated in corn embryos during A. flavus infection in both genotypes, although higher levels of transcripts were already present in Mp313E before fungal inoculation. Surprisingly, we found out that the AOS gene expression was repressed in Sc212M inoculated embryos compared to control uninoculated embryos. It might be possible that some metabolites produced during A. flavus infection may inhibit AOS expression in the embryos of the susceptible genotype, contributing to the susceptibility of Sc212M corn inbred line. Furthermore, we also compared the JA levels produced in embryos of both genotypes. Although the differences in JA concentracion were not significantly different between inoculated and control embryos, JA levels increased in the embryos of Mp313E genotype while they decreased in the embryos of Sc212M the susceptible genotype, after the inoculation with A. flavus. Taken together this results suggest that the expression of the LOX pathway is required to activate JA and another different oxylipins produced by the action of ZmLOX3 and HPL pathway to confer resistance to the embryos of Mp313E against Apergillus flavus to inhibit either the fungal growth and/or the aflatoxin synthesis. We conclude that LOX pathway has to be considered in maize breeding programs to enhance corn resistance against A. flavus.