Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/6/2005
Publication Date: 1/1/2006
Citation: Harfouche, A.L., Shivaji, R., Stocker, R., Williams, W.P., Luthe, D.S. 2006. Ethylene signaling mediates a maize defense response to insect herbivory. Molecular Plant-Microbe Interactions. 19:189-199. Interpretive Summary: Fall armyworm and southwestern corn borer are major insect pests of corn in the southern United States. Feeding by these insects on leaves of young corn plants reduces yield and quality of grain. The development and release of corn germplasm lines with genetic resistance to these pests provided an environmentally sound and cost effective way to reduce losses to these insects. Fall armyworm or southwestern corn borer larvae that feed on leaves of resistant plants are smaller than those that feed on susceptible plants. A unique protein, a cysteine protease, that accumulates at the sites where larvae feed on leaves within the plant whorls has been identified. In a series of experiments, the effects of blocking ethylene synthesis in a resistant, Mp708, and a susceptible, Tx601, inbred line of corn were investigated. When fall armyworm larvae were fed on leaves of resistant plants after ethylene synthesis was blocked, the larvae grew larger than when ethylene synthesis was not blocked. Blocking ethylene synthesis in the susceptible corn inbred did not affect larval growth. Blocking ethylene synthesis reduced, but did not completely inhibit, the synthesis of the unique cysteine protease in the resistant line. Ethylene is a component of a pathway leading to resistance of the inbred line, Mp708. These results give additional evidence that the cysteine protein found in Mp708 is important to resistance and that transferring the gene responsible for its production into susceptible corn genotypes lacking the gene should increase resistance to fall armyworm and reduce losses to the insect.
Technical Abstract: The signaling pathways that enable plants to mount defenses against insect herbivores are known to be complex. We have previously demonstrated that the insect-resistant maize (Zea mays) genotype Mp708 accumulates a unique defense cysteine proteinase Mir1-CP in response to caterpillar feeding. We furthered this study by exploring the role of ethylene in insect defense in Mp708 and an insect-susceptible line Tx601. Ethylene synthesis was blocked with either cobalt chloride (CoC12) or aminoethyoxyvinylglycine (AVG). Alternatively, ethylene perception was inhibited with 1-methylcyclopropene (1-MCP). Blocking ethylene synthesis and perception resulted in Mp708 plants that were less resistant to caterpillar feeding. In addition, fall armyworm (Spodoptera frugiperda) larvae that fed on inhibitor treated Mp708 plants had significantly higher growth rates than those reared on untreated plants. In contrast, these responses were not significantly altered in Tx601. The ethylene synthesis and perception inhibitors reduced but did not completely inhibit the accumulation of Mir1-CP and its transcript mir1 in response to herbivory, suggesting there is an additional ethylene-independent pathway for insect defense response. These results indicate that ethylene is a component of the signal transduction pathway leading to defense against insect herbivory in the resistant genotype Mp708.