Role of phenolic acids in maize resistance to the European corn borer, Ostrinia nubilalis (Huebner).
Creators: name: Bergvinson, David.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1993
Subjects: subject: Biology, Entomology.

description: Host plant resistance of maize, Zea mays L., to the European corn borer (ECB), Ostrinia nubilalis Hubner, has been largely attributed to the hydroxamic acid DIMBOA. However, DIMBOA does not consistently account for observed resistance in both field and greenhouse environments. It was hypothesized that phenolic acids, a major secondary metabolite in plants, make an important contribution to resistance, through cell wall carbohydrate complexes. Preliminary studies incorporating ferulic acid into the meridic diet of the ECB did not show any adverse effects until 10 mg/g, a dosage that is approximately 30 times that found in maize. Semi-preparative samples of sugar conjugates of ferulic and p-coumaric acids were tested using leaf-feeding bioassays that showed phagostimulant activity at ecologically relevant dosages. These observations suggested that soluble phenolics did not make a major contribution to host plant resistance but may be used for host plant recognition by the ECB. Bound phenolics can form dimers with adjacent phenolic acids. It was hypothesized that these dimers, cross-linked phenolic acid-carbohydrate complexes, could constitute a quantitative defence against the ECB. Developmental studies of a resistant maize synthetic using leaf bioassays and standard phytochemical procedures showed ECB larvae prefer to feed on immature tissue within the whorl and on plants between the 3- and 7-leaf stages of development. These preferred tissues had elevated levels of DIMBOA and lower levels of cell wall phenolics and lower levels of detergent fibre than more mature leaf tissue. Leaf toughness related inversely to leaf consumption. Microspectrophotometer readings of abaxial, epidermal cell walls best predicted insect preference for leaf tissues of different maturity. These studies showed that leaf toughness, vis a vis phenolic acid-carbohydrate complexes, is one of the major resistance factors which can account for larval feeding behavior. Environmental factors also influence maize resistance, with light quality having a significant impact. Under reduced UV light conditions, maize resistance in both greenhouse and field environments was reduced as were the levels of cell wall bound cyclobutane dimers. Cyclobutane dimers appear to increase the mechanical strength of leaf tissue by cross-linking hemicelluloses. The changes in maize phytochemistry and nutritional quality that accompanied increased resistance were monitored for leaf(mature and immature), sheath, stalk, node and pith tissues. DIMBOA levels increased with selection as did fibre content and the cell wall phenolics p-coumaric acid and dehydrodiferulic acid. Cultivars developed at the International Center for Maize and Wheat Improvement have resistance to several lepidopteran borer species but the mechanism of this resistance is unknown. Correlations of field resistance were highest for fibre, dehydrodiferulic acid content and protein content. A nutritional model for host plant resistance in maize based on these three plant components was proposed that accounts for 78 to 87 percent of the variation in field damage ratings observed over two years. According to Feeny's hypothesis of plant apparency, maize appears to rely heavily on quantitative defences. This is logical given the extensive areas planted in maize, with many areas employing no-till practices and continuous cropping which elevate the apparency of maize to insect pests. Phenolic acid-carbohydrate complexes have been demonstrated in this study to be a major component in this defence strategy. (Abstract shortened by UMI.)