BROWNING-ASSOCIATED MECHANISMS OF RESISTANCE TO INSECTS IN CORN CALLUS-TISSUE

Callus of the fungus- and insect-resistant corn inbred, Mp313E, of dif fering ages and degrees of brownness was tested for effects on neonate larvae of the corn earworm, Helicoverpa tea. Older, browner, 8-week-o ld callus caused significantly higher mortality of H. tea after two da ys compared to larvae fed 5-week-old callus (brown or not) or pinto be an diet. Larvae fed on the 5-week-old callus were significantly smalle r after nine days compared to those fed on nonbrown callus of the same age. Ferulic acid, p-coumaric acid, and sinapic acid, corn allelochem icals associated with cell walls, were oxidized (produced a brown prod uct) up to 10-fold more rapidly by peroxidase preparations of the brow n callus compared to nonbrown callus. Peroxidase isozymes from both ty pes of brown callus separated by conventional gel electrophoresis and isoelectric focusing showed more intense anodic/acidic bands compared to peroxidase isozymes separated from the nonbrown callus. Some additi onal peroxidase isozymes were also present in the brown callus and wer e able to oxidize ferulic acid. Conventional extraction techniques for phenolic acids indicated free ferulic acid levels were similar for sa me-age callus, but somewhat less for older callus; no free p-coumaric acid was detected. For esterified and insoluble-bound phenolic content , the level of p-coumaric acid was higher than that of ferulic acid an d was somewhat greater in insoluble-bound fractions for the old callus compared to the younger callus. Color ratings of aqueous extracts and phenolic acid extract residues indicated much brown material remained , which was proportional in color to the original material. The differ ential insect response to, and peroxidase activity of, brown vs. nonbr own callus were similar to results previously noted for brown pericarp s of corn compared to nonbrown pericarps. This information suggests th e peroxidases are contributing to the browning through oxidation of ph enolic acids and thereby enhancing resistance to insects.