Browning-associated mechanisms of resistance to insects in corn callus tissue

Abstract

Callus of the fungus- and insect-resistant corn inbred, Mp313E, of differing ages and degrees of brownness was tested for effects on neonate larvae of the corn earworm,Helicoverpa zea. Older, browner, 8-week-old callus caused significantly higher mortality ofH. zea after two days compared to larvae fed 5-week-old callus (brown or not) or pinto bean diet. Larvae fed on the 5-week-old callus were significantly smaller after nine days compared to those fed on nonbrown callus of the same age. Ferulic acid,p-coumaric acid, and sinapic acid, corn allelochemicals associated with cell walls, were oxidized (produced a brown product) up to 10-fold more rapidly by peroxidase preparations of the brown callus compared to nonbrown callus. Peroxidase isozymes from both types 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 additional peroxidase isozymes were also present in the brown callus and were able to oxidize ferulic acid. Conventional extraction techniques for phenolic acids indicated free ferulic acid levels were similar for same-age callus, but somewhat less for older callus; no freep-coumaric acid was detected. For esterified and insoluble-bound phenolic content, the level ofp-coumaric acid was higher than that of ferulic acid and 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 differential insect response to, and peroxidase activity of, brown vs. nonbrown callus were similar to results previously noted for brown pericarps of corn compared to nonbrown pericarps. This information suggests the peroxidases are contributing to the browning through oxidation of phenolic acids and thereby enhancing resistance to insects.

Differential leaf resistance to insects of transgenic sweetgum ( Liquidambar styraciflua ) expressing tobacco anionic peroxidase

“…These reactive compounds can be toxic and inhibitory, and may be involved in further binding and cross-linking, as has been recently reviewed [7]. There is evidence that peroxidase activity contributes to insect resistance in tomato, Lycopersicon esculentum [4,7,8,32]; tobacco, Nicotiana tabacum and N. syl6estris [7 -9]; soybeans, Glycine max [33]; and maize, Zea mays [5,6,[34][35][36][37]. Dark brown regions were noted in the transgenic compared with wild-type L. styraciflua leaves where insects fed, and the pellet of the transgenic leaf homogenates also turned much darker compared with that from wild-type leaves in the presence of guaiacol and peroxidase.…”

Differential resistance of switchgrass Panicum virgatum L. lines to fall armyworms Spodoptera frugiperda (J. E. Smith)

“…While all lines evidently expressed the same isozymes, the relative expression of the two main isozymes differed significantly among cultivars. Different peroxidase isozymes in maize can act on different natural substrates (Dowd 1994;Dowd and Norton 1995), and the peroxidase products can vary in toxicity to insects (Dowd and Vega 1996) and phytopathogenic fungi .…”

Distinguishing Defensive Characteristics in the Phloem of Ash Species Resistant and Susceptible to Emerald Ash Borer

“…Although little studied in relation to wood borer feeding, defensive proteins in the phloem are recognized as potentially important resistance factors to phloem-feeding insects (Kehr, 2006). Polyphenoloxidase and peroxidase activity are associated with resistance of plants to both insects and pathogens (Felton et al, 1989;Dowd and Norton, 1995;Cipollini and Redman, 1999;Cipollini et al, 2004), presumably by increasing the toxicity of phenolics or by increasing cell wall toughness. Trypsin inhibitor activity may slow herbivore growth and feeding by decreasing protein assimilation by inhibiting the action of serine proteases, and thus slow larval growth rate (e.g., Cipollini et al, 2004).…”