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Gene-for-Gene Defense of Wheat Against the Hessian Fly Lacks a Classical Oxidative Burst

¹Department of Agronomy, Purdue University, 915 W. State St., West Lafayette, IN 47907, U.S.A.; ²United States Department of Agriculture-Agricultural Research Service (USDA-ARS) Crop Production and Pest Control Research Unit, West Lafayette, IN 47907, U.S.A.; ³Department of Entomology, Purdue University, 901 W. State St., West Lafayette, IN 47907, U.S.A.

Genetic similarities between plant interactions with microbial pathogens and wheat interactions with Hessian fly larvae prompted us to investigate defense and counterdefense mechanisms. Plant oxidative burst, a rapid increase in the levels of active oxygen species (AOS) within the initial 24 h of an interaction with pathogens, commonly is associated with defenses that are triggered by gene-for-gene recognition events similar to those involving wheat and Hessian fly larvae. RNAs encoded by Hessian fly superoxide dismutase (SOD) and catalase (CAT) genes, involved in detoxification of AOS, increased in first-instar larvae during both compatible and incompatible interactions. However, mRNA levels of a wheat NADPH oxidase (NOX) gene that generates superoxide (O₂¯) did not increase. In addition, inhibiting wheat NOX enzyme with diphenyleneiodonium did not result in increased survival of avirulent larvae. However, nitro blue tetrazolium staining indicated that basal levels of O₂¯ are present in both uninfested and infested wheat tissue. mRNA encoded by wheat genes involved in detoxification of the cellular environment, SOD, CAT, and glutathione-S-transferase did not increase in abundance. Histo-chemical staining with 3,3-diaminobenzidine revealed no increases in wheat hydrogen peroxide (H₂O₂) during infestation that were correlated with the changes in larval SOD and CAT mRNA. However, treatment with 2′,7′-dichlorofluorescin demonstrated the presence of basal levels of H₂O₂ in the elongation zone of both infested and uninfested plants. The accumulation of a wheat flavanone 3-hydroxylase mRNA did show some parallels with larval gene mRNA profiles. These results suggested that larvae encounter stresses imposed by mechanisms other than an oxidative burst in wheat seedlings.

Additional keywords: flavonoid , oxidative stress , quantitative real-time polymerase chain reaction , SYBR Green .