Involvement of active oxygen species and phenylalanine ammonia-lyase in the resistance responsr of wheat to the Russian wheat aphid

Abstract

The effect of Russian wheat aphid (RW A), Diuraphis noxia (Mordvilko) infestation on phenylalanine ammonia-lyase (PAL) and superoxide dismutase (SOD) activities as well as internal hydrogen peroxide (H202) levels and phenolic acid composition was studied in resistant (Tugela DNI) and susceptible (Tugela) wheat cultivars. In this study we aimed to learn more about the resistance response of wheat against the RW A with special reference to the role of active oxygen species (AOS), in particular H202, in establishing the down stream defense response and furthermore to investigate the involvement of phenylalanine ammonia-lyase (PAL) and phenolic compounds in the resistance response. SOD, PAL, POD (peroxidase) and chitinase activities were determined by means of spectrophotometric essay procedures. The phenolic compounds were separated using HPLC and a C-18 reverse phase column. Allopurinol was used to inhibit xanthine oxidase activity and concomitant H202 generation. The expression of the down stream defense response was measured in terms of POD and chitinase activities. RWA infestation selectively induced H202 production to a maximum concentration 8 h.p.i. in resistant wheat. The increase in H202 concentration corresponded with the increase in SOD activity, which was also differentially induced in infested resistant wheat to a maximum value 8 h.p.i. These results suggest that SOD is involved in the generation of H202 during the hypersensitive reaction (HR). Inhibitory studies done, have proven that H202 was needed for the induction of the down stream defense response. Chitinase activity was extremely low when the production of H202 was inhibited by allopurinol. PAL activity increased differentially in infested resistant wheat to higher levels (200% increase) than in infested susceptible wheat. Peak activity of PAL was reached between 8-48 h.p.i. The increase in the activity of PAL, a key enzyme in the synthesis of phenolic compounds, was reflected in increased phenolic acid concentrations. Some of the phenolic acids that were selectively induced in resistant wheat upon RW A infestation were, p-hydroxybenzoic acid, gallie acid, resorcylic acid, gentisic acid, caffeic acid, p-hydroxyphenylacetic acid and ferulic acid. The P: hydroxybenzoic acid, p-hydroxyphenylacetic acid, caffeic acid and ferulic acid concentrations corresponded to the peak PAL activity. Selective increases in PAL activity, caffeic and ferulic acid concentrations probably contributed to the selective lignification of mesophyll cells in infested resistant wheat. This is a manifestation of the HR, which is part of the defense response against the RWA. Findings in this study were indicative of the involvement of active oxygen species in establishing the down stream defense response and of phenolic compounds in the resistance response of wheat against the RWA.