Alexin™-mediated defence responses in wheat during Russian wheat aphid (Diuraphis noxia) infestation
The Russian wheat aphid (RWA, Diuraphis noxia Kurdjumov) is a serious international pest occurring in wheat production areas in South Africa. The use of RWA-resistant cultivars is an effective pest management tactic, but the occurrence of resistance-breaking biotypes affects the durability of this strategy. We investigated the effect of Alexin™, a potential priming agent, on the defence responses of three different wheat cultivars challenged with two South African RWA biotypes-RWASA1 and RWASA2. Alexin™ (0.375%, v/v) pre-treatment selectively mediated the expression of resistance responses in the wheat cultivars. The resistance response was measured in terms of enzyme activities of β-1,3-glucanase and peroxidase. To compliment these findings three host plant resistance nodes (antixenosis, antibiosis and tolerance) were used to screen for Alexin™-mediated resistance. Alexin™ treatment mediated tolerance and antibiosis to RWASA1 and tolerance to RWASA2 in the cultivar SST387. These changes were evident as reduced intrinsic rate of aphid population and phenotypic damage rating score from susceptible to moderately resistant. Alexin™ treatment however, compromised the defence responses towards RWASA1 and RWASA2 in PAN3379, a cultivar with reported resistance to all four known South African biotypes. To understand the mechanisms of priming in resistance to RWA, we measured the levels of reactive oxygen species (H2O2), antioxidant enzyme activities and certain defence hormones, as well as, expression of some stress related genes. Alexin™ mediated accumulation of H2O2 in the susceptible cultivar, increase in antioxidant enzyme activities (CAT, GR, SOD) and levels of Salicylic acid (SA) relative to Jasmonic acid (JA). On the other hand, the treatment suppressed defence responses in the resistant cultivar, except the relatively high level of induced SA. We concluded that defence mechanisms might be induced with other defence signalling networks. Even though field studies were not repeatable due to extreme seasonal variations in rainfall and temperature, the results indicate that Alexin™ has a potential to express systemic acquired resistance in otherwise susceptible wheat cultivars, and as such can be incorporated into an integrated pest management system to improve aphid management in wheat.