Mechanism of silicon application to improve constitutive and inducible resistance to aphid during physiological metabolism of maize

Objectives Rhopalosiphum padi is one of the main pests in maize production, its growth and reproduction severely affect the yield and quality of maize. Silicon (Si) application is capable of inducing aphids resistance by maize. In this study, we explored the beneficial effects of Si on the antioxidant system, secondary metabolism and signal transduction pathway of maize.

Methods A maize hydroponic experiment was carried out, and the tested aphid was Rhopalosiphum padi. The four treatments were no Si application without aphid infestation (–Si–A), no Si application with aphid infestation (–Si+A), Si application without aphid infestation (+Si–A), and Si application with aphid infestation (+Si+A). At 48, 60, 72, 96 and 120 h after aphid infestation on maize plant, the density of aphids was surveyed. The CAT, SOD, H₂O₂, and MDA contents in the antioxidant system, the PAL, PPO, LOX, and lignin contents in the secondary metabolism and signal transduction substances (JA and SA) were analyzed. The content or enzyme activity of the tested indices in maize without aphid infestation was defined as the constitutive resistance. The difference between the indices before and after aphid infestation was defined as inducible resistance, and Si's effect on these resistances was discussed.

Results Aphids density increased with infection time. The density of aphids at 60−120 h after aphid infestation without Si application was 12.50%−40.18% higher, and that with Si application was 12.36%−49.44% higher than that at 48 h after aphid infestation. At the same infestation time, the aphid density with Si application was 15.29%–20.64% lower than without Si application. During the whole incubation period, Si application improved the activity of the constitutive CAT and SOD, reduced the constitutive H₂O₂ and MDA by 4.41%−15.35% and 5.35%−17.95% (P < 0.05), increased the activity of inducible CAT across the infestation time except at 72 h after aphid infestation, reduced (P < 0.05) inducible SOD activity, and inducible H₂O₂ increased first and subsequently declined. The activities of constitutive PAL, LOX, PPO, and constitutive lignin (P < 0.05) increased, while inducible PPO activity and lignin content decreased. The activity of inducible PAL increased first and then decreased, inducible LOX activity fluctuated, the content of constitutive JA and SA (P < 0.05) increased, the content of inducible SA increased by 43.77%−117.48%, and the content of inducible JA decreased. Similarly, irrespective of Si application, the activity of constitutive CAT, SOD, PAL, LOX, and PPO, the content of H₂O₂, MDA, lignin, JA and SA were (P < 0.05) higher than those of the inducible type. The PCA and correlation analysis results showed that constitutive and inducible CAT activity, SOD activity, inducible H₂O₂ content, and SA content were better indicators reflecting maize’s constitutive and inducible resistance to aphids.

Conclusions Silicon application improved the aphid resistance of maize by affecting the constitutive and inducible resistance of each substance in the antioxidant system, secondary metabolism and signal transduction pathways of maize, which provided a theoretical and scientific basis for the ecological regulation of aphids in maize fields.