Caracterização dos efeitos de indução de resistência, memória imune e tolerância em genótipos de milho a Spodoptera frugiperda (Lepidoptera: Noctuidae) pela aplicação de silício

Abstract

The fall armyworm Spodoptera frugiperda is considered to be the major defoliating pest of maize in Brazil and other countries, such as Africa and Asia. Studies have already shown that plants fertilized with silicon (Si) show increased resistance to insect pests due to the hardening of cell walls, activation of antioxidant enzymes, and expression of genes and secondary metabolites. However, there are controversies regarding the effects promoted by Si regarding the resistance of plants, leading to debates about whether the effects would be resistance- inducing; or induction of immune memory. Another approach is whether Si application could induce greater tolerance to plants. This work aimed to characterize the effects of Si as to resistance induction, priming of immune memory, and tolerance in a hybrid and a landrace variety of maize against the major pest S. frugiperda. The experiment was carried out in a 2 x 2 x 2 factorial (with or without Si x with or without herbivory x hybrid or landrace variety), forming with the combinations 8 treatments and 15 repetitions, using potted plants in a greenhouse. The dose of Si (1% silicic acid) used was proportional to 2 t ha-1 and was applied to plants at V2 stage. When the plants reached V3, two neonates of S. frugiperda were infested in the plants’ whorls in the treatments assigned with herbivory. After 5-7 days, plants at V4 were reinfested with a 4th instar larva that remained in the plant until V5. Next, plants of each treatment were divided into 4 groups for assessments of: 1) dry mass of the aerial part and root system, plant height, and relative chlorophyll index; 2) activity of the enzymes SOD, CAT, and APX, H2O2 and MAD; 3) induced effects on the weight and leaf consumption of newborn caterpillars; and 4) determination of the injury score based on the Davis scale. There was a reduction in injury scores only in plants with Si and subjected to herbivory. The landrace variety showed lower injury scores than the hybrid. Larval weight gain was lower in landrace plants with both Si and herbivory. Greater root dry matter was achieved in the landrace with Si and without herbivory. Landrace plants showed greater weight of the aerial part than the hybrid with herbivory only. Si fertilization to plants increased H2O2 concentrations. In the landrace variety higher activities of APX occurred with Si application and without herbivory, and of CAT and SOD in conditions with Si and without or with herbivory and without Si. The effects of resistance induction, priming of immune memory, and tolerance can occur jointly and be used by the plants as defense strategies against herbivory. In addition, the results suggest that Si effects are enhanced by herbivory, negatively affecting growth of larvae and reducing injury in the landrace variety. The landrace variety should have specific mechanisms of induced resistance and tolerance.