Fall Armyworm-Induced Secondary Metabolites in Sorghum Defend Against Its Attack

Abstract

The fall armyworm (FAW), Spodoptera frugiperda, is one of the major agricultural pests that has invaded China. The FAW is a polyphagous insect with the gramineous crop sorghum being a key host plant. However, the basis of sorghum's chemical defense against FAW feeding is still unclear. In this study, we investigated the potential defensive mechanism of sorghum against this insect species. It was found that FAW larvae preferred maize over sorghum, the selection and damage rates for sorghum plants by larvae were significantly lower than those of maize plants, and feeding on sorghum restricted larval weight. The non-target metabolomics revealed that the feeding of FAW larvae altered the plant secondary metabolite spectra in maize and sorghum, resulting in species-specific differential secondary metabolites (DSMs). Of these, 19 DSMs were specific in maize, and 51 in sorghum, and only 6 were found in both species. Two-choice and no-choice feeding assays found that gambogenic acid and chimonanthine, two DSMs unique to sorghum, were found to deter larval feeding and decrease the larval weight. These findings reveal that the defense of sorghum against FAW is regulated by changing the response spectra of secondary metabolites and that the induced metabolites have a defensive function by acting as antifeedants, which provides new insights into employing bioactive plant compounds against polyphagous insects.

Keywords: Spodoptera frugiperda; antifeedant; defense mechanism; secondary metabolites; sorghum.

Figure 1

Preferences of Spodoptera frugiperda larvae towards maize and sorghum. (A,B) Feeding area (A) and feeding preference index (B) of larvae on maize and sorghum leaves. n = 24. (C,D) Schematic diagram (C) and representative image (D) of the feeding effects of larvae on maize and sorghum plants. (E) Damage rate on maize and sorghum plants after being infested by larvae. n = 4. (F) Number of larvae collected on maize and sorghum. n = 3. Data are presented as mean ± SEM. * p < 0.05, *** p < 0.001.

Figure 2

Effects of maize and sorghum on Spodoptera frugiperda larval weight. The newly hatched S. frugiperda larvae were fed on the leaves of maize and sorghum, respectively, and the larvae were weighed every 2 days. n = 5. Five larvae were used for each replicate. Data are presented as mean ± SEM. ** p < 0.01, *** p < 0.001. n.s. indicates no significant difference (p > 0.05).

Figure 3

Detection of differential secondary metabolites of maize and sorghum after feeding by Spodoptera frugiperda larvae. (A) Flow diagram of detection of differential secondary metabolites (DSMs) in maize and sorghum. (B) Venn diagrams of DSMs among different maize and sorghum samples. (C) Numbers of induced and reduced DSMs in the maize and sorghum groups. BZm, maize plant before feeding by larvae; AZm, maize plant after feeding by larvae; BSb, sorghum plant before feeding by larvae; ASb, sorghum plant after feeding by larvae.

Figure 4

Hierarchical cluster analysis heatmap for the differential secondary metabolites of maize and sorghum. The heatmap of changes in DSMs from maize (A) and sorghum (B) before and after being fed on by S. frugiperda larvae was made by using non-target metabolomics. BZm, maize plant before feeding by larvae; AZm, maize plant after feeding by larvae; BSb, sorghum plant before feeding by larvae; ASb, sorghum plant after feeding by larvae. The horizontal axis represents the samples. The abundance of DSMs in each sample is shown in different colors. Red indicates high abundance, and blue indicates low abundance.

Figure 5

Feeding responses of Spodoptera frugiperda larvae to three secondary compounds. The feeding preference of 5th-instar larvae of S. frugiperda to gambogenic acid, baohuoside II, and chimonanthine ranged from 0.4 to 40 μg/cm². P.I., preference index. Data are presented as mean ± SEM. * p < 0.05, ** p < 0.01. Different letters labeled indicate significant differences and n.s. indicates no significant difference (p > 0.05).

Figure 6

Effect of three secondary compounds on the larval growth rate of Spodoptera frugiperda. The growth rate of 2nd-instar S. frugiperda larvae to gambogenic acid, baohuoside II, and chimonanthine on day 3 and day 5. Gam, gambogenic acid; Bao, baohuoside II; Chi, chimonanthine. Ctrl¹, acetone; Ctrl², methyl alcohol. Data are presented as mean ± SEM. The nonparametric test (Mann–Whitney U test) was used. ** p < 0.01, *** p < 0.001; n.s. indicates no significant difference between control and treatment (p > 0.05). The colors white, grey, and black represent gambogenic acid, baohuoside II, and chimonanthine, respectively.