Effects of sugarcane aphid herbivory on transcriptional responses of resistant and susceptible sorghum

Abstract

Background: Sugarcane aphid (Melanaphis sacchari) outbreaks in sorghum that were first reported in 2013 are now the most significant threat to this crop in all major sorghum production areas in the U.S. The outcomes of interactions between sugarcane aphid and sorghum and thus the severity of the outbreaks depend on sorghum genotype and potentially also on the phenology of sorghum. Mechanisms underlying these interactions are not known, however. Thus, the goal of this research was to characterize transcriptional changes in a commercially available resistant and a susceptible genotype of sorghum at 2- and 6-wk post-emergence exposed to M. sacchari herbivory. The effects of sorghum age and genotype on the daily change in aphid densities were also evaluated in separate greenhouse experiments.

Results: A higher number of diffentially expressed genes (DEGs) was recovered from the 2-wk plants exposed to aphid herbivory compared to the 6-wk plants across genotypes. Further, gene ontology and pathway analysis indicated a suite of transcriptional changes in the resistant genotype that were weak or absent in the susceptible sorghum. Specifically, the aphid-resistant genotype exposed to M. sacchari up-regulated several genes involved in defense, which was particularly evident in the 2-wk plants that showed the most robust transcriptional responses. These transcriptional changes in the younger resistant sorghum were characterized by induction of hormone-signaling pathways, pathways coding for secondary metabolites, glutathion metabolism, and plant-pathogen interaction. Furthermore, the 2-wk resistant plants appeared to compensate for the effects of oxidative stress induced by sugarcane aphid herbivory with elevated expression of genes involved in detoxification. These transcriptional responses were reflected in the aphid population growth, which was significantly faster in the susceptible and older sorghum than in the resistant and younger plants.

Conclusion: This experiment provided the first insights into molecular mechanisms underlying lower population growth of M. sacchari on the resistant sorghum genotype. Further, it appears that the younger resistant sorghum was able to mount a robust defense response following aphid herbivory, which was much weaker in the older sorghum. Several pathways and specific genes provide specific clues into the mechanisms underlying host plant resistance to this invasive insect.

Keywords: Melanaphis sacchari; Plant defenses; Plant-insect interaction; RNA-seq; Sorghum bicolor.

Fig. 1

Number of DEGs in sorghum genotypes in response to sugarcane aphid herbivory at 2- and 6-wk. Columns represent number of genes up-regulated or down-regulated following exposure to aphid herbivory compared to the equivalent genotype and age treatment free of the aphids. DEGs were defined as having a fold change ≥1.9 or ≤ − 1.9 with a false discovery rate (FDR) adjusted p-value < 0.05

Fig. 2

Venn diagram of DEGs in susceptible and resistant sorghum genotypes exposed to sugarcane aphid herbivory. DEGs were compared in 2-wk (a) and 6-wk (b) sorghum plants in response to aphid herbivory (A⁺) and in aphid-free (A⁻) plants. All overlapped and unique DEGs from different comparisons are shown

Fig. 3

Enriched Gene Ontology (GO) terms in the two sorghum genotypes in response to aphid herbivory. GO terms were compared among the 2-wk old resistant sorghum a, 2-wk old susceptible sorghum b, 6-wk old resistant sorghum c, and the 6-wk old susceptible sorghum (d). GO terms (FDR corrected p-value < 0.05) with significant numbers of up-regulated or down-regulated genes were identified by contrasting gene expression in response to aphid herbivory and control (aphid-free plants). DEGs were then grouped into and biological process, molecular function, and cellular component categories. Color of the bars refers to the –log² corrected p-value of the respective GO term. Red colored bars indicate enriched GO terms in up-regulated genes, and blue colored bars indicate enriched pathways in down-regulated genes. The darker the red color the higher statistical significance

Fig. 4

Pathways enriched in DEGs from two sorghum genotypes in response to sugarcane aphid herbivory. Pathways were compared among the 2-wk old resistant sorghum a, 2-wk old susceptible sorghum b, 6-wk old resistant sorghum c, and the 6-wk old susceptible sorghum (d) Pathways (FDR corrected p-value < 0.05) with significant numbers of up-regulated or down-regulated genes were identified by comparing gene expression in response to aphid herbivory and control (aphid-free plants). Color of the bars refers to the –log² corrected p-value of the respective pathway. Red colored bars indicate enriched pathways in up-regulated genes, and blue colored bars indicate enriched pathways in down-regulated genes. The darker the red color the higher statistical significance. Length of the bar represents the number of genes within the respective GO term

Fig. 5

Heat map of DEGs involved in hormone transduction pathways in response to aphid herbivory. Heat map shows DEGs across two resistant and susceptible sorghum genotypes at 2- and 6-wk post-emergence. The color key represents log2-transformed fold changes and red indicate increased expression in response to aphid, while blue represents a decrease in expression

Fig. 6

Heat map of DEGs involved in phenylpropanoid biosynthesis pathway. Heat map presents DEGs across two resistant and susceptible sorghum genotypes at 2- and 6-wk post emergence. The color key represents log2-transformed fold changes and red indicate increased expression in response to aphid, while blue represents a decrease in expression

Fig. 7

Heat map of differentially expressed transcription factors. Heat map shows DEGs across two resistant and susceptible sorghum genotypes at 2- and 6-wk post-emergence. The color key represents log2-transformed fold changes and red indicate increased expression in response to aphid, while blue represents a decrease in expression

Fig. 8

Impact of sorghum genotype and age on average daily change of aphid density. Daily change in aphid density was measured on susceptible and resistant sorghum 2- and 6-wk post emergence. Bars mark means ±1 SEM and means with different letters differed significantly (P ≤ 0.05)