Cassava (Manihot esculenta Crantz) Transcriptional Responses As a Result of Mealybug (Phenacoccus manihoti) Feeding

With regard to the organisms in the plants’ system, their response to herbivore feeding appears to be specific. Understanding plant responses to insect herbivory, by determining and identifying differentially expressed genes (DEGs), is a vital step towards the understanding of molecular mechanisms of defence and the development of resistant cultivars. Here we report the interaction between cassava (Manihot esculenta Crantz) and sap-sucking pest cassava mealybugs (Phenacoccus manihoti). Plants were exposed to mealybugs for a period of 24 and 72 h respectively, and gene expression levels were analyzed using illumina HiSeq 2500. Using a log2 fold change and P-value ≤ 0.05, 301 and 203 genes were differentially expressed compared to uninfested controls between the time-points for AR23.1 and P40/1 genotypes respectively. Most of these genes expressed have been reported in other studies in response to herbivore infestation. Biological pathway analysis using KEGG revealed significant enrichment of plant-pathogen interactions and plant hormonal signal transduction for both upregulated and downregulated DEGs. Putative defence-related genes such as 2-oxogluterate (2OG) and terpene synthase 21 were induced in the AR23.1 genotype, and were further validated by real-time RT-qPCR. The study revealed a significantly different response to mealybug infestation in the two genotypes studied, with the resistant genotype (AR23.1) showing a higher proportion of differentially expressed transcripts post infestation. Putative candidate defence-related genes that were overexpressed in the AR23.1 genotype post infestation will be useful in future functional studies towards the control of mealybugs.

Keywords: Differential gene expression, RNA-seq, signal transduction, cassava, resistance, plant-insect interaction