Induction of leaf curling in cassava plants by the cassava mealybug Phenacoccus manihoti (Hemiptera: Pseudococcidae)

Certain phytophagous insects can induce leaf curling in their host plants that may provide protected and nutrient-rich habitats. However, the mechanisms of this induction remain poorly understood. The cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Hemiptera: Pseudococcidae), is a serious pest of the cassava and causes leaf curling. To reveal the mechanisms of leaf-curl induction, we first inoculated varying numbers of mealybugs in different locations, namely, the apical meristem and the stem, on cassava seedlings. Second, we performed transcriptome analysis using the total RNA extracted from leaves. The results showed that a single insect was able to induce leaf curling, but the intensity and frequency of the leaf curling were positively correlated with the number of insects. Furthermore, the leaf curling occurred when the mealybugs fed on or close to the apical meristem but not when they fed on the stem. Transcriptome analysis identified a total of 3,931 differentially expressed genes (DEGs) from intact plants and the plants inoculated with mealybugs at different time points. GO analysis of the biological processes revealed that the DEGs contained a series of factors for leaf development of the adaxial–abaxial axis, and auxin biosynthesis and polarity. This suggests that alterations in these functions may cause leaf curling. Overall design: To investigate the effects of P. manihoti on deformation, varying numbers of second and third instar nymphs of P. manihoti were inoculated into the cassava plants. We inoculated 1, 5, or 10 nymphs into the top of the plants and placed the plants individually in a plastic container (80 mm × 80 mm × 205 mm) with a fine mesh plastic screen on the top and bottom. Eight plants were used for each treatment. The occurrence of deformation was checked every 2–3 days. The first and second leaves from the top stems of the control and inoculated plants were collected at 1, 3, 5, and 10 d after inoculation for RNA-seq analysis.