Salivary proteins in spider mites: dual roles in feeding and silk fiber coating

Spider mites are plant cell-sucking herbivores that spin nanoscale adhesive silk fibers released from their mouthpart appendages. Previous whole-genome sequencing predicted 17 silk genes in the two-spotted spider mite,Tetranychus urticae, while silk proteomics detected two candidates, Fibroin-1 and Fibroin-2. However, saliva proteomics also detected Fibroin-1, Fibroin-2, and sFibroin-1 (with high sequence similarity to Fibroin-1). We investigated whether these three proteins function in silk, saliva, or both. We foundFibroin-1,sFibroin-1, andFibroin-2genes expressed in the salivary glands but found no direct internal connection between salivary and silk glands. Our silk proteomics detected 8 silk proteins previously predicted and 66 salivary proteins, including Fibroin-1, sFibroin-1, and Fibroin-2. We detected a bead-on-a-string pattern formed by fluid (hypothesized to be saliva) on the silk fiber and a fluid patch that remains at the piercing site on the leaf surface after mite feeding. RNAi-mediated silencing ofFibroin-1andsFibroin-1genes reduced mite feeding duration, survival, and fecundity, and silencing ofFibroin-2gene showed a tendency to reduce the thickness of individual silk fibers. These results suggest that Fibroin-1, sFibroin-1, and Fibroin-2 are secreted from the salivary glands and then adhere to silk fibers. Fibroin-1 and sFibroin-1 may assist mites to adhere the tip of the mouthparts to the leaf surface when sucking with their stylets, while Fibroin-2 may coat silk fibers. This study reveals the dual role of saliva in plant cell-sucking herbivorous mites, namely, stabilizing the mouthparts during feeding and coating silk fibers to provide them with adhesive properties.