Sugar transporters enable a leaf beetle to accumulate plant defense compounds
Many herbivorous insects selectively accumulate plant toxins for defense against predators; however, little is known about the transport processes that enable insects to absorb and store defense compounds in the body. Here, we investigate how a specialist herbivore, the horseradish flea beetle, accumulates high amounts of glucosinolate defense compounds in the hemolymph. Using phylogenetic analyses of coleopteran membrane transporters of the major facilitator superfamily, we identified a clade of glucosinolate-specific transporters (PaGTRs) belonging to the sugar porter family. PaGTR expression was predominantly detected in the excretory system, the Malpighian tubules. Silencing of PaGTRs led to elevated glucosinolate excretion, significantly reducing the levels of sequestered glucosinolates in beetles. This suggests that PaGTRs reabsorb glucosinolates from the Malpighian tubule lumen to prevent their loss by excretion. Ramsay assays performed with dissected Malpighian tubules confirmed a selective retention of glucosinolates. Thus, the selective accumulation of plant defense compounds in herbivorous insects can depend on the ability to prevent excretion.