Caterpillar-Specific Gene Expression in the Legume, Medicago truncatula

Plants adjust their defense responses to target caterpillar herbivores. Their ability to do this is, in part, based on the recognition of salivary elicitors which are secreted during insect feeding. We are interested in identifying caterpillar-specific defense responses in themodel legume, Medicago truncatula Gaertn. (Fabaceae). To do this, we have taken advantage of the fact that labial salivary secretions of caterpillars of the beet armyworm, Spodoptera exigua Hubner (Lepidoptera: Noctuidae), can be impaired by cauterizing the spinneret. Plant tissues were removed within 1 h after the initiation of herbivory, and differentially expressed transcripts were identified by cDNA-amplified fragment length polymorphism (cDNA-AFLP). Gene expression patterns of five differentially expressed transcripts and one constitutive gene were verified by quantitative real time-polymerase chain reaction (qRT-PCR). Genes encoding ribulose-1,5-bisphosphate carboxylase/oxygenase activase (rubisco activase; MtRCA), strictosidine synthase (MtSTR), an unknown protein (MtUNK), a zinc RING finger protein (MtRFP), and a receptor-like protein kinase (MtRPK) were constitutively expressed. In plants subject to mechanical damage or herbivory by caterpillars, lower expression of MtRCA, MtSTR, Mt UNK, and MtRFP was observed. In comparison, higher levels of MtRPK were expressed in a caterpillar-specific manner.