Expression profiles of antioxidant genes in the emerald ash borer (Agrilus planipennis)

Phytophagous insects frequently encounter reactive oxygen species (ROS) from exogenous and endogenous sources. To overcome the effect of ROS, insects have evolved a suite of antioxidant defense genes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX). The emerald ash borer (Agrilus planipennis Fairmaire), an exotic invasive insect pest from Asia has killed millions of ash trees (Fraxinus spp.) and continues to invade North America at a rapid pace. From an on-going larval midgut expressed sequence tag (EST) project of A. planipennis, we identified ESTs coding for a Cu-Zn SOD (ApSOD1), a CAT (ApCAT1), and a GPX (ApGPX1). Real time quantitative PCR (RT-qPCR) analysis in third instar larval tissues (midgut, fatbody, Malpighian tubule and integument) feeding on green ash (F. pennsylvanica) revealed peak mRNA levels of ApCAT1 in the midgut whereas high mRNA levels for both ApSOD1 and ApGPX1 were observed in all the tissues studied. Assay of mRNA levels in developmental stages (larva, prepupa and adults) by RT-qPCR indicated high transcript levels of ApCAT1 and ApGPX1 in larval and prepupal stages with a decline in adults; while ApSOD1 was observed to be constitutive in all the stages. Further, a tissue-specific comparative analysis between larvae feeding on susceptible (F. pennsylvanica) and resistant ash (F. mandshurica) revealed higher transcript levels for ApSOD1 and ApGPX1 in the integument of larvae feeding on F. mandshurica. These results provide unique insights into the adaptive (tissue-specific) responses of A. planipennis during its interaction with ash.