RNA interference of glycerol biosynthesis suppresses rapid cold hardening of the beet armyworm, Spodoptera exigua

Abstract

Summary The beet armyworm, Spodoptera exigua, is a freeze-susceptible species that overwinters in temperate zones without diapause. The rapid cold hardening (RCH) and supercooling capacity usually play crucial roles in the survival during overwintering period. This study determined a cryoprotectant as an RCH factor of S. exigua. A pre-exposure of S. exigua larvae to 4°C significantly increased survival at -10°C in all developmental stages from egg to adult. The RCH was dependent on the duration of the pre-exposure period. The RCH also significantly enhanced the supercooling capacity. Cryoprotectant analysis using HPLC showed that the pre-exposure treatment allowed the larvae to accumulate glycerol in the hemolymph. Two genes, glycerol-3-phosphate dehydrogenase (GPDH) and glycerol kinase (GK), were identified as being associated with glycerol biosynthesis, and were cloned in S. exigua larvae. Both GPDH and GK were expressed in all developmental stages of S. exigua. Either RNA interference (RNAi) of GPDH or GK significantly inhibited glycerol accumulation in the hemolymph of S. exigua. Larvae treated with RNAi of GPDH or GK gene expression exhibited a significant decrease in RCH capacity. The glycerol accumulation in response to 4°C appeared to be under the control of humoral signal, because a ligation experiment prevented glycerol accumulation in the other half of the body. This study indicated that glycerol is a RCH factor of S. exigua and is induced to be synthesized in response to low temperature via humoral mediation.