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RNA Interference of Glycerol Production Patterns a Rapid Cold Hardiness of the Beet Armyworm, Spodoptera exigua
The beet armyworm, Spodoptera exigua, is a freeze-susceptible species and overwinters without diapause in temperate zone. Depression of supercooling point (SCP) and rapid cold hardiness (RCH) allow S. exigua to survive at low temperatures. This study reports a polyol which is responsible for the cold hardiness of S. exigua. Pre-exposure of S. exigua larvae to 4°C for 6 h significantly enhanced survival under a freezing temperature (-10°C). This pre-exposure treatment also significantly depressed larval SCPs. Analysis of polyols indicated that glycerol titers significantly increase with increase of pre-exposure time. Glycerol kinase (GK) and glycerol-3-phosphate dehydrogenase (GPDH) are involved in glycolysis pathway of insect. The S. exigua GK (SeGK1) and G3PDH (SeG3PDH1) genes were predicted from 454 pyrosequencing transcripts from fifth instar larvae of the beet armyworm, S. exigua. The SeGK1 and SeG3PDH1 genes both were expressed in all larval stage by RT-PCR analysis. Expression of SeGK1 and SeG3PDH1 genes were suppressed by its specific dsRNASeGK1 or dsRNASeG3PDH1 injection into hemocoel of 5th instar larva. Each 200 ng of dsRNASeGK1 or dsRNASeG3PDH1 injection also significantly decreased glycerol amount in hemolymph. Larval treated by either dsRNASeGK1 or dsRNASeG3PDH1 significantly lost the RCH under -10°C exposure. These results indicate that glycerol is a crucial RCH agent and its synthesis is regulated by SeGK1 and SeG3PDH1 genes in S. exigua.
