Cold tolerance strategies of the fall armyworm, Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae)

The fall armyworm (FAW), Spodoptera frugiperda, is native to the tropical and subtropical areas of the American continent and is one of the world's most destructive insect pests and invaded Africa and spread to most of Asia in two years. Glycerol is generally used as a cryoprotectant for overwintering insects in cold areas. In many studies, the increase in glycerol as a main rapid cold hardening (RCH) factor and enhancing the supercooling point was revealed at low temperatures. There are two genes, including glycerol-3-phosphate dehydrogenase (GPDH) and glycerol kinase (GK), that were identified as being associated with the glycerol synthesis pathway. In this study, one GPDH and two GK sequences (GK1 and GK2) were extracted from FAW transcriptome analysis. RNA interference (RNAi) specific to GPDH or GK1 and GK2 exhibited a significant down-regulation at the mRNA level as well as a reduction in survival rate when the RNAi-treated of FAW larvae post a RCH treatment. Following a cold period, an increase in glycerol accumulation was detected utilizing high-pressure liquid chromatography and colorimetric analysis of glycerol quantity in RCH treated hemolymph of FAW larvae. This research suggests that GPDH and GK isozymes are linked to the production of a high quantity of glycerol as an RCH factor, and glycerol as main cryoprotectant plays an important role in survival throughout the cold period in this quarantine pest studied.

A specific glycerol kinase induces rapid cold hardening of the diamondback moth, Plutella xylostella

Insects in temperate zones survive low temperatures by migrating or tolerating the cold. The diamondback moth, Plutella xylostella, is a serious insect pest on cabbage and other cruciferous crops worldwide. We showed that P. xylostella became cold-tolerant by expressing rapid cold hardiness (RCH) in response to a brief exposure to moderately low temperature (4°C) for 7h along with glycerol accumulation in hemolymph. Glycerol played a crucial role in the cold-hardening process because exogenously supplying glycerol significantly increased the cold tolerance of P. xylostella larvae without cold acclimation. To determine the genetic factor(s) responsible for RCH and the increase of glycerol, four glycerol kinases (GKs), and glycerol-3-phosphate dehydrogenase (PxGPDH) were predicted from the whole P. xylostella genome and analyzed for their function associated with glycerol biosynthesis. All predicted genes were expressed, but differed in their expression during different developmental stages and in different tissues. Expression of the predicted genes was individually suppressed by RNA interference (RNAi) using double-stranded RNAs specific to target genes. RNAi of PxGPDH expression significantly suppressed RCH and glycerol accumulation. Only PxGK1 among the four GKs was responsible for RCH and glycerol accumulation. Furthermore, PxGK1 expression was significantly enhanced during RCH. These results indicate that a specific GK, the terminal enzyme to produce glycerol, is specifically inducible during RCH to accumulate the main cryoprotectant.

RNA interference of a heat shock protein, Hsp70, loses its protection role in indirect chilling injury to the beet armyworm, Spodoptera exigua

The beet armyworm, Spodoptera exigua, is freeze-susceptible, in which glycerol plays a crucial role in depressing supercooling point (SCP) to avoid the freezing injury. This study focused on a non-freezing injury classified into indirect chilling injury of S. exigua after a prolonged exposure to low temperatures much above SCPs. Exposure to 0 and 5°C for longer than 2weeks was lethal to all the immature stages. Among immature stages, eggs were the most susceptible to the low temperature treatments and pupae were the next susceptible. Among larvae, the third instar (L3) appeared to be more tolerant than the fifth instar (L5). The temperature treatment at 15°C allowed both L3 and L5 to exhibit a feeding behavior and induced little non-freezing injury, suggesting a minimal temperature threshold for optimal overwintering conditions of S. exigua. Three heat shock protein genes (Hsp70, Hsp74, Hsp83) were expressed in the larvae at the low temperature treatments. Only Hsp70 was inducible to the low temperatures in both L3 and L5 stages. RNA interference of Hsp70 expression led to significantly lose the survival rates of the treated larvae in the conditions inducing the non-freezing injury. These results suggest that Hsp70 plays a role in protecting S. exigua from the indirect chilling injury.