Overwintering physiology of the rice stem borer larvae, Chilo suppressalis Walker (Lepidoptera: Pyralidae): Roles of glycerol, amino acids, low-molecular weight carbohydrates and antioxidant enzymes
The rice stem borer, Chilo suppressalis (Walker), is a major rice pest around the world. A strong ability of the rice stem borer to adapt/resist cold temperature (cold hardiness) contributes to its survival through winter. However, the physiological mechanism of its cold hardiness is poorly understood. In this study, we determined the supercooling points (SCPs), the contents of amino acids and low-molecular weight carbohydrates of overwintering rice stem borer larvae. We also assessed the levels of their superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) antioxidative enzymes in the overwintering larvae. Our results revealed an intimate relationship of larval SCP with environmental temperature; SCP change reflected proportionally to change of environmental temperature. Analysis of low molecular weight carbohydrates established that the concentrations of glycerol and trehalose change in a manner that is inversely proportional to that of the environmental temperatures. Changes or significant changes of some amino acids and antioxidative enzymes were also observed in overwintering larvae. Our data suggest that decrease of the SCP in overwintering larvae is due primarily to the increase of cryoprotective glycerol and trehalose and also to the increase of several amino acids to an extent. The decreased SCP in-turn enabled rice stem borer larvae to withstand low temperature. Our study therefore provides an overall picture regarding seasonal changes of the cryoprotective substances in relation to the ability of the rice stem borer to survive cold environmental conditions.
Keywords: Chilo suppressalis (Walker), cold hardiness, supercooling points (SCPs), amino acids, low-molecular weight carbohydrates, antioxidant enzymes