Regulation of cryoprotectant metabolism in the overwintering gall fly larva,Eurosta solidaginis: Temperature control of glycerol and sorbitol levels

Summary

Abrupt temperature change, from 23 to 13 °C, 13 to 3 °C or vice versa, was used to study the metabolic events associated with cryoprotectant polyol synthesis and the reversibility of polyol accumulations in the overwintering, freezing tolerant larvae of the gall fly,Eurosta solidaginis.

Sorbitol synthesis was induced when larvae acclimated to 13 °C were abruptly moved to 3 °C. A precursor-product relationship between glucose-6-P, glucose and sorbitol was apparent with elevated levels of the compounds in the larvae first detected after 1, 2 and 24 h at 3 °C, respectively. A negative cross-over (increase in fructose-6-P, decrease in fructose-1,6-P₂ levels) at phosphofructokinase at 3 °C demonstrated that inhibition at this locus was responsible for the diversion of carbon flow into sorbitol synthesis.

Glycerol synthesis was stimulated when larvae acclimated to 23 °C were chilled to 13 °C, with increased glycerol levels first apparent after 2 days at 13 °C. Synthesis was accomplished via an activation of glycogenolysis coupled with a facilitation of flux through the phosphofructokinase locus and an inhibition (negative cross-over) of flux at the pyruvate kinase reaction resulting in a diversion of triose phosphates into the pathway of glycerol synthesis.

Warming of the larvae resulted in a rapid catabolism of sorbitol, with a restoration of glycogen reserves, when larvae were switched from 3 to 13 °C. Glycerol content of the larvae, however, did not respond to warming and remained constant when larvae were moved from 13 to 23 °C.

The two cyoprotectants appear to have different roles in the overwintering larvae. Glycerol, once synthesized, provides a constant and permanent cryoprotection throughout the winter. Accumulation of this polyol also appears to be anticipatory occurring in response to chilling at relatively high temperatures, well above those at which cryoprotection is needed. Sorbitol, however, is produced only in direct response to cold when freezing temperatures are imminent. Sorbitol provides a variable cryoprotection, levels of the polyol responding to increases or decreases in ambient temperature.