A THERMOPROTECTIVE ROLE FOR SORBITOL IN THE SILVERLEAF WHITEFLY, BEMISIA-ARGENTIFOLII

Accumulation of polyols in insects is well known as a cold-hardening r esponse related to overwintering or to protection against cold shock. The silverleaf whitefly (Bemisia argentifolii, Bellows and Perring) is a major insect pest in tropical and subtropical regions where heat st ress and desiccation pose formidable threats to survival. We found tha t sorbitol levels increased ten-fold when whiteflies were exposed to e levated temperatures. Sorbitol levels rose from 0.16 nmol whitefly(-1) at 25 degrees C to 1.59 nmol whitefly(-1) at 42 degrees C. Sorbitol l evels fluctuated diurnally under glasshouse and field conditions incre asing ten-fold from morning to early afternoon. Feeding experiments on artificial diets showed that both temperature and dietary sucrose con centration were key factors influencing sorbitol accumulation. Cell fr ee extracts prepared from adult whiteflies catalyzed NADPH-dependent f ructose reduction, but were unable to reduce glucose with either NADPH or NADH. Radiotracer experiments with labeled glucose and fructose sh owed that fructose was the immediate precursor of sorbitol. Thus, sorb itol synthesis in the whitefly is apparently unconventional, involving conversion of fructose by a novel NPLDPH-dependent ketose reductase. We propose that sorbitol accumulation is a mechanism for thermoprotect ion and osmoregulation in the silverleaf whitefly, allowing the insect to thrive in environments conducive to thermal and osmotic stress. (C ) 1998 Elsevier Science Ltd. All rights reserved.