Action of brassinosteroids in the cotton leafworm Spodoptera littoralis

The two native plant hormones 24-epibrassinolide and 24-epicastasterone showed 50% competition for binding at IC50 of 1-3.6 muM with [H-3]ponasterone A using cultured imaginal wing discs from last-instar larvae of the cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae). However, culture of imaginal wing discs in different concentrations of brassinosteroids, even up to 100 muM, demonstrated no induction of evagination. In contrast, 20E and the non-steroidal agonist RH-5992 competed respectively about 23- and 42-fold more effectively with labeled ponasterone A, and their ability (EC50) to induce disc evagination in vitro was 158 and 87 nM, respectively. Injection of 10 mug of brassinosteroids in newly-moulted last-instar larvae did not cause mortality above controls; higher mortalities were scored when brassinosteroids were injected late in the last instar. (C) 2002 Elsevier Science Ltd. All rights reserved.