The ecdysteroid agonist/antagonist and brassinosteroid-like activities of synthetic brassinosteroid/ecdysteroid hybrid molecules

A series of synthetic hybrid brassinosteroid/ecdysteroid structures has bee n assessed for their ecdysteroid agonist/antagonist activities in the Droso phila melanogaster B-II cell bioassay and for brassinosteroid-like activity in the rice lamina inclination test. Most of the compounds proved inactive for ecdysteroid agonist activity, demonstrating the specificity of the ecd ysteroid receptor for compounds closely structurally related to 20-hydroxye cdysone. However, compound 18, with 14 alpha -hydroxy-7-en-6-one and 22S-hy droxy functionalities (as in most active ecdysteroids), possessed distinct agonist activity (median effective concentration = 1.4 x 10(-5) M), althoug h this is still almost 2000-fold less active than 20-hydroxyecdysone (25). Compounds 13 and 15 possessed weak agonist activity. Compounds 5, 11 and 14 weakly antagonised the action of 20-hydroxyecdysone (at 5 x 10(-8) M) on B -II cells. In the brassinosteroid bioassay, most of the tested compounds sh owed activity. This may reflect the metabolic capability of plant tissue to convert test compounds to more active analogues. However, it is clear that biological activity declines as the structure of the test compound deviate s further from that of castasterone (16). Three ecdysteroids (25, 26 and 27 ) are completely inactive in the rice lamina inclination test. These studie s demonstrate the high specificities of the insect ecdysteroid receptor and the plant brassinosteroid receptor and indicate that phytoecdysteroids, ev en in high concentrations, would not interfere with brassinosteroid signall ing pathways in plants where the two classes of compounds co-occur. Equally , brassinosteroids would not interfere with ecdysteroid signalling in insec ts, especially if one takes into account the low concentrations of brassino steroids in the diet of phytophagous insects.