Antiecdysteroid activity of brassinosteroids

Abstract

Summary. We report the discovery of the first antiecdysteroids which belong to a comparatively new class of plant growth regulators, the brassinosteroids. These compounds bind competitively to ecdysteroid receptors partially purified from larvae of the blowfly Calliphora vicina and inhibit biological responses to 20-hydroxyecdysone, the active form of the molting hormone.Key words. Ecdysteroid; brassinosteroid; hormone antagonist.Steroids with signal functions are found in all three kingdoms of eucaryotic organisms -fungi, plants and animals 2. The discovery that plants possess growth regulators with a steroidal structure has only been made recently 3 and since then a series of brassinosteroids have been isolated. They possess striking structural similarities to the ecdysteroids 4, 5 ( fig. 1). These similarities suggest that brassinosteroids and ecdysteroids share a common progenitor hormone which might be the ancestor of all steroid hormones existing today 6. To see whether the analogy between brassinosteroids and ecdysteroids even extends to the recognition by ecdysteroid receptors we investigated the binding of two brassinosteroids and their synthetic precursors to ecdysteroid receptors from the blowfly, Calliphora vicina. Results and discussion. Isolation, purification, and characterization of nuclear ecdysteroid receptors has been described in detail recently 7. Binding of the brassinosteroids was studied by means of a competitive binding assay using the radiolabeled ecdysteroid ponasterone A as a high affinity receptor ligand. The synthetic brassinosteroids 22S,23S-homobrassinolide and 22S,23S-homocastasterone 5 ( fig. 1) as well as the tetra-acetates of these compounds, which served as synthetic precursors, clearly competed with (3H)ponasterone A for the binding to ecdysteroid receptors. Figure 2 shows competition curves for homocastasterone, homobrassinolide and the ecdysteroids 20-hydroxyecdysone and ecdysone. An equilibrium dissociation constant (KD) of 5 x 10-6 M for the homocastasterone-receptor complex could be estimated from the competition curves. This K o is of the same order of magnitude as the Ko for the molting hormone secreted by the endocrine gland, ecdysone, which is 2 x 10 -6 M. Brassinosteroids bound to ecdysteroid receptors could be displaced again by an elevation of the (3H)ponasterone A concentration, demonstrating the competitive nature of the binding. It has been independently shown by two research groups that 22S,23S-homobrassinolide inhibits the evagination ofimagihal discs of the dipteran, Phormia terra-novae, in vitro 8 and that it causes a delay in the imaginal molt when fed to the last larval instar of the cockroach, Periplaneta americana 9. Our results show that these effects can be explained by the competitive displacement of 20-hydroxyecdysone from its receptor binding site. 22S,23S-homocastasterone, however, which was the most effective competitor in the binding assay, did not disturb the imaginal molt when fed to cockroaches 9. We suppose that the lack of effect in the in ...

Steroid plant hormones: Effects outside plant kingdom

“…The first experiments showed that steroidal phytohormones could affect normal growth and development of insects. A number of BS effects were revealed at different levels [22], including intact animals [15,23,24], isolated tissues [23,25,26], cultured cells [27,28], particular insect neurons [29], and protein molecules (ecdysteroid receptors) [23,28,30,31]. However, the results of these experiments are not always consistent with each other.…”

Assessment of natural products in the Drosophila melanogaster BII cell bioassay for ecdysteroid agonist and antagonist activities

“…Two other related compounds, speciophylline (13) and uncarine F (15) showed weak activity, but mitraphylline (8) was inactive. Veratridine (16), one of the insecticidal steroidal alkaloid components of sabadilla [prepared from the seeds of Schoenocaulon officinale (Liliaceae)], did not possess ecdysteroid (ant)agonistic activity. Two other steroidal alkaloids, chaconine (5) and solanine (12), were also inactive, as were the nonsteroidal alkaloids atropine (2), nicotine (9), strychnine (14), caulerpin (17) and yohimine (18).…”

“…Brassinosteroids. Brassinosteroids have been proposed to act as either ecdysteroid agonists [14] or antagonists [15,16], based on evidence from in vivo and in vitro experiments with insect systems. The apparent similarity of certain structural features between the ecdysteroids and brassinosteroids has been put forward as the basis of these activities.…”

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

“…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 deviates further from that of castasterone (16). Three ecdysteroids (25, 26 and 27) are completely inactive in the rice lamina inclination test.…”