An Assessment of Gibberellin Structure-activity Relationships

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Abstract
Information on the biological activities of gibberellins (GAs) in the barley aleurone, Tangin-bozu dwarf rice, dwarf pea, lettuce hypocotyl and cucumber hypocotyl bioassays is reviewed and discussed in the context of GA structure-activity relationships. The barley aleurone bioassay exhibits a limited response to GAs and it is suggested that this may be because the aleurone cells are able to carry out few GA interconversions. Consequently activity is determined by the degree of compatibility between the GAs and a receptor site. In this assay high biological activity is associated with GAs having a 3β-hydroxy-γ-lactone structure. This activity is substantially enhanced by the additional presence of a 13α-hydroxyl group. The substitution of a δ-lactone or a δ-lactol for a γ-lactone results in reduced activity while 3β,13α-dihydroxy GAs with either 20-carboxyl or 20-methyl functions are completely inactive. The Tanginbozu dwarf rice bioassay responds to many more GAs than the barley aleurone system possibly because the rice seedlings can carry out extensive GA interconversions. Under these circumstances GAs that are inactive per se can be metabolically converted to active forms. There is no interaction between the 3β- and 13α-hydroxy functions of GA molecules in the rice assay. Activity appears to be determined by the degree oxidation of the C-20 group. The order of activity is usually γ-lactone > δ-lactol > δ-lactone > methyl > carboxyl. It is suggested this may indicate that in rice seedlings C20-GAs are converted to C19-GAs via a Baeyer-Villiger type oxidation. Activity in the dwarf pea bioassay is dependent upon GAs possessing both 3β- and 13α-hydroxyl groups and is again related to the state of oxidation at the C-20 locus. In the lettuce bioassay activity is restricted to GAs with a γ-lactone function. In some instances a δ-lactone, but not a δ-lactol, can substitute effectively. This may imply that the applied C20-GAs are not converted to C19-GAs and that the response to the δ-lactone results from the six-membered ring mimicking the γ-lactone at the receptor site. Only GAs having a 19,10 or a 19,20 lactonic bridge show substantial activity in the cucumber bioassay. The additional presence of either a 12α- or a 13α-hydroxyl group severely reduces activity.

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