Biological activity of gibberellin analogues.

In order to determine the significance of the C-6 carboxyl group for the biological activity gibberellin A3, 6-epigibberellin A3, 7-norgibberellin A3, 6β-methyl-7-norgibberellin A3, and 7-homogibberellin A3 were studied using dwarf pea, dwarf maize, dwarf rice, dwarf barley and α-amylase bioassays. All gibberellin A3(GA3)derivatives tested were considerably less active than GA3. In all biossays, 6-epi-GA3 showed a low activity of the same order, whereas 6β-methyl-7-nor-GA3 was inactive. Surprisingly, 7-nor-GA3 had some activity in the dwarf rice (root application), dwarf barley, and α-amylase bioassay, in contrary to its low potency in the dwarf pea, dwarf maize, and dwarf rice (micro drop) bioassay. 7-Homo-GA3 was primarily active in the dwarf maize, dwarf barley and dwarf rice bioassay. It also caused antigibberellin effects in dwarf rice. The results demonstrate that the C-6 carboxyl group is not absolutely essential for biological activity of gibberellins. The different activities of 7-nor-GA3 observed in the various test systems may indicate that the C-6 carboxyl group is a structural requirement more for uptake and/or transport processes than for receptor affinity.