A plant growth retardant related to chlamydocin and its proposed mechanism of action

The plant growth retardant, 8, was discovered in the course of the investigation of the structure–activity relationship of 1, a fungal plant growth retardant. Compounds 1 and 8 affected the hormonal levels in rice seedlings, but their effects were quite different. A comparison of the plant growth retardant activity of the chlamydocin analogues, compound 1, six derivatives from 1 and 2, and two synthetic analogues revealed that there are two types of retardant in chlamydocin analogues. One, for example in compound 1, requires an oxygen atom at C-8 of the 2-aminodecanoic acid moiety to show retardant activity. The other, for example in compound 8, requires no oxygen atom at C-8 but requires a specific alkyl group chain length for activity. To determine the differences in mode of action of both types of retardant, rice seedlings were separately treated with compounds 1 and 8, and after appearance of dwarfism, their endogenous ABA and GA 1 levels were determined and compared to those of the control. Treatment with 1 (10 nmol/plant) increased ABA levels 4 times higher than that of the control and decreased GA 1 levels to 20% of that of the control. Treatment with 8 (30 nmol/plant) did not affect the ABA level but decreased GA 1 content to 5% of that of the control.