The Bryonia dioica tendril-coiling assay provides a rapid, sensitive and selective bioassay for jasmonates. Using this assay, a large number of jasmonate and coronatine analogs were analyzed for their biological activities. In a systematic study, C-3 analogs, C-2 analogs, C-1 homologs and -analogs, C-1(1¢) analogs of jasmonic acid, as well as analogs of coronatine altered in both the amino acid and the coronafacic acid moiety, were compared. The results demonstrated at least two structurally non-overlapping centers of biological activity, one centered around the structure of jasmonic acid allowing only minor C-1(1¢) modi®cations and a second center around the structure of 12-oxophytodienoic acid and having dierent structural requirements for activity, thus allowing quite dierent structural modi®cations. The C18-group of the jasmonates [12-oxophytodienoic acid and 3-oxo-2(2¢ (Z)-pentenyl)-cyclopentane-1-octanoic acid], for which coronatine is a structural mimic, was the much more potent inducer of tendril coiling, when applied externally. The levels of jasmonic acid and 3-oxo-2(2¢(Z)-pentenyl)-cyclopentane-1-octanoic acid in mechanically stimulated tendrils remained very low and did not change detectably, while the level of 12-oxophytodienoic acid had earlier been shown to change drastically and transiently during the onset and progression of the coiling reaction. Thus, 12-oxophytodienoic acid, rather than jasmonic acid or 3-oxo-2(2¢ (Z)-pentenyl)-cyclopentane-1-octanoic acid, has to be considered as an endogenous signal transducer in B. dioica mechanotransduction.

A facile test system based on the accumulation of benzo[c]phenanthridine alkaloids inthere is a large degree of plasticity allowed at the C-3 of jasmonic acid in the activation of defense genes. The carbonyl moiety is not strictly required, but exocyclic double bond character appears necessary. The pentenyl side chain at C-2 cannot tolerate bulky groups at the terminal carbon and still be biologically active. Substitutions to the C-1Ј position are tolerated if they can potentially undergo ␤-oxidation. Either an alkanoic acid or methyl ester is required at C-1, or a side chain that can be shortened by ␤-oxidation or by peptidase hydrolysis. Coronatine and various derivatives thereof are not as effective as jasmonic acid, and derivatives in inducing benzo[c]phenanthridine alkaloid accumulation. Jasmonic acid rather than the octadecanoic precursors is therefore considered to be a likely signal transducer of defense gene activation in planta.

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