Xanthoxin, a recently discovered plant growth inhibitor

Abstract

The naturally occurring plant growth inhibitor xanthoxin which was discovered in these laboratories has been prepared in vitro by the oxidation of the pigment violaxanthin with neutral zinc permanganate solution. By the use of chemical and spectroscopic techniques, xanthoxin has been characterized as a mixture of the 2- cis , 4- trans - and 2- trans , 4- trans -isomers of 5-(1', 2', -epoxy-4'-hydroxy-2', 6', 6'-tri-methyl-1'-cyclohexy)-3-methyl-pentadienal. These are also obtained by similar oxidations of neoxanthin, antheraxanthin and lutein epoxide. Cis, trans -xanthoxin probably arises from the corresponding cis -xanthophyll and its conversion to ( + )-ABA by simple chemical procedures is reported. The biological activity of cis, trans -xanthoxin has been shown to be considerably greater than that of the trans, trans -isomer. It is comparable with that of ( ± )-ABA in the wheat coleoptile section, the lettuce hypocotyl and bean petiole abscission tests, but greater than that of ( ± )-ABA in the cress seed germination test. Xanthoxin is shown to antagonize the growth-promoting effects of the plant hormones 3-indolylacetic acid, gibberellic acid and kinetin. Other xanthoxin derivatives have been prepared and their activities in several tests are also reported. The uptake of xanthoxin by tomato shoots leads to a subsequent large increase in the ABA content of the tissue. Evidence is presented for a possible biogenetic conversion of cis, trans -xanthoxin into ( + )-ABA by plants.