Ethionine Inhibition and Morphogenesis of Excised Tomato Roots

Replacement of the terminal methyl group of methionine by an ethyl group gives the substance ethionine. Ethionine has been shown to be an antimetabolite of methionine in a number of biological systems in that inhibition by ethionine is reversed by methionine (16, 35). The precise mode of action is not certain; it probably varies from one organism to another. Present evidence from work on animals (11, 14, 16, 26, 27, 35) indicates that ethionine exerts inhibitory effects upon 1 ) transmethylation, 2) conversion of methionine to cystine, 3) the lipotropic activity of methionine, 4) the incorporation of amino acids into proteins, and 5) the formation of certain adaptive enzymes. A recent report (20) attributes the inhibitory effect on transmethylation to the formation of S-adenosylethionine, which serves as an ethyl donor, with the consequent formation of various ethyl analogs of metabolites which are normally formed as a consequence of transmethylation from S-adenosylmethionine. This paper records effects, inhibitory and otherwise, of ethionine on the growth of excised tomato roots in sterile culture. The effects of some antagonistic substances are also described. The purpose of the experiments was to obtain information regarding the possible metabolic relationships and physiological importance of methionine in the excised tomato root. The main interest is in the morphogenetic consequences of controlling methionine metabolism in the root. A general discussion of growth and growth correlations, upon which the morphogenesis of the excised tomato root depends. is presented elsewhere (7). The present work, which has been presented in a preliminary report (5), is, to mv knowledge. the only record of the effects of ethionine on growth of plants, other than microorganisms. An exception is a paper by Schrank (22) who showed that the inhibition of growth of oat coleoptile cells by ethionine was reversed by methionine.