GAMMA-AMINOBUTYRIC-ACID STIMULATES ETHYLENE BIOSYNTHESIS IN SUNFLOWER

gamma-Aminobutyric acid (GABA), a nonprotein amino acid, is often accu mulated in plants following environmental stimuli that can also cause ethylene production. We have investigated the relationship between GAB A and ethylene production in excised sunflower (Helianthos annuus L.) tissues. Exogenous GABA causes up to a 14-fold increase in the ethylen e production rate after about 12 h. Cotyledons fed with [C-14]GABA did not release substantial amounts of radioactive ethylene despite its c hemical similarity to 1-aminocyclopropane-1-carboxylic acid (ACC), ind icating that GABA is not likely to be an alternative precursor for eth ylene. GABA causes increases in ACC synthase mRNA accumulation, ACC le vels, ACC oxidase mRNA levels, and in vitro ACC oxidase activity. In t he presence of aminoethoxyvinylglycine or alpha-aminoisobutyric acid, GABA did not stimulate ethylene production. We therefore conclude that GABA stimulates ethylene biosynthesis mainly by promoting ACC synthas e transcript abundance. Possible roles of GABA as a signal transducer are suggested.