Mechanical stimulation, 4-aminobutyric acid (GABA) synthesis, and growth inhibition in soybean hypocotyl tissue

Abstract

4-Aminobutyric acid (GABA), a ubiquitous four-carbon nonprotein, amino acid, is synthesized via glutamate decarboxylation. It accumulates in response to mechanical stimulation through the Ca2+/calmodulin activation of glutamate decarboxylase. In this study, dark- and light-grown soybean hypocotyl tissues were used to investigate the relationship between mechanical stimulation, GABA synthesis, and growth inhibition. Mechanical stimulation through stroking resulted in a 65% growth inhibition of dark- or light-grown tissue within 1 min. The 24-h growth increment was reduced by 50% or more. Growth inhibition was accompanied by rapid 4- and 10-fold increases in GABA levels in dark- and light-grown tissue, respectively. Treatment of tissue with exogenous GABA raised GABA levels approximately 6-fold, but did not inhibit growth. In addition, lanthanum or blue light treatment of dark-grown tissue reduced growth by over 60% with no increase in GABA levels. The data demonstrate for the first time, that GABA accumulation accompanies growth inhibition occurring in response to mechanical stimulation. However, GABA accumulation alone appears insufficient to cause growth inhibition.Key words: 4-aminobutyric acid, GABA, L-Glu decarboxylase, GAD, thigmomorphogenesis, mechanical stimulation.