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

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 glutama te decarboxylase. In this study, dark- and light-grown soybean hypocotyl ti ssues were used to investigate the relationship between mechanical stimulat ion, 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 level s in dark- and light-grown tissue, respectively. Treatment of tissue with e xogenous 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 demon strate for the first time, that GABA accumulation accompanies growth inhibi tion occurring in response to mechanical stimulation. However, GABA accumul ation alone appears insufficient to cause growth inhibition.