Cytosolic acidification and gamma-aminobutyric acid synthesis during the oxidative burst in isolated Asparagus sprengeri mesophyll cells

The four carbon, non-protein amino acid gamma -aminobutyrate (GABA) accumul ates rapidly in response to diverse stresses. Its synthesis is stimulated b y increases in intracellular Ca2+ or H+ levels. The pathogen-induced oxidat ive burst is also associated with increases in Ca2+ and H+ levels. This stu dy investigated the relationship between GABA synthesis and the oxidative b urst. A Mas-7-induced consumption of oxygen in isolated Asparagus sprengeri Regel mesophyll cells was accompanied by rapid GABA synthesis. At pH 5.0, a 300% increase occurred within 16 min from 6.6 to 26.3 nmol GABA.10(6) cel ls(-1). At pH 6.0, the increase was from 8.5 to 18.1 nmol GABA.10(6) cells( -1). Mas-7 also stimulated rapid external alkalinization and intracellular acidification. Intracellular pH decreased 0.44 pH units at pH 5.0, and 0.21 pH units at pH 6.0. The Mas-7-induced oxidative burst, GABA synthesis, ext racellular alkalinization, and intracellular acidification were all elimina ted when lanthanum, a Ca2+ channel blocker, replaced Ca2+ in the incubation medium. The data demonstrate that GABA accumulation is associated with the oxidative burst, and results from the fluxes of H+ and Ca2+, which are kno wn to accompany the oxidative burst. They are discussed in light of emergin g data that indicate a role for GABA in plant cell to cell signaling.