Cytosolic acidification and γ-aminobutyric acid synthesis during the oxidative burst in isolatedAsparagus sprengerimesophyll cells

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Abstract
The four carbon, non-protein amino acid γ-aminobutyrate (GABA) accumulates rapidly in response to diverse stresses. Its synthesis is stimulated by increases in intracellular Ca2+or H+levels. The pathogen-induced oxidative burst is also associated with increases in Ca2+and H+levels. This study investigated the relationship between GABA synthesis and the oxidative burst. 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·106cells–1. At pH 6.0, the increase was from 8.5 to 18.1 nmol GABA·106cells–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, extracellular alkalinization, and intracellular acidification were all eliminated 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 known to accompany the oxidative burst. They are discussed in light of emerging data that indicate a role for GABA in plant cell to cell signaling.Key words: γ-aminobutyric acid, GABA, oxidative burst.