GABA accumulation and the hypersensitive response in isolated mesophyll cells treated with the G protein activator mastoparan
Abstract
GABA (y-amino butyric acid) is a non-protein amino acid synthesized through
the a-decarboxylation of L-glutamate. This reaction is catalyzed by L-glutamate
decarboxylase (EC 4.1.1.15), a cytosolic Ca2+/calmodulin-stimulated enzyme. The
purpose of this study is to determine whether or not GABA accumulation is associated
with the hypersensitive response of isolated Asparagus sprengeri mesophyll cells.
The addition of 25 J.lM mastoparan, a G protein activator, to suspensions of
isolated asparagus mesophyll cells significantly increased GABA synthesis and cell
death. Cell death was assessed using Evan's blue dye and fluorescein diacetate tests
for cell viability. In addition, mastoparan stimulated pH-dependent alkalinization of the
external medium, and a rapid and large 02 consumption followed by a loss of
photosynthetic activity. The rate of 02 consumption and the net decrease in 02 in the
dark was enhanced by light. The inactive mastoparan analogue Mas17 was
ineffective in stimulating GABA accumulation, medium alkalinization, 02 uptake and
cell death. Accumulation of H202 in response tomastoparan was not detected,
however, mastoparan caused the cell-dependent degradation of added H202. The
pH dependence of mastoparan-stimulated alkalinization suggests cellular electrolyte
leakage, while the consumption of 02 corresponds to the oxidative burst in which 02
at the cell surface is reduced to form various active oxygen species. The results are
indicative of the "hypersensitive response" of plants to pathogen attack, namely, the
death of cells in the locality of pathogen invasion.
The data are compatible with a model in which mastoparan triggers G protein
activity, subsequent intracellular signal transduction pathway/s, and the hypersensitive
response. It is postulated that the physiological elicitation of the hypersensitive
response involves G protein signal transduction. The synthesis of GABA
during the hypersensitive response has not been documented previously; however the
role/s of GABA synthesis in the hypersensitive response, if any, remain unclear.