Abstract:
The hypothesis that rapid y-aminobutyric acid (GABA) accumulation is
a plant defense against phytophagous insects was investigated. Simulation
of mechanical damage resulting from phytophagous insect activity
increased soybean (Glycine max L.) leaf GABA 10- to 25-fold within 1 to 4
min. Pulverizing leaf tissue resulted in a value of 2. 15 (±O. 11 SE) ~mol
GABA per gram fresh weight. Increasing the GABA levels in a synthetic
diet from 1.6 to 2.6 Jlffiol GABA per gram fresh weight reduced the growth
rates, developmental rates, total biomass (50% reduction), and survival
rates (30% reduction) of cultured Oblique banded leaf-roller (OBLR)
(Choristonellra rosacealla Harris) larvae. In field experiments OBLR
larvae were found predominantly on young terminal leaves which have a
reduced capacity to produce GABA in response to mechanical damage.
Glutamate decarboxylase (GAD) is a cytosolic enzyme which catalyses the
decarboxylation of L-Glu to GABA. GAD is a calmodulin binding enzyme
whose activity is stimulated dramatically by increased cytosolic H+ or Ca2
+
ion concentrations. Phytophagous insect activity will disrupt the cellular
compartmentation of H+ and Ca2
+, activate GAD and subsequent GABA
accumulation. In animals GABA is a major inhibitory neurotransmitter.
The possible mechanisms resulting in GABA inhibited growth and
development of insects are discussed.
