Abstract:
Gamma-aminobutyric acid (GAB A) is a ubiquitous non-protein amino acid synthesized via
the decarboxylation of L-glutamate in a reaction catalyzed by the cytosolic enzyme L-glutamate
decarboxylase (GAD). In animals it functions as an inhibitory neurotransmitter. In plants it
accumulates rapidly in response to various stresses, but its function remains unclear. The hypothesis
that GABA accumulation in leaf tissue may function as a plant resistance mechanism against
phytophagous insect activity was investigated.
GABA accumulation in response to mechanical stimulation, mechanical damage and insect
activity was demonstrated. In wt tobacco (Nicotiana tabacum cv Samsun), mechanical stimulation
or damage caused GABA to accumulate within 2 min from mean levels of 14 to 37 and 1~9 nmol
g-l fresh weight (FW), respectively. In the transgenic tobacco strain CaMVGAD27c overexpressing
Petunia GAD, the same treatments caused GABA to accumulate from 12 to 59 and 279 nmol g-l FW,
respectively. In the transgenic tobacco strain CaMVGADilC 11 overexpressing Petunia GAD lacking
an autoinhibitory domain, mechanical stimulation or damage caused GABA to accumulate from 180
to 309 and 630 nmol g-l FW, respectively. Ambulatory activity by tobacco budworm (TBW) larvae
(Heliothis virescens) on leaves of CaMVGAD27c tobacco caused GABA to accumulate from 28 to
80 nmol g-l FW within 5 min. Ambulatory and leaf-rolling activity by oblique banded leaf roller
(OBLR) larvae (Choristoneura rosaceana cv Harris) on wt soybean leaves (Glycine max cv
Harovinton) caused GABA to accumulate from 60 to 1123 nmol g-l FW within 20 min. Increased
GABA levels in leaf tissue were shown to affect phytophagous preference in TBW larvae presented
with wt and transgenic tobacco leaves. When presented with leaves of Samsun wt and CaMVGAD27c plants, TBW larvae consumed more wt leaf tissue (640 ± 501 S.D. mm2
) than
transgenic leaf tissue (278 ± 338 S.D. mm2
) nine times out of ten. When presented with leaves of
Samsun wt and CaMVGAD~C11 plants, TBW larvae consumed more transgenic leaf tissue (1219
± 1009 S.D. mm2
) than wt leaf tissue (28 ± 31 S.D. mm2
) ten times out of ten.
These results indicate that: (1) ambulatory activity of insect larvae on leaves results in
increased GABA levels, (2) transgenic tobacco leaves with increased capacity for GABA synthesis
deter feeding, and (3) transgenic tobacco leaves with constitutively higher GABA levels stimulate
feeding.
