The impact of GABA in harpin-elicited biotic stress responses in Nicotiana tabaccum.

Harpin is a bacterial elicitor protein that was first isolated from Erwinia amylovora. Infiltration of this elicitor into the leaves of plants activates systemic acquired resistance against a variety of plant pathogens via the salicyclic acid defense pathway. The non-protein amino acid, neurotransmission inhibitor molecule of mammals-GABA- is found in all organisms and is known to be an important component of stress responses in plants. We hypothesized a possible interaction between harpin-induced defense responses and GABA shunt. Therefore, we conducted experiments on harpin-infiltrated tobacco and analyzed the components of GABA shunt in relation to growth, photosynthesis and H2O2 levels. RGR, RWC and photosynthetic efficiency were all affected in harpin-infiltrated tobacco leaves, but the rate of decline was more remarkable on RGR. H2O2 levels showed significant difference on 7 days after harpin infiltration when the necrotic lesions were also visible. GABA accumulation was increased and glutamate levels were decreased parallel to the differences in GDH and GAD enzyme activities, especially on days 5 and 7 of harpin infiltration. Transcript abundance of GDH and GAD encoding genes were differentially regulated in harpin-infiltrated leaves as compared to that of control and mock groups. In the present study, for the first time we showed a relationship between harpin-elicited responses and GABA in tobacco that was not mediated by H2O2 accumulation. Harpin infiltration significantly induced the first components of the GABA shunt such as GDH, GAD, glutamate and GABA in tobacco.