Polyamines are potential source of γ-aminobutyric acid (GABA) in plants under abiotic stress. However, the correlation between polyamine degradation and GABA accumulation in tea plants exposed to drought stress is unclear. In the present study, the contents of GABA and putrescine in tea leaves were increased significantly under drought stress, but the glutamate level did not change. At the same time, five copper-containing amine oxidase ( CuAO) and eight aminoaldehyde dehydrogenase ( AMADH) genes involved in putrescine-derived GABA pathway were identified from Tea Plant Information Archive. Expression analysis indicated that CsCuAO1, CsCuAO3, as well as CsAMADH1 were induced to play important function in response to exogenous putrescine or under drought stress. Thus, the three genes were cloned and their soluble recombinant proteins were produced in Escherichia coli. The catalytic efficiency, the pH and temperature optima of them were determined. CsCuAOs and CsAMADH1 exhibited indispensable function in the GABA production from putrescine in vitro. Subcellular localization assays indicated that CsAMADH1 was localized in plastid, while both CsCuAO1 and CsCuAO3 were localized in peroxisome. In addition, the synergistic effects of CsCuAOs and CsAMADH1 were investigated by transient co-expression system in Nicotiana benthamiana. The overexpression of CsAMADH1 lines in Arabidopsis showed increased GABA level supplemented with exogenous putrescine. Our data suggests that these three genes participate the GABA production through putrescine degradation in tea plants that associate with the response to drought stress.

Our previous study has shown that copper-containing amine oxidase ( CuAO) and aminoaldehyde dehydrogenase ( AMADH) could regulate the accumulation of γ-aminobutyric acid (GABA) in tea plants by participating in polyamine degradation pathway. However, the involvement of these genes in drought tolerance has not been underlined. In this study, CsCuAO1 associated with CsAMADH1 confers drought tolerance, which modulates GABA levels in tea plants, was conducted. The results showed that exogenous GABA spraying effectively alleviated the drought induced physical damage. Overexpression Arabidopsis lines of CsCuAO1 and CsAMADH1 exhibited enhanced resistance to drought, which promoted the synthesis of GABA and putrescine, by alerting reactive oxygen species scavenging capacity and stomatal movement. However, suppression of CsCuAO1 or CsAMADH1 in tea plants exhibited increased sensitivity during drought treatment. Moreover, the co-overexpressed plants increased the accumulation of GABA both in the Agrobacterium-mediated Nicotiana benthamiana transient assay and transgenic Arabidopsis plants. In addition, a GABA transporter, CsGAT1, was identified, whose expression is strongly correlated with GABA accumulation levels in different tissues under drought stress. Taken together, CsCuAO1 and CsAMADH1 were involved in response to drought stress through a dynamic balance between GABA and putrescine. Our data will greatly contribute to the characterization of GABA biological functions in response to environmental stresses in plants.