GABA shunt deficiencies and accumulation of reactive oxygen species under UV treatments: insight from Arabidopsis thaliana calmodulin mutants

Environmental stimuli such as UV, paraquat, and H2O2 can induce reactive oxygen species (ROS) production and impair the cellular redox equilibrium. ROS are controlled by a complex network of ROS metabolizing enzymes and play a major signaling role in different compartments of plants cell. GABA, alanine, and glutamate are all GABA shunt-related metabolites that are accumulated in response to oxidative stress. In this study, T-DNA insertion mutants of 7 calmodulin genes (CAM) in Arabidopsis thaliana were used to determine the role of specific CaM in tolerance of plants to oxidative stress induced by ultraviolet (UVA and UVB) treatments. Seedlings growth, seeds germination, reactive oxygen species accumulation, and changes in GABA shunt metabolites levels were determined. Only cam4 mutants showed significant tolerance to UVA and UVB treatments over the other cam mutants during seed germination. Oxidative damage measured as level of MDA caused by UV treatment was found in root and shoot tissues of cam1, cam4, cam5-4, and cam6-1 of Arabidopsis cam mutants. In response to UVA treatment, the shunt metabolites accumulated in root and shoot tissues after 30 min. As a result of UVB treatment, GABA accumulated after 30 min while alanine and glutamate accumulated after 60 min only in root tissue. There was a significant increase in GABA, alanine, and glutamate levels after 30, 60, and 90 min UVA treatments in root and shoot tissue of cam1, cam3-2, cam4, cam5-1, cam5-2, cam6-1, cam7-1 mutants. On the other hand, all shunt metabolites levels were significantly accumulated in root of cam1, cam4, cam5-4, and cam6-1 and only in shoot tissue of cam5-4 and cam6-1 mutants in response to 30, 60, and 90 min UVB treatment. Our results show that cam mutants are sensitive to induced-oxidative stress in response to both UV treatments especially cam1, cam4, cam5-4, cam6-1, and cam7-1 mutants for seed germination and ROS accumulation. Accumulation of GABA shunt metabolites under induced-oxidative stress via UV treatments demonstrates that GABA shunt pathway, GABA metabolites accumulation, and Ca+2/CaM-mediating signaling mechanisms are major components of antioxidant machinery associated with ROS scavenging, H2O2 equilibrium, maintaining balance of cellular redox state, and acquiring tolerance in cellular signaling in response to UV stress in Arabidopsis seedlings.