Arginine decarboxylase ADC2 enhances salt tolerance through increasing ROS scavenging enzyme activity in Arabidopsis thaliana

Soil salinity, one of the major abiotic stresses limiting plant growth and productivity, affects large terrestrial areas of the world. Putrescine (Put), one of major Polyamines (PAs) widely present in organisms, is a type of low-molecular-weight aliphatic nitrogenous bases with function of regulating cell activities. Arginine decarboxylase (ADC) is a key rate-limiting enzyme in Put synthesis. It has been reported that loss-of-function mutant of the encoding gene AtADC2 is intolerant to salt stress in Arabidopsis. However, the effect of overexpression of AtADC2 on plant salt tolerance and the mechanisms about how ADC is involved in the response of plants to salt stress remain unknown. In this study, both overexpression lines of AtADC2 (AtADC2-OE), which has higher endogenous Put concentration, and loss-of-function mutant (adc2-3) were treated by salt stress. Under 150 mM NaCl stress condition, the growth of the mutant adc2-3 was more strongly inhibited than the wild type (Col-0). However, AtADC2-OE was less affected and displayed an enhanced salt tolerance. The chlorophyll content was detected to be significantly enhanced in AtADC2-OE lines and reduced in adc2-3 compared with in Col-0. These results indicated AtADC2-OE lines had more tolerance to salt stress. In addition, AtADC2 expression is up-regulated by salt stress in primary root tip of Arabidopsis seedling. Under salt stress, the contents of malonaldehyde, superoxide (O-2.(-)) and hydrogen peroxide (H2O2), were increased in adc2-3, while H2O2 content was somewhat decreased in AtADC2-OE lines, as compared with those in Col-0. Determination of enzyme activities showed that after salt treatment, the activities of superoxide dismutase (SOD) and catalase (CAT) were enhanced significantly in AtADC2-OE lines, but varied inconspicuously in adc2-3. Taken together, our results reveal that, under salt stress, the increase in both expression of AtADC2 gene and Put accumulation regulates the activities of SOD and CAT to enhance the salt resistance of Arabidopsis thaliana.