The stress factor, exogenous ascorbic acid, affects plant growth and the antioxidant system in Arabidopsis thaliana

Ascorbic acid (AsA) is one of the most important soluble antioxidant molecules in plants, but excess AsA is a type of stress factor that inhibits plant growth. The exposure of Arabidopsis thaliana seedlings to 2 and 8 mM AsA decreased fresh weight to 78.6 and 64.3% of the control, respectively, after 5 days of treatment. A more than fivefold increase in the MDA content following the exposure to AsA suggests that the plant cellular structure is severely damaged by an increase in the ROS content. We also found that the transcripts of several antioxidant genes were down-regulated, which resulted in decreased activities of several antioxidant enzymes. These events caused an imbalance between oxidants and the antioxidant system. Real-time PCR demonstrated that the exogenous AsA reduced the transcript abundance of several aquaporins, whereas 2 mM exogenous AsA increased the transcripts of four aquaporins after 5 days of exposure. A high concentration of AsA significantly down-regulated aquaporins compared to a low concentration of AsA, especially PIP 2;1 and PIP 2;2, which were only 6 and 10% of the control, respectively. These results demonstrated that exogenous AsA was a stress factor that caused ROS overproduction, inhibited antioxidant ability, regulated aquaporin gene expression, and inhibited plant growth.