Recently we have reported that high intracellular levels of galactinol and raffinose in Arabidopsis plants overexpressing the heat shock transcription factor A2 or galactinol synthase are correlated with increased tolerance to methylviologen treatment and salinity or chilling stress, and galactinol and raffinose also are found to effectively protect salicylate from attack by hydroxyl radicals in vitro.1,2 These findings indicate that galactinol and raffinose act not only as osmoprotectants, but also as antioxidants in the leaves of Arabidopsis plants. At the same time, we found that the rate constant (1.1 +/- 0.29 x 10(10) M(-1)s(-1)) for the reaction between stachyose and hydroxyl radicals was higher than those of galactinol and raffinose and typical antioxidants. The accumulation of stachyose was only observed in seeds. Furthermore, glucose, fructose and sucrose which are abundant in higher plants efficiently scavenge hydroxyl radicals. Judging from the radical scavenging activity and the intracellular level of each compound reported here and previously, we suggest that carbohydrates including raffinose family oligosaccharides and sugar alcohols are present at high levels under normal and/or stressfull conditions, and act as antioxidants to protect plant cells from oxidative damage and maintain redox homeostasis.
