Vitamin B6 as a potential antioxidant : a study emanating from UV-B-stressed plants

Sammanfattning: Increased influx of solar UV-B radiation (280-320 nm), due to a decreasing stratospheric ozone layer, impacts severely on plants. Some of the UV-B effects on plants are related to stress induced by produced reactive oxygen species (ROS). Pyridoxine (vitamin B6) has been reported to be a potential quencher of ROS.Previous studies have shown that the PDX1.3 gene (encoding a key enzyme in vitamin B6 biosynthesis) is up-regulated by UV-B. We showed that this up-regulation also occurs at the protein level in UV-B exposed Col-0 wild-type plants. Studies performed using pdx1 knock-out mutants of Arabidopsis thaliana showed elevated transcripts levels for LHCB1'3 and PR-5 compared with the Col-0 wild-type. The pdx1 knock-out mutants showed an increased sensitivity towards H2O2-stress, but no increased sensitivity towards low dose UV-B-stress.To study the postulated role of pyridoxine as quencher of ROS in more detail, both theoretical and experimental studies were performed. Theoretical and experimental UV absorption spectra obtained at different protonation/deprotonation states of pyridoxine showed a very close resemblance at low and high pH. However, for the computational studies at physiological pH addition of counterions were required to accurately describe the experimental spectra.When theoretically studying the reactivity of pyridoxine towards three different ROS, .OH, .OOH and .O2-, the hydroxyl radical (.OH) was shown to be the most reactive species, while the superoxide radical (.O2-) showed no reactivity towards pyridoxine.In order to study the role of G-proteins in UV-B signal transduction pathways, UV-B irradiated G-protein mutants of Arabidopsis thaliana were studied. The regulation of the PDX1.3 gene was not affected in the G-protein mutants, regardless of the developmental stage of the plant. However, the expression of CHS was affected in the Ga subunit mutants.