| Summary: | <p>Abstract</p> <p>Background</p> <p>Vitamin B6 is a collective term for a group of six interconvertible compounds: pyridoxine, pyridoxal, pyridoxamine and their phosphorylated derivatives. Vitamin B6 plays essential roles as a cofactor in a range of biochemical reactions. In addition, vitamin B6 is able to quench reactive oxygen species <it>in vitro</it>, and exogenously applied vitamin B6 protects plant cells against cell death induced by singlet oxygen (<sup>1</sup>O<sub>2</sub>). These results raise the important question as to whether plants employ vitamin B6 as an antioxidant to protect themselves against reactive oxygen species.</p> <p>Results</p> <p>The <it>pdx1.3 </it>mutation affects the vitamin B6 biosynthesis enzyme, pyridoxal synthase (PDX1), and leads to a reduction of the vitamin B6 concentration in <it>Arabidopsis thaliana </it>leaves. Although leaves of the <it>pdx1.3 Arabidopsis </it>mutant contained less chlorophyll than wild-type leaves, we found that vitamin B6 deficiency did not significantly impact photosynthetic performance or shoot and root growth. Chlorophyll loss was associated with an increase in the chlorophyll <it>a</it>/<it>b </it>ratio and a selective decrease in the abundance of several PSII antenna proteins (Lhcb1/2, Lhcb6). These changes were strongly dependent on light intensity, with high light amplifying the difference between <it>pdx1.3 </it>and the wild type. When leaf discs were exposed to exogenous <sup>1</sup>O<sub>2</sub>, lipid peroxidation in <it>pdx1.3 </it>was increased relative to the wild type; this effect was not observed with superoxide or hydrogen peroxide. When leaf discs or whole plants were exposed to excess light energy, <sup>1</sup>O<sub>2</sub>-mediated lipid peroxidation was enhanced in leaves of the <it>pdx1.3 </it>mutant relative to the wild type. High light also caused an increased level of <sup>1</sup>O<sub>2 </sub>in vitamin B6-deficient leaves. Combining the <it>pdx1.3 </it>mutation with mutations affecting the level of 'classical' quenchers of <sup>1</sup>O<sub>2 </sub>(zeaxanthin, tocopherols) resulted in a highly photosensitive phenotype.</p> <p>Conclusion</p> <p>This study demonstrates that vitamin B6 has a function in the <it>in vivo </it>antioxidant defense of plants. Thus, the antioxidant activity of vitamin B6 inferred from <it>in vitro </it>studies is confirmed <it>in planta</it>. Together with the finding that chloroplasts contain vitamin B6 compounds, the data show that vitamin B6 functions as a photoprotector that limits <sup>1</sup>O<sub>2 </sub>accumulation in high light and prevents <sup>1</sup>O<sub>2</sub>-mediated oxidative damage.</p>
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