Vitamer levels, stress response, enzyme activity, and gene regulation of Arabidopsis lines mutant in the pyridoxine/pyridoxamine 5′-phosphate oxidase (PDX3) and the pyridoxal kinase (SOS4) genes involved in the vitamin B6 salvage pathway

PDX3 and SALT OVERLY SENSITIVE4 (SOS4), encoding pyridoxine/pyridoxamine 5'-phosphate oxidase and pyridoxal kinase, respectively, are the only known genes involved in the salvage pathway of pyridoxal 5'-phosphate in plants. In this study, we determined the phenotype, stress responses, vitamer levels, and regulation of the vitamin B-6 pathway genes in Arabidopsis (Arabidopsis thaliana) plants mutant in PDX3 and SOS4. sos4 mutant plants showed a distinct phenotype characterized by chlorosis and reduced plant size, as well as hypersensitivity to sucrose in addition to the previously noted NaCl sensitivity. This mutant had higher levels of pyridoxine, pyridoxamine, and pyridoxal 5'-phosphate than the wild type, reflected in an increase in total vitamin B-6 observed through HPLC analysis and yeast bioassay. The sos4 mutant showed increased activity of PDX3 as well as of the B-6 de novo pathway enzyme PDX1, correlating with increased total B-6 levels. Two independent lines with T-DNA insertions in the promoter region of PDX3 (pdx3-1 and pdx3-2) had decreased PDX3 activity. Both also had decreased activity of PDX1, which correlated with lower levels of total vitamin B-6 observed using the yeast bioassay; however, no differences were noted in levels of individual vitamers by HPLC analysis. Both pdx3 mutants showed growth reduction in vitro and in vivo as well as an inability to increase growth under high light conditions. Increased expression of salvage and some of the de novo pathway genes was observed in both the pdx3 and sos4 mutants. In all mutants, increased expression was more dramatic for the salvage pathway genes.