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

Abstract

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.

Nitric oxide negatively regulates AKT1-mediated potassium uptake through modulating vitamin B6 homeostasis in Arabidopsis

“…PDX3 encodes pyridoxine/ pyridoxamine 5′-phosphate oxidase, which functions in the other vitamin B6 salvage pathway parallel to SOS4 (SNO1 here; Fig. S2F), but losing function of this gene has no obvious effects on the levels of plant vitamers (37). Supporting our notion, pdx3 mutant showed similar SNP response as wild type (Fig.…”

“…SNO1 was identified as the same gene known as salt overly sensitive 4 (SOS4) (36) encoding a pyridoxal kinase that catabolizes the biosynthesis of an active form of vitamin B6, PLP. The sos4 mutant was reported to have unexpected elevated levels of PLP (37) and disturbed Na + and K + amounts (36). Here we show that NO induced accumulation of PLP.…”

“…S2E). Thus, we concluded that SNO1 is SOS4, a pyridoxal kinase that functions in the vitamin B6 synthesis pathway (36,37) (Fig. S2F).…”

“…Thus, it is conceivable that, evolutionally, the vitamin B6 salvage pathway appeared earlier than the de novo pathway (63,64), implicating the former pathwayderived vitamin in the harsh and volatile environments during long-term plant development. Considering the essential role of vitamin B6 PLP as an enzyme cofactor in basic cellular metabolism (37,40), the NO-accumulated PLP here may represent more profound roles in plant growth besides its regulation of the AKT1 channel.…”

“…In a sos4 mutant containing low K + and high Na + relative to wild-type plants (36), vitamin B6 pyridoxal-5′-phosphate (PLP) levels were unusually high (37). Interestingly, in this NaCl-hypersensitive mutant, the less retention of K + was more pronounced in roots than shoots, especially under salt stress (36), suggesting less K + uptake in this high PLP-containing mutant.…”