Balancing of B₆ Vitamers Is Essential for Plant Development and Metabolism in Arabidopsis

Abstract

Vitamin B 6 comprises a family of compounds that is essential for all organisms, most notable among which is the cofactor pyridoxal 59-phosphate (PLP). Other forms of vitamin B 6 include pyridoxamine 59-phosphate (PMP), pyridoxine 59-phosphate (PNP), and the corresponding nonphosphorylated derivatives. While plants can biosynthesize PLP de novo, they also have salvage pathways that serve to interconvert the different vitamers. The selective contribution of these various pathways to cellular vitamin B 6 homeostasis in plants is not fully understood. Although biosynthesis de novo has been extensively characterized, the salvage pathways have received comparatively little attention in plants. Here, we show that the PMP/PNP oxidase PDX3 is essential for balancing B 6 vitamer levels in Arabidopsis thaliana. In the absence of PDX3, growth and development are impaired and the metabolite profile is altered. Surprisingly, RNA sequencing reveals strong induction of stress-related genes in pdx3, particularly those associated with biotic stress that coincides with an increase in salicylic acid levels. Intriguingly, exogenous ammonium rescues the growth and developmental phenotype in line with a severe reduction in nitrate reductase activity that may be due to the overaccumulation of PMP in pdx3. Our analyses demonstrate an important link between vitamin B 6 homeostasis and nitrogen metabolism.

Interaction between vitamin B6metabolism, nitrogen metabolism and autoimmunity

“…Arabidopsis thaliana; autoimmunity; nitrogen metabolism; PDX3; plant defense; pyridoxamine/ pyridoxine phosphate oxidase; vitamin B 6 salvage; vitamin B 6 The essential water-soluble vitamin, B 6 , comprises 6 different vitamers, the alcohol pyridoxine (PN), the amine pyridoxamine (PM), the aldehyde pyridoxal (PL) and their respective 5 0 phosphorylated forms (PNP, PMP, PLP). 1 The enzymatic cofactor PLP is essential for more than 140 enzymatic reactions (http:// bioinformatics.unipr.it/cgi-bin/bioinformatics/B6db/home.pl).…”

“…While its biosynthesis de novo exclusively occurs in plants, bacteria and fungi; humans and animals have to take it up from their diet. Independent from biosynthesis de novo, most organisms possess a salvage pathway through which the different B 6 vitamers are interconverted enzymatically. In plants, the main salvage pathway enzymes are the kinase SALT OVERLY SEN-SITIVE 4 (SOS4) 2,3 ) that phosphorylates all non-phosphorylated vitamers and the oxidase PDX3 that recycles PLP from PMP and PNP.…”

“…4,5 Recently, we showed that mutants knocked out in PDX3, pdx3-3 (SALK_054167C) and pdx3-4 (GK-260E03), are strongly impaired in vegetative and reproductive development. 6 In addition to aberrant leaf growth, the strong allele (pdx3-3) flowers early, is dwarf and has reduced seed yield. 6 At the biochemical level, these pdx3 mutants accumulate high amounts of PMP and show a strongly reduced activity for nitrate reductase (NR) compared to the wild-type.…”

“…6 In addition to aberrant leaf growth, the strong allele (pdx3-3) flowers early, is dwarf and has reduced seed yield. 6 At the biochemical level, these pdx3 mutants accumulate high amounts of PMP and show a strongly reduced activity for nitrate reductase (NR) compared to the wild-type. 6 They thus become dependent on an exogenous supply of ammonium as a nitrogen source, which in turn rescues the developmental phenotype of the mutants.…”

“…6 At the biochemical level, these pdx3 mutants accumulate high amounts of PMP and show a strongly reduced activity for nitrate reductase (NR) compared to the wild-type. 6 They thus become dependent on an exogenous supply of ammonium as a nitrogen source, which in turn rescues the developmental phenotype of the mutants. Moreover, we demonstrated that the levels of the vitamer PMP respond (i.e.…”