Vitamin B₆ is a family of compounds and in its form as pyridoxal 5ʹ-phosphate (PLP) functions as a coenzyme in every form of life in hundreds of enzymatic reactions. Biosynthetic pathways yielding vitamin B₆ de novo and through so-called salvage pathways have been described in plants to service provision of PLP and to maintain a balance between the various forms of vitamin B₆ (vitamers) - critical for plant health. Over the past decade, an enigmatic PLP-binding protein has emerged as being vital to balance vitamin B₆ levels and maintain vitamin B₆-dependent metabolism. The protein named PLP HOMEOSTASIS PROTEIN (PLPHP) has been associated with vitamin B₆ dependent epilepsy as well as other severe brain disorders in humans. Strikingly, at the outset of this thesis, the PLPHP protein had not been explored in plants. So, this study aimed to investigate the contribution of PLPHPs to vitamin B₆ homeostasis and consequently plant fitness using the model plant Arabidopsis thaliana.

A phylogenetic analysis performed here shows that core Angiosperms exceptionally have two PLPHP homologues with residues conserved that are predicted to bind PLP. Recombinant versions of Arabidopsis PLPHP homologues (AtPLPHP1/2) harbour a Schiff-base bound PLP at a conserved lysine residue and appeared as monomeric proteins with PLP surface exposed, in stark contrast to the higher order nature of PLP-dependent proteins where the active site is shielded. Analysis of AtPLPHP1/2 mutants revealed that the two homologues contribute redundantly to vitamin B₆ balance throughout the whole plant. Characteristic features are a general deficit in PLP with accumulation of pyridoxic acid (the degradation product of PLP/PL) in shoots and roots, accumulation of PNP in shoots and a deficit of PL in roots. While this imbalance appears to be dispensable under standard laboratory conditions in soil-grown plants, AtPLPHP1/2 was found to be necessary for root development on culture plates with root illumination, i.e. root photomorphogenesis. Surprisingly, the presence of AtPLPHP1/2 in the shoot was found to be sufficient to control root PLP levels and rescue root photomorphogenesis, suggesting that AtPLPHP1/2 may service PLP homeostasis distally. Further investigation revealed that in the absence of AtPLPHP1/2, Arabidopsis is probably unable to achieve auxin/ethylene homeostasis, thereby compromising root development under illumination. Although the shoot morphology of soil-grown AtPLPHP1/2 mutants is apparently unaffected, metabolic profiling of these plants revealed that the homeostasis of certain aspartate and chorismate derived amino acids is compromised in the shoot, providing a basis for future investigations into the physiological role of AtPLPHP1/2 in plants. In general, as plant foods are the main source of vitamin B₆ in the human diet, a better understanding of how plants regulate levels could help us to improve not only plant health but also human health by increasing the nutritional quality of crops. This work presents PLPHP as an unexplored factor regulating vitamin B₆ levels in plants and provides a basis for future investigations into the role of PLPHP in the green lineage.