Two PYRIDOXAL PHOSPHATE HOMEOSTASIS PROTEINs are essential for management of the coenzyme in plants

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Abstract
Coenzyme management is believed to be important for the required pool of active enzymes driving metabolic routes to facilitate homeostasis and match environmental circumstance. The coenzyme pyridoxal 5’-phosphate (PLP) (a vitamin B 6 derivative) is involved in a diverse array of enzyme reactions spanning amino acid to hormone metabolism. However, dedicated proteins that contribute to PLP homeostasis have not yet been studied in plants. Here we demonstrate the importance of proteins annotated PLP HOMEOSTASIS PROTEINs (PLPHPs) for control of PLP in Arabidopsis. A systematic analysis indicates that while most kingdoms have a single PLPHP homolog, Angiosperms within the plant kingdom have two. PLPHPs from Arabidopsis bind PLP and exist as monomers in solution in contrast to reported PLP-dependent enzymes from all kingdoms. Disrupting functionality of both homologs perturbs vitamin B 6 content including a PLP deficit accompanied by impaired and light hypersensitive root growth, unlike biosynthesis mutants. Micrografting studies show that the PLP deficit can be relieved distally between shoots and roots. Yet, supplementation experiments do not restore vitamin B 6 homeostasis in the absence of PLPHP. A series of chemical treatments probing PLP-dependent reactions, notably those for auxin and ethylene, provide evidence that the physiological role of PLPHPs is dynamic management of PLP. Assays in vitro show that Arabidopsis PLPHP can coordinate both PLP transfer and withdrawal. This study expands our broader knowledge of vitamin B 6 biology and highlights the importance of PLP coenzyme homeostasis in plants, providing a platform for further investigations in boosting adaptive responses. One sentence summary: PLPHPs contribute to surveillance of vitamin B 6 homeostasis, likely acting as a rheostat in adaptive responses as a function of the use of the coenzyme PLP.

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