Phosphorylated B6 vitamer deficiency in SALT OVERLY SENSITIVE 4 mutants compromises shoot and root development

Two pyridoxal phosphate homeostasis proteins are essential for management of the coenzyme pyridoxal 5'-phosphate in Arabidopsis

Coenzyme management is important for homeostasis of the pool of active metabolic enzymes. The coenzyme pyridoxal 5'-phosphate (PLP) is involved in diverse enzyme reactions including amino acid and hormone metabolism. Regulatory proteins that contribute to PLP homeostasis remain to be explored in plants. Here, we demonstrate the importance of proteins annotated as PLP homeostasis proteins (PLPHPs) for controlling PLP in Arabidopsis (Arabidopsis thaliana). A systematic analysis indicates that while most organisms across kingdoms have a single PLPHP homolog, Angiosperms have two. PLPHPs from Arabidopsis bind PLP and exist as monomers, in contrast to reported PLP-dependent enzymes, which exist as multimers. Disrupting the function of both PLPHP homologs perturbs vitamin B6 (pyridoxine) content, inducing a PLP deficit accompanied by light hypersensitive root growth, unlike PLP biosynthesis mutants. Micrografting studies show that the PLP deficit can be relieved distally between shoots and roots. Chemical treatments probing PLP-dependent reactions, notably those for auxin and ethylene, provide evidence that PLPHPs function in the dynamic management of PLP. Assays in vitro show that Arabidopsis PLPHP can coordinate PLP transfer and withdrawal from other enzymes. This study thus expands our knowledge of vitamin B6 biology and highlights the importance of PLP coenzyme homeostasis in plants.

PYRIDOX(AM)INE 5'-PHOSPHATE OXIDASE3 of Arabidopsis thaliana maintains carbon/nitrogen balance in distinct environmental conditions

The identification of factors that regulate C/N utilization in plants can make a substantial contribution to optimization of plant health. Here, we explored the contribution of pyridox(am)ine 5'-phosphate oxidase3 (PDX3), which regulates vitamin B6 homeostasis, in Arabidopsis (Arabidopsis thaliana). Firstly, N fertilization regimes showed that ammonium application rescues the leaf morphological phenotype of pdx3 mutant lines but masks the metabolite perturbance resulting from impairment in utilizing soil nitrate as a source of N. Without fertilization, pdx3 lines suffered a C/N imbalance and accumulated nitrogenous compounds. Surprisingly, exploration of photorespiration as a source of endogenous N driving this metabolic imbalance, by incubation under high CO2, further exacerbated the pdx3 growth phenotype. Interestingly, the amino acid serine, critical for growth and N management, alleviated the growth phenotype of pdx3 plants under high CO2, likely due to the requirement of pyridoxal 5'-phosphate for the phosphorylated pathway of serine biosynthesis under this condition. Triggering of thermomorphogenesis by growth of plants at 28 °C (instead of 22 °C) did not appear to require PDX3 function, and we observed that the consequent drive toward C metabolism counters the C/N imbalance in pdx3. Further, pdx3 lines suffered a salicylic acid-induced defense response, probing of which unraveled that it is a protective strategy mediated by nonexpressor of pathogenesis related1 (NPR1) and improves fitness. Overall, the study demonstrates the importance of vitamin B6 homeostasis as managed by the salvage pathway enzyme PDX3 to growth in diverse environments with varying nutrient availability and insight into how plants reprogram their metabolism under such conditions.

B vitamin supply in plants and humans: the importance of vitamer homeostasis

B vitamins are a group of water-soluble micronutrients that are required in all life forms. With the lack of biosynthetic pathways, humans depend on dietary uptake of these compounds, either directly or indirectly, from plant sources. B vitamins are frequently given little consideration beyond their role as enzyme accessory factors and are assumed not to limit metabolism. However, it should be recognized that each individual B vitamin is a family of compounds (vitamers), the regulation of which has dedicated pathways. Moreover, it is becoming increasingly evident that individual family members have physiological relevance and should not be sidelined. Here, we elaborate on the known forms of vitamins B₁ , B₆ and B₉ , their distinct functions and importance to metabolism, in both human and plant health, and highlight the relevance of vitamer homeostasis. Research on B vitamin metabolism over the past several years indicates that not only the total level of vitamins but also the oft-neglected homeostasis of the various vitamers of each B vitamin is essential to human and plant health. We briefly discuss the potential of plant biology studies in supporting human health regarding these B vitamins as essential micronutrients. Based on the findings of the past few years we conclude that research should focus on the significance of vitamer homeostasis - at the organ, tissue and subcellular levels - which could improve the health of not only humans but also plants, benefiting from cross-disciplinary approaches and novel technologies.