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To survive and proliferate in diverse environments with varying climate and nutrient availability, plants modulate their metabolism. Achieving a balance between carbon (C) and nitrogen (N) use such that growth and defense mechanisms can be appropriately controlled is critical for plant fitness. The identification of factors that regulate C/N utilization in plants can make a significant contribution to optimization of plant health. Here we show that pyridox(am)ine 5’-phosphate oxidase (PDX3), which regulates vitamin B6homeostasis, influences C/N balance. The B6vitamer imbalance resulting from loss of PDX3 leads to over-accumulation of nitrogenous compounds. A combination of increased glutamate dehydrogenase activity, impairment in the photorespiratory cycle and inappropriate use of endogenous ammonium fuel the metabolic imbalance. Growth at elevated CO2levels further exacerbates thepdx3phenotypes. Interestingly, serine supplementation rescues growth under high CO2likely bypassing the phosphorylated pathway of biosynthesis suggesting that this amino acid is an important commodity. We show that PDX3 function appears dispensable upon thermomorphogenesis, a condition that favors C metabolism. Furthermore, while a low ammonium to nitrate ratio likely accounts for overstimulation of salicylic acid (SA) defense responses inpdx3lines that compromises growth, a basal level of SA protects against loss of PDX3 biochemical function. Overall, the study highlights environmental scenarios where vitamin B6homeostasis, as managed by the salvage pathway enzyme PDX3, is critical and provides insight into how plants reprogram their metabolism under such conditions.

Triphosphate tunnel metalloenzymes (TTMs) are found in all biological kingdoms and have been characterized in microorganisms and animals. Members of the TTM family already characterized have divergent biological functions and act on a range of triphosphorylated substrates (RNA, thiamine triphosphate, inorganic polyphosphate). TTM proteins in plants have received considerably less attention and are unique in that some homologs harbor additional domains including a P-loop kinase and transmembrane domain. Here we report on structural and functional aspects of the multimodular TTM1 and TTM2 of Arabidopsis thaliana. Tissue and cellular microscopy studies show that both AtTTM1 and AtTTM2 are expressed in actively dividing (meristem) tissue and are tail-anchored proteins at the outer mitochondrial membrane - mediated by the single transmembrane domain at the C-terminus, supporting earlier studies. Crystal structures of AtTTM1 in the presence and absence of a non-hydrolyzable ATP analog reveal a catalytically incompetent TTM tunnel domain tightly interacting with the P-loop kinase domain that is locked in an inactive conformation. Structural comparison reveals that a helical hairpin may facilitate movement of the TTM domain thereby activating the kinase. Genetic studies show that AtTTM2 is important for the developmental transition from the vegetative to the reproductive phase in Arabidopsis, whereas its closest paralog AtTTM1 is not. Rational design of mutations based on the 3D structure demonstrates that both the P-loop kinase and TTM tunnel modules of AtTTM2 are required for the developmental switch.