The role of nitrogen metabolism in the survival of prolonged periods of waterlogging was investigated in highly flood-tolerant, nodulated Lotus japonicus plants. Alanine production revealed to be a critical hypoxic pathway. Alanine is the only amino acid whose biosynthesis is not inhibited by nitrogen deficiency resulting from RNA interference silencing of nodular leghemoglobin. The metabolic changes that were induced following waterlogging can be best explained by the activation of alanine metabolism in combination with the modular operation of a split tricarboxylic acid pathway. The sum result of this metabolic scenario is the accumulation of alanine and succinate and the production of extra ATP under hypoxia. The importance of alanine metabolism is discussed with respect to its ability to regulate the level of pyruvate, and this and all other changes are discussed in the context of current models concerning the regulation of plant metabolism.

The essential mineral nutrient potassium (K + ) is the most important inorganic cation for plants and is recognized as a limiting factor for crop yield and quality. Nonetheless, it is only partially understood how K + contributes to plant productivity. K + is used as a major active solute to maintain turgor and to drive irreversible and reversible changes in cell volume. K + also plays an important role in numerous metabolic processes, for example, by serving as an essential cofactor of enzymes. Here, we provide evidence for an additional, previously unrecognized role of K + in plant growth. By combining diverse experimental approaches with computational cell simulation, we show that K + circulating in the phloem serves as a decentralized energy storage that can be used to overcome local energy limitations. Posttranslational modification of the phloem-expressed Arabidopsis K + channel AKT2 taps this "potassium battery," which then efficiently assists the plasma membrane H + -ATPase in energizing the transmembrane phloem (re) loading processes.channel gating | energy limiting condition | phloem reloading | posttranslational regulation | potassium channel

Summary• The variation in tolerance to low oxygen is likely explained by divergent sets of molecular and metabolic responses between species.• We analysed the versatility of the response to low oxygen of primary metabolism by comparing nine previously published metabolome profiling studies. Data were juxtaposed with expression profiles of genes encoding enzymes involved in the metabolic pathways of rice, Arabidopsis and poplar. Furthermore, full transcript profiles were compared to determine commonalities in the expression of orthologous genes and genes that serve similar functions.• Activation of fermentation and the accumulation of alanine plus succinate were observed in all species, but transcriptional regulation of these metabolic pathways varied. Global analysis of orthologue expression revealed that most differentially expressed genes either had no orthologues or were not affected in the other species. Expression analysis of nearly all gene clusters with common functions varied significantly between species.• The resemblance of the metabolic response to hypoxia indicates that this occurs independent of the level of tolerance. However, regulation of these processes at transcriptional level varied between species. An important role is suggested for signalling and post-transcriptional regulation to be involved in the mechanisms that lead to tolerance to hypoxia.

Based on enzyme activity assays and metabolic responses to waterlogging of the legume Lotus japonicus, it was previously suggested that, during hypoxia, the tricarboxylic acid cycle switches to a noncyclic operation mode. Hypotheses were postulated to explain the alternative metabolic pathways involved, but as yet, a direct analysis of the relative redistribution of label through the corresponding pathways was not made. Here, we describe the use of stable isotope-labeling experiments for studying metabolism under hypoxia using wild-type roots of the crop legume soybean (Glycine max). [ 13 C]Pyruvate labeling was performed to compare metabolism through the tricarboxylic acid cycle, fermentation, alanine metabolism, and the g-aminobutyric acid shunt, while [ 13 C]glutamate and [ 15 N]ammonium labeling were performed to address the metabolism via glutamate to succinate. Following these labelings, the time course for the redistribution of the 13 C/ 15 N label throughout the metabolic network was evaluated with gas chromatography-time of flight-mass spectrometry. Our combined labeling data suggest the inhibition of the tricarboxylic acid cycle enzyme succinate dehydrogenase, also known as complex II of the mitochondrial electron transport chain, providing support for the bifurcation of the cycle and the down-regulation of the rate of respiration measured during hypoxic stress. Moreover, up-regulation of the g-aminobutyric acid shunt and alanine metabolism explained the accumulation of succinate and alanine during hypoxia.