Metabolic control of arginine and ornithine levels paces the progression of leaf senescence

Pools of arginine and ornithine generated during protein degradation can pace the progression of leaf senescence by affecting the TCA cycle, polyamine biosynthesis and the ethylene signaling pathway.Leaf senescence can be induced by stress or aging, sometimes in a synergistic manner. It is generally acknowledged that the ability to withstand senescence-inducing conditions can provide plants with stress resilience. Although the signaling and transcriptional networks responsible for a delayed senescence phenotype, often referred to as a functional stay-green trait, have been actively investigated, very little is known about the subsequent metabolic adjustments conferring this aptitude to survival. First, using the individually darkened leaf (IDL) experimental setup, we compared IDLs of wild-type (WT) Arabidopsis (Arabidopsis thaliana) to several stay-green contexts, that is IDLs of two functional stay-green mutant lines, oresara1-2 (ore1-2) and an allele of phytochrome-interacting factor 5 (pif5), as well as to leaves from a WT plant entirely darkened (DP). We provide compelling evidence that arginine and ornithine, which accumulate in all stay-green contexts-likely due to the lack of induction of amino acids (AAs) transport-can delay the progression of senescence by fueling the Krebs cycle or the production of polyamines (PAs). Secondly, we show that the conversion of putrescine to spermidine (SPD) is controlled in an age-dependent manner. Thirdly, we demonstrate that SPD represses senescence via interference with ethylene signaling by stabilizing the ETHYLENE BINDING FACTOR1 and 2 (EBF1/2) complex. Taken together, our results identify arginine and ornithine as central metabolites influencing the stress- and age-dependent progression of leaf senescence. We propose that the regulatory loop between the pace of the AA export and the progression of leaf senescence provides the plant with a mechanism to fine-tune the induction of cell death in leaves, which, if triggered unnecessarily, can impede nutrient remobilization and thus plant growth and survival

In comparison with nitrate nutrition, ammonium nutrition increases growth of the frostbite1 Arabidopsis mutant

Ammonium nutrition inhibits the growth of many plant species, including Arabidopsis thaliana. The toxicity of ammonium is associated with changes in the cellular redox state. The cellular oxidant/antioxidant balance is controlled by mitochondrial electron transport chain. In this study, we analysed the redox metabolism of frostbite1 (fro1) plants, which lack mitochondrial respiratory chain complex I. Surprisingly, the growth of fro1 plants increased under ammonium nutrition. Ammonium nutrition increased the reduction level of pyridine nucleotides in the leaves of wild-type plants, but not in the leaves of fro1 mutant plants. The observed higher activities of type II NADH dehydrogenases and cytochrome c oxidase in the mitochondrial electron transport chain may improve the energy metabolism of fro1 plants grown on ammonium. Additionally, the observed changes in reactive oxygen species (ROS) metabolism in the apoplast may be important for determining the growth of fro1 under ammonium nutrition. Moreover, bioinformatic analyses showed that the gene expression changes in fro1 plants significantly overlap with the changes previously observed in plants with a modified apoplastic pH. Overall, the results suggest a pronounced connection between the mitochondrial redox system and the apoplastic pH and ROS levels, which may modify cell wall plasticity and influence growth. In this paper, we analysed the redox metabolism of frostbite1 (fro1) plants lacking Complex I under ammonium nutrition. We showed that, although ammonium leads to stress in wild type plants, ammonium does not cause reductive stress in fro1 plants. Our experimental and bioinformatic analyses indicated that mtETC dysfunction strongly influences apoplastic reactive oxygen species content and pH, and suggested that the faster growth of fro1 plants under ammonium nutrition probably results from modification of the cell wall

Long-term ammonium nutrition of Arabidopsis increases the extrachloroplastic NAD(P)H/NAD(P)(+) ratio and mitochondrial reactive oxygen species level in leaves but does not impair photosynthetic capacity

Ammonium nutrition has been suggested to be associated with alterations in the oxidation-reduction state of leaf cells. Herein, we show that ammonium nutrition in Arabidopsis thaliana increases leaf NAD(P)H/NAD(P)(+) ratio, reactive oxygen species content and accumulation of biomolecules oxidized by free radicals. We used the method of rapid fractionation of protoplasts to analyse which cellular compartments were over-reduced under ammonium supply and revealed that observed changes in NAD(P)H/NAD(P)(+) ratio involved only the extrachloroplastic fraction. We also showed that ammonium nutrition changes mitochondrial electron transport chain activity, increasing mitochondrial reactive oxygen species production. Our results indicate that the functional impairment associated with ammonium nutrition is mainly associated with redox reactions outside the chloroplast

Temperature-dependent changes in respiration rates and redox poise of the ubiquinone pool in protoplasts and isolated mitochondria of potato leaves

In many environments, leaves experience large diurnal variations in temperature. Such short-term changes in temperature are likely to have important implications for respiratory metabolism in leaves. Here, we used intact leaf, protoplasts and isolated mitochondria to determine the impact of short-term changes in temperature on respiration rates (R), adenylate concentrations and the redox poise of the ubiquinone (UQ) pool in mitochondria of potato leaves. The Q10 (i.e. proportional change in R for each 10°C rise in temperature) of respiration was 1.8, both for intact leaves and protoplasts. In protoplasts, the redox poise of the extracted UQ pool (UQR/UQT) increased from 0.33 at 22°C, to 0.76 at 15°C. Further decreases in temperature (from 15 to 5°C) resulted in UQR/UQT decreasing to 0.40. Adenylate ratios in protoplasts were also temperature dependent. At high adenosine 5′-triphosphate (ATP) adenosine 5′-diphosphate (ADP) ratios (i.e. low ADP concentrations), UQR/UQT values were low, suggesting that adenylates restricted flux via the UQ-reducing pathways more than they restricted flux via pathways that oxidized UQH2. To assess whether high rates of alternative oxidase (AOX) activity could have uncoupled respiratory flux (and thus UQR/UQT) from adenylate restriction of the cytochrome (Cyt) pathway, we constructed kinetic curves of O2 uptake (via the two pathways) vs UQR/UQT in isolated mitochondria, measured at two temperatures (15 and 25°C); measurements were made for mitochondria operating under state 3 (i.e. +ADP) and state 4 (i.e. -ADP) conditions. In contrast to the Cyt pathway, flux via the AOX was temperature insensitive, with maximal rates of AOX activity representing 21-57% of total O2 uptake in isolated mitochondria. We conclude that temperature-dependent variations in UQR/UQT are largely dependent on temperature-dependent changes in adenylate ratios, and that flux via the AOX could in some circumstances help reduce maximal UQ values

The mitochondrial type II peroxiredoxin from poplar

Gama F, Keech O, Eymery F, et al. The mitochondrial type II peroxiredoxin from poplar. PHYSIOLOGIA PLANTARUM. 2007;129(1):196-206.Mitochondria are a major site of reactive oxygen species production and controlling the peroxide levels in this compartment is essential. Peroxiredoxins (Prx) are heme-free peroxidases, which use reactive cysteines for their catalysis and reducing systems for their regeneration. One of the two Prxs present in poplar mitochondria, Prx IIF, expressed as a recombinant protein, was found to reduce a broad range of peroxides with electrons provided preferentially by glutaredoxin and to a lesser extent by glutathione, all the thioredoxins tested being inefficient. This protein is constitutively expressed because it is found in all tissues analyzed. Its expression is modified during a biotic interaction between poplar and the rust fungus Melampsora laricii populina. On the other hand, Prx IIF expression does not substantially vary under abiotic stress conditions. Nevertheless, water deficit or chilling and probably induced senescence, but not photooxidative conditions or heavy metal treatment, also led to a small increase in PrxIIF abundance in Arabidopsis thaliana plants