The Biochemistry of the Mitochondrial Matrix. In: Levings CS, Vasil IK, editors. The molecular biology of plant mitochondria. Dordrecht: Springer Netherlands; 1995. pp. 237-80. [DOI: 10.1007/978-94-011-0163-9_7]

Primary Metabolite Responses to Oxidative Stress in Early-Senescing and Paraquat Resistant Arabidopsis thaliana rcd1 (Radical-Induced Cell Death1)

Rcd1 (radical-induced cell death1) is an Arabidopsis thaliana mutant, which exhibits high tolerance to paraquat [methyl viologen (MV)], herbicide that interrupts photosynthetic electron transport chain causing the formation of superoxide and inhibiting NADPH production in the chloroplast. To understand the biochemical mechanisms of MV-resistance and the role of RCD1 in oxidative stress responses, we performed metabolite profiling of wild type (Col-0) and rcd1 plants in light, after MV exposure and after prolonged darkness. The function of RCD1 has been extensively studied at transcriptomic and biochemical level, but comprehensive metabolite profiling of rcd1 mutant has not been conducted until now. The mutant plants exhibited very different metabolic features from the wild type under light conditions implying enhanced glycolytic activity, altered nitrogen and nucleotide metabolism. In light conditions, superoxide production was elevated in rcd1, but no metabolic markers of oxidative stress were detected. Elevated senescence-associated metabolite marker levels in rcd1 at early developmental stage were in line with its early-senescing phenotype and possible mitochondrial dysfunction. After MV exposure, a marked decline in the levels of glycolytic and TCA cycle intermediates in Col-0 suggested severe plastidic oxidative stress and inhibition of photosynthesis and respiration, whereas in rcd1 the results indicated sustained photosynthesis and respiration and induction of energy salvaging pathways. The accumulation of oxidative stress markers in both plant lines indicated that MV-resistance in rcd1 derived from the altered regulation of cellular metabolism and not from the restricted delivery of MV into the cells or chloroplasts. Considering the evidence from metabolomic, transcriptomic and biochemical studies, we propose that RCD1 has a negative effect on reductive metabolism and rerouting of the energy production pathways. Thus, the altered, highly active reductive metabolism, energy salvaging pathways and redox transfer between cellular compartments in rcd1 could be sufficient to avoid the negative effects of MV-induced toxicity.

Overexpression of ALDH10A8 and ALDH10A9 Genes Provides Insight into Their Role in Glycine Betaine Synthesis and Affects Primary Metabolism in Arabidopsis thaliana

Betaine aldehyde dehydrogenases oxidize betaine aldehyde to glycine betaine in species that accumulate glycine betaine as a compatible solute under stress conditions. In contrast, the physiological function of betaine aldehyde dehydrogenase genes is at present unclear in species that do not accumulate glycine betaine, such as Arabidopsis thaliana. To address this question, we overexpressed the Arabidopsis ALDH10A8 and ALDH10A9 genes, which were identified to code for betaine aldehyde dehydrogenases, in wild-type A. thaliana. We analysed changes in metabolite contents of transgenic plants in comparison with the wild type. Using exogenous or endogenous choline, our results indicated that ALDH10A8 and ALDH10A9 are involved in the synthesis of glycine betaine in Arabidopsis. Choline availability seems to be a factor limiting glycine betaine synthesis. Moreover, the contents of diverse metabolites including sugars (glucose and fructose) and amino acids were altered in fully developed transgenic plants compared with the wild type. The plant metabolic response to salt and the salt stress tolerance were impaired only in young transgenic plants, which exhibited a delayed growth of the seedlings early after germination. Our results suggest that a balanced expression of the betaine aldehyde dehydrogenase genes is important for early growth of A. thaliana seedlings and for salt stress mitigation in young seedlings.

Transgenic tobacco (Nicotiana tabacum L.) plants with increased expression levels of mitochondrial NADP+-dependent isocitrate dehydrogenase: evidence implicating this enzyme in the redox activation of the alternative oxidase

Many metabolic reactions are coupled to NADPH in the mitochondrial matrix, including those involved in thiol group reduction. One enzyme linked to such processes is mitochondrial NADP+-dependent isocitrate dehydrogenase (mtICDH; EC 1.1.1.42), although the precise role of this enzyme is not yet known. Previous work has implicated mtICDH as part of a biochemical mechanism to reductively activate the alternative oxidase (AOX). We have partially purified mtICDH from tobacco (Nicotiana tabacum L. cv. Petit Havana SR1) cell suspension cultures and localized this to a 46-kDa protein on SDS-PAGE, which was verified by peptide sequencing. In the inflorescence of the aroid Sauromatum guttatum Schott (voodoo lily), mtICDH appears to be developmentally regulated, presenting maximal specific activity during the thermogenic period of anthesis when the capacity for AOX respiration is also at its peak. Transgenic tobacco plants were generated that overexpress mtICDH and lines were obtained that demonstrated up to a 7-fold increase in mtICDH activity. In isolated mitochondria, this resulted in a measurable increase in the reductive activation of AOX in comparison with wild type. When examined in planta in response to citrate feeding, a strong conversion of AOX from its oxidized to its reduced form was observed in the transgenic line. These data support the hypothesis that mtICDH may be a regulatory switch involved in tricarboxylic acid cycle flux and the reductive modulation of AOX.

Integrated temporal regulation of the photorespiratory pathway. Circadian regulation of two Arabidopsis genes encoding serine hydroxymethyltransferase

The photorespiratory pathway is comprised of enzymes localized within three distinct cellular compartments: chloroplasts, peroxisomes, and mitochondria. Photorespiratory enzymes are encoded by nuclear genes, translated in the cytosol, and targeted into these distinct subcellular compartments. One likely means by which to regulate the expression of the genes encoding photorespiratory enzymes is coordinated temporal control. We have previously shown in Arabidopsis that a circadian clock regulates the expression of the nuclear genes encoding both chloroplastic (Rubisco small subunit and Rubisco activase) and peroxisomal (catalase) components of the photorespiratory pathway. To determine whether a circadian clock also regulates the expression of genes encoding mitochondrial components of the photorespiratory pathway, we characterized a family of Arabidopsis serine hydroxymethyltransferase (SHM) genes. We examined mRNA accumulation for two of these family members, including one probable photorespiratory gene (SHM1) and a second gene expressed maximally in roots (SHM4), and show that both exhibit circadian oscillations in mRNA abundance that are in phase with those described for other photorespiratory genes. In addition, we show that SHM1 mRNA accumulates in light-grown seedlings, although this response is probably an indirect consequence of the induction of photosynthesis and photorespiration by illumination.

Molecular characterization of higher plant NAD-dependent isocitrate dehydrogenase: evidence for a heteromeric structure by the complementation of yeast mutants

NAD-dependent isocitrate dehydrogenase (IDH) is a key enzyme controlling the activity of the citric acid cycle. Despite more than 30 years of work, the plant enzyme remains poorly characterized. In this paper, a molecular characterization of the plant IDH is presented. Starting from probes defined according to sequence comparisons, three full-length cDNAs named Ntidha, Ntidhb and Ntidhc encoding different IDH subunits have been isolated from a Nicotiana tabacum cell suspension library. Sequence comparisons of the tobacco IDH subunits with the E. coli NADP-dependent enzyme, and the yeast IDH1 and IDH2 subunits suggested that only IDHa had the capacity to be catalytic as IDHb and IDHc were lacking certain residues implied in catalysis. The ability of antibodies raised against the recombinant IDHa protein to preferentially cross-react with IDH2 indicated that IDHa was more closely related to IDH2 than to IDH1. Complementation of yeast single IDH mutants showed that IDHb and IDHc could replace the function of the yeast regulatory IDH1 subunit. Although IDHa was unable to complement the IDH2 mutant, its catalytic function was revealed by the ability of two heteromeric enzymes, composed of either IDHa with IDHb or IDHa with IDHc, to replace IDH function in a yeast double mutant lacking both subunits. Expression studies at the protein and mRNA levels show that each subunit is present in both root and leaf tissues and that the three IDH genes respond in the same way to nitrate addition. Taken together, such observations suggest that the physiologically active enzyme is composed of the three different subunits. These results show for the first time that the plant IDH is heteromeric and that IDH subunit composition appears to be conserved between plant and animal kingdoms.

Glycine decarboxylase: protein chemistry and molecular biology of the major protein in leaf mitochondria

The four component proteins of the glycine decarboxylase multienzyme complex (the P-, H-, T-, and L-proteins) comprise over one-third of the soluble proteins in mitochondria isolated from the leaves of C3 plants. Together with serine hydroxymethyltransferase, glycine decarboxylase converts glycine to serine and is the site of photorespiratory CO2 and NH3 release. The component proteins of the complex are encoded on nuclear genes with N-terminal presequences that target them to the mitochondria. The isolated complex readily dissociates into its component proteins and reassociates into the intact complex in vitro. Because of the intimate association between photosynthesis and photorespiration, the proteins of the complex are present at higher levels in leaves in the light. The expression of these genes is controlled at the transcriptional level and the kinetics of expression are closely related to those of the small subunit of Rubisco. Deletion analysis of fusions between the promoter of the H-protein of the complex and the reporter gene beta-glucuronidase in transgenic tobacco has identified a region responsible for the tissue specificity and light dependence of gene expression. Gel shift experiments show that a nuclear protein in leaves binds to this region. Glycine decarboxylase has proven to be an excellent system for studying problems in plant biochemistry ranging from protein-protein interactions to control of gene expression.