Non-coordinate expression of peroxisome biogenesis, beta-oxidation and glyoxylate cycle genes in mature Arabidopsis plants

Genome-Wide Identification and Characterization of Tomato Fatty Acid β-Oxidase Family Genes KAT and MFP

Fatty acids and their derivatives play a variety of roles in living organisms. Fatty acids not only store energy but also comprise membrane lipids and act as signaling molecules. There are three main proteins involved in the fatty acid β-oxidation pathway in plant peroxisomes, including acyl-CoA oxidase (ACX), multifunctional protein (MFP), and 3-ketolipoyl-CoA thiolase (KAT). However, genome-scale analysis of KAT and MFP has not been systemically investigated in tomatoes. Here, we conducted a bioinformatics analysis of KAT and MFP genes in tomatoes. Their physicochemical properties, protein secondary structure, subcellular localization, gene structure, phylogeny, and collinearity were also analyzed. In addition, a conserved motif analysis, an evolutionary pressure selection analysis, a cis-acting element analysis, tissue expression profiling, and a qRT-PCR analysis were conducted within tomato KAT and MFP family members. There are five KAT and four MFP family members in tomatoes, which are randomly distributed on four chromosomes. By analyzing the conserved motifs of tomato KAT and MFP family members, we found that both KAT and MFP members are highly conserved. In addition, the results of the evolutionary pressure selection analysis indicate that the KAT and MFP family members have evolved mainly from purifying selection, which makes them more structurally stable. The results of the cis-acting element analysis show that SlKAT and SlMFP with respect may respond to light, hormones, and adversity stresses. The tissue expression analysis showed that KAT and MFP family members have important roles in regulating the development of floral organs as well as fruit ripening. The qRT-PCR analysis revealed that the expressions of SlKAT and SlMFP genes can be regulated by ABA, MeJA, darkness, NaCl, PEG, UV, cold, heat, and H2O2 treatments. These results provide a basis for the involvement of the SlKAT and SlMFP genes in tomato floral organ development and abiotic stress response, which lay a foundation for future functional study of SlKAT and SlMFP in tomatoes.

An Arabidopsis GCMS chemical ionization technique to quantify adaptive responses in central metabolism

We present a methodology to survey central metabolism in 13CO2-labeled Arabidopsis (Arabidopsis thaliana) rosettes by ammonia positive chemical ionization-gas chromatography-mass spectrometry. This technique preserves the molecular ion cluster of methyloxime/trimethylsilyl-derivatized analytes up to 1 kDa, providing unambiguous nominal mass assignment of >200 central metabolites and 13C incorporation rates into a subset of 111 from the tricarboxylic acid (TCA) cycle, photorespiratory pathway, amino acid metabolism, shikimate pathway, and lipid and sugar metabolism. In short-term labeling assays, we observed plateau labeling of ∼35% for intermediates of the photorespiratory cycle except for glyoxylate, which reached only ∼4% labeling and was also present at molar concentrations several fold lower than other photorespiratory intermediates. This suggests photorespiratory flux may involve alternate intermediate pools besides the generally accepted route through glyoxylate. Untargeted scans showed that in illuminated leaves, noncyclic TCA cycle flux and citrate export to the cytosol revert to a cyclic flux mode following methyl jasmonate (MJ) treatment. MJ also caused a block in the photorespiratory transamination of glyoxylate to glycine. Salicylic acid treatment induced the opposite effects in both cases, indicating the antagonistic relationship of these defense signaling hormones is preserved at the metabolome level. We provide complete chemical ionization spectra for 203 Arabidopsis metabolites from central metabolism, which uniformly feature the unfragmented pseudomolecular ion as the base peak. This unbiased, soft ionization technique is a powerful screening tool to identify adaptive metabolic trends in photosynthetic tissue and represents an important advance in methodology to measure plant metabolic flux.

The Impact of Photorespiratory Glycolate Oxidase Activity on Arabidopsis thaliana Leaf Soluble Amino Acid Pool Sizes during Acclimation to Low Atmospheric CO2 Concentrations

Photorespiration is a metabolic process that removes toxic 2-phosphoglycolate produced by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. It is essential for plant growth under ambient air, and it can play an important role under stress conditions that reduce CO₂ entry into the leaf thus enhancing photorespiration. The aim of the study was to determine the impact of photorespiration on Arabidopsis thaliana leaf amino acid metabolism under low atmospheric CO₂ concentrations. To achieve this, wild-type plants and photorespiratory glycolate oxidase (gox) mutants were given either short-term (4 h) or long-term (1 to 8 d) low atmospheric CO₂ concentration treatments and leaf amino acid levels were measured and analyzed. Low CO₂ treatments rapidly decreased net CO₂ assimilation rate and triggered a broad reconfiguration of soluble amino acids. The most significant changes involved photorespiratory Gly and Ser, aromatic and branched-chain amino acids as well as Ala, Asp, Asn, Arg, GABA and homoSer. While the Gly/Ser ratio increased in all Arabidopsis lines between air and low CO₂ conditions, low CO₂ conditions led to a higher increase in both Gly and Ser contents in gox1 and gox2.2 mutants when compared to wild-type and gox2.1 plants. Results are discussed with respect to potential limiting enzymatic steps with a special emphasis on photorespiratory aminotransferase activities and the complexity of photorespiration.

Molecular Evolution of Maize Ascorbate Peroxidase Genes and Their Functional Divergence

Ascorbate peroxidase (APX) is an important antioxidant enzyme. APXs in maize are encoded by multiple genes and exist as isoenzymes. The evolutionary history and functional divergence of the maize APX gene family were analyzed through comparative genomic and experimental data on the Internet in this paper. APX genes in higher plants were divided into classes A, B, and C. Each type of APX gene in angiosperms only had one ancestral gene that was duplicated along with the genome duplication or local (or tandem) duplication of the angiosperm. A total of eight genes were retained in maize and named APXa1, APXa2, APXa3, APXb1, APXb2, APXc1.1, APXc1.2, and APXc2. The APX genes of class A were located in the chloroplasts or mitochondria, and the class B and C genes were localized in the peroxisomes and cytoplasm, respectively. The expression patterns of eight APXs were different in vegetative and reproductive organs at different growth and development stages. APXa1 and APXb1 of maize may participate in the antioxidant metabolism of vegetative organs under normal conditions. APXa2, APXb2, APXc1.1, and APXc1.2 may be involved in the stress response, and APXb2 and APXc2 may participate in the senescence response. These results provide a basis for cultivating high-yield and resistant maize varieties.

Reserve lipids and plant autophagy

Autophagy is a universal mechanism that facilitates the degradation of unwanted cytoplasmic components in eukaryotic cells. In this review, we highlight recent developments in the investigation of the role of autophagy in lipid homeostasis in plants by comparison with algae, yeast, and animals. We consider the storage compartments that form the sources of lipids in plants, and the roles that autophagy plays in the synthesis of triacylglycerols and in the formation and maintenance of lipid droplets. We also consider the relationship between lipids and the biogenesis of autophagosomes, and the role of autophagy in the degradation of lipids in plants.

Overexpression of AtAGT1 promoted root growth and development during seedling establishment

Arabidopsis photorespiratory gene AtAGT1 is important for the growth and development of root, the non-photosynthetic organ, and it is involved in a complex metabolic network and salt resistance. AtAGT1 in Arabidopsis encodes an aminotransferase that has a wide range of donor:acceptor combinations, including Asn:glyoxylate. Although it is one of the photorespiratory genes, its encoding protein has been suggested to function also in roots to metabolize Asn. However, experimental data are still lacking. In this study, we investigated experimentally the function of AtAGT1 in roots and our results uncovered its importance in root development during seedling establishment after seed germination. Overexpression of AtAGT1 in roots promoted both the growth of primary root and outgrowth of lateral roots. To further elucidate the molecular mechanisms underlying, amino acid content and gene expression in roots were analyzed, and results revealed that AtAGT1 is involved in a complex metabolic network and salt resistance of roots.

Retrograde response by reactive oxygen/nitrogen species in plants involving different cellular organelles

During oxidative and nitrosative stress conditions cellular organelles convey information to the nucleus to express specific sets of genes to withstand the stress condition and to reorganize their growth and developmental pattern. This organelle to nucleus communication is termed retrograde signaling. In the plant system chloroplast and peroxisomes are mainly involved with little involvement of mitochondria and other organelles in oxidative stress-mediated retrograde signaling. In this review, we will discuss retrograde signaling in plant systems with factors that regulate this signaling cascade.

In situ transcriptomic and metabolomic study of the loss of photosynthesis in the leaves of mixotrophic plants exploiting fungi

Mycoheterotrophic plants have lost photosynthesis and obtain carbon through mycorrhizal fungi colonizing their roots. They are likely to have evolved from mixotrophic ancestors, which rely on both photosynthesis and fungal carbon for their development. Whereas our understanding of the ecological and genomic changes associated with the evolutionary shift to mycoheterotrophy is deepening, little information is known about the specific metabolic and physiological features driving this evolution. We investigated this issue in naturally occurring achlorophyllous variants of temperate mixotrophic orchids. We carried out an integrated transcriptomic and metabolomic analysis of the response to achlorophylly in the leaves of three mixotrophic species sampled in natura. Achlorophyllous leaves showed major impairment of their photosynthetic and mineral nutrition functions, strong accumulation of free amino acids, overexpression of enzymes and transporters related to sugars, amino acids and fatty acid catabolism, as well as induction of some autophagy-related and biotic stress genes. Such changes were reminiscent of these reported for variegated leaves and appeared to be symptomatic of a carbon starvation response. Rather than decisive metabolic innovations, we suggest that the evolution towards mycoheterotrophy in orchids is more likely to be reliant on the versatility of plant metabolism and an ability to exploit fungal organic resources, especially amino acids, to replace missing photosynthates.

Conserved and differential transcriptional responses of peroxisome associated pathways to drought, dehydration and ABA

Plant peroxisomes are important components of cellular antioxidant networks, dealing with ROS generated by multiple metabolic pathways. Peroxisomes respond to environmental and cellular conditions by changing their size, number, and proteomic content. To investigate the role of peroxisomes in response to drought, dehydration and ABA treatment we took an evolutionary and comparative genomics approach. Colonisation of land required evolution of dehydration tolerance in the absence of subsequent anatomical adaptations. Therefore, the model bryophyte Physcomitrella patens, the model dicot Arabidopsis thaliana and wheat (Tricitcum aestivum), a globally important cereal crop were compared. Three sets of genes namely 'PTS1 genes' (a proxy for genes encoding peroxisome targeted proteins), PEX genes (involved in peroxisome biogenesis) and genes involved in plant antioxidant networks were identified in all 3 species and their expression compared under drought (dehydration) and ABA treatment. Genes encoding enzymes of β-oxidation and gluconeogenesis, antioxidant enzymes including catalase and glutathione reductase and PEX3 and PEX11 isoforms showed conserved up-regulation, and peroxisome proliferation was induced by ABA in moss. Interestingly, expression of some of these genes differed between drought sensitive and resistant genotypes of wheat in line with measured photosynthetic and biochemical differences. These results point to an underappreciated role for peroxisomes in drought response.

They Can Handle the Stress: MPK17 and PMD1 act in a salt-specific pathway

Arabidopsis MAP KINASE17 (MPK17) was recently identified as a novel regulator of peroxisome division in response to salt stress. Further, the known peroxisome division factor PEROXISOME AND MITOCHONDRIAL DIVISION FACTOR1 (PMD1) genetically acts downstream of MPK17. We previously showed that mutants defective in either MPK17 or PMD1 fail to proliferate peroxisomes in response to NaCl stress. Here, we show that, unlike their abnormal NaCl responses, mpk17 and pmd1 mutants display wild type responses to other stresses known to alter peroxisome proliferation, suggesting that plants distinguish among peroxisome division-inducing stresses and alter the peroxisome division pathway based on the stress applied.

Integrating transcriptomic and metabolomic analysis to understand natural leaf senescence in sunflower

Leaf senescence is a complex process, which has dramatic consequences on crop yield. In sunflower, gap between potential and actual yields reveals the economic impact of senescence. Indeed, sunflower plants are incapable of maintaining their green leaf area over sustained periods. This study characterizes the leaf senescence process in sunflower through a systems biology approach integrating transcriptomic and metabolomic analyses: plants being grown under both glasshouse and field conditions. Our results revealed a correspondence between profile changes detected at the molecular, biochemical and physiological level throughout the progression of leaf senescence measured at different plant developmental stages. Early metabolic changes were detected prior to anthesis and before the onset of the first senescence symptoms, with more pronounced changes observed when physiological and molecular variables were assessed under field conditions. During leaf development, photosynthetic activity and cell growth processes decreased, whereas sucrose, fatty acid, nucleotide and amino acid metabolisms increased. Pathways related to nutrient recycling processes were also up-regulated. Members of the NAC, AP2-EREBP, HB, bZIP and MYB transcription factor families showed high expression levels, and their expression level was highly correlated, suggesting their involvement in sunflower senescence. The results of this study thus contribute to the elucidation of the molecular mechanisms involved in the onset and progression of leaf senescence in sunflower leaves as well as to the identification of candidate genes involved in this process.

Lipid turnover during senescence

Rapid turnover of stored triacylglycerol occurs after seed germination, releasing fatty acids that provide carbon and energy for seedling establishment. Glycerolipid and fatty acid turnover that occurs at other times in the plant life cycle, including senescence is less studied. Although the entire pathway of β-oxidation is induced during senescence, Arabidopsis leaf fatty acids turnover at rates 50 fold lower than in seedlings. Major unknowns in lipid turnover include the identity of lipases responsible for degradation of the wide diversity of galactolipid, phospholipid, and other lipid class structures. Also unknown is the relative flux of the acetyl-CoA product of β-oxidation into alternative metabolic pathways. We present an overview of senescence-related glycerolipid turnover and discuss its function(s) and speculate about how it might be controlled to increase the energy density and nutritional content of crops. To better understand regulation of lipid turnover, we developed a database that compiles and plots transcript expression of lipid-related genes during natural leaf senescence of Arabidopsis. The database allowed identification of coordinated patterns of down-regulation of lipid biosynthesis genes and the contrasting groups of genes that increase, including 68 putative lipases.

Comparison of intact Arabidopsis thaliana leaf transcript profiles during treatment with inhibitors of mitochondrial electron transport and TCA cycle

Plant mitochondria signal to the nucleus leading to altered transcription of nuclear genes by a process called mitochondrial retrograde regulation (MRR). MRR is implicated in metabolic homeostasis and responses to stress conditions. Mitochondrial reactive oxygen species (mtROS) are a MRR signaling component, but whether all MRR requires ROS is not established. Inhibition of the cytochrome respiratory pathway by antimycin A (AA) or the TCA cycle by monofluoroacetate (MFA), each of which initiates MRR, can increase ROS production in some plant cells. We found that for AA and MFA applied to leaves of soil-grown Arabidopsis thaliana plants, ROS production increased with AA, but not with MFA, allowing comparison of transcript profiles under different ROS conditions during MRR. Variation in transcript accumulation over time for eight nuclear encoded mitochondrial protein genes suggested operation of both common and distinct signaling pathways between the two treatments. Consequences of mitochondrial perturbations for the whole transcriptome were examined by microarray analyses. Expression of 1316 and 606 genes was altered by AA and MFA, respectively. A subset of genes was similarly affected by both treatments, including genes encoding photosynthesis-related proteins. MFA treatment resulted in more down-regulation. Functional gene category (MapMan) and cluster analyses showed that genes with expression levels affected by perturbation from AA or MFA inhibition were most similarly affected by biotic stresses such as pathogens. Overall, the data provide further evidence for the presence of mtROS-independent MRR signaling, and support the proposed involvement of MRR and mitochondrial function in plant responses to biotic stress.

Defining the plant peroxisomal proteome: from Arabidopsis to rice

Peroxisomes are small subcellular organelles mediating a multitude of processes in plants. Proteomics studies over the last several years have yielded much needed information on the composition of plant peroxisomes. In this review, the status of peroxisome proteomics studies in Arabidopsis and other plant species and the cumulative advances made through these studies are summarized. A reference Arabidopsis peroxisome proteome is generated, and some unique aspects of Arabidopsis peroxisomes that were uncovered through proteomics studies and hint at unanticipated peroxisomal functions are also highlighted. Knowledge gained from Arabidopsis was utilized to compile a tentative list of peroxisome proteins for the model monocot plant, rice. Differences in the peroxisomal proteome between these two model plants were drawn, and novel facets in rice were expounded upon. Finally, we discuss about the current limitations of experimental proteomics in decoding the complete and dynamic makeup of peroxisomes, and complementary and integrated approaches that would be beneficial to defining the peroxisomal metabolic and regulatory roadmaps. The synteny of genomes in the grass family makes rice an ideal model to study peroxisomes in cereal crops, in which these organelles have received much less attention, with the ultimate goal to improve crop yield.

Arabidopsis LON2 is necessary for peroxisomal function and sustained matrix protein import

Relatively little is known about the small subset of peroxisomal proteins with predicted protease activity. Here, we report that the peroxisomal LON2 (At5g47040) protease facilitates matrix protein import into Arabidopsis (Arabidopsis thaliana) peroxisomes. We identified T-DNA insertion alleles disrupted in five of the nine confirmed or predicted peroxisomal proteases and found only two-lon2 and deg15, a mutant defective in the previously described PTS2-processing protease (DEG15/At1g28320)-with phenotypes suggestive of peroxisome metabolism defects. Both lon2 and deg15 mutants were mildly resistant to the inhibitory effects of indole-3-butyric acid (IBA) on root elongation, but only lon2 mutants were resistant to the stimulatory effects of IBA on lateral root production or displayed Suc dependence during seedling growth. lon2 mutants displayed defects in removing the type 2 peroxisome targeting signal (PTS2) from peroxisomal malate dehydrogenase and reduced accumulation of 3-ketoacyl-CoA thiolase, another PTS2-containing protein; both defects were not apparent upon germination but appeared in 5- to 8-d-old seedlings. In lon2 cotyledon cells, matrix proteins were localized to peroxisomes in 4-d-old seedlings but mislocalized to the cytosol in 8-d-old seedlings. Moreover, a PTS2-GFP reporter sorted to peroxisomes in lon2 root tip cells but was largely cytosolic in more mature root cells. Our results indicate that LON2 is needed for sustained matrix protein import into peroxisomes. The delayed onset of matrix protein sorting defects may account for the relatively weak Suc dependence following germination, moderate IBA-resistant primary root elongation, and severe defects in IBA-induced lateral root formation observed in lon2 mutants.

The ABC transporter PXA1 and peroxisomal beta-oxidation are vital for metabolism in mature leaves of Arabidopsis during extended darkness

Fatty acid beta-oxidation is essential for seedling establishment of oilseed plants, but little is known about its role in leaf metabolism of adult plants. Arabidopsis thaliana plants with loss-of-function mutations in the peroxisomal ABC-transporter1 (PXA1) or the core beta-oxidation enzyme keto-acyl-thiolase 2 (KAT2) have impaired peroxisomal beta-oxidation. pxa1 and kat2 plants developed severe leaf necrosis, bleached rapidly when returned to light, and died after extended dark treatment, whereas the wild type was unaffected. Dark-treated pxa1 plants showed a decrease in photosystem II efficiency early on and accumulation of free fatty acids, mostly alpha-linolenic acid [18:3(n-3)] and pheophorbide a, a phototoxic chlorophyll catabolite causing the rapid bleaching. Isolated wild-type and pxa1 chloroplasts challenged with comparable alpha-linolenic acid concentrations both showed an 80% reduction in photosynthetic electron transport, whereas intact pxa1 plants were more susceptible to the toxic effects of alpha-linolenic acid than the wild type. Furthermore, starch-free mutants with impaired PXA1 function showed the phenotype more quickly, indicating a link between energy metabolism and beta-oxidation. We conclude that the accumulation of free polyunsaturated fatty acids causes membrane damage in pxa1 and kat2 plants and propose a model in which fatty acid respiration via peroxisomal beta-oxidation plays a major role in dark-treated plants after depletion of starch reserves.

Peroxisome-associated matrix protein degradation in Arabidopsis

Peroxisomes are ubiquitous eukaryotic organelles housing diverse enzymatic reactions, including several that produce toxic reactive oxygen species. Although understanding of the mechanisms whereby enzymes enter peroxisomes with the help of peroxin (PEX) proteins is increasing, mechanisms by which damaged or obsolete peroxisomal proteins are degraded are not understood. We have exploited unique aspects of plant development to characterize peroxisome-associated protein degradation (PexAD) in Arabidopsis. Oilseed seedlings undergo a developmentally regulated remodeling of peroxisomal matrix protein composition in which the glyoxylate cycle enzymes isocitrate lyase (ICL) and malate synthase (MLS) are replaced by photorespiration enzymes. We found that mutations expected to increase or decrease peroxisomal H(2)O(2) levels accelerated or delayed ICL and MLS disappearance, respectively, suggesting that oxidative damage promotes peroxisomal protein degradation. ICL, MLS, and the beta-oxidation enzyme thiolase were stabilized in the pex4-1 pex22-1 double mutant, which is defective in a peroxisome-associated ubiquitin-conjugating enzyme and its membrane tether. Moreover, the stabilized ICL, thiolase, and an ICL-GFP reporter remained peroxisome associated in pex4-1 pex22-1. ICL also was stabilized and peroxisome associated in pex6-1, a mutant defective in a peroxisome-tethered ATPase. ICL and thiolase were mislocalized to the cytosol but only ICL was stabilized in pex5-10, a mutant defective in a matrix protein import receptor, suggesting that peroxisome entry is necessary for degradation of certain matrix proteins. Together, our data reveal new roles for PEX4, PEX5, PEX6, and PEX22 in PexAD of damaged or obsolete matrix proteins in addition to their canonical roles in peroxisome biogenesis.

Expression of the beta-oxidation gene 3-ketoacyl-CoA thiolase 2 (KAT2) is required for the timely onset of natural and dark-induced leaf senescence in Arabidopsis

The onset of leaf senescence is regulated by a complex mechanism involving positive and negative regulators. Among positive regulators, jasmonic acid (JA) accumulates in senescing leaves and the JA-insensitive coi1-1 mutant displays delayed leaf senescence in Arabidopsis. A strong activated expression of the gene coding for the JA-biosynthetic beta-oxidation enzyme 3-ketoacyl-CoA thiolase 2 (KAT2) in natural and dark-induced senescing leaves of Arabidopsis thaliana is reported here. By using KAT2::GUS and KAT2::LUC transgenic plants, it was observed that dark-induced KAT2 activation occurred both in excised leaves as well as in whole darkened plants. The KAT2 activation associated with dark-induced senescence occurred soon after a move to darkness, and it preceded the detection of symptoms and the expression of senescence-associated gene (SAG) markers. Transgenic plants with reduced expression of the KAT2 gene showed a significant delayed senescence both in natural and dark-induced processes. The rapid induction of the KAT2 gene in senescence-promoting conditions as well as the delayed senescence phenotype and the reduced SAG expression in KAT2 antisense transgenic plants, point to KAT2 as an essential component for the timely onset of leaf senescence in Arabidopsis.

The COMATOSE ATP-binding cassette transporter is required for full fertility in Arabidopsis

COMATOSE (CTS) encodes a peroxisomal ATP-binding cassette transporter required not only for beta-oxidation of storage lipids during germination and establishment, but also for biosynthesis of jasmonic acid and conversion of indole butyric acid to indole acetic acid. cts mutants exhibited reduced fertilization, which was rescued by genetic complementation, but not by exogenous application of jasmonic acid or indole acetic acid. Reduced fertilization was also observed in thiolase (kat2-1) and peroxisomal acyl-Coenzyme A synthetase mutants (lacs6-1,lacs7-1), indicating a general role for beta-oxidation in fertility. Genetic analysis revealed reduced male transmission of cts alleles and both cts pollen germination and tube growth in vitro were impaired in the absence of an exogenous carbon source. Aniline blue staining of pollinated pistils demonstrated that pollen tube growth was affected only when both parents bore the cts mutation, indicating that expression of CTS in either male or female tissues was sufficient to support pollen tube growth in vivo. Accordingly, abundant peroxisomes were detected in a range of maternal tissues. Although gamma-aminobutyric acid levels were reduced in flowers of cts mutants, they were unchanged in kat2-1, suggesting that alterations in gamma-aminobutyric acid catabolism do not contribute to the reduced fertility phenotype through altered pollen tube targeting. Taken together, our data support an important role for beta-oxidation in fertility in Arabidopsis (Arabidopsis thaliana) and suggest that this pathway could play a role in the mobilization of lipids in both pollen and female tissues.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

Yeast aconitase in two locations and two metabolic pathways: seeing small amounts is believing

The distribution of identical enzymatic activities between different subcellular compartments is a fundamental process of living cells. At present, the Saccharomyces cerevisiae aconitase enzyme has been detected only in mitochondria, where it functions in the tricarboxylic acid (TCA) cycle and is considered a mitochondrial matrix marker. We developed two strategies for physical and functional detection of aconitase in the yeast cytosol: 1) we fused the alpha peptide of the beta-galactosidase enzyme to aconitase and observed alpha complementation in the cytosol; and 2) we created an ACO1-URA3 hybrid gene, which allowed isolation of strains in which the hybrid protein is exclusively targeted to mitochondria. These strains display a specific phenotype consistent with glyoxylate shunt elimination. Together, our data indicate that yeast aconitase isoenzymes distribute between two distinct subcellular compartments and participate in two separate metabolic pathways; the glyoxylate shunt in the cytosol and the TCA cycle in mitochondria. We maintain that such dual distribution phenomena have a wider occurrence than recorded currently, the reason being that in certain cases there is a small fraction of one of the isoenzymes, in one of the locations, making its detection very difficult. We term this phenomenon of highly uneven isoenzyme distribution "eclipsed distribution."