A protective association between catalase and isocitrate lyase in peroxisomes

Deciphering peroxisomal reactive species interactome and redox signalling networks

Plant peroxisomes are highly dynamic organelles with regard to metabolic pathways, number and morphology and participate in different metabolic processes and cell responses to their environment. Peroxisomes from animal and plant cells house a complex system of reactive oxygen species (ROS) production associated to different metabolic pathways which are under control of an important set of enzymatic and non enzymatic antioxidative defenses. Nitric oxide (NO) and its derivate reactive nitrogen species (RNS) are also produced in these organelles. Peroxisomes can regulate ROS and NO/RNS levels to allow their role as signalling molecules. The metabolism of other reactive species such as carbonyl reactive species (CRS) and sulfur reactive species (SRS) in peroxisomes and their relationship with ROS and NO have not been explored in depth. In this review, we define a peroxisomal reactive species interactome (PRSI), including all reactive species ROS, RNS, CRS and SRS, their interaction and effect on target molecules contributing to the dynamic redox/ROS homeostasis and plasticity of peroxisomes, enabling fine-tuned regulation of signalling networks associated with peroxisome-dependent H₂O₂. Particular attention will be paid to update the information available on H₂O₂-dependent peroxisomal retrograde signalling and to discuss a specific peroxisomal footprint.

2-Methoxy-1,4-naphthoquinone regulated molecular alternation of Fusarium proliferatum revealed by high-dimensional biological data

Fungi Fusarium proliferatum and the toxins it produces are hazardous to agricultural plants, animals, and human health. However, there is a lack of more effective and environment-friendly natural anti-F. proliferatum agents. In the search for natural anti-fungal agents, we found that naturally originated 2-methoxy-1,4-naphthoquinone (MNQ) with a minimal inhibitory dose of 8.0 mg L^-1 possessed a potential inhibitory effect on F. proliferatum. The results of transcriptomic, proteomic, and metabolomic reveal a total of 1314 differential expression genes (DEGs, 873 up-regulated and 441 down-regulated), 259 differential expression proteins (DEPs, 104 up-regulated and 155 down-regulated), and 86 differential accumulation metabolites (DAMs, 49 up-regulated and 37 down-regulated) in MNQ-induced F. proliferatum. Further, the correlation analysis of transcriptomic, proteomic, and metabolomic indicated that these DEGs, DEPs, and DAMs were co-mapped in the pathways of glyoxylate and dicarboxylate metabolism, glycine, serine, and threonine metabolism, and pyruvate metabolism that linked to the TCA cycle. Furthermore, the key DEGs of the significantly co-mapped pathways were verified with qPCR analysis, which was related to the permeability of the cell membrane of F. proliferatum. Thus, these findings will provide fundamental scientific data on the molecular shifts of MNQ-induced F. proliferatum.

Nuclear and peroxisomal targeting of catalase

Catalase is a well-known component of the cellular antioxidant network, but there have been conflicting conclusions reached regarding the nature of its peroxisome targeting signal. It has also been reported that catalase can be hijacked to the nucleus by effector proteins of plant pathogens. Using a physiologically relevant system where native untagged catalase variants are expressed in a cat2-1 mutant background, the C terminal most 18 amino acids could be deleted without affecting activity, peroxisomal targeting or ability to complement multiple phenotypes of the cat2-1 mutant. In contrast, converting the native C terminal tripeptide PSI to the canonical PTS1 sequence ARL resulted in lower catalase specific activity. Localisation experiments using split superfolder green fluorescent protein revealed that catalase can be targeted to the nucleus in the absence of any pathogen effectors, and that C terminal tagging in combination with alterations of the native C terminus can interfere with nuclear localisation. These findings provide fundamental new insights into catalase targeting and pave the way for exploration of the mechanism of catalase targeting to the nucleus and its role in non-infected plants.

In vitro and in silico analysis reveals antifungal activity and potential targets of curcumin on Paracoccidioides spp

The search for new compounds with activity against Paracoccidioides, etiologic agents of Paracoccidioidomycosis (PCM), is extremely necessary due to the current scenario of the available therapeutic arsenal. Treatment is restricted to three classes of antifungals with side effects. Curcumin is a polyphenol with antifungal effects that is extracted from Curcuma longa. The present work aimed to evaluate the activity of curcumin in different species of Paracoccidioides and to evaluate the potential molecular targets of curcumin using computational strategies. In addition, interactions with classic antifungals used in the treatment of PCM were evaluated. Curcumin inhibits the growth of Paracoccidioides spp. exerting a fungicidal effect. The combination of curcumin with amphotericin B, co-trimoxazole, and itraconazole showed a synergistic or additive interaction. Molecular targets as superoxide dismutase, catalase, and isocitrate lyase were proposed based on in silico approaches. Curcumin affects the fungal plasma membrane and increases the production of reactive oxygen species. Therefore, curcumin is a good alternative for the treatment of PCM.

Peroxisomes as redox-signaling nodes in intracellular communication and stress responses

Peroxisomes are redox nodes playing a diverse range of roles in cell functionality and in the perception of and responses to changes in their environment.

How does drought affect native grasses' photosynthesis on the revegetation of iron ore tailings?

The revegetation of areas degraded by iron ore mining is a difficult challenge mainly due to water availability and impoverished metal-rich substrates. We sought to understand the photosynthetic responses to drought of native tropical grasses Paspalum densum (Poir.) and Setaria parviflora (Poir.) grown in iron ore tailing. The grass P. densum presented better photosynthetic adjustments when grown in the iron ore tailing and S. paviflora in response to water stress. Both species accumulated iron above the phytotoxic threshold when grown in an iron ore tailing. The net photosynthesis, stomatal conductance, transpiration, and water use efficiency decreased followed by a reduction in leaf relative water content in response to water stress for both species. The photochemical efficiency of photosystem II only decreased at the point of maximum drought. At this point, the water-stressed grass grown in the iron ore tailing presented higher H₂O₂ concentrations, particularly S. parviflora. After rehydration, full recovery of photosynthetic variables was achieved with decreased malondialdehyde concentrations, increased catalase activity, and, consequently, decreased H₂O₂ concentrations in leaves for both species. The fast recovery of the native grasses P. densum and S. parviflora to drought in the iron ore tailing substrate is indicative of their resistance and potential use in the revegetation of impoverished mined areas with high iron content and seasonal water deficit.

Photochemical Efficiency and Oxidative Metabolism of Tree Species during Acclimation to High and Low Irradiance

The balance between efficiency of absorption and use of light energy is fundamental for plant metabolism and to avoid photoinhibition. Here, we investigated the effects of light environments on the photosynthetic apparatus of tropical tree species of three successional groups (pioneer, mid-, and late successional) subjected to different light conditions: full sunlight (FS), moderate shade (MS), and deep shade (DS). Twenty-nine ecophysiological parameters were correlated with each other. The pioneer species exhibited better photochemical performance and a more efficient antioxidant enzymatic system in comparison with the other successional groups. Plants in FS showed higher intensity of lipid peroxidation, with superoxide dismutase having a prominent role in the antioxidant system. At lower irradiance the enzymatic activity was reduced, and the photochemical efficiency was the preferred way to reduce oxidative damages. P was highly related to photochemical yield, and the N modulation amplified the light harvesting complex in DS to the detriment of the antioxidant system. Despite evidence of cell damage, most species exhibited the ability to adjust to high irradiance. Contrary to expectations, Hymenea courbaril (late-successional) exhibited higher plasticity to fluorescence, nutritional, and antioxidant parameters. Only Carapa guianensis (late-successional) displayed photoinhibitory damage in FS, and Ochroma pyramidale (pioneer) did not survive in DS, suggesting that acclimation to shade is more challenging than to high irradiance.

Reactive Oxygen Species and Gibberellin Acid Mutual Induction to Regulate Tobacco Seed Germination

Seed germination is a complex process controlled by various mechanisms. To examine the potential contribution of reactive oxygen species (ROS) and gibberellin acid (GA) in regulating seed germination, diphenylene iodonium chloride (DPI) and uniconazole (Uni), as hydrogen peroxide (H2O2) and GA synthesis inhibitor, respectively, were exogenously applied on tobacco seeds using the seed priming method. Seed priming with DPI or Uni decreased germination percentage as compared with priming with H2O, especially the DPI + Uni combination. H2O2 and GA completely reversed the inhibition caused by DPI or Uni. The germination percentages with H2O2 + Uni and GA + DPI combinations kept the same level as with H2O. Meanwhile, GA or H2O2 increased GA content and deceased ABA content through corresponding gene expressions involving homeostasis and signal transduction. In addition, the activation of storage reserve mobilization and the enhancement of soluble sugar content and isocitrate lyase (ICL) activity were also induced by GA or H2O2. These results strongly suggested that H2O2 and GA were essential for tobacco seed germination and by downregulating the ABA/GA ratio and inducing reserve composition mobilization mutually promoted seed germination. Meanwhile, ICL activity was jointly enhanced by a lower ABA/GA ratio and a higher ROS concentration.

Superoxide serves as a putative signal molecule for plant cell division: overexpression of CaRLK1 promotes the plant cell cycle via accumulation of O2- and decrease in H2 O2

Reactive oxygen species (ROS) exert both positive and negative effects on plant growth and development and therefore receive a great deal of attention in current research. A hot pepper, Capsicum annuum receptor-like kinase 1 (CaRLK1) was ectopically expressed in Nicotiana tabacum BY-2 cell and Nicotiana benthamiana plants. This ectopic expression of CaRLK1 enhanced cell division and proliferation in both heterologous systems. Apparently, CaRLK1 is involved in controlling the cell cycle, possibly by inducing expressions of cyclin B1, cyclin D3, cyclin-dependent protein kinase 3, condensin complex subunit 2 and anaphase-promoting complex subunit 11 genes. CaRLK1 overexpression also increased transcript accumulation of NADPH oxidase genes, generation of O2- and catalase (CAT) activity/protein levels. In parallel, it decreased cellular H2 O2 levels and cell size. Treatment with Tiron or diphenyleneiodonium (DPI) both decreased the cell division rate and O2- concentrations, but increased cellular H2 O2 levels. Tobacco BY-2 cells overexpressing CaRLK1 were more sensitive to amino-1,2,4-triazole (3-AT), a CAT inhibitor, than control cells, suggesting that the increased H2 O2 levels may not function as a signal for cell division and proliferation. Overexpression of CaRLK1 stimulated progression of the cell cycle from G0 /G1 phase into the S phase. It is concluded that the CaRLK1 protein plays a pivotal role in controlling the level of O2- as signaling molecule which promotes cell division, concomitant with a reduction in H2 O2 by the induction of CAT activity/protein.

2-Hydroxy Acids in Plant Metabolism

Glycolate, malate, lactate, and 2-hydroxyglutarate are important 2-hydroxy acids (2HA) in plant metabolism. Most of them can be found as D- and L-stereoisomers. These 2HA play an integral role in plant primary metabolism, where they are involved in fundamental pathways such as photorespiration, tricarboxylic acid cycle, glyoxylate cycle, methylglyoxal pathway, and lysine catabolism. Recent molecular studies in Arabidopsis thaliana have helped elucidate the participation of these 2HA in in plant metabolism and physiology. In this chapter, we summarize the current knowledge about the metabolic pathways and cellular processes in which they are involved, focusing on the proteins that participate in their metabolism and cellular/intracellular transport in Arabidopsis.

New Insight into the Mechanism and Function of Autophagy in Plant Cells

Autophagy is a degradation pathway that is conserved throughout eukaryotic organisms and plays important roles in the tolerance of abiotic and biotic stresses. It functions as a housekeeping process to remove unwanted cell components under normal conditions, and is induced during stress and senescence to break down damaged cellular contents and to recycle materials. The target components are engulfed into specialized transport structures termed autophagosomes and are subsequently delivered to the vacuole for degradation. Here, we review milestones in the study of autophagy in plants, discuss recent advances in our understanding of the mechanism and physiological roles of plant autophagy, and highlight potential future directions of research.

Ectopic expression of CaRLK1 enhances hypoxia tolerance with increasing alanine production in Nicotiana spp

In a previous report, the pepper receptor-like kinase 1 (CaRLK1) gene was shown to be responsible for negatively regulating plant cell death caused by pathogens via accumulation of superoxide anions. Here, we examined whether this gene also plays a role in regulating cell death under abiotic stress. The total concentrations of free amino acids in CaRLK1-overexpressed cells (RLKox) increased by twofold compared with those of the wild-type Nicotiana tabacum BY-2 cells. Additionally, alanine and pyruvate concentrations increased by approximately threefold. These accumulations were associated with both the expression levels of the isocitrate lyase (ICL) and malate synthase genes and their specific activities, which were preferentially up-regulated in the RLKox cells. The expression levels of ethylene biosynthetic genes (ACC synthase and ACC oxidase) were suppressed, but those of both the metallothionein and lesion simulating disease 1 genes increased in the RLKox cells during submergence-induced hypoxia. The specific activity of catalase, which is involved in protecting ICL from reactive oxygen species, was also induced threefold in the RLKox cells. The primary roots of the transgenic plants that were exposed to hypoxic conditions grew at similar rates to those in normal conditions. We propose that CaRLK1 maintains a persistent hypoxia-resistant phenotype.

Autophagy-related proteins are required for degradation of peroxisomes in Arabidopsis hypocotyls during seedling growth

Plant peroxisomes play a pivotal role during postgerminative growth by breaking down fatty acids to provide fixed carbons for seedlings before the onset of photosynthesis. The enzyme composition of peroxisomes changes during the transition of the seedling from a heterotrophic to an autotrophic state; however, the mechanisms for the degradation of obsolete peroxisomal proteins remain elusive. One candidate mechanism is autophagy, a bulk degradation pathway targeting cytoplasmic constituents to the lytic vacuole. We present evidence supporting the autophagy of peroxisomes in Arabidopsis thaliana hypocotyls during seedling growth. Mutants defective in autophagy appeared to accumulate excess peroxisomes in hypocotyl cells. When degradation in the vacuole was pharmacologically compromised, both autophagic bodies and peroxisomal markers were detected in the wild-type vacuole but not in that of the autophagy-incompetent mutants. On the basis of the genetic and cell biological data we obtained, we propose that autophagy is important for the maintenance of peroxisome number and cell remodeling in Arabidopsis hypocotyls.

Permeability of the peroxisomal membrane: lessons from the glyoxylate cycle

Glyoxylate serves as intermediate in various metabolic pathways, although high concentrations of this metabolite are toxic to the cell. In many organisms glyoxylate is fed into the glyoxylate cycle. Enzymes participating in this metabolism are located on both sides of the peroxisomal membrane. The permeability of this membrane for small metabolites paves the way for exchange of intermediates between proteins catalyzing consecutive reactions. A model, in which soluble enzymes accumulate in close proximity to both ends of pore-like structures forming a transmembrane metabolon could explain the rapid and targeted exchange of intermediates. The metabolites passing the membrane differ between the three model organisms Saccharomyces cerevisiae, Arabidopsis thaliana, and Candida albicans, which reflects the ease of evolutionary adaptation processes whenever specific transporter proteins are not involved. The atypical permeability properties of the peroxisomal membrane together with a flexible structural arrangement ensuring the swift and selective transport across the membrane might represent the molecular basis for the functional versatility of peroxisomes.

Sir2-dependent asymmetric segregation of damaged proteins in ubp10 null mutants is independent of genomic silencing

Carbonylation of proteins is an irreversible oxidative damage that increases during both chronological and replicative yeast aging. In the latter, a spatial protein quality control system that relies on Sir2 is responsible for the asymmetrical damage segregation in the mother cells. Proper localization of Sir2 on chromatin depends on the deubiquitinating enzyme Ubp10, whose loss of function deeply affects the recombination and gene-silencing activities specific to Sir2. Here, we have analyzed the effects of SIR2 and UBP10 inactivations on carbonylated protein patterns obtained in two aging models such as stationary phase cells and size-selected old mother ones. In line with the endogenous situation of higher oxidative stress resulting from UBP10 inactivation, an increase of protein carbonylation has been found in the ubp10Delta stationary phase cells compared with sir2Delta ones. Moreover, Calorie Restriction had a salutary effect for both mutants by reducing carbonylated proteins accumulation. Remarkably, in the replicative aging model, whereas SIR2 inactivation resulted in a failure to establish damage asymmetry, the Sir2-dependent damage inheritance is maintained in the ubp10Delta mutant which copes with the increased oxidative damage by retaining it in the mother cells. This indicates that both Ubp10 and a correct association of Sir2 with the silenced chromatin are not necessary in such a process but also suggests that additional Sir2 activities on non-chromatin substrates are involved in the establishment of damage asymmetry.

Hydrogen peroxide induced oxidation of peroxisomal malate synthase and catalase

Peroxisomes contain oxidases that produce H(2)O(2), which can result in protein oxidation. To test the vulnerability of peroxisomal proteins to oxidation in vivo the organelles were isolated from castor bean endosperm incubated with H(2)O(2). When peroxisomes were exposed to H(2)O(2)in vivo, the peroxisomal proteins exhibited an increase in carbonylation as detected in avidin blots of biotin hydrazide derivatized samples. Biotin-tagged peptides from trypsin digests of the proteins were analyzed by mass spectroscopy and compared to the masses of peptides from the same protein that had not been biotin-tagged and from proteins not exposed to excess H(2)O(2). H(2)O(2) exposure was found to increase the activity of catalase (CAT), and to increase the number of oxidized peptides found in CAT and malate synthase (MS). CAT had 10 peptides that were affected by in vivo exposure to H(2)O(2) and MS had 8. These sites of oxidation have definable locations within the proteins' structures.

Peroxisome biogenesis and function

Peroxisomes are small and single membrane-delimited organelles that execute numerous metabolic reactions and have pivotal roles in plant growth and development. In recent years, forward and reverse genetic studies along with biochemical and cell biological analyses in Arabidopsis have enabled researchers to identify many peroxisome proteins and elucidate their functions. This review focuses on the advances in our understanding of peroxisome biogenesis and metabolism, and further explores the contribution of large-scale analysis, such as in sillco predictions and proteomics, in augmenting our knowledge of peroxisome function In Arabidopsis.

Increasing sulfur supply enhances tolerance to arsenic and its accumulation in Hydrilla verticillata (Lf.) Royle

The present study was aimed to analyze the effects of variable S supply on arsenic (As) accumulation potential of Hydrilla verticillata (Lf.) Royle. Plants were exposed to either arsenate (AsV; 50 microM) or arsenite (AsIII; 5 microM) for 4 h and 1 day while S supply was varied as deficient (2 microM, -S), normal (1 mM, +S) and excess (2 mM, +HS). The level of As accumulation (microg g(-1) dw) after 1 day was about 2-fold higher upon exposure to either AsV (30) or AsIII (50) in +HS plants than that being in +S (12 and 24) and -S (14 and 26) plants. The +HS plants showed a significant stimulation of the thiol metabolism upon As exposure. Besides, they did not experience significant toxicity, measured in terms of malondialdehyde accumulation; an indicator of oxidative stress. By contrast, -S plants suffered from oxidative stress probably due to negative impact to thiol metabolism. Variable S supply also modulated the activity of enzymes of glycine and serine biosynthesis indicating an interconnection between S and N metabolism. In conclusion, an improved supply of S to plants was found to augment their ability for As accumulation through stimulated thiol metabolism.

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.

Purification, identification, and characterization of peanut isocitrate lyase

Isocitrate lyase (ICL, EC 4.1.3.1) is commonly present in oil-rich seeds in catalyzing the cleavage of isocitrate to glyoxylate and succinate and plays an essential role in lipid metabolism and gluconeogenesis. When peanut kernels (Tainan 14) were germinated at 30 degrees C, the cotyledon ICL activities increased substantially in the initial 4 days, and the 4-day-germinated cotyledons were subjected to ICL purification by Tris-HCl buffer extraction, heat treatment at 55 degrees C for 1 h, (NH4)2SO4 fractionation at 25-35% saturation, DEAE-cellulose chromatography, and Sephacryl S-300 gel filtration. A single 64 kDa SDS-PAGE protein band was obtained with 7.7% recovery and 37.5-fold purity. It was identified as ICL by LC-MS/MS analyses and Mascot Search with 494 as the highest Probability Based Mowse Score (PBMS). On the basis of the sequence of the homologous ICL of Glycine max, 26% of the peptide sequences of the peanut ICL were identified. During gel filtration, separation of peanut catalase (identified by LC-MS/MS and Mascot Search with 405 as the highest PBMS) from peanut ICL was achieved. The highest measured peanut ICL enzymatic activities were obtained at 45 degrees C and pH 7.0-7.8, respectively. The enzyme activities were stable (>80%) as stored for 8 h at 30 degrees C, 15 days at 4 degrees C, or 60 days at -25 degrees C. As affected by the supplements in the reactants for activity determinations, ICL activity was not affected by glucose up to 4%, sucrose up to 5%, or ethanol up to 8.33%.

MONODEHYROASCORBATE REDUCTASE4 is required for seed storage oil hydrolysis and postgerminative growth in Arabidopsis

Hydrogen peroxide is a major by-product of peroxisomal metabolism and has the potential to cause critical oxidative damage. In all eukaryotes, catalase is thought to be instrumental in removing this H(2)O(2). However, plants also contain a peroxisomal membrane-associated ascorbate-dependent electron transfer system, using ascorbate peroxidase and monodehydroascorbate reductase (MDAR). Here, I report that the conditional seedling-lethal sugar-dependent2 mutant of Arabidopsis thaliana is deficient in the peroxisomal membrane isoform of MDAR (MDAR4). Following germination, Arabidopsis seeds rely on storage oil breakdown to supply carbon skeletons and energy for early seedling growth, and massive amounts of H(2)O(2) are generated within the peroxisome as a by-product of fatty acid beta-oxidation. My data suggest that the membrane-bound MDAR4 component of the ascorbate-dependent electron transfer system is necessary to detoxify H(2)O(2), which escapes the peroxisome. This function appears to be critical to protect oil bodies that are in close proximity to peroxisomes from incurring oxidative damage, which otherwise inactivates the triacylglycerol lipase SUGAR-DEPENDENT1 and cuts off the supply of carbon for seedling establishment.

A central role for the peroxisomal membrane in glyoxylate cycle function

The glyoxylate cycle provides the means to convert C2-units to C4-precursors for biosynthesis, allowing growth on fatty acids and C2-compounds. The conventional view that the glyoxylate cycle is contained within peroxisomes in fungi and plants is no longer valid. Glyoxylate cycle enzymes are located both inside and outside the peroxisome. Thus, the operation of the glyoxylate cycle requires transport of several intermediates across the peroxisomal membrane. Glyoxylate cycle progression is also dependent upon mitochondrial metabolism. An understanding of the operation and regulation of the glyoxylate cycle, and its integration with cellular metabolism, will require further investigation of the participating metabolite transporters in the peroxisomal membrane.

Plant peroxisomes as a source of signalling molecules

Peroxisomes are pleiomorphic, metabolically plastic organelles. Their essentially oxidative function led to the adoption of the name 'peroxisome'. The dynamic and diverse nature of peroxisome metabolism has led to the realisation that peroxisomes are an important source of signalling molecules that can function to integrate cellular activity and multicellular development. In plants defence against predators and a hostile environment is of necessity a metabolic and developmental response--a plant has no place to hide. Mutant screens are implicating peroxisomes in disease resistance and signalling in response to light. Characterisation of mutants disrupted in peroxisomal beta-oxidation has led to a growing appreciation of the importance of this pathway in the production of jasmonic acid, conversion of indole butyric acid to indole acetic acid and possibly in the production of other signalling molecules. Likewise the role of peroxisomes in the production and detoxification of reactive oxygen, and possibly reactive nitrogen species and changes in redox status, suggests considerable scope for peroxisomes to contribute to perception and response to a wide range of biotic and abiotic stresses. Whereas the peroxisome is the sole site of beta-oxidation in plants, the production and detoxification of ROS in many cell compartments makes the specific contribution of the peroxisome much more difficult to establish. However progress in identifying peroxisome specific isoforms of enzymes associated with ROS metabolism should allow a more definitive assessment of these contributions in the future.

Molecular organization of peroxisomal enzymes: protein-protein interactions in the membrane and in the matrix

The beta-oxidation of fatty acids in peroxisomes produces hydrogen peroxide (H2O2), a toxic metabolite, as a bi-product. Fatty acids beta-oxidation activity is deficient in X-linked adrenoleukodystrophy (X-ALD) because of mutation in ALD-gene resulting in loss of very long chain acyl-CoA synthetase (VLCS) activity. It is also affected in disease with catalase negative peroxisomes as a result of inactivation by H2O2. Therefore, the following studies were undertaken to delineate the molecular interactions between both the ALD-gene product (adrenoleukodystrophy protein, ALDP) and VLCS as well as H2O2 degrading enzyme catalase and proteins of peroxisomal beta-oxidation. Studies using a yeast two hybrid system and surface plasmon resonance techniques indicate that ALDP, a peroxisomal membrane protein, physically interacts with VLCS. Loss of these interactions in X-ALD cells may result in a deficiency in VLCS activity. The yeast two-hybrid system studies also indicated that catalase physically interacts with L-bifunctional enzyme (L-BFE). Interactions between catalase and L-BFE were further supported by affinity purification, using a catalase-linked resin. The affinity bound 74-kDa protein, was identified as L-BFE by Western blot with specific antibodies and by proteomic analysis. Additional support for their interaction comes from immunoprecipitation of L-BFE with antibodies against catalase as a catalase- L-BFE complex. siRNA for L-BFE decreased the specific activity and protein levels of catalase without changing its subcellular distribution. These observations indicate that L-BFE might help in oligomerization and possibly in the localization of catalase at the site of H2O2 production in the peroxisomal beta-oxidation pathway.