Cytochrome c levels affect the TOR pathway to regulate growth and metabolism under energy-deficient conditions

Mitochondrial function is essential for plant growth, but the mechanisms involved in adjusting growth and metabolism to changes in mitochondrial energy production are not fully understood. We studied plants with reduced expression of CYTC-1, one of two genes encoding the respiratory chain component cytochrome c (CYTc) in Arabidopsis, to understand how mitochondria communicate their status to coordinate metabolism and growth. Plants with CYTc deficiency show decreased mitochondrial membrane potential and lower ATP content, even when carbon sources are present. They also exhibit higher free amino acid content, induced autophagy, and increased resistance to nutritional stress caused by prolonged darkness, similar to plants with triggered starvation signals. CYTc deficiency affects target of rapamycin (TOR)-pathway activation, reducing S6 kinase (S6K) and RPS6A phosphorylation, as well as total S6K protein levels due to increased protein degradation via proteasome and autophagy. TOR overexpression restores growth and other parameters affected in cytc-1 mutants, even if mitochondrial membrane potential and ATP levels remain low. We propose that CYTc-deficient plants coordinate their metabolism and energy availability by reducing TOR-pathway activation as a preventive signal to adjust growth in anticipation of energy exhaustion, thus providing a mechanism by which changes in mitochondrial activity are transduced to the rest of the cell.

Knockdown of double-stranded RNases (dsRNases) enhances oral RNA interference (RNAi) in the corn leafhopper, Dalbulus maidis

The leafhopper Dalbulus maidis is a harmful pest that causes severe damage to corn crops. Conventional chemical pesticides have negative environmental impacts, emphasizing the need for alternative solutions. RNA interference (RNAi) is a more specific and environmentally friendly method for controlling pests and reducing the negative impacts of current pest management practices. Previous studies have shown that orally administered double-stranded RNA (dsRNA) is less effective than injection protocols in silencing genes. This study focuses on identifying and understanding the role of double-stranded ribonucleases (dsRNases) in limiting the efficiency of oral RNAi in D. maidis. Three dsRNases were identified and characterized, with Dmai-dsRNase-2 being highly expressed in the midgut and salivary glands. An ex vivo degradation assay revealed significant nuclease activity, resulting in high instability of dsRNA when exposed to tissue homogenates. Silencing Dmai-dsRNase-2 improved the insects' response to the dsRNA targeting the gene of interest, providing evidence of dsRNases involvement in oral RNAi efficiency. Therefore, administering both dsRNase-specific and target gene-specific-dsRNAs simultaneously is a promising approach to increase the efficiency of oral RNAi and should be considered in future control strategies.

Mitochondrial H2S donor AP39 induces stomatal closure by modulating guard cell mitochondrial activity

Hydrogen sulfide (H2S) is a gaseous signaling molecule involved in numerous physiological processes in plants, including gas exchange with the environment through the regulation of stomatal pore width. Guard cells (GCs) are pairs of specialized epidermal cells that delimit stomatal pores and have a higher mitochondrial density and metabolic activity than their neighboring cells. However, there is no clear evidence on the role of mitochondrial activity in stomatal closure induction. In this work, we showed that the mitochondrial-targeted H2S donor AP39 induces stomatal closure in a dose-dependent manner. Experiments using inhibitors of the mitochondrial electron transport chain (mETC) or insertional mutants in cytochrome c (CYTc) indicated that the activity of mitochondrial CYTc and/or complex IV are required for AP39-dependent stomatal closure. By using fluorescent probes and genetically encoded biosensors we reported that AP39 hyperpolarized the mitochondrial inner potential (Δψm) and increased cytosolic ATP, cytosolic hydrogen peroxide levels, and oxidation of the glutathione pool in GCs. These findings showed that mitochondrial-targeted H2S donors induce stomatal closure, modulate guard cell mETC activity, the cytosolic energetic and oxidative status, pointing to an interplay between mitochondrial H2S, mitochondrial activity, and stomatal closure.

Cytochrome c and the transcription factor ABI4 establish a molecular link between mitochondria and ABA-dependent seed germination

During germination, seed reserves are mobilised to sustain the metabolic and energetic demands of plant growth. Mitochondrial respiration is presumably required to drive germination in several species, but only recently its role in this process has begun to be elucidated. Using Arabidopsis thaliana lines with changes in the levels of the respiratory chain component cytochrome c (CYTc), we investigated the role of this protein in germination and its relationship with hormonal pathways. Cytochrome c deficiency causes delayed seed germination, which correlates with decreased cyanide-sensitive respiration and ATP production at the onset of germination. In addition, CYTc affects the sensitivity of germination to abscisic acid (ABA), which negatively regulates the expression of CYTC-2, one of two CYTc-encoding genes in Arabidopsis. CYTC-2 acts downstream of the transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), which binds to a region of the CYTC-2 promoter required for repression by ABA and regulates its expression. The results show that CYTc is a main player during seed germination through its role in respiratory metabolism and energy production. In addition, the direct regulation of CYTC-2 by ABI4 and its effect on ABA-responsive germination establishes a link between mitochondrial and hormonal functions during this process.

Development of efficient RNAi methods in the corn leafhopper Dalbulus maidis, a promising application for pest control

BACKGROUND

The corn leafhopper Dalbulus maidis is the main vector of important stunting pathogens that affect maize production. Currently, there are no effective methods available to manage this pest without adverse impact on the environment. In this context, genomic-based technologies such as RNA interference (RNAi) provide a more environmentally friendly pest control strategy. Therefore, we aimed to assess the application of RNAi in D. maidis and determine the function of a candidate gene related to insect reproduction and propagation.

RESULTS

We have characterized the core RNAi genes and evaluated the functionality of the RNAi machinery. We assessed the potential of RNAi technology in D. maidis via injection or ingestion of double-stranded RNA (dsRNA) to adult females. We chose Bicaudal C (BicC) as a target gene due to its important role during insect oogenesis. Administration of dsRNA^BicC caused significant reductions in the transcript levels (fold changes up to 170 times) and ovipositions. Phenotypic analysis of the ovaries revealed alterations in oocyte development, providing additional confirmation for our results and supporting the idea that Dmai-BicC is a key player of D. maidis oogenesis.

CONCLUSION

This is, to our knowledge, the first report of efficient RNAi in D. maidis. We believe our findings provide a starting point for future control strategies against one of the most important maize pests in the Americas. © 2022 Society of Chemical Industry.

Breaking boundaries: exploring short- and long-distance mitochondrial signalling in plants

Communication of mitochondria with other cell compartments is essential for the coordination of cellular functions. Mitochondria send retrograde signals through metabolites, redox changes, direct organelle contacts and protein trafficking. Accumulating evidence indicates that, in animal systems, changes in mitochondrial function also trigger responses in other, either neighbouring or distantly located, cells. Although not clearly established, there are indications that this type of communication may also be operative in plants. Grafting experiments suggested that the translocation of entire mitochondria or submitochondrial vesicles between neighbouring cells is possible in plants, as already documented in animals. Changes in mitochondrial function also regulate cell-to-cell communication via plasmodesmata and may be transmitted over long distances through plant hormones acting as mitokines to relay mitochondrial signals to distant tissues. Long-distance movement of transcripts encoding mitochondrial proteins involved in crucial aspects of metabolism and retrograde signalling was also described. Finally, changes in mitochondrial reactive species (ROS) production may affect the 'ROS wave' that triggers systemic acquired acclimation throughout the plant. In this review, we summarise available evidence suggesting that mitochondria establish sophisticated communications not only within the cell but also with neighbouring cells and distant tissues to coordinate plant growth and stress responses in a cell nonautonomous manner.

The tip of the iceberg: emerging roles of TORC1, and its regulatory functions in plant cells

Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in coordinating cell growth with light availability, the diurnal cycle, energy availability, and hormonal pathways. TOR Complex 1 (TORC1) controls cell proliferation, growth, metabolism, and defense in plants. Sugar availability is the main signal for activation of TOR in plants, as it also is in mammals and yeast. Specific regulators of the TOR kinase pathway in plants are inorganic compounds in the form of major nutrients in the soils, and light inputs via their impact on autotrophic metabolism. The lack of TOR is embryo-lethal in plants, whilst dysregulation of TOR signaling causes major alterations in growth and development. TOR exerts control as a regulator of protein translation via the action of proteins such as S6K, RPS6, and TAP46. Phytohormones are central players in the downstream systemic physiological TOR effects. TOR has recently been attributed to have roles in the control of DNA methylation, in the abundance of mRNA splicing variants, and in the variety of regulatory lncRNAs and miRNAs. In this review, we summarize recent discoveries in the plant TOR signaling pathway in the context of our current knowledge of mammalian and yeast cells, and highlight the most important gaps in our understanding of plants that need to be addressed in the future.

Cross-talk between mitochondrial function, growth, and stress signalling pathways in plants

Plant mitochondria harbour complex metabolic routes that are interconnected with those of other cell compartments, and changes in mitochondrial function remotely influence processes in different parts of the cell. This implies the existence of signals that convey information about mitochondrial function to the rest of the cell. Increasing evidence indicates that metabolic and redox signals are important for this process, but changes in ion fluxes, protein relocalization, and physical contacts with other organelles are probably also involved. Besides possible direct effects of these signalling molecules on cellular functions, changes in mitochondrial physiology also affect the activity of different signalling pathways that modulate plant growth and stress responses. As a consequence, mitochondria influence the responses to internal and external factors that modify the activity of these pathways and associated biological processes. Acting through the activity of hormonal signalling pathways, mitochondria may also exert remote control over distant organs or plant tissues. In addition, an intimate cross-talk of mitochondria with energy signalling pathways, such as those represented by TARGET OF RAPAMYCIN and SUCROSE NON-FERMENTING1-RELATED PROTEIN KINASE 1, can be envisaged. This review discusses available evidence on the role of mitochondria in shaping plant growth and stress responses through various signalling pathways.

Cytochrome c Deficiency Differentially Affects the In Vivo Mitochondrial Electron Partitioning and Primary Metabolism Depending on the Photoperiod

Plant respiration provides metabolic flexibility under changing environmental conditions by modulating the activity of the nonphosphorylating alternative pathways from the mitochondrial electron transport chain, which bypass the main energy-producing components of the cytochrome oxidase pathway (COP). While adjustments in leaf primary metabolism induced by changes in day length are well studied, possible differences in the in vivo contribution of the COP and the alternative oxidase pathway (AOP) between different photoperiods remain unknown. In our study, in vivo electron partitioning between AOP and COP and expression analysis of respiratory components, photosynthesis, and the levels of primary metabolites were studied in leaves of wild-type (WT) plants and cytochrome c (CYTc) mutants, with reduced levels of COP components, under short- and long-day photoperiods. Our results clearly show that differences in AOP and COP in vivo activities between WT and cytc mutants depend on the photoperiod likely due to energy and stress signaling constraints. Parallel responses observed between in vivo respiratory activities, TCA cycle intermediates, amino acids, and stress signaling metabolites indicate the coordination of different pathways of primary metabolism to support growth adaptation under different photoperiods.

The sunflower TLDc-containing protein HaOXR2 confers tolerance to oxidative stress and waterlogging when expressed in maize plants

The sunflower (Helianthus annuus L.) genome encodes six proteins containing a TLDc domain, typical of the eukaryotic OXidation Resistance (OXR) protein family. Expression of sunflower HaOXR2 in Arabidopsis generated plants with increased rosette diameter, higher number of leaves and increased seed production. Maize inbred lines expressing HaOXR2 also showed increased total leaf area per plant. In addition, heterologous expression of HaOXR2 induced an increase in the oxidative stress tolerance in Arabidopsis and maize. Maize transgenic plants expressing HaOXR2 experienced less oxidative damage and exhibited increased photosynthetic performance and efficiency than non-transgenic segregant plants after treatment of leaves with the reactive oxygen species generating compound Paraquat. Expression of HaOXR2 in maize also improved tolerance to waterlogging. The number of expanded leaves, aerial biomass, and stem height and cross-section area were less affected by waterlogging in HaOXR2 expressing plants, which also displayed less aerial tissue damage under these conditions. Transgenic plants also showed an increased production of roots, a typical adaptive stress response. The results show the existence of functional conservation of OXR proteins in dicot and monocot plants and indicate that HaOXR2 could be useful to improve plant performance under conditions that increase oxidative stress.

OXR2 Increases Plant Defense against a Hemibiotrophic Pathogen via the Salicylic Acid Pathway

Arabidopsis (Arabidopsis thaliana) OXIDATION RESISTANCE2 (AtOXR2) is a mitochondrial protein belonging to the Oxidation Resistance (OXR) protein family, recently described in plants. We analyzed the impact of AtOXR2 in Arabidopsis defense mechanisms against the hemibiotrophic bacterial pathogen Pseudomonas syringae oxr2 mutant plants are more susceptible to infection by the pathogen and, conversely, plants overexpressing AtOXR2 (oeOXR2 plants) show enhanced disease resistance. Resistance in these plants is accompanied by higher expression of WRKY transcription factors, induction of genes involved in salicylic acid (SA) synthesis, accumulation of free SA, and overall activation of the SA signaling pathway. Accordingly, defense phenotypes are dependent on SA synthesis and SA perception pathways, since they are lost in isochorismate synthase1/salicylic acid induction deficient2 and nonexpressor of pathogenesis-related genes1 (npr1) mutant backgrounds. Overexpression of AtOXR2 leads to faster and stronger oxidative burst in response to the bacterial flagellin peptide flg22 Moreover, AtOXR2 affects the nuclear localization of the transcriptional coactivator NPR1, a master regulator of SA signaling. oeOXR2 plants have increased levels of total glutathione and a more oxidized cytosolic redox cellular environment under normal growth conditions. Therefore, AtOXR2 contributes to establishing plant protection against infection by P. syringae acting on the activity of the SA pathway.

Arabidopsis thaliana SURFEIT1-like genes link mitochondrial function to early plant development and hormonal growth responses

Mutations in SURFEIT1 (SURF1) genes affect cytochrome c oxidase (COX) levels in different prokaryotic and eukaryotic organisms. In this work, we report that Arabidopsis thaliana has two genes that potentially encode SURF1 proteins, as a result of a duplication that took place in Brassicaceae. Both genes encode mitochondrial proteins and mutation in AtSURF1a causes embryonic lethality. Mutation in AtSURF1b, instead, causes defects in hypocotyl elongation under growth-stimulating conditions, such as low light intensity, increased ambient temperature and incubation with glucose. Mutants in AtSURF1b show reduced expression of the auxin reporter DR5:GUS and increased levels of the gibberellin reporter GFP-RGA, suggesting that auxin and gibberellin homeostasis are affected. In agreement, growth defects caused by AtSURF1b mutation can be overcome by treatment with indole-3-acetic acid and gibberellin A₃ , and also by increasing expression of the auxin biosynthesis gene YUC8 or the transcription factor PIF4, which shows lower abundance in AtSURF1b-deficient plants. Mutants in AtSURF1b display lower COX levels, higher alternative oxidase and superoxide levels, and increased expression of genes that respond to mitochondrial dysfunction. Decreased hypocotyl growth and DR5:GUS expression can be reversed by treatment with reduced glutathione, suggesting that redox changes, probably related to mitochondrial dysfunction, are responsible for the effect of AtSURF1b deficiency on hormone responses. The results indicate that changes in AtSURF1b affect mitochondrial function and the production of reactive oxygen species, which, in turn, impinges on a growth regulatory circuit that involves auxin, gibberellins and the transcription factor PIF4.

Arabidopsis SCO Proteins Oppositely Influence Cytochrome c Oxidase Levels and Gene Expression during Salinity Stress

SCO (synthesis of cytochrome c oxidase) proteins are involved in the insertion of copper during the assembly of cytochrome c oxidase (COX), the final enzyme of the mitochondrial respiratory chain. Two SCO proteins, namely, homolog of copper chaperone 1 and 2 (HCC1 and HCC2) are present in seed plants, but HCC2 lacks the residues involved in copper binding, leading to uncertainties about its function. In this study, we performed a transcriptomic and phenotypic analysis of Arabidopsis thaliana plants with reduced expression of HCC1 or HCC2. We observed that a deficiency in HCC1 causes a decrease in the expression of several stress-responsive genes, both under basal growth conditions and after applying a short-term high salinity treatment. In addition, HCC1 deficient plants show a faster decrease in chlorophyll content, photosystem II quantum efficiency and COX levels after salinity stress, as well as a faster increase in alternative oxidase capacity. Notably, HCC2 deficiency causes opposite changes in most of these parameters. Bimolecular fluorescence complementation analysis indicated that both proteins are able to interact. We postulate that HCC1 is a limiting factor for COX assembly during high salinity conditions and that HCC2 probably acts as a negative modulator of HCC1 activity through protein-protein interactions. In addition, a direct or indirect role of HCC1 and HCC2 in the gene expression response to stress is proposed.

The mitochondrial oxidation resistance protein AtOXR2 increases plant biomass and tolerance to oxidative stress

This study demonstrates the existence of the oxidation resistance (OXR) protein family in plants. There are six OXR members in Arabidopsis that contain the highly conserved TLDc domain that is characteristic of this eukaryotic protein family. AtOXR2 is a mitochondrial protein able to alleviate the stress sensitivity of a yeast oxr1 mutant. It was induced by oxidative stress and its overexpression in Arabidopsis (oeOXR2) increased leaf ascorbate, photosynthesis, biomass, and seed production, as well as conferring tolerance to methyl viologen, antimycin A, and high light intensities. The oeOXR2 plants also showed higher ABA content, changes in ABA sensitivity, and modified expression of ABA- and stress-regulated genes. While the oxr2 mutants had a similar shoot phenotype to the wild-type, they exhibited increased sensitivity to stress. We propose that by influencing the levels of reactive oxygen species (ROS), AtOXR2 improves the efficiency of photosynthesis and elicits basal tolerance to environmental challenges that increase oxidative stress, allowing improved plant growth and biomass production.

Assembly of the Complexes of the Oxidative Phosphorylation System in Land Plant Mitochondria

Plant mitochondria play a major role during respiration by producing the ATP required for metabolism and growth. ATP is produced during oxidative phosphorylation (OXPHOS), a metabolic pathway coupling electron transfer with ADP phosphorylation via the formation and release of a proton gradient across the inner mitochondrial membrane. The OXPHOS system is composed of large, multiprotein complexes coordinating metal-containing cofactors for the transfer of electrons. In this review, we summarize the current state of knowledge about assembly of the OXPHOS complexes in land plants. We present the different steps involved in the formation of functional complexes and the regulatory mechanisms controlling the assembly pathways. Because several assembly steps have been found to be ancestral in plants-compared with those described in fungal and animal models-we discuss the evolutionary dynamics that lead to the conservation of ancestral pathways in land plant mitochondria.

The mitochondrial copper chaperone COX19 influences copper and iron homeostasis in arabidopsis

The mitochondrial metallochaperone COX19 influences iron and copper responses highlighting a role of mitochondria in modulating metal homeostasis in Arabidopsis. The mitochondrial copper chaperone COX19 participates in the biogenesis of cytochrome c oxidase (COX) in yeast and humans. In this work, we studied the function of COX19 in Arabidopsis thaliana, using plants with either decreased or increased COX19 levels. A fusion of COX19 to the red fluorescent protein localized to mitochondria in vivo, suggesting that Arabidopsis COX19 is a mitochondrial protein. Silencing of COX19 using an artificial miRNA did not cause changes in COX activity levels or respiration in plants grown under standard conditions. These amiCOX19 plants, however, showed decreased expression of the low-copper responsive miRNA gene MIR398b and an induction of the miR398 target CSD1 relative to wild-type plants. Plants with increased COX19 levels, instead, showed induction of MIR398b and other low-copper responsive genes. In addition, global transcriptional changes in rosettes of amiCOX19 plants resembled those observed under iron deficiency. Phenotypic analysis indicated that the roots of amiCOX19 plants show altered growth responses to copper excess and iron deficiency. COX activity levels and COX-dependent respiration were lower in amiCOX19 plants than in wild-type plants under iron deficiency conditions, suggesting that COX19 function is particularly important for COX assembly under iron deficiency. The results indicate that the mitochondrial copper chaperone COX19 has a role in regulating copper and iron homeostasis and responses in plants.

Interplay between cytochrome c and gibberellins during Arabidopsis vegetative development

We studied the effect of reducing the levels of the mitochondrial electron carrier cytochrome c (CYTc) in Arabidopsis thaliana. Plants with CYTc deficiency have delayed growth and development, and reach flowering several days later than the wild-type but with the same number of leaves. CYTc-deficient plants accumulate starch and glucose during the day, and contain lower levels of active gibberellins (GA) and higher levels of DELLA proteins, involved in GA signaling. GA treatment abolishes the developmental delay and reduces glucose accumulation in CYTc-deficient plants, which also show a lower raise in ATP levels in response to glucose. Treatment of wild-type plants with inhibitors of mitochondrial energy production limits plant growth and increases the levels of DELLA proteins, thus mimicking the effects of CYTc deficiency. In addition, an increase in the amount of CYTc decreases DELLA protein levels and expedites growth, and this depends on active GA synthesis. We conclude that CYTc levels impinge on the activity of the GA pathway, most likely through changes in mitochondrial energy production. In this way, hormone-dependent growth would be coupled to the activity of components of the mitochondrial respiratory chain.

The Complexity of Mitochondrial Complex IV: An Update of Cytochrome c Oxidase Biogenesis in Plants

Mitochondrial respiration is an energy producing process that involves the coordinated action of several protein complexes embedded in the inner membrane to finally produce ATP. Complex IV or Cytochrome c Oxidase (COX) is the last electron acceptor of the respiratory chain, involved in the reduction of O₂ to H₂O. COX is a multimeric complex formed by multiple structural subunits encoded in two different genomes, prosthetic groups (heme a and heme a₃), and metallic centers (Cu_A and Cu_B). Tens of accessory proteins are required for mitochondrial RNA processing, synthesis and delivery of prosthetic groups and metallic centers, and for the final assembly of subunits to build a functional complex. In this review, we perform a comparative analysis of COX composition and biogenesis factors in yeast, mammals and plants. We also describe possible external and internal factors controlling the expression of structural proteins and assembly factors at the transcriptional and post-translational levels, and the effect of deficiencies in different steps of COX biogenesis to infer the role of COX in different aspects of plant development. We conclude that COX assembly in plants has conserved and specific features, probably due to the incorporation of a different set of subunits during evolution.

d-Lactate Dehydrogenase Links Methylglyoxal Degradation and Electron Transport through Cytochrome c

Glycolysis generates methylglyoxal (MGO) as an unavoidable, cytotoxic by-product in plant cells. MGO scavenging is performed by the glyoxalase system, which produces d-lactate as an end product. d-Lactate dehydrogenase (d-LDH) is encoded by a single gene in Arabidopsis (Arabidopsis thaliana; At5g06580). It catalyzes in vitro the oxidation of d-lactate to pyruvate using flavin adenine dinucleotide as a cofactor; knowledge of its function in the context of the plant cell remains sketchy. Blue native-polyacrylamide gel electrophoresis of mitochondrial extracts combined with in gel activity assays using different substrates and tandem mass spectrometry allowed us to definitely show that d-LDH acts specifically on d-lactate, is active as a dimer, and does not associate with respiratory supercomplexes of the inner mitochondrial membrane. The combined use of cytochrome c (CYTc) loss-of-function mutants and respiratory complex III inhibitors showed that CYTc acts as the in vivo electron acceptor of d-LDH. CYTc loss-of-function mutants, as well as the d-LDH mutants, were more sensitive to d-lactate and MGO, indicating that they function in the same pathway. In addition, overexpression of d-LDH and CYTc increased tolerance to d-lactate and MGO Together with fine-localization of d-LDH, the functional interaction with CYTc in vivo strongly suggests that d-lactate oxidation takes place in the mitochondrial intermembrane space, delivering electrons to the respiratory chain through CYTc These results provide a comprehensive picture of the organization and function of d-LDH in the plant cell and exemplify how the plant mitochondrial respiratory chain can act as a multifunctional electron sink for reductant from cytosolic pathways.

Cytochrome c, a hub linking energy, redox, stress and signaling pathways in mitochondria and other cell compartments

Cytochrome c (CYTc) is a soluble redox-active heme protein that transfers electrons from complex III to complex IV in the cyanide-sensitive mitochondrial respiratory pathway. CYTc biogenesis is a complex process that requires multiple steps until the mature active protein is obtained. CYTc levels and activity are finely regulated, revealing the importance of this protein not only as electron carrier but also in many other processes. In this article, we describe the role of CYTc in mitochondrial respiration, from its canonical role as electron carrier for ATP production to its involvement in protein import and the stabilization of respiratory complexes and supercomplexes. In plants, CYTc is connected to the synthesis of the antioxidant ascorbate and the detoxification of toxic compounds. Finally, CYTc is also a multi-functional signaling molecule that influences the balance between life and death, acting in energy provision for cellular functions or triggering programmed cell death. The confluence of several metabolic routes into a single protein that links redox reactions with energy producing pathways seems logical from the point of view of cellular economy, control and organization.

The cytochrome c oxidase biogenesis factor AtCOX17 modulates stress responses in Arabidopsis

COX17 is a soluble protein from the mitochondrial intermembrane space that participates in the transfer of copper for cytochrome c oxidase (COX) assembly in eukaryotic organisms. In this work, we studied the function of both Arabidopsis thaliana AtCOX17 genes using plants with altered expression levels of these genes. Silencing of AtCOX17-1 in a cox17-2 knockout background generates plants with smaller rosettes and decreased expression of genes involved in the response of plants to different stress conditions, including several genes that are induced by mitochondrial dysfunctions. Silencing of either of the AtCOX17 genes does not affect plant development or COX activity but causes a decrease in the response of genes to salt stress. In addition, these plants contain higher reactive oxygen and lipid peroxidation levels after irrigation with high NaCl concentrations and are less sensitive to abscisic acid. In agreement with a role of AtCOX17 in stress and abscisic acid responses, both AtCOX17 genes are induced by several stress conditions, abscisic acid and mutation of the transcription factor ABI4. The results indicate that AtCOX17 is required for optimal expression of a group of stress-responsive genes, probably as a component of signalling pathways that link stress conditions to gene expression responses.

AtCOX10, a protein involved in haem o synthesis during cytochrome c oxidase biogenesis, is essential for plant embryogenesis and modulates the progression of senescence

Cytochrome c oxidase (CcO) biogenesis requires several accessory proteins implicated, among other processes, in copper and haem a insertion. In yeast, the farnesyltransferase Cox10p that catalyses the conversion of haem b to haem o is the limiting factor in haem a biosynthesis and is essential for haem a insertion in CcO. In this work, we characterized AtCOX10, a putative Cox10p homologue from Arabidopsis thaliana. AtCOX10 was localized in mitochondria and was able to restore growth of a yeast Δcox10 null mutant on non-fermentable carbon sources, suggesting that it also participates in haem o synthesis. Plants with T-DNA insertions in the coding region of both copies of AtCOX10 could not be recovered, and heterozygous mutant plants showed seeds with embryos arrested at early developmental stages that lacked CcO activity. Heterozygous mutant plants exhibited lower levels of CcO activity and cyanide-sensitive respiration but normal levels of total respiration at the expense of an increase in alternative respiration. AtCOX10 seems to be implicated in the onset and progression of senescence, since heterozygous mutant plants showed a faster decrease in chlorophyll content and photosynthetic performance than wild-type plants after natural and dark-induced senescence. Furthermore, complementation of mutants by expressing AtCOX10 under its own promoter allowed us to obtain plants with T-DNA insertions in both AtCOX10 copies, which showed phenotypic characteristics comparable to those of wild type. Our results highlight the relevance of haem o synthesis in plants and suggest that this process is a limiting factor that influences CcO activity levels, mitochondrial respiration, and plant senescence.

TCP transcription factors: architectures of plant form

After its initial definition in 1999, the TCP family of transcription factors has become the focus of a multiplicity of studies related with plant development at the cellular, organ, and tissue levels. Evidence has accumulated indicating that TCP transcription factors are the main regulators of plant form and architecture and constitute a tool through which evolution shapes plant diversity. The TCP transcription factors act in a multiplicity of pathways related with cell proliferation and hormone responses. In recent years, the molecular pathways of TCP protein action and biochemical studies on their mode of interaction with DNA have begun to shed light on their mechanism of action. However, the available information is fragmented and a unifying view of TCP protein action is lacking, as well as detailed structural studies of the TCP-DNA complex. Also important, the possible role of TCP proteins as integrators of plant developmental responses to the environment has deserved little attention. In this review, we summarize the current knowledge about the structure and functions of TCP transcription factors and analyze future perspectives for the study of the role of these proteins and their use to modify plant development.

Mitochondria and copper homeostasis in plants

Copper (Cu) and other transition metals are essential for living organisms but also toxic when present in excess. To cope with this apparent paradox, organisms have developed sophisticated mechanisms to acquire, transport and store these metals. Particularly, plant mitochondria require Cu for the assembly and function of cytochrome c oxidase (COX), the terminal enzyme of the respiratory chain. COX assembly is a complex process that requires the action of multiple factors, many of them involved in the delivery and insertion of Cu into the enzyme. In this review, we summarize what is known about the processes involved in Cu delivery to mitochondria and how these processes impact in Cu homeostasis at the cellular level. We also discuss evidence indicating that metallochaperones involved in COX assembly play additional roles in signaling pathways related to changes in Cu and redox homeostasis and the response of plants to stress. We propose that cysteine-rich proteins present in the mitochondrial intermembrane space are excellent candidates as sensors of these changes and transducers of signals originated in the organelle to the rest of the cell.

Coordination of plant mitochondrial biogenesis: keeping pace with cellular requirements

Plant mitochondria are complex organelles that carry out numerous metabolic processes related with the generation of energy for cellular functions and the synthesis and degradation of several compounds. Mitochondria are semiautonomous and dynamic organelles changing in shape, number, and composition depending on tissue or developmental stage. The biogenesis of functional mitochondria requires the coordination of genes present both in the nucleus and the organelle. In addition, due to their central role, all processes held inside mitochondria must be finely coordinated with those in other organelles according to cellular demands. Coordination is achieved by transcriptional control of nuclear genes encoding mitochondrial proteins by specific transcription factors that recognize conserved elements in their promoter regions. In turn, the expression of most of these transcription factors is linked to developmental and environmental cues, according to the availability of nutrients, light-dark cycles, and warning signals generated in response to stress conditions. Among the signals impacting in the expression of nuclear genes, retrograde signals that originate inside mitochondria help to adjust mitochondrial biogenesis to organelle demands. Adding more complexity, several nuclear encoded proteins are dual localized to mitochondria and either chloroplasts or the nucleus. Dual targeting might establish a crosstalk between the nucleus and cell organelles to ensure a fine coordination of cellular activities. In this article, we discuss how the different levels of coordination of mitochondrial biogenesis interconnect to optimize the function of the organelle according to both internal and external demands.

Delta subclass HD-Zip proteins and a B-3 AP2/ERF transcription factor interact with promoter elements required for expression of the Arabidopsis cytochrome c oxidase 5b-1 gene

We have identified transcription factors that interact with a promoter region involved in expression of the Arabidopsis thaliana COX5b-1 gene, which encodes an isoform of the cytochrome c oxidase zinc binding subunit. Elements with the core sequence ATCATT, involved in induction by sugars, are recognized both in vitro and in one-hybrid assays in yeast by HD-Zip proteins from the delta subclass and, though less efficiently, by the trihelix transcription factor GT-3b. DistalB-like elements (CCACTTG), required for induction by abscisic acid (ABA), interact with ESE1, a member of the B-3 subgroup of AP2/ERF transcription factors. The HD-Zip protein Athb-21 and ESE1 are able to interact in yeast two-hybrid assays with the ABA responsive element binding factor AREB2/ABF4, which binds to a G-box absolutely required for expression of the COX5b-1 gene. Overexpression of the identified transcription factors in plants produces an increase in COX5b-1 transcript levels. Moreover, these factors are able to induce the expression of a reporter gene located in plants under the control of the relevant COX5b-1 promoter regions required for expression. Analysis of promoter regions of COX5b genes from different plant species suggests that the identified transcription factors were recruited for the regulation of COX5b gene expression at different stages during the evolution of dicot plants.

Lack of cytochrome c in Arabidopsis decreases stability of Complex IV and modifies redox metabolism without affecting Complexes I and III

We studied the role of cytochrome c (CYTc), which mediates electron transfer between Complexes III and IV, in cellular events related with mitochondrial respiration, plant development and redox homeostasis. We analyzed single and double homozygous mutants in both CYTc-encoding genes from Arabidopsis: CYTC-1 and CYTC-2. While individual mutants were similar to wild-type, knock-out of both genes produced an arrest of embryo development, showing that CYTc function is essential at early stages of plant development. Mutants in which CYTc levels were extremely reduced respective to wild-type had smaller rosettes with a pronounced decrease in parenchymatic cell size and an overall delay in development. Mitochondria from these mutants had lower respiration rates and a relative increase in alternative respiration. Furthermore, the decrease in CYTc severely affected the activity and the amount of Complex IV, without affecting Complexes I and III. Reactive oxygen species levels were reduced in these mutants, which showed induction of genes encoding antioxidant enzymes. Ascorbic acid levels were not affected, suggesting that a small amount of CYTc is enough to support its normal synthesis. We postulate that, in addition to its role as an electron carrier between Complexes III and IV, CYTc influences Complex IV levels in plants, probably reflecting a role of this protein in Complex IV stability. This double function of CYTc most likely explains why it is essential for plant survival.

Plants contain two SCO proteins that are differentially involved in cytochrome c oxidase function and copper and redox homeostasis

Two Arabidopsis thaliana genes (HCC1 and HCC2), resulting from a duplication that took place before the emergence of flowering plants, encode proteins with homology to the SCO proteins involved in copper insertion during cytochrome c oxidase (COX) assembly in other organisms. Heterozygote HCC1 mutant plants produce 25% abnormal seeds with defective embryos arrested at the heart or torpedo stage. These embryos lack COX activity, suggesting that the requirement of HCC1 during the early stages of plant development is related with its COX assembly function. Homozygote HCC2 mutant plants develop normally and do not show changes in COX2 levels. These plants display increased sensitivity of root growth to increased copper and a higher expression of miR398 and other genes that respond to copper limitation, in spite of the fact that they have a higher copper content than the wild type. HCC2 mutant plants also show increased expression of stress-responsive genes. The results suggest that HCC1 is the protein involved in COX biogenesis and that HCC2, that lacks the cysteines and histidine putatively involved in copper binding, functions in copper sensing and redox homeostasis. In addition, plants that overexpress HCC1 have an altered response of root elongation to changes in copper in the growth medium and increased expression of two low-copper-responsive genes, suggesting that HCC1 may also have a role in copper homeostasis.

A segment containing a G-box and an ACGT motif confers differential expression characteristics and responses to the Arabidopsis Cytc-2 gene, encoding an isoform of cytochrome c

Sequences required for the expression of Cytc-2 (At4g10040), one of two cytochrome c genes from Arabidopsis thaliana, were characterized using plants transformed with deleted and mutagenized promoter fragments fused to gus. These studies indicated that a region containing a G-box and an ACGT motif is essential for expression. Mutation of the ACGT motif causes a complete loss of expression, while mutation of the G-box causes decreased expression in aerial parts and abolishes expression in roots and induction by environmental factors. Upstream located site II elements are required for maximal expression, mainly in reproductive tissues, and maximal induction by different factors. One-hybrid screenings allowed the identification of transcription factors from the bZIP and bHLH families that interact mainly with the G-box. Four of these factors were able to bind to the Cytc-2 promoter in vitro and in transactivation assays in Arabidopsis. Analysis of available microarray data indicated that the bZIP transcription factors share expression characteristics with the Cytc-2 gene, suggesting that they act as mediators of its response to tissue-specific, environmental, and metabolic conditions. Site II elements interact with a TCP family protein and may co-ordinate the expression of the Cytc-2 gene with that of other respiratory chain components. A model is proposed for the evolution of the Cytc-2 gene through the incorporation of a segment containing a G-box and an ACGT motif into an ancestral gene that contained site II elements. This may have reduced the importance of site II elements for basal expression and conferred new responses to environmental factors.

Characterization of Arabidopsis thaliana genes encoding functional homologues of the yeast metal chaperone Cox19p, involved in cytochrome c oxidase biogenesis

The Arabidopsis thaliana genome contains two nearly identical genes which encode proteins showing similarity with the yeast metal chaperone Cox19p, involved in cytochrome c oxidase biogenesis. One of these genes (AtCOX19-1) produces two transcript forms that arise from an alternative splicing event and encode proteins with different N-terminal portions. Both AtCOX19 isoforms are imported into mitochondria in vitro and are found attached to the inner membrane facing the intermembrane space. The smaller AtCOX19-1 isoform, but not the larger one, is able to restore growth on non-fermentable carbon sources when expressed in a yeast cox19 null mutant. AtCOX19 transcript levels increase by treatment with copper or compounds that produce reactive oxygen species. Young roots and anthers are highly stained in AtCOX19-1::GUS plants. Expression in leaves is only observed when cuts are produced, suggesting an induction by wounding. Infection of plants with the pathogenic bacterium Pseudomonas syringae pv. tomato also induces AtCOX19 gene expression. The results suggest that AtCOX19 genes encode functional homologues of the yeast metal chaperone. Induction by biotic and abiotic stress factors may indicate a relevant role of this protein in the biogenesis of cytochrome c oxidase to replace damaged forms of the enzyme or a more general role in the response of plants to stress.

Transcriptional coordination of the biogenesis of the oxidative phosphorylation machinery in plants

Publicly available microarray experiments were used to analyze Arabidopsis thaliana genes whose expression is correlated with that of nuclear genes encoding components of the oxidative phosphorylation machinery (OxPhos genes). This analysis indicated the existence of coordination in the expression of genes encoding components of the five respiratory complexes. For these genes, preferential expression was observed in anthers and roots, especially in the elongation zone, while reduced or very low relative expression was evident in leaves and mature pollen grains. A global induction of OxPhos genes by carbohydrates, photo-destruction of chloroplasts, inhibition of cellulose synthesis, release from dormancy and germination, among other conditions, was also observed. Cluster analysis of the response of Arabidopsis genes to a set of 15 treatments allowed the identification of DNA motifs, known as site II, that are frequently present in the upstream regions of genes with responses like those of OxPhos genes. Mutagenic analysis of site II motifs in several genes encoding respiratory chain components showed that they actively participate in transcription of these genes. We conclude that an important number of nuclear genes encoding components of the five respiratory complexes show coordinated expression under various circumstances, and that site II elements and the putative proteins that interact with them are, together with as yet unidentified factors, important actors in this coordinated response.

Differential expression of the Arabidopsis cytochrome c genes Cytc-1 and Cytc-2. Evidence for the involvement of TCP-domain protein-binding elements in anther- and meristem-specific expression of the Cytc-1 gene

The promoters of the Arabidopsis (Arabidopsis thaliana) cytochrome c genes, Cytc-1 and Cytc-2, were analyzed using plants transformed with fusions to the beta-glucuronidase coding sequence. Histochemical staining of plants indicated that the Cytc-1 promoter directs preferential expression in root and shoot meristems and in anthers. In turn, plants transformed with the Cytc-2 promoter fusions showed preferential expression in vascular tissues of cotyledons, leaves, roots, and hypocotyls, and also in anthers. Quantitative measurements in extracts prepared from different organs suggested that expression of Cytc-1 is higher in flowers, while that of Cytc-2 is higher in leaves. The analysis of a set of deletions and site-directed mutants of the Cytc-1 promoter indicated that a segment located between -147 and -156 from the translation start site is required for expression and that site II elements (TGGGCC/T) located in this region, coupled with a downstream internal telomeric repeat (AAACCCTAA), are responsible for the expression pattern of this gene. Proteins present in cauliflower nuclear extracts, as well as a recombinant protein from the TCP-domain family, were able to specifically bind to the region required for expression. We propose that expression of the Cytc-1 gene is linked to cell proliferation through the elements described above. The fact that closely located site II motifs are present in similar locations in several genes encoding proteins involved in cytochrome c-dependent respiration suggests that these elements may be the target of factors that coordinate the expression of nuclear genes encoding components of this part of the mitochondrial respiratory chain.

Engineering the Loop Region of a Homeodomain-Leucine Zipper Protein Promotes Efficient Binding to a Monomeric DNA Binding Site

Plant homeodomain-leucine zipper (HD-Zip) proteins, unlike many animal homeodomains (HDs), are unable to bind DNA as monomers. To investigate the molecular basis of their different behavior, we have constructed chimeras between the HD of the sunflower HD-Zip protein Hahb-4 and that of Drosophila engrailed (EN). Analysis of the interaction of these proteins with the pseudopalindromic Hahb-4 binding site and the monomeric EN binding site suggests that the loop located between helix I and helix II (amino acids 21-28) of EN is enough to confer efficient DNA binding activity to the Hahb-4 HD. Accordingly, the combined mutation of residues 24 and 25 of Hahb-4 to those present in EN (S24R/R25Y) originated an HD able to interact with the EN binding site, while single mutations were ineffective. We have also determined that a protein with the leucine zipper and helix III of Hahb-4 fused to the rest of the EN HD binds to the Hahb-4 pseudopalindomic binding site with increased affinity and shows extended contacts with DNA respective to Hahb-4. We conclude that the loop located between helix I and helix II of the HD must be regarded as one of the segments that contribute to the present-day diversity in the properties of different HDs.

The promoter of the Arabidopsis nuclear gene COX5b-1, encoding subunit 5b of the mitochondrial cytochrome c oxidase, directs tissue-specific expression by a combination of positive and negative regulatory elements

In the present work, the promoter of the Arabidopsis thaliana nuclear gene COX5b-1, encoding subunit 5b of the mitochondrial cytochrome c oxidase, has been analysed. For this purpose, plants, stably transformed with different promoter fragments fused to the beta-glucuronidase reporter gene, have been obtained. Histochemical staining indicated that the COX5b-1 promoter directs expression in meristems and in vascular tissues of cotyledons, roots, and hypocotyls, as well as in anthers and pollen and the central leaf vein. Quantitative measurements in extracts prepared from different organs suggested that expression is higher in roots. The analysis of progressive upstream deletions of the promoter suggested the presence of negative regulatory elements, preferentially active in leaves, between nucleotides -609 and -387 from the translation start site. A further deletion down to nucleotide -195 completely abolished expression. The inclusion of sucrose or the cytokinin 6-benzylaminopurine in the culture medium induced COX5b-1 promoter-dependent beta-glucuronidase expression. This induction was observed with all constructs that produced beta-glucuronidase activity. Putative regulatory elements involved in the regulation of other genes were detected in the promoter fragment required for expression. A detailed analysis of these elements will help to elucidate the molecular mechanisms that participate in the expression of this and, possibly, other components of the cytochrome c-dependent respiratory pathway.

Purification of the 67-kDa lectin-like glycoprotein of Trypanosoma cruzi, LLGP-67, and its evaluation as a relevant antigen for the diagnosis of human infection

In the present work we propose a simple method for affinity purification of the 67-kDa lectin-like glycoprotein (LLGP-67) from Trypanosoma cruzi, the causative agent of Chagas' disease. The LLGP-67, which presents galactose binding activity and participates in the host cell recognition process, was previously purified by methods based on its interaction with galactose residues on erythrocytic membranes. We describe herein results showing that this protein can be purified from T. cruzi in a direct way using non-derivatized agarose as a chromatographic ligand. We also demonstrate the relevance of LLGP-67 as an antigen for human diagnosis of chagasic infection. Sensitivity and specificity for this antigen were calculated, being 98 and 98.11% respectively.