A role for mitochondria in the establishment and maintenance of the maize root quiescent center

Enhanced meristem development, tolerance to oxidative stress and hyposensitivity to nitric oxide in the hypermorphic vq10-H mutant in AtVQ10 gene

Nitric oxide (NO) and reactive oxygen are common factors in multiple plant responses to stress, and their involvement in hypoxia-triggered responses is key to ensure growth under adverse environmental conditions. Here, we analyse the regulatory functions exerted by hypoxia-, NO- and oxidative stress-inducible Arabidopsis gene coding for the VQ motif-containing protein 10 (VQ10). A hypermorphic vq10-H mutant allowed identifying VQ10-exerted regulation on root and shoot development as well as its role in regulating responses to NO and oxidative stress. Enhanced VQ10 expression in vq10-H plants led to enhanced elongation of the primary root, and increased root cell division and meristem size during early postgermination development. In shoots, VQ10 activation of cell division was counteracted by WRKY33-exerted repression, thus leading to a dwarf bushy phenotype in plants with enhanced VQ10 expression in a wrky33 knock-out background. Low number of differentially expressed genes were identified when vq10-H versus Col-0 plants were compared either under normoxia or hypoxia. vq10-H and VQ10ox plants displayed less tolerance to submergence but, in turn, were more tolerant to oxidative stress and less sensitive to NO than wild-type plants. VQ10 could be a node integrating redox-related regulation on development and stress responses.

Preserving root stem cell functionality under low oxygen stress: the role of nitric oxide and phytoglobins

MAIN CONCLUSION

The preservation of quiescent center stem cell integrity in hypoxic roots by phytoglobins is exercised through their ability to scavenge nitric oxide and attenuate its effects on auxin transport and cell degradation. Under low oxygen stress, the retention or induction of phytoglobin expression maintains cell viability while loss or lack of induction of phytoglobin leads to cell degradation. Plants have evolved unique attributes to ensure survival in the environment in which they must exist. Common among the attributes is the ability to maintain stem cells in a quiescent (or low proliferation) state in unfriendly environments. From the seed embryo to meristematic regions of the plant, quiescent stem cells exist to regenerate the organism when environmental conditions are suitable to allow plant survival. Frequently, plants dispose of mature cells or organs in the process of acclimating to the stresses to ensure survival of meristems, the stem cells of which are capable of regenerating cells and organs that have been sacrificed, a feature not generally available to mammals. Most of the research on plant stress responses has dealt with how mature cells respond because of the difficulty of specifically examining plant meristem responses to stress. This raises the question as to whether quiescent stem cells behave in a similar fashion to mature cells in their response to stress and what factors within these critical cells determine whether they survive or degrade when exposed to environmental stress. This review attempts to examine this question with respect to the quiescent center (QC) stem cells of the root apical meristem. Emphasis is put on how varying levels of nitric oxide, influenced by the expression of phytoglobins, affect QC response to hypoxic stress.

Plant stem cells under low oxygen: metabolic rewiring by phytoglobin underlies stem cell functionality

Root growth in maize (Zea mays L.) is regulated by the activity of the quiescent center (QC) stem cells located within the root apical meristem. Here, we show that despite being highly hypoxic under normal oxygen tension, QC stem cells are vulnerable to hypoxic stress, which causes their degradation with subsequent inhibition of root growth. Under low oxygen, QC stem cells became depleted of starch and soluble sugars and exhibited reliance on glycolytic fermentation with the impairment of the TCA cycle through the depressed activity of several enzymes, including pyruvate dehydrogenase (PDH). This finding suggests that carbohydrate delivery from the shoot might be insufficient to meet the metabolic demand of QC stem cells during stress. Some metabolic changes characteristic of the hypoxic response in mature root cells were not observed in the QC. Hypoxia-responsive genes, such as PYRUVATE DECARBOXYLASE (PDC) and ALCOHOL DEHYDROGENASE (ADH), were not activated in response to hypoxia, despite an increase in ADH activity. Increases in phosphoenolpyruvate (PEP) with little change in steady-state levels of succinate were also atypical responses to low-oxygen tensions. Overexpression of PHYTOGLOBIN 1 (ZmPgb1.1) preserved the functionality of the QC stem cells during stress. The QC stem cell preservation was underpinned by extensive metabolic rewiring centered around activation of the TCA cycle and retention of carbohydrate storage products, denoting a more efficient energy production and diminished demand for carbohydrates under conditions where nutrient transport may be limiting. Overall, this study provides an overview of metabolic responses occurring in plant stem cells during oxygen deficiency.

The Aqueous Extract of Brassica oleracea L. Exerts Phytotoxicity by Modulating H2O2 and O2- Levels, Antioxidant Enzyme Activity and Phytohormone Levels

Allelopathic interactions between plants serve as powerful tools for weed control. Despite the increasing understanding of the allelopathic mechanisms between different plant species, the inhibitory effects of B. oleracea on weed growth remain poorly understood. In this study, we conducted experiments to demonstrate that B. oleracea extract can suppress the germination of Panicum miliaceum L.varruderale Kit. seeds as well as of the roots, shoots and hypocotyl elongation of P. miliaceum seedlings. Furthermore, we observed that B. oleracea extract reduced the levels of hydrogen peroxide and superoxide anion in the roots while increasing the activities of catalase and ascorbate peroxidase. In the shoots, B. oleracea extract enhanced the activities of superoxide dismutase and peroxidase. Moreover, the use of the extract led to an increase in the content of phytohormones (indole-3-acetic acid, indole-3-acetaldehyde, methyl indole-3-acetate, N6-isoPentenyladenosine, dihydrozeatin-7-glucoside, abscisic acid and abscisic acid glucose ester) in P. miliaceum seedlings. Interestingly, the aqueous extract contained auxins and their analogs, which inhibited the germination and growth of P. miliaceum. This may contribute to the mechanism of the B. oleracea-extract-induced suppression of P. miliaceum growth.

Metabolic regulation of quiescence in plants

Quiescence is a crucial survival attribute in which cell division is repressed in a reversible manner. Although quiescence has long been viewed as an inactive state, recent studies have shown that it is an actively monitored process that is influenced by environmental stimuli. Here, we provide a perspective of the quiescent state and discuss how this process is tuned by energy, nutrient and oxygen status, and the pathways that sense and transmit these signals. We not only highlight the governance of canonical regulators and signalling mechanisms that respond to changes in nutrient and energy status, but also consider the central significance of mitochondrial functions and cues as key regulators of nuclear gene expression. Furthermore, we discuss how reactive oxygen species and the associated redox processes, which are intrinsically linked to energy carbohydrate metabolism, also play a key role in the orchestration of quiescence.

Oxygen uptake rates have contrasting responses to temperature in the root meristem and elongation zone

Growing at either 15 or 25°C, roots of Arabidopsis thaliana, Columbia accession, produce cells at the same rate and have growth zones of the same length. To determine whether this constancy is related to energetics, we measured oxygen uptake by means of a vibrating oxygen-selective electrode. Concomitantly, the spatial distribution of elongation was measured kinematically, delineating meristem and elongation zone. All seedlings were germinated, grown, and measured at a given temperature (15 or 25°C). Columbia was compared to lines where cell production rate roughly doubles between 15 and 25°C: Landsberg and two Columbia mutants, er-105 and ahk3-3. For all genotypes and temperatures, oxygen uptake rate at any position was highest at the root cap, where mitochondrial density was maximal, based on the fluorescence of a reporter. Uptake rate declined through the meristem to plateau within the elongation zone. For oxygen uptake rate integrated over a zone, the meristem had steady-state Q₁₀ values ranging from 0.7 to 2.1; by contrast, the elongation zone had values ranging from 2.6 to 3.3, implying that this zone exerts a greater respiratory demand. These results highlight a substantial energy consumption by the root cap, perhaps helpful for maintaining hypoxia in stem cells, and suggest that rapid elongation is metabolically more costly than is cell division.

Stress effects on the reactive oxygen species-dependent regulation of plant growth and development

Plant growth is mediated by cell proliferation and expansion. Both processes are controlled by a network of endogenous factors such as phytohormones, reactive oxygen species (ROS), sugars, and other signals, which influence gene expression and post-translational regulation of proteins. Stress resilience requires rapid and appropriate responses in plant growth and development as well as defence. Regulation of ROS accumulation in different cellular compartments influences growth responses to abiotic and biotic stresses. While ROS are essential for growth, they are also implicated in the stress-induced cessation of growth and, in some cases, programmed cell death. It is widely accepted that redox post-translational modifications of key proteins determine the growth changes and cell fate responses to stress, but the molecular pathways and factors involved remain poorly characterized. Here we discuss ROS as a signalling molecule, the mechanisms of ROS-dependent regulation that influence protein-protein interactions, protein function, and turnover, together with the relocation of key proteins to different intracellular compartments in a manner that can alter cell fate. Understanding how the redox interactome responds to stress-induced increases in ROS may provide a road map to tailoring the dynamic ROS interactions that determine growth and cell fate in order to enhance stress resilience.

The microRNA476a-RFL module regulates adventitious root formation through a mitochondria-dependent pathway in Populus

For woody plants, clonal propagation efficiency is largely determined by adventitious root (AR) formation at the bases of stem cuttings. However, our understanding of the molecular mechanisms contributing to AR morphogenesis in trees remains limited, despite the importance of vegetative propagation, currently the most common practice for tree breeding and commercialization. Here, we identified Populus-specific miR476a as a regulator of wound-induced adventitious rooting that acts by orchestrating mitochondrial homeostasis. MiR476a exhibited inducible expression during AR formation and directly targeted several Restorer of Fertility like (RFL) genes encoding mitochondrion-localized pentatricopeptide repeat proteins. Genetic modification of miR476a-RFL expression revealed that miR476a/RFL-mediated dynamic regulation of mitochondrial homeostasis influences AR formation in poplar. Mitochondrial perturbation via exogenous application of a chemical inhibitor indicated that miR476a/RFL-directed AR formation depends on mitochondrial regulation that acts via auxin signaling. Our results thus establish a microRNA-directed mitochondrion-auxin signaling cascade required for AR development, providing insights into the role of mitochondrial regulation in the developmental plasticity of plants.

Stem cell fate in hypoxic root apical meristems is influenced by phytoglobin expression

Root survival to flooding-induced hypoxic stress is dependent upon maintaining the functionality of the root apical meristem quiescent center (QC), a process that is governed by the basipetal flow of auxin leading to the formation of an auxin maximum, which is needed for the establishment of a highly oxidized environment specifying the QC niche. Perturbations in auxin flow and distribution along the root profile occurring during hypoxia can shift the redox state of the QC towards a more reduced environment, leading to the activation of the QC, degradation of the meristem, and root abortion. The maize phytoglobin gene ZmPgb1.1 is involved in minimizing these damaging effects during hypoxia in processes that result in sustaining the PIN-mediated auxin maximum and an oxidized environment in the QC. The oxidized environment is accomplished by maintaining the activity of redox enzymes oxidizing ascorbate and glutathione. These events, compromised in QCs suppressing ZmPgb1.1, ensure the functionality of the QC and root meristems under conditions of low oxygen, resulting in stable root performance.

Identification and characterization of the TCA cycle genes in maize

BACKGROUND

The tricarboxylic acid (TCA) cycle is crucial for cellular energy metabolism and carbon skeleton supply. However, the detailed functions of the maize TCA cycle genes remain unclear.

RESULTS

In this study, 91 TCA genes were identified in maize by a homology search, and they were distributed on 10 chromosomes and 1 contig. Phylogenetic results showed that almost all maize TCA genes could be classified into eight major clades according to their enzyme families. Sequence alignment revealed that several genes in the same subunit shared high protein sequence similarity. The results of cis-acting element analysis suggested that several TCA genes might be involved in signal transduction and plant growth. Expression profile analysis showed that many maize TCA cycle genes were expressed in specific tissues, and replicate genes always shared similar expression patterns. Moreover, qPCR analysis revealed that some TCA genes were highly expressed in the anthers at the microspore meiosis phase. In addition, we predicted the potential interaction networks among the maize TCA genes. Next, we cloned five TCA genes located on different TCA enzyme complexes, Zm00001d008244 (isocitrate dehydrogenase, IDH), Zm00001d017258 (succinyl-CoA synthetase, SCoAL), Zm00001d025258 (α-ketoglutarate dehydrogenase, αKGDH), Zm00001d027558 (aconitase, ACO) and Zm00001d044042 (malate dehydrogenase, MDH). Confocal observation showed that their protein products were mainly localized to the mitochondria; however, Zm00001d025258 and Zm00001d027558 were also distributed in the nucleus, and Zm00001d017258 and Zm00001d044042 were also located in other unknown positions in the cytoplasm. Through the bimolecular fluorescent complimentary (BiFC) method, it was determined that Zm00001d027558 and Zm00001d044042 could form homologous dimers, and both homologous dimers were mainly distributed in the mitochondria. However, no heterodimers were detected between these five genes. Finally, Arabidopsis lines overexpressing the above five genes were constructed, and those transgenic lines exhibited altered primary root length, salt tolerance, and fertility.

CONCLUSION

Sequence compositions, duplication patterns, phylogenetic relationships, cis-elements, expression patterns, and interaction networks were investigated for all maize TCA cycle genes. Five maize TCA genes were overexpressed in Arabidopsis, and they could alter primary root length, salt tolerance, and fertility. In conclusion, our findings may help to reveal the molecular function of the TCA genes in maize.

OPENER Is a Nuclear Envelope and Mitochondria Localized Protein Required for Cell Cycle Progression in Arabidopsis

Currently one-third of the proteins encoded by the Arabidopsis (Arabidopsis thaliana) genome are of unknown function. Some of these unknown proteins are likely to be involved in uncharacterized vital biological processes. Evolutionarily conserved single copy genes in flowering plants have been shown to be enriched in essential housekeeping functions. This together with publicly available gene expression data allows for a focused search for uncharacterized essential genes. Here we identify an essential single copy gene called OPENER (OPNR) in Arabidopsis. We show that OPNR is predominantly expressed in actively dividing cells and performs essential functions in seed development and root meristem maintenance. Cell cycle tracking using 5-ethynyl-2'-deoxyuridine staining and fluorescent cell cycle markers together with the increased size of nucleolus and nucleus in opnr mutants indicate that OPNR is required for cell cycle progression through the S or G2 phases. Intriguingly, OPNR localizes to the nuclear envelope and mitochondria. Furthermore, the nuclear envelope localization of OPNR is dependent on its interaction with nuclear inner membrane Sad1/UNC-84 (SUN) domain proteins SUN1 and SUN2. Taken together our results open a line of investigation into an evolutionarily conserved essential cellular process occurring in both the nuclear envelopes and mitochondria of dividing cells.

Redox Balance-DDR-miRNA Triangle: Relevance in Genome Stability and Stress Responses in Plants

Plants are continuously faced with complex environmental conditions which can affect the oxidative metabolism and photosynthetic efficiency, thus leading to the over-production of reactive oxygen species (ROS). Over a certain threshold, ROS can damage DNA. DNA damage, unless repaired, can affect genome stability, thus interfering with cell survival and severely reducing crop productivity. A complex network of pathways involved in DNA damage response (DDR) needs to be activated in order to maintain genome integrity. The expression of specific genes belonging to these pathways can be used as indicators of oxidative DNA damage and effective DNA repair in plants subjected to stress conditions. Managing ROS levels by modulating their production and scavenging systems shifts the role of these compounds from toxic molecules to key messengers involved in plant tolerance acquisition. Oxidative and anti-oxidative signals normally move among the different cell compartments, including the nucleus, cytosol, and organelles. Nuclei are dynamically equipped with different redox systems, such as glutathione (GSH), thiol reductases, and redox regulated transcription factors (TFs). The nuclear redox network participates in the regulation of the DNA metabolism, in terms of transcriptional events, replication, and repair mechanisms. This mainly occurs through redox-dependent regulatory mechanisms comprising redox buffering and post-translational modifications, such as the thiol-disulphide switch, glutathionylation, and S-nitrosylation. The regulatory role of microRNAs (miRNAs) is also emerging for the maintenance of genome stability and the modulation of antioxidative machinery under adverse environmental conditions. In fact, redox systems and DDR pathways can be controlled at a post-transcriptional level by miRNAs. This review reports on the interconnections between the DDR pathways and redox balancing systems. It presents a new dynamic picture by taking into account the shared regulatory mechanism mediated by miRNAs in plant defense responses to stress.

Analysis of Redox Relationships in the Plant Cell Cycle: Determination of Ascorbate, Glutathione, and Poly(ADPribose)polymerase (PARP) in Plant Cell Cultures

Reactive oxygen species (ROS) and low molecular weight antioxidants, such as glutathione and ascorbate, are powerful signalling molecules that participate in the control of plant growth and development, and modulate progression through the mitotic cell cycle. Enhanced ROS accumulation or low levels of ascorbate or glutathione cause the cell cycle to arrest and halt progression especially through the G1 checkpoint. Plant cell suspension cultures have proved to be particularly useful tools for the study of cell cycle regulation. Here we provide effective and accurate methods for the measurement of changes in the cellular ascorbate and glutathione pools and the activities of related enzymes such poly(ADP-ribose)polymerase (PARP) during mitosis and cell expansion, particularly in cell suspension cultures. These methods can be used in studies seeking to improve current understanding of the roles of redox controls on cell division and cell expansion.

Impairment of Meristem Proliferation in Plants Lacking the Mitochondrial Protease AtFTSH4

Shoot and root apical meristems (SAM and RAM, respectively) are crucial to provide cells for growth and organogenesis and therefore need to be maintained throughout the life of a plant. However, plants lacking the mitochondrial protease AtFTSH4 exhibit an intriguing phenotype of precocious cessation of growth at both the shoot and root apices when grown at elevated temperatures. This is due to the accumulation of internal oxidative stress and progressive mitochondria dysfunction. To explore the impacts of the internal oxidative stress on SAM and RAM functioning, we study the expression of selected meristem-specific (STM, CLV3, WOX5) and cell cycle-related (e.g., CYCB1, CYCD3;1) genes at the level of the promoter activity and/or transcript abundance in wild-type and loss-of-function ftsh4-1 mutant plants grown at 30 °C. In addition, we monitor cell cycle progression directly in apical meristems and analyze the responsiveness of SAM and RAM to plant hormones. We show that growth arrest in the ftsh4-1 mutant is caused by cell cycle dysregulation in addition to the loss of stem cell identity. Both the SAM and RAM gradually lose their proliferative activity, but with different timing relative to CYCB1 transcriptional activity (a marker of G2-M transition), which cannot be compensated by exogenous hormones.

Learning To Breathe: Developmental Phase Transitions in Oxygen Status

Plants are developmentally disposed to significant changes in oxygen availability, but our understanding of the importance of hypoxia is almost entirely limited to stress biology. Differential patterns of the abundance of oxygen, nitric oxide (^•NO), and reactive oxygen species (ROS), as well as of redox potential, occur in organs and meristems, and examples are emerging in the literature of mechanistic relationships of these to development. We describe here the convergence of these cues in meristematic and reproductive tissues, and discuss the evidence for regulated hypoxic niches within which oxygen-, ROS-, ^•NO-, and redox-dependent signalling curate developmental transitions in plants.

The role of mitochondria in plant development and stress tolerance

Eukaryotic cells require orchestrated communication between nuclear and organellar genomes, perturbations in which are linked to stress response and disease in both animals and plants. In addition to mitochondria, which are found across eukaryotes, plant cells contain a second organelle, the plastid. Signaling both among the organelles (cytoplasmic) and between the cytoplasm and the nucleus (i.e. nuclear-cytoplasmic interactions (NCI)) is essential for proper cellular function. A deeper understanding of NCI and its impact on development, stress response, and long-term health is needed in both animal and plant systems. Here we focus on the role of plant mitochondria in development and stress response. We compare and contrast features of plant and animal mitochondrial genomes (mtDNA), particularly highlighting the large and highly dynamic nature of plant mtDNA. Plant-based tools are powerful, yet underutilized, resources for enhancing our fundamental understanding of NCI. These tools also have great potential for improving crop production. Across taxa, mitochondria are most abundant in cells that have high energy or nutrient demands as well as at key developmental time points. Although plant mitochondria act as integrators of signals involved in both development and stress response pathways, little is known about plant mtDNA diversity and its impact on these processes. In humans, there are strong correlations between particular mitotypes (and mtDNA mutations) and developmental differences (or disease). We propose that future work in plants should focus on defining mitotypes more carefully and investigating their functional implications as well as improving techniques to facilitate this research.

The mitochondrial protease AtFTSH4 safeguards Arabidopsis shoot apical meristem function

The shoot apical meristem (SAM) ensures continuous plant growth and organogenesis. In LD 30 °C, plants lacking AtFTSH4, an ATP-dependent mitochondrial protease that counteracts accumulation of internal oxidative stress, exhibit a puzzling phenotype of premature SAM termination. We aimed to elucidate the underlying cellular and molecular processes that link AtFTSH4 with SAM arrest. We studied AtFTSH4 expression, internal oxidative stress accumulation, and SAM morphology. Directly in the SAM we analysed H₂O₂ accumulation, mitochondria behaviour, and identity of stem cells using WUS/CLV3 expression. AtFTSH4 was expressed in proliferating tissues, particularly during the reproductive phase. In the mutant, SAM, in which internal oxidative stress accumulates predominantly at 30 °C, lost its meristematic fate. This process was progressive and stage-specific. Premature meristem termination was associated with an expansion in SAM area, where mitochondria lost their functionality. All these effects destabilised the identity of the stem cells. SAM termination in ftsh4 mutants is caused both by internal oxidative stress accumulation with time/age and by the tissue-specific role of AtFTSH4 around the flowering transition. Maintaining mitochondria functionality within the SAM, dependent on AtFTSH4, is vital to preserving stem cell activity throughout development.

Nitric Oxide-Mediated Maize Root Apex Responses to Nitrate are Regulated by Auxin and Strigolactones

Nitrate (NO3 (-)) is a key element for crop production but its levels in agricultural soils are limited. Plants have developed mechanisms to cope with these NO3 (-) fluctuations based on sensing nitrate at the root apex. Particularly, the transition zone (TZ) of root apex has been suggested as a signaling-response zone. This study dissects cellular and molecular mechanisms underlying NO3 (-) resupply effects on primary root (PR) growth in maize, confirming nitric oxide (NO) as a putative modulator. Nitrate restoration induced PR elongation within the first 2 h, corresponding to a stimulation of cell elongation at the basal border of the TZ. Xyloglucans (XGs) immunolocalization together with Brefeldin A applications demonstrated that nitrate resupply induces XG accumulation. This effect was blocked by cPTIO (NO scavenger). Transcriptional analysis of ZmXET1 confirmed the stimulatory effect of nitrate on XGs accumulation in cells of the TZ. Immunolocalization analyses revealed a positive effect of nitrate resupply on auxin and PIN1 accumulation, but a transcriptional regulation of auxin biosynthesis/transport/signaling genes was excluded. Short-term nitrate treatment repressed the transcription of genes involved in strigolactones (SLs) biosynthesis and transport, mainly in the TZ. Enhancement of carotenoid cleavage dioxygenases (CCDs) transcription in presence of cPTIO indicated endogenous NO as a negative modulator of CCDs activity. Finally, treatment with the SLs-biosynthesis inhibitor (TIS108) restored the root growth in the nitrate-starved seedlings. Present report suggests that the NO-mediated root apex responses to nitrate are accomplished in cells of the TZ via integrative actions of auxin, NO and SLs.

Glutathione--linking cell proliferation to oxidative stress

SIGNIFICANCE

The multifaceted functions of reduced glutathione (gamma-glutamyl-cysteinyl-glycine; GSH) continue to fascinate plants and animal scientists, not least because of the dynamic relationships between GSH and reactive oxygen species (ROS) that underpin reduction/oxidation (redox) regulation and signalling. Here we consider the respective roles of ROS and GSH in the regulation of plant growth, with a particular focus on regulation of the plant cell cycle. Glutathione is discussed not only as a crucial low molecular weight redox buffer that shields nuclear processes against oxidative challenge but also a flexible regulator of genetic and epigenetic functions.

RECENT ADVANCES

The intracellular compartmentalization of GSH during the cell cycle is remarkably consistent in plants and animals. Moreover, measurements of in vivo glutathione redox potentials reveal that the cellular environment is much more reducing than predicted from GSH/GSSG ratios measured in tissue extracts. The redox potential of the cytosol and nuclei of non-dividing plant cells is about -300 mV. This relatively low redox potential maintained even in cells experiencing oxidative stress by a number of mechanisms including vacuolar sequestration of GSSG. We propose that regulated ROS production linked to glutathione-mediated signalling events are the hallmark of viable cells within a changing and challenging environment.

CRITICAL ISSUES

The concept that the cell cycle in animals is subject to redox controls is well established but little is known about how ROS and GSH regulate this process in plants. However, it is increasingly likely that redox controls exist in plants, although possibly through different pathways. Moreover, redox-regulated proteins that function in cell cycle checkpoints remain to be identified in plants. While GSH-responsive genes have now been identified, the mechanisms that mediate and regulate protein glutathionylation in plants remain poorly defined.

FUTURE DIRECTIONS

The nuclear GSH pool provides an appropriate redox environment for essential nuclear functions. Future work will focus on how this essential thiol interacts with the nuclear thioredoxin system and nitric oxide to regulate genetic and epigenetic mechanisms. The characterization of redox-regulated cell cycle proteins in plants, and the elucidation of mechanisms that facilitate GSH accumulation in the nucleus are keep steps to unravelling the complexities of nuclear redox controls.

Implications of mutation of organelle genomes for organelle function and evolution

Organelle genomes undergo more variation, including that resulting from damage, than eukaryotic nuclear genomes, or bacterial genomes, under the same conditions. Recent advances in characterizing the changes to genomes of chloroplasts and mitochondria of Zea mays should, when applied more widely, help our understanding of how damage to organelle genomes relates to how organelle function is maintained through the life of individuals and in succeeding generations. Understanding of the degree of variation in the changes to organelle DNA and its repair among photosynthetic organisms might help to explain the variations in the rate of nucleotide substitution among organelle genomes. Further studies of organelle DNA variation, including that due to damage and its repair might also help us to understand why the extent of DNA turnover in the organelles is so much greater than that in their bacterial (cyanobacteria for chloroplasts, proteobacteria for mitochondria) relatives with similar rates of production of DNA-damaging reactive oxygen species. Finally, from the available data, even the longest-lived organelle-encoded proteins, and the RNAs needed for their synthesis, are unlikely to maintain organelle function for much more than a week after the complete loss of organelle DNA.

Low concentrations of the toxin ophiobolin A lead to an arrest of the cell cycle and alter the intracellular partitioning of glutathione between the nuclei and cytoplasm

Ophiobolin A, a tetracyclic sesterpenoid produced by phytopathogenic fungi, is responsible for catastrophic losses in crop yield but its mechanism of action is not understood. The effects of ophiobolin A were therefore investigated on the growth and redox metabolism of Tobacco Bright Yellow-2 (TBY-2) cell cultures by applying concentrations of the toxin that did not promote cell death. At concentrations between 2 and 5 μM, ophiobolin A inhibited growth and proliferation of the TBY-2 cells, which remained viable. Microscopic and cytofluorimetric analyses showed that ophiobolin A treatment caused a rapid decrease in mitotic index, with a lower percentage of the cells at G1 and increased numbers of cells at the S/G2 phases. Cell size was not changed following treatment suggesting that the arrest of cell cycle progression was not the result of a block on cell growth. The characteristic glutathione redox state and the localization of glutathione in the nucleus during cell proliferation were not changed by ophiobolin A. However, subsequent decreases in glutathione and the re-distribution of glutathione between the cytoplasm and nuclei after mitosis occurring in control cells, as well as the profile of glutathionylated proteins, were changed in the presence of the toxin. The profile of poly ADP-ribosylated proteins were also modified by ophiobolin A. Taken together, these data provide evidence of the mechanism of ophiobolin A action as a cell cycle inhibitor and further demonstrate the link between nuclear glutathione and the cell cycle regulation, suggesting that glutathione-dependent redox controls in the nuclei prior to cell division are of pivotal importance.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Dysfunctional mitochondria regulate the size of root apical meristem and leaf development in Arabidopsis

Mitochondria play an important role in maintaining metabolic and energy homeostasis in the plant cell. Thus, perturbation of mitochondrial structure and function will affect plant growth and development. Arabidopsis slow growth3 (slo3) is defective in At3g61360 that encodes a pentatricopeptide repeat (PPR) protein. Analysis of slo3 mitochondrial RNA metabolism revealed that the splicing of nad7 intron 2 is impaired, which leads to a dramatic reduction in complex I activity. So the SLO3 PPR protein is a splicing factor that is required for the removal of nad7 intron 2 in Arabidopsis. The slo3 mutant plants have obvious phenotypes with severe growth retardation and delayed development. The size of root apical meristem (RAM) is reduced and the production of meristem cells is decreased in slo3. Furthermore, the rosette leaves of slo3 are curled or crinkled, which may be derived from uneven growth of the leaf surface. The underlying mechanisms by which dysfunctional mitochondria affect these growth and developmental phenotypes have yet to be established. Nonetheless, plant hormone auxin is known to play an important role in orchestrating the development of RAM and leaf shape. It is possible that dysfunctional mitochondria may interact with auxin signaling pathways to regulate the boundary of RAM and the cell division arrest front during leaf growth in Arabidopsis.

Tomato fruit chromoplasts behave as respiratory bioenergetic organelles during ripening

During tomato (Solanum lycopersicum) fruit ripening, chloroplasts differentiate into photosynthetically inactive chromoplasts. It was recently reported that tomato chromoplasts can synthesize ATP through a respiratory process called chromorespiration. Here we show that chromoplast oxygen consumption is stimulated by the electron donors NADH and NADPH and is sensitive to octyl gallate (Ogal), a plastidial terminal oxidase inhibitor. The ATP synthesis rate of isolated chromoplasts was dependent on the supply of NAD(P)H and was fully inhibited by Ogal. It was also inhibited by the proton uncoupler carbonylcyanide m-chlorophenylhydrazone, suggesting the involvement of a chemiosmotic gradient. In addition, ATP synthesis was sensitive to 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a cytochrome b6f complex inhibitor. The possible participation of this complex in chromorespiration was supported by the detection of one of its components (cytochrome f) in chromoplasts using immunoblot and immunocytochemical techniques. The observed increased expression of cytochrome c6 during ripening suggests that it could act as electron acceptor of the cytochrome b6f complex in chromorespiration. The effects of Ogal on respiration and ATP levels were also studied in tissue samples. Oxygen uptake of mature green fruit and leaf tissues was not affected by Ogal, but was inhibited increasingly in fruit pericarp throughout ripening (up to 26% in red fruit). Similarly, Ogal caused a significant decrease in ATP content of red fruit pericarp. The number of energized mitochondria, as determined by confocal microscopy, strongly decreased in fruit tissue during ripening. Therefore, the contribution of chromoplasts to total fruit respiration appears to increase in late ripening stages.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.

A novel function for a redox-related LEA protein (SAG21/AtLEA5) in root development and biotic stress responses

SAG21/AtLEA5 belongs to the late embryogenesis-associated (LEA) protein family. Although it has been implicated in growth and redox responses, its precise roles remain obscure. To address this problem, we characterized root and shoot development and response to biotic stress in SAG21/AtLEA5 over-expressor (OEX) and antisense (AS) lines. AS lines exhibited earlier flowering and senescence and reduced shoot biomass. Primary root length was reduced in AS lines, as was the number of laterals relative to the primary root. Root hair number was unchanged but root hair length was proportional to SAG21/AtLEA5 expression level, with longer root hairs in OEX lines and shorter root hairs in AS, relative to wild type. Growth of the fungal nectroph, Botrytis cinerea and of a virulent bacterial pathogen (Pseudomonas syringae pv. tomato) was affected by SAG21/AtLEA5 expression; however, growth of an avirulent P.syringae strain was unaffected. A SAG21/AtLEA5-YFP fusion was localized to mitochondria, raising the intriguing possibility that SAG21 interacts with proteins involved in mitochondrial ROS signalling, which in turn, impacts on root development and pathogen responses.

The Arabidopsis RETARDED ROOT GROWTH gene encodes a mitochondria-localized protein that is required for cell division in the root meristem

To develop a growing root, cell division in the root meristem has to be properly regulated in order to generate or propagate new cells. How cell division is regulated in the root meristem remains largely unknown. Here, we report the identification and characterization of the Arabidopsis (Arabidopsis thaliana) RETARDED ROOT GROWTH (RRG) gene that plays a role in the regulation of root meristem cell division. In the root, RRG is predominantly expressed in the root meristem. Disruption of RRG function reduced numbers of dividing cells, the rate of cell production, and endoreduplication, and thus affected meristem size and root growth. Quantitative reverse transcription-polymerase chain reaction and marker-assisted analyses revealed that expression levels of several cell cycle genes were decreased in the mutant roots, indicating a defect in cell cycle progression. Mutations in RRG, however, did not affect the expression of key root-patterning genes and an auxin-responsive marker, suggesting that RRG is not essential for root patterning and auxin signaling. RRG is a mitochondria-localized protein conserved in plants and shares a DUF155 domain with proteins related to cell division in yeast, and rrg mutants displayed extensive vacuolization in mitochondria. We propose that Arabidopsis RRG is a conserved mitochondrial protein required for cell division in the root meristem.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

Redox regulation of root apical meristem organization: connecting root development to its environment

Post-embryonic root growth relies on the proliferative activity of the root apical meristem (RAM), consisting, in part, of cells with juvenile characteristics (stem cells). It is generally, but erroneously held that the RAM indefinitely produces new cells throughout the lifespan of a plant, resulting in indeterminate root growth. On the contrary, convincing data, mainly from the lab of Thomas L. Rost, show in all species analyzed so far, including Arabidopsis, that RAM organization changes over time in parallel with both a cessation of the production of new cells, and a consequent reduction in root growth, even under optimal conditions. In addition, RAM organization evolved to become highly plastic and dynamic in response to environmental triggers (e.g. water and nutrient availability, pollutants). Under unfavourable conditions, the RAM is rapidly reorganized, and, as a result of the cessation of new cell production at the root tip, root growth is altered, and lateral root production is enhanced, thus providing the plant additional strategies to overcome the stress. It is now becoming increasingly clear that this environment-responsive developmental plasticity is linked to reactive oxygen/nitrogen species, antioxidants, and related enzymes, which form part of a complex signalling module specifically operating in the regulation of RAM functioning, in strict relationship with hormonal control of root development exerted by auxin, gibberellins and cytokinins. In turn, such redox/hormone crosstalk regulates gene expression.

Pyridine nucleotide cycling and control of intracellular redox state in relation to poly (ADP-ribose) polymerase activity and nuclear localization of glutathione during exponential growth of Arabidopsis cells in culture

Pyridine nucleotides, ascorbate and glutathione are major redox metabolites in plant cells, with specific roles in cellular redox homeostasis and the regulation of the cell cycle. However, the regulation of these metabolite pools during exponential growth and their precise functions in the cell cycle remain to be characterized. The present analysis of the abundance of ascorbate, glutathione, and pyridine nucleotides during exponential growth of Arabidopsis cells in culture provides evidence for the differential regulation of each of these redox pools. Ascorbate was most abundant early in the growth cycle, but glutathione was low at this point. The cellular ascorbate to dehydroascorbate and reduced glutathione (GSH) to glutathione disulphide ratios were high and constant but the pyridine nucleotide pools were largely oxidized over the period of exponential growth and only became more reduced once growth had ceased. The glutathione pool increased in parallel with poly (ADP-ribose) polymerase (PARP) activities and with increases in the abundance of PARP1 and PARP2 mRNAs at a time of high cell cycle activity as indicated by transcriptome information. Marked changes in the intracellular partitioning of GSH between the cytoplasm and nucleus were observed. Extension of the exponential growth phase by dilution or changing the media led to increases in the glutathione and nicotinamide adenine dinucleotide, oxidized form (NAD)-plus-nicotinamide adenine dinucleotide, reduced form (NADH) pools and to higher NAD/NADH ratios but the nicotinamide adenine dinucleotide phosphate, oxidized form (NADP)-plus-nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) pool sizes, and NAPD/NADPH ratios were much less affected. The ascorbate, glutathione, and pyridine nucleotide pools and PARP activity decreased before the exponential growth phase ended. We conclude that there are marked changes in intracellular redox state during the growth cycle but that redox homeostasis is maintained by interplay of the major redox pyridine nucleotides, glutathione, and ascorbate pools. The correlation between PARP expression and activity and GSH accumulation and the finding that GSH can be recruited to the nucleus suggest a relationship between redox regulation and nuclear enzyme activity.

Redox regulation in photosynthetic organisms: signaling, acclimation, and practical implications

Reactive oxygen species (ROS) have multifaceted roles in the orchestration of plant gene expression and gene-product regulation. Cellular redox homeostasis is considered to be an "integrator" of information from metabolism and the environment controlling plant growth and acclimation responses, as well as cell suicide events. The different ROS forms influence gene expression in specific and sometimes antagonistic ways. Low molecular antioxidants (e.g., ascorbate, glutathione) serve not only to limit the lifetime of the ROS signals but also to participate in an extensive range of other redox signaling and regulatory functions. In contrast to the low molecular weight antioxidants, the "redox" states of components involved in photosynthesis such as plastoquinone show rapid and often transient shifts in response to changes in light and other environmental signals. Whereas both types of "redox regulation" are intimately linked through the thioredoxin, peroxiredoxin, and pyridine nucleotide pools, they also act independently of each other to achieve overall energy balance between energy-producing and energy-utilizing pathways. This review focuses on current knowledge of the pathways of redox regulation, with discussion of the somewhat juxtaposed hypotheses of "oxidative damage" versus "oxidative signaling," within the wider context of physiological function, from plant cell biology to potential applications.

Oxidative stress and formation and maintenance of root stem cells

The hypothesis of L. Feldman and his coworkers, according to which a more oxidizing environment in the cells of root quiescent center results from high activity of ascorbate oxidase activated by indoleacetic acid (IAA) accumulating in these cells, is discussed. The high activity of ascorbate oxidase is responsible for lowered concentrations of the reduced form of ascorbic acid and glutathione and high content of reactive oxygen species in quiescent center cells. The oxidative stress represses proliferation of the cells. Inhibitors of IAA transport attenuate the oxidative stress, thus suggesting a role of IAA as an activator of ascorbate oxidase. Interestingly, the high concentration of IAA in dividing cap cells adjacent to the quiescent center cells did not cause retardation of cell proliferation and oxidative state in these cells.

Analysis of redox relationships in the plant cell cycle: determinations of ascorbate, glutathione and poly (ADPribose)polymerase (PARP) in plant cell cultures

Reactive oxygen species (ROS) and low molecular weight antioxidants, such as glutathione and ascorbate, are powerful signaling molecules that participate in the control of plant growth and development, and modulate progression through the mitotic cell cycle. Enhanced reactive oxygen species accumulation or low levels of ascorbate or glutathione cause the cell cycle to arrest and halt progression especially through the G1 checkpoint. Plant cell suspension cultures have proved to be particularly useful tools for the study of cell cycle regulation. Here we provide effective and accurate methods for the measurement of changes in the cellular ascorbate and glutathione pools and the activities of related enzymes such poly (ADP-ribose) polymerase during mitosis and cell expansion, particularly in cell suspension cultures. These methods can be used in studies seeking to improve current understanding of the roles of redox controls on cell division and cell expansion.

Alternative oxidases in Arabidopsis: A comparative analysis of differential expression in the gene family provides new insights into function of non-phosphorylating bypasses

The emergence of Arabidopsis as a model plant provides an opportunity to gain insights into the role of the alternative oxidase that cannot be as readily achieved in other plant species. The analysis of extensive mRNA expression data indicates that all five Aox genes (Aox1a, 1b, 1c, 1d and 2) are expressed, but organ and developmental regulation are evident, suggesting regulatory specialisation of Aox gene members. The stress-induced nature of the alternative pathway in a variety of plants is further supported in Arabidopsis as Aox1a and Aox1d are amongst the most stress responsive genes amongst the hundreds of known genes encoding mitochondrial proteins. Analysis of genes co-expressed with Aoxs from studies of responses to various treatments altering mitochondrial functions and/or from plants with altered Aox levels reveals that: (i) this gene set encodes more functions outside the mitochondrion than functions in mitochondria, (ii) several pathways for induction exist and there is a difference in the magnitude of the induction in each pathway, (iii) the magnitude of induction may depend on the endogenous levels of Aox, and (iv) induction of Aox can be oxidative stress-dependent or -independent depending on the gene member and the tissue analysed. An overall role for Aox in re-programming cellular metabolism in response to the ever changing environment encountered by plants is proposed.

Expression and characterization of a redox-sensing green fluorescent protein (reduction-oxidation-sensitive green fluorescent protein) in Arabidopsis

Arabidopsis (Arabidopsis thaliana) was transformed with a redox-sensing green fluorescent protein (reduction-oxidation-sensitive green fluorescent protein [roGFP]), with expression targeted to either the cytoplasm or to the mitochondria. Both the mitochondrial and cytosolic forms are oxidation-reduction sensitive, as indicated by a change in the ratio of 510 nm light (green light) emitted following alternating illumination with 410 and 474 nm light. The 410/474 fluorescence ratio is related to the redox potential (in millivolts) of the organelle, cell, or tissue. Both forms of roGFP can be reduced with dithiothreitol and oxidized with hydrogen peroxide. The average resting redox potentials for roots are -318 mV for the cytoplasm and -362 mV for the mitochondria. The elongation zone of the Arabidopsis root has a more oxidized redox status than either the root cap or meristem. Mitochondria are much better than the cytoplasm, as a whole, at buffering changes in redox. The data show that roGFP is redox sensitive in plant cells and that this sensor makes it possible to monitor, in real time, dynamic changes in redox in vivo.