The size of quiescent centre in roots of Allium cepa L. grown with ascorbic acid

The quiescent centre of the root apical meristem: conceptual developments from Clowes to modern times

In this review we discuss the concepts of the quiescent centre (QC) of the root apical meristem (RAM) and their change over time, from their formulation by F.A.L. Clowes to the present. This review is dedicated to the 100th anniversary of the birth of Clowes, and we present his short biography and a full bibliography of Clowes' work. Over time, the concept of the QC proved to be useful for the understanding of RAM organization and behaviour. We focus specifically on conceptual developments, from the organization of the QC to understanding its functions in RAM maintenance and activity, ranging from a model species, Arabidopsis thaliana, to crops. Concepts of initial cells, stem cells, and heterogeneity of the QC cells in the context of functional and structural stem cells are considered. We review the role of the QC in the context of cell flux in the RAM and the nature of quiescence of the QC cells. We discuss the origin of the QC and fluctuation of its size in ontogenesis and why the QC cells are more resistant to stress. Contemporary concepts of the organizer and stem cell niche are also considered. We also propose how the stem cell niche in the RAM can be defined in roots of a non-model species.

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.

Compartment-Specific Importance of Ascorbate During Environmental Stress in Plants

SIGNIFICANCE

Ascorbate is an essential antioxidant in plants. Total contents and its redox state in organelles are crucial to fight and signal oxidative stress. Recent Advances: With quantitative immunoelectron microscopy and biochemical methods, highest ascorbate contents have recently been measured in peroxisomes (23 mM) and the cytosol (22 mM), lowest ones in vacuoles (2 mM), and intermediate concentrations (4-16 mM) in all other organelles.

CRITICAL ISSUES

The accumulation of ascorbate in chloroplasts and peroxisomes is crucial for plant defense. Its depletion in chloroplasts, peroxisomes, and mitochondria during biotic stress leads to the accumulation of reactive oxygen species (ROS) and the development of chlorosis and necrosis. In the apoplast and vacuoles, ascorbate is the most important antioxidant for the detoxification of ROS. The cytosol acts as a hub for ascorbate metabolism as it reduces its oxidized forms that are produced in the cytosol or imported from other cell compartments. It is a sink for ascorbate that is produced in mitochondria, distributes ascorbate to all organelles, and uses ascorbate to detoxify ROS. As ascorbate and its redox state are involved in protein synthesis and modifications, it can be concluded that ascorbate in the cytosol senses oxidative stress and regulates plant growth, development, and defense.

FUTURE DIRECTIONS

Future research should focus on (1) dissecting roles of ascorbate in vacuoles and the lumen of the endoplasmic reticulum, (2) identifying the physiological relevance of ascorbate transporters, and (3) correlating current data with changes in the subcellular distribution of related enzymes, ROS, and gene expression patterns.

Origin of the concept of the quiescent centre of plant roots

Concepts in biology feed into general theories of growth, development and evolution of organisms and how they interact with the living and non-living components of their environment. A well-founded concept clarifies unsolved problems and serves as a focus for further research. One such example of a constructive concept in the plant sciences is that of the quiescent centre (QC). In anatomical terms, the QC is an inert group of cells maintained within the apex of plant roots. However, the evidence that established the presence of a QC accumulated only gradually, making use of strands of different types of observations, notably from geometrical-analytical anatomy, radioisotope labelling and autoradiography. In their turn, these strands contributed to other concepts: those of the mitotic cell cycle and of tissue-related cell kinetics. Another important concept to which the QC contributed was that of tissue homeostasis. The general principle of this last-mentioned concept is expressed by the QC in relation to the recovery of root growth following a disturbance to cell proliferation; the resulting activation of the QC provides new cells which not only repair the root meristem but also re-establish a new QC.

Two tomato GDP-D-mannose epimerase isoforms involved in ascorbate biosynthesis play specific roles in cell wall biosynthesis and development

GDP-D-mannose epimerase (GME, EC 5.1.3.18) converts GDP-D-mannose to GDP-L-galactose, and is considered to be a central enzyme connecting the major ascorbate biosynthesis pathway to primary cell wall metabolism in higher plants. Our previous work demonstrated that GME is crucial for both ascorbate and cell wall biosynthesis in tomato. The aim of the present study was to investigate the respective role in ascorbate and cell wall biosynthesis of the two SlGME genes present in tomato by targeting each of them through an RNAi-silencing approach. Taken individually SlGME1 and SlGME2 allowed normal ascorbate accumulation in the leaf and fruits, thus suggesting the same function regarding ascorbate. However, SlGME1 and SlGME2 were shown to play distinct roles in cell wall biosynthesis, depending on the tissue considered. The RNAi-SlGME1 plants harbored small and poorly seeded fruits resulting from alterations of pollen development and of pollination process. In contrast, the RNAi-SlGME2 plants exhibited vegetative growth delay while fruits remained unaffected. Analysis of SlGME1- and SlGME2-silenced seeds and seedlings further showed that the dimerization state of pectin rhamnogalacturonan-II (RG-II) was altered only in the RNAi-SlGME2 lines. Taken together with the preferential expression of each SlGME gene in different tomato tissues, these results suggest sub-functionalization of SlGME1 and SlGME2 and their specialization for cell wall biosynthesis in specific tomato tissues.

Increasing vitamin C content in plant foods to improve their nutritional value-successes and challenges

Vitamin C serves as a cofactor in the synthesis of collagen needed to support cardiovascular function, maintenance of cartilage, bones, and teeth, as well as being required in wound healing. Although vitamin C is essential, humans are one of the few mammalian species unable to synthesize the vitamin and must obtain it through dietary sources. Only low levels of the vitamin are required to prevent scurvy but subclinical vitamin C deficiency can cause less obvious symptoms such as cardiovascular impairment. Up to a third of the adult population in the U.S. obtains less than the recommended amount of vitamin C from dietary sources of which plant-based foods constitute the major source. Consequently, strategies to increase vitamin C content in plants have been developed over the last decade and include increasing its synthesis as well as its recycling, i.e., the reduction of the oxidized form of ascorbic acid that is produced in reactions back into its reduced form. Increasing vitamin C levels in plants, however, is not without consequences. This review provides an overview of the approaches used to increase vitamin C content in plants and the successes achieved. Also discussed are some of the potential limitations of increasing vitamin C and how these may be overcome.

The role of L-ascorbic acid recycling in responding to environmental stress and in promoting plant growth

L-Ascorbic acid (Asc) is the most abundant water-soluble antioxidant in plants. It serves as a cofactor for enzymes involved in photosynthesis, hormone biosynthesis, and the regeneration of antioxidants such as α-tocopherol. Once used, Asc can be recycled by several different mechanisms. The short-lived monodehydroascorbate (MDHA) radical, produced following Asc oxidation, can be recycled following reduction by ferredoxin or monodehydroascorbate reductase (MDAR). MDHA can also undergo disproportionation into dehydroascorbate (DHA) and Asc. DHA can be recycled into Asc by dehydroascorbate reductase (DHAR) before it undergoes irrevocable hydrolysis. Through its recycling, Asc content and its redox state are maintained, which is critical under conditions of high demand, for example during high light or other stress conditions that increase reactive oxygen species (ROS) production. This review provides an overview of research in the last decade revealing the role that Asc recycling plays during germination, growth, and reproduction, as well as in response to environmental stress. These findings highlight the importance of DHAR- and MDAR-mediated mechanisms of Asc recycling in maintaining ROS at non-damaging levels while modulating ROS signalling function.

Induction of monozygotic twinning by ascorbic acid in tobacco

Embryo development in plants initiates following the transverse division of a zygote into an apical, proembryo cell and a basal cell that gives rise to the suspensor. Although mutants affected in embryo development through changes in cell division have been described, little is known about the control of the first zygotic division that gives rise to the proembryo. Ascorbic acid (Asc) promotes cell division by inducing G(1) to S progression but its role in embryo development has not been examined. In this study, we show that the level of dehydroascorbate reductase (DHAR) expression, which recycles Asc and regulates Asc pool size, affects the rate of monozygotic twinning and polycotyly. DHAR-induced twinning resulted from altered cell polarity and longitudinal instead of transverse cell division that generated embryos of equal size. Direct injection of Asc into ovaries phenocopied DHAR-induced twinning. Twinning induced by Asc was developmentally limited to the first two days after pollination whereas polycotyly was induced when the level of Asc was elevated just prior to cotyledon initiation. This work describes the first example of gene-directed monozygotic twinning and shows that Asc regulates cell polarity during embryo development.

Generation and properties of ascorbic acid-overproducing transgenic tobacco cells expressing sense RNA for l-galactono-1,4-lactone dehydrogenase

L-Galactono-1,4-lactone dehydrogenase (GalLDH; EC 1.3.2.3) is the last enzyme in the putative L-ascorbic acid (AsA) biosynthetic pathway of plants. Here, we show for the first time that the overexpression of GalLDH can increase AsA content in tobacco (Nicotiana tabacum L.) BY-2 cells. To see the effect, we analyzed the properties of these AsA-overproducing transgenic cell lines, especially in relation to AsA content of cells, cell division, senescence and resistance to oxidative stress. The mitotic index in AsA-overproducing cells was higher than in wild-type cells. Moreover, the browning of these cells was markedly restrained, and the proportion of dead cells was reduced, especially in the later period of culture. These AsA-overproducing cells also acquired resistance to paraquat (methyl viologen), which produces active oxygen species. These results contribute to the previous insights about AsA and raise the possibility of the generation of plants that have resistance to environmental stresses by increasing their AsA content.

Dehydroascorbate influences the plant cell cycle through a glutathione-independent reduction mechanism

Glutathione is generally accepted as the principal electron donor for dehydroascorbate (DHA) reduction. Moreover, both glutathione and DHA affect cell cycle progression in plant cells. But other mechanisms for DHA reduction have been proposed. To investigate the connection between DHA and glutathione, we have evaluated cellular ascorbate and glutathione concentrations and their redox status after addition of dehydroascorbate to medium of tobacco (Nicotiana tabacum) L. cv Bright Yellow-2 (BY-2) cells. Addition of 1 mm DHA did not change the endogenous glutathione concentration. Total glutathione depletion of BY-2 cells was achieved after 24-h incubation with 1 mm of the glutathione biosynthesis inhibitor l-buthionine sulfoximine. Even in these cells devoid of glutathione, complete uptake and internal reduction of 1 mm DHA was observed within 6 h, although the initial reduction rate was slower. Addition of DHA to a synchronized BY-2 culture, or depleting its glutathione content, had a synergistic effect on cell cycle progression. Moreover, increased intracellular glutathione concentrations did not prevent exogenous DHA from inducing a cell cycle shift. It is therefore concluded that, together with a glutathione-driven DHA reduction, a glutathione-independent pathway for DHA reduction exists in vivo, and that both compounds act independently in growth control.

Gene expression of ascorbic acid-related enzymes in tobacco

GDP-D-mannose pyrophosphorylase (GMPase) and L-galactono-1, 4-lactone dehydrogenase (GalLDH) are key enzymes in L-ascorbic acid (AsA) biosynthesis of plants, and a full-length cDNA for GMPase was isolated from tobacco using PCR. Additionally, expression of GMPase, GalLDH and other AsA-related enzymes was examined in tobacco tissues and cultured BY-2 cells, and the relationship between their expression patterns and AsA content is discussed. It was found that the expression of GalLDH and GMPase mRNAs was markedly suppressed by loading AsA, suggesting that AsA concentration in the cells may regulate AsA biosynthesis. Moreover, the expression of GMPase and GalLDH mRNAs in tobacco leaf also suggested that AsA biosynthesis may be induced by light.

Generation and properties of ascorbic acid-deficient transgenic tobacco cells expressing antisense RNA for L-galactono-1,4-lactone dehydrogenase

In higher plants, the terminal step of L-ascorbic acid (AsA) biosynthesis is catalyzed by the enzyme L-galactono-1,4-lactone dehydrogenase (EC 1.3.2.3, GalLDH). We generated AsA-deficient transgenic tobacco BY-2 cell lines by antisense expression of the GalLDH cDNA that was amplified from BY-2 cells using PCR. Two transgenic cell-lines, AS1-1 and AS2-2, having a marked expression of antisense RNA were analyzed. Antisense suppression of GalLDH mRNA led to a significant decline in the GalLDH activity. The AsA levels in the transgenic cell lines were found to be 30% lower than the wild-type BY-2 cells. In synchronous cultures, division of AS1-1 and AS2-2 cells was restrained with a concomitant decrease in mitotic index that was probably due to a decline in AsA levels. The rate of cell growth was also found to be less than that of the wild-type cells. Interestingly, there was a significant phenotypic difference between the transgenic and wild-type cells. The calli of AS1-1 and AS2-2 appeared to be sticky and soft. Back extrusion method also showed that AsA-deficient BY-2 callus was rheologically soft. Furthermore, microscopic analysis revealed that AS1-1 and AS2-2 cells were abnormally slender, suggesting a potential for a significant and a uni-axial elongation. Thus, we observed that decline in the AsA levels has an adverse effect on the division, growth and structure of a plant cell.

The redox state of the ascorbate-dehydroascorbate pair as a specific sensor of cell division in tobacco BY-2 cells

The effects of ascorbate (ASC) and dehydroascorbate (DHA) on cell proliferation were examined in the tobacco Bright Yellow 2 (TBY-2) cell line to test the hypothesis that the ASC-DHA pair is a specific regulator of cell division. The hypothesis was tested by measuring the levels of ASC and DHA or another general redox pair, glutathione (GSH) and glutathione disulfide (GSSG), during the exponential-growth phase of TBY-2 cells. A peak in ASC, but not GSH, levels coincided with a peak in the mitotic index. Moreover, when the cells were enriched with ascorbate, a stimulation of cell division occurred whereas, when the cells were enriched with DHA, the mitotic index was reduced. In contrast, glutathione did not affect the mitotic-index peak during this exponential-growth phase. The data are consistent in showing that the ASC-DHA pair acts as a specific redox sensor which is part of the mechanism that regulates cell cycle progression in this cell line.

Ascorbate system in plant development

By using lycorine, a specific inhibitor of ascorbate biosynthesis, it was possible to demonstrate that plant cells consume a high quantity of ascorbate (AA). The in vivo metabolic reactions utilizing ascorbate are the elimination of H2O2 by ascorbate peroxidase and the hydroxylation of proline residues present in the polypeptide chains by means of peptidyl-proline hydroxylase. Ascorbate acts in the cell metabolism as an electron donor, and consequently ascorbate free radical (AFR) is continuously produced. AFR can be reconverted to AA by means of AFR reductase or can undergo spontaneous disproportion, thus generating dehydroascorbic acid (DHA). During cell division and cell expansion ascorbate consumption is more or less the same; however, the AA/DHA ratio is 6-10 during cell division and 1-3 during cell expansion. This ratio depends essentially on the different AFR reductase activity in these cells. In meristematic cells AFR reductase is very high, and consequently a large amount of AFR is reduced to AA and a small amount of AFR undergoes disproportionation; in expanding cells the AFR reductase activity is lower, and therefore AFR is massively disproportionated, thus generating a large quantity of DHA. Since the transition from cell division to cell expansion is marked by a large drop of AFR reductase activity in the ER, it is suggested here that AFR formed in this compartment may be involved in the enlargement of the ER membranes and provacuole acidification. DHA is a toxic compound for the cell metabolism and as such the cell has various strategies to counteract its effects: (i) meristematic cells, having an elevated AFR reductase, prevent large DHA production, limiting the quantity of AFR undergoing disproportionation (ii) Expanding cells, which contain a lower AFR reductase, are, however, provided with a developed vacuolar system and segregate the toxic DHA in the vacuole. (iii) Chloroplast strategy against DHA toxicity is efficient DHA reduction to AA using GSH as electron donor. This strategy is usually poorly utilized by the surrounding cytoplasm. DHA reduction does play an important role at one point in the life of the plant, that is, during the early stage of seed germination. The dry seed does not store ascorbate, but contains DHA, and several DHA-reducing proteins are detectable. In this condition, DHA reduction is necessary to form a limited AA pool in the seed for the metabolic requirements of the beginning of germination. After 30-40 h ascorbate ex novo synthesis starts, DHA reduction declines until a single isoform remains, as is typical in the roots, stem, and leaves of seedlings.(ABSTRACT TRUNCATED AT 400 WORDS)