Aspects of programmed cell death during early senescence of barley leaves: possible role of nitric oxide

Plant organ senescence - regulation by manifold pathways

Senescence is the final stage of plant ontogeny before death. Senescence may occur naturally because of age or may be induced by various endogenous and exogenous factors. Despite its destructive character, senescence is a precisely controlled process that follows a well-defined order. It is often inseparable from programmed cell death (PCD), and a correlation between these processes has been confirmed during the senescence of leaves and petals. Despite suggestions that senescence and PCD are two separate processes, with PCD occurring after senescence, cell death responsible for senescence is accompanied by numerous changes at the cytological, physiological and molecular levels, similar to other types of PCD. Independent of the plant organ analysed, these changes are focused on initiating the processes of cellular structural degradation via fluctuations in phytohormone levels and the activation of specific genes. Cellular structural degradation is genetically programmed and dependent on autophagy. Phytohormones/plant regulators are heavily involved in regulating the senescence of plant organs and can either promote [ethylene, abscisic acid (ABA), jasmonic acid (JA), and polyamines (PAs)] or inhibit [cytokinins (CKs)] this process. Auxins and carbohydrates have been assigned a dual role in the regulation of senescence, and can both inhibit and stimulate the senescence process. In this review, we introduce the basic pathways that regulate senescence in plants and identify mechanisms involved in controlling senescence in ephemeral plant organs. Moreover, we demonstrate a universal nature of this process in different plant organs; despite this process occurring in organs that have completely different functions, it is very similar. Progress in this area is providing opportunities to revisit how, when and which way senescence is coordinated or decoupled by plant regulators in different organs and will provide a powerful tool for plant physiology research.

Cell-Type Specific Chromatin Analysis in Whole-Mount Plant Tissues by Immunostaining

Chromatin organization in eukaryotes is highly dynamic, playing fundamental roles in regulating diverse nuclear processes including DNA replication, transcription, and repair. Thus, the analysis of chromatin organization is of great importance for the elucidation of chromatin-mediated biological processes. Immunostaining coupled with imaging is one of the most powerful tools for chromatin analysis at the cellular level. However, in plants, it is sometimes technically challenging to apply this method due to the inaccessibility of certain cell types and/or poor penetration of the reagents into plant tissues and cells. To circumvent these limitations, we developed a highly efficient protocol enabling the analysis of chromatin modifications and nuclear organization at the single-cell level with high resolution in whole-mount plant tissues. The main procedure consists of five steps: (1) tissue fixation; (2) dissection and embedding; (3) tissue processing; (4) antibody incubation; and (5) imaging. This protocol has been simplified for the processing of multiple samples without the need for laborious tissue sectioning. Additionally, it preserves cellular morphology and chromatin organization, allowing comparative analyses of chromatin organization between different cell types or developmental stages. This protocol was successfully used for various tissues of different plant species, including Arabidopsis thaliana, Oryza sativa (rice), and Zea mays (maize). Importantly, this method is very useful to analyze poorly accessible tissues, such as female meiocytes, gametophytes, and embryos.

Label-free and dynamic monitoring of cytotoxicity to the blood-brain barrier cells treated with nanometre copper oxide

A cytotoxicity study was conducted with a primary culture of the nervous system cells, including brain microvascular endothelial cells (BMECs) and astrocytes, which are important components of the blood-brain barrier. The real-time cell analysis (RTCA) was used to determine the cytotoxicity of copper-oxide nanoparticles (CuO NPs). The IC50 values of CuO NPs in astrocytes and BMECs were determined by the RTCA at different exposure times and were used as base values for further research. DNA damage after exposure to CuO NPs for 3 and 24 h was assessed using comet assay at the IC50 obtained from RTCA. The onset time of cytotoxicity induced by CuO NPs was 2 and 2-4 h post-exposure in BMECs and astrocytes, respectively. Furthermore, the degree of cytotoxicity induced by exposure to CuO NPs for 24-48 h in the BMECs and astrocytes was similar. Treatment with CuO NPs at 1/2*IC50 and 1/5*IC50 for 3 h induced genotoxicity in both cells as assessed by a measurement of DNA damage, although no cytotoxicity was observed. However, significant DNA damage was observed at all concentrations of CuO NPs used in this study, when the treatment time was 24 h.

The mystery of underground death: cell death in roots during ontogeny and in response to environmental factors

Programmed cell death (PCD) is an essential part of the ontogeny of roots and their tolerance/resistance mechanisms, allowing adaptation and growth under adverse conditions. It occurs not only at the cellular and subcellular level, but also at the levels of tissues, organs and even whole plants. This process involves a wide spectrum of mechanisms, from signalling and the expression of specific genes to the degradation of cellular structures. The major goals of this review were to broaden current knowledge about PCD processes in roots, and to identify mechanisms associated with both developmental and stress-associated cell death in roots. Vacuolar cell death, when cell contents are removed by a combination of an autophagy-associated process and the release of hydrolases from a collapsed vacuole, is responsible for programming self-destruction. Regardless of the conditions and factors inducing PCD, its subcellular events usually include the accumulation of autophagosome-like structures, and the formation of massive lytic compartments. In some cases these are followed by the nuclear changes of chromatin condensation and DNA fragmentation. Tonoplast disruption and vacuole implosion occur very rapidly, are irreversible and constitute a definitive step toward cell death in roots. Active cell elimination plays an important role in various biological processes in the life history of plants, leading to controlled cellular death during adaptation to changing environmental conditions, and organ remodelling throughout development and senescence.

Caffeine-Induced Premature Chromosome Condensation Results in the Apoptosis-Like Programmed Cell Death in Root Meristems of Vicia faba

We have demonstrated that the activation of apoptosis-like programmed cell death (AL-PCD) was a secondary result of caffeine (CF) induced premature chromosome condensation (PCC) in hydroxyurea-synchronized Vicia faba root meristem cells. Initiation of the apoptotic-like cell degradation pathway seemed to be the result of DNA damage generated by treatment with hydroxyurea (HU) [double-stranded breaks (DSBs) mostly] and co-treatment with HU/CF [single-stranded breaks (SSBs) mainly]. A single chromosome comet assay was successfully used to study different types of DNA damage (neutral variant–DSBs versus alkaline–DSBs or SSBs). The immunocytochemical detection of H2AXS139Ph and PARP-2 were used as markers for DSBs and SSBs, respectively. Acridine orange and ethidium bromide (AO/EB) were applied for quantitative immunofluorescence measurements of dead, dying and living cells. Apoptotic-type DNA fragmentation and positive TUNEL reaction finally proved that CF triggers AL-PCD in stressed V. faba root meristem cells. In addition, the results obtained under transmission electron microscopy (TEM) further revealed apoptotic-like features at the ultrastructural level of PCC-type cells: (i) extensive vacuolization; (ii) abnormal chromatin condensation, its marginalization and concomitant degradation; (iii) formation of autophagy-like vesicles (iv) protoplast shrinkage (v) fragmentation of cell nuclei and (vi) extensive degeneration of the cells. The results obtained have been discussed with respect to the vacuolar/autolytic type of plant-specific AL-PCD.

Licensed to Kill: Mitochondria, Chloroplasts, and Cell Death

Programmed cell death (PCD) is crucial in plant organogenesis and survival. In this review the involvement of mitochondria and chloroplasts in PCD execution is critically assessed. Recent findings support a central role for mitochondria in PCD, with newly identified components of the mitochondrial electron transport chain (mETC), FOF1 ATP synthase, cardiolipins, and ATPase AtOM66. While chloroplasts received less attention, their contribution to PCD is well supported, suggesting that they possibly contribute by producing reactive oxygen species (ROS) in the presence of light or even contribute through cytochrome f release. Finally we discuss two working models where mitochondria and chloroplasts could cooperatively execute PCD: mitochondria initiate the commitment steps and recruit chloroplasts for swift execution or, alternatively, mitochondria and chloroplasts could operate in parallel.

Epigenetic control of plant senescence and linked processes

Senescence processes are part of the plant developmental programme. They involve reprogramming of gene expression and are under the control of a complex regulatory network closely linked to other developmental and stress-responsive pathways. Recent evidence indicates that leaf senescence is regulated via epigenetic mechanisms. In the present review, the epigenetic control of plant senescence is discussed in the broader context of environment-sensitive plant development. The review outlines the concept of epigenetic control of interconnected regulatory pathways steering stress responses and plant development. Besides giving an overview of techniques used in the field, it summarizes recent findings on global alterations in chromatin structure, histone and DNA modifications, and ATP-dependent chromatin remodelling during plant senescence and linked processes.

The age-dependent epigenetic and physiological changes in an Arabidopsis T87 cell suspension culture during long-term cultivation

Plant cell suspension cultures represent good model systems applicable for both basic research and biotechnological purposes. Nevertheless, it is widely known that a prolonged in vitro cultivation of plant cells is associated with genetic and epigenetic instabilities, which may limit the usefulness of plant lines. In this study, the age-dependent epigenetic and physiological changes in an asynchronous Arabidopsis T87 cell culture were examined. A prolonged cultivation period was found to be correlated with a decrease in the proliferation rate and a simultaneous increase in the expression of senescence-associated genes, indicating that the aging process started at the late growth phase of the culture. In addition, increases in the heterochromatin-specific epigenetic markers, i.e., global DNA methylation, H3K9 dimethylation, and H3K27 trimethylation, were observed, suggesting the onset of chromatin condensation, a hallmark of the early stages of plant senescence. Although the number of live cells decreased with an increase in the age of the culture, the remaining viable cells retained a high potential to efficiently perform photosynthesis and did not exhibit any symptoms of photosystem II damage.

Epigenetic and small RNA regulation of senescence

Leaf senescence is regulated through a complex regulatory network triggered by internal and external signals for the reprogramming of gene expression. In plants, the major developmental phase transitions and stress responses are under epigenetic control. In this review, the underlying molecular mechanisms are briefly discussed and evidence is shown that epigenetic processes are also involved in the regulation of leaf senescence. Changes in the chromatin structure during senescence, differential histone modifications determining active and inactive sites at senescence-associated genes and DNA methylation are addressed. In addition, the role of small RNAs in senescence regulation is discussed.

Phytohormones and microRNAs as sensors and regulators of leaf senescence: assigning macro roles to small molecules

Ageing or senescence is an intricate and highly synchronized developmental phase in the life of plant parts including leaf. Senescence not only means death of a plant part, but during this process, different macromolecules undergo degradation and the resulting components are transported to other parts of the plant. During the period from when a leaf is young and green to the stage when it senesces, a multitude of factors such as hormones, environmental factors and senescence associated genes (SAGs) are involved. Plant hormones including salicylic acid, abscisic acid, jasmonic acid and ethylene advance leaf senescence, whereas others like cytokinins, gibberellins, and auxins delay this process. The environmental factors which generally affect plant development and growth, can hasten senescence, the examples being nutrient dearth, water stress, pathogen attack, radiations, high temperature and light intensity, waterlogging, and air, water or soil contamination. Other important influences include carbohydrate accumulation and high carbon/nitrogen level. To date, although several genes involved in this complex process have been identified, still not much information exists in the literature on the signalling mechanism of leaf senescence. Now, the Arabidopsis mutants have paved our way and opened new vistas to elucidate the signalling mechanism of leaf senescence for which various mutants are being utilized. Recent studies demonstrating the role of microRNAs in leaf senescence have reinforced our knowledge of this intricate process. This review provides a comprehensive and critical analysis of the information gained particularly on the roles of several plant growth regulators and microRNAs in regulation of leaf senescence.

Application of comet assay for the assessment of DNA damage caused by chemical genotoxins in the dairy yeast Kluyveromyces lactis

Kluyveromyces lactis, also known as dairy yeast, has numerous applications in scientific research and practice. It has been approved as a GRAS (Generally Recognized As Safe) organism, a probiotic, a biotechnological producer of important enzymes at industrial scale and a bioremediator of waste water from the dairy industry. Despite these important practical applications the sensitivity of this organism to genotoxic substances has not yet been assessed. In order to evaluate the response of K. lactis cells to genotoxic agents we have applied several compounds with well-known cyto- and genotoxic activity. The method of comet assay (CA) widely used for the assessment of DNA damages is presented here with new special modifications appropriate for K. lactis cells. The comparison of the response of K. lactis to genotoxins with that of Saccharomyces cerevisiae showed that both yeasts, although considered close relatives, exhibit species-specific sensitivity toward the genotoxins examined.

Oxidative biomarkers in leaf tissue of barley seedlings in response to aluminum stress

Cellular responses to Al-stress in Hordeum vulgare seedling bioassay were evaluated with an objective to identify the possible biomarkers in leaf tissue that would be best suited to biomonitor aluminum (Al) in the environment. Germinating seeds were treated with different concentrations of AlCl(3) at pH 4.5 for 12h. Al-uptake and accumulation in root and leaf, generation of reactive oxygen species (ROS: O(2)(-), H(2)O(2) and ()OH), cell death, activity of antioxidant enzymes: catalase, superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase, lipid peroxidation, protein oxidation, DNase activity and DNA damage were measured in leaf tissue of the seedlings on day 6 after treatment. The above parameters assessed in leaf tissue that followed a dose-response exhibited significant correlation with concentration of Al(3+) in experimental solution as well as in root tissue. The findings underscored the sensitivity as well as potential of Hordeum vulgare seedling bioassay for biomonitoring of Al in the ambient environment.