Abiotic and biotic factors induce oxidative stress involving the production and scavenging of reactive oxygen species (ROS). This review is a survey of well-known and possible roles of ...serine-threonine protein phosphatases in plant oxidative stress signaling, with special emphasis on PP2A. ROS mediated signaling involves three interrelated pathways: (i) perception of extracellular ROS triggers signal transduction pathways, leading to DNA damage and/or the production of antioxidants; (ii) external signals induce intracellular ROS generation that triggers the relevant signaling pathways and (iii) external signals mediate protein phosphorylation dependent signaling pathway(s), leading to the expression of ROS producing enzymes like NADPH oxidases. All pathways involve inactivation of serine-threonine protein phosphatases. The metal dependent phosphatase PP2C has a negative regulatory function during ABA mediated ROS signaling. PP2A is the most abundant protein phosphatase in eukaryotic cells. Inhibitors of PP2A exert a ROS inducing activity as well and we suggest that there is a direct relationship between these two effects of drugs. We present current findings and hypotheses regarding PP2A-ROS signaling connections related to all three ROS signaling pathways and anticipate future research directions for this field. These mechanisms have implications in the understanding of stress tolerance of vascular plants, having applications regarding crop improvement.
The protein phosphatase PP2A is essential for the control of integrated eukaryotic cell functioning. Several cellular and developmental events, e.g., plant growth regulator (PGR) mediated signaling ...pathways are regulated by reversible phosphorylation of vesicle traffic proteins. Reviewing present knowledge on the relevant role of PP2A is timely. We discuss three aspects: (1) PP2A regulates microtubule-mediated vesicle delivery during cell plate assembly. PP2A dephosphorylates members of the microtubule associated protein family MAP65, promoting their binding to microtubules. Regulation of phosphatase activity leads to changes in microtubule organization, which affects vesicle traffic towards cell plate and vesicle fusion to build the new cell wall between dividing cells. (2) PP2A-mediated inhibition of target of rapamycin complex (TORC) dependent signaling pathways contributes to autophagy and this has possible connections to the brassinosteroid signaling pathway. (3) Transcytosis of vesicles transporting PIN auxin efflux carriers. PP2A regulates vesicle localization and recycling of PINs related to GNOM (a GTP-GDP exchange factor) mediated pathways. The proper intracellular traffic of PINs is essential for auxin distribution in the plant body, thus in whole plant development. Overall, PP2A has essential roles in membrane interactions of plant cell and it is crucial for plant development and stress responses.
Overproduction of toxic cyanobacteria is a type of harmful algal blooms (HABs). The heptapeptide microcystins (MCs) are one of the most common cyanotoxins. There is increasing research concerning the ...effects of MCs on growth and physiology of vascular plants, however there is a lack of studies on their direct effects on aquatic macrophytes in the real environment. Here we report the occurrence of a MC producing HAB in Lake Bárdos, Hungary in 2012 with harmful effects on cytological, histological and biochemical parameters of Ceratophyllum submersum (soft hornwort) plants naturally growing at the blooming site. Blue-Green Sinapis Test (BGST) showed high toxicity of HAB samples. Cell-free water samples contained a significant amount of MCs (7.31 ± 0.17 μg L−1) while C. submersum plants contained 1.01 ± 0.21 μg g DW−1 MCs. Plants showed significant increases of protein content and decreases of anthocyanin content and carotenoid/chlorophyll ratio, indicating physiological stress- as compared to plants from the control (MC free) sampling site of the same water body. Histological and cytological studies showed (i) radial swelling and the abnormal formation of lateral buds at the shoot tip leading to abnormal development; (ii) the fragmentation of nuclei as well as accumulation of phenolics in the nucleus indicating that the HAB induced cell death and stress reactions at the nuclear level. The most relevant effect was the increase of histone H3 phosphorylation in metaphase chromosomes: since MCs are strong inhibitors of protein phosphatases, this alteration is related to the biochemical targets of these toxins. The HAB decreased peroxidase activity, but increased nuclease and protease activities, showing the decreased capacity of plants to face biotic stress and as the cytological changes, the induction of cell death. This study is one of the first to show the complex harmful changes in aquatic plants that co-exist with HABs.
Display omitted
•This is a study of cyanobacterium-plant interaction in the real environment.•Microcystis blooming affected a C. submersum (macrophyte) population in Hungary.•Microcystins were found in the cyanobacterial cells, aquatic plants and water body.•Histone H3 hyperphosphorylation in plant cells is directly related to microcystins.
There is increasing evidence for the induction of programmed cell death (PCD) in vascular plants by the cyanobacterial toxin microcystin-LR (MC-LR). Our aim was to detect the occurrence of ...PCD-related DNA strand breaks and their possible connections to specific nuclease and protease activities. DNA breaks were studied by the deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) method in the photoperiodically grown dicot model of white mustard (Sinapis alba). In-gel nuclease and protease activity assays showed changes in the activities of specific isoenzymes during treatments with MC-LR. Strand breaks occurred both in the developing root epidermis and cortex. Several isoenzyme activities were related to these breaks, for example: an increase in the activity of neutral 80–75 kDa, acidic high MW (100–120 kDa) and, most importantly, an increase in the activity of neutral 26–20 kDa nucleases, all of them having single-stranded DNA cleaving (SSP nuclease) activities. Increases in the activities of alkaline proteases in the 61–41 kDa range were also detected and proved to be in relation with MC-LR-induced PCD. This is one of the first pieces of evidence on the correlation of PCD-related DNA strand breaks with specific hydrolase activities in a model dicot treated with a cyanobacterial toxin known to have environmental importance.
Horseradish hairy root cultures are suitable plant tissue organs to study the glucosinolate-myrosinase-isothiocyanate system and also to produce the biologically active isothiocyanates and ...horseradish peroxidase, widely used in molecular biology. Fifty hairy root clones were isolated after
infection of surface sterilized
petioles and leaf blades, from which 21 were viable after antibiotic treatment. Biomass properties (e.g. dry weight %, daily growth index), glucosinolate content (analyzed by liquid chromatography-electronspray ionization-mass spectrometry (LC-ESI-MS/MS)), isothiocyanate and nitrile content (analyzed by gas chromatography-mass spectrometry (GC-MS)), myrosinase (on-gel detection) and horseradish peroxidase enzyme patterns (on-gel detection and spectrophotometry), and morphological features were examined with multi-variable statistical analysis. In addition to the several positive and negative correlations, the most outstanding phenomenon was many parameters of the hairy root clones showed dependence on the organ of origin. Among others, the daily growth index, sinigrin, glucobrassicin, 3-phenylpropionitrile, indole-3-acetonitrile and horseradish peroxidase values showed significantly higher levels in horseradish hairy root cultures initiated from leaf blades.
Cylindrospermopsin (CYN) is a tricyclic guanidino alkaloid toxin produced by several cyanobacterial genera. It alters cellular functioning in eukaryotes, including animal and plant organisms. Over ...the past decades, more and more evidence shows its potential hazardous effects on animal and human health. In this review, we give a critical survey and interpretation of data currently available on its biochemical and consequently, cellular effects. CYN is considered to be a cytotoxin. Several reports suggest that it is a potent inhibitor of eukaryotic protein synthesis, though the exact mechanisms are not completely understood. Here we show that the biochemical changes induced by CYN are complex, possibly involving multiple modes of action. Glutathione metabolism and pyrimidine nucleotide synthesis is affected besides the proposed protein synthesis inhibition. Biochemical alterations lead to the following cellular/subcellular alterations both in animals and plants: (i) changes in cell division rates due to perturbations in chromatin and cytoskeleton; (ii) perturbations of structure and functioning of endomembranes including endoplasmic reticulum; (iii) general metabolic alterations leading to genotoxicity and programmed cell death/apoptosis. The underlying mechanisms and possible health consequences are discussed.
Microcystins (MCYs) are cyanobacterial heptapeptides known for their high toxicity in eukaryotic cells and for their potential human health hazards. They are potent and specific inhibitors of type 1 ...and 2A, serine-threonine protein phosphatases (PP1 and PP2A) and as such, interfere with key cellular and metabolic events. Moreover, they induce oxidative stress involving reactive oxygen species (ROS) generation. Their cytoskeletal effects involve both mitotic and differentiated eukaryotic cells. The main objective of the present review is to summarize the most important cytoskeletal effects of MCY on human, animal and plant cells known to date and to give an insight into the cellular and molecular background of these alterations. Disruptions of microtubule (MTs), microfilament (MF) and intermediate filament (IF) organization have all been described, having consequences on cell shape, tissue integrity and functionality and mitotic division. Most of these subcellular changes are closely related to PP1 and PP2A inhibition and involve misfunctioning of cytoskeleton associated proteins. However, several cytoskeletal alterations are likely to be related to the induction of oxidative stress. MCY induced changes in MT, MF and IF assembly may have severe human health consequences. The main target of cyanotoxin in human/ animal cells is liver and cytoskeletal disruption alters structure and functioning of hepatocytes. However, many other cell types undergo alterations similar to those observed in hepatocytes. Both PP1/PP2A inhibition and ROS generation are involved and the activation of mitogen activated protein kinases (MAPKs) seems to play a crucial role in the molecular events leading to cytoskeletal disruption.
Cyanobacteria produce metabolites with diverse bioactivities, structures and pharmacological properties. The effects of microcystins (MCYs), a family of peptide type protein-phosphatase inhibitors ...and cylindrospermopsin (CYN), an alkaloid type of protein synthesis blocker will be discussed in this review. We are focusing mainly on cyanotoxin-induced changes of chromatin organization and their possible cellular mechanisms. The particularities of plant cells explain the importance of such studies. Preprophase bands (PPBs) are premitotic cytoskeletal structures important in the determination of plant cell division plane. Phragmoplasts are cytoskeletal structures involved in plant cytokinesis. Both cyanotoxins induce the formation of multipolar spindles and disrupted phragmoplasts, leading to abnormal sister chromatid segregation during mitosis. Thus, MCY and CYN are probably inducing alterations of chromosome number. MCY induces programmed cell death: chromatin condensation, nucleus fragmentation, necrosis, alterations of nuclease and protease enzyme activities and patterns. The above effects may be related to elevated reactive oxygen species (ROS) and/or disfunctioning of microtubule associated proteins. Specific effects: MCY-LR induces histone H3 hyperphosphorylation leading to incomplete chromatid segregation and the formation of micronuclei. CYN induces the formation of split or double PPB directly related to protein synthesis inhibition. Cyanotoxins are powerful tools in the study of plant cell organization.
Phytotoxicity of cyanobacterial toxins has been confirmed at the subcellular level with consequences on whole plant physiological parameters and thus growth and productivity. Most of the data are ...available for two groups of these toxins: microcystins (MCs) and cylindrospermopsins (CYNs). Thus, in this review we present a timely survey of subcellular cyanotoxin effects with the main focus on these two cyanotoxins. We provide comparative insights into how peculiar plant cellular structures are affected. We review structural changes and their physiological consequences induced in the plastid system, peculiar plant cytoskeletal organization and chromatin structure, the plant cell wall, the vacuolar system, and in general, endomembrane structures. The cyanotoxins have characteristic dose-and plant genotype-dependent effects on all these structures. Alterations in chloroplast structure will influence the efficiency of photosynthesis and thus plant productivity. Changing of cell wall composition, disruption of the vacuolar membrane (tonoplast) and cytoskeleton, and alterations of chromatin structure (including DNA strand breaks) can ultimately lead to cell death. Finally, we present an integrated view of subcellular alterations. Knowledge on these changes will certainly contribute to a better understanding of cyanotoxin–plant interactions.
The serine-threonine protein phosphatases PP2A regulate many cellular processes, however their role in oxidative stress responses and defence is less known. We show the involvement of its C ...(catalytic) and B” (a regulatory) subunits. The c3c4 (C subunit) and fass (B”) subunit mutants and Col wt of Arabidopsis were used. Controls and treatments with the PP2A inhibitor microcystin-LR (MCY-LR) and reactive oxygen species (ROS) inducer diquat (DQ) were employed. ROS levels of primary roots were largely genotype dependent and both C and B” subunit mutants had increased sensitivity to MCY-LR and DQ indicating the involvement of these subunits in oxidative stress induction. Superoxide dismutases (SOD), mainly the Cu/Zn-SOD isoform, as key enzymes involved in ROS scavenging are also showing altered (mostly increased) activities in both c3c4 and fass mutants and have opposite relations to ROS induction. This indicates that the two types of subunits involved have partially different regulatory roles. In relation to this, control and MCY-LR/DQ treated B” subunit mutants were proven to have altered levels of phosphorylation of histone H2AX. γH2AX, the phosphorylated form indicates double stranded DNA damage during oxidative stress. Overall we point out the probable pivotal role of several PP2A subunits in the regulation of oxidative stress responses in plants and pave the way for future research to reveal the signaling pathways involved.
•C (catalytic) and B” subunits of PP2A modulate ROS induction and scavenging.•B” subunit regulates histone H2AX phosphorylation, an indicator of DNA damage repair.•PP2A regulates ROS levels and their physiological effects in Arabidopsis.
PDF
