Okadaic acid (1 microM) accelerates S phase and mitosis but inhibits heterochromatin replication and metaphase anaphase transition in Vicia faba meristem cells

Histone Modification Is Involved in Okadaic Acid (OA) Induced DNA Damage Response and G2-M Transition Arrest in Maize

Histone modifications are involved in regulation of chromatin structure. To investigate the relationship between chromatin modification and cell cycle regulation during plant cell proliferation, Okadaic acid (OA), a specific inhibitor of serine/threonine protein phosphatase, was applied in this study. The results showed that OA caused the cell cycle arrest at preprophase, leading to seedling growth inhibition. Western blotting assay revealed that the spatial distribution of phosphorylation of Ser10 histone H3 tails (H3S10ph) signals was altered under OA treatment. Reactive oxygen species (ROS) was found to be at higher levels and TdT-mediated dUTP nick end labeling (TUNEL) assay displayed DNA breaks happened at the chromatin after treatment with OA, companied with an increase in the acetylation of histone H4 at lysine 5 (H4K5ac) level. From these observations, we speculated that the alteration of the spatial distribution of H3S10ph and the level of H4K5ac was involved in the procedure that OA induced DNA breaks and G2-M arrested by the accumulation of ROS, and that the histone H3S10ph and H4K5ac might facilitate DNA repair by their association with the chromatin decondensation.

The crucial elements of the 'last step' of programmed cell death induced by kinetin in root cortex of V. faba ssp. minor seedlings

Kinetin-induced programmed cell death, manifested by condensation, degradation and methylation of DNA and fluctuation of kinase activities and ATP levels, is an autolytic and root cortex cell-specific process. The last step of programmed cell death (PCD) induced by kinetin in the root cortex of V. faba ssp. minor seedlings was explained using morphologic (nuclear chromatin/aggregation) and metabolic (DNA degradation, DNA methylation and kinases activity) analyses. This step involves: (1) decrease in nuclear DNA content, (2) increase in the number of 4',6-diamidino-2-phenylindole (DAPI)-stained chromocenters, and decrease in chromomycin A3 (CMA3)-stained chromocenters, (3) increase in fluorescence intensity of CMA3-stained chromocenters, (4) condensation of DAPI-stained and loosening of CMA3-stained chromatin, (5) fluctuation of the level of DNA methylation, (6) fluctuation of activities of exo-/endonucleolytic Zn(2+) and Ca(2+)/Mg(2+)-dependent nucleases, (7) changes in H1 and core histone kinase activities and (8) decrease in cellular ATP amount. These results confirmed that kinetin-induced PCD was a specific process. Additionally, based on data presented in this paper (DNA condensation and ATP depletion) and previous studies [increase in vacuole, increase in amount of cytosolic calcium ions, ROS production and cytosol acidification "in Byczkowska et al. (Protoplasma 250:121-128, 2013)"], we propose that the process resembles autolytic type of cell death, the most common type of death during development of plants. Lastly, the observations also suggested that regulation of these processes might be under control of epigenetic (methylation/phosphorylation) mechanisms.

Sucrose synthase activity and carbohydrates content in relation to phosphorylation status of Vicia faba root meristems during reactivation from sugar depletion

Carbohydrate starvation of Vicia faba root meristems leads to readjustment of carbohydrate metabolism and blocks the cell cycle in two principal control points (PCP1/2). The cell cycle reactivation is possible after sucrose provision, although with a delay of about 12h. During this period, the cells are sensitive to 6-dimethylaminopurine (6-DMAP) and okadaic acid (OA), inhibitors of protein kinases and phosphatases, respectively. The aim of the present study was to investigate whether those inhibitors are involved in inhibition of cell cycle revival through interference with the activities of two sucrose-cleaving enzymes: sucrose synthase (SuSy; EC 2.4.1.13) and invertase (INV; EC 3.2.1.26). In sugar-starved cells, the in situ activity of both enzymes decreased significantly. Following supplementation of root meristems with sugar, INV remained inactive, but SuSy activity increased. Despite the lack of INV activity, glucose was present in meristem cells, but its content was low in cells treated with OA. In the latter case, the size of plastids was reduced, they had less starch, and Golgi structures were affected. In sugar-starved cells, SuSy activity was induced more by exogenous sucrose than by glucose. The sucrose-induced activity was strongly inhibited by OA (less by 6-DMAP) at early stages of regeneration, but not at the stages preceding DNA replication or mitotic activities. The results indicate that prolongation of regeneration and a marked decrease in the number of cells resuming proliferation (observed in previous studies) and resulting from the action of inhibitors, are correlated with the process of SuSy activation at the beginning of regeneration from sugar starvation.

Microcystin-LR, a protein phosphatase inhibitor, induces alterations in mitotic chromatin and microtubule organization leading to the formation of micronuclei in Vicia faba

BACKGROUND AND AIMS

Microcystin-LR (MCY-LR) is a cyanobacterial toxin, a specific inhibitor of type 1 and 2A protein phosphatases (PP1 and PP2A) with significant impact on aquatic ecosystems. It has the potential to alter regulation of the plant cell cycle. The aim of this study was improved understanding of the mitotic alterations induced by cyanotoxin in Vicia faba, a model organism for plant cell biology studies.

METHODS

Vicia faba seedlings were treated over the long and short term with MCY-LR purified in our laboratory. Short-term treatments were performed on root meristems synchronized with hydroxylurea. Sections of lateral root tips were labelled for chromatin, phosphorylated histone H3 and β-tubulin via histochemical and immunohistochemical methods. Mitotic activity and the occurrence of mitotic alterations were detected and analysed by fluorescence microscopy. The phosphorylation state of histone H3 was studied by Western blotting.

KEY RESULTS

Long-term MCY-LR exposure of lateral root tip meristems increased the percentage of either early or late mitosis in a concentration-dependent manner. We observed hypercondensed chromosomes and altered sister chromatid segregation (lagging chromosomes) leading to the formation of micronuclei, accompanied by the formation of disrupted, multipolar and monopolar spindles, disrupted phragmoplasts and the hyperphosphorylation of histone H3 at Ser10. Short-term MCY-LR treatment of synchronized cells showed that PP1 and PP2A inhibition delayed the onset of anaphase at 1 µg mL(-1) MCY-LR, accelerated cell cycle at 10 µg mL(-1) MCY-LR and induced the formation of lagging chromosomes. In this case mitotic microtubule alterations were not detected, but histone H3 was hyperphosphorylated.

CONCLUSIONS

MCY-LR delayed metaphase-anaphase transition. Consequently, it induced aberrant chromatid segregation and micronucleus formation that could be associated with both H3 hyperphosphorylation and altered microtubule organization. However, these two phenomena seemed to be independent. The toxin may be a useful tool in the study of plant cell cycle regulation.

The Ph1 locus suppresses Cdk2-type activity during premeiosis and meiosis in wheat

Despite possessing multiple sets of related (homoeologous) chromosomes, hexaploid wheat (Triticum aestivum) restricts pairing to just true homologs at meiosis. Deletion of a single major locus, Pairing homoeologous1 (Ph1), allows pairing of homoeologs. How can the same chromosomes be processed as homologs instead of being treated as nonhomologs? Ph1 was recently defined to a cluster of defective cyclin-dependent kinase (Cdk)-like genes showing some similarity to mammalian Cdk2. We reasoned that the cluster might suppress Cdk2-type activity and therefore affect replication and histone H1 phosphorylation. Our study does indeed reveal such effects, suggesting that Cdk2-type phosphorylation has a major role in determining chromosome specificity during meiosis.

In situ activities of hexokinase and fructokinase in relation to phosphorylation status of root meristem cells of Vicia faba during reactivation from sugar starvation

The plant cell cycle is equipped with two principal control points: PCP1 in G1 and PCP2 in G2 phase. These checkpoints can arrest the cell cycle in response to carbohydrate starvation, while sugar presence can revive the replication and mitotic activity. The process of cell cycle revival is strongly repressed by okadaic acid (OA) or 6-dimethylaminopurine (6-DMAP), inhibitors of specific protein phosphatases 1 or 2A or kinases (cyclin-dependent kinases), respectively. In the present study, it was investigated whether inhibition of cell cycle revival is performed through interference of the above-mentioned inhibitors with the metabolic pathway of sucrose applied to the cells. Changes of hexokinase (HK) and fructokinase (FK) activities, key enzymes of hexose metabolism, were analyzed in Vicia faba root meristem cells arrested in G1 and G2 phase by carbohydrate starvation as well as in those recovered with glucose or sucrose in the presence of OA or 6-DMAP. It was shown that in the sugar-starved cells, the activity of both enzymes decreased significantly. During cell regeneration with carbohydrates, the activity of HK was induced more by sucrose than by glucose, while FK remained inactive after glucose addition. Moreover, in situ investigation of the activities of HK and FK showed that OA-induced and 6-DMAP-induced repression of the cell cycle revival is connected with the interference of these drugs in the metabolic pathway of sucrose. It was also indicated that stronger OA-induced and 6-DMAP-induced inhibition of the replication and mitosis revival, at the early stages of sucrose regeneration, was correlated with the stronger influence of these inhibitors on HK and FK activities.

Protein phosphorylation in Vicia faba root meristem cells during the first steps of leaving principal control points after sucrose application

Before Vicia faba root meristem cells stopped by carbohydrate starvation in principal control points (PCP1 and PCP2) start sucrose induced replication and division they go through a phase of metabolic regeneration. This interval is characterised st great sensitivity to the inhibitors of cyclin-dependent protein kinases and protein phosphatases (PPs). In the present research, changes of phosphoprotein levels in the nucleolus, nucleus and cytoplasm were analysed using okadaic acid and 6-dimethylaminopurine (6-DMAP) during the first period of cell regeneration in sucrose (0-3 h). It was established that when the cells start to leave checkpoints, the balance between protein phosphorylation and dephosphorylation shifts towards the intensified activity of PPs. Furthermore, it was also established that the structures appearing during cell regeneration, which were located around cell nuclei and which contained large amounts of phosphorylated proteins, were plastids. The reactions of protein phosphorylation which took place in the plastids were directly correlated with starch synthesis and were stopped by inactivation of protein phosphatases (PP1 and/or PP2A).