DELLA signaling mediates stress-induced cell differentiation in Arabidopsis leaves through modulation of anaphase-promoting complex/cyclosome activity

Drought is responsible for considerable yield losses in agriculture due to its detrimental effects on growth. Drought responses have been extensively studied, but mostly on the level of complete plants or mature tissues. However, stress responses were shown to be highly tissue and developmental stage specific, and dividing tissues have developed unique mechanisms to respond to stress. Previously, we studied the effects of osmotic stress on dividing leaf cells in Arabidopsis (Arabidopsis thaliana) and found that stress causes early mitotic exit, in which cells end their mitotic division and start endoreduplication earlier. In this study, we analyzed this phenomenon in more detail. Osmotic stress induces changes in gibberellin metabolism, resulting in the stabilization of DELLAs, which are responsible for mitotic exit and earlier onset of endoreduplication. Consequently, this response is absent in mutants with altered gibberellin levels or DELLA activity. Mitotic exit and onset of endoreduplication do not correlate with an up-regulation of known cell cycle inhibitors but are the result of reduced levels of DP-E2F-LIKE1/E2Fe and UV-B-INSENSITIVE4, both inhibitors of the developmental transition from mitosis to endoreduplication by modulating anaphase-promoting complex/cyclosome activity, which are down-regulated rapidly after DELLA stabilization. This work fits into an emerging view of DELLAs as regulators of cell division by regulating the transition to endoreduplication and differentiation.     

Stem Cell Maintenance and Abiotic Stress Response in Shoot Apical Meristem for Developmental Plasticity

The shoot apical meristem (SAM) serves as a non-drying reservoir of pluripotent stem cells to supply new daughter cells forming above-ground tissues and organs such as leaves, stems, flowers and fruits throughout the life cycle of plants. Accordingly, the homeostasis control of stem cell division and differentiation must be an essential core mechanism for harmonic growth and development of plants as multicellular higher eukaryotes. Unlike animals, plants are sessile organisms and thus constantly face environmental factors, including abiotic stresses. Therefore, post-embryonic development derived from stem cells in the SAM likely interacts with surrounding abiotic stresses for plant adaptation and plastic development. For this reason, this review provides the most recent findings regarding comprehensive signaling networks involved in stem cell maintenance in the SAM, and then describes how stem cell signaling is related with abiotic stress response through involvement of phytohormones and reactive oxygen species in the SAM.     

Phytohormone Priming: Regulator for Heavy Metal Stress in Plants

Phytohormones act as chemical messengers and, under a complex regulation, allow plants to sustain biotic and abiotic stresses. Thus, phytohormones are known for their regulatory role in plant growth and development. Heavy metals (HMs) play an important role in metabolism and have roles in plant growth and development as micronutrients. However, at a level above threshold, these HMs act as contaminants and pose a worldwide environmental threat. Thus, finding eco-friendly and economical deliverables to tackle this problem is a priority. In addition to physicochemical methods, exogenous application of phytohormones, i.e., auxins, cytokinins, and gibberellins, can positively influence the regulation of the ascorbate–glutathione cycle, transpiration rate, cell division, and the activities of nitrogen metabolism and assimilation, which improve plant growth activity. Brassinosteroids, ethylene and salicylic acid have been reported to enhance the level of the anti-oxidant system, decrease levels of ROS, lipid peroxidation and improve photosynthesis in plants, when applied exogenously under a HM effect. There is a crosstalk between phytohormones which is activated upon exogenous application. Research suggests that plants are primed by phytohormones for stress tolerance. Chemical priming has provided good results in plant physiology and stress adaptation, and phytohormone priming is underway. We have reviewed promising phytohormones, which can potentially confer enhanced tolerance when used exogenously. Exogenous application of phytohormones may increase plant performance under HM stress and can be used for agro-ecological benefits under environmental conditions with high HMs level.

     

Studies on endoreduplication. III. Endoreduplication induced by mitomycin C in PHA-stimulated tonsillar lymphocyte cultures

The activity of mitomycin C (MMC) to induce endoreduplication was examined in PHA-stimulated tonsillar lymphocyte cultures. Cellular kinetics of both diploid and endoreduplicated cells was studied by the combined techniques of thymidine and BrdU incorporation. The kinetic data showed that the increase of endoreduplicated cells induced by MMC correlated with the increase of the second-generation diploid cells. Also, it showed that the majority of endoreduplicated cells found at the time when the maximal number of endoreduplicated cells was obtained had incorporated BrdU only once into their diplochromosomes. This suggests that MMC-induced endoreduplicated cells originate from MMC-induced G2-arrested cells.     

Genetic,molecular, and humoral endocycle-regulating mechanisms

The review presents data on the molecular genetic mechanisms controlling endoreduplication. The issues concerning the activity of the main cycle cell regulators, such as cyclins, cyclin-dependent kinases, and their inhibitors, under conditions of a modified cell cycle of polytene cells are considered. Specific features of regulation at the replication origin points and the role of hormones and phytohormones in the ontogenetic control of endoreduplication are analyzed.     

Regulatory mechanism of cell division : I. Colchicine-induced endoreduplication

Experiments were carried out to study the induction of endoreduplication by colchicine in Chinese hamster cells cultivated in vitro. The cells that endoreduplicate are those that, at the moment of treatment, are in late-S and, in particular, in G2. The endoreduplication cycle consists of two periods of synthesis (S1 and S2), as already noted by Schwarzacher & Schnedl [42], separated by an intervening period that we call G? The S2 synthesis begins in a highly synchronous manner, without the cells having gone into a c-mitosis. The quantity of endoreduplicated cells induced is proportional to the 3.5th root of the colchicine concentration, above a threshold value, and does not depend on the duration of the treatment. When the cultures are treated twice with colchicine, the second treatment is also able to induce endoreduplication and, after it, there appear double endoreduplicated cells (with quadruplochromosomes).     

Crop Improvement and Abiotic Stress Tolerance Promoted by Moringa Leaf Extract

Moringa leaf extract (MLE) has been shown to promote beneficial outcomes in animals and plants. It is rich in amino acids, antioxidants, phytohormones, minerals, and many other bioactive compounds with nutritional and growth-promoting potential. Recent reports indicated that MLE improved abiotic stress tolerance in plants. Our understanding of the mechanisms underlying MLE-mediated abiotic stress tolerance remains limited. This review summarizes the existing literature on the role of MLE in promoting plant abiotic stress acclimation processes. MLE is applied to plants in a variety of ways, including foliar spray, rooting media, and seed priming. Exogenous application of MLE promoted crop plant growth, photosynthesis, and yield under both nonstress and abiotic stress conditions. MLE treatment reduced the severity of osmotic and oxidative stress in plants by regulating osmolyte accumulation, antioxidant synthesis, and secondary metabolites. MLE also improves mineral homeostasis in the presence of abiotic stress. Overall, this review describes the potential mechanisms underpinning MLE-mediated stress tolerance.     

植物血红素加氧酶研究进展

本文综述了血红素加氧酶（HO）的催化机理、结构特征、遗传学突变体以及在光敏色素发色团合成、调控根形态发生和参与植物对非生物胁迫应答中的作用及信号转导。同时,介绍了紫外线和盐胁迫等逆境条件以及生长素和脱落酸等植物激素对HO基因表达调控等方面的研究进展。     

Cytogenetic demonstration of out-of-phase DNA synthesis in endoreduplicated CHO cells: evidence for partial endoreduplication

Out-of-phase DNA synthesis, which is demonstrated cytogenetically as premature chromosome condensation (PCC), was analyzed in endoreduplicated Chinese hamster ovary (CHO) cells induced by colchicine or vincristine. Like conventional polyploid cells, endoreduplicated cells exhibited PCC in either S or G2. The former was more frequently observed in drug-treated cultures. In addition to these two types of PCC, other mitotic figures showing out-of-phase DNA synthesis were found. Such cells contained both conventional chromosomes (monochromosomes) and diplochromosomes. Differential FPG staining of chromatids in these cells showed that diplochromosomes incorporated BrdU twice while monochromosomes did so once, indicating the occurrence of partial endoreduplication in one of the sister nuclei of multinucleate cells. The possible mechanisms underlying induction of out-of-phase DNA synthesis and production of partial endoreduplication are discussed.     

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids — including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids — also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.     

Neuroprotective and Neurological Properties of <Emphasis Type="Italic">Melissa officinalis</Emphasis>

Melissa officinalis has traditionally been used due to its effects on nervous system. Both methanolic and aqueous extracts were tested for protective effects on the PC12 cell line, free radical scavenging properties and neurological activities (inhibition of MAO-A and acetylcholinesterase enzymes and affinity to the GABA_A-benzodiazepine receptor). The results suggest that the plant has a significant (P < 0.05) protective effect on hydrogen peroxide induced toxicity in PC12 cells. The radical scavenging properties were also investigated in cells and in cell free systems, where this plant was shown to be a good free radical scavenger. The MAO-A bioassay was also performed to detect possible antidepressant activities demonstrating that both extracts inhibited this enzyme, which has a key role in neurotransmitters metabolism. However, no activity was detected in the acetylcholinesterase and GABA assays. In general, the methanolic extract was more effective than the aqueous.     

The C2H2 zinc‐finger protein SlZF3 regulates AsA synthesis and salt tolerance by interacting with CSN5B

Abiotic stresses are a major cause of crop loss. Ascorbic acid (AsA) promotes stress tolerance by scavenging reactive oxygen species (ROS), which accumulate when plants experience abiotic stress. Although the biosynthesis and metabolism of AsA are well established, the genes that regulate these pathways remain largely unexplored. Here, we report on a novel regulatory gene from tomato (Solanum lycopersicum) named SlZF3 that encodes a Cys2/His2‐type zinc‐finger protein with an EAR repression domain. The expression of SlZF3 was rapidly induced by NaCl treatments. The overexpression of SlZF3 significantly increased the levels of AsA in tomato and Arabidopsis. Consequently, the AsA‐mediated ROS‐scavenging capacity of the SlZF3‐overexpressing plants was increased, which enhanced the salt tolerance of these plants. Protein–protein interaction assays demonstrated that SlZF3 directly binds CSN5B, a key component of the COP9 signalosome. This interaction inhibited the binding of CSN5B to VTC1, a GDP‐mannose pyrophosphorylase that contributes to AsA biosynthesis. We found that the EAR domain promoted the stability of SlZF3 but was not required for the interaction between SlZF3 and CSN5B. Our findings indicate that SlZF3 simultaneously promotes the accumulation of AsA and enhances plant salt‐stress tolerance.     

Reducing AsA Leads to Leaf Lesion and Defence Response in Knock-Down of the AsA Biosynthetic Enzyme GDP-D-Mannose Pyrophosphorylase Gene in Tomato Plant

As a vital antioxidant, L-ascorbic acid (AsA) affects diverse biological processes in higher plants. Lack of AsA in cell impairs plant development. In the present study, we manipulated a gene of GDP-mannose pyrophosphorylase which catalyzes the conversion of D-mannose-1-P to GDP-D-mannose in AsA biosynthetic pathway and found out the phenotype alteration of tomato. In the tomato genome, there are four members of GMP gene family and they constitutively expressed in various tissues in distinct expression patterns. As expected, over-expression of SlGMP3 increased total AsA contents and enhanced the tolerance to oxidative stress in tomato. On the contrary, knock-down of SlGMP3 significantly decreased AsA contents below the threshold level and altered the phenotype of tomato plants with lesions and further senescence. Further analysis indicated the causes for this symptom could result from failing to instantly deplete the reactive oxygen species (ROS) as decline of free radical scavenging activity. More ROS accumulated in the leaves and then triggered expressions of defence-related genes and mimic symptom occurred on the leaves similar to hypersensitive responses against pathogens. Consequently, the photosynthesis of leaves was dramatically fallen. These results suggested the vital roles of AsA as an antioxidant in leaf function and defence response of tomato.     

The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants.

Plant organs develop mostly post-embryonically from persistent or newly formed meristems. After cell division arrest, differentiation frequently involves endoreduplication and cell enlargement. Factors controlling transition from mitotic cycles to differentiation programmes have not been identified yet in plants. Here we describe ccs52, a plant homologue of APC activators involved in mitotic cyclin degradation. The ccs52 cDNA clones were isolated from Medicago sativa root nodules, which exhibit the highest degree of endopolyploidy in this plant. ccs52 represents a small multigenic family and appears to be conserved in plants. Overexpression of ccs52 in yeast triggered mitotic cyclin degradation, cell division arrest, endoreduplication and cell enlargement. In Medicago, enhanced expression of ccs52 was found in differentiating cells undergoing endoreduplication. In transgenic M.truncatula plants, overexpression of the ccs52 gene in the antisense orientation resulted in partial suppression of ccs52 expression and decreased the number of endocycles and the volume of the largest cells. Thus, the ccs52 product may switch proliferating cells to differentiation programmes which, in the case of endocycles, result in cell size increments.     

植物根边缘细胞的抗逆性研究进展

综述了近几年来国内外有关植物根边缘细胞抗逆性方面的研究,重点概述植物根边缘细胞对生物与非生物胁迫的响应及其相应的抗性机理。在生物胁迫下,边缘细胞能吸引和固定病原根结线虫,排斥或约束致病性细菌,可作为真菌感染的假目标,减少或避免各种病原菌对根尖的伤害。在非生物胁迫下,边缘细胞通过分泌粘液、诱导ROS产生刺激细胞死亡以抵抗铝毒,并通过其数量的改变来调节高温、高浓度CO2等多种生理反应。最后在当前植物根边缘细胞研究的基础上,提出了今后的研究方向。     

Compatible solutes reduce ROS-induced potassium efflux in Arabidopsis roots

Reactive oxygen species (ROS) are known to be primarily responsible for the impairment of cellular function under numerous abiotic and biotic stress conditions. In this paper, using non-invasive microelectrode ion flux measuring (MIFE) system, we show that the application of a hydroxyl radical (OH*)-generating Cu2+/ascorbate (Cu/a) mixture to Arabidopsis roots results in a massive, dose-dependent efflux of K+ from epidermal cells in the elongation zone. Pharmacological experiments suggest that both outward-rectifying K+ channels and non-selective cation channels (NSCCs) mediate such effluxes. Low (5 mM) concentrations of compatible solutes (glycine betaine, proline, mannitol, trehalose or myo-inositol) significantly reduces OH*-induced K+ efflux, similar to our previous reports for NaCl-induced K+ efflux. Importantly, a significant reduction in K+ efflux was found using osmolytes with no reported free radical scavenging activity, as well as those for which a role in free radical scavenging has been demonstrated. This indicates that compatible solutes must play other (regulatory) roles, in addition to free radical scavenging, in mitigating the damaging effects of oxidative stress.     

Cell cycle regulation in the course of nodule organogenesis in Medicago

The molecular mechanisms of de novo meristem formation, cell differentiation and the integration of the cell cycle machinery into appropriate stages of the developmental programmes are still largely unknown in plants. Legume root nodules, which house nitrogen-fixing rhizobia, are unique plant organs and their development may serve as a model for organogenetic processes in plants. Nodules form and are essential for the plant only under limitation of combined nitrogen in the soil. Moreover, their development is triggered by external mitogenic signals produced by their symbiotic partners, the rhizobia. These signals, the lipochitooligosaccharide Nod factors, act as host-specific morphogens and induce the re-entry of root cortical cells into mitotic cycles. Maintenance of cell division activity leads to the formation of a persistent nodule meristem from which cells exit continuously and enter the nodule differentiation programme, involving multiple cycles of endoreduplication and enlargement of nuclear and cell volumes. While the small diploid 2C cells remain uninfected, the large polyploid cells can be invaded and, after completing the differentiation programme, host the nitrogen-fixing bacteroids. This review summarizes the present knowledge on cell cycle reactivation and meristem formation in response to Nod factors and reports on a novel plant cell cycle regulator that can switch mitotic cycles to differentiation programmes.     

Endoreduplication preferentially occurs at the proximal side of the abscission zone during abscission of tomato leaf

Endoreduplication is a cell cycle variant in which multiple rounds of DNA replication occur without subsequent mitosis, resulting in polyploid cells. Although cells with endoreduplicated nuclei were ubiquitously distributed throughout the abscission zone (AZ) of tomato leaf before abscission induction by ethylene, endoreduplication was detected mostly on the proximal side of the AZ after induction. The possible association between endoreduplication and intensive membrane trafficking in cells at the proximal side of the AZ is discussed.