Mechanistic study of programmed cell death of root border cells of cucumber (Cucumber sativus L.) induced by copper

Receptor Plants Alleviated Allelopathic Stress from Invasive Chenopodium ambrosioides L. by Upregulating the Production and Autophagy of Their Root Border Cells

Chenopodium ambrosioides L. is an invasive plant native to the Neotropics that has seriously threatened the ecological security of China, and allelopathy is one of the mechanisms underlying its successful invasion. Maize (Zea mays L.) and soybean (Glycine max (L.) Merr.), as the main food crops, are usually affected by C. ambrosioides in their planting areas. The purpose of this study was to investigate the ultrastructure, autophagy, and release-related gene expression of receptor plant root border cells (RBCs) after exposure to volatile oil from C. ambrosioides and its main component α-terpene, which were studied using maize and soybean as receptor plants. The volatiles inhibited root growth and promoted a brief increase in the number of RBCs. As the volatile concentration increased, the organelles in RBCs were gradually destroyed, and intracellular autophagosomes were produced and continuously increased in number. Transcriptomic analysis revealed that genes involved in the synthesis of the plasma membrane and cell wall components in receptor root cells were significantly up-regulated, particularly those related to cell wall polysaccharide synthesis. Meanwhile, polygalacturonase and pectin methylesterases (PME) exhibited up-regulated expression, and PME activity also increased. The contribution of α-terpene to this allelopathic effect of C. ambrosioides volatile oil exceeded 70%. Based on these results, receptor plant root tips may increase the synthesis of cell wall substances while degrading the intercellular layer, accelerating the generation and release of RBCs. Meanwhile, their cells survived through autophagy of RBCs, indicating the key role of RBCs in alleviating allelopathic stress from C. ambrosioides volatiles.

Transmembrane potential, an indicator in situ reporting cellular senescence and stress response in plant tissues

BACKGROUND

Plant cells usually sustain a stable membrane potential due to influx and/or efflux of charged ions across plasma membrane. With the growth and development of plants, different tissues and cells undergo systemic or local programmed decline. Whether the membrane potential of plasma membrane could report senescence signal of plant tissues and cells is unclear.

RESULTS

We applied a maneuverable transmembrane potential (TMP) detection method with patch-clamp setup to examine the senescence signal of leaf tissue cells in situ over the whole life cycle in Arabidopsis thaliana. The data showed that the TMPs of plant tissues and cells were varied at different growth stages, and the change of TMP was higher at the vegetative growth stage than at the reproductive stage of plant growth. The distinct change of TMP was detectable between the normal and the senescent tissues and cells in several plant species. Moreover, diverse abiotic stimuli, such as heat stress, hyperpolarized the TMP in a short time, followed by depolarized membrane potential with the senescence occurring. We further examined the TMP of plant chloroplasts, which also indicates the senescence signal in organelles.

CONCLUSIONS

This convenient TMP detection method can report the senescence signal of plant tissues and cells, and can also indicate the potential of plant tolerance to environmental stress.

Involvement of abscisic acid in silicon-mediated enhancement of copper stress tolerance in Artemisia annua

Heavy metal (HM) toxicity is a well-known hazard which causes deleterious impact on the growth and development of plants. The impact of abscisic acid (ABA) in presence of silicon (Si) on plant development and quality traits has largely gone unexplored. The effects of ABA and Si on the growth, yield, and quality characteristics of Artemisia annua L. plants growing under copper (Cu) stress (20 and 40 mg kg^-1) were investigated in a pot experiment. During this investigation, Cu stress caused severe damage to the plants but exogenous administration of Si and ABA ameliorated the harmful effects of Cu toxicity, and the plants displayed higher biomass and improved physio-biochemical attributes. Copper accumulated in the roots and shoots and its toxicity caused oxidative stress as demonstrated by the increased 2-thiobarbituric acid reactive substance (TBARS) content. It also resulted in the increased activity of antioxidant enzymes, however, the exogenous Si and ABA supplementation decreased the buildup of reactive oxygen species (ROS) and lipid peroxidation, alleviating the oxidative damage produced by HM stress. Copper toxicity had a considerable negative impact on glandular trichome density, ultrastructure as well as artemisinin production. However, combined Si and ABA enhanced the size and density of glandular trichomes, resulting in higher artemisinin production. Taken together, our results demonstrated that exogenous ABA and Si supplementation protect A. annua plants against Cu toxicity by improving photosynthetic characteristics, enhancing antioxidant enzyme activity, protecting leaf structure and integrity, avoiding excess Cu deposition in shoot and root tissues, and helping in enhanced artemisinin biosynthesis. Our results indicate that the combined application of Si and ABA improved the overall growth of plants and may thus be used as an effective approach for the improvement of growth and yield of A. annua in Cu-contaminated soils.

Programmed cell death process in freshwater Microcystis aeruginosa and marine Phaeocystis globosa induced by a plant derived allelochemical

Harmful algal blooms (HAB) are a serious problem worldwide. Allelochemicals from natural plants were recently thought to be promising anti-algaecide in controlling harmful algae. However, the programmed cell death (PCD) process of algae under allelopathic pressure induced by 5,4'-dihydroxyflavone (5,4'-DHF) was poorly understood. In this study, two common and worldwide distributed microalgae, Microcystis aeruginosa and Phaeocystis globosa were selected as target algae, and the PCD processes induced by 5,4'-DHF were cross-compared between the two species. Both algae species were inhibited significantly by 5,4'-DHF with the relative sensitivity of 0.11. To uncover the PCD progress systematically, signals for PCD triggering, antioxidant enzyme activity, photosynthetic ability variation, caspase-like activities and typical indicators were investigated. In both species, typical indicators of PCD - phosphatidylserine externalization and chromatin condensation - were detected. The intracellular reactive oxygen species (ROS), nitric oxide (NO) and H₂O₂ were the potential signal molecules to stimulate PCD, and caspase-like activities were activated with an elevation of cytochrome c indicating the initiation of PCD in both species. However, P. globosa responded to 5,4'-DHF immediately after 3 h with the elevation of ROS and not in M. aeruginosa. Antioxidant enzyme activities of superoxide dismutase (SOD) and catalase (CAT) in M. aeruginosa and P. globosa also showed different patterns on day 3. Specifically, SOD activity in M. aeruginosa increased significantly while it decreased significantly in P. globosa, CAT activity in M. aeruginosa decreased significantly while it increased significantly in P. globosa (p < 0.05). Malondialdehyde (MDA) content in P. globosa increased significantly (p < 0.001) while it showed no variation in M. aeruginosa. Overall, this study is one of the earliest studies to explore the inhibition and action mechanism of plant derived flavonoids on harmful algae from the perspective of PCD, and provide new insights into the antialgal mechanism of allelochemicals.

Phytotoxicity mechanism of the natural radionuclide thorium in Vicia faba

Elucidation of the phytotoxic mechanisms of thorium (Th) is important for controlling Th accumulation in crops and improving the efficiency of phytoremediation. Here, we analyzed the subcellular distribution of Th in Vicia faba seedlings and the toxic reaction of seedlings to Th (5-40 μmol·L^-1) at the subcellular and cellular levels. Increasing the phosphate level in the culture medium from 0.01 to 0.1 mmol·L^-1 decreased the Th accumulation by the roots by 47-57%. Th was mainly distributed in the root cell walls (94-96%) and existed mainly in the form of residue (92-94%). Th accumulation in the root was similar to the changes observed for P, Ni, Cu, and Fe. High concentrations of Th (40 μmol·L^-1) induced abnormal root growth and leaf photosynthetic metabolism. At the cellular level, Th (40 μmol·L^-1) induced root edge cell death and inhibited root respiration and cell mitosis. SOD, POD and CAT activities were involved in the regulation of reactive oxygen species accumulation in the roots. Untargeted metabolomics identified 580 and 262 differentially expressed metabolites in roots and leaves. At the metabolic level, its toxicological mechanism involved a severe inhibition of the expression of nucleotides in roots and leaves.

Rhizobium Inoculation Enhances the Resistance of Alfalfa and Microbial Characteristics in Copper-Contaminated Soil

Some studies have reported the importance of rhizobium in mitigating heavy metal toxicity, however, the regulatory mechanism of the alfalfa-rhizobium symbiosis to resist copper (Cu) stress in the plant-soil system through biochemical reactions is still unclear. This study assessed the effects of rhizobium (Sinorhizobium meliloti CCNWSX0020) inoculation on the growth of alfalfa and soil microbial characteristics under Cu-stress. Further, we determined the regulatory mechanism of rhizobium inoculation to alleviate Cu-stress in alfalfa through plant-soil system. The results showed that rhizobium inoculation markedly alleviated Cu-induced growth inhibition in alfalfa by increasing the chlorophyll content, height, and biomass, in addition to nitrogen and phosphorus contents. Furthermore, rhizobium application alleviated Cu-induced phytotoxicity by increasing the antioxidant enzyme activities and soluble protein content in tissues, and inhibiting the lipid peroxidation levels (i.e., malondialdehyde content). In addition, rhizobium inoculation improved soil nutrient cycling, which increased soil enzyme activities (i.e., β-glucosidase activity and alkaline phosphatase) and microbial biomass nitrogen. Both Pearson correlation coefficient analysis and partial least squares path modeling (PLS-PM) identified that the interactions between soil nutrient content, enzyme activity, microbial biomass, plant antioxidant enzymes, and oxidative damage could jointly regulate plant growth. This study provides comprehensive insights into the mechanism of action of the legume-rhizobium symbiotic system to mitigate Cu stress and provide an efficient strategy for phytoremediation of Cu-contaminated soils.

Insights into Plant Programmed Cell Death Induced by Heavy Metals-Discovering a Terra Incognita

Programmed cell death (PCD) is a process that plays a fundamental role in plant development and responses to biotic and abiotic stresses. Knowledge of plant PCD mechanisms is still very scarce and is incomparable to the large number of studies on PCD mechanisms in animals. Quick and accurate assays, e.g., the TUNEL assay, comet assay, and analysis of caspase-like enzyme activity, enable the differentiation of PCD from necrosis. Two main types of plant PCD, developmental (dPCD) regulated by internal factors, and environmental (ePCD) induced by external stimuli, are distinguished based on the differences in the expression of the conserved PCD-inducing genes. Abiotic stress factors, including heavy metals, induce necrosis or ePCD. Heavy metals induce PCD by triggering oxidative stress via reactive oxygen species (ROS) overproduction. ROS that are mainly produced by mitochondria modulate phytotoxicity mechanisms induced by heavy metals. Complex crosstalk between ROS, hormones (ethylene), nitric oxide (NO), and calcium ions evokes PCD, with proteases with caspase-like activity executing PCD in plant cells exposed to heavy metals. This pathway leads to very similar cytological hallmarks of heavy metal induced PCD to PCD induced by other abiotic factors. The forms, hallmarks, mechanisms, and genetic regulation of plant ePCD induced by abiotic stress are reviewed here in detail, with an emphasis on plant cell culture as a suitable model for PCD studies. The similarities and differences between plant and animal PCD are also discussed.

Do plants use root-derived proteases to promote the uptake of soil organic nitrogen?

AIMS

The capacity of plant roots to directly acquire organic nitrogen (N) in the form of oligopeptides and amino acids from soil is well established. However, plants have poor access to protein, the central reservoir of soil organic N. Our question is: do plants actively secrete proteases to enhance the breakdown of soil protein or are they functionally reliant on soil microorganisms to undertake this role?

METHODS

Growing maize and wheat under sterile hydroponic conditions with and without inorganic N, we measured protease activity on the root surface (root-bound proteases) or exogenously in the solution (free proteases). We compared root protease activities to the rhizosphere microbial community to estimate the ecological significance of root-derived proteases.

RESULTS

We found little evidence for the secretion of free proteases, with almost all protease activity associated with the root surface. Root protease activity was not stimulated under N deficiency. Our findings suggest that cereal roots contribute one-fifth of rhizosphere protease activity.

CONCLUSIONS

Our results indicate that plant N uptake is only functionally significant when soil protein is in direct contact with root surfaces. The lack of protease upregulation under N deficiency suggests that root protease activity is unrelated to enhanced soil N capture.

Assessment of cyto- and genotoxic effects of Cesium-133 in Vicia faba using single-cell gel electrophoresis and random amplified polymorphic DNA assays

The aim of this study was to evaluate the ecotoxic effect of high concentration cesium (Cs) exposure on plant root growth and its toxicological mechanism. The radicle of broad bean (Vicia faba) was selected as experimental material. The cytotoxic and genotoxic effects of plants exposed to different Cs levels (0.19-1.5 mM) for 48 h were evaluated using scanning electron microscopy (SEM), X-ray fluorescence (XRF) analysis, single-cell gel electrophoresis (SCGE) and random amplified polymorphic DNA (RAPD) assays. The results showed that radicle elongation decreased clearly after 48 h of exposure treatment with different concentrations of Cs solution. The root cell structure was obviously damaged in the Cs treatment groups (0.19-1.5 mM). At a Cs concentration of 1.5 mM, the percentages of viable non-apoptotic cells, viable apoptotic cells, non-viable apoptotic cells, and non-viable cells were 40.09%, 20.67%, 28.73%, and 10.52%, respectively. SCGE showed DNA damage in radicle cells 48 h after Cs exposure. Compared with the control group, the percentage of tail DNA in Cs exposed group (0.38-1.5 mM) increased by 0.56-1.12 times (P < 0.05). RAPD results showed that the genomic stability of V. faba radicles decreased by 4.44%-15.56%. This study confirmed that high concentration Cs exposure had cytotoxicity and genotoxicity effects on plants.

The leaf extract of crofton weed (Eupatorium adenophorum) inhibits primary root growth by inducing cell death in maize root border cells

The extract of crofton weed (Eupatorium adenophorum) inhibits seed germination and weed growth; however, the physiological mechanisms underlying the effect of crofton weed extract on the modulation of seedling growth and root system development remain largely unclear. In this study, we investigated the effects of the leaf extract of crofton weed (LECW) on primary root (PR) growth in maize seedlings. Treatment with LECW markedly inhibited seed germination and seedling growth in a dose-dependent manner. Physiological analysis indicated that the LECW induced reactive oxygen species (ROS) accumulation in root tips, thereby leading to cell swelling and deformation both in the root cap and elongation zone of root tips, finally leading to cell death in root border cells (RBCs) and PR growth inhibition. The LECW also inhibited pectin methyl esterase (PME) activity, thereby decreasing the RBC number. Taken together, our results indicated that the LECW inhibited PR growth by inducing ROS accumulation and subsequent cell death in RBCs. The present study provides a better understanding of how the LECW modifies root system development and provides insight for evaluating the toxicity of crofton weed extracts in plants.

Toxic mechanism of eucalyptol and β-cyclocitral on Chlamydomonas reinhardtii by inducing programmed cell death

Eucalyptol and β-cyclocitral are 2 main compounds in cyanobacterial volatile organic compounds and can poison other algae. To uncover the toxic mechanism of the 2 compounds, the cell growth, photosynthetic abilities, H₂O₂ production, caspase-like activities, nuclear variation and DNA laddering were investigated in Chlamydomonas reinhardtii treated with eucalyptol and β-cyclocitral. Eucalyptol at ≥ 0.1 mM and β-cyclocitral at ≥ 0.05 mM showed toxic effects on C. reinhardtii cells, and 1.2 mM eucalyptol and 0.4 mM β-cyclocitral killed the whole of the cells during 24 h. During the death process, the photosynthetic pigment gradually degraded, and Fv/Fm gradually declined, indicating that the death is not a necrosis due to the gradual disappearance of the physiological process. In the treatments with 1.2 mM eucalyptol and 0.4 mM β-cyclocitral, H₂O₂ content burst at 10 min and 30 min, respectively. Caspase-9-like and caspase-3-like were activated, and cell nucleuses concentrated firstly and then broke with prolonging the treatment time. Meanwhile, DNA showed laddering after 1 h, and was gradually cleaved by Ca²⁺-dependent endonucleases to mainly about 100-250 bp fragments. These hallmarks indicated that eucalyptol and β-cyclocitral may poison other algal cells by inducing programmed cell death triggered by the increased H₂O₂.

Copper treatment of peach leaves causes lesion formation similar to the biotic stress response

Peach (Prunus persica (L.) Batsch) leaves are sensitive to copper (Cu) exposure. The symptoms of Cu exposure are similar to those of bacterial spot disease; however, the mechanism underlying lesion formation caused by Cu exposure is not clear. Here, we investigated whether lesion formation caused by Cu exposure was related to the mechanism underlying plant resistance to microbial pathogens. When Cu was applied to the centre of a pinhole on peach leaves, a two-step process was observed. A pale green section in the shape of a doughnut, located far from a Cu treatment point, first appeared on a leaf treated with 2 mM CuSO₄. Next, a yellow-white section gradually spread from the Cu treatment point to the pale green section. Finally, a gap was formed in the middle of the pale green section. The inner part of the pale green section contained 96% of the Cu applied, indicating that Cu is retained in the lesion area. Real-time PCR analysis of the expression of genes encoding pathogenesis-related proteins and enzymes involved in phytoalexin synthesis revealed that three genes (encoding chitinase, pathogenesis-related protein 4, and β-1,3-glucanase-3) of the eight tested were upregulated by Cu treatment. Furthermore, treatment with caspase-1 inhibitors reduced lesion formation. These results show that Cu treatment of peach leaves causes cell death similar to that occurring during the biotic stress response.

De novo assembly of the Platycladus orientalis (L.) Franco transcriptome provides insight into the development and pollination mechanism of female cone based on RNA-Seq data

For seed-bearing plants, the basis of seed and fruit formation is pollination. The normal progression of pollination is through advances in continuous signal exchange and material transfer, which occur mainly in female reproductive organs; thus, the molecular mechanism of development in female reproductive organs is vital for understanding the principle of pollination. However, molecular biology studies on the development of female cones related to pollination are rare and unclear in gymnosperms, especially in Cupressaceae. In this study, Platycladus orientalis, a monotypic genus within Cupressaceae, was chosen to examine female cone transcriptomes at pre-pollination and pollination stages by Illumina paired-end sequencing technology to de novo sequence six libraries with 3 biological replicates. These libraries were used to construct a P. orientalis transcriptome database containing 71,669 unigenes (4,963 upregulated unigenes and 11,747 downregulated unigenes at the pollination stage) for subsequent analysis. Based on the annotations and expression levels, the functions of differentially expressed unigenes and enriched pathways between the developmental processes of female cones were analysed to detail the preliminary development and pollination mechanism of the female cone. Targeted investigations were specifically performed to determine the elementary mechanism of secretion and functioning of the pollination drop, a vital ovule secretion at the pollination stage. Ultimately, the expression of 15 unigenes selected between two stages were further assessed and confirmed using qRT-PCR, which demonstrated reliable data and significant differences in the expression profiles of key genes. As one of the largest available transcriptomic resources of this species, the database is constructed to prospectively adapt to the physiological and genomic data of woody plants. This work provided the first transcriptome profile of P. orientalis female cones at different developmental stages, and will promote the illumination of the pollination mechanism of P. orientalis, and will serve as the basis for in-depth genomic study in the Cupressaceae family. This initiative will arouse the interest and attention of scholars and pave the way for future studies.

Linalool- and α-terpineol-induced programmed cell death in Chlamydomonas reinhardtii

Plant allelochemicals effectively inhibit and/ or control algal growth, and have potential to use as algaecide. To uncover the lethal mechanism of 2 anti-algal compounds linalool and α-terpineol identified from Cinnamomum camphora extracts, and promote their development as algaecide, the H₂O₂ production, photosynthetic abilities, caspase-like activities, nuclear changes and DNA degradation were investigated in Chlamydomonas reinhardtii treated with the 2 compounds. H₂O₂ content burst in linalool treatment at 0.5 h and in α-terpineol treatment at 1 h, with increases of 2.7 folds and 1.3 folds, respectively, compared to that at 0 h. The photosynthetic pigments gradually degraded, and Fv/Fm gradually declined to zero, indicating that the cell death was not a necrosis due to the gradual disappearance of physiological process. In C. reinhardtii cells, the caspase-9-like and caspase-3-like were activated in the treatments with the 2 compounds for 1 h. With prolonging the treatment time, the fluorescent intensity of the cell nucleuses stained by DAPI gradually enhanced and then faded, and the genomic DNA isolated from the cells gradually degraded. These hallmarks indicated that the death of C. reinhardtii cells in linalool and α-terpineol treatments was a programmed cell death (PCD) triggered by the increased reactive oxygen species (ROS). Compared to α-terpineol treatment, linalool treatment showed stronger promoting effects on PCD at the same time point, which may be caused by the higher ROS content inducing higher caspase-9-like and caspase-3-like activities in a short time.

Rhizobium inoculation enhances copper tolerance by affecting copper uptake and regulating the ascorbate-glutathione cycle and phytochelatin biosynthesis-related gene expression in Medicago sativa seedlings

Despite numerous reports that legume-rhizobium symbiosis alleviates Cu stress in plants, the possible roles of legume-rhizobium symbiosis and the regulatory mechanisms in counteracting Cu toxicity remain unclear. Here, Sinorhizobium meliloti CCNWSX0020 was used for analyzing the effects of rhizobium inoculation on plant growth in Medicago sativa seedlings under Cu stress. Our results showed that rhizobium inoculation alleviated Cu-induced growth inhibition, and increased nitrogen concentration in M. sativa seedlings. Moreover, the total amount of Cu uptake in inoculated plants was significantly increased compared with non-inoculated plants, and the increase in the roots was much higher than that in the shoots, thus decreasing the transfer coefficient and promoting Cu phytostabilization. Cu stress induced lipid peroxidation and reactive oxygen species production, but rhizobium inoculation reduced these components' accumulation through altering antioxidant enzyme activities and regulating ascorbate-glutathione cycles. Furthermore, legume-rhizobium symbiosis regulated the gene expression involved in antioxidant responses, phytochelatin (PC) biosynthesis, and metallothionein biosynthesis in M. sativa seedlings under Cu stress. Our results demonstrate that rhizobium inoculation enhanced Cu tolerance by affecting Cu uptake, regulating antioxidant enzyme activities and the ascorbate-glutathione cycle, and influencing PC biosynthesis-related gene expression in M. sativa. The results provide an efficient strategy for phytoremediation of Cu-contaminated soils.

Modulating role of ROS in re-establishing desiccation tolerance in germinating seeds of Caragana korshinskii Kom

In close agreement with visible germination, orthodox seeds lose desiccation tolerance (DT). This trait can be regained under osmotic stress, but the mechanisms are poorly understood. In this study, germinating seeds of Caragana korshinskii Kom. were investigated, focusing on the potential modulating roles of reactive oxygen species (ROS) in the re-establishment of DT. Germinating seeds with 2 mm long radicles can be rendered tolerant to desiccation by incubation in a polyethylene glycol (PEG) solution (-1.7 MPa). Upon PEG incubation, ROS accumulation was detected in the radicles tip by nitroblue tetrazolium chloride staining and further confirmed by confocal microscopy. The PEG-induced re-establishment of DT was repressed when ROS scavengers were added to the PEG solution. Moreover, ROS act downstream of abscisic acid (ABA) to modulate PEG-mediated re-establishment of DT and serve as a new inducer to re-establish DT. Transcriptomic analysis revealed that re-establishment of DT by ROS involves the up-regulation of key genes in the phenylpropanoid-flavonoid pathway, and total flavonoid content and key enzyme activity increased after ROS treatment. Furthermore, DT was repressed by an inhibitor of phenylalanine ammonia lyase. Our data suggest that ROS play a key role in the re-establishment of DT by regulating stress-related genes and the phenylpropanoid-flavonoid pathway.