Changes in growth and structure of pea primary roots (Pisum sativum L. cv. Alaska) as a result of sudden flooding

Proteomic analysis reveals the effects of melatonin on soybean root tips under flooding stress

Flooding constrains soybean growth, while melatonin enhances the ability of plants to tolerate abiotic stresses. To interpret the melatonin-mediated flooding response in soybeans, proteomic analysis was performed in root tips. Retarded growth and severe cell death were observed in flooded soybeans, but these phenotypes were ameliorated by melatonin treatment. A total of 634, 1401, and 1205 proteins were identified under control, flood, and flood plus melatonin conditions, respectively; and these proteins were predominantly associated with metabolism of protein, RNA, and the cell wall. Among these melatonin-induced proteins, eukaryotic aspartyl protease family protein was increased after flood compared with melatonin treatment group, in accordance with its upregulated transcript levels during stress. Eukaryotic translation initiation factor 5A was decreased after flood compared with melatonin. When stress was prolonged, its transcript levels were upregulated by flood, while they were not changed by melatonin. Furthermore, 13-hydroxylupanine O-tigloyltransferase was decreased by flood compared with melatonin; however, its transcription was upregulated by melatonin. In addition, reduced lignification in root tips of flooded soybeans was restored by melatonin. These results suggest that factors related to protein degradation and functional states of RNA play critical roles in promoting the effects of melatonin on soybean plants under flooding. SIGNIFICANCE: Flooding stress threatens soybean growth, while melatonin treatment enhances plant tolerance to stress stimuli. To examine the effects of melatonin on flooded soybeans, morphological analysis was performed. Melatonin promoted soybean growth as judged from greater fresh weight of plant, longer seedling length, and less evident cell death in flooding-stressed soybeans treated with melatonin than those plants exposed to flood alone. Proteomic analysis was conducted to explore the promoting effects of melatonin on soybeans under flooding stress. As a result, metabolism of protein metabolism, RNA regulation, and cell wall was enriched by proteins identified under control, flood, and flood plus melatonin conditions. Among these melatonin-induced proteins, abundance of eukaryotic aspartyl protease family protein, eukaryotic translation initiation factor 5A, and 13-hydroxylupanine O-tigloyltransferase displayed similar change patterns between the control and melatonin compared with flood; and transcript levels of genes encoding these proteins responded to flooding stress and melatonin treatment. In addition, activated cell degradation, expanded intercellular spaces, and reduced lignification in root tips of flooded soybeans were ameliorated by melatonin treatment.

Immunoprofiling of Cell Wall Carbohydrate Modifications During Flooding-Induced Aerenchyma Formation in Fabaceae Roots

Understanding plant adaptation mechanisms to prolonged water immersion provides options for genetic modification of existing crops to create cultivars more tolerant of periodic flooding. An important advancement in understanding flooding adaptation would be to elucidate mechanisms, such as aerenchyma air-space formation induced by hypoxic conditions, consistent with prolonged immersion. Lysigenous aerenchyma formation occurs through programmed cell death (PCD), which may entail the chemical modification of polysaccharides in root tissue cell walls. We investigated if a relationship exists between modification of pectic polysaccharides through de-methyl esterification (DME) and the formation of root aerenchyma in select Fabaceae species. To test this hypothesis, we first characterized the progression of aerenchyma formation within the vascular stele of three different legumes-Pisum sativum, Cicer arietinum, and Phaseolus coccineus-through traditional light microscopy histological staining and scanning electron microscopy. We assessed alterations in stele morphology, cavity dimensions, and cell wall chemistry. Then we conducted an immunolabeling protocol to detect specific degrees of DME among species during a 48-hour flooding time series. Additionally, we performed an enzymatic pretreatment to remove select cell wall polymers prior to immunolabeling for DME pectins. We were able to determine that all species possessed similar aerenchyma formation mechanisms that begin with degradation of root vascular stele metaxylem cells. Immunolabeling results demonstrated DME occurs prior to aerenchyma formation and prepares vascular tissues for the beginning of cavity formation in flooded roots. Furthermore, enzymatic pretreatment demonstrated that removal of cellulose and select hemicellulosic carbohydrates unmasks additional antigen binding sites for DME pectin antibodies. These results suggest that additional carbohydrate modification may be required to permit DME and subsequent enzyme activity to form aerenchyma. By providing a greater understanding of cell wall pectin remodeling among legume species, we encourage further investigation into the mechanism of carbohydrate modifications during aerenchyma formation and possible avenues for flood-tolerance improvement of legume crops.

The effect of chitosan-PMAA-NPK nanofertilizer on Pisum sativum plants

The use of chitosan (CS) as a carrier for slow fertilizer release is a novel trend. The potential effect of this system in agriculture is still debatable. Here, chitosan (CS) nanoparticles were obtained by polymerizing methacrylic acid (PMAA) for the entrapment of nitrogen, phosphorous and potassium (NPK) nanoparticles (NP), each at a time to form CS-PMAA-NPK NPs complex. The impact of this complex was evaluated using garden pea (Pisum sativum var. Master B) plants. Five-day-old pea seedlings were treated through their root system with CS-PMAA-NPK NPs at concentrations of 1, 0.5, 0.25, 0.125 and 0.0625 of the stock solution (R) for 1, 2, 4 and 7 days. In general, CS-PMAA-NPK NP complex reduced root elongation rate and resulted in the accumulation of starch at the root tip in a dose-dependent manner within the treated plants. Interestingly, the lowest concentrations of 0.0625 and 0.125 R had induced mitotic cell division (MI = 22.45 ± 2.68 and 19.72 ± 3.48, respectively) compared with the control (MI = 9.09 ± 3.28). In addition, some of major proteins such as convicilin, vicilin and legumin β were upregulated in plants treated with these low concentrations too. However, all concentrations used exhibited genotoxic effect on DNA based on the comet assay data after 48 h of treatment. Thus, it is highly recommended to consider the negative effects of this carrier system on plants and environment that may arise due to its accumulation in the agricultural fields.

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.

DNA and Flavonoids Leach out from Active Nuclei of Taxus and Tsuga after Extreme Climate Stresses

Severe over-stresses of climate caused dramatic changes in the intracellular distribution of the flavonoids. This was studied in needles from the current year’s growth of the following species and varieties: Tsuga canadensis, Taxus baccata, T. aurea, T. repens, T. nana, and T. compacta. The mode of steady changes in flavonoids was evaluated by microscopic techniques. Most of the flavonoids stain visibly yellow by themselves. The colorless flavanol subgroup can be stained blue by the DMACA reagent. In mid-summer 2013, outstanding high temperatures and intense photo-oxidative irradiation caused in a free-standing tree of Taxus baccata dramatic heat damage in a limited number of cells of the palisade layers. In these cells, the cytoplasm was burned brown. However, the nucleus maintained its healthy “blue” colored appearance which apparently was a result of antioxidant barrier effects by these flavanols. In late May 2014, excessive rainfall greatly affected all study trees. Collectively, in all study trees, a limited number of the mesophyll nuclei from the needless grown in 2013 and 2014 became overly turgid, enlarged in size and the flavanols leached outward through the damaged nuclear membranes. This diffusive stress event was followed one to three days later by a similar efflux of DNA. Such a complete dissolution of the nuclei in young tissues was the most spectacular phenomenon of the present study. As a common feature, leaching of both flavanols and DNA was markedly enhanced with increasing size and age of the cells. There is evidence that signalling flavonoids are sensitized to provide in nuclei and cytoplasm multiple mutual protective mechanisms. However, this well-orchestrated flavonoid system is broken down by extreme climate events.

Leaf-shape remodeling: programmed cell death in fistular leaves of Allium fistulosum

Some species of Allium in Liliaceae have fistular leaves. The fistular lamina of Allium fistulosum undergoes a process from solid to hollow during development. The aims were to reveal the process of fistular leaf formation involved in programmed cell death (PCD) and to compare the cytological events in the execution of cell death to those in the unusual leaf perforations or plant aerenchyma formation. In this study, light and transmission electron microscopy were used to characterize the development of fistular leaves and cytological events. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assays and gel electrophoresis were used to determine nuclear DNA cleavage during the PCD. The cavity arises in the leaf blade by degradation of specialized cells, the designated pre-cavity cells, in the center of the leaves. Nuclei of cells within the pre-cavity site become TUNEL-positive, indicating that DNA cleavage is an early event. Gel electrophoresis revealed that DNA internucleosomal cleavage occurred resulting in a characteristic DNA ladder. Ultrastructural analysis of cells at the different stages showed disrupted vacuoles, misshapen nuclei with condensed chromatin, degraded cytoplasm and organelles and emergence of secondary vacuoles. The cell walls degraded last, and residue of degraded cell walls aggregated together. These results revealed that PCD plays a critical role in the development of A. fistulosum fistular leaves. The continuous cavity in A. fistulosum leaves resemble the aerenchyma in the pith of some gramineous plants to improve gas exchange.

Anatomical aspects of angiosperm root evolution

BACKGROUND AND AIMS

Anatomy had been one of the foundations in our understanding of plant evolutionary trends and, although recent evo-devo concepts are mostly based on molecular genetics, classical structural information remains useful as ever. Of the various plant organs, the roots have been the least studied, primarily because of the difficulty in obtaining materials, particularly from large woody species. Therefore, this review aims to provide an overview of the information that has accumulated on the anatomy of angiosperm roots and to present possible evolutionary trends between representatives of the major angiosperm clades.

SCOPE

This review covers an overview of the various aspects of the evolutionary origin of the root. The results and discussion focus on angiosperm root anatomy and evolution covering representatives from basal angiosperms, magnoliids, monocots and eudicots. We use information from the literature as well as new data from our own research.

KEY FINDINGS

The organization of the root apical meristem (RAM) of Nymphaeales allows for the ground meristem and protoderm to be derived from the same group of initials, similar to those of the monocots, whereas in Amborellales, magnoliids and eudicots, it is their protoderm and lateral rootcap which are derived from the same group of initials. Most members of Nymphaeales are similar to monocots in having ephemeral primary roots and so adventitious roots predominate, whereas Amborellales, Austrobaileyales, magnoliids and eudicots are generally characterized by having primary roots that give rise to a taproot system. Nymphaeales and monocots often have polyarch (heptarch or more) steles, whereas the rest of the basal angiosperms, magnoliids and eudicots usually have diarch to hexarch steles.

CONCLUSIONS

Angiosperms exhibit highly varied structural patterns in RAM organization; cortex, epidermis and rootcap origins; and stele patterns. Generally, however, Amborellales, magnoliids and, possibly, Austrobaileyales are more similar to eudicots, and the Nymphaeales are strongly structurally associated with the monocots, especially the Acorales.

Hypoxic stress triggers a programmed cell death pathway to induce vascular cavity formation in Pisum sativum roots

Flooding at warm temperatures induces hypoxic stress in Pisum sativum seedling roots. In response, some undifferentiated cells in the primary root vascular cylinder start degenerating and form a longitudinal vascular cavity. Changes in cellular morphology and cell wall ultrastructure detected previously in the late stages of cavity formation suggest possible involvement of programmed cell death (PCD). In this study, cytological events occurring in the early stages of cavity formation were investigated. Systematic DNA fragmentation, a feature of many PCD pathways, was detected in the cavity-forming roots after 3 h of flooding in situ by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay and in isolated total DNA by gel electrophoresis. High molecular weight DNA fragments of about 20-30 kb were detected by pulse-field gel electrophoresis, but no low-molecular weight internucleosomal DNA fragments were detected by conventional gel electrophoresis. Release of mitochondrial cytochrome c protein into the cytosol, an integral part of mitochondria-dependent PCD pathways, was detected in the cavity-forming roots within 2 h of flooding by fluorescence microscopy of immunolabeled cytochrome c in situ and in isolated mitochondrial and cytosolic protein fractions by western blotting. DNA fragmentation and cytochrome c release remained confined to the undifferentiated cells in center of the root vascular cylinders, even after 24 h of flooding, while outer vascular cylinder cells and cortical cells maintained cellular integrity and normal activity. These findings confirm that hypoxia-induced vascular cavity formation in P. sativum roots involves PCD, and provides a chronological model of cytological events involved in this rare and understudied PCD system.

Changes in cell wall ultrastructure induced by sudden flooding at 25{degrees}C in Pisum sativum (Fabaceae) primary roots

Cellular degeneration is essential for many developmental and stress acclimation processes. Undifferentiated parenchymatous cells in the central vascular cylinder of pea primary roots degenerate under hypoxic conditions created by flooding at temperatures >15°C, forming a long vascular cavity that seems to provide a conduit for longitudinal oxygen transport in the roots. We show that specific changes in the cell wall ultrastructure accompanied previously detected cytoplasmic and organellar degradation in the cavity-forming roots. The degenerating cells had thinner primary cell walls, less electron-dense middle lamellae, and less abundant cell wall homogalacturonans in altered patterns, compared to healthy cells of roots grown under cold, nonflooded conditions. Cellular breakdown and changes in wall ultrastructure, however, remained confined to cells within a 50-μm radius around the root center, even after full development of the cavity. Cells farther away maintained cellular integrity and had signs of wall synthesis, perhaps from tight regulation of wall metabolism over short distances. These observations suggest that the cell degeneration might involve programmed cell death. We also show that warm, nonflooded or cold, flooded conditions that typically do not induce vascular cavity formation can also induce variations in cell wall ultrastructure.

Apoptosis-like programmed cell death occurs in procambium and ground meristem of pea (Pisum sativum) root tips exposed to sudden flooding

BACKGROUND AND AIMS

Pea (Pisum sativum) primary roots form long vascular cavities when grown under wet or flooded conditions at 25 degrees C. It is thought that the cavities are a form of aerenchyma. At 25 degrees C short roots continue to grow after flooding. After roots reach 10 cm long flooding causes rapid cessation of growth, and root tips often become curled. In longer roots the cavities do not extend into the base of the roots, perhaps rendering them ineffective as aerenchyma. It was hypothesized that the resulting growth arrest was due to programmed cell death (PCD) rather than necrosis.

METHODS AND KEY RESULTS

Histological examination by light microscope showed that some cells in the primary meristem (elongation) zone of the primary root tips had morphological abnormalities, including misshapen and fragmented nuclei, and cytoplasmic shrinking and fragmentation. Transmission electron microscopy revealed lobing, invagination and chromatin aggregation in nuclei. The affected cells were positive for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling. Extracted DNA formed a "DNA ladder" during electrophoresis. Cell death usually began in procambium at one or two protoxylem poles and seemed to spread out to nearby tissues, which asymmetrically inhibited growth and resulted in tip curling.

CONCLUSIONS

The above are symptoms of apoptosis-like PCD. Programmed root tip death may rapidly reduce oxygen demand and sink strength, allowing more rapid diversion of resources to lateral roots growing in more permissive conditions.