Polyamines affect the cellular growth and structure of pro-embryogenic masses in Araucaria angustifolia embryogenic cultures through the modulation of proton pump activities and endogenous levels of polyamines

Somatic Embryogenesis in Conifers: One Clade to Rule Them All?

Somatic embryogenesis (SE) in conifers is usually characterized as a multi-step process starting with the development of proembryogenic cell masses and followed by histodifferentiation, somatic embryo development, maturation, desiccation, and plant regeneration. Our current understanding of conifers' SE is mainly derived from studies using Pinaceae species as a model. However, the evolutionary relationships between conifers are not clear. Some hypotheses consider conifers as a paraphyletic group and Gnetales as a closely related clade. In this review, we used an integrated approach in order to cover the advances in knowledge on SE in conifers and Gnetales, discussing the state-of-the-art and shedding light on similarities and current bottlenecks. With this approach, we expect to be able to better understand the integration of these clades within current studies on SE. Finally, the points discussed raise an intriguing question: are non-Pinaceae conifers less prone to expressing embryogenic competence and generating somatic embryos as compared to Pinaceae species? The development of fundamental studies focused on this morphogenetic route in the coming years could be the key to finding a higher number of points in common between these species, allowing the success of the SE of one species to positively affect the success of another.

Putrescine transformation to other forms of polyamines in filling grain embryos functioned in enhancing the resistance of maize plants to drought stress

Polyamines (PAs), one of plant growth regulators, play an important role in the plant resistance to drought stress. However, the precise function of putrescine (Put) transformation to other forms of PAs is not clear in filling maize grain embryos. In this study, two maize (Zea mays L.) cultivars, Yedan No. 13 (drought-resistant) and Xundan No. 22 (drought-sensitive), were used as experimental materials. Maize was planted in big plastic basins during whole growth period, and from the 25th day after fertilization, the plants were treated with drought (-1.0 MPa), PAs and inhibitors for 12 d. The experiments were performed during three consecutive years. The changes in the levels of three main free PAs, Put, spermidine (Spd) and spermine (Spm), covalently conjugated PAs (perchloric acid-soluble), covalently bound PAs (perchloric acid-insoluble), the activities of arginine decarboxylase, S-adenosylmethionine decarboxylase, and transglutaminase were investigated in embryos of filling grains. During drought stress, free Put increased from 109 to 367 nmol g^-1 FW and from 107 to 142 nmol g^-1 FW in Xundan 22 and in Yedan 13, respectively. Meanwhile, free Spd, free Spm and bound Put increased 2.7, 3.0 and 4.2 times in Yedan 13, respectively, and they merely increased about 1.5 times in Xundan 22. These results suggested that free Spd/Spm and bound Put, which were transformed from free Put, were possibly involved in drought resistance. Exogenous Spd treatment enhanced the drought-induced increase in endogenous free Spd/Spm content in drought-sensitive Xundan 22, coupled with the increase in drought resistance, as judged by the decrease in ear leaf relative plasma membrane permeability and increases in ear leaf relative water content, 1000-grain weight and grain number per ear. The suggestion was further testified with methylglyoxal-bis guanylhydrazone and o-phenanthrolin treatments. Collectively, it could be inferred that transformation of free Put to free Spd/Spm and bound Put in filling grain embryos functioned in enhancing the resistance of maize plants to soil drought.

Potassium ameliorates cotton (Gossypium hirsutum L.) fiber length by regulating osmotic and K+ /Na+ homeostasis under salt stress

Potassium (K) application can alleviate cotton salt stress, but the regulatory mechanisms affecting cotton fiber elongation and ion homeostasis are still unclear. A two-year field experiment was conducted to explore the effects of K on the osmolyte contents (soluble sugar, K⁺ content, and malate) and related enzyme activities during the fiber elongation of two cotton cultivars with contrasting salt sensitivity (CCRI-79; salt tolerant cultivar, and Simian 3; salt-sensitive cultivar). Three K application treatments (0, 150, and 300 kg K₂ O ha^-1 ) were applied at three soil salinity levels (low salinity, EC = 1.73 ± 0.05 dS m^-1 ; medium salinity, EC = 6.32 ± 0.10 dS m^-1 ; high salinity, EC = 10.84 ± 0.24 dS m^-1 ). K application improved fiber length and alleviated salt stress by increasing the maximum velocity of fiber elongation (V_max ). The increase rate of K on fiber length decreased with elevating salt stress, and the increase rate of K on V_max of CCRI-79 was greater than that of Simian3. K application can increase the enzyme activities (phosphoenolpyruvate carboxylase, PEPC, E.C. 4.1.1.31; pyrophosphatase, PPase, E.C. 3.6.1.1; and plasma membrane H⁺ -ATPase, PM H⁺ -ATPase, E.C. 3.6.3.14) as well as the content of osmolytes associated with the enzymes mentioned above. K increased the osmolyte contents under salt stress, and the increase in the K⁺ content of the fibers was much higher than that of soluble sugar and malate. The results of this study indicated K fertilizer application rates regulate the metabolism of osmolytes in cotton fiber development under salt stress, K⁺ is more critical to fiber elongation.

Transcriptomic and Metabolomic Analysis of Korean Pine Cell Lines with Different Somatic Embryogenic Potential

The embryogenesis capacity of conifer callus is not only highly genotype-dependent, but also gradually lost after long-term proliferation. These problems have seriously limited the commercialization of conifer somatic embryogenesis (SE) technology. In this study, the responsive SE cell line (R-EC), the blocked SE cell line (B-EC), and the loss of SE cell line (L-EC) were studied. The morphological, physiological, transcriptomic, and metabolomic profiles of these three types of cells were analyzed. We found that R-EC had higher water content, total sugar content, and putrescine (Put) content, as well as lower superoxide dismutase (SOD) activity and H2O2 content compared to B-EC and L-EC. A total of 2566, 13,768, and 13,900 differentially expressed genes (DEGs) and 219, 253, and 341 differentially expressed metabolites (DEMs) were found in the comparisons of R-EC versus B-EC, R-EC versus B-EC, and B-EC versus L-EC, respectively. These DEGs and DEMs were mainly found to be involved in plant signal transduction, starch and sugar metabolism, phenylpropane metabolism, and flavonoid metabolism. We found that the AUX1 and AUX/IAA families of genes were significantly up-regulated after the long-term proliferation of callus, resulting in higher auxin content. Most phenylpropane and flavonoid metabolites, which act as antioxidants to protect cells from damage, were found to be significantly up-regulated in R-EC.

Morphological and Physiological Indicators for Screening Cell Lines with High Potential for Somatic Embryo Maturation at an Early Stage of Somatic Embryogenesis in Pinus Koraiensis

Many cell lines in the embryogenic callus cannot produce somatic embryos (SEs) even if they meet the optimal SE maturation culture conditions during conifer somatic embryogenesis. This phenomenon hinders the progress of the industrial-scale reproduction of conifers. Therefore, there is an urgent need to obtain morphological and physiological markers to screen embryogenic calli in response to SE maturation conditions. To detect cell lines with high somatic embryogenesis potential during the proliferation process, we counted the number of pro-embryos and early SEs (ESEs) in different cell lines and storage substances, endogenous hormones, and polyamine contents. The results showed that the yield of P. koraiensis SEs was heavily dependent on genotype (p = 0.001). There were high levels of PE III (pro-embryo III) number, ESE number, and soluble protein content, in the response cell lines (R cell lines), which were 1.6-, 3-, and 1.1-fold those of the obstructive cell lines (B cell lines), respectively. The B cell line had high levels of starch, auxin (IAA), Put, Spd, and putrescine: spermine (Put: Spm) compared to the R cell line. In addition, the numbers of PE III, ESEs, and soluble protein content were significantly positively correlated with SE yield. In contrast, the contents of starch, abscisic acid (ABA), Put, Spm, and Spd were significantly negatively correlated with SE yield. To ensure the accuracy of the results, we used nine cell lines to test the results. The PE III and ESE numbers and the Spm and Spd contents were positively correlated with SE yield, while the levels of starch, ABA, IAA, Put: Spd, and Put: Spm were negatively correlated with SE yield. Thus, we recommend using high PE III and ESEs as morphological indicators and low levels of starch, IAA, ABA, and Put: Spm as physiological markers to screen cell lines with a high somatic embryogenesis potential. In addition, we also found that the relationship between Spd, Spm, and SE yield was opposite in the two experimental results. Therefore, we speculate that the differences in Spd and Spm content are mainly affected by genotype. In conclusion, this study obtained the morphological and physiological markers of some high-somatic embryogenic cell lines by comparing the differences between nine somatic embryogenic cell lines. Our results can guide the improvement of conifer somatic embryogenesis technology and can provide a theoretical basis for accelerating the application of biotechnology in large-scale artificial breeding.

Conjugated Polyamines in Root Plasma Membrane Enhanced the Tolerance of Plum Seedling to Osmotic Stress by Stabilizing Membrane Structure and Therefore Elevating H+-ATPase Activity

Polyamines are small positively charged molecules in plants and play important functions in many biological processes under various environmental stresses. One of the most confounding problems relating to polyamines (PAs) in stresses is the lack of understanding of the mechanisms underlying their function(s). Furthermore, a limited number of studies have addressed this issue at the sub-cellular level, especially in tree plants under drought stress. Therefore, in this research, by simulating natural drought stress with polyethylene glycol (PEG) osmotic stress, the relationship between the levels of conjugated polyamines and the activity of H⁺-ATPase in the plasma membrane was elucidated with the roots of two plum (Prunus salicina L.) cultivars, which were different in drought tolerance, as experimental materials. Furthermore, free PA levels and the activities of S-adenosylmethionine decarboxylase (SAMDC) and transglutaminase (TGase), which were closely associated with the levels of free and conjugated PAs, were also detected. Results showed that under osmotic stress, the increases of the levels of non-covalently conjugated (non-CC) spermidine (Spd) and spermine (Spm), covalently conjugated (CC) putrescine (Put) and Spd in the plasma membrane of drought-tolerant Ganli No. 5 were more significant than those of drought-sensitive Suli No. 3, indicating that these conjugated PAs might be involved in the tolerance of plum seedlings to stress. Furthermore, the conjugated PAs were closely correlated with plum seedling growth, water retention capacity, plasma membrane damage degree, and hydrogen (H⁺)-ATPase activity in the plasma membrane. To get more complementary pieces of evidence, we subjected plum seedlings to combined treatments of PEG and exogenous PA (Spd and Spm), and an inhibitor of SAMDC [methylglyoxal-bis (guanylhydrazone), (MGBG)] or TGase (o-phenanthroline). These results collectively suggested that non-CC Spd and Spm, CC Put and Spd in plasma membrane might function in enhancing the tolerance of plum seedlings to osmotic stress by stabilizing membrane structure and therefore elevating H⁺-ATPase activity.

Exogenously applied Spd and Spm enhance drought tolerance in tea plants by increasing fatty acid desaturation and plasma membrane H+-ATPase activity

Polyamines, due to their positive charges, bind to ROS Reactive oxygen species (ROS) thereby stabilizing the plasma membrane (PM). Drought is one of the main limiting factors affecting tea plant yield and quality. However, the effect of Spermidine (Spd) or Spermine (Spm) on membrane stability and fluidity in tea plants under drought stress is poorly understood. In this investigation, an exogenous supply of 1 mM Spd or Spm did not mitigate drought stress-induced damage, however, an exogenous supply of 0.2 mM Spd or Spm application significantly alleviated drought-induced damage in tea plants. To further illustrate the role of 0.2 mM Spd or Spm in maintaining membrane integrity and fluidity, the fatty acid percentage and PM H⁺-ATPase activity were analyzed. Spd and Spm application significantly increased PM H⁺-ATPase activity by 43.79% compared with that without the addition of polyamine under drought stress. In addition, exogenous application of Spd and Spm also significantly increased C18:3 by approximately 10%, hence alleviating drought-reduced fatty acid unsaturation. In contrast, Spd and Spm metabolic inhibitors dicyclohexylamine (DCHA) further impaired PM H⁺-ATPase activity and fatty acid desaturation under the drought + DCHA treatment compared with the drought treatment, respectively. Taken together, 0.2 mM Spd and Spm application significantly enhanced drought tolerance by increasing fatty acid unsaturation and maintaining PM H⁺-ATPase activity in tea plants. Therefore, foliar application of 0.2 mM Spd or Spm can be a potential foliar-spraying substances for improving tea drought tolerance.

Polyamines conjugated to plasma membrane functioned in enhancing the tolerance of cucumber seedlings to osmotic stress via elevating H+-ATPase activity

Polyamine (PA), one of the important plant growth regulators, is closely associated with drought stress. However, the function of conjugated PA is not still clear in the roots of cucumber seedlings under polyethylene glycol (PEG) osmotic stress. Therefore, in this study the relationship between the levels of conjugated polyamines and the activity of H⁺-ATPase in plasma membrane was elucidated with the roots of two cucumber (Cucumis sativus L.) cultivars, which were different in drought tolerance, as experimental materials. Furthermore, the contents of free PAs and the activities of S-adenosylmethionine decarboxylase (SAMDC) and transglutaminase (TGase), which were closely related to the levels of conjugated polyamines, were also determined. Results showed that under osmotic stress, the increases of the levels of non-covalently conjugated (non-CC) spermidine (Spd) and spermine (Spm), covalently conjugated (CC) putrescine (Put) and Spd in plasma membrane of drought-tolerant Tangshan 5 were more obvious than those of drought-sensitive Jinyou 1. Furthermore, the conjugated PAs mentioned above were closely correlated with increase rate of seedling dry weight, plasma membrane permeability, water content and H⁺-ATPase activity in plasma membrane. Results of the additional tests, in which exogenous Spd, Spm and two inhibitors, MGBG and phenanthrolin were used, were complementary to the results above. From these results, it could be concluded that non-CC Spd and Spm, CC Put and Spd in plasma membrane functioned in enhancing the tolerance of cucumber seedlings to osmotic stress via elevating H⁺-ATPase activity.

Citrus Polyamines: Structure, Biosynthesis, and Physiological Functions

Polyamines (PAs) are ubiquitous biogenic amines found in all living organisms from bacteria to Archaea, and Eukaryotes including plants and animals. Since the first description of putrescine conjugate, feruloyl-putrescine (originally called subaphylline), from grapefruit leaves and juice, many research studies have highlighted the importance of PAs in growth, development, and other physiological processes in citrus plants. PAs appear to be involved in a wide range of physiological processes in citrus plants; however, their exact roles are not fully understood. Accordingly, in the present review, we discuss the biosynthesis of PAs in citrus plants, with an emphasis on the recent advances in identifying and characterizing PAs-biosynthetic genes and other upstream regulatory genes involved in transcriptional regulation of PAs metabolism. In addition, we will discuss the recent metabolic, genetic, and molecular evidence illustrating the roles of PAs metabolism in citrus physiology including somatic embryogenesis; root system formation, morphology, and architecture; plant growth and shoot system architecture; inflorescence, flowering, and flowering-associated events; fruit set, development, and quality; stomatal closure and gas-exchange; and chlorophyll fluorescence and photosynthesis. We believe that the molecular and biochemical understanding of PAs metabolism and their physiological roles in citrus plants will help citrus breeding programs to enhance tolerance to biotic and abiotic stresses and provide bases for further research into potential applications.

Spermine modulates fungal morphogenesis and activates plasma membrane H+-ATPase during yeast to hyphae transition

Polyamines play a regulatory role in eukaryotic cell growth and morphogenesis. Despite many molecular advances, the underlying mechanism of action remains unclear. Here, we investigate a mechanism by which spermine affects the morphogenesis of a dimorphic fungal model of emerging relevance in plant interactions, Yarrowia lipolytica, through the recruitment of a phytohormone-like pathway involving activation of the plasma membrane P-type H⁺-ATPase. Morphological transition was followed microscopically, and the H⁺-ATPase activity was analyzed in isolated membrane vesicles. Proton flux and acidification were directly probed at living cell surfaces by a non-invasive selective ion electrode technique. Spermine and indol-3-acetic acid (IAA) induced the yeast-hypha transition, influencing the colony architecture. Spermine induced H⁺-ATPase activity and H⁺ efflux in living cells correlating with yeast-hypha dynamics. Pharmacological inhibition of spermine and IAA pathways prevented the physio-morphological responses, and indicated that spermine could act upstream of the IAA pathway. This study provides the first compelling evidence on the fungal morphogenesis and colony development as modulated by a spermine-induced acid growth mechanism analogous to that previously postulated for the multicellular growth regulation of plants.

Summary: This study presents a new mechanistic model for the integrative role of the polyamine spermine and hormone auxin in the signaling of yeast-to-hypha transition, filling an important gap in fungal morphogenesis.

Bone mesenchymal stem cell secretion of sRANKL/OPG/M-CSF in response to macrophage-mediated inflammatory response influences osteogenesis on nanostructured Ti surfaces

Although it has been well established that osteogenic differentiation of bone mesenchymal stem cells (bMSCs) as well as osteoclastic differentiation of macrophages can be manipulated by the nanostructure of biomaterial surfaces, the interactions among the effects of the surface on immune cells and bMSCs remained unknown. Therefore, in this study, the osteogenic behaviors and secretion of osteoclastogenesis-related cytokines of human bMSCs on TiO₂ nanotubular (NT) surfaces in conditioned medium (CM) generated by macrophages cultured on the respective NT surfaces (NT-CM) were analyzed. Although bMSCs showed consistent osteogenic behaviors on the NT5 and NT20 surfaces in both standard culture medium and both types of NT-CM, collagen synthesis and extracellular matrix mineralization were partially impeded on the NT20 surface in NT20-CM and bMSC cytokine secretions on the NT20 surface in NT20-CM elicited remarkable multinuclear giant cell and osteoclast formation compared with that observed on the NT5 surface in NT5-CM. After implantation in vivo, mineralized bone formation was significantly delayed around the NT20 implant compared with the NT5 implant, but both surfaces contributed to good bone formation after 12 weeks. The results obtained in this study advance our understanding of the confounding influence of the implant surface nanostructure, macrophage inflammatory response, and osteogenic differentiation of bMSCs as well as the retro-regulative effects of bMSCs on the osteoclastic differentiation of macrophages, and the culture system based on different NT surfaces and CM generated on the respective surfaces may provide a systematic research model for evaluating the performance of endosseous implants as well as a prospective approach for improving implant osseointegration via immune-regulation.

Toward establishing a morphological and ultrastructural characterization of proembryogenic masses and early somatic embryos of Araucaria angustifolia (Bert.) O. Kuntze

Somatic embryogenesis is a morphogenetic route useful for the study of embryonic development, as well as the large-scale propagation of endangered species, such as the Brazilian pine (Araucaria angustifolia). In the present study, we investigated the morphological and ultrastructural organization of A. angustifolia somatic embryo development by means of optical and electron microscopy. The proembryogenic stage was characterized by the proliferation of proembryogenic masses (PEMs), which are cellular aggregates composed of embryogenic cells (ECs) attached to suspensor-like cells (SCs). PEMs proliferate through three developmental stages, PEM I, II, and III, by changes in the number of ECs and SCs. PEM III-to-early somatic embryo (SE) transition was characterized by compact clusters of ECs growing out of PEM III, albeit still connected to it by SCs. Early SEs showed a dense globular embryonic mass (EM) and suspensor region (SR) connected by embryonic tube cells (TCs). By comparison, early somatic and zygotic embryos showed similar morphology. ECs are round with a large nucleus, nucleoli, and many cytoplasmic organelles. In contrast, TCs and SCs are elongated and vacuolated with cellular dismantling which is associated with programmed cell death of SCs. Abundant starch grains were observed in the TCs and SCs, while proteins were more abundant in the ECs. Based on the results of this study, a fate map of SE development in A. angustifolia is, for the first time, proposed. Additionally, this study shows the cell biology of SE development of this primitive gymnosperm which may be useful in evolutionary studies in this area.

Somatic Embryogenesis in Araucaria angustifolia (Bertol.) Kuntze (Araucariaceae)

This chapter deals with the features of somatic embryogenesis (SE) in Araucaria angustifolia, an endangered and native conifer from south Brazil. In this species SE includes the induction and proliferation of embryogenic cultures composed of pro-embryogenic masses (PEMs), which precede somatic embryos development. A. angustifolia SE model encompasses induction, proliferation, pre-maturation, and maturation steps. Double-staining with acetocarmine and Evan's blue is useful to evaluate the embryonic somatic structures. In this chapter we describe A. angustifolia SE protocols and analyzes morphological features in the different SE developmental stages.

Natural polyamines and synthetic analogs modify the growth and the morphology of Pyrus communis pollen tubes affecting ROS levels and causing cell death

Polyamines (PAs) are small molecules necessary for pollen maturation and tube growth. Their role is often controversial, since they may act as pro-survival factors as well as factors promoting Programmed Cell Death (PCD). The aim of the present work was to evaluate the effect of exogenous PAs on the apical growth of pear (Pyrus communis) pollen tube and to understand if PAs and reactive oxygen species (ROS) are interconnected in the process of tip-growth. In the present study besides natural PAs, also aryl-substituted spermine and methoctramine (Met 6-8-6) analogs were tested. Among the natural PAs, Spm showed strongest effects on tube growth. Spm entered through the pollen tube tip, then diffused in the sub-apical region that underwent drastic morphological changes, showing enlarged tip. Analogs were mostly less efficient than natural PAs but BD23, an asymmetric synthetic PAs bearing a pyridine ring, showed similar effects. These effects were related to the ability of PAs to cause the decrease of ROS level in the apical zone, leading to cell death, counteracted by the caspase-3 inhibitor Ac-DEVD-CHO (DEVD). In conclusions, ROS are essential for pollen germination and a strict correlation between ROS regulation and PA concentration is reported. Moreover, an imbalance between ROS and PAs can be detrimental thereby driving pollen toward cell death.

Apomictic and sexual germline development differ with respect to cell cycle, transcriptional, hormonal and epigenetic regulation

Seeds of flowering plants can be formed sexually or asexually through apomixis. Apomixis occurs in about 400 species and is of great interest for agriculture as it produces clonal offspring. It differs from sexual reproduction in three major aspects: (1) While the sexual megaspore mother cell (MMC) undergoes meiosis, the apomictic initial cell (AIC) omits or aborts meiosis (apomeiosis); (2) the unreduced egg cell of apomicts forms an embryo without fertilization (parthenogenesis); and (3) the formation of functional endosperm requires specific developmental adaptations. Currently, our knowledge about the gene regulatory programs underlying apomixis is scarce. We used the apomict Boechera gunnisoniana, a close relative of Arabidopsis thaliana, to investigate the transcriptional basis underlying apomeiosis and parthenogenesis. Here, we present the first comprehensive reference transcriptome for reproductive development in an apomict. To compare sexual and apomictic development at the cellular level, we used laser-assisted microdissection combined with microarray and RNA-Seq analyses. Conservation of enriched gene ontologies between the AIC and the MMC likely reflects functions of importance to germline initiation, illustrating the close developmental relationship of sexuality and apomixis. However, several regulatory pathways differ between sexual and apomictic germlines, including cell cycle control, hormonal pathways, epigenetic and transcriptional regulation. Enrichment of specific signal transduction pathways are a feature of the apomictic germline, as is spermidine metabolism, which is associated with somatic embryogenesis in various plants. Our study provides a comprehensive reference dataset for apomictic development and yields important new insights into the transcriptional basis underlying apomixis in relation to sexual reproduction.

In flowering plants, asexual reproduction through seeds (apomixis) likely evolved from sexual ancestors several times independently. Only three key developmental steps differ between sexual reproduction and apomixis. In contrast to sexual reproduction, in apomicts the first cell of the female reproductive lineage omits or aborts meiosis (apomeiosis) to initiate gamete formation. Subsequently, the egg cell develops into an embryo without fertilization (parthenogenesis), and endosperm formation can either be autonomous or depend on fertilization. Consequently, the offspring of apomicts is genetically identical to the mother plant. The production of clonal seeds bears great promise for agricultural applications. However, the targeted manipulation of reproductive pathways for seed production has proven difficult as knowledge about the underlying gene regulatory processes is limited. We performed cell type-specific transcriptome analyses to study apomictic germline development in Boechera gunnisoniana, an apomictic species closely related to Arabidopsis thaliana. To facilitate these analyses, we first characterized a floral reference transcriptome. In comparison, we identified several regulatory pathways, including core cell cycle regulation, protein degradation, transcription factor activity, and hormonal pathways to be differentially regulated between sexual and apomictic plants. Apart from new insights into the underlying transcriptional networks, our dataset provides a valuable starting point for functional investigations.