ROS and Ions in Cell Signaling during Sexual Plant Reproduction

Interspecific Hybridization Barrier Between Paeonia ostii and P. ludlowii

Paeonia ludlowii is a threatened and valuable germplasm in the cultivated tree peony gene pool, with distinctive traits such as tall stature, pure yellow flowers, and scarlet foliage in autumn. However, the crossability barrier limits gene transfer from P. ludlowii to cultivated tree peony. Therefore, our study investigated the reasons for the lack of crossability between P. ludlowii and Paeonia ostii 'Fengdan'. Distant cross pollination (DH) resulted in the formation of many calloses at the ends of the pollen tubes, which grew non-polar, twisted, entangled, and often stopped in the style. Pollen tubes elongated the fastest in self-pollination (CK), and pollen tubes elongated faster and fewer pollen tube abnormalities were observed in stigmas treated with KCl solution before pollination (KH) than in DH. During pollen-pistil interactions, the absence of stigma exudates, high levels of H2O2, O2-, MDA, •OH, ABA, and MeJA, and lower levels of BR and GA3 may negatively affect pollen germination and pollen tube elongation in the pistil of P. ostii 'Fengdan'. Pollen tubes in CK and KH penetrated the ovule into the embryo sac at 24 h after pollination, whereas only a few pollen tubes in DH penetrated the ovule at 36 h after pollination. Pre-embryo abnormalities and the inhibition of free nuclear endosperm division resulted in embryo abortion in most of the fruits of DH and many fruits of KH, which occurred between 10 and 20 days after pollination, whereas embryos in CK developed well. Early embryo abortion and endosperm abortion in most of the fruits of DH and KH led to seed abortion. Seed abortion in KH and DH was mainly due to an insufficient supply of auxins and gibberellins and lower content of soluble protein and soluble sugars. The cross failure between P. ludlowii and P. ostii 'Fengdan' is mostly caused by a pre-fertilization barrier. KH treatment can effectively promote pollen tube growth and facilitate normal development of hybrid embryos. These findings provide new insights into overcoming the interspecific hybridization barrier between cultivated tree peony varieties and wild species.

Loss of flavonoids homeostasis leads to pistillody in sua-CMS of Nicotiana tabacum

The homeotic transformation of stamens into pistil-like structures (pistillody) causes cytoplasmic male sterility (CMS). This phenomenon is widely present in plants, and might be induced by intracellular communication (mitochondrial retrograde signaling), but its systemic regulating mechanism is still unclear. In this study, morphological observation showed that the stamens transformed into pistil-like structures, leading to flat and dehiscent pistils, and fruit set decrease in sua-CMS (MS K326, somatic fusion between Nicotiana. tabacum L. K326 and Nicotiana suaveolens). Transcriptome data analysis presented that the expression levels of B-class MADS genes, including pMADS1, GLO1, GLO2, pMADS2.1, pMADS2.2, significantly reduced in the pistil-like structure of sua-CMS. DEGs were enriched in flavonoid and phenylpropanoid biosynthesis pathways. Transcriptome and metabolomics analysis revealed that the expression levels of CHI/CHS (key enzymes regulating flavonoid synthesis), and the contents of flavonoids reduced significantly in the pistil-like structures of sua-CMS. Chemical fluorescence staining assay showed that reactive oxygen species (ROS) levels were higher in the pistil-like structure of sua-CMS. Application of external flavonoids (hesperetin) reduced the frequency of pistillody and ROS levels. These results suggested that the metabolism of flavonoids played important roles in regulating pistillody through ROS in sua-CMS. Our study provides new insights into the regulatory mechanism of pistillody in plants.

Dynamics of ROS production, SOD, POD and CAT activity during stigma maturation and pollination in Nicotiana tabacum and Lilium longiflorum

Reactive oxygen species (ROS) are a key regulator of physiological processes in pollen grains, and an essential component of stigma exudate. The mechanisms of this redox-based regulatory system and its features in different plant groups are still unclear. For two species from different families (tobacco and lily), the dynamics of total ROS, O• 2 - generation, and H2O2 concentration in stigma exudate were examined using EPR spectroscopy and quantitative colorimetric analysis. Dynamics of all major enzymes of redox homeostasis were analysed using native electrophoresis and zymography for four stages of stigma development, before and after pollination. There were completely different patterns of ROS production and interconversion in the two species. In tobacco, the initially high level of ROS generation decreased before pollination but remained high. There was no CAT activity in fresh stigma tissues, which apparently contribute to the high level of H2O2. Lilium had peak O• 2 - generation at the fertile stage and high activity of H2O2-reducing enzymes, including CAT, hence, H2O2 level remained relatively low. We suggest that Lilium pollen germination is largely controlled by the SOD radical, while in Nicotiana H2O2 is the main form of ROS in the stigma.

Reactive oxygen species are signaling molecules that modulate plant reproduction

Reactive oxygen species (ROS) can serve as signaling molecules that are essential for plant growth and development but abiotic stress can lead to ROS increases to supraoptimal levels resulting in cellular damage. To ensure efficient ROS signaling, cells have machinery to locally synthesize ROS to initiate cellular responses and to scavenge ROS to prevent it from reaching damaging levels. This review summarizes experimental evidence revealing the role of ROS during multiple stages of plant reproduction. Localized ROS synthesis controls the formation of pollen grains, pollen-stigma interactions, pollen tube growth, ovule development, and fertilization. Plants utilize ROS-producing enzymes such as respiratory burst oxidase homologs and organelle metabolic pathways to generate ROS, while the presence of scavenging mechanisms, including synthesis of antioxidant proteins and small molecules, serves to prevent its escalation to harmful levels. In this review, we summarized the function of ROS and its synthesis and scavenging mechanisms in all reproductive stages from gametophyte development until completion of fertilization. Additionally, we further address the impact of elevated temperatures induced ROS on impairing these reproductive processes and of flavonol antioxidants in maintaining ROS homeostasis to minimize temperature stress to combat the impact of global climate change on agriculture.

Synergistic interplay of redox homeostasis and polysaccharide synthesis promotes cotton fiber elongation

Cell polarity is the foundation of cell development and tissue morphogenesis. The investigation of polarized growth provides opportunities to gain profound insights into morphogenesis and tissue functionality in organisms. Currently, there are still many mysteries surrounding the mechanisms that regulate polarized cell growth. Cotton fiber cells serve as an excellent model for studying polarized growth, and provide important clues for unraveling the molecular mechanisms, signaling pathways, and regulatory networks of polarized growth. In this study, we characterized two functional genes, GhMDHAR1AT/DT and GhDHAR2AT/DT with predominant expression during fiber elongation. Loss of function of both genes contributed to a significant increase in fiber length. Transcriptomic data revealed up-regulated expression of antioxidant genes in CRISPR mutant lines, along with delayed expression of secondary wall-related genes and temporally prolonged expression of primary wall-related genes. Experimental evidence demonstrated that the increase in GSH content and glutathione peroxidase (GPX) enzyme activity led to enhanced total antioxidant capacity (T-AOC), resulting in reduced H2O2 levels, which contributed to the extension of fiber elongation stage in CRISPR mutant lines. Moreover, the increased polysaccharide synthesis in CRISPR mutant lines was found to provide an abundant supply of raw materials for fiber cell wall elongation, suggesting that synergistic interplay between redox homeostasis and polysaccharide synthesis in fiber cells may facilitate cell wall remodeling and fiber elongation. This study provides valuable insights for deciphering the mechanisms of cell polarized growth and improving cotton fiber quality.

Bio-positive effects of ionizing radiation on pollen: The role of ROS

The concept of 'hormesis' is defined as a dose-response relationship whereby low doses of various toxic substances or physical stressors trigger bio-positive effects in diverse biological systems, whereas high doses cause inhibition of cellular performance (e.g. growth, viability). The two-sided phenomenon of specific low-dose stimulation and high-dose inhibition imposed by a 'hormetic-factor' has been well documented in toxicology and pharmacology. Multitudinous factors have been identified that correspondingly cause hormetic effects in diverse taxa of animals, fungi, and plants. This study particularly aims to elucidate the molecular basis for stimulatory implications of ionizing radiation (IR) on plant male gametophytes (pollen). Beyond that, this analysis impacts general research on cell growth, plant breeding, radiation protection, and, in a wider sense, medical treatment. For this purpose, IR-related data were surveyed and discussed in connection with the present knowledge about pollen physiology. It is concluded that IR-induced reactive oxygen species (ROS) have a key role here. Moreover, it is hypothesized that IR-exposure shifts the ratio between diverse types of ROS in the cell. The interrelation between ROS, intracellular Ca2+-gradient, NADPH oxidases, ROS-scavengers, actin dynamics, and cell wall properties are most probably involved in IR-hormesis of pollen germination and tube growth. Modulation of gene expression, phytohormone signalling, and cellular antioxidant capacity are also implicated in IR-hormesis.

Plant Hormone and Fatty Acid Screening of Nicotiana tabacum and Lilium longiflorum Stigma Exudates

Pollen germination in vivo on wet stigmas is assisted by the receptive fluid-stigma exudate. Its exact composition is still unknown because only some components have been studied. For the first time, hormonal screening was carried out, and the fatty acid (FA) composition of lipid-rich (Nicotiana tabacum) and sugar-rich (Lilium longiflorum) exudates was studied. Screening of exudate for the presence of plant hormones using HPLC-MS revealed abscisic acid (ABA) in tobacco stigma exudate at the two stages of development, at pre-maturity and in mature stigmas awaiting pollination, increasing at the fertile stage. To assess physiological significance of ABA on stigma, we tested the effect of this hormone in vitro. ABA concentration found in the exudate strongly stimulated the germination of tobacco pollen, a lower concentration had a weaker effect, increasing the concentration did not increase the effect. GC-MS analysis showed that both types of exudate are characterized by a predominance of saturated FAs. The lipids of tobacco stigma exudate contain significantly more myristic, oleic, and linoleic acids, resulting in a higher unsaturation index relative to lily stigma exudate lipids. The latter, in turn, contain more 14-hexadecenoic and arachidic acids. Both exudates were found to contain significant amounts of squalene. The possible involvement of saturated FAs, ABA, and squalene in various exudate functions, as well as their potential relationship on the stigma, is discussed.

Polyphenol oxidases regulate pollen development through modulating flavonoids homeostasis in tobacco

Pollen development is critical in plant reproduction. Polyphenol oxidases (PPOs) genes encode defense-related enzymes, but the role of PPOs in pollen development remains largely unexplored. Here, we characterized NtPPO genes, and then investigated their function in pollen via creating NtPPO9/10 double knockout mutant (cas-1), overexpression 35S::NtPPO10 (cosp) line and RNAi lines against all NtPPOs in Nicotiana tabacum. NtPPOs were abundantly expressed in the anther and pollen (especially NtPPO9/10). The pollen germination, polarity ratio and fruit weights were significantly reduced in the NtPPO-RNAi and cosp lines, while they were normal in cas-1 likely due to compensation by other NtPPO isoforms. Comparisons of metabolites and transcripts between the pollen of WT and NtPPO-RNAi, or cosp showed that decreased enzymatic activity of NtPPOs led to hyper-accumulation of flavonoids. This accumulation might reduce the content of ROS. Ca²⁺ and actin levels also decreased in pollen of the transgenic lines.Thus, the NtPPOs regulate pollen germination through the flavonoid homeostasis and ROS signal pathway. This finding provides novel insights into the native physiological functions of PPOs in pollen during reproduction.

Signaling network controlling ROP-mediated tip growth in Arabidopsis and beyond

Cell polarity operates across a broad range of spatial and temporal scales and is essential for specific biological functions of polarized cells. Tip growth is a special type of polarization in which a single and unique polarization site is established and maintained, as for the growth of root hairs and pollen tubes in plants. Extensive studies in past decades have demonstrated that the spatiotemporal localization and activity of Rho of Plants (ROPs), the only class of Rho GTPases in plants, are critical for tip growth. ROPs are switched on or off by different factors to initiate dynamic intracellular activities, leading to tip growth. Recent studies have also uncovered several feedback modules for ROP signaling. In this review, we summarize recent progress on ROP signaling in tip growth, focusing on molecular mechanisms that underlie the dynamic distribution and activity of ROPs in Arabidopsis. We also highlight feedback modules that control ROP-mediated tip growth and provide a perspective for building a complex ROP signaling network. Finally, we provide an evolutionary perspective for ROP-mediated tip growth in Physcomitrella patens and during plant-rhizobia interaction.

The Balance between Different ROS on Tobacco Stigma during Flowering and Its Role in Pollen Germination

The concept of ROS as an important factor controlling pollen germination and tube growth has become generally accepted in the last decade. However, the relationship between various ROS and their significance for the success of in vivo germination and fertilization remained unexplored. For the present study, we collected Nicotiana tabacum stigma exudate on different stages of stigma maturity before and after pollination. Electron paramagnetic resonance (EPR) and colorimetric analysis were used to assess levels of O^•₂⁻ and H₂O₂ on stigma. Superoxide dismutase activity in the stigma tissues at each stage was evaluated zymographically. As the pistil matured, the level of both ROS decreased markedly, while the activity of SOD increased, and, starting from the second stage, the enzyme was represented by two isozymes: Fe SOD and Cu/Zn SOD, which was demonstrated by the in-gel inhibitory analysis. Selective suppression of Cu/Zn SOD activity shifted the ROS balance, which was confirmed by EPR. This shift markedly reduced the rate of pollen germination in vivo and the fertilization efficiency, which was estimated by the seed set. This result showed that hydrogen peroxide is a necessary component of stigma exudate, accelerates germination and ensures successful reproduction. A decrease in O^•₂⁻ production due to NADPH oxidase inhibition, although it slowed down germination, did not lead to a noticeable decrease in the seed set. Thus, the role of the superoxide radical can be characterized as less important.

Global survey of alternative splicing and gene modules associated with fertility regulation in a thermosensitive genic male sterile wheat

Thermosensitive genic male sterile (TGMS) wheat lines are the core of two-line hybrid systems. Understanding the mechanism that regulates male sterility in TGMS wheat lines is helpful for promoting wheat breeding. Several studies have obtained information regarding the mechanisms associated with male sterility at the transcriptional level, but it is not clear how the post-transcriptional process of alternative splicing might contribute to controlling male sterility. In this study, we performed genome-wide analyses of alternative splicing during the meiosis stage in TGMS line BS366 using PacBio and RNA-Seq hybrid sequencing. Cytological observations indicated that cytoskeleton assembly in pollen cells, calcium deposition in pollen and tapetal cells, and vesicle transport in tapetal cells were deficient in BS366. According to our cytological findings, 49 differentially spliced genes were isolated. Moreover, 25 long non-coding RNA targets and three bHLH transcription factors were identified. Weighted gene co-expression network analysis detected four candidate differentially spliced genes that had strong co-relation with the seed setting percentage, which is the direct representation of male sterility in BS366. In this study, we obtained comprehensive data regarding the alternative splicing-mediated regulation of male sterility in TGMS wheat. The candidates identified may provide the molecular basis for an improved understanding of male sterility.

How to Cope with the Challenges of Environmental Stresses in the Era of Global Climate Change: An Update on ROS Stave off in Plants

With the advent of human civilization and anthropogenic activities in the shade of urbanization and global climate change, plants are exposed to a complex set of abiotic stresses. These stresses affect plants’ growth, development, and yield and cause enormous crop losses worldwide. In this alarming scenario of global climate conditions, plants respond to such stresses through a highly balanced and finely tuned interaction between signaling molecules. The abiotic stresses initiate the quick release of reactive oxygen species (ROS) as toxic by-products of altered aerobic metabolism during different stress conditions at the cellular level. ROS includes both free oxygen radicals {superoxide (O₂^•−) and hydroxyl (OH⁻)} as well as non-radicals [hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂)]. ROS can be generated and scavenged in different cell organelles and cytoplasm depending on the type of stimulus. At high concentrations, ROS cause lipid peroxidation, DNA damage, protein oxidation, and necrosis, but at low to moderate concentrations, they play a crucial role as secondary messengers in intracellular signaling cascades. Because of their concentration-dependent dual role, a huge number of molecules tightly control the level of ROS in cells. The plants have evolved antioxidants and scavenging machinery equipped with different enzymes to maintain the equilibrium between the production and detoxification of ROS generated during stress. In this present article, we have focused on current insights on generation and scavenging of ROS during abiotic stresses. Moreover, the article will act as a knowledge base for new and pivotal studies on ROS generation and scavenging.

Links between Regulatory Systems of ROS and Carbohydrates in Reproductive Development

To thrive on the earth, highly sophisticated systems to finely control reproductive development have been evolved in plants. In addition, deciphering the mechanisms underlying the reproductive development has been considered as a main research avenue because it leads to the improvement of the crop yields to fulfill the huge demand of foods for the growing world population. Numerous studies revealed the significance of ROS regulatory systems and carbohydrate transports and metabolisms in the regulation of various processes of reproductive development. However, it is poorly understood how these mechanisms function together in reproductive tissues. In this review, we discuss mode of coordination and integration between ROS regulatory systems and carbohydrate transports and metabolisms underlying reproductive development based on the hitherto findings. We then propose three mechanisms as key players that integrate ROS and carbohydrate regulatory systems. These include ROS-dependent programmed cell death (PCD), mitochondrial and respiratory metabolisms as sources of ROS and energy, and functions of arabinogalactan proteins (AGPs). It is likely that these key mechanisms govern the various signals involved in the sequential events required for proper seed production.

Pollen Germination and Pollen Tube Growth in Gymnosperms

Pollen germination and pollen tube growth are common to all seed plants, but these processes first developed in gymnosperms and still serve for their successful sexual reproduction. The main body of data on the reproductive physiology, however, was obtained on flowering plants, and one should be careful to extrapolate the discovered patterns to gymnosperms. In recent years, physiological studies of coniferous pollen have been increasing, and both the features of this group and the similarities with flowering plants have already been identified. The main part of the review is devoted to physiological studies carried out on conifer pollen. The main properties and diversity of pollen grains and pollination strategies in gymnosperms are described.

Oxygen radicals and cytoplasm zoning in growing lily pollen tubes

Differential modulation of ROS content of the microenvironment (O ¯/MnTMPP/OH·) affects growth speed and morphology in lily pollen tubes. Oxygen radicals influence ionic zoning: membrane potential and pH gradients. Recently, redox-regulation of tip growth has been extensively studied, but differential sensitivity of growing cells to particular ROS and their subcellular localization is still unclear. Here, we used specific dyes to provide mapping of H₂O₂ and O^·₂¯ in short and long pollen tubes. We found apical accumulation of H₂O₂ and H₂O₂-producing organelles in the shank that were not colocalized with O^·₂¯-producing mitochondria. Differential modulation of ROS content of the germination medium affected both growth speed and pollen tube morphology. Oxygen radicals affected ionic zoning: membrane potential and pH gradients. OH· caused depolarization all along the tube while O^·₂¯ provoked hyperpolarization and cytoplasm alkalinization. O^·₂¯accelerated growth and reduced tube diameter, indicating that this ROS can be considered as pollen tube growth stimulator. Serious structural disturbances were observed upon exposure to OH· and ROS quencher MnTMPP: pollen tube growth slowed down and ballooned tips formed in both cases, but OH· affected membrane transport and organelle distribution as well. OH·, thus, can be considered as a negative regulator of pollen tube growth. Pollen tubes, in turn, are able to reduce OH· concentration, which was assessed by electron paramagnetic resonance spectroscopy (EPR).

cROStalk for Life: Uncovering ROS Signaling in Plants and Animal Systems, from Gametogenesis to Early Embryonic Development

This review explores the role of reactive oxygen species (ROS)/Ca2+ in communication within reproductive structures in plants and animals. Many concepts have been described during the last years regarding how biosynthesis, generation products, antioxidant systems, and signal transduction involve ROS signaling, as well as its possible link with developmental processes and response to biotic and abiotic stresses. In this review, we first addressed classic key concepts in ROS and Ca2+ signaling in plants, both at the subcellular, cellular, and organ level. In the plant science field, during the last decades, new techniques have facilitated the in vivo monitoring of ROS signaling cascades. We will describe these powerful techniques in plants and compare them to those existing in animals. Development of new analytical techniques will facilitate the understanding of ROS signaling and their signal transduction pathways in plants and mammals. Many among those signaling pathways already have been studied in animals; therefore, a specific effort should be made to integrate this knowledge into plant biology. We here discuss examples of how changes in the ROS and Ca2+ signaling pathways can affect differentiation processes in plants, focusing specifically on reproductive processes where the ROS and Ca2+ signaling pathways influence the gametophyte functioning, sexual reproduction, and embryo formation in plants and animals. The study field regarding the role of ROS and Ca2+ in signal transduction is evolving continuously, which is why we reviewed the recent literature and propose here the potential targets affecting ROS in reproductive processes. We discuss the opportunities to integrate comparative developmental studies and experimental approaches into studies on the role of ROS/ Ca2+ in both plant and animal developmental biology studies, to further elucidate these crucial signaling pathways.