Polyamines treatment during pollen germination and pollen tube elongation in tobacco modulate reactive oxygen species and nitric oxide homeostasis

Subfunctionalization of Parental Polyamine Oxidase (PAO) Genes in the Allopolyploid Tobacco Nicotiana tabacum (L.)

Polyamines play an important role in developmental and environmental stress responses in plants. Polyamine oxidases (PAOs) are flavin-adenine-dinucleotide-dependent enzymes associated with polyamine catabolism. In this study, 14 genes were identified in the tobacco genome that code for PAO proteins being named based on their sequence homology with Arabidopsis PAOs (AtPAO1-5): NtPAO1A-B; NtPAO2A-C, NtPAO4A-D, and NtPAO5A-E. Sequence analysis confirmed that the PAO gene family of the allopolyploid hybrid Nicotiana tabacum is not an exact combination of the PAO genes of the maternal Nicotiana sylvestris and paternal Nicotiana tomentosiformis ones. The loss of the N. sylvestris homeolog of NtPAO5E and the gain of an extra NtPAO2 copy, likely of Nicotiana othophora origin, was revealed. The latter adds to the few pieces of evidence suggesting that the paternal parent of N. tabacum was an introgressed hybrid of N. tomentosiformis and N. othophora. Gene expression analysis indicated that all 14 PAO genes kept their expression following the formation of the hybrid species. The homeologous gene pairs showed similar or opposite regulation depending on the investigated organ, applied stress, or hormone treatment. The data indicate that the expression pattern of the homeologous genes is diversifying in a process of subfunctionalization.

TGase-induced Cd tolerance by boosting polyamine, nitric oxide, cell wall composition and phytochelatin synthesis in tomato

In highly intensive greenhouse vegetable production, soil acidification was caused by excessive fertilization, increasing cadmium (Cd) concentrations in the vegetables, which bears environmental hazards and is a negative influence on vegetables and humans. Transglutaminases (TGases), a central mediator for certain physiological effects of polyamines (PAs) in the plant kingdom, play important roles in plant development and stress response. Despite increased research on the crucial role of TGase in protecting against environmental stresses, relatively little is known about the mechanisms of Cd tolerance. In this study, we found, TGase activity and transcript level, which was upregulated by Cd, and TGase-induced Cd tolerance related to endogenous bound PAs increase and formation of nitric oxide (NO). Plant growth of tgase mutants was hypersensitive to Cd, chemical complementation by putrescine, sodium nitroprusside (SNP, nitric oxide donor) or gain of function TGase experiments restore Cd tolerance. α-diflouromethylornithine (DFMO, a selective ODC inhibitor) and 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, NO scavenger), were respectively found declined drastically endogenous bound PA and NO content in TGase overexpression plants. Likewise, we reported that TGase interacted with polyamine uptake protein 3 (Put3), and the silencing of Put3 largely reduced TGase-induced Cd tolerance and bound PAs formation. This salvage strategy depends on TGase-regulated synthesis of bound PAs and NO that is able to positively increase the concentration of thiol and phytochelatins, elevate Cd in the cell wall, as well as induce the levels of expression Cd uptake and transport genes. Collectively, these findings indicate that TGase-mediated enhanced levels of bound PA and NO acts as a vital mechanism to protect the plant from Cd-caused toxicity.

H2 O2 , NO, and H2 S networks during root development and signalling under physiological and challenging environments: Beneficial or toxic?

Hydrogen peroxide (H₂ O₂ ) is a reactive oxygen species (ROS) and a key modulator of the development and architecture of the root system under physiological and adverse environmental conditions. Nitric oxide (NO) and hydrogen sulphide (H₂ S) also exert myriad functions on plant development and signalling. Accumulating pieces of evidence show that depending upon the dose and mode of applications, NO and H₂ S can have synergistic or antagonistic actions in mediating H₂ O₂ signalling during root development. Thus, H₂ O₂ -NO-H₂ S crosstalk might essentially impart tolerance to elude oxidative stress in roots. Growth and proliferation of root apex involve crucial orchestration of NO and H₂ S-mediated ROS signalling which also comprise other components including mitogen-activated protein kinase, cyclins, cyclin-dependent kinases, respiratory burst oxidase homolog (RBOH), and Ca²⁺ flux. This assessment provides a comprehensive update on the cooperative roles of NO and H₂ S in modulating H₂ O₂ homoeostasis during root development, abiotic stress tolerance, and root-microbe interaction. Furthermore, it also analyses the scopes of some fascinating future investigations associated with strigolactone and karrikins concerning H₂ O₂ -NO-H₂ S crosstalk in plant roots.

Absence of Arabidopsis Polyamine Oxidase 5 Influences the Cytokinin-Induced Shoot Meristem Formation from Lateral Root Primordia

Lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems (SMs) by the application of exogenous cytokinin. Here, we report that Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) contributes to this process, since the rate of SM formation from LRPs was significantly lower in the pao5-2 knockout mutant. Furthermore, the presented experiments showed that AtPAO5 influences SM formation via controlling the thermospermine (T-Spm) level. Gene expression analyses supported the view that the pao5-2 mutation as well as exogenous T-Spm downregulate the expression of the class 3 haemoglobin coding genes AtGLB1 and AtGLB2. AtGLB1 and 2 have been reported to augment cytokinin sensitivity, indirectly inhibiting the expression of type-A ARABIDOPSIS RESPONSE REGULATORs (ARRs). In agreement, the same ARR-coding genes were found to be upregulated in the pao5-2 mutant. Although GLB proteins might also control cytokinin-induced nitric oxide (NO) accumulation, we could not find experimental evidence for it. Rather, the negative effect of NO-donor treatment on AtPAO5 gene expression and SM formation was seen. Nevertheless, a hypothetical pathway is set up explaining how AtPAO5 may affect direct shoot meristem formation, controlling cytokinin sensitivity through T-Spm and GLBs.

Cold Stress Response Mechanisms in Anther Development

Unlike animals that can escape threats, plants must endure and adapt to biotic and abiotic stresses in their surroundings. One such condition, cold stress, impairs the normal growth and development of plants, in which most phases of reproductive development are particularly susceptible to external low temperature. Exposed to uncomfortably low temperature at the reproductive stage, meiosis, tapetal programmed cell death (PCD), pollen viability, and fertilization are disrupted, resulting in plant sterility. Of them, cold-induced tapetal dysfunction is the main cause of pollen sterility by blocking nutrition supplements for microspore development and altering their timely PCD. Further evidence has indicated that the homeostatic imbalances of hormones, including abscisic acid (ABA) and gibberellic acid (GA), and sugars have occurred in the cold-treated anthers. Among them, cold stress gives rise to the accumulation of ABA and the decrease of active GA in anthers to affect tapetal development and represses the transport of sugar to microspores. Therefore, plants have evolved lots of mechanisms to alleviate the damage of external cold stress to reproductive development by mainly regulating phytohormone levels and sugar metabolism. Herein, we discuss the physiological and metabolic effects of low temperature on male reproductive development and the underlying mechanisms from the perspective of molecular biology. A deep understanding of cold stress response mechanisms in anther development will provide noteworthy references for cold-tolerant crop breeding and crop production under cold stress.

Polyamine Oxidase-Generated Reactive Oxygen Species in Plant Development and Adaptation: The Polyamine Oxidase-NADPH Oxidase Nexus

Metabolism and regulation of cellular polyamine levels are crucial for living cells to maintain their homeostasis and function. Polyamine oxidases (PAOs) terminally catabolize polyamines or catalyse the back-conversion reactions when spermine is converted to spermidine and Spd to putrescine. Hydrogen peroxide (H₂O₂) is a by-product of both the catabolic and back-conversion processes. Pharmacological and genetic approaches have started to uncover the roles of PAO-generated H₂O₂ in various plant developmental and adaptation processes such as cell differentiation, senescence, programmed cell death, and abiotic and biotic stress responses. Many of these studies have revealed that the superoxide-generating Respiratory Burst Oxidase Homolog (RBOH) NADPH oxidases control the same processes either upstream or downstream of PAO action. Therefore, it is reasonable to suppose that the two enzymes co-ordinately control the cellular homeostasis of reactive oxygen species. The intricate relationship between PAOs and RBOHs is also discussed, posing the hypothesis that these enzymes indirectly control each other's abundance/function via H₂O₂.

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.

Male Fertility under Environmental Stress: Do Polyamines Act as Pollen Tube Growth Protectants?

Although pollen structure and morphology evolved toward the optimization of stability and fertilization efficiency, its performance is affected by harsh environmental conditions, e.g., heat, cold, drought, pollutants, and other stressors. These phenomena are expected to increase in the coming years in relation to predicted environmental scenarios, contributing to a rapid increase in the interest of the scientific community in understanding the molecular and physiological responses implemented by male gametophyte to accomplish reproduction. Here, after a brief introduction summarizing the main events underlying pollen physiology with a focus on polyamine involvement in its development and germination, we review the main effects that environmental stresses can cause on pollen. We report the most relevant evidence in the literature underlying morphological, cytoskeletal, metabolic and signaling alterations involved in stress perception and response, focusing on the final stage of pollen life, i.e., from when it hydrates, to pollen tube growth and sperm cell transport, with these being the most sensitive to environmental changes. Finally, we hypothesize the molecular mechanisms through which polyamines, well-known molecules involved in plant development, stress response and adaptation, can exert a protective action against environmental stresses in pollen by decoding the essential steps and the intersection between polyamines and pollen tube growth mechanisms.

Translational and post-translational regulation of polyamine metabolic enzymes in plants

Polyamines are small organic and basic polycations that perform essential regulatory functions in all living organisms. Fluctuations in polyamine content have been observed to occur during growth, development and under stress conditions, implying that polyamines play pivotal roles in diverse cellular and physiological processes. To achieve polyamine homeostasis, the entire metabolic pathway is subjected to a fine-tuned regulation of its biosynthetic and catabolic genes and enzymes. In this review, we describe and discuss the most important mechanisms implicated in the translational and post-translational regulation of polyamine metabolic enzymes in plants. At the translational level, we emphasize the role of polyamines in the modulation of upstream open reading frame (uORF) activities that control the translation of polyamine biosynthetic and catabolic mRNAs. At the post-translational level, different aspects of the regulation of polyamine metabolic proteins are depicted, such as the proteolytic activation of enzyme precursors, the importance of dimerization in protein stability as well as in protein intracellular localization.

The Arabidopsis Rho of Plants GTPase ROP1 Is a Potential Calcium-Dependent Protein Kinase (CDPK) Substrate

Plant Rho-type GTPases (ROPs) are versatile molecular switches involved in a number of signal transduction pathways. Although it is well known that they are indirectly linked to protein kinases, our knowledge about their direct functional interaction with upstream or downstream protein kinases is scarce. It is reasonable to suppose that similarly to their animal counterparts, ROPs might also be regulated by phosphorylation. There is only, however, very limited experimental evidence to support this view. Here, we present the analysis of two potential phosphorylation sites of AtROP1 and two types of potential ROP-kinases. The S74 site of AtROP1 has been previously shown to potentially regulate AtROP1 activation dependent on its phosphorylation state. However, the kinase phosphorylating this evolutionarily conserved site could not be identified: we show here that despite of the appropriate phosphorylation site consensus sequences around S74 neither the selected AGC nor CPK kinases phosphorylate S74 of AtROP1 in vitro. However, we identified several phosphorylation sites other than S74 for the CPK17 and 34 kinases in AtROP1. One of these sites, S97, was tested for biological relevance. Although the mutation of S97 to alanine (which cannot be phosphorylated) or glutamic acid (which mimics phosphorylation) somewhat altered the protein interaction strength of AtROP1 in yeast cells, the mutant proteins did not modify pollen tube growth in an in vivo test.

Polyamine Metabolism Is Involved in the Direct Regeneration of Shoots from Arabidopsis Lateral Root Primordia

Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. Most of these regeneration pathways are indirect and involve callus formation. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. In this system, no callus formation takes place. We report that the level of PAs, especially that of spermidine (Spd), increased during meristem conversion and the application of exogenous Spd improved its efficiency. The high endogenous Spd level could be due to enhanced synthesis as indicated by the augmented relative expression of PA synthesis genes (AtADC1,2, AtSAMDC2,4, AtSPDS1,2) during the process. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. The expression of Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) was shown to be specifically high during the process and its ectopic overexpression increased the LRP-to-shoot conversion efficiency. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.

Do fungicides affect the physiology, reproductive development and productivity of healthy soybean plants?

Fungicides are widely used to control diseases in soybean crops. We hypothesized that fungicides applied to healthy soybean plants compromise the plant's physiology, affect the reproductive process and reduce crop productivity. We aimed to evaluate the photosynthetic process, pollen grain viability and yield components of soybean plants exposed to three commercial fungicides. The experiment was performed twice using soybean cultivar SYN 1378C, disease-free plants, with four treatments: i) control treatment (without any fungicide application); ii) cyproconazole 150 g L-1 + difenoconazole 250 g L-1 (CPZ + DFZ; 250 mL ha-1; without adjuvant); iii) azoxystrobin 300 g Kg-1 + benzovindiflupyr 150 g Kg-1 (AZB + BZP; 200 g ha-1; Nimbus® adjuvant (Syngenta)); and iv) propiconazole 250 g L-1 + difenoconazole 250 g L-1 (PPZ + DFZ; 150 mL ha-1; without adjuvant) in both soybean pre-bloom (V8) and bloom (R1) developmental stages. The experimental design was randomized blocks with four replicates. Phytotoxicity, gas exchange and chlorophyll a fluorescence traits, pollen grain viability, pollen grain germination, flower abortion and soybean production components were evaluated. The fungicides did not affect the physiological traits, pollen grain germination and crop yield.

Putrescine Promotes Betulin Accumulation in Suspension Cell Cultures of Betula platyphylla by Regulating NO and NH4+ Production

Putrescine (Put) can enhance secondary metabolite production, but its intrinsic regulatory mechanism remains unclear. In this study, Put treatment promoted betulin production and gene expression of lupeol synthase (LUS), one of betulin synthetic enzymes. The maximum betulin content and gene expression level of LUS was 4.25 mg·g−1 DW and 8.25 at 12 h after 1 mmol·L−1 Put treatment, approximately two- and four-times that in the control, respectively. Put treatment increased the content of nitric oxide (NO) and its biosynthetic enzyme activity of nitrate reductase (NR) and NO synthase (NOS). Pretreatment of the birch suspension cells with NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline- 1-oxyl-3-oxide (cPTIO), NR inhibitor sodium azide (NaN3), and NOS inhibitor NG-nitro-L-Arg methyl ester (L-NAME) decreased Put-triggered NO generation and blocked Put-induced betulin production. Put treatment improved the content of NH4+ and its assimilation enzyme activity of glutamate synthase and glutamate dehydrogenase. NH4+ supplementation also promoted NO and betulin production. Thus, the above data indicated that Put-induced NO was essential for betulin production. NO derived from NR, NOS, and NH4+ mediated betulin production in birch suspension cell cultures under Put treatment.

ROS and Ions in Cell Signaling during Sexual Plant Reproduction

Pollen grain is a unique haploid organism characterized by two key physiological processes: activation of metabolism upon exiting dormancy and polar tube growth. In gymnosperms and flowering plants, these processes occur in different time frames and exhibit important features; identification of similarities and differences is still in the active phase. In angiosperms, the growth of male gametophyte is directed and controlled by its microenvironment, while in gymnosperms it is relatively autonomous. Recent reviews have detailed aspects of interaction between angiosperm female tissues and pollen such as interactions between peptides and their receptors; however, accumulated evidence suggests low-molecular communication, in particular, through ion exchange and ROS production, equally important for polar growth as well as for pollen germination. Recently, it became clear that ROS and ionic currents form a single regulatory module, since ROS production and the activity of ion transport systems are closely interrelated and form a feedback loop.

Exogenous Nitric Oxide Delays Plant Regeneration from Protoplast and Protonema Development in Physcomitrella patens

Nitric oxide (NO) has been recognized as a major player in the regulation of plant physiology and development. NO regulates cell cycle progression and cell elongation in flowering plants and green algae, although the information about NO function in non-vascular plants is scarce. Here, we analyze the effect of exogenous NO on Physcomitrella patens protonema growth. We find that increasing concentrations of the NO donor sodium nitroprusside (SNP) inhibit protonema relative growth rate and cell length. To further comprehend the effect of NO on moss development, we analyze the effect of SNP 5 and 10 µM on protoplast regeneration and, furthermore, protonema formation compared with untreated plants (control). Isolated protoplasts were left to regenerate for 24 h before starting the SNP treatments that lasted five days. The results show that SNP restrains the protoplast regeneration process and the formation of new protonema cells. When SNP treatments started five days after protoplast isolation, a decrease in cell number per protonema filament was observed, indicating an inhibition of cell cycle progression. Our results show that in non-vascular plants, NO negatively regulates plant regeneration, cell cycle and cell elongation.

Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.

Fertilization in flowering plants: an odyssey of sperm cell delivery

In light of the available discoveries in the field, this review manuscript discusses on plant reproduction mechanism and molecular players involved in the process. Sperm cells in angiosperms are immotile and are physically distant to the female gametophytes (FG). To secure the production of the next generation, plants have devised a clever approach by which the two sperm cells in each pollen are safely delivered to the female gametophyte where two fertilization events occur (by each sperm cell fertilizing an egg cell and central cell) to give rise to embryo and endosperm. Each of the successfully fertilized ovules later develops into a seed. Sets of macromolecules play roles in pollen tube (PT) guidance, from the stigma, through the transmitting tract and funiculus to the micropylar end of the ovule. Other sets of genetic players are involved in PT reception and in its rupture after it enters the ovule, and yet other sets of genes function in gametic fusion. Angiosperms have come long way from primitive reproductive structure development to today's sophisticated, diverse, and in most cases flamboyant organ. In this review, we will be discussing on the intricate yet complex molecular mechanism of double fertilization and how it might have been shaped by the evolutionary forces focusing particularly on the model plant Arabidopsis.