Transcriptional evidence for inferred pattern of pollen tube-stigma metabolic coupling during pollination

Emerging Roles of γ Aminobutyric Acid (GABA) Gated Channels in Plant Stress Tolerance

The signaling role for γ-Aminobutyric acid (GABA) has been documented in animals for over seven decades. However, a signaling role for GABA in plants is just beginning to emerge with the discovery of putative GABA binding site/s and GABA regulation of anion channels. In this review, we explore the role of GABA in plant growth and development under abiotic stress, its interactions with other signaling molecules and the probability that there are other anion channels with important roles in stress tolerance that are gated by GABA.

Glycosylphosphatidylinositol-anchored proteins mediate the interactions between pollen/pollen tube and pistil tissues

In flowering plants, pollen germination on the stigma and pollen tube growth in pistil tissues are critical for sexual plant reproduction, which are involved in the interactions between pollen/pollen tube and pistil tissues. GPI-anchored proteins (GPI-APs) are located on the external surface of the plasma membrane and function in various processes of sexual plant reproduction. The evidences suggest that GPI-APs participate in endosome machinery, Ca²⁺ oscillations, the development of the transmitting tract, the maintenance of the integrity of pollen tube, the enhancement of interactions of the receptor-like kinase (RLK) and ligand, and guidance of the growth of pollen tube, and so on. In this review, we will summarize the recent progress on the roles of GPI-APs in the interactions between pollen/pollen tube and pistil tissues during pollination, such as pollen germination on the stigma, pollen tube growth in the transmitting tract, pollen tube guidance to the ovule, and pollen tube reception in the embryo sac. We will also discuss the future outlook of GPI-APs in the interactions between pollen/pollen tube and pistil tissues.

ROS in the Male-Female Interactions During Pollination: Function and Regulation

The male-female interactions in pollination mediate pollen hydration and germination, pollen tube growth and fertilization. Reactive oxygen species (ROS) derived from both male and female tissues play regulatory roles for the communication between the pollen/pollen tube and female tissues at various stages, such as pollen hydration and germination on the stigma, pollen tube growth in the pistil and pollen tube reception in the female gametophyte. In this minireview, we primarily summarize the recent progress on the roles of ROS signaling in male-female interactions during pollination and discuss several ROS-regulated downstream signaling pathways for these interactions. Furthermore, several ROS-involved downstream pathways are outlined, such as Ca²⁺ signaling, cell wall cytomechanics, the redox modification of CRP, and cell PCD. At the end, we address the roles of ROS in pollen tube guidance and fertilization as future questions that merit study.

The Ca2+ signaling, Glu, and GABA responds to Cd stress in duckweed

Cadmium (Cd) affects plants and animal health seriously. Ca²⁺ signals in plant cells are important for adaptive responses to environmental stresses. Here we showed that 50 μM Cd shock stimulated the Ca²⁺ signal via modifying the instantaneous Ca²⁺ flux from influx of 17 pmol·cm^-2·s^-1 to the efflux of 240 pmol·cm^-2·s^-1 at 100 μm from rhizoid tip. And the Ca²⁺ signal transferred to the vein and mesophyll cell. The Ca addition decreased the accumulation of Cd. The gene expression of glutamate receptor-like (GLR) proteins, which is activated by Glu and triggers Ca²⁺ flux, was increased significantly by 24 h Cd stress. Glu content was increased under Cd stress and exogenous Glu triggered the Ca²⁺ signal in duckweed, while Ca²⁺ addition caused no influence to Glu content. GABA, which is synthesized from Glu and acts as an inhibitory neurotransmitter, has been decreased with 24 h Cd treatment. GABA addition increased the abscission rate and Glu addition decreased the abscission rate during Cd stress, suggesting that the Glu/GABA ratio is important for responding to Cd. This research shows the sight of the Glu, Ca²⁺, GABA signaling networks during Cd stress.

Untargeted metabolomic profiling reveals that different responses to self and cross pollination in each flower morph of the heteromorphic plant Plumbago auriculata

Heteromorphic self-incompatibility (HetSI), which is regulated by sporophytes, occurs in some species as a strategy to promote cross-pollination. This research aimed to reveal metabolic changes occurring in HetSI. We used fluorescence microscopy as a tool to compare growth behavior in self-incompatible (SI) and self-compatible (SC) pollination in both pin and thrum flowers of Plumbago auriculata and to identify the ideal timepoint for sample collection for subsequent experiments. We also employed scanning electron microscopy (SEM) to evaluate intermorph structural differences in the pollen grains and stigmas in relation to HetSI. Importantly, UPLC-MS/MS was applied in this study to identify metabolites, compare metabolic differences between pin and thrum styles and monitor metabolic changes in SC and SI pollinations in the two types of flowers. The metabolites mainly included amino acids/peptides, flavonoids, glycosides/sugars, phenols, other organic acids, fatty acids (derivatives)/lipids, amines, aldehydes, alkaloids, alcohols and other compounds. Surprisingly, energy-related nutrients such as amino acids/peptides and tricarboxylic acid cycle-related metabolites were found at higher levels in SI pollinations than in SC pollinations. This result indicates that physiological changes in pollen-stigma interactions differ in pin and thrum styles and SC and SI pollinations and that energy deficiency is not one of the reasons for HetSI.

Male Sterility in Maize after Transient Heat Stress during the Tetrad Stage of Pollen Development

Shifts in the duration and intensity of ambient temperature impair plant development and reproduction, particularly male gametogenesis. Stress exposure causes meiotic defects or premature spore abortion in male reproductive organs, leading to male sterility. However, little is known about the mechanisms underlying stress and male sterility. To elucidate these mechanisms, we imposed a moderate transient heat stress on maize (Zea mays) plants at the tetrad stage of pollen development. After completion of pollen development at optimal conditions, stress responses were assessed in mature pollen. Transient heat stress resulted in reduced starch content, decreased enzymatic activity, and reduced pollen germination, resulting in sterility. A transcriptomic comparison pointed toward misregulation of starch, lipid, and energy biosynthesis-related genes. Metabolomic studies showed an increase of Suc and its monosaccharide components, as well as a reduction in pyruvate. Lipidomic analysis showed increased levels of unsaturated fatty acids and decreased levels of saturated fatty acids. In contrast, the majority of genes involved in developmental processes such as those required for auxin and unfolded protein responses, signaling, and cell wall biosynthesis remained unaltered. It is noteworthy that changes in the regulation of transcriptional and metabolic pathway genes, as well as heat stress proteins, remained altered even though pollen could recover during further development at optimal conditions. In conclusion, our findings demonstrate that a short moderate heat stress during the highly susceptible tetrad stage strongly affects basic metabolic pathways and thus generates germination-defective pollen, ultimately leading to severe yield losses in maize.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Metabolite-Centric Reporter Pathway and Tripartite Network Analysis of Arabidopsis Under Cold Stress

The study of plant resistance to cold stress and the metabolic processes underlying its molecular mechanisms benefit crop improvement programs. Here we investigate the effects of cold stress on the metabolic pathways of Arabidopsis when directly inferred at system level from transcriptome data. A metabolite-centric reporter pathway analysis approach enabled the computation of metabolites associated with transcripts at four time points of cold treatment. Tripartite networks of gene-metabolite-pathway connectivity outlined the response of metabolites and pathways to cold stress. Our metabolome-independent analysis revealed stress-associated metabolites in pathway routes of the cold stress response, including amino acid, carbohydrate, lipid, hormone, energy, photosynthesis, and signaling pathways. Cold stress first triggered the mobilization of energy from glycolysis and ethanol degradation to enhance TCA cycle activity via acetyl-CoA. Interestingly, tripartite networks lacked power law behavior and scale free connectivity, favoring modularity. Network rewiring explicitly involved energetics, signal, carbon and redox metabolisms and membrane remodeling.

γ-Aminobutyric acid (GABA) signalling in plants

The role of γ-aminobutyric acid (GABA) as a signal in animals has been documented for over 60 years. In contrast, evidence that GABA is a signal in plants has only emerged in the last 15 years, and it was not until last year that a mechanism by which this could occur was identified-a plant 'GABA receptor' that inhibits anion passage through the aluminium-activated malate transporter family of proteins (ALMTs). ALMTs are multigenic, expressed in different organs and present on different membranes. We propose GABA regulation of ALMT activity could function as a signal that modulates plant growth, development, and stress response. In this review, we compare and contrast the plant 'GABA receptor' with mammalian GABAA receptors in terms of their molecular identity, predicted topology, mode of action, and signalling roles. We also explore the implications of the discovery that GABA modulates anion flux in plants, its role in signal transduction for the regulation of plant physiology, and predict the possibility that there are other GABA interaction sites in the N termini of ALMT proteins through in silico evolutionary coupling analysis; we also explore the potential interactions between GABA and other signalling molecules.

The Arabidopsis phytohormone crosstalk network involves a consecutive metabolic route and circular control units of transcription factors that regulate enzyme-encoding genes

Background

Phytohormone synergies and signaling interdependency are important topics in plant developmental biology. Physiological and genetic experimental evidence for phytohormone crosstalk has been accumulating and a genome-scale enzyme correlation model representing the Arabidopsis metabolic pathway has been published. However, an integrated molecular characterization of phytohormone crosstalk is still not available.

Results

A novel modeling methodology and advanced computational approaches were used to construct an enzyme-based Arabidopsis phytohormone crosstalk network (EAPCN) at the biosynthesis level. The EAPCN provided the structural connectivity architecture of phytohormone biosynthesis pathways and revealed a surprising result; that enzymes localized at the highly connected nodes formed a consecutive metabolic route. Furthermore, our analysis revealed that the transcription factors (TFs) that regulate enzyme-encoding genes in the consecutive metabolic route formed structures, which we describe as circular control units operating at the transcriptional level. Furthermore, the downstream TFs in phytohormone signal transduction pathways were found to be involved in the circular control units that included the TFs regulating enzyme-encoding genes. In addition, multiple functional enzymes in the EAPCN were found to be involved in ion and pH homeostasis, environmental signal perception, cellular redox homeostasis, and circadian clocks. Last, publicly available transcriptional profiles and a protein expression map of the Arabidopsis root apical meristem were used as a case study to validate the proposed framework.

Conclusions

Our results revealed multiple scales of coupled mechanisms in that hormonal crosstalk networks that play a central role in coordinating internal developmental processes with environmental signals, and give a broader view of Arabidopsis phytohormone crosstalk. We also uncovered potential key regulators that can be further analyzed in future studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0333-9) contains supplementary material, which is available to authorized users.

Dissect style response to pollination using metabolite profiling in self-compatible and self-incompatible tomato species

Tomato style is the pathway for pollen germination and pollen tubes growth from the stigma to the ovules where fertilization occurs. It is essential to supplying the nutrients for pollen tube growth and guidance for the pollen tubes. To our knowledge, style also regulates gametophytic self-incompatibility (SI) in tomato species. This study identified the metabolites and monitored the metabolic changes of self-incompatible and self-compatible tomato with self-pollinated or unpollinated styles by gas chromatography-mass spectrometry (GC-MS). A total of 9 classes of compounds were identified in SI and self-compatibility (SC) self-pollinated and unpollinated styles which included amino acids, sugars, fatty acids/lipids, amines, organic acids, alcohols, nitriles, inorganic acids and other compounds. The contents of d-Mannose-6-phosphate, Cellobiose, Myristic acid, 2,4-Diaminobutyric acid, Inositol and Urea were significantly decreased and the rest did not significantly change in SI styles. But change of metabolites content significantly happened in SC styles. In addition, among the total 9 classes of compounds, the different metabolites accounted for a different proportion in amino acids, sugars, amines, organic acids and alcohols compared SC and SI. The result indicated that the physiological changes of styles existed differences in SC and SI after self pollination.

Cytological and proteomic analyses of horsetail (Equisetum arvense L.) spore germination

Spermatophyte pollen tubes and root hairs have been used as single-cell-type model systems to understand the molecular processes underlying polar growth of plant cells. Horsetail (Equisetum arvense L.) is a perennial herb species in Equisetopsida, which creates separately growing spring and summer stems in its life cycle. The mature chlorophyllous spores produced from spring stems can germinate without dormancy. Here we report the cellular features and protein expression patterns in five stages of horsetail spore germination (mature spores, rehydrated spores, double-celled spores, germinated spores, and spores with protonemal cells). Using 2-DE combined with mass spectrometry, 80 proteins were found to be abundance changed upon spore germination. Among them, proteins involved in photosynthesis, protein turnover, and energy supply were over-represented. Thirteen proteins appeared as proteoforms on the gels, indicating the potential importance of post-translational modification. In addition, the dynamic changes of ascorbate peroxidase, peroxiredoxin, and dehydroascorbate reductase implied that reactive oxygen species homeostasis is critical in regulating cell division and tip-growth. The time course of germination and diverse expression patterns of proteins in photosynthesis, energy supply, lipid and amino acid metabolism indicated that heterotrophic and autotrophic metabolism were necessary in light-dependent germination of the spores. Twenty-six proteins were involved in protein synthesis, folding, and degradation, indicating that protein turnover is vital to spore germination and rhizoid tip-growth. Furthermore, the altered abundance of 14-3-3 protein, small G protein Ran, actin, and caffeoyl-CoA O-methyltransferase revealed that signaling transduction, vesicle trafficking, cytoskeleton dynamics, and cell wall modulation were critical to cell division and polar growth. These findings lay a foundation toward understanding the molecular mechanisms underlying fern spore asymmetric division and rhizoid polar growth.

Evolutionarily conserved phenylpropanoid pattern on angiosperm pollen

The male gametophyte of higher plants appears as a solid box containing the essentials to transmit genetic material to the next generation. These consist of haploid generative cells that are required for reproduction, and an invasive vegetative cell producing the pollen tube, both mechanically protected by a rigid polymer, the pollen wall, and surrounded by a hydrophobic pollen coat. This coat mediates the direct contact to the biotic and abiotic environments. It contains a mixture of compounds required not only for fertilization but also for protection against biotic and abiotic stressors. Among its metabolites, the structural characteristics of two types of phenylpropanoids, hydroxycinnamic acid amides and flavonol glycosides, are highly conserved in Angiosperm pollen. Structural and functional aspects of these compounds will be discussed.

Circadian rhythms synchronise intracellular calcium dynamics and ATP production for facilitating Arabidopsis pollen tube growth

Experimental evidences support that the circadian rhythm regulates the transcription levels of genes encoding the enzymes involved in plant metabolism. However, there is no paper to refer the correlation of the circadian rhythms and the metabolic processes for facilitating pollen tube growth. In this study, we found that many central components of the circadian clock were highly enriched and specifically present in the in vivo grown Arabidopsis pollen tubes. Our analysis also identified the significant differentially expressed genes encoding co-expressed enzymes in the consecutive steps of fatty acid β-oxidation II, pentose phosphate pathway (oxidative branch) and phosphatidic acid biosynthesis pathway in the in vivo grown Arabidopsis pollen tubes during pollination. Thus, it is implicated that the circadian rhythms of pollen tube may be adjusted and have a greater probability of the direct or indirect functional relationship with enhanced intracellular Ca(2+) dynamics and ATP production for facilitating pollen tube growth in vivo.