Peptide signaling in pollen-pistil interactions

Pectate lyase-like lubricates the male gametophyte's path toward its mating partner

The pollen tube is an extension of the male gametophyte in plants and mediates sexual reproduction by delivering the sperm cells to the female gametophyte. To accomplish this task, the elongating pollen tube must break through the thick wall of the pollen grain and penetrate multiple pistillar tissues. Both processes require the loosening of cell wall material-that of the pollen intine and that of the apoplast of the transmitting tract. The enzymatic toolbox for these cell wall modifying processes employed by the invading male gametophyte is elusive. We investigated the role of the pectin-digesting pectate lyase-like (PLL) by combining mutant analysis with microscopy observations, fluorescence recovery after photo-bleaching experiments, and immuno-detection. We show that in Arabidopsis (Arabidopsis thaliana), PLLs are required for intine loosening during the first steps of pollen tube germination. We provide evidence that during pollen tube elongation, PLLs are released by the pollen tube into the extracellular space, suggesting that they may be employed to soften the apoplast of the transmitting tissue. The synergistic enzymatic action of PLLs in the pollen grain, the pollen tube, and the transmitting track contribute to an effective fertilization process.

Peptide signaling in anther development and pollen-stigma interactions

Polypeptides play irreplaceable roles in cell-cell communication by binding to receptor-like kinases. Various types of peptide-receptor-like kinase-mediated signaling have been identified in anther development and male-female interactions in flowering plants. Here, we provide a comprehensive summary of the biological functions and signaling pathways of peptides and receptors involved in anther development, self-incompatibility, pollen tube growth and pollen tube guidance.

Thermosensitivity of pollen: a molecular perspective

A current trend in climate comprises adverse weather anomalies with more frequent and intense temperature events. Heatwaves are a serious threat to global food security because of the susceptibility of crop plants to high temperatures. Among various developmental stages of plants, even a slight rise in temperature during reproductive development proves detrimental, thus making sexual reproduction heat vulnerable. In this context, male gametophyte or pollen development stages are the most sensitive ones. High-temperature exposure induces pollen abortion, reducing pollen viability and germination rate with a concomitant effect on seed yield. This review summarizes the ultrastructural, morphological, biochemical, and molecular changes underpinning high temperature-induced aberrations in male gametophytes. Specifically, we highlight the temperature sensing cascade operating in pollen, involving reactive oxygen species (ROS), heat shock factors (HSFs), a hormones and transcriptional regulatory network. We also emphasize integrating various omics approaches to decipher the molecular events triggered by heat stress in pollen. The knowledge of genes, proteins, and metabolites conferring thermotolerance in reproductive tissues can be utilized to breed/engineer thermotolerant crops to ensure food security.

Eco-Geography and Phenology Are the Major Drivers of Reproductive Isolation in the Royal Irises, a Species Complex in the Course of Speciation

The continuous nature of speciation implies that different species are found at different stages of divergence, from no- to complete reproductive isolation. This process and its underlying mechanisms are best viewed in incipient species. Moreover, the species complex can offer unique insight into how reproductive isolation (RI) has evolved. The royal irises (Iris section Oncocyclus) are a young group of species in the course of speciation, providing an ideal system for speciation study. We quantified pre- and post-zygotic reproductive barriers between the eight Israeli species of this complex and estimated the total RI among them. We tested for both pre-pollination and post-pollination reproductive barriers. Pre-pollination barriers, i.e., eco-geographic divergence and phenological differentiation were the major contributors to RI among the Iris species. On the other hand, post-pollination barriers, namely pollen-stigma interactions, fruit set, and seed viability had negligible contributions to total RI. The strength of RI was not uniform across the species complex, suggesting that species may have diverged at different rates. Overall, this study in a young, recently diverged group of species provides insight into the first steps of speciation, suggesting a crucial role of the pre-zygotic barriers.

A Cysteine-Rich Protein, SpDIR1L, Implicated in S-RNase-Independent Pollen Rejection in the Tomato (Solanum Section Lycopersicon) Clade

Tomato clade species (Solanum sect. Lycopersicon) display multiple interspecific reproductive barriers (IRBs). Some IRBs conform to the SI x SC rule, which describes unilateral incompatibility (UI) where pollen from SC species is rejected on SI species’ pistils, but reciprocal pollinations are successful. However, SC x SC UI also exists, offering opportunities to identify factors that contribute to S-RNase-independent IRBs. For instance, SC Solanum pennellii LA0716 pistils only permit SC Solanum lycopersicum pollen tubes to penetrate to the top third of the pistil, while S. pennellii pollen penetrates to S. lycopersicum ovaries. We identified candidate S. pennellii LA0716 pistil barrier genes based on expression profiles and published results. CRISPR/Cas9 mutants were created in eight candidate genes, and mutants were assessed for changes in S. lycopersicum pollen tube growth. Mutants in a gene designated Defective in Induced Resistance 1-like (SpDIR1L), which encodes a small cysteine-rich protein, permitted S. lycopersicum pollen tubes to grow to the bottom third of the style. We show that SpDIR1L protein accumulation correlates with IRB strength and that species with weak or no IRBs toward S. lycopersicum pollen share a 150 bp deletion in the upstream region of SpDIR1L. These results suggest that SpDIR1L contributes to an S-RNase-independent IRB.

Seed-derived defensins from Scots pine: structural and functional features

The recombinant PsDef5.1 defensin inhibits the growth of phytopathogenic fungi, Gram-positive and Gram-negative bacteria, and human pathogen Candida albicans. Expression of seed-derived Scots pine defensins is tissue-specific and developmentally regulated. Plant defensins are ubiquitous antimicrobial peptides that possess a broad spectrum of activities and multi-functionality. The genes for these antimicrobial proteins form a multigenic family in the plant genome and are expressed in every organ. Most of the known defensins have been isolated from seeds of various monocot and dicot species, but seed-derived defensins have not yet been characterized in gymnosperms. This study presents the isolation of two new 249 bp cDNA sequences from Scots pine seeds with 97.9% nucleotide homology named PsDef5.1 and PsDef5.2. Their deduced amino acid sequences have typical plant defensin features, including an endoplasmic reticulum signal sequence of 31 amino acids (aa), followed by a characteristic defensin domain of 51 aa. To elucidate the functional activity of new defensins, we expressed the mature form of PsDef5.1 in a prokaryotic system. The purified recombinant peptide exhibited activity against the phytopathogenic fungi and Gram-negative and Gram-positive bacteria with the IC₅₀ of 5-18 µM. Moreover, it inhibited the growth of the human pathogen Candida albicans with the IC₅₀ of 6.0 µM. Expression analysis showed that transcripts of PsDef5.1-2 genes were present in immature and mature pine seeds and different parts of seedlings at the early stage of germination. In addition, unlike the PsDef5.2, the PsDef5.1 gene was expressed in the reproductive organs. Our findings indicate that novel defensins are promising candidates for transgenic application and the development of new antimicrobial drugs.

The role of diverse LURE-type cysteine-rich peptides as signaling molecules in plant reproduction

In angiosperm sexual reproduction, the male pollen tube undergoes a series of interactions with female tissues. For efficient growth and precise guidance, the pollen tube perceives extracellular ligands. In recent decades, various types of secreted cysteine-rich peptides (CRPs) have been identified as peptide ligands that regulate diverse angiosperm reproduction processes, including pollen tube germination, growth, guidance, and rupture. Notably, in two distant core eudicot plants, multiple LURE-type CRPs were found to be secreted from egg-accompanying synergid cells, and these CRPs act as a cocktail of pollen tube attractants for the final step of pollen tube guidance. LURE-type CRPs have species-preferential activity, even among close relatives, and exhibit remarkably divergent molecular evolution with conserved cysteine frameworks, demonstrating that they play a key role in species recognition in pollen tube guidance. In this review, I focus on "reproductive CRPs," particularly LURE-type CRPs, which underlie common but species-specific mechanisms in angiosperm sexual reproduction, and discuss their action, functional regulation, receptors, and evolution.

Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness

Many organisms consume pollen, yet mechanisms of its digestion remain a fundamental enigma in pollination biology,^1-3 as pollen is protected by a recalcitrant outer shell.^4-8 Pollen is commonly found in floral nectar,^9,10 as are nectar microbes, which are nearly ubiquitous among flowers.^11-13 Nectar specialist bacteria, like Acinetobacter, can reach high densities (up to 10⁹ cells/mL), despite the fact that floral nectar is nitrogen poor.^14-17 Here, we show evidence that the genus Acinetobacter, prevalent nectar- and bee-associated bacteria,^12,18-20 can induce pollen germination and bursting, gain access to protoplasm nutrients, and thereby grow to higher densities. Although induced germination had been suggested as a potential method in macroscopic pollen consumers,^2,21-23 and fungal inhibition of pollen germination has been shown,^24-27 direct biological induction of germination has not been empirically documented outside of plants.^28-32Acinetobacter pollinis SCC477¹⁹ induced over 5× greater pollen germination and 20× greater pollen bursting than that of uninoculated pollen by 45 min. When provided with germinable pollen, A. pollinis stimulates protein release and grows to nearly twice the density compared to growth with ungerminable pollen, indicating that stimulation of germination benefits bacterial fitness. In contrast, a common nectar-inhabiting yeast (Metschnikowia)³³ neither induced nor benefited from pollen germination. We conclude that Acinetobacter both specifically causes and benefits from inducing pollen germination and bursting. Further study of microbe-pollen interactions may inform many aspects of pollination ecology, including floral microbial ecology,^34,35 pollinator nutrient acquisition from pollen,^2,3,21,36 and cues of pollen germination for plant reproduction.^37-39.

Hyperoside regulates its own biosynthesis via MYB30 in promoting reproductive development and seed set in okra

Flavonoids are secondary metabolites that play important roles in fruit and vegetable development. Here, we examined the function of hyperoside, a unique flavonoid in okra (Abelmoschus esculentus), known to promote both flowering and seed set. We showed that the exogenous application of hyperoside significantly improved pollen germination rate and pollen tube growth by almost 50%, resulting in a 42.7% increase in the seed set rate. Of several genes induced by the hyperoside treatment, AeUF3GaT1, which encodes an enzyme that catalyzes the last step of hyperoside biosynthesis, was the most strongly induced. The transcription factor AeMYB30 enhanced AeUFG3aT1 transcription by directly binding to the AeUFG3aT1 promoter. We studied the effect of the hyperoside application on the expression of 10 representative genes at four stages of reproductive development, from pollination to seed maturity. We firstly developed an efficient transformation system that uses seeds as explants to study the roles of AeMYB30 and AeUFG3aT1. Overexpression of AeMYB30 or AeUF3GaT1 promoted seed development. Moreover, exogenous application of hyperoside partially restored the aberrant phenotype of AeUF3GaT1 RNA-interference plants. Thus, hyperoside promotes seed set in okra via a pathway involving AeUF3GaT and AeMYB30, and the exogenous application of this flavonoid is a simple method that can be used to improve seed quality and yield in okra.

A gamma-thionin protein from apple, MdD1, is required for defence against S-RNase-induced inhibition of pollen tube prior to self/non-self recognition

Apple exhibits S-RNase-mediated self-incompatibility. Although the cytotoxic effect of S-RNase inside the self-pollen tube has been studied extensively, the underlying defence mechanism in pollen tube in Rosaceae remains unclear. On exposure to stylar S-RNase, plant defence responses are activated in the pollen tube; however, how these are regulated is currently poorly understood. Here, we show that entry of both self and non-self S-RNase into pollen tubes of apple (Malus domestica) stimulates jasmonic acid (JA) production, in turn inducing the accumulation of MdMYC2 transcripts, a transcription factor in the JA signalling pathway widely considered to be involved in plant defence processes. MdMYC2 acts as a positive regulator in the pollen tube activating expression of MdD1, a gene encoding a defence protein. Importantly, MdD1 was shown to bind to the RNase activity sites of S-RNase leading to inhibition of enzymatic activity. This work provides intriguing insights into an ancient defence mechanism present in apple pollen tubes where MdD1 likely acts as a primary line of defence to inhibit S-RNase cytotoxicity prior to self/non-self recognition.

Insights into secrets along the pollen tube pathway in need to be discovered

The process of plant fertilization provides an outstanding example of refined control of gene expression. During this elegant process, subtle communication occurs between neighboring cells, based on chemical signals, that induces cellular mechanisms of patterning and growth. Having faced an immediate issue of self-incompatibility responses, the pathway to fertilization starts once the stigmatic cells recognize a compatible pollen grain, and it continues with numerous players interacting to affect pollen tube growth and the puzzling process of navigation along the transmitting tract. The pollen tube goes through a guidance process that begins with a preovular stage (i.e. prior to the influence of the target ovule), with interactions with factors from the transmitting tissue. In the subsequent ovular-guidance stage a specific relationship develops between the pollen tube and its target ovule. This stage is divided into the funicular and micropylar guidance steps, with numerous receptors working in signalling cascades. Finally, just after the pollen tube has passed beyond the synergids, fusion of the gametes occurs and the developing seed-the ultimate aim of the process-will start to mature. In this paper, we review the existing knowledge of the crucial biological processes involved in pollen-pistil interactions that give rise to the new seed.

The Paradox of Self-Fertile Varieties in the Context of Self-Incompatible Genotypes in Olive

Olive, representing one of the most important fruit crops of the Mediterranean area, is characterized by a general low fruit yield, due to numerous constraints, including alternate bearing, low flower viability, male-sterility, inter-incompatibility, and self-incompatibility (SI). Early efforts to clarify the genetic control of SI in olive gave conflicting results, and only recently, the genetic control of SI has been disclosed, revealing that olive possesses an unconventional homomorphic sporophytic diallelic system of SI, dissimilar from other described plants. This system, characterized by the presence of two SI groups, prevents self-fertilization and regulates inter-compatibility between cultivars, such that cultivars bearing the same incompatibility group are incompatible. Despite the presence of a functional SI, some varieties, in particular conditions, are able to set seeds following self-fertilization, a mechanism known as pseudo-self-compatibility (PSC), as widely reported in previous literature. Here, we summarize the results of previous works on SI in olive, particularly focusing on the occurrence of self-fertility, and offer a new perspective in view of the recent elucidation of the genetic architecture of the SI system in olive. Recent advances in research aimed at unraveling the molecular bases of SI and its breakdown in olive are also presented. The clarification of these mechanisms may have a huge impact on orchard management and will provide fundamental information for the future of olive breeding programs.

Impact of Plant Peptides on Symbiotic Nodule Development and Functioning

Ribosomally synthesized peptides have wide ranges of functions in plants being, for example, signal molecules, transporters, alkaloids, or antimicrobial agents. Legumes are an unprecedented rich source of peptides, which are used to control the symbiosis of these plants with the nitrogen-fixing Rhizobium bacteria. Here, we discuss the function and the evolution of these peptides playing an important role in the formation or functioning of the symbiotic organs, the root nodules. We distinguish peptides that can be either cell-autonomous or secreted short-range or long-range signals, carrying messages in or between plant cells or that can act as effectors interacting with the symbiotic bacteria. Peptides are further classified according to the stage of the symbiotic process where they act. Several peptide classes, including RALF, DLV, ENOD40, and others, control Rhizobium infection and the initiation of cell divisions and the formation of nodule primordia. CLE and CEP peptides are implicated in systemic and local control of nodule initiation during autoregulation of nodulation and in response to the nutritional demands of the plant. Still other peptides act at later stages of the symbiosis. The PSK peptide is thought to be involved in the suppression of immunity in nodules and the nodule-specific cysteine-rich, GRP, and SNARP (LEED..PEED) peptide families are essential in the functioning of the nitrogen fixing root nodules. The NCRs and possibly also the GRP and SNARPs are targeted to the endosymbionts and play essential roles in the terminal differentiation of these bacteria.

Multilayered signaling pathways for pollen tube growth and guidance

Sexual reproductive success is essential for the survival of all higher organisms. As the most prosperous and diverse group of land plants on earth, flowering plants evolved highly sophisticated fertilization mechanisms. To adapt to the terrestrial environment, a tubular structure pollen tube has been evolved to deliver the immobile sperm cells to the egg and central cell enclosed within the ovule. The pollen tube is generated from the vegetative cell of the pollen (male gametophyte), where two sperm cells are hosted. Pollen tube elongation in the maternal tissue and navigation to the ovule require intimate cell-cell interactions between the tube and female tissues. Questions on how the single-celled pollen tube accomplishes such task and how the female tissues accommodate the tube have attracted many plant biologists. Here, we review recent progresses and concepts in understanding the molecular mechanisms governing pollen tube growth and its interactions with the female tissues. We will also discuss the future perspective in this field.

The Extracellular Domain of Pollen Receptor Kinase 3 is structurally similar to the SERK family of co-receptors

During reproduction in flowering plants, the male gametophyte delivers an immotile male gamete to the female gametophyte in the pistil by formation of pollen tubes. In Arabidopsis thaliana, two synergid cells situated on either side of the egg cell produce cysteine-rich chemoattractant peptide LURE that guides the pollen tube to the female gametophyte for sexual reproduction. Recently, in Arabidopsis thaliana, Pollen Receptor Kinase 3 (PRK3), along with PRK1, PRK6, and PRK8, have been predicted to be the receptors responsible for sensing LURE. These receptors belong to the Leucine Rich Repeat Receptor Like Kinases (LRR-RLKs), the largest family of receptor kinases found in Arabidopsis thaliana. How PRKs regulate the growth and development of the pollen tube remains elusive. In order to better understand the PRK-mediated signaling mechanism in pollen tube growth and guidance, we have determined the crystal structure of the extracellular domain (ecd) of PRK3 at 2.5 Å, which resembles the SERK family of plant co-receptors. The structure of ecdPRK3 is composed of a conserved surface that coincides with the conserved receptor-binding surface of the SERK family of co-receptors. Our structural analyses of PRK3 have provided a template for future functional studies of the PRK family of LRR-RLK receptors in the regulation of pollen tube development.

Chemical signaling for pollen tube guidance at a glance

Pollen tube guidance is a unique navigating system that is required for the successful sexual reproduction of plants. As plant sperm cells are non-motile and egg cells are embedded deep inside the female tissues, a pollen tube delivers the two sperm cells that it contains by growing towards the ovule, in which the egg cell resides. Pollen tube growth towards the ovule is precisely controlled and divided into two stages, preovular and ovular guidance. In this Cell Science at a Glance article and accompanying poster, we provide a comprehensive overview of pollen tube guidance and highlight some of the attractant peptides used during ovular guidance. We further discuss the precise one-to-one guidance system that exists in multi-ovular plants. The pollen tube-blocking system, which is mediated by male–female crosstalk communication, to avoid attraction of multiple pollen tubes, is also reviewed.

Male gametophyte development and function in angiosperms: a general concept

Overview of pollen development. Male gametophyte development of angiosperms is a complex process that requires coordinated activity of different cell types and tissues of both gametophytic and sporophytic origin and the appropriate specific gene expression. Pollen ontogeny is also an excellent model for the dissection of cellular networks that control cell growth, polarity, cellular differentiation and cell signaling. This article describes two sequential phases of angiosperm pollen ontogenesis-developmental phase leading to the formation of mature pollen grains, and a functional or progamic phase, beginning with the impact of the grains on the stigma surface and ending at double fertilization. Here we present an overview of important cellular processes in pollen development and explosive pollen tube growth stressing the importance of reserves accumulation and mobilization and also the mutual activation of pollen tube and pistil tissues, pollen tube guidance and the communication between male and female gametophytes. We further describe the recent advances in regulatory mechanisms involved such as posttranscriptional regulation (including mass transcript storage) and posttranslational modifications to modulate protein function, intracellular metabolic signaling, ionic gradients such as Ca(2+) and H(+) ions, cell wall synthesis, protein secretion and intercellular signaling within the reproductive tissues.

Quantitative proteomics of the tobacco pollen tube secretome identifies novel pollen tube guidance proteins important for fertilization

BACKGROUND

As in animals, cell-cell communication plays a pivotal role in male-female recognition during plant sexual reproduction. Prelaid peptides secreted from the female reproductive tissues guide pollen tubes towards ovules for fertilization. However, the elaborate mechanisms for this dialogue have remained elusive, particularly from the male perspective.

RESULTS

We performed genome-wide quantitative liquid chromatography-tandem mass spectrometry analysis of a pistil-stimulated pollen tube secretome and identified 801 pollen tube-secreted proteins. Interestingly, in silico analysis reveals that the pollen tube secretome is dominated by proteins that are secreted unconventionally, representing 57 % of the total secretome. In support, we show that an unconventionally secreted protein, translationally controlled tumor protein, is secreted to the apoplast. Remarkably, we discovered that this protein could be secreted by infiltrating through the initial phases of the conventional secretory pathway and could reach the apoplast via exosomes, as demonstrated by co-localization with Oleisin1 exosome marker. We demonstrate that translationally controlled tumor protein-knockdown Arabidopsis thaliana plants produce pollen tubes that navigate poorly to the target ovule and that the mutant allele is poorly transmitted through the male. Further, we show that regulators of the endoplasmic reticulum-trans-Golgi network protein secretory pathway control secretion of Nicotiana tabacum Pollen tube-secreted cysteine-rich protein 2 and Lorelei-like GPI-anchor protein 3 and that a regulator of endoplasmic reticulum-trans-Golgi protein translocation is essential for pollen tube growth, pollen tube guidance and ovule-targeting competence.

CONCLUSIONS

This work, the first study on the pollen tube secretome, identifies novel genome-wide pollen tube-secreted proteins with potential functions in pollen tube guidance towards ovules for sexual reproduction. Functional analysis highlights a potential mechanism for unconventional secretion of pollen tube proteins and reveals likely regulators of conventional pollen tube protein secretion. The association of pollen tube-secreted proteins with marker proteins shown to be secreted via exosomes in other species suggests exosome secretion is a possible mechanism for cell-cell communication between the pollen tube and female reproductive cells.

Transcript Analysis and Regulative Events during Flower Development in Olive (Olea europaea L.)

The identification and characterization of transcripts involved in flower organ development, plant reproduction and metabolism represent key steps in plant phenotypic and physiological pathways, and may generate high-quality transcript variants useful for the development of functional markers. This study was aimed at obtaining an extensive characterization of the olive flower transcripts, by providing sound information on the candidate MADS-box genes related to the ABC model of flower development and on the putative genetic and molecular determinants of ovary abortion and pollen-pistil interaction. The overall sequence data, obtained by pyrosequencing of four cDNA libraries from flowers at different developmental stages of three olive varieties with distinct reproductive features (Leccino, Frantoio and Dolce Agogia), included approximately 465,000 ESTs, which gave rise to more than 14,600 contigs and approximately 92,000 singletons. As many as 56,700 unigenes were successfully annotated and provided gene ontology insights into the structural organization and putative molecular function of sequenced transcripts and deduced proteins in the context of their corresponding biological processes. Differentially expressed genes with potential regulatory roles in biosynthetic pathways and metabolic networks during flower development were identified. The gene expression studies allowed us to select the candidate genes that play well-known molecular functions in a number of biosynthetic pathways and specific biological processes that affect olive reproduction. A sound understanding of gene functions and regulatory networks that characterize the olive flower is provided.

A small multigene hydroxyproline-O-galactosyltransferase family functions in arabinogalactan-protein glycosylation, growth and development in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are ubiquitous components of cell walls throughout the plant kingdom and are extensively post translationally modified by conversion of proline to hydroxyproline (Hyp) and by addition of arabinogalactan polysaccharides (AG) to Hyp residues. AGPs are implicated to function in various aspects of plant growth and development, but the functional contributions of AGP glycans remain to be elucidated. Hyp glycosylation is initiated by the action of a set of Hyp-O-galactosyltransferase (Hyp-O-GALT) enzymes that remain to be fully characterized.

RESULTS

Three members of the GT31 family (GALT3-At3g06440, GALT4-At1g27120, and GALT6-At5g62620) were identified as Hyp-O-GALT genes by heterologous expression in tobacco leaf epidermal cells and examined along with two previously characterized Hyp-O-GALT genes, GALT2 and GALT5. Transcript profiling by real-time PCR of these five Hyp-O-GALTs revealed overlapping but distinct expression patterns. Transiently expressed GALT3, GALT4 and GALT6 fluorescent protein fusions were localized within Golgi vesicles. Biochemical analysis of knock-out mutants for the five Hyp-O-GALT genes revealed significant reductions in both AGP-specific Hyp-O-GALT activity and β-Gal-Yariv precipitable AGPs. Further phenotypic analysis of these mutants demonstrated reduced root hair growth, reduced seed coat mucilage, reduced seed set, and accelerated leaf senescence. The mutants also displayed several conditional phenotypes, including impaired root growth, and defective anisotropic growth of root tips under salt stress, as well as less sensitivity to the growth inhibitory effects of β-Gal-Yariv reagent in roots and pollen tubes.

CONCLUSIONS

This study provides evidence that all five Hyp-O-GALT genes encode enzymes that catalyze the initial steps of AGP galactosylation and that AGP glycans play essential roles in both vegetative and reproductive plant growth.

Proteomic Analysis of Silk Viability in Maize Inbred Lines and Their Corresponding Hybrids

A long period of silk viability is critical for a good seed setting rate in maize (Zea mays L.), especially for inbred lines and hybrids with a long interval between anthesis and silking. To explore the molecular mechanism of silk viability and its heterosis, three inbred lines with different silk viability characteristics (Xun928, Lx9801, and Zong3) and their two hybrids (Xun928×Zong3 and Lx9801×Zong3) were analyzed at different developmental stages by a proteomic method. The differentially accumulated proteins were identified by mass spectrometry and classified into metabolism, protein biosynthesis and folding, signal transduction and hormone homeostasis, stress and defense responses, and cellular processes. Proteins involved in nutrient (methionine) and energy (ATP) supply, which support the pollen tube growth in the silk, were important for silk viability and its heterosis. The additive and dominant effects at a single locus, as well as complex epistatic interactions at two or more loci in metabolic pathways, were the primary contributors for mid-parent heterosis of silk viability. Additionally, the proteins involved in the metabolism of anthocyanins, which indirectly negatively regulate local hormone accumulation, were also important for the mid-parent heterosis of silk viability. These results also might imply the developmental dependence of heterosis, because many of the differentially accumulated proteins made distinct contributions to the heterosis of silk viability at specific developmental stages.

The Plant Ovule Secretome: A Different View toward Pollen-Pistil Interactions

During plant sexual reproduction, continuous exchange of signals between the pollen and the pistil (stigma, style, and ovary) plays important roles in pollen recognition and selection, establishing breeding barriers and, ultimately, leading to optimal seed set. After navigating through the stigma and the style, pollen tubes (PTs) reach their final destination, the ovule. This ultimate step is also regulated by numerous signals emanating from the embryo sac (ES) of the ovule. These signals encompass a wide variety of molecules, but species-specificity of the pollen-ovule interaction relies mainly on secreted proteins and their receptors. Isolation of candidate genes involved in pollen-pistil interactions has mainly relied on transcriptomic approaches, overlooking potential post-transcriptional regulation. To address this issue, ovule exudates were collected from the wild potato species Solanum chacoense using a tissue-free gravity-extraction method (tf-GEM). Combined RNA-seq and mass spectrometry-based proteomics led to the identification of 305 secreted proteins, of which 58% were ovule-specific. Comparative analyses using mature ovules (attracting PTs) and immature ovules (not attracting PTs) revealed that the last maturation step of ES development affected almost half of the ovule secretome. Of 128 upregulated proteins in anthesis stage, 106 were not regulated at the mRNA level, emphasizing the importance of post-transcriptional regulation in reproductive development.

A Receptor-Like Kinase, Related to Cell Wall Sensor of Higher Plants, is Required for Sexual Reproduction in the Unicellular Charophycean Alga, Closterium peracerosum-strigosum-littorale Complex

Here, we cloned the CpRLK1 gene, which encodes a receptor-like protein kinase expressed during sexual reproduction, from the heterothallic Closterium peracerosum-strigosum-littorale complex, one of the closest unicellular alga to land plants. Mating-type plus (mt(+)) cells with knockdown of CpRLK1 showed reduced competence for sexual reproduction and formed an abnormally enlarged conjugation papilla after pairing with mt(-) cells. The knockdown cells were unable to release a naked gamete, which is indispensable for zygote formation. We suggest that the CpRLK1 protein is an ancient cell wall sensor that now functions to regulate osmotic pressure in the cell to allow proper gamete release.

Same same but different: sperm-activating EC1 and ECA1 gametogenesis-related family proteins

During double fertilization in Arabidopsis thaliana, the egg cell secretes small cysteine-rich EC1 (egg cell 1) proteins, which enable the arriving sperm pair to rapidly interact with the two female gametes. EC1 proteins are members of the large and unexplored group of ECA1 (early culture abundant 1) gametogenesis-related family proteins, characterized by a prolamin-like domain with six conserved cysteine residues that may form three pairs of disulfide bonds. The distinguishing marks of egg-cell-expressed EC1 proteins are, however, two short amino acid sequence motifs present in all EC1-like proteins. EC1 genes appear to encode the major CRPs (cysteine-rich proteins) expressed by the plant egg cell, and they are restricted to flowering plants, including the most basal extant flowering plant Amborella trichopoda. Many other ECA1 gametogenesis-related family genes are preferentially expressed in the synergid cell. Functional diversification among the ECA1 gametogenesis-related family is suggested by the different patterns of expression in the female gametophyte and the low primary sequence conservation.

Young Genes out of the Male: An Insight from Evolutionary Age Analysis of the Pollen Transcriptome

The birth of new genes in genomes is an important evolutionary event. Several studies reveal that new genes in animals tend to be preferentially expressed in male reproductive tissues such as testis (Betrán et al., 2002; Begun et al., 2007; Dubruille et al., 2012), and thus an "out of testis" hypothesis for the emergence of new genes has been proposed (Vinckenbosch et al., 2006; Kaessmann, 2010). However, such phenomena have not been examined in plant species. Here, by employing a phylostratigraphic method, we dated the origin of protein-coding genes in rice and Arabidopsis thaliana and observed a number of young genes in both species. These young genes tend to encode short extracellular proteins, which may be involved in rapid evolving processes, such as reproductive barriers, species specification, and anti-microbial processes. Further analysis of transcriptome age indexes across different tissues revealed that male reproductive cells express a phylogenetically younger transcriptome than other plant tissues. Compared with sporophytic tissues, the young transcriptomes of the male gametophyte displayed greater complexity and diversity, which included a higher ratio of anti-sense and inter-genic transcripts, reflecting a pervasive transcription state that facilitated the emergence of new genes. Here, we propose that pollen may act as an "innovation incubator" for the birth of de novo genes. With cases of male-biased expression of young genes reported in animals, the "new genes out of the male" model revealed a common evolutionary force that drives reproductive barriers, species specification, and the upgrading of defensive mechanisms against pathogens.

Live Imaging and Laser Disruption Reveal the Dynamics and Cell-Cell Communication During Torenia fournieri Female Gametophyte Development

The female gametophytes of many flowering plants contain one egg cell, one central cell, two synergid cells and three antipodal cells with respective morphological characteristics and functions. These cells are formed by cellularization of a multinuclear female gametophyte. However, the dynamics and mechanisms of female gametophyte development remain largely unknown due to the lack of a system to visualize directly and manipulate female gametophytes in living material. Here, we established an in vitro ovule culture system to examine female gametophyte development in Torenia fournieri, a unique plant species with a protruding female gametophyte. The four-nucleate female gametophyte became eight nucleate by the final (third) mitosis and successively cellularized and matured to attract a pollen tube. The duration of final mitosis was 28 ± 6.5 min, and cellularization was completed in 54 ± 20 min after the end of the third mitosis. Fusion of polar nuclei in the central cell occurred in 13.1 ± 1.1 h, and onset of expression of LURE2, a pollen tube attractant gene, was visualized by a green fluorescent protein reporter 10.7 ± 2.3 h after cellularization. Laser disruption analysis demonstrated that the egg and central cells were required for synergid cells to acquire the pollen tube attraction function. Moreover, aberrant nuclear positioning and down-regulation of LURE2 were observed in one of the two synergid cells after disrupting an immature egg cell, suggesting that cell specification was affected. Our system provides insights into the precise dynamics and mechanisms of female gametophyte development in T. fournieri.

The ejaculatory biology of leafcutter ants

The eusocial ants are unique in that females (queens) acquire and store sperm on a single mating flight early in adult life. This event largely determines the size (possibly millions of workers), longevity (possibly decades) and genetic variation of the colonies that queens found, but our understanding of the fundamental biology of ejaculate production, transfer and physiological function remains extremely limited. We studied the ejaculation process in the leafcutter ant Atta colombica and found that it starts with the appearance of a clear pre-ejaculatory fluid (PEF) at the tip of the endophallus that is followed by the joint expulsion of the remainder of accessory gland (AG) secretion, sperm, accessory testes (AT) secretion, and a small mating plug. PEF, AG secretion and AT secretion all contribute to sperm survival, but PEF and AG secretion also reduce the survival of sperm from other males. We show that PEF is produced in the AGs and is likely identical to AG secretion because protein-banding patterns of PEF and AG secretion were similar on 1D electrophoresis gels, but differed from the protein-banding pattern of AT secretion. We show that proteins in AG secretion are responsible for the incapacitation of rival sperm and infer that transfer of AG secretion prior to sperm may allow these components to interact with rival sperm, while at the same time providing a supportive biochemical environment for the arrival of own sperm.

KAPPA, a simple algorithm for discovery and clustering of proteins defined by a key amino acid pattern: a case study of the cysteine-rich proteins

MOTIVATION

Proteins defined by a key amino acid pattern are key players in the exchange of signals between bacteria, animals and plants, as well as important mediators for cell-cell communication within a single organism. Their description and characterization open the way to a better knowledge of molecular signalling in a broad range of organisms, and to possible application in medical and agricultural research. The contrasted pattern of evolution in these proteins makes it difficult to detect and cluster them with classical sequence-based search tools. Here, we introduce Key Aminoacid Pattern-based Protein Analyzer (KAPPA), a new multi-platform program to detect them in a given set of proteins, analyze their pattern and cluster them by comparison to reference patterns (ab initio search) or internal pairwise comparison (de novo search).

RESULTS

In this study, we use the concrete example of cysteine-rich proteins (CRPs) to show that the similarity of two cysteine patterns can be precisely and efficiently assessed by a quantitative tool created for KAPPA: the κ-score. We also demonstrate the clear advantage of KAPPA over other classical sequence search tools for ab initio search of new CRPs. Eventually, we present de novo clustering and subclustering functionalities that allow to rapidly generate consistent groups of CRPs without a seed reference.

AVAILABILITY AND IMPLEMENTATION

KAPPA executables are available for Linux, Windows and Mac OS at http://kappa-sequence-search.sourceforge.net.

Pollination triggers female gametophyte development in immature Nicotiana tabacum flowers

In Nicotiana tabacum, female gametophytes are not fully developed at anthesis, but flower buds pollinated 12 h before anthesis produce mature embryo sacs. We investigated several pollination-associated parameters in N. tabacum flower buds to determine the developmental timing of important events in preparation for successful fertilization. First, we performed hand pollinations in flowers from stages 4 to 11 to study at which developmental stage pollination would produce fruits. A Peroxtesmo test was performed to correlate peroxidase activity on the stigma surface, indicative of stigma receptivity, with fruit set. Pollen tube growth and female gametophyte development were microscopically analyzed in pistils of different developmental stages. Fruits were obtained only after pollinations of flower buds at late stage 7 and older; fruit weight and seed germination capacity increased as the developmental stage of the pollinated flower approached anthesis. Despite positive peroxidase activity and pollen tube growth, pistils at stages 5 and 6 were unable to produce fruits. At late stage 7, female gametophytes were undergoing first mitotic division. After 24 h, female gametophytes of unpollinated pistils were still in the end of the first division, whereas those of pollinated pistils showed egg cells. RT-qPCR assay showed that the expression of the NtEC1 gene, a marker of egg cell development, is considerably higher in pollinated late stage 7 ovaries compared with unpollinated ovaries. To test whether ethylene is the signal eliciting female gametophyte maturation, the expression of ACC synthase was examined in unpollinated and pollinated stage 6 and late stage 7 stigmas/styles. Pollination induced NtACS expression in stage 6 pistils, which are unable to produce fruits. Our results show that pollination is a stimulus capable of triggering female gametophyte development in immature tobacco flowers and suggests the existence of a yet undefined signal sensed by the pistil.

The mechanism and key molecules involved in pollen tube guidance

During sexual reproduction of flowering plants, pollen tube guidance by pistil tissue is critical for the delivery of nonmotile sperm cells to female gametes. Multistep controls of pollen tube guidance can be divided into two phases: preovular guidance and ovular guidance. During preovular guidance, various female molecules, including stimulants for pollen germination and pollen tube growth, are provided to support tube growth toward the ovary, where the ovules are located. After entering the ovary, pollen tubes receive directional cues from their respective target ovules, including attractant peptides for precise, species-preferential attraction. Successful pollen tube guidance in the pistil requires not only nutritional and directional controls but also competency controls to make pollen tubes responsive to guidance cues, regulation to terminate growth once a pollen tube arrives at the target, and strategies to stop ovular attraction depending on the fertilization of female gametes.

Emerging roles for microtubules in angiosperm pollen tube growth highlight new research cues

In plants, actin filaments have an important role in organelle movement and cytoplasmic streaming. Otherwise microtubules (MTs) have a role in restricting organelles to specific areas of the cell and in maintaining organelle morphology. In somatic plant cells, MTs also participate in cell division and morphogenesis, allowing cells to take their definitive shape in order to perform specific functions. In the latter case, MTs influence assembly of the cell wall, controlling the delivery of enzymes involved in cellulose synthesis and of wall modulation material to the proper sites. In angiosperm pollen tubes, organelle movement is generally attributed to the acto-myosin system, the main role of which is in distributing organelles in the cytoplasm and in carrying secretory vesicles to the apex for polarized growth. Recent data on membrane trafficking suggests a role of MTs in fine delivery and repositioning of vesicles to sustain pollen tube growth. This review examines the role of MTs in secretion and endocytosis, highlighting new research cues regarding cell wall construction and pollen tube-pistil crosstalk, that help unravel the role of MTs in polarized growth.

Identification of small secreted peptides (SSPs) in maize and expression analysis of partial SSP genes in reproductive tissues

Maize 1,491 small secreted peptides were identified, which were classified according to the character of peptide sequences. Partial SSP gene expressions in reproductive tissues were determined by qRT-PCR. Small secreted peptides (SSPs) are important cell-cell communication messengers in plants. Most information on plant SSPs come from Arabidopsis thaliana and Oryza sativa, while little is known about the SSPs of other grass species such as maize (Zea mays). In this study, we identified 1,491 SSP genes from maize genomic sequences. These putative SSP genes were distributed throughout the ten maize chromosomes. Among them, 611 SSPs were classified into 198 superfamilies according to their conserved domains, and 725 SSPs with four or more cysteines at their C-termini shared similar cysteine arrangements with their counterparts in other plant species. Moreover, the SSPs requiring post-translational modification, as well as defensin-like (DEFL) proteins, were identified. Further, the expression levels of 110 SSP genes were analyzed in reproductive tissues, including male flower, pollen, silk, and ovary. Most of the genes encoding basal-layer antifungal peptide-like, small coat proteins-like, thioredoxin-like proteins, γ-thionins-like, and DEFL proteins showed high expression levels in the ovary and male flower compared with their levels in silk and mature pollen. The rapid alkalinization factor-like genes were highly expressed only in the mature ovary and mature pollen, and pollen Ole e 1-like genes showed low expression in silk. The results of this study provide basic information for further analysis of SSP functions in the reproductive process of maize.

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

It is difficult to derive all qualitative proteomic and metabolomic experimental data in male (pollen tube) and female (pistil) reproductive tissues during pollination because of the limited sensitivity of current technology. In this study, genome-scale enzyme correlation network models for plants (Arabidopsis/maize) were constructed by analyzing the enzymes and metabolic routes from a global perspective. Then, we developed a data-driven computational pipeline using the "guilt by association" principle to analyze the transcriptional coexpression profiles of enzymatic genes in the consecutive steps for metabolic routes in the fast-growing pollen tube and stigma during pollination. The analysis identified an inferred pattern of pollen tube-stigma ethanol coupling. When the pollen tube elongates in the transmitting tissue (TT) of the pistil, this elongation triggers the mobilization of energy from glycolysis in the TT cells of the pistil. Energy-rich metabolites (ethanol) are secreted that can be taken up by the pollen tube, where these metabolites are incorporated into the pollen tube's tricarboxylic acid (TCA) cycle, which leads to enhanced ATP production for facilitating pollen tube growth. In addition, our analysis also provided evidence for the cooperation of kaempferol, dTDP-alpha-L-rhamnose and cell-wall-related proteins; phosphatidic-acid-mediated Ca2+ oscillations and cytoskeleton; and glutamate degradation IV for γ-aminobutyric acid (GABA) signaling activation in Arabidopsis and maize stigmas to provide the signals and materials required for pollen tube tip growth. In particular, the "guilt by association" computational pipeline and the genome-scale enzyme correlation network models (GECN) developed in this study was initiated with experimental "omics" data, followed by data analysis and data integration to determine correlations, and could provide a new platform to assist inachieving a deeper understanding of the co-regulation and inter-regulation model in plant research.

Brassinosteroids promote Arabidopsis pollen germination and growth

Pollen tubes are among the fastest tip-growing plant cells and represent an excellent experimental system for studying the dynamics and spatiotemporal control of polarized cell growth. However, investigating pollen tube tip growth in the model plant Arabidopsis remains difficult because in vitro pollen germination and pollen tube growth rates are highly variable and largely different from those observed in pistils, most likely due to growth-promoting properties of the female reproductive tract. We found that in vitro grown Arabidopsis pollen respond to brassinosteroid (BR) in a dose-dependent manner. Pollen germination and pollen tube growth increased nine- and fivefold, respectively, when media were supplemented with 10 µM epibrassinolide (epiBL), resulting in growth kinetics more similar to growth in vivo. Expression analyses show that the promoter of one of the key enzymes in BR biosynthesis, CYP90A1/CPD, is highly active in the cells of the reproductive tract that form the pathway for pollen tubes from the stigma to the ovules. Pollen tubes grew significantly shorter through the reproductive tract of a cyp90a1 mutant compared to the wild type, or to a BR perception mutant. Our results show that epiBL promotes pollen germination and tube growth in vitro and suggest that the cells of the reproductive tract provide BR compounds to stimulate pollen tube growth.

De novo assembly of the perennial ryegrass transcriptome using an RNA-Seq strategy

BACKGROUND

Perennial ryegrass is a highly heterozygous outbreeding grass species used for turf and forage production. Heterozygosity can affect de-Bruijn graph assembly making de novo transcriptome assembly of species such as perennial ryegrass challenging. Creating a reference transcriptome from a homozygous perennial ryegrass genotype can circumvent the challenge of heterozygosity. The goals of this study were to perform RNA-sequencing on multiple tissues from a highly inbred genotype to develop a reference transcriptome. This was complemented with RNA-sequencing of a highly heterozygous genotype for SNP calling.

RESULT

De novo transcriptome assembly of the inbred genotype created 185,833 transcripts with an average length of 830 base pairs. Within the inbred reference transcriptome 78,560 predicted open reading frames were found of which 24,434 were predicted as complete. Functional annotation found 50,890 transcripts with a BLASTp hit from the Swiss-Prot non-redundant database, 58,941 transcripts with a Pfam protein domain and 1,151 transcripts encoding putative secreted peptides. To evaluate the reference transcriptome we targeted the high-affinity K+ transporter gene family and found multiple orthologs. Using the longest unique open reading frames as the reference sequence, 64,242 single nucleotide polymorphisms were found. One thousand sixty one open reading frames from the inbred genotype contained heterozygous sites, confirming the high degree of homozygosity.

CONCLUSION

Our study has developed an annotated, comprehensive transcriptome reference for perennial ryegrass that can aid in determining genetic variation, expression analysis, genome annotation, and gene mapping.

A pollen-specific calmodulin-binding protein, NPG1, interacts with putative pectate lyases

Previous genetic studies have revealed that a pollen-specific calmodulin-binding protein, No Pollen Germination 1 (NPG1), is required for pollen germination. However, its mode of action is unknown. Here we report direct interaction of NPG1 with pectate lyase-like proteins (PLLs). A truncated form of AtNPG1 lacking the N-terminal tetratricopeptide repeat 1 (TPR1) failed to interact with PLLs, suggesting that it is essential for NPG1 interaction with PLLs. Localization studies with AtNPG1 fused to a fluorescent reporter driven by its native promoter revealed its presence in the cytosol and cell wall of the pollen grain and the growing pollen tube of plasmolyzed pollen. Together, our data suggest that the function of NPG1 in regulating pollen germination is mediated through its interaction with PLLs, which may modify the pollen cell wall and regulate pollen tube emergence and growth.

Two Mitogen-Activated Protein Kinases, MPK3 and MPK6, Are Required for Funicular Guidance of Pollen Tubes in Arabidopsis

Double fertilization in flowering plants requires the delivery of two immotile sperm cells to the female gametes by a pollen tube, which perceives guidance cues, modifies its tip growth direction, and eventually enters the micropyle of the ovule. In spite of the recent progress, so far, little is known about the signaling events in pollen tubes in response to the guidance cues. Here, we show that MPK3 and MPK6, two Arabidopsis (Arabidopsis thaliana) mitogen-activated protein kinases, mediate the guidance response in pollen tubes. Genetic analysis revealed that mpk3 mpk6 double mutant pollen has reduced transmission. However, direct observation of mpk3 mpk6 mutant pollen phenotype was hampered by the embryo lethality of double homozygous mpk3^-/- mpk6^-/- plants. Utilizing a fluorescent reporter-tagged complementation method, we showed that the mpk3 mpk6 mutant pollen had normal pollen tube growth but impaired pollen tube guidance. In vivo pollination assays revealed that the mpk3 mpk6 mutant pollen tubes were defective in the funicular guidance phase. By contrast, semi-in vitro guidance assay showed that the micropylar guidance of the double mutant pollen tube was normal. Our results provide direct evidence to support that the funicular guidance phase of the pollen tube requires an in vivo signaling mechanism distinct from the micropyle guidance. Moreover, our finding opened up the possibility that the MPK3/MPK6 signaling pathway may link common signaling networks in plant stress response and pollen-pistil interaction.

Posttranslationally modified small-peptide signals in plants

Cell-to-cell signaling is essential for many processes in plant growth and development, including coordination of cellular responses to developmental and environmental cues. Cumulative studies have demonstrated that peptide signaling plays a greater-than-anticipated role in such intercellular communication. Some peptides act as signals during plant growth and development, whereas others are involved in defense responses or symbiosis. Peptides secreted as signals often undergo posttranslational modification and proteolytic processing to generate smaller peptides composed of approximately 10 amino acid residues. Such posttranslationally modified small-peptide signals constitute one of the largest groups of secreted peptide signals in plants. The location of the modification group incorporated into the peptides by specific modification enzymes and the peptide chain length defined by the processing enzymes are critical for biological function and receptor interaction. This review covers 20 years of research into posttranslationally modified small-peptide signals in plants.

Substantial expression of novel small open reading frames in Oryza sativa

In our previous integrated study combining informatics and molecular biology analyses, we revealed that Arabidopsis small open reading frames (sORFs) predicted by computational analysis have biological functions in morphogenesis. Here, we report that sequences homologous to Arabidopsis sORFs are abundant in intergenic regions of the rice genome. These sequences represent a subset of non-protein-coding DNA, and some are transcribed into mRNA. These results indicate that many sORFs associated with morphogenesis are hidden in the genomes of crop species.

Transcript profiles of maize embryo sacs and preliminary identification of genes involved in the embryo sac-pollen tube interaction

The embryo sac, the female gametophyte of flowering plants, plays important roles in the pollination and fertilization process. Maize (Zea mays L.) is a model monocot, but little is known about the interactions between its embryo sac and the pollen tube. In this study, we compared the transcript profiles of mature embryo sacs, mature embryo sacs 14-16 h after pollination, and mature nucelli. Comparing the transcript profiles of the embryo sacs before and after the entry of the pollen tube, we identified 3467 differentially expressed transcripts (3382 differentially expressed genes; DEGs). The DEGs were grouped into 22 functional categories. Among the DEGs, 221 genes were induced upon the entry of the pollen tube, and many of them encoded proteins involved in RNA binding, processing, and transcription, signaling, miscellaneous enzyme family processes, and lipid metabolism processes. Genes in the DEG dataset were grouped into 17 classes in a gene ontology enrichment analysis. The DEGs included many genes encoding proteins involved in protein amino acid phosphorylation and protein ubiquitination, implying that these processes might play important roles in the embryo sac-pollen tube interaction. Additionally, our analyses indicate that the expression of 112 genes encoding cysteine-rich proteins (CRPs) is induced during pollination and fertilization. The CRPs likely regulate pollen tube guidance and embryo sac development. These results provide important information on the genes involved in the embryo sac-pollen tube interaction in maize.

Selection on plant male function genes identifies candidates for reproductive isolation of yellow monkeyflowers

Understanding the genetic basis of reproductive isolation promises insight into speciation and the origins of biological diversity. While progress has been made in identifying genes underlying barriers to reproduction that function after fertilization (post-zygotic isolation), we know much less about earlier acting pre-zygotic barriers. Of particular interest are barriers involved in mating and fertilization that can evolve extremely rapidly under sexual selection, suggesting they may play a prominent role in the initial stages of reproductive isolation. A significant challenge to the field of speciation genetics is developing new approaches for identification of candidate genes underlying these barriers, particularly among non-traditional model systems. We employ powerful proteomic and genomic strategies to study the genetic basis of conspecific pollen precedence, an important component of pre-zygotic reproductive isolation among yellow monkeyflowers (Mimulus spp.) resulting from male pollen competition. We use isotopic labeling in combination with shotgun proteomics to identify more than 2,000 male function (pollen tube) proteins within maternal reproductive structures (styles) of M. guttatus flowers where pollen competition occurs. We then sequence array-captured pollen tube exomes from a large outcrossing population of M. guttatus, and identify those genes with evidence of selective sweeps or balancing selection consistent with their role in pollen competition. We also test for evidence of positive selection on these genes more broadly across yellow monkeyflowers, because a signal of adaptive divergence is a common feature of genes causing reproductive isolation. Together the molecular evolution studies identify 159 pollen tube proteins that are candidate genes for conspecific pollen precedence. Our work demonstrates how powerful proteomic and genomic tools can be readily adapted to non-traditional model systems, allowing for genome-wide screens towards the goal of identifying the molecular basis of genetically complex traits.

Developmental transcriptional profiling reveals key insights into Triticeae reproductive development

Despite their importance, there remains a paucity of large-scale gene expression-based studies of reproductive development in species belonging to the Triticeae. As a first step to address this deficiency, a gene expression atlas of triticale reproductive development was generated using the 55K Affymetrix GeneChip(®) wheat genome array. The global transcriptional profiles of the anther/pollen, ovary and stigma were analyzed at concurrent developmental stages, and co-expressed as well as preferentially expressed genes were identified. Data analysis revealed both novel and conserved regulatory factors underlying Triticeae floral development and function. This comprehensive resource rests upon detailed gene annotations, and the expression profiles are readily accessible via a web browser.

Spatio-temporal expression patterns of Arabidopsis thaliana and Medicago truncatula defensin-like genes

Plant genomes contain several hundred defensin-like (DEFL) genes that encode short cysteine-rich proteins resembling defensins, which are well known antimicrobial polypeptides. Little is known about the expression patterns or functions of many DEFLs because most were discovered recently and hence are not well represented on standard microarrays. We designed a custom Affymetrix chip consisting of probe sets for 317 and 684 DEFLs from Arabidopsis thaliana and Medicago truncatula, respectively for cataloging DEFL expression in a variety of plant organs at different developmental stages and during symbiotic and pathogenic associations. The microarray analysis provided evidence for the transcription of 71% and 90% of the DEFLs identified in Arabidopsis and Medicago, respectively, including many of the recently annotated DEFL genes that previously lacked expression information. Both model plants contain a subset of DEFLs specifically expressed in seeds or fruits. A few DEFLs, including some plant defensins, were significantly up-regulated in Arabidopsis leaves inoculated with Alternaria brassicicola or Pseudomonas syringae pathogens. Among these, some were dependent on jasmonic acid signaling or were associated with specific types of immune responses. There were notable differences in DEFL gene expression patterns between Arabidopsis and Medicago, as the majority of Arabidopsis DEFLs were expressed in inflorescences, while only a few exhibited root-enhanced expression. By contrast, Medicago DEFLs were most prominently expressed in nitrogen-fixing root nodules. Thus, our data document salient differences in DEFL temporal and spatial expression between Arabidopsis and Medicago, suggesting distinct signaling routes and distinct roles for these proteins in the two plant species.

ScRALF3, a secreted RALF-like peptide involved in cell-cell communication between the sporophyte and the female gametophyte in a solanaceous species

Small peptides have been shown to regulate numerous aspects of plant development through cell-cell communication. These signaling events are particularly important during reproduction, regulating gamete development and embryogenesis. Rapid alkalinization factor (RALF)-like genes, a large gene family that encodes secreted peptides, have specific or ubiquitous expression patterns. Previously, five RALF-like genes with potential involvement during reproduction were isolated from Solanum chacoense. Here, we show that ScRALF3 is an important peptide regulator of female gametophyte development. Its expression, which is auxin-inducible, is strictly regulated before and after fertilization. Down-regulation of ScRALF3 expression by RNA interference leads to the production of smaller fruits that produce fewer seeds, due to improper development of the embryo sacs. Defects include loss of embryo sac nuclei polarization, as well as an increase in asynchronous division, accounting for cellular dysfunctions and premature embryo sac development arrest during megagametogenesis. ScRALF3 is expressed in the sporophytic tissue surrounding the embryo sac, the integument and the nucellus, as revealed by in situ hybridization and GUS staining. As expected for a secreted peptide, fluorescence from an ScRALF3-GFP fusion construct is detected throughout the secretory pathway. Therefore, the ScRALF3 secreted peptide may be directly involved in the regulation of multiple aspects of cell-cell communication between the female gametophyte and its surrounding sporophytic tissue during ovule development.

Antifungal proteins: More than antimicrobials?

Antimicrobial proteins (AMPs) are widely distributed in nature. In higher eukaryotes, AMPs provide the host with an important defence mechanism against invading pathogens. AMPs of lower eukaryotes and prokaryotes may support successful competition for nutrients with other microorganisms of the same ecological niche. AMPs show a vast variety in structure, function, antimicrobial spectrum and mechanism of action. Most interestingly, there is growing evidence that AMPs also fulfil important biological functions other than antimicrobial activity. The present review focuses on the mechanistic function of small, cationic, cysteine-rich AMPs of mammals, insects, plants and fungi with antifungal activity and specifically aims at summarizing current knowledge concerning additional biological properties which opens novel aspects for their future use in medicine, agriculture and biotechnology.

A species-specific cluster of defensin-like genes encodes diffusible pollen tube attractants in Arabidopsis

AtLURE1 defensin-like peptides, which show species-specific evolution, are essential in Arabidopsis for attracting pollen tubes and can function in the breakdown of reproductive isolation barriers.

Genes directly involved in male/female and host/parasite interactions are believed to be under positive selection. The flowering plant Arabidopsis thaliana has more than 300 defensin-like (DEFL) genes, which are likely to be involved in both natural immunity and cell-to-cell communication including pollen–pistil interactions. However, little is known of the relationship between the molecular evolution of DEFL genes and their functions. Here, we identified a recently evolved cluster of DEFL genes in A. thaliana and demonstrated that these DEFL (cysteine-rich peptide [CRP810_1]) peptides, named AtLURE1 peptides, are pollen tube attractants guiding pollen tubes to the ovular micropyle. The AtLURE1 genes formed the sole species-specific cluster among DEFL genes compared to its close relative, A. lyrata. No evidence for positive selection was detected in AtLURE1 genes and their orthologs, implying neutral evolution of AtLURE1 genes. AtLURE1 peptides were specifically expressed in egg-accompanying synergid cells and secreted toward the funicular surface through the micropyle. Genetic analyses showed that gametophytic mutants defective in micropylar guidance (myb98, magatama3, and central cell guidance) do not express AtLURE1 peptides. Downregulation of the expression of these peptides impaired precise pollen tube attraction to the micropylar opening of some populations of ovules. Recombinant AtLURE1 peptides attracted A. thaliana pollen tubes at a higher frequency compared to A. lyrata pollen tubes, suggesting that these peptides are species-preferential attractants in micropylar guidance. In support of this idea, the heterologous expression of a single AtLURE1 peptide in the synergid cell of Torenia fournieri was sufficient to guide A. thaliana pollen tubes to the T. fournieri embryo sac and to permit entry into it. Our results suggest the unique evolution of AtLURE1 genes, which are directly involved in male–female interaction among the DEFL multigene family, and furthermore suggest that these peptides are sufficient to overcome interspecific barriers in gametophytic attraction and penetration.

Defensin-like (DEFL) peptides commonly function as effector peptides and are involved in male-female and host-parasite interactions in eukaryotes. In higher plants, DEFL genes belong to a large multigene family and are highly variable between species. However, little is known about the relationship between the molecular evolution of DEFL genes and their functions. By comparing multiply duplicated DEFL genes between A. thaliana and its close relative A. lyrata, we have now identified pollen tube attractant peptides called AtLURE1 peptides, in A. thaliana. We find that AtLURE1 genes form a species-specific gene cluster and that the AtLURE1 peptides these genes encode are specifically expressed in the synergid (egg-accompanying) cells and are secreted along the path down which the pollen tube elongates to reach the female gametophyte. AtLURE1 peptides attract pollen tubes in a species-preferential manner and their downregulation impairs pollen tube guidance. Interestingly, the genetic introduction of a single AtLURE1 gene from A. thaliana into another plant, T. fournieri, is sufficient to breakdown reproductive isolation barriers in pollen tube guidance and penetration. These results suggest that AtLURE1 peptides, which show species-specific evolution, are key molecules that attract pollen tubes from a plant's own species to the embryo sac to enable successful reproduction.