Gametophytic self-incompatibility: understanding the cellular mechanisms involved in "self" pollen tube inhibition

The Nβ motif of NaTrxh directs secretion as an endoplasmic reticulum transit peptide and variations might result in different cellular targeting

Soluble secretory proteins with a signal peptide reach the extracellular space through the endoplasmic reticulum-Golgi conventional pathway. During translation, the signal peptide is recognised by the signal recognition particle and results in a co-translational translocation to the endoplasmic reticulum to continue the secretory pathway. However, soluble secretory proteins lacking a signal peptide are also abundant, and several unconventional (endoplasmic reticulum/Golgi independent) pathways have been proposed and some demonstrated. This work describes new features of the secretion signal called Nβ, originally identified in NaTrxh, a plant extracellular thioredoxin, that does not possess an orthodox signal peptide. We provide evidence that other proteins, including thioredoxins type h, with similar sequences are also signal peptide-lacking secretory proteins. To be a secretion signal, positions 5, 8 and 9 must contain neutral residues in plant proteins-a negative residue in position 8 is suggested in animal proteins-to maintain the Nβ motif negatively charged and a hydrophilic profile. Moreover, our results suggest that the NaTrxh translocation to the endoplasmic reticulum occurs as a post-translational event. Finally, the Nβ motif sequence at the N- or C-terminus could be a feature that may help to predict protein localisation, mainly in plant and animal proteins.

Hormonal Signaling during dPCD: Cytokinin as the Determinant of RNase-Based Self-Incompatibility in Solanaceae

Research into molecular mechanisms of self-incompatibility (SI) in plants can be observed in representatives of various families, including Solanaceae. Earlier studies of the mechanisms of S-RNase-based SI in petunia (Petunia hybrida E. Vilm.) demonstrate that programmed cell death (PCD) is an SI factor. These studies suggest that the phytohormon cytokinin (CK) is putative activator of caspase-like proteases (CLPs). In this work, data confirming this hypothesis were obtained in two model objects-petunia and tomato (six Solanaceae representatives). The exogenous zeatin treatment of tomato and petunia stigmas before a compatible pollination activates CLPs in the pollen tubes in vivo, as shown via the intravital imaging of CLP activities. CK at any concentration slows down the germination and growth of petunia and tomato male gametophytes both in vitro and in vivo; shifts the pH of the cytoplasm (PHc) to the acid region, thereby creating the optimal conditions for CLP to function and inhibiting the F-actin formation and/or destructing the cytoskeleton in pollen tubes to point foci during SI-induced PCD; and accumulates in style tissues during SI response. The activity of the ISOPENTENYLTRANSFERASE 5 (IPT5) gene at this moment exceeds its activity in a cross-compatible pollination, and the levels of expression of the CKX1 and CKX2 genes (CK OXIDASE/DEHYDROGENASE) are significantly lower in self-incompatible pollination. All this suggests that CK plays a decisive role in the mechanism underlying SI-induced PCD.

Heterosis patterns and sources of self-compatibility, cross-compatibility and key nut traits within single and double hybrid crosses of kola [Cola nitida (Vent) Schott and Endl.]

Sexual incompatibility among kola genotypes accounted for over 50% yield loss. Compatible and high yielding varieties are in demand to develop commercial orchards. The objective of this study was to assess self-compatibility and cross-compatibility of kola (C. nitida) genotypes within self, single and double hybrid crosses and to determine heterosis pattern in the resulting hybrids for sexual compatibility and key nut yield and quality traits. Crosses among kola genotypes from three field gene banks (JX1, GX1, MX2) and one advanced germplasm (Bunso progeny) in Ghana were evaluated along their parents for sexual compatibility, nut yield and nut quality. Data were collected on pod set, pseudo-pod set, pod weight, number of nuts per pod, nut weight, brix, potential alcohol and nut firmness. Significant (P < 0.001) differential pod set was observed within Bunso progeny, JX1, GX1 and MX2 crosses; while pseudo-pod set differed only within JX1 and MX2 crosses (P < 0.001). Very large prevalence of mid-parent, heterobeltiosis, and economic heterosis was observed for sexual compatibility, outturn and brix for the single and double hybrid crosses. Heterosis was prominent among the double hybrid crosses as compared to the single hybrid crosses suggesting that recurrent selection of compatible varieties from advanced generations could result in genetic gain in kola improvement. The top five crosses with best heterosis for sexual compatibility and an appreciable positive heterosis for outturn and brix were B1/11 × B1/71 × B1/157 × B1/149, B1/11 × B1/71 × B1/296 × B1/177, GX1/46 × GX1/33 × B1/212 × B1/236, JX1/90 × JX1/51 and JX1/51 × JX1/36. These materials could serve as sources of beneficial alleles for improving Ghanaian kola hybrids and populations for yield and sexual compatibility.

Combined transcriptomic and proteomic analysis reveals multiple pathways involved in self-pollen tube development and the potential roles of FviYABBY1 in self-incompatibility in Fragaria viridis

Fragaria viridis exhibits S-RNase-based gametophytic self-incompatibility, in which S-RNase is the major factor inhibiting pollen tube growth. However, the pathways involved in and the immediate causes of the inhibition of pollen tube growth remain unknown. Here, interactive RNA sequencing and proteome analysis revealed changes in the transcriptomic and proteomic profiles of F. viridis styles harvested at 0 and 24 h after self-pollination. A total of 2,181 differentially expressed genes and 200 differentially abundant proteins were identified during the pollen development stage of self-pollination. Differentially expressed genes and differentially abundant proteins associated with self-incompatible pollination were further mined, and multiple pathways were found to be involved. Interestingly, the expression pattern of the transcription factor FviYABBY1, which is linked to polar growth, differed from those of other genes within the same family. Specifically, FviYABBY1 expression was extremely high in pollen, and its expression trend in self-pollinated styles was consistent with that of S-RNase. Furthermore, FviYABBY1 interacted with S-RNase in a non-S haplotype way. Therefore, FviYABBY1 affects the expression of polar growth-related genes in self-pollen tubes and is positively regulated by S-RNase.

The origin and evolution of RNase T2 family and gametophytic self-incompatibility system in plants

Ribonuclease (RNase) T2 genes are found widely in both eukaryotes and prokaryotes, and genes from this family have been revealed to have various functions in plants. In particular, S-RNase is known to be the female determinant in the S-RNase-based gametophytic self-incompatibility (GSI) system. However, the origin and evolution of the RNase T2 gene family and GSI system are not well understood. In this study, 785 RNase T2 genes were identified in 81 sequenced plant genomes representing broad-scale diversity and divided into three subgroups (Class I, II, and III) based on phylogenetic and synteny network analysis. Class I was found to be of ancient origin and to emerge in green algae, Class II was shown to originate with the appearance of angiosperms, while Class III was discovered to be eudicot-specific. Each of the three major classes could be further classified into several subclasses of which some subclasses were found to be lineage-specific. Furthermore, duplication, deletion, or inactivation of the S/S-like-locus was revealed to be linked to repeated loss and gain of self-incompatibility in different species from distantly related plant families with GSI. Finally, the origin and evolutionary history of S-locus in Rosaceae species was unraveled with independent loss and gain of S-RNase occurred in different subfamilies of Rosaceae. Our findings provide insights into the origin and evolution of the RNase T2 family and the GSI system in plants.

Genetic control of compatibility in crosses between wheat and its wild or cultivated relatives

In the recent years, the agricultural world has been progressing towards integrated crop protection, in the context of sustainable and reasoned agriculture to improve food security and quality, and to preserve the environment through reduced uses of water, pesticides, fungicides or fertilisers. For this purpose, one possible issue is to cross-elite varieties widely used in fields for crop productions with exotic or wild genetic resources in order to introduce new diversity for genes or alleles of agronomical interest to accelerate the development of new improved cultivars. However, crossing ability (or crossability) often depends on genetic background of the recipient varieties or of the donor, which hampers a larger use of wild resources in breeding programmes of many crops. In this review, we tried to provide a comprehensive summary of genetic factors controlling crossing ability between Triticeae species with a special focus on the crossability between wheat (Triticum aestivum L.) and rye (Secale cereale), which lead to the creation of Triticale (x Triticosecale Wittm.). We also discussed potential applications of newly identified genes or markers associated with crossability for accelerating wheat and Triticale improvement by application of modern genomics technologies in breeding programmes.

Evidence for human-caused founder effect in populations of Solanum jamesii found at archaeological sites: I. Breeding experiments and the geography of sexual reproduction

PREMISE

Plant domestication can be detected when transport, use, and manipulation of propagules impact reproductive functionality, especially in species with self-incompatible breeding systems.

METHODS

Evidence for human-caused founder effect in the Four Corners potato (Solanum jamesii Torr.) was examined by conducting 526 controlled matings between archaeological and non-archaeological populations from field-collected tubers grown in a greenhouse. Specimens from 24 major herbaria and collection records from >160 populations were examined to determine which produced fruits.

RESULTS

Archaeological populations did not produce any fruits when self-crossed or outcrossed between individuals from the same source. A weak ability to self- or outcross within populations was observed in non-archaeological populations. Outcrossing between archaeological and non-archaeological populations, however, produced fully formed, seed-containing fruits, especially with a non-archaeological pollen source. Fruit formation was observed in 51 of 162 occurrences, with minimal evidence of constraint by monsoonal drought, lack of pollinators, or spatial separation of suitable partners. Some archaeological populations (especially those along ancient trade routes) had records of fruit production (Chaco Canyon), while others (those in northern Arizona, western Colorado, and southern Utah) did not.

CONCLUSIONS

The present study suggests that archaeological populations could have different origins at different times-some descending directly from large gene pools to the south and others derived from gardens already established around occupations. The latter experienced a chain of founder events, which presumably would further reduce genetic diversity and mating capability. Consequently, some archaeological populations lack the genetic ability to sexually reproduce, likely as the result of human-caused founder effect.

Research advances in and prospects of ornamental plant genomics

The term 'ornamental plant' refers to all plants with ornamental value, which generally have beautiful flowers or special plant architectures. China is rich in ornamental plant resources and known as the "mother of gardens". Genomics is the science of studying genomes and is useful for carrying out research on genome evolution, genomic variations, gene regulation, and important biological mechanisms based on detailed genome sequence information. Due to the diversity of ornamental plants and high sequencing costs, the progress of genome research on ornamental plants has been slow for a long time. With the emergence of new sequencing technologies and a reduction in costs since the whole-genome sequencing of the first ornamental plant (Prunus mume) was completed in 2012, whole-genome sequencing of more than 69 ornamental plants has been completed in <10 years. In this review, whole-genome sequencing and resequencing of ornamental plants will be discussed. We provide analysis with regard to basic data from whole-genome studies of important ornamental plants, the regulation of important ornamental traits, and application prospects.

Cytological Observation and Transcriptome Comparative Analysis of Self-Pollination and Cross-Pollination in Dendrobium Officinale

Dendrobium officinale is a rare and traditional medicinal plant with high pharmacological and nutritional value. The self-incompatibility mechanism of D. officinale reproductive isolation was formed in the long-term evolution process, but intraspecific hybridization of different germplasm resources leads to a large gap in the yield, quality, and medicinal value of D. officinale. To investigate the biological mechanism of self-incompatibility in D. officinale, cytological observation and the transcriptome analysis was carried out on the samples of self-pollination and cross-pollination in D. officinale. Results for self-pollination showed that the pollen tubes could grow in the style at 2 h, but most of pollen tubes stopped growing at 4 h, while a large number of cross-pollinated pollen tubes grew along the placental space to the base of ovary, indicating that the self-incompatibility of D. officinale may be gametophyte self-incompatibility. A total of 63.41 G basesum of D. officinale style samples from non-pollinated, self-pollination, and cross-pollination by RNA-seq were obtained, and a total of 1944, 1758, and 475 differentially expressed genes (DEGs) in the comparison of CK (non-pollinated) vs. HF (cross-pollination sample), CK vs. SF (self-pollination sample) and SF vs. HF were identified, respectively. Forty-one candidate genes related to self-incompatibility were found by function annotation of DEGs, including 6 Ca²⁺ signal genes, 4 armed repeat containing (ARC) related genes, 11 S-locus receptor kinase (SRK) related genes, 2 Exo70 family genes, 9 ubiquitin related genes, 1 fatty acid related gene, 6 amino acid-related genes, 1 pollen-specific leucine-rich repeat extensin-like protein (LRX) related gene and 1 lectin receptor-like kinases (RLKs) related gene, showed that self-incompatibility mechanism of D. officinale involves the interaction of multiple genes and pathways. The results can provide a basis for the study of the self-incompatibility mechanism of D. officinale, and provide ideas for the preservation and utilization of high-quality resources of D. officinale.

Insights into the molecular control of cross-incompatibility in Zea mays

Gametophytic cross-incompatibility systems in corn have been the subject of genetic studies for more than a century. They have tremendous economic potential as a genetic mechanism for controlling fertilization without controlling pollination. Three major genetically distinct and functionally equivalent cross-incompatibility systems exist in Zea mays: Ga1, Tcb1, and Ga2. All three confer reproductive isolation between maize or teosinte varieties with different haplotypes at any one locus. These loci confer genetically separable functions to the silk and pollen: a female function that allows the silk to block fertilization by non-self-type pollen and a male function that overcomes the block of the female function from the same locus. Identification of some of these genes has shed light on the reproductive isolation they confer. The identification of both male and female factors as pectin methylesterases reveals the importance of pectin methylesterase activity in controlling the decision between pollen acceptance versus rejection, possibly by regulating the degree of methylesterification of the pollen tube cell wall. The appropriate level and spatial distribution of pectin methylesterification is critical for pollen tube growth and is affected by both pectin methylesterases and pectin methylesterase inhibitors. We present a molecular model that explains how cross-incompatibility systems may function that can be tested in Zea and uncharacterized cross-incompatibility systems. Molecular characterization of these loci in conjunction with further refinement of the underlying molecular and cellular mechanisms will allow researchers to bring new and powerful tools to bear on understanding reproductive isolation in Zea mays and related species.

NaTrxh is an essential protein for pollen rejection in Nicotiana by increasing S-RNase activity

In self-incompatible Solanaceae, the pistil protein S-RNase contributes to S-specific pollen rejection in conspecific crosses, as well as to rejecting pollen from foreign species or whole clades. However, S-RNase alone is not sufficient for either type of pollen rejection. We describe a thioredoxin (Trx) type h from Nicotiana alata, NaTrxh, which interacts with and reduces S-RNase in vitro. Here, we show that expressing a redox-inactive mutant, NaTrxhSS , suppresses both S-specific pollen rejection and rejection of pollen from Nicotiana plumbaginifolia. Biochemical experiments provide evidence that NaTrxh specifically reduces the Cys155 -Cys185 disulphide bond of SC10 -Rnase, resulting in a significant increase of its ribonuclease activity. This reduction and increase in S-RNase activity by NaTrxh helps to explain why S-RNase alone could be insufficient for pollen rejection.

Interaction Between AtCML9 and AtMLO10 Regulates Pollen Tube Development and Seed Setting

In higher-plant reproduction, the compatibility of pollen tube germination in the pistil is essential for successful double fertilization. It has been reported that Mildew Locus O (MLO) family gene NTA (MLO7), expressing in synergid cells, can correctly guide pollen tubes. However, the molecular mechanism underlying the interacting partners to MLOs in the fertilization is still unknown. In our study, we identified the direct protein interaction between CML9 and MLO10 within a non-canonical CaMBD. In GUS reporter assays, CML9 expresses in a high level in pollens, whereas MLO10 can be specifically detected in stigma which reaches up to a peaking level before fertilization. Therefore, the spatio-temporal expression patterns of MLO10 and CML9 are required for the time-window of pollination. When we observed the pollen germination in vitro, two cml9 mutant alleles dramatically reduced germination rate by 15% compared to wild-type. Consistently, the elongation rate of pollen tubes in planta was obviously slow while manually pollinating cml9-1 pollens to mlo10-1 stigmas. Additionally, cml9-1 mlo10-1 double mutant alleles had relatively lower rate of seed setting. Taken together, protein interaction between MLO10 and CML9 is supposed to affect pollen tube elongation and further affect seed development.

Temperature-Dependent Compatible and Incompatible Pollen-Style Interactions in Citrus clementina Hort. ex Tan. Show Different Transglutaminase Features and Polyamine Pattern

In clementine, failure of fertilization can result in parthenocarpic fruit development, which has several advantages, such as seedless fruit, longer shelf-life, and greater consumer appeal. Recently, S-RNases have been identified in Citrus grandis, thus revealing that the self-incompatibility (SI) reaction relies on the S-RNase gametophytic mechanism. The fundamental role of environmental factors, mostly temperature, in determining the numbers of pollen tubes reaching the ovary is also well established in Citrus. In the present work, temperature-dependent pollen-pistil interactions in C. clementina were analyzed, focusing on several morphological aspects, as well as on polyamine (PA) content and the activity and distribution of transglutaminase (TGase), both reported to be involved in the SI response in pear and in pummelo. Results clearly indicate that temperature contributed to a different activation of the SI response, which occurs at optimal temperature of 25°C but was by-passed at 15°C. TGase activity was stimulated during the SI response, and it localized differently in the compatible and incompatible interaction: in compatible pollinated styles, TGase localized inside the style canal, while it was detected all around it in incompatible crosses. TGase localization and activity were congruent with the levels of soluble and insoluble conjugated PAs and with morphological evidences, which highlighted cell wall modification occurring as a result of SI.

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.

Compatible and Incompatible Pollen-Styles Interaction in Pyrus communis L. Show Different Transglutaminase Features, Polyamine Pattern and Metabolomics Profiles

Pollen-stigma interaction is a highly selective process, which leads to compatible or incompatible pollination, in the latter case, affecting quantitative and qualitative aspects of productivity in species of agronomic interest. While the genes and the corresponding protein partners involved in this highly specific pollen-stigma recognition have been studied, providing important insights into pollen-stigma recognition in self-incompatible (SI), many other factors involved in the SI response are not understood yet. This work concerns the study of transglutaminase (TGase), polyamines (PAs) pattern and metabolomic profiles following the pollination of Pyrus communis L. pistils with compatible and SI pollen in order to deepen their possible involvement in the reproduction of plants. Immunolocalization, abundance and activity of TGase as well as the content of free, soluble-conjugated and insoluble-bound PAs have been investigated. 1H NMR-profiling coupled with multivariate data treatment (PCA and PLS-DA) allowed to compare, for the first time, the metabolic patterns of not-pollinated and pollinated styles. Results clearly indicate that during the SI response TGase activity increases, resulting in the accumulation of PAs conjugated to hydroxycinnamic acids and other small molecules. Metabolomic analysis showed a remarkable differences between pollinated and not-pollinated styles, where, except for glucose, all the other metabolites where less concentrated. Moreover, styles pollinated with compatible pollen showed the highest amount of sucrose than SI pollinated ones, which, in turn, contained highest amount of all the other metabolites, including aromatic compounds, such as flavonoids and a cynnamoil derivative.

Global Transcriptional Insights of Pollen-Pistil Interactions Commencing Self-Incompatibility and Fertilization in Tea [Camellia sinensis (L.) O. Kuntze]

This study explicates molecular insights commencing Self-Incompatibility (SI) and CC (cross-compatibility/fertilization) in self (SP) and cross (CP) pollinated pistils of tea. The fluorescence microscopy analysis revealed ceased/deviated pollen tubes in SP, while successful fertilization occurred in CP at 48 HAP. Global transcriptome sequencing of SP and CP pistils generated 109.7 million reads with overall 77.9% mapping rate to draft tea genome. Furthermore, concatenated de novo assembly resulted into 48,163 transcripts. Functional annotations and enrichment analysis (KEGG & GO) resulted into 3793 differentially expressed genes (DEGs). Among these, de novo and reference-based expression analysis identified 195 DEGs involved in pollen-pistil interaction. Interestingly, the presence of 182 genes [PT germination & elongation (67), S-locus (11), fertilization (43), disease resistance protein (30) and abscission (31)] in a major hub of the protein-protein interactome network suggests a complex signaling cascade commencing SI/CC. Furthermore, tissue-specific qRT-PCR analysis affirmed the localized expression of 42 DE putative key candidates in stigma-style and ovary, and suggested that LSI initiated in style and was sustained up to ovary with the active involvement of csRNS, SRKs & SKIPs during SP. Nonetheless, COBL10, RALF, FERONIA-rlk, LLG and MAPKs were possibly facilitating fertilization. The current study comprehensively unravels molecular insights of phase-specific pollen-pistil interaction during SI and fertilization, which can be utilized to enhance breeding efficiency and genetic improvement in tea.

A farnesyl pyrophosphate synthase gene expressed in pollen functions in S-RNase-independent unilateral incompatibility

Multiple independent and overlapping pollen rejection pathways contribute to unilateral interspecific incompatibility (UI). In crosses between tomato species, pollen rejection usually occurs when the female parent is self-incompatible (SI) and the male parent self-compatible (SC) (the 'SI × SC rule'). Additional, as yet unknown, UI mechanisms are independent of self-incompatibility and contribute to UI between SC species or populations. We identified a major quantitative trait locus on chromosome 10 (ui10.1) which affects pollen-side UI responses in crosses between cultivated tomato, Solanum lycopersicum, and Solanum pennelliiLA0716, both of which are SC and lack S-RNase, the pistil determinant of S-specificity in Solanaceae. Here we show that ui10.1 is a farnesyl pyrophosphate synthase gene (FPS2) expressed in pollen. Expression is about 18-fold higher in pollen of S. pennellii than in S. lycopersicum. Pollen with the hypomorphic S. lycopersicum allele is selectively eliminated on pistils of the F₁ hybrid, leading to transmission ratio distortion in the F₂ progeny. CRISPR/Cas9-generated knockout mutants (fps2) in S. pennelliiLA0716 are self-sterile due to pollen rejection, but mutant pollen is fully functional on pistils of S. lycopersicum. F₂ progeny of S. lycopersicum × S. pennellii (fps2) show reversed transmission ratio distortion due to selective elimination of pollen bearing the knockout allele. Overexpression of FPS2 in S. lycopersicum pollen rescues the pollen elimination phenotype. FPS2-based pollen selectivity does not involve S-RNase and has not been previously linked to UI. Our results point to an entirely new mechanism of interspecific pollen rejection in plants.

Comparative Proteomics Analyses of Pollination Response in Endangered Orchid Species Dendrobium Chrysanthum

Pollination is a crucial stage in plant reproductive process. The self-compatibility (SC) and self-incompatibility (SI) mechanisms determined the plant genetic diversity and species survival. D. chrysanthum is a highly valued ornamental and traditional herbal orchid in Asia but has been declared endangered. The sexual reproduction in D. chrysanthum relies on the compatibility of pollination. To provide a better understanding of the mechanism of pollination, the differentially expressed proteins (DEP) between the self-pollination (SP) and cross-pollination (CP) pistil of D. chrysanthum were investigated using proteomic approaches—two-dimensional electrophoresis (2-DE) coupled with tandem mass spectrometry technique. A total of 54 DEP spots were identified in the two-dimensional electrophoresis (2-DE) maps between the SP and CP. Gene ontology analysis revealed an array of proteins belonging to following different functional categories: metabolic process (8.94%), response to stimulus (5.69%), biosynthetic process (4.07%), protein folding (3.25%) and transport (3.25%). Identification of these DEPs at the early response stage of pollination will hopefully provide new insights in the mechanism of pollination response and help for the conservation of the orchid species.

A comparative study of distant hybridization in plants and animals

Distant hybridization refers to crosses between two different species, genera, or higher-ranking taxa, which can break species limits, increase genetic variation, and combine the biological characteristics of existing species. It is an important way of creating genetic variation, fertile strains, and excellent characteristics in new strains and populations. Combining analyses and summaries from many inter-related documents in plants and animals, both domestic and international, including examples and long-standing research on distant hybridization in fish from our laboratory, we summarize and compare the similarities and differences in plant and animal distant hybridization. In addition, we analyze and review the biological characteristics of their different ploidy progenies and the possible causes of disparity in survival rates. Mechanisms of sterility in animal and plant distant hybrids are also discussed, and research methods for the study of biological characteristics of hybrids, including morphology, cytology, and molecular cytogenetics are presented. This paper aims to provide comprehensive research materials and to systematically compare the general and specific characteristics of plant and animal hybrids with regards to reproduction, genetics, growth traits, and other biological characteristics. It is hoped that this paper will have great theoretical and practical significance for the study of genetic breeding and biological evolution of plant and animal distant hybridization.

Mating system transitions in Solanum habrochaites impact interactions between populations and species

In plants, transitions in mating system from outcrossing to self-fertilization are common; however, the impact of these transitions on interspecific and interpopulation reproductive barriers is not fully understood. We examined the consequences of mating system transition for reproductive barriers in 19 populations of the wild tomato species Solanum habrochaites. We identified S. habrochaites populations with self-incompatible (SI), self-compatible (SC) and mixed population (MP) mating systems, and characterized pollen-pistil interactions among S. habrochaites populations and between S. habrochaites and other tomato species. We examined the relationship between mating system, floral morphology, interspecific and interpopulation compatibility and pistil SI factors. We documented five distinct phenotypic groups by combining reproductive behavior with molecular data. Transitions from SI to MP were not associated with weakened interspecific reproductive barriers or loss of known pistil SI factors. However, transitions to SC at the northern range margin were accompanied by loss of S-RNase, smaller flowers, and weakened (or absent) interspecific pollen-pistil barriers. Finally, we identified a subset of SC populations that exhibited a partial interpopulation reproductive barrier with central SI populations. Our results support the hypothesis that shifts in mating system, followed by additional loss-of-function mutations, impact reproductive barriers within and between species.

Polyamines in Pollen: From Microsporogenesis to Fertilization

The entire pollen life span is driven by polyamine (PA) homeostasis, achieved through fine regulation of their biosynthesis, oxidation, conjugation, compartmentalization, uptake, and release. The critical role of PAs, from microsporogenesis to pollen-pistil interaction during fertilization, is suggested by high and dynamic transcript levels of PA biosynthetic genes, as well as by the activities of the corresponding enzymes. Moreover, exogenous supply of PAs strongly affects pollen maturation and pollen tube elongation. A reduction of endogenous free PAs impacts pollen viability both in the early stages of pollen development and during fertilization. A number of studies have demonstrated that PAs largely function by modulating transcription, by structuring pollen cell wall, by modulating protein (mainly cytoskeletal) assembly as well as by modulating the level of reactive oxygen species. Both free low-molecular weight aliphatic PAs, and PAs conjugated to proteins and hydroxyl-cinnamic acids take part in these complex processes. Here, we review both historical and recent evidence regarding molecular events underlying the role of PAs during pollen development. In the concluding remarks, the outstanding issues and directions for future research that will further clarify our understanding of PA involvement during pollen life are outlined.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.

Testing the SI × SC rule: Pollen-pistil interactions in interspecific crosses between members of the tomato clade (Solanum section Lycopersicon, Solanaceae)

PREMISE OF THE STUDY

Interspecific reproductive barriers (IRBs) act to ensure species integrity by preventing hybridization. Previous studies on interspecific crosses in the tomato clade have focused on the success of fruit and seed set. The SI × SC rule (SI species × SC species crosses are incompatible, but the reciprocal crosses are compatible) often applies to interspecific crosses. Because SI systems in the Solanaceae affect pollen tube growth, we focused on this process in a comprehensive study of interspecific crosses in the tomato clade to test whether the SI × SC rule was always followed.

METHODS

Pollen tube growth was assessed in reciprocal crosses between all 13 species of the tomato clade using fluorescence microscopy.

KEY RESULTS

In crosses between SC and SI species, pollen tube growth follows the SI × SC rule: interspecific pollen tube rejection occurs when SI species are pollinated by SC species, but in the reciprocal crosses (SC × SI), pollen tubes reach ovaries. However, pollen tube rejection occurred in some crosses between pairs of SC species, demonstrating that a fully functional SI system is not necessary for pollen tube rejection in interspecific crosses. Further, gradations in the strength of both pistil and pollen IRBs were revealed in interspecific crosses using SC populations of generally SI species.

CONCLUSION

The SI × SC rule explains many of the compatibility relations in the tomato clade, but exceptions occur with more recently evolved SC species and accessions, revealing differences in strength of both pistil and pollen IRBs.

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.

Extensive trans-specific polymorphism at the mating type locus of the root decay fungus Heterobasidion

Incompatibility systems in which individuals bearing identical alleles reject each other favor the maintenance of a diversity of alleles. Mushroom mating type loci (MAT) encode for dozens or hundreds of incompatibility alleles whose loss from the population is greatly restricted through negative frequency selection, leading to a system of alleles with highly divergent sequences. Here, we use DNA sequences of homeodomain (HD) encoding genes at the MAT locus of five closely related species of the root rot basidiomycete Heterobasidion annosum sensu lato to show that the extended coalescence time of MAT alleles greatly predates speciation in the group, contrasting loci outside of MAT that show allele divergences largely consistent with the species phylogeny with those of MAT, which show rampant trans-species polymorphism. We observe a roughly 6-fold greater genealogical depth and polymorphism of MAT compared with non-MAT that argues for the maintenance of balanced polymorphism for a minimum duration of 24 My based on a molecular-clock calibrated species phylogeny. As with other basidiomycete HD genes, balancing selection appears to be concentrated at the specificity-determining region in the N-terminus of the protein based on identification of codons under selection and the absence of recombination within the region. However, the elevated polymorphism extends into the nonspecificity determining regions as well as a neighboring non-MAT gene, the mitochondrial intermediate peptidase (MIP). In doing so, increased divergence should decrease recombination among alleles and as a by-product create incompatibilities in the functional domains not involved in allele recognition but in regulating sexual development.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Isolation and characterization of multiple F-box genes linked to the S9- and S10-RNase in apple (Malus × domestica Borkh.)

Using 11 consensus primer pairs designed from S-linked F-box genes of apple and Japanese pear, 10 new F-box genes (MdFBX21 to 30) were isolated from the apple cultivar 'Spartan' (S(9)S(10)). MdFBX21 to 23 and MdFBX24 to 30 were completely linked to the S(9) -RNase and S(10-)RNase, respectively, and showed pollen-specific expression and S-haplotype-specific polymorphisms. Therefore, these 10 F-box genes are good candidates for the pollen determinant of self-incompatibility in apple. Phylogenetic analysis and comparison of deduced amino acid sequences of MdFBX21 to 30 with those of 25 S-linked F-box genes previously isolated from apple showed that a deduced amino acid identity of greater than 88.0 % can be used as the tentative criterion to classify F-box genes into one type. Using this criterion, 31 of 35 F-box genes of apple were classified into 11 types (SFBB1-11). All types included F-box genes derived from S(3-) and S(9-)haplotypes, and seven types included F-box genes derived from S(3-), S(9-), and S(10-)haplotypes. Moreover, comparison of nucleotide sequences of S-RNases and multiple F-box genes among S(3-), S(9-), and S(10-)haplotypes suggested that F-box genes within each type showed high nucleotide identity regardless of the identity of the S-RNase. The large number of F-box genes as candidates for the pollen determinant and the high degree of conservation within each type are consistent with the collaborative non-self-recognition model reported for Petunia. These findings support that the collaborative non-self-recognition system also exists in apple.

Molecular characterization and expression analysis of S1 self-incompatibility locus-linked pollen 3.15 gene in Citrus reticulata

Gametophytic self-incompatibility (GSI) is controlled by a highly polymorphic locus called the S-locus, which is an important factor that can result in seedless fruit in Citrus. The S1 self-incompatibility locus-linked pollen 3.15 gene (S1-3.15 ) belongs to a type of S locus gene. The role of S1-3.15 in the SI reaction of Citrus has not yet been reported. In this study, full-length sequences of cDNA and DNA encoding the S1-3.15 gene, referred to as CrS1-3.15 , were isolated from 'Wuzishatangju' (Self-incompatibility, SI) and 'Shatangju' (Self-compatibility, SC). The predicted amino acid sequences of CrS1-3.15 between 'Wuzishatangju' and 'Shatangju' differ by only three amino acids. Compared to 'Wuzishatangju', three bases were substituted in the genomic DNA of CrS1-3.15 from 'Shatangju'. Southern blot results showed that one copy of CrS1-3.15 existed in the genomic DNA of both 'Wuzishatangju' and 'Shatangju'. The expression level of the CrS1-3.15 gene in the ovaries of 'Shatangju' was approximately 60-fold higher than that in the ovaries of 'Wuzishatangju'. When 'Wuzishatangju' was cross-pollinated, the expression of CrS1-3.15 was upregulated in the ovaries at 3 d, and the highest expression levels were detected in the ovaries at 6 d after cross-pollination of 'Wuzishatangju' × 'Shatangju'. To obtain the CrS1-3.15 protein, the full-length cDNA of CrS1-3.15 genes from 'Wuzishatangju' and 'Shatangju' was successfully expressed in Pichia pastoris. Pollen germination frequency of 'Wuzishatangju' was inhibited significantly with increasing CrS1-3.15 protein concentrations from SI 'Wuzishatangju'.

Developmental onset of reproductive barriers and associated proteome changes in stigma/styles of Solanum pennellii

Although self-incompatibility (SI) in plants has been studied extensively, far less is known about interspecific reproductive barriers. One interspecific barrier, known as unilateral incongruity or incompatibility (UI), occurs when species display unidirectional compatibility in interspecific crosses. In the wild tomato species Solanum pennellii, both SI and self-compatible (SC) populations express UI when crossed with domesticated tomato, offering a useful model system to dissect the molecular mechanisms involved in reproductive barriers. In this study, the timing of reproductive barrier establishment during pistil development was determined in SI and SC accessions of S. pennellii using a semi-in vivo system to track pollen-tube growth in developing styles. Both SI and UI barriers were absent in styles 5 days prior to flower opening, but were established by 2 days before flower opening, with partial barriers detected during a transition period 3-4 days before flower opening. The developmental expression dynamics of known SI factors, S-RNases and HT proteins, was also examined. The accumulation of HT-A protein coincided temporally and spatially with UI barriers in developing pistils. Proteomic analysis of stigma/styles from key developmental stages showed a switch in protein profiles from cell-division-associated proteins in immature stigma/styles to a set of proteins in mature stigma/styles that included S-RNases, HT-A protein and proteins associated with cell-wall loosening and defense responses, which could be involved in pollen-pistil interactions. Other prominent proteins in mature stigma/styles were those involved in lipid metabolism, consistent with the accumulation of lipid-rich material during pistil maturation.

Thiol-based redox regulation in sexual plant reproduction: new insights and perspectives

The success of sexual reproduction in plants involves (i) the proper formation of the plant gametophytes (pollen and embryo sac) containing the gametes, (ii) the accomplishment of specific interactions between pollen grains and the stigma, which subsequently lead to (iii) the fusion of the gametes and eventually to (iv) the seed setting. Owing to the lack of mobility, plants have developed specific regulatory mechanisms to control all developmental events underlying the sexual plant reproduction according to environmental challenges. Over the last decade, redox regulation and signaling have come into sight as crucial mechanisms able to manage critical stages during sexual plant reproduction. This regulation involves a complex redox network which includes reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione and other classic buffer molecules or antioxidant proteins, and some thiol/disulphide-containing proteins belonging to the thioredoxin superfamily, like glutaredoxins (GRXs) or thioredoxins (TRXs). These proteins participate as critical elements not only in the switch between the mitotic to the meiotic cycle but also at further developmental stages of microsporogenesis. They are also implicated in the regulation of pollen rejection as the result of self-incompatibility. In addition, they display precise space-temporal patterns of expression and are present in specific localizations like the stigmatic papillae or the mature pollen, although their functions and subcellular localizations are not clear yet. In this review we summarize insights and perspectives about the presence of thiol/disulphide-containing proteins in plant reproduction, taking into account the general context of the cell redox network.

Histochemical location of key enzyme activities involved in receptivity and self-incompatibility in the olive tree (Olea europaea L.)

Stigma-surface and style enzymes are important for pollen reception, selection and germination. This report deals with the histochemical location of the activity of four basic types of enzyme involved in these processes in the olive (Olea europaea L.). The detection of peroxidase, esterase and acid-phosphatase activities at the surface of the stigma provided evidence of early receptivity in olive pistils. The stigma maintained its receptivity until the arrival of pollen. Acid-phosphatase activity appeared in the style at the moment of anthesis and continued until the fertilization of the ovule. RNase activity was detected in the extracellular matrix of the styles of flowers just before pollination and became especially evident in pistils after self-pollination. This activity gradually decreased until it practically disappeared in more advanced stages. RNase activity was also detected in pollen tubes growing in pollinated pistils and appeared after in vitro germination in the presence of self-incompatible pistils. These findings suggest that RNases may well be involved in intraspecific pollen rejection in olive flowers. To the best of our knowledge this is the first time that evidence of enzyme activity in stigma receptivity and pollen selection has been described in this species.

Self-sterility in flowering plants: preventing self-fertilization increases family diversification rates

BACKGROUND AND SCOPE

New data are presented on the distribution and frequency of self-sterility (SS) - predominantly pre-zygotic self-incompatibility (SI) systems - in flowering plants and the hypothesis is tested that families with self-sterile taxa have higher net diversification rates (DRs) than those with exclusively self-compatible taxa using both absolute and relative rate tests.

KEY RESULTS

Three major forms of SI systems (where pollen is rejected at the stigmatic, stylar or ovarian interface) are found to occur in the oldest families of flowering plants, with times of divergence >100 million years before the present (mybp), while post-fertilization SS and heterostyly appear in families with crown ages of 81 and 87 mybp, respectively. It is also founnd that many (22) angiosperm families exhibit >1 SI phenotype and that the distribution of different types of SS does not show strong phylogenetic clustering, collectively suggesting that SS and SI systems have evolved repeatedly de novo in angiosperm history. Families bearing self-sterile taxa have higher absolute DRs using all available calibrations of the angiosperm tree, and this affect is caused mostly by the high DR of families with homomorphic SI systems (in particular stigmatic SI) or those in which multiple SS/SI phenotypes have been observed (polymorphic). Lastly, using sister comparisons, it is further demonstrated that in 29 of 38 sister pairs (including 95 families), the self-sterile sister group had higher species richness and DR than its self-compatible sister based on either the total number of taxa in the clade with SS or only the estimated fraction to harbour SS based on literature surveys.

CONCLUSIONS

Collectively, these analyses point to the importance of SS, particularly pre-zygotic SI in the evolution of flowering plants.

The post-pollination ethylene burst and the continuation of floral advertisement are harbingers of non-random mate selection in Nicotiana attenuata

The self-compatible plant Nicotiana attenuata grows in genetically diverse populations after fires, and produces flowers that remain open for 3 days and are visited by assorted pollinators. To determine whether and when post-pollination non-random mate selection occurs among self and non-self pollen, seed paternity and semi-in vivo pollen tube growth were determined in controlled single/mixed pollinations. Despite all pollen sources being equally proficient in siring seeds in single-genotype pollinations, self pollen was consistently selected in mixed pollinations, irrespective of maternal genotype. However, clear patterns of mate discrimination occurred amongst non-self pollen when mixed pollinations were performed soon after corollas open, including selection against hygromycin B resistance (transformation selectable marker) in wild-type styles and for it in transformed styles. However, mate choice among pollen genotypes was completely shut down in plants transformed to be unable to produce (irACO) or perceive (ETR1) ethylene. The post-pollination ethylene burst, which originates primarily from the stigma and upper style, was strongly correlated with mate selection in single and mixed hand-pollinations using eight pollen donors in two maternal ecotypes. The post-pollination ethylene burst was also negatively correlated with the continuation of emission of benzylacetone, the most abundant pollinator-attracting corolla-derived floral volatile. We conclude that ethylene signaling plays a pivotal role in mate choice, and the post-pollination ethylene burst and the termination of benzylacetone release are accurate predictors, both qualitatively and quantitatively, of pre-zygotic mate selection and seed paternity.

A time course of GFP expression and mRNA stability in pollen tubes following compatible and incompatible pollinations in Solanum chacoense

The self-incompatibility (SI) reaction in the Solanaceae involves molecular recognition of stylar haplotypes by pollen and is mediated by the S-locus from which a stylar-localized S-RNase and several pollen-localized F-box proteins are expressed. S-RNase activity has been previously shown to be essential for the SI reaction, leading to the hypothesis that pollen rejection in incompatible crosses is due to degradation of pollen RNA. We used pollen expressing the fluorescent marker GFP, driven by the LAT52 promoter, to monitor the accumulation of mRNA and protein in pollen after compatible and incompatible pollinations. We find that GFP mRNA and protein gradually accumulate in pollen tubes until at least 18-h post-pollination and, up to this time, are only slightly more abundant in compatible compared with incompatible crosses. However, between 18- and 24-h post-pollination, pollen tube GFP mRNA and protein levels show a dramatic increase in compatible crosses and either remain constant or decrease in incompatible crosses. In contrast to these molecular correlates, the growth rates of compatible and incompatible pollen tubes begin to differ after 6-h post-pollination. We interpret the changes in growth rate at 6-h post-pollination as the previously described transition from autotrophic to heterotrophic growth. Thus, while pollen rejection is generally considered to result from the cytotoxic effects of S-RNase activity, this time course reveals that a difference in the growth rate of compatible and incompatible pollen appears prior to any marked effects on at least some types of pollen RNA.

The pollen-specific R-SNARE/longin PiVAMP726 mediates fusion of endo- and exocytic compartments in pollen tube tip growth

The growing pollen tube apex is dedicated to balancing exo- and endocytic processes to form a rapidly extending tube. As perturbation of either tends to cause a morphological phenotype, this system provides tractable model for studying these processes. Vesicle-associated membrane protein 7s (VAMP7s) are members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family that mediate cognate membrane fusion but their role in pollen tube growth has not been investigated. This manuscript identifies PiVAMP726 of Petunia inflata as a pollen-specific VAMP7 that localizes to the inverted cone of transport vesicles at the pollen tube tip. The endocytic marker FM4-64 was found to colocalize with yellow fluorescent protein (YFP)-PiVAMP726, which is consistent with PiVAMP726 containing an amino-acid motif implicated in endosomal localization, At high overexpression levels, YFP- PiVAMP726 inhibited growth and caused the formation of novel membrane compartments within the pollen tube tip. Functional dissection of PiVAMP726 implicated the N-terminal longin domain in negative regulation of the SNARE activity, but not localization of PiVAMP726. Expression of the constitutively active C-terminal SNARE domain alone, in pollen tubes, generated similar phenotypes to the full-length protein, but the truncated domain was more potent than the wild-type protein at both inhibiting growth and forming the novel membrane compartments. Both endo- and exocytic markers localized to these compartments in addition to YFP-PiVAMP726, leading to the speculation that PiVAMP726 might be involved in the recycling of endocytic vesicles in tip growth.

Polyamines and transglutaminase activity are involved in compatible and self-incompatible pollination of Citrus grandis

Pollination of pummelo (Citrus grandis L. Osbeck) pistils has been studied in planta by adding compatible and self-incompatible (SI) pollen to the stigma surface. The pollen germination has been monitored inside the pistil by fluorescent microscopy showing SI altered morphologies with irregular depositions of callose in the tube walls, and heavy callose depositions in enlarged tips. The polyamine (PA) content as free, perchloric acid (PCA)-soluble and -insoluble fractions and transglutaminase (TGase) activity have been analyzed in order to deepen their possible involvement in the progamic phase of plant reproduction. The conjugated PAs in PCA-soluble fraction were definitely higher than the free and the PCA-insoluble forms, in both compatible and SI pollinated pistils. In pistils, pollination caused an early decrease of free PAs and increase of the bound forms. The SI pollination, showed highest values of PCA-soluble and -insoluble PAs with a maximum in concomitance with the pollen tube arrest. As TGase mediates some of the effects of PAs by covalently binding them to proteins, its activity, never checked before in Citrus, was examined with two different assays. In addition, the presence of glutamyl-PAs confirmed the enzyme assay data and excluded the possibility of a misinterpretation. The SI pollination caused an increase in TGase activity, whereas the compatible pollination caused its decrease. Similarly to bound PAs, the glutamyl-PAs and the enzyme activity peaked in the SI pollinated pistils in concomitance with the observed block of the pollen tube growth, suggesting an involvement of TGase in SI response.

Organisation and regulation of the cytoskeleton in plant programmed cell death

Programmed cell death (PCD) involves precise integration of cellular responses to extracellular and intracellular signals during both stress and development. In recent years much progress in our understanding of the components involved in PCD in plants has been made. Signalling to PCD results in major reorganisation of cellular components. The plant cytoskeleton is known to play a major role in cellular organisation, and reorganization and alterations in its dynamics is a well known consequence of signalling. There are considerable data that the plant cytoskeleton is reorganised in response to PCD, with remodelling of both microtubules and microfilaments taking place. In the majority of cases, the microtubule network depolymerises, whereas remodelling of microfilaments can follow two scenarios, either being depolymerised and then forming stable foci, or forming distinct bundles and then depolymerising. Evidence is accumulating that demonstrate that these cytoskeletal alterations are not just a consequence of signals mediating PCD, but that they also may have an active role in the initiation and regulation of PCD. Here we review key data from higher plant model systems on the roles of the actin filaments and microtubules during PCD and discuss proteins potentially implicated in regulating these alterations.

Expression and trans-specific polymorphism of self-incompatibility RNases in coffea (Rubiaceae)

Self-incompatibility (SI) is widespread in the angiosperms, but identifying the biochemical components of SI mechanisms has proven to be difficult in most lineages. Coffea (coffee; Rubiaceae) is a genus of old-world tropical understory trees in which the vast majority of diploid species utilize a mechanism of gametophytic self-incompatibility (GSI). The S-RNase GSI system was one of the first SI mechanisms to be biochemically characterized, and likely represents the ancestral Eudicot condition as evidenced by its functional characterization in both asterid (Solanaceae, Plantaginaceae) and rosid (Rosaceae) lineages. The S-RNase GSI mechanism employs the activity of class III RNase T2 proteins to terminate the growth of "self" pollen tubes. Here, we investigate the mechanism of Coffea GSI and specifically examine the potential for homology to S-RNase GSI by sequencing class III RNase T2 genes in populations of 14 African and Madagascan Coffea species and the closely related self-compatible species Psilanthus ebracteolatus. Phylogenetic analyses of these sequences aligned to a diverse sample of plant RNase T2 genes show that the Coffea genome contains at least three class III RNase T2 genes. Patterns of tissue-specific gene expression identify one of these RNase T2 genes as the putative Coffea S-RNase gene. We show that populations of SI Coffea are remarkably polymorphic for putative S-RNase alleles, and exhibit a persistent pattern of trans-specific polymorphism characteristic of all S-RNase genes previously isolated from GSI Eudicot lineages. We thus conclude that Coffea GSI is most likely homologous to the classic Eudicot S-RNase system, which was retained since the divergence of the Rubiaceae lineage from an ancient SI Eudicot ancestor, nearly 90 million years ago.

Related polymorphic F-box protein genes between haplotypes clustering in the BAC contig sequences around the S-RNase of Japanese pear

Most fruit trees in the Rosaceae exhibit self-incompatibility, which is controlled by the pistil S gene, encoding a ribonuclease (S-RNase), and the pollen S gene at the S-locus. The pollen S in Prunus is an F-box protein gene (SLF/SFB) located near the S-RNase, but it has not been identified in Pyrus and Malus. In the Japanese pear, various F-box protein genes (PpSFBB(-α-γ)) linked to the S-RNase are proposed as the pollen S candidate. Two bacterial artificial chromosome (BAC) contigs around the S-RNase genes of Japanese pear were constructed, and 649 kb around S(4)-RNase and 378 kb around S(2)-RNase were sequenced. Six and 10 pollen-specific F-box protein genes (designated as PpSFBB(4-u1-u4, 4-d1-d2) and PpSFBB(2-u1-u5,) (2-d1-d5), respectively) were found, but PpSFBB(4-α-γ) and PpSFBB(2-γ) were absent. The PpSFBB(4) genes showed 66.2-93.1% amino acid identity with the PpSFBB(2) genes, which indicated clustering of related polymorphic F-box protein genes between haplotypes near the S-RNase of the Japanese pear. Phylogenetic analysis classified 36 F-box protein genes of Pyrus and Malus into two major groups (I and II), and also generated gene pairs of PpSFBB genes and PpSFBB/Malus F-box protein genes. Group I consisted of gene pairs with 76.3-94.9% identity, while group II consisted of gene pairs with higher identities (>92%) than group I. This grouping suggests that less polymorphic PpSFBB genes in group II are non-S pollen genes and that the pollen S candidates are included in the group I PpSFBB genes.

Receptor-like kinases as surface regulators for RAC/ROP-mediated pollen tube growth and interaction with the pistil

BACKGROUND

RAC/ROPs are RHO-type GTPases and are known to play diverse signalling roles in plants. Cytoplasmic RAC/ROPs are recruited to the cell membrane and activated in response to extracellular signals perceived and mediated by cell surface-located signalling assemblies, transducing the signals to regulate cellular processes. More than any other cell types in plants, pollen tubes depend on continuous interactions with an extracellular environment produced by their surrounding tissues as they grow within the female organ pistil to deliver sperm to the female gametophyte for fertilization.

SCOPE

We review studies on pollen tube growth that provide compelling evidence indicating that RAC/ROPs are crucial for regulating the cellular processes that underlie the polarized cell growth process. Efforts to identify cell surface regulators that mediate extracellular signals also point to RAC/ROPs being the molecular switches targeted by growth-regulating female factors for modulation to mediate pollination and fertilization. We discuss a large volume of work spanning more than two decades on a family of pollen-specific receptor kinases and some recent studies on members of the FERONIA family of receptor-like kinases (RLKs).

SIGNIFICANCE

The research described shows the crucial roles that two RLK families play in transducing signals from growth regulatory factors to the RAC/ROP switch at the pollen tube apex to mediate and target pollen tube growth to the female gametophyte and signal its disintegration to achieve fertilization once inside the female chamber.

S-RNase disrupts tip-localized reactive oxygen species and induces nuclear DNA degradation in incompatible pollen tubes of Pyrus pyrifolia

Pear (Pyrus pyrifolia L.) has an S-RNase-based gametophytic self-incompatibility (SI) mechanism, and S-RNase has also been implicated in the rejection of self-pollen and genetically identical pollen. However, RNA degradation might be only the beginning of the SI response, not the end. Recent in vitro studies suggest that S-RNase triggers mitochondrial alteration and DNA degradation in the incompatible pollen tube of Pyrus pyrifolia, and it seems that a relationship exists between self S-RNase, actin depolymerization and DNA degradation. To further uncover the SI response in pear, the relationship between self S-RNase and tip-localized reactive oxygen species (ROS) was evaluated. Our results show that S-RNase specifically disrupted tip-localized ROS of incompatible pollen tubes via arrest of ROS formation in mitochondria and cell walls. The mitochondrial ROS disruption was related to mitochondrial alteration, whereas cell wall ROS disruption was related to a decrease in NADPH. Tip-localized ROS disruption not only decreased the Ca(2+) current and depolymerized the actin cytoskeleton, but it also induced nuclear DNA degradation. These results indicate that tip-localized ROS disruption occurs in Pyrus pyrifolia SI. Importantly, we demonstrated nuclear DNA degradation in the incompatible pollen tube after pollination in vivo. This result validates our in vitro system in vivo.

[Control of fertilization by self-incompatibility mechanisms]

Flowering plants (angiosperms) are the most prevalent and evolutionarily advanced group of plants. Reproductive strategies that promote cross-fertilization have played an essential role in the success of angiosperms as they contribute to genetic variability among plant species. A major genetic barrier to self-fertilization is self-incompatibility (SI), which allows female reproductive cells to discriminate between self- and non-self pollen and specifically reject self-pollen. In this review, we describe three SI mechanisms showing that different flowering plant families use distinct molecules for recognition of self as well as diverse biochemical pathways to arrest pollen tube growth.

Two neutral variants segregating at the gametophytic self-incompatibility locus of European pear (Pyrus communis L.) (Rosaceae, Pyrinae)

Extensive survey of the S-locus diversity of plant species with RNase-based gametophytic self-incompatibility has failed to identify neutral variation segregating within S-allele specificities. Although this is the expected result according to population genetics theory, it conflicts with recent models of S-allele evolution, which suggest that new specificities might arise by a continuous process of subtle changes that individually do not alter the specificity of the S-genes, but whose cumulative effects result in new S-allele functions. Genomic analysis of S-RNase sequences associated with the S(104) (=S(4), =S(b)) allele of European pear (Pyrus communis L.) cultivars yielded two distinct variants (named herein S(104-1) and S(104-2)) that differed at five nucleotide positions within the open reading frame, two of which resulted in changes in the predicted protein sequence. Test-cross experiments indicated that the S-alleles associated with the S(104-1) and S(104-2)RNases exhibit the same pollen and pistil functions, suggesting that they are two neutral variants segregating within the S(104) haplotype of European pear. These allelic forms might represent transitional states in the process of generating new specificities in the species, in accordance with models that predict S-function transition through neutral intermediates. This possibility was further evaluated through the pattern of molecular evolution of functionally distinct European pear S-RNases, which indicated that most recent S-allele diversification in this species proceeded in the absence of adaptive selective pressure.

Self-compatibility in 'Cristobalina' sweet cherry is not associated with duplications or modified transcription levels of S-locus genes

Sweet cherry shows S-RNase-based gametophytic self-incompatibility, which prevents self- and cross-fertilization between genetically related individuals. The specificity of the self-incompatible reaction is determined by two genes located in the S-locus. These encode a pistil-expressed ribonuclease (S-RNase) that inhibits self pollen tube growth, and a pollen-expressed F-box protein (SFB) that may be involved in the cytotoxicity of self-S-RNases. Initial genetic and pollination studies in a self-compatible sweet cherry cultivar, 'Cristobalina' (S (3) S (6)), showed that self-compatibility was caused by the loss of pollen function of both haplotypes (S (3) and S (6)). In this study, we further characterize self-compatibility in this genotype by molecular analysis of the S-locus. DNA blot analyses using S-RNase and SFB probes show no duplications of 'Cristobalina' S-locus genes or differences in the restriction patterns when compared with self-incompatible cultivars with the same S-genotype. Furthermore, reverse transcriptase-PCR of S-locus genes and quantitative reverse transcription-PCR of SFBs revealed no differences at the transcription level when compared with a self-incompatible genotype. The results of this study show that no differences at the S-locus can be correlated with self-compatibility, indicating the possible involvement of non-S-locus modifiers in self-incompatibility breakdown in this cultivar.

Sporophytic control of pollen tube growth and guidance in grasses

Pollen tube growth and guidance in the female tissues of flowering plants is a long-studied and anatomically well-described process. A large number of gene products and chemical compounds involved have been identified in the last 20 years, and some underlying molecular mechanisms including self-incompatibility in the Brassicaceae, Solanaceae and Papaveraceae are now well understood. However, the largest part of the pollen tube pathway inside the transmitting tract towards the ovule harbouring the female gametophyte still requires intensive investigations. Especially in the economically most import plant family, the Poaceae or grasses, progamic pollen tube development is barely understood. Using maize as a model, we propose to divide pollen tube germination, growth and guidance towards the female gametophyte into five distinct phases. The model is adapted from Arabidopsis thaliana, taking anatomical differences and novel genetic and cellular studies into consideration. With the exception of Phase V, all phases seem to be under sporophytic control in grasses.

Self-incompatibility in Papaver: identification of the pollen S-determinant PrpS

Many flowering plants are hermaphrodite, posing the problem of self-fertilization and the subsequent loss of the genetic fitness of the offspring. To prevent this, many plants have developed a genetically controlled mechanism called self-incompatibility (SI). When the male and female S-determinants match, self (incompatible) pollen is recognized and rejected before fertilization can occur. In poppy (Papaver rhoeas), the pistil S-determinant (PrsS) is a small secreted protein that interacts with incompatible pollen, initiating a Ca(2+)-dependent signalling network. SI triggers several downstream events, including depolymerization of the cytoskeleton, phosphorylation of two soluble inorganic pyrophosphatases and an MAPK (mitogen-activated protein kinase). This culminates in PCD (programmed cell death) involving several caspase-like activities. The recent discovery of the Papaver pollen S-determinant PrpS marks a significant step forward in the understanding of the Papaver SI system. PrpS encodes a ~20 kDa predicted transmembrane protein which has no homology with known proteins. It is specifically expressed in pollen, linked to the pistil S-determinant, and displays the high polymorphism expected of an S-locus determinant. The present review focuses on the discovery and characterization of PrpS which strongly support the hypothesis that Papaver SI is triggered by the interaction of PrsS and PrpS.

Petunia nectar proteins have ribonuclease activity

Plants requiring an insect pollinator often produce nectar as a reward for the pollinator's visitations. This rich secretion needs mechanisms to inhibit microbial growth. In Nicotiana spp. nectar, anti-microbial activity is due to the production of hydrogen peroxide. In a close relative, Petunia hybrida, limited production of hydrogen peroxide was found; yet petunia nectar still has anti-bacterial properties, suggesting that a different mechanism may exist for this inhibition. The nectar proteins of petunia plants were compared with those of ornamental tobacco and significant differences were found in protein profiles and function between these two closely related species. Among those proteins, RNase activities unique to petunia nectar were identified. The genes corresponding to four RNase T2 proteins from Petunia hybrida that show unique expression patterns in different plant tissues were cloned. Two of these enzymes, RNase Phy3 and RNase Phy4 are unique among the T2 family and contain characteristics similar to both S- and S-like RNases. Analysis of amino acid patterns suggest that these proteins are an intermediate between S- and S-like RNases, and support the hypothesis that S-RNases evolved from defence RNases expressed in floral parts. This is the first report of RNase activities in nectar.

Pollen-pistil interactions and the endomembrane system

The endomembrane system offers many potential points where plant mating can be effectively controlled. This results from two basic features of angiosperm reproduction: the requirement for pollen tubes to pass through sporophytic tissues to gain access to ovules and the physiology of pollen tube growth that provides it with the capacity to do so. Rapid pollen tube growth requires extravagant exocytosis and endocytosis activity as cell wall material is deposited and membrane is recovered from the actively growing tip. Moreover, recent results show that pollen tubes take up a great deal of material from the pistil extracellular matrix. Regarding the stigma and style as organs specialized for mate selection focuses attention on their complementary roles in secreting material to support the growth of compatible pollen tubes and discourage the growth of undesirable pollen. Since these processes also involve regulated activities of the endomembrane system, the potential for regulating mating by controlling endomembrane events exists in both pollen and pistil.