The influence of the pollination compatibility type on the pistil S-RNase expression in European pear (Pyrus communis).

The S-RNase gene plays an essential role in the gametophytic self-incompatibility (GSI) system of Pyrus. It codes for the stylar-expressed S-RNase protein which inhibits the growth of incompatible pollen tubes through cytotoxicity and the induction of programmed cell death in the pollen tube. While research on the Pyrus GSI system has primarily focused on the S-RNase gene, there is still a lack of insight into its spatiotemporal expression profile and the factors that regulate it. Previous studies have suggested that S-RNase expression in the style is influenced by pollination and is dependent on the compatibility type. We here continue on this basic hypothesis by analyzing the spatiotemporal expression of the S-RNase alleles in Pyrus communis "Conference" styles in response to different types of pollination; namely, upon full- and semi-compatible pollination and upon incompatible selfing. The results revealed that temporal dynamics of S-RNase expression are influenced by the pollen's compatibility type, indicating the presence of a signaling mechanism between pollen and style to control S-RNase production during pollen tube growth. In our experiment, S-RNase expression continuously decreased after cross-pollination and in the unpollinated control. However, after a fully incompatible pollination, S-RNase expression remained constant. Finally, semi-compatible pollination showed a initially constant S-RNase expression for both alleles followed by a strong decrease in expression. Based on these results and previous findings, we propose a regulatory mechanism to explain the effect of pollination and the associated compatibility type on S-RNase expression in the style. This proposed mechanism could be used as a starting point for future research.

Phenotyping, genetics, and "-omics" approaches to unravel and introgress enhanced resistance against apple scab (Venturia inaequalis) in apple cultivars (Malus × domestica).

Apple scab disease, caused by the fungus Venturia inaequalis, endangers commercial apple production globally. It is predominantly managed by frequent fungicide sprays that can harm the environment and promote the development of fungicide-resistant strains. Cultivation of scab-resistant cultivars harboring diverse qualitative Rvi resistance loci and quantitative trait loci associated with scab resistance could reduce the chemical footprint. A comprehensive understanding of the host-pathogen interaction is, however, needed to efficiently breed cultivars with enhanced resistance against a variety of pathogenic strains. Breeding efforts should not only encompass pyramiding of Rvi loci and their corresponding resistance alleles that directly or indirectly recognize pathogen effectors, but should also integrate genes that contribute to effective downstream defense mechanisms. This review provides an overview of the phenotypic and genetic aspects of apple scab resistance, and currently known corresponding defense mechanisms. Implementation of recent "-omics" approaches has provided insights into the complex network of physiological, molecular, and signaling processes that occur before and upon scab infection, thereby revealing the importance of both constitutive and induced defense mechanisms. Based on the current knowledge, we outline advances toward more efficient introgression of enhanced scab resistance into novel apple cultivars by conventional breeding or genetic modification techniques. However, additional studies integrating different "-omics" approaches combined with functional studies will be necessary to unravel effective defense mechanisms as well as key regulatory genes underpinning scab resistance in apple. This crucial information will set the stage for successful knowledge-based breeding for enhanced scab resistance.

Mind the (CRISPR) gaps: The European Commission's proposal for the use of NGTs in the EU.

The proposal by the European Commission to regulate New Genome Technique (NGT) plants is a leap forward, but it does not revise the current legislation on GMOs and includes many inconsistencies that may hinder the adoption of specific NGTs.

Asynapsis and meiotic restitution in tomato male meiosis induced by heat stress.

Susceptibility of the reproductive system to temperature fluctuations is a recurrent problem for crop production under a changing climate. The damage is complex as multiple processes in male and female gamete formation are affected, but in general, particularly pollen production is impaired. Here, the impact of short periods of elevated temperature on male meiosis of tomato (Solanum lycopersicon L.) is reported. Meiocytes in early stage flower buds exposed to heat stress (>35°C) exhibit impaired homolog synapsis resulting in partial to complete omission of chiasmata formation. In the absence of chiasmata, univalents segregate randomly developing unbalanced tetrads and polyads resulting in aneuploid spores. However, most heat-stressed meiotic buds primarily contain balanced dyads, indicating a propensity to execute meiotic restitution. With most meiocytes exhibiting a complete loss of chiasma formation and concomitantly showing a mitotic-like division, heat stress triggers first division restitution resulting in clonal spores. These findings corroborate with the plasticity of male meiosis under heat and establish a natural route for the induction of sexual polyploidization in plants and the engineering of clonal seed.

Structural regulation and dynamic behaviour of organelles during plant meiosis.

Eukaryotes use various mechanisms to maintain cell division stability during sporogenesis, and in particular during meiosis to achieve production of haploid spores. In addition to establishing even chromosome segregation in meiosis I and II, it is crucial for meiotic cells to guarantee balanced partitioning of organelles to the daughter cells, to properly inherit cellular functions. In plants, cytological studies in model systems have yielded insights into the meiotic behaviour of different organelles, i.e., clearly revealing a distinct organization at different stages throughout meiosis indicating for an active regulatory mechanism determining their subcellular dynamics. However, how, and why plant meiocytes organize synchronicity of these elements and whether this is conserved across all plant genera is still not fully elucidated. It is generally accepted that the highly programmed intracellular behaviour of organelles during meiosis serves to guarantee balanced cytoplasmic inheritance. However, recent studies also indicate that it contributes to the regulation of key meiotic processes, like the organization of cell polarity and spindle orientation, thus exhibiting different functionalities than those characterized in mitotic cell division. In this review paper, we will outline the current knowledge on organelle dynamics in plant meiosis and discuss the putative strategies that the plant cell uses to mediate this programmed spatio-temporal organization in order to safeguard balanced separation of organelles. Particular attention is thereby given to putative molecular mechanisms that underlie this dynamic organelle organization taken into account existing variations in the meiotic cell division program across different plant types. Furthermore, we will elaborate on the structural role of organelles in plant meiosis and discuss on organelle-based cellular mechanisms that contribute to the organization and molecular coordination of key meiotic processes, including spindle positioning, chromosome segregation and cell division. Overall, this review summarizes all relevant insights on the dynamic behaviour and inheritance of organelles during plant meiosis, and discusses on their functional role in the structural and molecular regulation of meiotic cell division.

A semi in vivo pollination technique to assess the level of gametophytic self-incompatibility and pollen tube growth in pear (Pyrus communis L.).

KEY MESSAGE: We describe a semi in vivo pollination technique to determine the compatibility relation between different pear cultivars. This assay provides a valuable addition to existing tools in GSI research. The gametophytic self-incompatibility (GSI) system in Pyrus inhibits fertilization by pollen that shares one of the two S-alleles of the style. Depending on their S-locus genotype, two pear cultivars therefore either show a cross-compatible, semi-compatible or incompatible interaction. Because GSI greatly influences seed and fruit set, accurate knowledge of the compatibility type of a cultivar is key for both pear fruit production and breeding. Currently, compatibility relations between different pear cultivars are generally assessed via S-genotyping. However, this approach is restricted to the currently known S-alleles in pear, and does not provide functional assessment of the level of (self-)incompatibility. We here present an optimized semi in vivo pollination assay, that enables quantitative analysis of (self-)incompatibility in pear, and that can also serve useful for more fundamental studies on pollen tube development and pollen-style interactions. This assay involves in vitro incubation of cut pollinated styles followed by microscopic counting of emerging pollen tubes at a specific time interval. The validity and selectivity of this method to determine compatibility interactions in pear is demonstrated in the cultivars "Celina" and "Packham's Triumph." Overall, this technique constitutes a valuable tool for quantitatively determining in vivo pollen tube growth and (cross-)compatibility in pear.

A Hypomorphic Mutant of PHD Domain Protein Male Meiocytes Death 1.

Meiosis drives reciprocal genetic exchanges and produces gametes with halved chromosome number, which is important for the genetic diversity, plant viability, and ploidy consistency of flowering plants. Alterations in chromosome dynamics and/or cytokinesis during meiosis may lead to meiotic restitution and the formation of unreduced microspores. In this study, we isolated an Arabidopsis mutant male meiotic restitution 1 (mmr1), which produces a small subpopulation of diploid or polyploid pollen grains. Cytological analysis revealed that mmr1 produces dyads, triads, and monads indicative of male meiotic restitution. Both homologous chromosomes and sister chromatids in mmr1 are separated normally, but chromosome condensation at metaphase I is slightly affected. The mmr1 mutant displayed incomplete meiotic cytokinesis. Supportively, immunostaining of the microtubular cytoskeleton showed that the spindle organization at anaphase II and mini-phragmoplast formation at telophase II are aberrant. The causative mutation in mmr1 was mapped to chromosome 1 at the chromatin regulator Male Meiocyte Death 1 (MMD1/DUET) locus. mmr1 contains a C-to-T transition at the third exon of MMD1/DUET at the genomic position 2168 bp from the start codon, which causes an amino acid change G618D that locates in the conserved PHD-finger domain of histone binding proteins. The F1 progenies of mmr1 crossing with knockout mmd1/duet mutant exhibited same meiotic defects and similar meiotic restitution rate as mmr1. Taken together, we here report a hypomorphic mmd1/duet allele that typically shows defects in microtubule organization and cytokinesis.

The Role of Chromatid Interference in Determining Meiotic Crossover Patterns.

Plants, like all sexually reproducing organisms, create genetic variability by reshuffling parental alleles during meiosis. Patterns of genetic variation in the resulting gametes are determined by the independent assortment of chromosomes in meiosis I and by the number and positioning of crossover (CO) events during meiotic recombination. On the chromosome level, spatial distribution of CO events is biased by multiple regulatory mechanisms, such as CO assurance, interference and homeostasis. However, little is known about how multiple COs are distributed among the four chromatids of a bivalent. Chromatid interference (CI) has been proposed as a regulatory mechanism that biases distribution of multiple COs toward specific chromatid partners, however, its existence has not been well-studied and its putative mechanistic basis remains undescribed. Here, we introduce a novel method to quantitatively express CI, and take advantage of available tetrad-based genotyping data from Arabidopsis and maize male meiosis to quantify CI effects on a genome-wide and chromosomal scale. Overall, our analyses reveal random involvement of sister chromatids in double CO events across paired chromosomes, indicating an absence of CI. However, on a genome-wide level, CI was found to vary with physical distance between COs, albeit with different effects in Arabidopsis and maize. While effects of CI are minor in Arabidopsis and maize, the novel methodology introduced here enables quantitative interpretation of CI both on a local and genome-wide scale, and thus provides a key tool to study CI with relevance for both plant genetics and crop breeding.

Effect of Whole-Genome Duplication on the Evolutionary Rescue of Sterile Hybrid Monkeyflowers.

Hybridization is a creative evolutionary force, increasing genomic diversity and facilitating adaptation and even speciation. Hybrids often face significant challenges to establishment, including reduced fertility that arises from genomic incompatibilities between their parents. Whole-genome duplication in hybrids (allopolyploidy) can restore fertility, cause immediate phenotypic changes, and generate reproductive isolation. Yet the survival of polyploid lineages is uncertain, and few studies have compared the performance of recently formed allopolyploids and their parents under field conditions. Here, we use natural and synthetically produced hybrid and polyploid monkeyflowers (Mimulus spp.) to study how polyploidy contributes to the fertility, reproductive isolation, phenotype, and performance of hybrids in the field. We find that polyploidization restores fertility and that allopolyploids are reproductively isolated from their parents. The phenotype of allopolyploids displays the classic gigas effect of whole-genome duplication, in which plants have larger organs and are slower to flower. Field experiments indicate that survival of synthetic hybrids before and after polyploidization is intermediate between that of the parents, whereas natural hybrids have higher survival than all other taxa. We conclude that hybridization and polyploidy can act as sources of genomic novelty, but adaptive evolution is key in mediating the establishment of young allopolyploid lineages.

Sweet Immunity: The Effect of Exogenous Fructans on the Susceptibility of Apple (Malus × domestica Borkh.) to Venturia inaequalis.

There is an urgent need for novel, efficient and environmentally friendly strategies to control apple scab (Venturia inaequalis), for the purpose of reducing overall pesticide use. Fructans are recently emerging as promising "priming" compounds, standing out for their safety and low production costs. The objective of this work was to test a fructan-triggered defense in the leaves of apple seedlings. It was demonstrated that exogenous leaf spraying can reduce the development of apple scab disease symptoms. When evaluated macroscopically and by V. inaequalis-specific qPCR, levan-treated leaves showed a significant reduction of sporulation and V. inaequalis DNA in comparison to mock- and inulin-treated leaves, comparable to the levels in fosetyl-aluminum-treated leaves. Furthermore, we observed a significant reduction of in vitro mycelial growth of V. inaequalis on plates supplemented with levans when compared to controls, indicating a direct inhibition of fungal growth. Variations in endogenous sugar contents in the leaves were followed during priming and subsequent infection, revealing complex dynamics as a function of time and leaf ontogeny. Our data are discussed in view of the present theories on sugar signaling and fructan-based immunity, identifying areas for future research and highlighting the potential use of fructans in apple scab management in orchards.

High temperatures alter cross-over distribution and induce male meiotic restitution in Arabidopsis thaliana.

Plant fertility is highly sensitive to elevated temperature. Here, we report that hot spells induce the formation of dyads and triads by disrupting the biogenesis or stability of the radial microtubule arrays (RMAs) at telophase II. Heat-induced meiotic restitution in Arabidopsis is predominantly SDR-type (Second Division Restitution) indicating specific interference with RMAs formed between separated sister chromatids. In addition, elevated temperatures caused distinct deviations in cross-over formation in male meiosis. Synapsis at pachytene was impaired and the obligate cross-over per chromosome was discarded, resulting in partial univalency in meiosis I (MI). At diakinesis, interconnections between non-homologous chromosomes tied separate bivalents together, suggesting heat induces ectopic events of non-homologous recombination. Summarized, heat interferes with male meiotic cross-over designation and cell wall formation, providing a mechanistic basis for plant karyotype change and genome evolution under high temperature conditions.

Directionality of Plasmodesmata-Mediated Transport in Arabidopsis Leaves Supports Auxin Channeling.

The plant hormone auxin serves as central regulator of growth and development. Auxin transporters in the plasma membrane are assumed to define tissue-level patterns of auxin distribution [1, 2]. However, auxin is small enough to diffuse through the plasmodesmata that connect neighboring cells [3], presenting an alternative pathway, whose contribution to auxin transport remained largely unexplored [4]. Here, photoactivation microscopy [5, 6] was used to measure the capacity for small-molecule diffusion in the epidermis of Arabidopsis thaliana leaves. In the elongated epidermis cells covering the midrib and petiole, the plasmodesmata-mediated cell-wall permeability was found to be several times higher in the longitudinal than in the transverse direction. The physiological relevance of this asymmetry was tested through quantification of the shade-avoidance response, which depends on auxin transport from the leaf tip to the petiole in the abaxial side of the leaf [7], with the hypothesis that directionality of diffusion supplements transporter-mediated auxin movement [8]. Triggering the response by auxin application at the tip led to stronger leaf movement in wild-type plants than in gsl8 mutants [9], which lack the callose synthase necessary to establish directionality. The results match the predictions of a mathematical model of auxin transport based on the permeabilities measured in wild-type and mutant plants. It is concluded that plasmodesmata permeability can be selectively modulated within a plant cell and that the conferred directionality in diffusion can influence the tissue-specific distribution patterns of small molecules, like auxin. VIDEO ABSTRACT.

Dynamics of ascorbic acid content in apple (Malus x domestica) during fruit development and storage.

Vitamin C is a crucial antioxidant and cofactor for both plants and humans. Apple fruits generally contain low levels of vitamin C, making vitamin C content an interesting trait for apple crop improvement. With the aim of breeding high vitamin C apple cultivars it is important to get an insight in the natural biodiversity of vitamin C content in apple fruits. In this study, quantification of ascorbic acid (AsA), dehydroascorbic acid (DHA), and total AsA (AsA + DHA) in apple pulp of 79 apple accessions at harvest revealed significant variation, indicating a large genetic biodiversity. High density genotyping using an 8 K SNP array identified 21 elite and 58 local cultivars in this germplasm, with local accessions showing similar levels of total AsA but higher amounts of DHA compared to elite varieties. Out of the 79 apple cultivars screened, ten genotypes with either the highest or the lowest concentration of total AsA at harvest were used for monitoring vitamin C dynamics during fruit development and storage. For all these cultivars, the AsA/DHA ratio in both apple pulp and peel increased throughout fruit development, whereas the AsA/DHA balance always shifted towards the oxidized form during storage and shelf life, putatively reflecting an abiotic stress response. Importantly, at any point during apple fruit development and storage, the apple peel contained a higher level of vitamin C compared to the pulp, most likely because of its direct exposure to abiotic and biotic stresses.

Induction and Characterization of Diploid Pollen Grains in Arabidopsis thaliana.

Polyploidization or whole genome duplication (WGD) is one of the main forces driving plant genome evolution and biodiversity with major implications for plant breeding and crop improvement. In nature, de novo formation of polyploid plant genomes most likely occurs through a modification of the sexual reproductive pathway. By interfering with reproductive genome stability, for example, via induction of meiotic restitution, diploid or polyploid gametes are ectopically formed that may participate in fertilization to yield polyploid offspring. This mechanism of WGD is generally referred to as sexual polyploidization. Considering the central role of sexual polyploidization in speciation, genome evolution and crop breeding, we provide here a set of methodologies to induce and characterize 2n pollen grain formation in plants. Using Arabidopsis thaliana as a model, we outline two different methods, that is, one chemical and one environmental, to induce male meiotic restitution and high frequency 2n pollen grain formation. In addition, we provide a set of simple and straightforward techniques to characterize alterations in male meiotic cell division and gametophytic ploidy stability underpinning 2n pollen formation. This comprehensive toolbox is applicable in a broad range of plant species to enable quick induction and assessment of 2n gamete formation during plant male reproduction.

Finding a Compatible Partner: Self-Incompatibility in European Pear (Pyrus communis); Molecular Control, Genetic Determination, and Impact on Fertilization and Fruit Set.

Pyrus species display a gametophytic self-incompatibility (GSI) system that actively prevents fertilization by self-pollen. The GSI mechanism in Pyrus is genetically controlled by a single locus, i.e., the S-locus, which includes at least two polymorphic and strongly linked S-determinant genes: a pistil-expressed S-RNase gene and a number of pollen-expressed SFBB genes (S-locus F-Box Brothers). Both the molecular basis of the SI mechanism and its functional expression have been widely studied in many Rosaceae fruit tree species with a particular focus on the characterization of the elusive SFBB genes and S-RNase alleles of economically important cultivars. Here, we discuss recent advances in the understanding of GSI in Pyrus and provide new insights into the mechanisms of GSI breakdown leading to self-fertilization and fruit set. Molecular analysis of S-genes in several self-compatible Pyrus cultivars has revealed mutations in both pistil- or pollen-specific parts that cause breakdown of self-incompatibility. This has significantly contributed to our understanding of the molecular and genetic mechanisms that underpin self-incompatibility. Moreover, the existence and development of self-compatible mutants open new perspectives for pear production and breeding. In this framework, possible consequences of self-fertilization on fruit set, development, and quality in pear are also reviewed.

Cold Influences Male Reproductive Development in Plants: A Hazard to Fertility, but a Window for Evolution.

Being sessile organisms, plants suffer from various abiotic stresses including low temperature. In particular, male reproductive development of plants is extremely sensitive to cold which may dramatically reduce viable pollen shed and plant fertility. Cold stress disrupts stamen development and prominently interferes with the tapetum, with the stress-responsive hormones ABA and gibberellic acid being greatly involved. In particular, low temperature stress delays and/or inhibits programmed cell death of the tapetal cells which consequently damages pollen development and causes male sterility. On the other hand, studies in Arabidopsis and crops have revealed that ectopically decreased temperature has an impact on recombination and cytokinesis during meiotic cell division, implying a putative role for temperature in manipulating plant genomic diversity and architecture during the evolution of plants. Here, we review the current understanding of the physiological impact of cold stress on the main male reproductive development processes including tapetum development, male meiosis and gametogenesis. Moreover, we provide insights into the genetic factors and signaling pathways that are involved, with putative mechanisms being discussed.

Polyploidy Affects Plant Growth and Alters Cell Wall Composition.

Polyploidization has played a key role in plant breeding and crop improvement. Although its potential to increase biomass yield is well described, the effect of polyploidization on biomass composition has largely remained unexplored. Here, we generated a series of Arabidopsis (Arabidopsis thaliana) plants with different somatic ploidy levels (2n, 4n, 6n, and 8n) and performed rigorous phenotypic characterization. Kinematic analysis showed that polyploids developed slower compared to diploids; however, tetra- and hexaploids, but not octaploids, generated larger rosettes due to delayed flowering. In addition, morphometric analysis of leaves showed that polyploidy affected epidermal pavement cells, with increased cell size and reduced cell number per leaf blade with incrementing ploidy. However, the inflorescence stem dry weight was highest in tetraploids. Cell wall characterization revealed that the basic somatic ploidy level negatively correlated with lignin and cellulose content, and positively correlated with matrix polysaccharide content (i.e. hemicellulose and pectin) in the stem tissue. In addition, higher ploidy plants displayed altered sugar composition. Such effects were linked to the delayed development of polyploids. Moreover, the changes in polyploid cell wall composition promoted saccharification yield. The results of this study indicate that induction of polyploidy is a promising breeding strategy to further tailor crops for biomass production.

PROTEIN PHOSHATASE 2A B'α and ß Maintain Centromeric Sister Chromatid Cohesion during Meiosis in Arabidopsis.

The correct separation of homologous chromosomes during meiosis I, and sister chromatids during meiosis II, relies on the tight control of the cohesion complex. The phosphorylation and subsequent cleavage of the meiotic recombination protein REC8 (REC8-like family protein [SYN1] in Arabidopsis [Arabidopsis thaliana]), the α-kleisin subunit of the cohesion ring, along the chromosome arms at meiosis I allows crossovers and separation of homologous chromosomes without chromatid dissociation. REC8 continues to localize and function at the centromeres up to metaphase II and, in yeast and vertebrates, is protected from cleavage by means of protein phosphatase 2A (PP2A)-mediated dephosphorylation. Here, we show that, in plants, centromeric sister chromatid cohesion until meiosis II also requires the activity of a PP2A-type phosphatase complex. The combined absence of the regulatory subunits PP2AB'α and PP2AB'ß leads to the premature loss of chromosome cohesion in meiosis I. Male meiocytes of the pp2ab'αß double mutant display premature depletion of SYN1. The PP2AA1 structural and B'α regulatory subunit localize specifically to centromeres until metaphase II, supporting a role for the PP2A complex in the SYN1-mediated maintenance of centromeric cohesion in plant meiosis.

Cold-Induced Male Meiotic Restitution in Arabidopsis thaliana Is Not Mediated by GA-DELLA Signaling.

Short periods of cold stress induce male meiotic restitution and diploid pollen formation in Arabidopsis thaliana by specifically interfering with male meiotic cytokinesis. Similar alterations in male meiotic cell division and gametophytic ploidy stability occur when gibberellic acid (GA) signaling is perturbed in developing anthers. In this study, we found that exogenous application of GA primarily induces second division restitution (SDR)-type pollen in Arabidopsis, similar to what cold does. Driven by the close similarity in cellular defects, we tested the hypothesis that cold-induced meiotic restitution is mediated by GA-DELLA signaling. Using a combination of chemical, genetic and cytological approaches, however, we found that both exogenously and endogenously altered GA signaling do not affect the cold sensitivity of male meiotic cytokinesis. Moreover, in vivo localization study using a GFP-tagged version of RGA protein revealed that cold does not affect the expression pattern and abundance of DELLA in Arabidopsis anthers at tetrad stage. Expression study found that transcript of RGA appears enhanced in cold-stressed young flower buds. Since our previous work demonstrated that loss of function of DELLA causes irregular male meiotic cytokinesis, we here conclude that cold-induced meiotic restitution is not mediated by DELLA-dependent GA signaling.