Interspecific Incompatibility Research Articles

Peterhof collection of rye and its use in genetic studies

The article provides information about the history and methods of development of Peterhof rye genetic collection, founded by V.S. Fedorov, Associate Professor of the Leningrad University. Isolation of self-compatible mutants, their crosses with self-incompatible rye plants, and subsequent self-pollination of hybrids allowed to reveal the allele diversity in heterogeneous and heterozygous rye varieties. In the course of genetic collection assembly the study of inheritance of qualitative and quantitative morphological traits, genetic control of self-compatibility, genetics of meiosis, genetics of interspecific incompatibility was performed. The corresponding genes were identified and, in most cases, mapped using isozymes and molecular markers. Fundamental research was introduced into practical breeding. Under the direction of V. S. Fedorov, and V. G. Smirnov the first in Russia tetraploid rye variety Leningradskaja Tetra was produced. Currently, based on the study of the genetics of self-fertility, the initial material is being obtained and used for improving rye population varieties. The possibility of using the genetic collection of rye to solve the fundamental problems of plant genetics is discussed.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

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.

Duplicated pollen-pistil recognition loci control intraspecific unilateral incompatibility in Brassica rapa.

In plants, cell-cell recognition is a crucial step in the selection of optimal pairs of gametes to achieve successful propagation of progeny. Flowering plants have evolved various genetic mechanisms, mediated by cell-cell recognition, to enable their pistils to reject self-pollen, thus preventing inbreeding and the consequent reduced fitness of progeny (self-incompatibility, SI), and to reject foreign pollen from other species, thus maintaining species identity (interspecific incompatibility)1. In the genus Brassica, the SI system is regulated by an S-haplotype-specific interaction between a stigma-expressed female receptor (S receptor kinase, SRK) and a tapetum cell-expressed male ligand (S locus protein 11, SP11), encoded by their respective polymorphic genes at the S locus2-6. However, the molecular mechanism for recognition of foreign pollen, leading to reproductive incompatibility, has not yet been identified. Here, we show that recognition between a novel pair of proteins, a pistil receptor SUI1 (STIGMATIC UNILATERAL INCOMPATIBILITY 1) and a pollen ligand PUI1 (POLLEN UNILATERAL INCOMPATIBILITY 1), triggers unilateral reproductive incompatibility between plants of two geographically distant self-incompatible Brassica rapa lines, even though crosses would be predicted to be compatible based on the S haplotypes of pollen and stigma. Interestingly, SUI1 and PUI1 are similar to the SI genes, SRK and SP11, respectively, and are maintained as cryptic incompatibility genes in these two populations. The duplication of the SRK and SP11 followed by reciprocal loss in different populations would provide a molecular mechanism of the emergence of a reproductive barrier in allopatry.

Interspecific incompatibility in the wide hybridization of plants and ways to overcome it

Wide hybridization is an important factor of angiosperm speciation and provides an introgression of genes between species. In experimental conditions, wide hybridization is used to increase the genetic diversity of cultivated plants. Since the emergence of reproductive barriers is of great importance for speciation, plants’ capacity for wide crosses is determined by the possibility of overcoming these barriers. This review discusses the main types of incompatibility in wide crossing plants, as well as the factors and methodological approaches that contribute to overcoming them. The role of Kr genes, which determine incompatibility between wheat and rye, is one of the examples of a prezygotic isolation mechanism. Postzygotic incompatibility may be associated with a highly pronounced genetic and epigenetic variability induced by wide crossings. The early stages of the postzygotic period are crucial for developing hybrid seeds due to an embryo’s death, including those associated with uniparental chromosome elimination in hybrid cells and abnormal development of the endosperm. The depression and lethality of F1 hybrids may be the result of interaction between complementary genes, which cause hybrid necrosis, hybrid chlorosis, and hybrid dwarfness. The causes of hybrid sterility are discussed. Nuclear-cytoplasmic interactions are regarded as one of the mechanisms of incompatibility in wide crosses. Reciprocal hybrids and alloplasmic lines are the main models for studying cytoplasm effects and nuclear-cytoplasmic interactions. Problems concerning work with their models are emphasized. There are some examples underlying the fact that alloplasmic lines are not a plain combination of the nuclear genome and cytoplasm of different species. The development of alloplasmic lines is connected with the structural and functional variability of nuclear and organelle genomes.

Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre- and postzygotic barriers in populations of individuals with varying ploidy levels.

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid-diploid life cycles that include the principal genetic model organism for the brown algae. In haploid-diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E.siliculosus and E.crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly postzygotic and suggests that abnormal chromosome segregation during meiosis following hybridization of species with different genome sizes could be a major cause of interspecific incompatibility in this system.

Identification of complementary genes of hybrid lethality in crosses of bread wheat and rye. Results and prospects

Genes, leading to the death or sterility of distant hybrids, are well known in cultivated plants. The paper summarizes own data on the genetics of hybrid lethality, which was discovered in the crosses of bread wheat and inbred lines of rye. It shows that hybrid lethality is a consequence of a negative interaction between the complementary genes of wheat and rye. However, embryonic lethality, which is typical for some wheatrye hybrids, cannot be attributed to the evolutionary originated interspecific incompatibility, which is a wellstudied prezygotic incompatibility of common wheat and its related genera.

Interspecific hybridization of potato: theoretical and applied aspects

The common potato Solanum tuberosum is the fourth most important crop in the world and like many other cultivated plants is characterized by low level of genetic diversity. The most effective approach to broaden genetic diversity of breeding varieties is based on the introgression of genetic material of wild and cultivated species belonging to section Petota Dumort. of the genus Solanum L. This section, according to estimates by different taxonomists, includes from 112 to 235 species. At the same time, because of the barriers of prezygotic and postzygotic incompatibility, only about 10 % of wild potato species have been actively involved into the breeding process and their genetic material has been introgressed into the gene pool of varieties. To date, molecular genetics, genomics, and biotechnology have accumulated a large amount of information, allowing the problems of extension of the genetic diversity of common potato to be addressed. The article presents a review of modern data about the genetic factors determining prezygotic and postzygotic incompatibility, about the features of involvement in the hybridization process of the self-compatible and self-incompatible species of section Petota and species with different values of the endosperm balance number (EBN), as well as possible mechanisms of interaction between the parental genomes during the formation of the endosperm of the hybrid seed. Data are provided on the value of the theory of EBN for the further development of applied research aimed at increasing the genetic diversity of the breeding gene pool. This review discusses the traditional and modern approaches and methods of overcoming interspecific incompatibility and gives examples of successful studies on introgressive hybridization of potato. The article is preceded by information about the genetic diversity of potato species belonging to section Petota of the genus Solanum.

Determining epistatic selection in admixed populations.

When two diverging species begin hybridizing, selection against hybridization is likely driven not by single substitutions, but by interactions between incompatible mutations. To identify these incompatibilities in natural populations, researchers examine the extent of nonrandom associations between ancestry at physically unlinked loci in admixed populations. In this approach, which we call 'AD scans', locus pairs with significantly positive 'ancestry disequilibrium' (AD, i.e. locus pairs that positively covary by ancestry) represent incompatible alleles. Past research has uniformly revealed an excess of locus pairs with significantly positive AD, suggesting that dozens to hundreds of incompatibilities separate species. With forward simulations, we show that many realistic demographic scenarios, including recent and/or ongoing hybridization, generate a bias towards positive ancestry disequilibrium. We suggest steps that researchers can take to avoid pitfalls in interpreting AD scans, and present a novel measure of AD, which minimizes but does not fully eliminate bias in the AD distribution. We also show, by simulation, that the tail of the AD distribution is enriched for true incompatibilities. To illustrate the potential power and appropriate caution in interpretation of AD scans, we reanalyse previously published data from two admixed populations of Xiphophorus fishes. Our results imply that the prevalence of positive LD in admixed populations does not in itself support the idea that two-locus incompatibilities are widespread, but the co-enrichment of top AD hits across the two Xiphophorus populations supports the idea that AD scans can identify candidate interspecific incompatibilities.

Interspecific incompatibility in wide hybridization of plants and ways to overcome

Wide hybridization is an important factor of angiosperm speciation and provides an introgression of genes between species. In experimental conditions wide hybridization is used to increase the genetic diversity of cultivated plants. Since the emergence of reproductive barriers is of great importance for speciation, plant capacity for wide crosses is determined by a possibility of overcoming these barriers. The review discusses the main types of incompatibility in wide crossing plants, as well as factors and methodological approaches that contribute to overcoming them. The role of Kr genes, which determine incompatibility between wheat and rye, is one of the examples of prezygotic isolation mechanism. Postzygotic incompatibility may be associated with a highly pronounced genetic and epigenetic variability induced by wide crossings. Early stages of the postzygotic period are crucial for developing hybrid seeds due to embryo’s death, including those associated with uniparental chromosome elimination in hybrid cells and abnormal development of the endosperm. A depression and a lethality of F1 hybrids may be the result of interaction between complementary genes, which cause hybrid necrosis, hybrid chlorosis, and hybrid dwarfness. The causes of hybrid sterility are discussed. Nuclearcytoplasmic interactions are regarded as one of the mechanisms of incompatibility in wide crosses. Reciprocal hybrids and alloplasmic lines are the main models for studying cytoplasm effects and nuclearcytoplasmic interactions. Problems concerning work with their models are emphasized. There are some examples underlining the fact that alloplasmic lines are not a plain combination of nuclear genome and cytoplasm of different species. Development of alloplasmatic lines is connected with structural and functional variability of nuclear and organelle genomes.

A new synthetic allotetraploid (A1A1G2G2) between Gossypium herbaceum and G. australe: bridging for simultaneously transferring favorable genes from these two diploid species into upland cotton.

Gossypium herbaceum, a cultivated diploid cotton species (2n = 2x = 26, A1A1), has favorable traits such as excellent drought tolerance and resistance to sucking insects and leaf curl virus. G. australe, a wild diploid cotton species (2n = 2x = 26, G2G2), possesses numerous economically valuable characteristics such as delayed pigment gland morphogenesis (which is conducive to the production of seeds with very low levels of gossypol as a potential food source for humans and animals) and resistance to insects, wilt diseases and abiotic stress. Creating synthetic allotetraploid cotton from these two species would lay the foundation for simultaneously transferring favorable genes into cultivated tetraploid cotton. Here, we crossed G. herbaceum (as the maternal parent) with G. australe to produce an F1 interspecific hybrid and doubled its chromosome complement with colchicine, successfully generating a synthetic tetraploid. The obtained tetraploid was confirmed by morphology, cytology and molecular markers and then self-pollinated. The S1 seedlings derived from this tetraploid gradually became flavescent after emergence of the fifth true leaf, but they were rescued by grafting and produced S2 seeds. The rescued S1 plants were partially fertile due to the existence of univalents at Metaphase I of meiosis, leading to the formation of unbalanced, nonviable gametes lacking complete sets of chromosomes. The S2 plants grew well and no flavescence was observed, implying that interspecific incompatibility, to some extent, had been alleviated in the S2 generation. The synthetic allotetraploid will be quite useful for polyploidy evolutionary studies and as a bridge for transferring favorable genes from these two diploid species into Upland cotton through hybridization.

Unilateral incompatibility gene ui1.1 encodes an S-locus F-box protein expressed in pollen of Solanum species

Unilateral interspecific incompatibility (UI) is a postpollination, prezygotic reproductive barrier that prevents hybridization between related species when the female parent is self-incompatible (SI) and the male parent is self-compatible (SC). In tomato and related Solanum species, two genes, ui1.1 and ui6.1, are required for pollen compatibility on pistils of SI species or hybrids. We previously showed that ui6.1 encodes a Cullin1 (CUL1) protein. Here we report that ui1.1 encodes an S-locus F-box (SLF) protein. The ui1.1 gene was mapped to a 0.43-cM, 43.2-Mbp interval at the S-locus on chromosome 1, but positional cloning was hampered by low recombination frequency. We hypothesized that ui1.1 encodes an SLF protein(s) that interacts with CUL1 and Skp1 proteins to form an SCF-type (Skp1, Cullin1, F-box) ubiquitin E3 ligase complex. We identified 23 SLF genes in the S. pennellii genome, of which 19 were also represented in cultivated tomato (S. lycopersicum). Data from recombination events, expression analysis, and sequence annotation highlighted 11 S. pennellii genes as candidates. Genetic transformations demonstrated that one of these, SpSLF-23, is sufficient for ui1.1 function. A survey of cultivated and wild tomato species identified SLF-23 orthologs in each of the SI species, but not in the SC species S. lycopersicum, S. cheesmaniae, and S. galapagense, pollen of which lacks ui1.1 function. These results demonstrate that pollen compatibility in UI is mediated by protein degradation through the ubiquitin-proteasome pathway, a mechanism related to that which controls pollen recognition in SI.

Interspecific hybridization between Brassica napus and Brassica rapa ssp. chinensis genotypes through embryo rescue and their evaluation for crossability

The genus Brassica contains several important crop species that are used for a variety of purposes. Brassica napus and Brassica rapa are the most important ones for their use as oilseed crops. In Brassica , interspecific hybridization is a potential and useful method for transferring valuable traits between species of commercial interest. For breeding Brassicas resistant to some diseases, i.e. clubroot, attempts of transferring resistant genes (CR) through interspecific hybridization have also been reported. The main step in the introgression process is the production of interspecific hybrids between the two species. However, the cross-incompatibility occurring in wide hybridization might hamper the possibility of obtaining hybrid progenies. For that reason, the crossability study is essential and may give an insight into the cross-compatibility relationship among the species, the direction of success of crossing, and the crossability barriers of some combinations, if any. In the present study, interspecific reciprocal crosses between B. napus and B. rapa ssp. chinensis genotypes were carried out in order to determine their crossability and to produce their F1 hybrids. Crossability was analyzed based on the pollen germination index (PGI) and the development of hybrid embryos in in vitro cultures. It was observed that the unilateral interspecific incompatibility occurred in crosses between B. napus × B. rapa ssp. chinensis. When B. napus cultivars were used as a maternal parent, pollen grains of B. rapa germinated well on the stigmas, while only a small number of pollen tubes could elongate near the ovules, resulting in a low development of hybrid embryos. Using the B. rapa ssp. chinensis as the pistillate parent in the crosses, it was found that the crossability was relatively higher than that in the case of the reciprocal cross. This resulted in a greater number of embryos obtained. Generally, it was concluded that the selection of parental components for hybridization is an important step for obtaining breeding success.

Performance of a Hybrid Fungal Pathogen on Pure-Species and Hybrid Host Plants

Premise of research. Recent hybridization events in fungi have produced emerging pathogens characterized by novel host specificities, increased infectivity, and/or elevated severity. We investigated the potential for host shifts and increased infectivity following hybridization of fungal pathogens in the genus Microbotryum, which causes anther-smut disease on caryophyllaceous hosts. Hybrids of the closely related species Microbotryum lychnidis-dioicae (MvSl) and Microbotryum silenes-dioicae (MvSd) are viable and fertile. Although historical genetic exchange between MvSl and MvSd is rare, there is evidence of recent hybridization of these fungal species, as well as of their plant hosts, Silene latifolia and Silene dioica. Methodology. We examined the fitness of hybrid pathogens and hosts by using F1 hybrids of MvSl × MvSd to inoculate S. latifolia × S. dioica hybrids. Experimental inoculations of S. latifolia and hybrid hosts with pure-species and hybrid pathogens allowed assessment of the likelihood of hybrid emergence on a novel host (the hybrid plant) and of increased infectiousness of the hybrid pathogen on the pure-species host. Pivotal results. We found no evidence for pathogen hybrid inferiority, arguing against interspecific incompatibilities at small genetic distances. Instead, we found that hybrid pathogens are more infectious on pure-species hosts, while pure-species pathogens are more infectious on hybrid hosts, indicating an interaction of host and parasite genotypes. Conclusions. This finding argues against emergence of hybrid pathogens on a novel hybrid host. However, our study suggests that hybridization of pathogens and hosts in natural populations may lead to elevated disease prevalence overall, thus furthering the impact of anther-smut disease in these Silene species.

Hybridization of cultivated lentil Lens culinaris Medik. and wild species Lens tomentosus Ladizinsky

Cultivated lentil L. culinaris was crossed to the wild species L. tomentosus ILWL90 and ILWL120. An ovule rescue technique was used to overcome interspecific incompatibility. Out of 296 hybrid ovules being planted in vitro 27 explants began to grow and three hybrids were recovered. A hybrid between L. culinaris and L. tomentosus accession ILWL90 was obtained by means of ovule recovery only. F<sub>1</sub> plant and next generations of the hybrid were either sterile or partly fertile. Hybridization with L. tomentosus accession ILWL120 was achieved by ovule culture as well as in a usual way i.e. without ovule culture. Seed progenies of these hybrids were fertile in both cases. Breeding lines recombinant in flower, seed coat and cotyledon coloring were developed as a result of multiple regular selection for highly productive plants in F<sub>2</sub>–F<sub>7</sub> (L. culinaris × L. tomentosus ILWL120).

Interspecific crossing and genetic mapping reveal intrinsic genomic incompatibility between two Senecio species that form a hybrid zone on Mount Etna, Sicily

Studies of hybridizing species can reveal much about the genetic basis and maintenance of species divergence in the face of gene flow. Here we report a genetic segregation and linkage analysis conducted on F2 progeny of a reciprocal cross between Senecio aethnensis and S. chrysanthemifolius that form a hybrid zone on Mount Etna, Sicily, aimed at determining the genetic basis of intrinsic hybrid barriers between them. Significant transmission ratio distortion (TRD) was detected at 34 (∼27%) of 127 marker loci located in nine distinct clusters across seven of the ten linkage groups detected, indicating genomic incompatibility between the species. TRD at these loci could not be attributed entirely to post-zygotic selective loss of F2 individuals that failed to germinate or flower (16.7%). At four loci tests indicated that pre-zygotic events, such as meiotic drive in F1 parents or gametophytic selection, contributed to TRD. Additional tests revealed that cytonuclear incompatibility contributed to TRD at five loci, Bateson-Dobzhansky-Muller (BDM) incompatibilities involving epistatic interactions between loci contributed to TRD at four loci, and underdominance (heterozygote disadvantage) was a possible cause of TRD at one locus. Major chromosomal rearrangements were probably not a cause of interspecific incompatibility at the scale that could be examined with current map marker density. Intrinsic genomic incompatibility between S. aethnensis and S. chrysanthemifolius revealed by TRD across multiple genomic regions in early-generation hybrids is likely to impact the genetic structure of the natural hybrid zone on Mount Etna by limiting introgression and promoting divergence across the genome.

The Role of a Pollen-Expressed Cullin1 Protein in Gametophytic Self-Incompatibility in Solanum

We previously isolated a pollen factor, ui6.1, which encodes a Cullin1 protein (CUL1) that functions in unilateral interspecific incompatibility (UI) in Solanum. Here we show that CUL1 is also required for pollen function in self-incompatibility (SI). We used RNA interference (RNAi) to reduce CUL1 expression in pollen of Solanum arcanum, a wild SI tomato relative. Hemizygous T0 plants showed little or no transmission of the transfer DNA (T-DNA) through pollen when crossed onto nontransgenic SI plants, indicating that CUL1-deficient pollen are selectively eliminated. When crossed onto a related self-compatible (SC) accession lacking active S-RNase, pollen transmission of the T-DNA followed Mendelian ratios. These results provide further evidence for functional overlap between SI and UI on the pollen side and suggest that CUL1 mutations will reinforce SI-to-SC transitions in natural populations only if preceded by loss of pistil S-RNase expression.

Self-incompatibility in Brassicaceae crops: lessons for interspecific incompatibility.

Most wild plants and some crops of the Brassicaceae express self-incompatibility, which is a mechanism that allows stigmas to recognize and discriminate against “self” pollen, thus preventing self-fertilization and inbreeding. Self-incompatibility in this family is controlled by a single S locus containing two multiallelic genes that encode the stigma-expressed S-locus receptor kinase and its pollen coat-localized ligand, the S-locus cysteine-rich protein. Physical interaction between receptor and ligand encoded in the same S locus activates the receptor and triggers a signaling cascade that results in inhibition of “self” pollen. Sequence information for many S-locus haplotypes in Brassica species has spurred studies of dominance relationships between S haplotypes and of S-locus structure, as well as the development of methods for S genotyping. Furthermore, molecular genetic studies have begun to identify genes that encode putative components of the self-incompatibility signaling pathway. In parallel, standard genetic analysis and QTL analysis of the poorly understood interspecific incompatibility phenomenon have been initiated to identify genes responsible for the inhibition of pollen from other species by the stigma. Herewith, we review recent studies of self-incompatibility and interspecific incompatibility, and we propose a model in which a universal pollen-inhibition pathway is shared by these two incompatibility systems.

Induction of direct shoot organogenesis and in vitro flowering from shoot tip explants of cucumber (Cucumis sativus L. cv. ‘Green long’)

In vitro flowering is an alternative breeding tool for generating hybrid Cucumis spp. as it is able to overcome limitations caused by interspecific incompatibility. The present study describes an efficient method for induction of multiple shoots and in vitro flowering from shoot tip explants of cucumber (Cucumis sativus L.). Shoot tip explants were excised from 7-day-old seedlings and cultured on Murashige and Skoog (MS) medium fortified with different concentrations of 6-benzylaminopurine (BAP; 0.5–2.5 mg/L) alone or in combination with 0.5 mg/L kinetin (KIN). The highest frequency (93.1%) of multiple shoot formation with maximum number of shoots (15.2 shoots/explant) was achieved on MS medium supplemented with 1.0 mg/L BAP. For in vitro flowering, shoots were cultured on MS medium supplemented with 0.5 mg/L BAP and different concentrations of sucrose. Flowering occurred on about 95% of in vitro shoots cultured on MS medium fortified with 6% (w/v) sucrose and 0.5 mg/L BAP after 15 d. For rooting, shoots (>2 cm) were cultured on MS medium augmented with various concentrations of indole-3-butyric acid (IBA; 0.5–2.5 mg/L) alone or in combination with 0.5 mg/L KIN. Among the combinations tested, supplementation with IBA (1.5 mg/L) and KIN (0.5 mg/L) induced maximum rooting rates (95.4%) with 7.8 roots/shoot. Rooted plantlets were successfully transferred into plastic cups containing a mixture of soil and sand (1:1), established in the greenhouse, and subsequently acclimatized in the field. The in vitro flowering reported in this study may facilitate rapid hybridization in Cucumis species and offers a model system for studying the physiological mechanisms involved in flowering.

Analysis of incompatibility barriers during cross-pollinations among <i>Phaseolus</i> L. and other leguminous plants

Plant breeding is intended to seek for ways of realizing a genetic structure adapted to criteria and population needs. In this context, cross-pollinations are for great interest. They offer the means of creating a wide genotypic variability. However, a limited number of species are able to undergo hybridization studies. In Phaseolus L., improvement can be envisioned by either exploiting the primary genetic pool of the species, or the genetic variability of other species. In fact, there are wild or wild-like species more resistant to stresses or diseases than the cultivated ones. The methods of embryo rescues facilitate the interspecific crosses and contribute to exploiting the available genetic diversity within plant germplasm. Difficulties related to obtaining hybrids in grain legumes, and Phaseolus L. in particular, are reported in this research. It is possible to avoid the incompatibility barriers based on their appearance time. When they are pre-zygotic, the use of growth regulators during pollination is recommended. The in vitro explants culture is the most usual method to overcome the post-zygotic barriers. The mechanisms leading to abortion of hybrid embryos, as well as adequate methods to overcome the interspecific incompatibilities should be subjected to further studies.Keywords: Abortion, hybridization, interspecific barriers, Phaseolus L., plant embryos.