A glycoprotein associated with the acquisition of the self-incompatibility system by maturing stigmas of Brassica oleracea

Unilateral incompatibility within the brassicaceae: further evidence for the involvement of the self-incompatibility (S)-locus

Unilateral pollen-pistil incompatibility within the Brassicaceae has been re-examined in a series of interspecific and intergeneric crosses using 13 self-compatible (SC, Sc) species and 12 self-incompatible (SI) species from ten tribes. SC x SC crosses were usually compatible, SI x SC crosses showed unilateral incompatibility, while SI x SI crosses were often incompatible or unilaterally incompatible. Unilateral incompatibility (UI) is shown to be overcome by bud pollination or treating stigmas with cycloheximide - features in common with self-incompatibility. Treating stigmas with pronase prevents pollen tubes from penetrating the stigma in normally compatible intra-and interspecific pollinations. The results presented show that the presence of an incompatibility system is important in predicting the outcome of interspecific and intergeneric crosses and, combined with the physiological similarities between UI and SI, would suggest an involvement of the S-locus in UI.

Cell-cell communication in plants: self-incompatibility in flower development

Self-incompatibility, a mechanism that prevents self-fertilization in plants, is based on the ability of the pistil to discern the presence of self-pollen and on the female tissue's capacity to inhibit the growth or germination of self-related, but not of genetically unrelated, pollen. As a self-recognition system, self-incompatibility responds to specific cellular products and signals and thus offers a unique system in which to study the components of cellular communication in plants. The cytological manifestations of self-incompatibility have been well studied, and, with the cloning of cDNAs for several proteins associated with this recognition process, a detailed molecular view of self-incompatibility is emerging.

Gamete self-discrimination in ascidians: a role for the follicle cells

Gamete self-incompatibility in the hermaphrodite tunicate Ciona intestinalis is a useful system with which to study self-nonself recognition. We have used in vitro fertilization of oocytes isolated from the gonad of Ciona intestinalis to identify the cellular source of self-sterility elements present on the egg envelopes. Here we show for the first time that self-discrimination, which occurs on the egg vitelline coat, is established there in late oogenesis and is contributed or controlled by products of the overlying follicle cells. The acquisition of self-sterility by the oocyte is prevented by the ionophore monensin, which suggests that the follicle cell self-sterility controlling factor is a glycoprotein.

The S-locus specific glycoproteins of Brassica accumulate in the cell wall of developing stigma papillae

Self-incompatibility in Brassica oleracea is now viewed as a cellular interaction between pollen and the papillar cells of the stigma surface. In this species, the inhibition of self-pollen occurs at the stigma surface under the influence of S-locus specific glycoproteins (SLSG). We used antibodies specific for a protein epitope of SLSG to study the subcellular distribution of these molecules in the stigmatic papillae. The antibodies have uncovered an interesting epitope polymorphism in SLSG encoded by subsets of S-alleles, thus providing us with useful genetic controls to directly verify the specificity of the immunolocalization data. Examination of thin sections of Brassica stigmas following indirect immunogold labeling showed that SLSG accumulate in the papillar cell wall, at the site where inhibition of self-pollen tube development has been shown to occur. In addition, the absence of gold particles over the papillar cell walls in the immature stigmas of very young buds, and the intense labeling of these walls in the stigmas of mature buds and open flowers, correlates well with the acquisition of the self-incompatibility response by the developing stigma.

Biosynthesis of glycoproteins involved in the pollen-stigma interaction of incompatibility in developing flowers of Brassica oleracea L

De-novo synthesis of the S-allele-specific glycoproteins of Brassica oleracea is demonstrated in stigmas at different developmental stages. Excised stigmas incorporate (14)C-labeled amino acids into their S-glycoproteins early in development and before the self-incompatibility response is acquired, but the rate of synthesis accelerates prior to anthesis, resulting in the accumulation of high levels of the S-glycoproteins in the stigma and coinciding with the acquisition of the pollen-stigma incompatibility response. Since the self-compatible and self-incompatible zones of developing inflorescences are very sharply delineated, a threshold quantity of S-glycoproteins appears to be critical for the onset of self-incompatibility. Incorporation experiments in which [(35S)methionine was applied to intact stigma surfaces indicate that the papillae are the main sites of synthesis of the S-specific glycoproteins.

Callose and determination of pistil viability and incompatibility

Callose provides a useful phenotypic bioassay in plant breeding to determine: incompatibility system; gametophytic competition; and stigma and ovule viability. Callose appearance in ovules may be associated with senescence, and used to determine the effective pollination period. In incompatible matings, callose formation is specific and related to rejection phenomena. The stigma callose response is induced by informational molecules carried by the germinal line, i.e. self or interspecific pollen, but not by the somatic line. Several methods of visualizing callose are reviewed. The role of callose in pollen-stigma interactions has many analogies with host-parasite interactions, and a model is proposed based on relationships between callose, boron and inhibitor (phytoalexin-like) synthesis. The callose response provides a useful tool for the biotechnology of seed production.

Pollen-stigma interactions in Brassica oleracea; a new pollen germination medium and its use in elucidating the mechanism of self incompatibility

A simple medium is described which supports the germination of Brassica pollen in vitro. The method is completely reliable and the percentage germination achieved is comparable to that found on a compatible stigma. The pollen tubes produced attain lengths equal to those growing in the style and exhibit similar growth kinetics. The major difference between this medium and other less-reliable media is the addition of 1 mM Tris. The efficacy of Tris is shown to be due in part to the establishment of an optimum pH but an "amine effect" is also identified. The optimum pH for growth in vitro may be different from that encountered on the stigma. Atmospheric concentration of CO2 and pollen population density are also shown to have pronounced effects on germination in vitro.Germination in vitro is not significantly affected by prior incubation on the stigma surface. Neither crossnor self-stigmas exert any apparent influence on pollen viability in this respect. Similarly the numbers of tube initials produced in atmospheres of 98% relative humidity (RH) are not altered by such treatments. Conversely, incubation in germination medium and in 98% RH prior to pollination appears to modify pollen-stigma interactions to some extent.Stigma extracts added to the germination medium cause an increase in the rate of hydration of both cross- and self-pollen, but this effect is greater in the case of cross-pollen. Evidence for specific inhibition of self-pollen germination by stigma extracts added to the medium was not obtained. The relevance of these phenomena to current concepts of the mechanism of self incompatibility (SI) are discussed and a potential use of the medium in overcoming the SI response is suggested.

Studies on heteromorphic self-incompatibility systems: Physiological aspects of the incompatibility system of Primula obconica

In Primula obconica, a species with a heteromorphic self-incompatibility system, the distinction between compatible and incompatible pollen tubes takes place on the stigma surface in thrum flowers, self tubes growing randomly over the papillar cells. No differences were seen between self and cross tube behaviour on the pin stigma surface, but self tubes were inhibited within the stigmatic tissue with differences in tube length evident after 24 h. The stigma surface bears a proteinaceous pellicle and binds the lectin Concanavalin A. Removal of the stigma removes the incompatibility barrier in mature gynoecia. Bud pollination shows that pollen tubes cannot grow in a normal manner on immature stigmas; the random growth of tubes over the stigma surface resembles that of mature thrum selfs. Fewer compatible tubes reach the style base of young gynoecia and smaller numbers of seeds are set than in mature flowers. Pin and thrum pollen grains germinate and grow in aqueous media, thrum tubes growing longer than pin. The presence of H3BO4 and CaCl2 in the growth medium promotes tube elongation and lengths equivalent to compatible styles can be obtained. The pollen grains have proteinaceous materials in their walls which diffuse out on moistening. Prolonged washing in aqueous media removes these materials but the incompatibility reaction remains unchanged. Thus the incompatibility reaction is between pollen tubes and stigmatic tissue and differs from the homomorphic, sporophytic system where pollen wall proteins elicit the incompatibility response.

Pollen-Stigma Interaction and Cross-Incompatibility in the Grasses

The failure of interspecific and intergeneric crosses in the grasses is commonly due to zygotic inviability, embryo abortion, or endosperm abnormalities, all of which occur after fertilization. Often, however, there is difficulty in obtaining fertilization because the pollen tube fails to reach the embryo sac. This article concerns recent research on pollen-stigma interaction in the grasses and the light it is beginning to throw on prefertilization self- and cross-incompatibilities.

Pollen stigma interactions in Brassica oleracea

Recent studies on the mechanism of self-incompatibility in Brassica indicate the location, nature and mode of action of the molecules involved. Characteristics of the pollen surface and the stigma surface are described in detail, together with new information pertaining to the recognition molecules located therein. A sequence of events is outlined leading from pollination, through adhesion, hydration, germination, and tube growth to acceptance and ultimate compatibility. The characteristics of rejection of incompatible grains are described for each stage of the pollen-stigma interaction. It is proposed that recognition of proteins from the coating of self-pollen by the molecules in the pellicle results in the formation of a biologically-active complex which inhibits water supply to the incompatible grain, and that all other manifestations of incompatibility are a consequence of this initial response.

Pollen-pistil interaction in Linum grandiflorum : Scanning electron microscopic observations and proteins of the stigma surface

A detailed investigation of the receptive surface of the stigma of a dimorphic taxon, Linum grandiflorum, was carried out using scanning electron microscopy, cytochemistry and polyacrylamide gel electrophoresis. The stigma surfaces of the pin and thrum morphs showed distinct differences. The stigma of the pin morph was of the dry type and the papillae were covered with a uniform cuticle-pellicle layer. The stigma of the thrum morph, on the other hand, resembled the wet type; the cuticle-pellicle layer was disrupted at places and a secretion product was released onto the surface of the stigma. Coomassie blue staining material was present on the surface of only the thrum stigma. Although esterases and acid phosphatases were present on the stigma of both the morphs, their activity was invariably higher on the thrum stigma. Polyacrylamide gel electrophoresis of stigma leachates also showed distinct differences in the protein profiles of the two morphs.

Ultrastructural and physiological differences between buds and mature flowers of Petunia hybrida prior to and following pollination

The structural events accompanying the maturation of the pistil of Petunia hybrida have been studied in detail, together with the changes in the protein spectrum of the transmitting tissue that occur over this period. These events have been considered in terms of the acquisition of the self-incompatibility response, which occurs while the pistil is enclosed in the bud. Apart from several minor differences, the young pistils differ only from the mature in that their transmitting tissue cells fail to respond to pollination by undergoing characteristic ultrastructural changes. Data from electrofocusing indicates that several proteins, mobilised in the mature transmitting tissue some three hours after pollination, are absent from bud pistils. It is proposed that the pollination-stimulated release of certain polypeptides, believed to be involved in the self-incompatibility response, does not take place in young pistils. These observations are considered with reference to currently-accepted models of the operation of the self-incompatibility mechanism in Petunia.