Analysis of S-specific proteins in stigma of Brassica oleracea L. by isoelectric focusing

Molecular mechanisms of self-incompatibility in Brassicaceae and Solanaceae

Self-incompatibility (SI) is a mechanism for preventing self-fertilization in flowering plants. SI is controlled by a single S-locus with multiple haplotypes (S-haplotypes). When the pistil and pollen share the same S-haplotype, the pollen is recognized as self and rejected by the pistil. This review introduces our research on Brassicaceae and Solanaceae SI systems to identify the S-determinants encoded at the S-locus and uncover the mechanisms of self/nonself-discrimination and pollen rejection. The recognition mechanisms of SI systems differ between these families. A self-recognition system is adopted by Brassicaceae, whereas a collaborative nonself-recognition system is used by Solanaceae. Work by our group and subsequent studies indicate that plants have evolved diverse SI systems.

Comprehensive computational analysis of the SRK-SP11 molecular interaction underlying self-incompatibility in Brassicaceae using improved structure prediction for cysteine-rich proteins

Plants employ self-incompatibility (SI) to promote cross-fertilization. In Brassicaceae, this process is regulated by the formation of a complex between the pistil determinant S receptor kinase (SRK) and the pollen determinant S-locus protein 11 (SP11, also known as S-locus cysteine-rich protein, SCR). In our previous study, we used the crystal structures of two eSRK-SP11 complexes in Brassica rapa S8 and S9 haplotypes and nine computationally predicted complex models to demonstrate that only the SRK ectodomain (eSRK) and SP11 pairs derived from the same S haplotype exhibit high binding free energy. However, predicting the eSRK-SP11 complex structures for the other 100 + S haplotypes and genera remains difficult because of SP11 polymorphism in sequence and structure. Although protein structure prediction using AlphaFold2 exhibits considerably high accuracy for most protein monomers and complexes, 46% of the predicted SP11 structures that we tested showed < 75 mean per-residue confidence score (pLDDT). Here, we demonstrate that the use of curated multiple sequence alignment (MSA) for cysteine-rich proteins significantly improved model accuracy for SP11 and eSRK-SP11 complexes. Additionally, we calculated the binding free energies of the predicted eSRK-SP11 complexes using molecular dynamics (MD) simulations and observed that some Arabidopsis haplotypes formed a binding mode that was critically different from that of B. rapa S8 and S9. Thus, our computational results provide insights into the haplotype-specific eSRK-SP11 binding modes in Brassicaceae at the residue level. The predicted models are freely available at Zenodo, https://doi.org/10.5281/zenodo.8047768.

Aminooxyacetic acid (АОА), inhibitor of 1-aminocyclopropane-1-carboxilic acid (AСС) synthesis, suppresses self-incompatibility-induced programmed cell death in self-incompatible Petunia hybrida L. pollen tubes

Self-incompatibility (SI) is genetically determined reproductive barrier preventing inbreeding and thereby providing the maintenance of plant species diversity. At present, active studies of molecular bases of SI mechanisms are underway. S-RNAse-based SI in Petunia hybrida L. is a self-/non-self recognition system that allows the pistil to reject self pollen and to accept non-self pollen for outcrossing. In the present work, using fluorescent methods including the TUNEL method allowed us to reveal the presence of markers of programmed cell death (PCD), such as DNA fragmentation, in growing in vivo petunia pollen tubes during the passage of the SI reaction. The results of statistical analysis reliably proved that PCD is the factor of S-RNAse-based SI. It was found that preliminary treatment before self-pollination of stigmas of petunia self-incompatible line with aminooxyacetic acid (AOA), inhibitor of ACC synthesis, led to stimulation of pollen tubes growth when the latter did not exhibit any hallmarks of PCD. These data argue in favor of assumption that ethylene controls the passage of PCD in incompatible pollen tubes in the course of S-RNAse-based SI functioning. The involvement of the hormonal regulation in SI mechanism in P. hybrida L. is the finding observed by us for the first time.

Progress on deciphering the molecular aspects of cell-to-cell communication in Brassica self-incompatibility response

The sporophytic system of self-incompatibility is a widespread genetic phenomenon in plant species, promoting out-breeding and maintaining genetic diversity. This phenomenon is of commercial importance in hybrid breeding of Brassicaceae crops and is controlled by single S locus with multiple S haplotypes. The molecular genetic studies of Brassica 'S' locus has revealed the presence of three tightly linked loci viz. S-receptor kinase (SRK), S-locus cysteine-rich protein/S-locus protein 11 (SCR/SP11), and S-locus glycoprotein (SLG). On self-pollination, the allele-specific ligand-receptor interaction activates signal transduction in stigma papilla cells and leads to rejection of pollen tube on stigmatic surface. In addition, arm-repeat-containing protein 1 (ARC1), M-locus protein kinase (MLPK), kinase-associated protein phosphatase (KAPP), exocyst complex subunit (Exo70A1) etc. has been identified in Brassica crops and plays a key role in self-incompatibility signaling pathway. Furthermore, the cytoplasmic calcium (Ca2+) influx in papilla cells also mediates self-incompatibility response in Brassicaceae, but how this cytoplasmic Ca2+ influx triggers signal transduction to inhibit pollen hydration is still obscure. There are many other signaling components which are not well characterized yet. Much progress has been made in elucidating the downstream multiple pathways of Brassica self-incompatibility response. Hence, in this review, we have made an effort to describe the recent advances made on understanding the molecular aspects of genetic mechanism of self-incompatibility in Brassicaceae.

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.

Proteomics approaches advance our understanding of plant self-incompatibility response

Self-incompatibility (SI) in plants is a genetic mechanism that prevents self-fertilization and promotes out-crossing needed to maintain genetic diversity. SI has been classified into two broad categories: the gametophytic self-incompatibility (GSI) and the sporophytic self-incompatibility (SSI) based on the genetic mechanisms involved in 'self' pollen rejection. Recent proteomic approaches to identify potential candidates involved in SI have shed light onto a number of previously unidentified mechanisms required for SI response. SI proteome research has progressed from the use of isoelectric focusing in early days to the latest third-generation technique of comparative isobaric tag for relative and absolute quantitation (iTRAQ) used in recent times. We will focus on the proteome-based approaches used to study self-incompatibility (GSI and SSI), recent developments in the field of incompatibility research with emphasis on SSI and future prospects of using proteomic approaches to study self-incompatibility.

Isolation and characterization of a polymorphic stigma-specific class III peroxidase gene from Senecio squalidus L. (Asteraceae)

A novel stigma-specific class III peroxidase gene, SSP (Stigma-Specific Peroxidase), has been isolated from the self-incompatible daisy Senecio squalidus L. (Asteraceae). Expression of SSP in flower buds is developmentally regulated, with maximal levels of expression coinciding with anthesis, when stigmas are most receptive to pollen and when self-incompatibility is fully developed. In situ hybridization revealed SSP expression to be localized exclusively to the specialized secretory epidermal cells (papillae) of the stigma, which receive and discriminate pollen. SSP is therefore the first tissue-specific and cell-specific peroxidase gene identified in a plant. SSP belongs to a distinct clade of class III plant peroxidases that possess two introns, instead of the more normal situation of three conserved introns. The deduced amino acid sequence of SSP revealed a 27 amino acid signal peptide, suggesting that the SSP protein is secreted to the cell wall of the stigmatic papillae. In-gel peroxidase activity assays showed that SSP has relatively low peroxidase activity compared to other, as yet uncharacterized, peroxidases present in stigmatic extracts. Six SSP alleles have been cloned from different lines of S. squalidus carrying a range of self-incompatibility (S)-alleles but there was no consistent association between the presence of a particular SSP allele and S-genotype indicating that SSP is not the female determinant of SSI in S. squalidus. Nevertheless, the precise expression of SSP in stigmatic papillae suggests that it may have a more general function in pollen-stigma interactions, or alternatively in protection of stigmas from pathogen attack. Extensive database screens have identified homologues of SSP in other plant species, but available expression data for these genes indicates that none are flower-specific, suggesting that SSP represents a new functional type of class III peroxidase specific to the stigma. We discuss the possible function(s) of S. squalidus SSP in pollen-stigma interactions and in protection of stigmas from pathogen attack.

Just how complex is the Brassica S-receptor complex?

Of the plant self-incompatibility (SI) systems investigated to date, that possessed by members of the Brassicaceae is currently the best understood. Whilst the recent demonstrations of interactions between the male determinant (S-locus cysteine rich protein, SCR) and the female determinant (S-locus receptor kinase, SRK) indicate the minimal requirement for SI in Brassica, no consensus exists as to the nature of these molecules in vivo and the potential involvement of accessory molecules in establishing the active S-receptor complex. Variation between S haplotypes appears to be present in the molecular composition of the receptor complex, the regulation of downstream signalling and the requirement for accessory molecules. This review discusses what constitutes an active receptor complex and highlights potential differences between haplotypes. The role of accessory molecules, in particular SLG (S-locus glycoprotein) and low molecular weight pollen coat proteins (PCPs), in pollination are discussed, as is the link between SI and unilateral incompatibility (UI).

Molecular mechanism of self-recognition in Brassica self-incompatibility

In most self-incompatible plant species, recognition of self-pollen is controlled by a single locus, termed the S-locus. In Brassica, genetic dissection of the S-locus has revealed the presence of three highly-polymorphic genes: S-receptor kinase (SRK), S-locus protein 11 (SP11) (also known as S-locus cysteine-rich protein; SCR) and S-locus glycoprotein (SLG). SRK encodes a membrane-spanning serine/threonine kinase that determines the S-haplotype specificity of the stigma. SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen. SLG encodes a secreted form of stigma protein similar to the extracellular domain of SRK. Recent biochemical studies have revealed that SP11 functions as the sole ligand for its cognate SRK receptor complex. Their interaction induces the autophosphorylation of SRK, which is expected to trigger the signalling cascade that results in the rejection of self-pollen. This so-called ligand-receptor complex interaction and receptor activation occur in an S-haplotype-specific manner, and this specificity is almost certainly the basis for self-pollen recognition.

Molecular aspects of self-incompatibility in Brassica species

Many flowering plants possess self-incompatibility (SI) systems to prevent inbreeding. SI in Brassica species is controlled by a single S locus with multiple alleles. In recent years, much progress has been made in determining the male and female S determinant in Brassica species. In the female, a gain-of-function experiment clearly demonstrated that SRK was the sole S determinant, and that SLG enhanced the SI recognition process. By contrast, the male S determinant (termed SP11/SCR) was identified in the course of genome analysis of S locus to be a small cysteine-rich protein, which was classified as a pollen coat protein. This SP11/SCR may function as a ligand for the S domain of SRK in the SI recognition reaction of Brassica species.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Use of a fast protein electrophoretic purification procedure for N-terminal sequence analysis to identify S-locus related proteins in stigmas of Brassica oleracea

In the cruciferous plant Brassica oleracea L. (cabbage), the S-locus specific glycoproteins (SLSGs) isolated only in stigmas are considered to play an important role in the normal prevention of self-fertilization. Recent molecular data have shown that the gene encoding these glycoproteins (the SLG gene) belonged to a multigenic family consisting of about 10 homologous copies among which another member is expressed, the S-locus related gene (SLR1gene). Our aim was to determine whether the SLR1-gene proteins were expressed in the stigmatic tissues. We first identified the putative SLSGs or SLR1-proteins by Con A-peroxidase detection of glycoproteins separated after isoelectric focusing in polyacrylamide gels. We describe a fast purification procedure for the glycoproteins of interest, based on analytical isoelectric focusing, electrophoresis, and electroblotting of proteins onto polyvinylidene difluoride membranes. Blotted proteins were sequenced for N-terminal amino acid determination. By comparison of the N-terminal sequences of the purified proteins with the peptide sequence predicted from the SLR1-cDNA, we demonstrate the expression of SLR1-like proteins in stigmas of B. oleracea.

A homozygous S genotype of Brassica oleracea expresses two S-like genes

The sporophytic self-incompatibility system of Brassica species is controlled by a single locus, S. Recognition of self between pollen and stigma is probably mediated by S locus-specific glycoproteins (SLSGs). We describe the isolation, from an S29 homozygote of Brassica oleracea, of two different cDNA clones for transcripts which are equally abundant in stigmas competent for self-incompatibility and each of which is homologous to previously reported SLSG sequences. Extensive DNA sequence divergence between the two clones precludes their cross-hybridisation and each acts as a gene-specific probe. All S genotypes appear to have a single copy of each gene but there are significantly different levels of polymorphism associated with each. The clear structural homology between the two indicates a gene duplication involving the S locus and, perhaps, related to the evolution of self-incompatibility.

Repressing the expression of self-incompatibility in crucifers by short-term high temperature treatment

The effect of short-term high temperature on the expression of self-incompatibility was studied in detached flowers of Brassica oleracea, B. campestris and Raphanus sativus. The expression of self-incompatibility was repressed by treatment of pistils at 40 °C for 15 minutes. Treatment at 50 °C repressed self-incompatibility but it also disturbed pollen tube elongation into stylar tissue. S-glycoproteins did not show any quantitative changes during the intact pistil treatment under 50 °C. Callose was occasionally found in the treated papilla where the self pollen tube penetrated. The repressing effect of the 40 °C treatment was found to be reversible, and this reversibility depended upon the environmental temperature of plant. Plants grown at 15/5 °C (day/night temperature) completely recovered self-incompatibility 2 h after treatment, while those grown at 20/10°, 25/15 °C did not. The reversibility of the expression of self-incompatibility correlated with the distortion of plasma membrane in the papilla. It is considered that high temperature affects the pollen tube penetration system in pistils rather than the recognition system between pistils and pollen. The treatment of dehiscing anthers at 40 °C killed the pollen.

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.

Con A - Peroxidase method: an improved procedure for staining S-glycoproteins in cellulose-acetate electrofocusing in crucifers

In the analysis of stigma glycoproteins by cellulose acetate electrofocusing in self-incompatible crucifers, the staining method of the glycoproteins, described in the earlier report, has been improved by using Con A - peroxidase reactions to obtain a permanent profile of band patterns which are visible under day-light conditions. Identifying S alleles by the corresponding S-glycoproteins can be facilitated by the present S-glycoprotein analysis.

S-specific proteins in styles of self-incompatible Nicotiana alata

A comparison of the stigma protein patterns of individual plants of the inbred- and cross-progenies in Nicotiana alata by isoelectric focusing revealed the presence of S-specific proteins. The S allele-protein relationship was found for three different S alleles. The S-specific proteins occurred in both stigma and stylar parts of the pistil whereas they were absent in leaves. In clone OWL the concentration of S-specific proteins in the stigma increased gradually during floral development. The shift from compatibility to incompatibility was not accompanied by an abrupt increase in concentration of the S-proteins.

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.

Do S allele-specific peroxidase isoenzymes exist in self-incompatible Nicotiana alata?

In order to test Pandey's hypothesis that peroxidase isoenzymes determine S-gene specificity in Nicotiana alata, peroxidase isoenzymes in styles and pollen from various plants of an inbred- and a cross progeny were compared by means of starch gel electrophoresis and electrofocusing.No relation between the S-genotype and the peroxidase isoenzyme patterns of pollen or of styles could be established. The differences between the isoenzyme patterns of different S-genotypes were ascribed to differences in the genetic background of various plants that had the same S-genotype.

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

Iso-electric focusing of extracts derived from stigmatic homogenates of Brassica oleracea reveals that the mature stigma possesses large quantities of a glycoprotein not present in earlier stages of development in the bud. Pollen germination experiments carried out in parallel with the biochemical tests suggest that the appearance of this glycoprotein, which has an isoelectric point of pH 5.8, is coincident with the development of the self-incompatibility response. The site of this protein, and the role it may play in pollen-stigma interactions are discussed in terms of current models of the self-incompatibility system in Brassica.