Rivka Dulberger

Floral polymorphisms and their functional significance in the heterostylous syndrome

The emblem of the heterostylous syndrome is the positioning at equivalent levels of the stigmas of one floral morph and the anthers of one or two alternate morphs. Of less prominence in the syndrome are distinguishing characters of the pistils and stamens of the morphs, particularly of the stigmas and pollen grains. At the core of heterostyly there is typically a sporophytically controlled, diallelic incompatibility system which prevents or reduces self- and intra-morph fertilizations. Thus, in distyly the entire syndrome is controlled by a “supergene”, i.e., a tightly linked group of genes, each with two alleles and apparently controlling three groups of characters; in tristyly the control is by genes at two loci, each with two alleles and epistatic interaction.

Intermorph Structural Differences between Stigmatic Papillae and Pollen Grains in Relation to Incompatibility in Plumbaginaceae

In dimorphic Plumbaginaceae the floral morphs differ in the wall structure at the apical region of the stigmatic papilla. In Ceratostigma willmottianum, Plumbago europaea and P. capensis, papillae of the long-styled and short-styled morphs differ in the way the cuticle is attached to the cellulose layer. In the short-styled morph of P. capensis, the cuticle of the papillae has minute protuberances. The ‘cob’ and ‘papillate’ stigmas of dimorphic Limonium species and of Armeria maritima are distinguished by the thickness of the cuticle layer at the papilla apex. In all species examined, intramorph pollinations are incompatible and inhibition of the pollen occurs at the stigma surface. Structural stigma dimorphism is probably involved in the incompatibility mechanism. In Limonium meyeri the stigma dimorphism controls the incompatibility at least at the stage of adherence of the pollen to the stigmas. Pollen grains of type A adhere to both types of stigmas; pollen of type B adheres to cob stigmas, but usually does not adhere to papillate stigmas. It is possible that a chemical ‘recognition’ is reinforced by a topographical complementarity between the outline of the stigma surface and of the pollen exine. It is suggested that the exine dimorphism may play a role in recognition by means of a different distribution of incompatibility substances held in the pollen wall. The view that the stigma and pollen dimorphism reflects the incompatibility mechanism is considered in relation to various breeding systems known in the Staticeae.

Heterostyly in species ofNarcissus (Amaryllidaceae) andHugonia (Linaceae) and other disputed cases

The hypothesis ofHenriques andFernandes that several Iberian species ofNarcissus (Amaryllidaceae) are tristylous is reconsidered. Contrary to the opinion ofBateman and most subsequent authors, we believe that the available evidence indicates that some populations ofN. triandrus andN. fernandesii, at least, are tristylous; other populations ofN. triandrus are distylous.Hugonia cf.penicillanthemum (Linaceae) from new Caledonia is distylous, but it remains possible that other species ofHugonia are tristylous. The disputed occurrence of heterostyly in S. African species ofBauhinia (Leguminosae),Cleome (Capparaceae) andAneilema (Commelinaceae), and inAgelaea (Connaraceae) is discussed.

S -Gene action and the significance of characters in the heterostylous syndrome

SummaryHeteromorphic characters of stigmas and pollen grains participate in the physiological mechanism of incompatibility. This points to incompatibility as a major factor in the evolutionary moulding of pollen and stigmas. A first reconstruction of S-gene action shows that it controls normal growth and differentiation of floral parts. The S gene operates through a stepwise progression of events. The independently mutable subunits of the S gene revealed by breakdowns of the heterostylous syndrome control stages in the chain of activities of the gene. Inception of pollen size heteromorphism during early meiosis indicates that sporophytic incompatibility factors may be derived not only from the tapetum, but from the pollen mother cells as well. It is suggested that the differential growth of styles and stamens in the morphs mediates the production of chemical recognition specificities and that metabolism related to wall extension plays an important role in incompatibility. The physiological basis of differential growth control in the morphs and the significance of pollen size heteromorphism are discussed.

Overexpression of the Wheat FK506-Binding Protein 73 (FKBP73) and the Heat-Induced Wheat FKBP77 in Transgenic Wheat Reveals Different Functions of the Two Isoforms

The FK506-binding proteins (FKBPs) belong to the peptidyl prolyl cis-trans isomerase (PPIase) family, and catalyse the rotation of the peptide bond preceding a proline. They are conserved in organisms from bacteria to man. In order to understand the function of plant FKBP isoforms, we have produced transgenic wheat plants overexpressing each of the two wheat FKBPs: wFKBP73 (which is expressed in young vegetative and reproductive tissues under normal growth conditions) and wFKBP77 (which is induced by heat stress). Transgenic lines overexpressing wFKBP77 at 25°C showed major morphological abnormalities, specifically relating to height, leaf shape, spike morphology and sterility. In these plants, the levels of hsp90 mRNA were over two fold higher than in controls, indicating a common regulatory pathway shared between wFKBP77 and Hsp90. Transgenic lines overexpressing wFKBP73 showed normal vegetative morphology, but the grain weight and composition was altered, corresponding to changes in amylase activity during seed development.

Deletion of the C-terminal 138 amino acids of the wheat FKBP73 abrogates calmodulin binding, dimerization and male fertility in transgenic rice.

Wheat FKBP73 (wFKBP73) belongs to the FK506-binding protein (FKBP) family which, in common with the cyclophilin and parvulin families, possesses peptidyl prolylcis-trans isomerase (PPIase) activity. Wheat FKBP73 has been shown to contain three FKBP12-like domains, a tetratricopeptide repeat (TPR) via which it binds heat shock protein 90 and a calmodulin-binding domain (CaMbd). In this study we investigated: (1) the contribution of the N-terminal and C-terminal moieties of wFKBP73 to its biological activity by over-expression of the prolyl isomerase domains in transgenic rice, and (2) the biochemical characteristics of the C-terminal moiety. The recombinant wFKBP73 was found to bind calmodulin via the CaMbd and to be present mainly as a dimer in solution. The dimerization was abrogated when 138 amino acids from the C-terminal half were deleted. Expression of the full-length FKBP73 produced fertile rice plants, whereas the expression of the peptidyl prolyl cis-trans isomerase domains in transgenic rice resulted in male-sterile plants. The male sterility was expressed at various stages of anther development with arrest of normal pollen development occurring after separation of the microspores from the tetrads. Although the direct cause of the dominant male sterility is not yet defined, we suggest that it is associated with a novel interaction of the prolyl isomerase domains with anther specific target proteins.

Dimorphic exine sculpturing in three distylous species ofLinum (Linaceae)

DistylousLinum grandiflorum, L. mucronatum, andL. pubescens have dimorphic pollen grains. In short-styled flowers the exine has monomorphic processes each with a ring of papillae. In long-styled flowers the exine has two types of processes: small processes terminating in a spinule and larger processes with a central spinule surrounded by a ring of spinuels, papillae or buttresses. The distylousLinum species of four different sections, in which morph-specific differences in the wall structure of stigmatic papillae have been reported, also exhibit exine dimorphism. The patterns of these dimorphisms are similar in all four sections in which distyly is present.

The genetic basis of gender in Silene vulgaris

SummaryInheritance of gender was studied in tetraploid Silene vulgaris in Israel. Three main phenotypes, hermaphrodite, H, female, F, and intermediate HF, were recognized. With the support of data from crosses within and between the phenotypes it is postulated that basic gender is controlled at a single tetrasomic locus AA/aa. Quadruplex and triplex AAAA and AAAa individuals exhibit phenotype H, simplex and nulliplex aaaa and Aaaa individuals phenotype F, and AaAa individuals phenotype HF. Predominantly hermaphrodite or predominantly female subgroups of the HF phenotype are liable to be confused with true H or F types, so that misclassifications may blurr the underlying genetic pattern. In the HF phenotype, in which A is assumed to be incompletely dominant over a, expression of male sterility is confined to single flowers or single anthers in a pattern with the appearance of randomness. The question is asked whether the mosaic pattern of sex expression in heterozygotes may reflect vegetative segregation of dissimilar cytoplasmic male sterility factors which are controlled by nuclear restorer genes.

Stigma morphology in distylous and non-heterostylous species ofVillarsia (Menyanthaceae)

Stigma morphology was examined with the SEM in 14 of the 16 species ofVillarsia. In nine of the ten distylous species studied, stigmas of the floral morphs were strongly dimorphic in length, shape, configuration of the receptive surface, and in the size and density of their papillae. Thrum stigmas ofVillarsia, in contrast to those of most other distylous species, are not simply smaller versions of the conspecific pin stigmas, but generally exhibit an array of morph-specific characters. Thrum stigma lobes may be broader than those of pins, they may have undulate margins, lobes subdivided into secondary lobes, papillae more extensively distributed than in pins, and various combinations of these traits occur. The traits that distinguish thrum from pin stigmas achieve an increase in the receptive area and may enhance more efficient pollen capture by the shorter and less accessible thrum stigmas. The morphogenesis of the stigma shape dimorphism appears to involve processes more complex than inhibition of elongation in thrum styles. InVillarsia, the stigma dimorphisms are species-specific. No correlations were found between morphologies of the stigma and the different breeding systems in distylous species. Stigmas of the four non-heterostylous species examined resemble the thrum stigma type found in most distylousVillarsia species.

Studies on Wheat Prolyl Isomerase in Transgenic Plants

Peptidyl prolyl isomerases catalyse the interconversion between cis and trans forms of the peptide bond preceding proline residues in proteins. Three distinct families of prolyl isomerases have been identified: the cyclophilins, the FKBPs and the parvulins (Schmid 1997; Dolinski, Heitman 1997). Cyclophilins and FKBPs have attracted attention as the cellular receptors for the immunosuppresants cyclosporin A, FK506 and rapamycin, but this role in drug action is distinct from the enzymatic activity. Members of the three prolyl isomerase families are highly conserved, abundant and expressed in multiple cellular compartments, suggesting that these enzymes play a critical role in cell physiology. Because prolyl isomerases can accelerate slow protein refolding steps in vitro, it has been speculated that this is what they do in vivo. A number of proteins physically associated with different prolyl isomerases such as kinases and transcription factors have been identified but the physiological significance of these associations remains to be established. The prolyl isomerases have been implicated in various cellular functions (independent of their prolyl isomerase activity) such as DNA degradation activity and RNA maturation.