Characterization and localization of short-specific polygalacturonase in distylous Turnera subulata (Turneraceae)

Two floral forms in the same species-distyly

MAIN CONCLUSION

This paper reviews the progress of research on the morphology, physiology and molecular biology of distyly in plants. It will help to elucidate the mysteries of distyly in plants. Distyly is a unique representative type of heterostyly in plants, primarily characterized by the presence of long style and short style within the flowers of the same species. This interesting trait has always fascinated researchers. With the rapid development of molecular biology, the molecular mechanism for the production of dimorphic styles in plants is also gaining ground. Researchers have been studying plant dimorphic styles from various perspectives. The researchers are gradually unravelling the mechanisms by which plants produce distyly traits. This paper reviews advances in the study of plant dimorphic style characteristics, mainly in terms of the morphology, physiology and molecular biology of plants with dimorphic styles. The aim is to provide a theoretical basis for the study of the mechanism of distyly formation in plants.

Identification of pollen and pistil polygalacturonases in Nicotiana tabacum and their function in interspecific stigma compatibility

KEY MESSAGE

The pollen and pistil polygalacturonases in Nicotiana tabacum were identified and found to regulate pollen tube growth and interspecific compatibility. Polygalacturonase (PG) is one of the enzymes catalyzing the hydrolysis of pectin. This process plays important roles in the pollen and pistil. In this research, the pollen and pistil PGs in Nicotiana tabacum (NtPGs) were identified, and their expression, localization and the potential function in the pollen and interspecific stigma incompatibility were explored. The results showed that 118 NtPGs were retrieved from the genome of N. tabacum. The phylogenetic tree and RT-qPCR analysis led to the identification of 10 pollen PGs; among them, two, seven and one showed specifically higher expression levels in the early development of anthers, during pollen maturation and in mature anthers, respectively, indicating their function difference. Immunofluorescence analysis showed that PGs were located in the cytoplasm of (1) mature pollen and (2) in vitro grown pollen tubes, as well as in the wall of in vivo grown pollen tubes. Four NtPGs in clade A were identified as the pistil PGs, and the pistil PGs were not found in clade E. Significantly higher PGs expression was recorded after incompatible pollination in comparison with the compatible stigma, indicating a potential function of PGs in regulating stigma incompatibility. The influence of PGs on pollen tube growth was explored in vitro and partly in vivo, showing that high PGs activity inhibited pollen tube growth. The application of PGs on the otherwise compatible stigma resulted in pollen tube growth inhibition or failure of germination. These results further supported that increased PGs expression in incompatible stigma might be partially responsible for the interspecific stigma incompatibility in Nicotiana.

Transcriptome and Network Analyses of Heterostyly in Turnera subulata Provide Mechanistic Insights: Are S-Loci a Red-Light for Pistil Elongation?

Heterostyly employs distinct hermaphroditic floral morphs to enforce outbreeding. Morphs differ structurally in stigma/anther positioning, promoting cross-pollination, and physiologically blocking self-fertilization. Heterostyly is controlled by a self-incompatibility (S)-locus of a small number of linked S-genes specific to short-styled morph genomes. Turnera possesses three S-genes, namely TsBAHD (controlling pistil characters), TsYUC6, and TsSPH1 (controlling stamen characters). Here, we compare pistil and stamen transcriptomes of floral morphs of T. subulata to investigate hypothesized S-gene function(s) and whether hormonal differences might contribute to physiological incompatibility. We then use network analyses to identify genetic networks underpinning heterostyly. We found a depletion of brassinosteroid-regulated genes in short styled (S)-morph pistils, consistent with hypothesized brassinosteroid-inactivating activity of TsBAHD. In S-morph anthers, auxin-regulated genes were enriched, consistent with hypothesized auxin biosynthesis activity of TsYUC6. Evidence was found for auxin elevation and brassinosteroid reduction in both pistils and stamens of S- relative to long styled (L)-morph flowers, consistent with reciprocal hormonal differences contributing to physiological incompatibility. Additional hormone pathways were also affected, however, suggesting S-gene activities intersect with a signaling hub. Interestingly, distinct S-genes controlling pistil length, from three species with independently evolved heterostyly, potentially intersect with phytochrome interacting factor (PIF) network hubs which mediate red/far-red light signaling. We propose that modification of the activities of PIF hubs by the S-locus could be a common theme in the evolution of heterostyly.

Buckwheat heteromorphic self-incompatibility: genetics, genomics and application to breeding

Common buckwheat (Fagopyrum esculentum Moench 2n = 2x = 16) is an outcrossing crop with heteromorphic self-incompatibility due to its distylous flowers, called pin and thrum. In pin plants, a long style is combined with short stamens and small pollen grains; in thrum plants, a short style is combined with long stamens and large pollen grains. Both the intra-morph self-incompatibility and flower morphology are controlled by a single genetic locus named the S locus; thrum plants are heterozygous (Ss) and pin plants are homozygous recessive (ss) at this locus. Self-incompatibility is an obstacle for establishing pure lines and fixation of agronomically useful genes. Elucidation of the molecular mechanism of heterostylous self-incompatibility of common buckwheat has continued for a quarter of a century. Recent advances in genomic and transcriptomic analyses using next-generation sequencing have made it possible to determine the genomic region harboring the buckwheat S locus and to identify novel genes at this locus. In this review, we summarize the current knowledge on buckwheat heterostyly gained from conventional and molecular genetics and genomics. We also discuss the application of these studies to breeding of common buckwheat.

Identification of a gene encoding polygalacturonase expressed specifically in short styles in distylous common buckwheat (Fagopyrum esculentum)

Common buckwheat (Fagopyrum esculentum) is a heteromorphic self-incompatible (SI) species with two types of floral architecture: thrum (short style) and pin (long style). The floral morphology and intra-morph incompatibility are controlled by a single genetic locus, S. However, the molecular mechanisms underlying the heteromorphic self-incompatibility of common buckwheat remain unclear. To identify these mechanisms, we performed proteomic, quantitative reverse-transcription PCR, and linkage analyses. Comparison of protein profiles between the long and short styles revealed a protein unique to the short style. Amino-acid sequencing revealed that it was a truncated form of polygalacturonase (PG); we designated the gene encoding this protein FePG1. Phylogenetic analysis classified FePG1 into the same clade as PGs that function in pollen development and floral morphology. FePG1 expression was significantly higher in short styles than in long styles. It was expressed in flowers of a short-homostyle line but not in flowers of a long-homostyle line. Linkage analysis indicated that FePG1 was not linked to the S locus; it could be a factor downstream of this locus. Our finding of a gene putatively working under the regulation of the S locus provides useful information for elucidation of the mechanism of heteromorphic self-incompatibility.

A short story gets longer: recent insights into the molecular basis of heterostyly

Heterostyly is a fascinating adaptation to promote outbreeding and a classical paradigm of botany. In the most common type of heterostyly, plants either form flowers with long styles and short stamens, or short styles and long stamens. This reciprocal organ positioning reduces pollen wastage and promotes cross-pollination, thus increasing male fitness. In addition, in many heterostylous species selfing and the generation of unfit progeny due to inbreeding depression is limited by a self-incompatibility system, thus promoting female fitness. The two floral forms are genetically determined by the S locus as a complex supergene, namely a chromosomal region containing several individual genes that control the different traits, such as style or stamen length, and are held together by very tight linkage due to suppressed recombination. Recent molecular-genetic studies in several systems, including Turnera, Fagopyrum, Linum, and Primula have begun to identify and characterize the causal heterostyly genes residing at the S locus. An emerging theme from several families is that the dominant S haplotype represents a hemizygous region not present on the recessive s haplotype. This provides an explanation for the suppressed recombination and suggests a scenario for the chromosomal evolution of the S locus. In this review, we discuss the results from recent molecular-genetic analyses in light of the classical models on the genetics and evolution of heterostyly.

Integration of genetic and physical maps of the Primula vulgaris S locus and localization by chromosome in situ hybridization

Heteromorphic flower development in Primula is controlled by the S locus. The S locus genes, which control anther position, pistil length and pollen size in pin and thrum flowers, have not yet been characterized. We have integrated S-linked genes, marker sequences and mutant phenotypes to create a map of the P. vulgaris S locus region that will facilitate the identification of key S locus genes. We have generated, sequenced and annotated BAC sequences spanning the S locus, and identified its chromosomal location. We have employed a combination of classical genetics and three-point crosses with molecular genetic analysis of recombinants to generate the map. We have characterized this region by Illumina sequencing and bioinformatic analysis, together with chromosome in situ hybridization. We present an integrated genetic and physical map across the P. vulgaris S locus flanked by phenotypic and DNA sequence markers. BAC contigs encompass a 1.5-Mb genomic region with 1 Mb of sequence containing 82 S-linked genes anchored to overlapping BACs. The S locus is located close to the centromere of the largest metacentric chromosome pair. These data will facilitate the identification of the genes that orchestrate heterostyly in Primula and enable evolutionary analyses of the S locus.

Isolation of the floral morph-related genes in heterostylous flax (Linum grandiflorum): the genetic polymorphism and the transcriptional and post-transcriptional regulations of the S locus

Heterostylous species have two types of flowers, thrum and pin morphs, and these are controlled by a single diallelic locus designated the 'S locus'; fertilization between these two types of flowers is successful. The S gene and the molecular mechanism by which it operates remain to be uncovered, although heterostyly has been studied since the time of Darwin. We compared transcripts and proteins of the thrum and pin flowers of heterostylous flax (Linum grandiflorum) to characterize the molecular differences between them and to elucidate the molecular machinery of heterostyly. Twelve floral morph-related genes were eventually isolated by an integrated study of subtraction and 2D-PAGE analyses, and four genes, TSS1, LgAP1, LgMYB21 and LgSKS1, were predicted to be related to heterostyly. TSS1, a thrum style-specific gene, showed some features suitable for the S gene. Although its biological function is unclear, TSS1 was expressed only in the thrum style and is probably linked to the S locus. LgMYB21, another thrum style gene, would be involved in floral morphogenesis. LgMYB21 was highly expressed in the thrum style, which is shorter than the pin style, and its overexpression in Arabidopsis reduced pistil length. Furthermore, a comparison of transcript and protein accumulations showed no differences in the mRNA accumulation of some thrum-specific proteins, including LgSKS1, suggesting that these are regulated by floral morph-specific post-transcriptional regulation. The Linum S locus regulates not only S specificity but also many floral phenotypes. Dynamic regulation of transcripts and proteins would be necessary for the pleiotropic function of the Linum S locus.

Cryptic self-incompatibility and distyly in Hedyotis acutangula Champ. (Rubiaceae)

Distyly, floral polymorphism frequently associated with reciprocal herkogamy, self- and intramorph incompatibility and secondary dimorphism, constitutes an important sexual system in the Rubiaceae. Here we report an unusual kind of distyly associated with self- and/or intramorph compatibility in a perennial herb, Hedyotis acutangula. Floral morphology, ancillary dimorphisms and compatibility of the two morphs were studied. H. acutangula did not exhibit precise reciprocal herkogamy, but this did not affect the equality of floral morphs in the population, as usually found in distylous plants. Both pin and thrum pollen retained relatively high viability for 8 h. The pollen to ovule ratio was 72.5 in pin flowers and 54.4 in thrum flowers. Pistils of pin flowers remained receptive for longer than those of thrum flowers. No apparent difference in the germination rate of pin and thrum pollen grains was observed when cultured in vitro, although growth of thrum pollen tubes was much faster than that of pin pollen tubes. Artificial pollination revealed that pollen tube growth in legitimate intermorph crosses was faster than in either intramorph crosses or self-pollination, suggesting the occurrence of cryptic self-incompatibility in this species. Cryptic self-incompatibility functioned differently in the two morphs, with pollen tube growth rates after legitimate and illegitimate pollination much more highly differentiated in pin flowers than in thrum flowers. No fruit was produced in emasculated netted flowers, suggesting the absence of apomixis. Our results indicate that H. acutangula is distylous, with a cryptic self-incompatibility breeding system.

"A case to which no parallel exists": The influence of Darwin's Different Forms of Flowers

PREMISE OF THE STUDY

Research on the subject of heterostyly is often traced back to 1877 when Charles Darwin published the landmark book The Different Forms of Flowers on Plants of the Same Species. This book synthesized heterostyly research at the time, much of which Darwin conducted, and it continues to be a major contribution to the study of the breeding system. In this book, Darwin discussed the ecology, morph-specific differences, self- and intramorph-incompatibility, evolution and origin, and floral development of heterostyly. Many of the hypotheses he proposed have been and continue to be tested.

KEY RESULTS

Throughout the 20(th) and 21(st) centuries, researchers have continued to identify new and different morph-specific floral characters, discover the mechanisms that underlie heteromorphic self-incompatibility, use phylogenies to examine the evolution of heterostyly, and determine novel floral developmental patterns in heterostylous species. From all of these studies, we have learned a great deal about the function, evolution, and development of heterostyly.

CONCLUSIONS

However, almost 150 years after Darwin's publications on the subject of heterostyly, we still have a great deal to learn concerning the breeding system, and new technologies and techniques are allowing for new advances in heterostyly research to occur.

BcMF9, a novel polygalacturonase gene, is required for both Brassica campestris intine and exine formation

BACKGROUND AND AIMS

The polygalacturonase (PG) gene family has been found to be enriched in pollen of several species; however, little is currently known about the function of the PG gene in pollen development. To investigate the exact role that the PG gene has played in pollen development and about this family in general, one putative PG gene, Brassica campestris Male Fertility 9 (BcMF9), was isolated from Chinese cabbage (Brassica campestris ssp. chinensis, syn. B. rapa ssp. chinensis) and characterized.

METHODS

RT-PCR, northern blotting and in situ hybridization were used to analyse the expression pattern of BcMF9, and antisense RNA technology was applied to study the function of this gene.

KEY RESULTS

BcMF9 is expressed in particular in the tapetum and microspore during the late stages of pollen development. Antisense RNA transgenic plants that displayed decreased expression of BcMF9 showed pollen morphological defects that resulted in reduced pollen germination efficiency. Transmission electron microscopy revealed that the homogeneous pectic exintine layer of pollen facing the exterior was over-developed and predominantly occupied the intine, reversing the normal proportional distribution of the internal endintine layer and the external exintine in transgenic pollen. Inhibition of BcMF9 also resulted in break-up of the previously formed tectum and baculae from the beginning of the binucleate stage, as a result of premature degradation of tapetum.

CONCLUSIONS

Several lines of evidence, including patterns of BcMF9 expression and phenotypic defects, suggest a sporophytic role in exine patterning, and a gametophytic mode of action of BcMF9 in intine formation. BcMF9 might act as a co-ordinator in the late stages of tapetum degeneration, and subsequently in the regulation of wall material secretion and, in turn, exine formation. BcMF9 might also play a role in intine formation, possibly via regulation of the dynamic metabolism of pectin.

High-resolution mapping of the S-locus in Turnera leads to the discovery of three genes tightly associated with the S-alleles

While the breeding system known as distyly has been used as a model system in genetics, and evolutionary biology for over a century, the genes determining this system remain unknown. To positionally clone genes determining distyly, a high-resolution map of the S-locus region of Turnera has been constructed using segregation data from 2,013 backcross progeny. We discovered three putative genes tightly linked with the S-locus. An N-acetyltransferase (TkNACE) flanks the S-locus at 0.35 cM while a sulfotransferase (TkST1) and a non-LTR retroelement (TsRETRO) show complete linkage to the S-locus. An assay of population samples of six species revealed that TsRETRO, initially discovered in diploid Turnera subulata, is also associated with the S-allele in tetraploid T. subulata and diploid Turnera scabra. The sulfotransferase gene shows some level of differential expression in long versus short styles, indicating it might be involved in some aspect of distyly. The complete linkage of TkST1 and TsRETRO to the S-locus suggests that both genes may reside within, or in the immediate vicinity of the S-locus. Chromosome walking has been initiated using one of the genes discovered in the present study to identify the genes determining distyly.

The polygalacturonase gene BcMF2 from Brassica campestris is associated with intine development

Brassica campestris Male Fertility 2 (BcMF2) is a putative polygalacturonase (PG) gene previously isolated from the flower bud of Chinese cabbage (Brassica campestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis). This gene was found to be expressed specifically in tapetum and pollen after the tetrad stage of anther development. Antisense RNA technology was used to study the function of BcMF2 in Chinese cabbage. Scanning and transmission electron microscopy revealed that there were deformities in the transgenic mature pollen grains such as abnormal location of germinal furrows. In addition, the homogeneous pectic exintine layer facing the exterior seemed to be overdeveloped and predominantly occupied the intine, thus reversing the normal proportional distribution of the internal endintine layer and the external exintine layer. Since it is a continuation of the intine layer, the pollen tube wall could not grow normally. This resulted in the formation of a balloon-like swelling structure in the pollen tube tip in nearly 80% of the transgenic pollen grains. Premature degradation of tapetum was also found in these transgenic plants, which displayed decreased expression of the BcMF2 gene. BcMF2 might therefore encode a new PG with an important role in pollen wall development, possibly via regulation of pectin's dynamic metabolism.

Construction of a first genetic map of distylous Turnera and a fine-scale map of the S-locus region

As a prelude to discovery of genes involved in floral dimorphism and incompatibility, a genetic map of distylous Turnera was constructed along with a fine-scale map of the S-locus region. The genetic map consists of 79 PCR-based molecular markers (48 AFLP, 18 RAPD, 9 ISSR, 4 RAMP), 5 isozyme loci, one additional gene, and the S-locus, spanning a total distance of 683.3 cM. The 86 markers are distributed in 5 linkage groups, corresponding to the haploid chromosome number. Molecular markers tightly linked or co-segregating with the S-locus in an initial mapping population of 94 individuals were used to assay an additional 642 progeny to construct a map of the S-locus region. The fine-scale map consists of 2 markers (IS864a and RP45E9) flanking the S-locus at distances of 0.41 and 0.54 cM, respectively, and 3 additional markers (OPK14c, RP45G18, and RP81E18) co-segregating with the S-locus in the total mapping population of 736 individuals. The genetic map constructed will serve as a framework for localization of genes outside the S-locus affecting distyly, while molecular markers of the fine-scale map will be used to initiate chromosome walking to find the genes residing at the S-locus.

Pectin methylesterase and polygalacturonase in the developing grape skin

Ripening of grape (Vitis vinifera L.) berry immediately precedes harvesting and the evolution of the skin tissue is important as it contains the key compounds for wine quality. Grape softening is thought to result from extensive cell wall modifications that occur during ripening. These modifications result from the activity of different cell wall-modifying enzymes. Two of the most significant pectin-degrading enzymes are pectin methylesterase (EC 3.1.1.11) and polygalacturonase (EC 3.2.1.15). In this work, the activities of both enzymes were monitored in skin tissue throughout berry development. Pectin methylesterase activity was present before the onset of veraison and increased during skin maturation. No polygalacturonase activity could be detected. The accumulation of mRNA encoding a pectin methylesterase and two polygalacturonase isoforms was examined using RT-PCR. Transcripts for pectin methylesterase were present in all stages analyzed with a maximal accumulation at the end of color change. Accumulation of VvPG1 transcript was closely correlated with berry softening, and expression of this gene was markedly increased during the color change. VvPG2 mRNA accumulation began before veraison and was low during skin ripening. A phylogenic analysis showed that this gene is classified in a different group than VvPG1. These findings suggest that both genes are associated with different mechanisms during skin development. VvPG1, in particular, is most likely to play a role in skin softening and VvPG2 in triggering the ripening process.

Identification and characterization of pin and thrum alleles of two genes that co-segregate with the Primula S locus

The study of heteromorphy in Primula over the past 140 years has established the reproductive significance of this breeding system. Plants produce either thrum or pin flowers that demonstrate reciprocal herkogamy. Thrums have short styles and produce large pollen from anthers at the mouth of the flower; pins have long styles and produce small pollen from anthers located within the corolla tube. The control of heteromorphy is orchestrated by the S locus with dominant (S) and recessive (s) alleles that comprise a co-adapted linkage group of genes. Thrum plants are heterozygous (Ss) and pin plants are homozygous (ss). Reciprocal crosses between the two forms are required for fertilization; within-morph crosses are impeded by a sporophytic self-incompatibility system. Rare recombination events within the S locus produce self-fertile homostyles. As a first step towards identifying genes located at the S locus, we used fluorescent differential display to screen for differential gene expression in pin and thrum flowers. Rather than only detecting differentially regulated genes, we identified two S locus linked genes by virtue of allelic variation between pin and thrum transcripts. Analysis of pin and thrum plants together with homostyle recombinant reveals that one gene flanks the locus, whereas the other shows complete linkage. One gene is related to Arabidopsis flower-timing genes Col9 and Col10; the other encodes a small predicted membrane protein of unknown function. Notwithstanding the diallelic behaviour of the Primula S locus, analysis of pin and thrum plants reveal three alleles for each gene: two pin and one thrum.

Meiotic recombination in Turnera (Turneraceae): extreme sexual difference in rates, but no evidence for recombination suppression associated with the distyly (S) locus

To explore the rate of recombination resulting from male vs female meiosis, crosses were performed using distylous Turnera subulata as well as a cross involving the introgression of genes from T. krapovickasii into T. subulata. We assayed four loci on the chromosome bearing the S-locus as well as two loci on each of two other linkage groups. Substantial and consistent dimorphism in recombination rates was found with female meiosis resulting in as much as a approximately 6-fold increase relative to male. Aberrant single locus segregation ratios occurred for some loci, particularly when the male (pollen) parent was heterozygous and the cross involved introgressed genes. The extreme trend of greater recombination resulting from female meiosis was, however, maintained in crosses where no aberrant ratios occurred, indicating that the sex dimorphism in recombination is not the result of aberrant segregation. We also exploited this distylous species and tested whether there is recombination suppression around the S-locus because of an inversion or other chromosome rearrangement(s). We found no significant evidence for recombination suppression.

Breeding system variation, genetics and evolution in the Turneraceae

We review the genetics and evolution of breeding systems in the Turneraceae. Distyly occurs in seven of 10 genera and 81% of species. The remaining species are homostylous. Polyploid evolution has been significant in Turnera. Approximately 60% of species are polyploid ranging from diploid through decaploid. No relationship between breeding system and polyploidy is evident. The genetics of distyly involves a one-locus two-allele system (S and s). Evidence from crosses with homostylous species and mutants is consistent with the possibility that a "Primula-type" supergene underlies distyly but does not prove this to be the case. A polygalacturonase, and an alpha-dioxygenase specific to the transmitting tissue of short-styled plants both exhibit morph-limited expression in concert with predictions from an evolutionary model. The function of the proteins in distyly, if any, is unknown. We have begun constructing a fine-scale genetic map of Turnera. Two genetic markers lie within 0.2 cm of the distyly locus. This should provide a starting point for positional cloning of the distyly locus and reveal the genetic architecture and molecular basis of distyly.

Inheritance of spontaneous mutant homostyles in Turnera subulata x krapovickasii and in autotetraploid T. scabra (Turneraceae)

To explore the genetic architecture of distyly in Turnera spp., we determined the inheritance and compatibility behaviour of two spontaneous homostyled mutants. A long-homostyled mutant shoot arose on an otherwise short-styled plant that was an artificial hybrid (Turnera subulata x T. krapovickasii) between two diploid distylous species. The mutation appears to be an allele, SH, of the distyly locus with the dominance relationships, S>SH>s, where S confers the short-styled phenotype, SH confers homostyly in SHSH and SHs genotypes, and ss genotypes are long-styled. Aberrant segregation ratios were observed among some crosses and might be the result of pollen competition. Compatibility relationships are consistent with the hypothesis that a gene complex determines distyly. Infrequently, revertant short-styled flowers have appeared on cuttings of the T. subulata x T. krapovickasii mutant and on occasion, short-styled progeny have appeared in crosses where none were expected. A second mutant homostyle was discovered in autotetraploid T. scabra. The mutation is inherited as above, however, tetrasomic inheritance occurs at the locus. This homostyled mutant carries two copies of the SH allele and has the duplex genotype SHSHss. Compatibility relationships were as observed above. The occurrence of homostyled mutants is consistent with the hypothesis that a linked gene complex underlies the inheritance of distyly in Turnera but we cannot discount the hypothesis that an allelic series is responsible.

Distribution of style and pollen polygalacturonases among distylous and homostylous Turnera and Piriqueta spp. (Turneraceae)

We explore the distribution of a style and pollen polygalacturonase in a number of distylous and homostylous species of Turnera, and two species of Piriqueta (Turneraceae). We show, using immunoblotting with antibodies made against these proteins, that the style polygalacturonase is specific to styles of short-styled plants of all the six distylous species of Turnera we have investigated. Styles of a somatic homostylous mutant derived from a short-styled plant do not possess the style polygalacturonase. Distylous P. caroliniana did not appear to possess this protein. We show that the pollen polygalacturonase, while associated with the short-styled morph in three species, is polymorphic among short-styled plants of T. krapovickasii, and absent from T. joelii, T. grandiflora and P. caroliniana. These data support a role for the style polygalacturonase in distyly, possibly in the incompatibility system, but cast doubt on any role for the pollen polygalacturonase. In concert with the predictions for the mode of origin, and the response of styles of homostylous species to pollen from long- and short-styled plants, we find that none of the homostylous species possess the style polygalacturonase. The pollen polygalacturonase does occur in some homostylous species, but not in others. It is not clear that the pollen polygalacturonase, however, provides a marker for the mode of origin of homostyly.

From pollen tubes to infection threads: recruitment of Medicago floral pectic genes for symbiosis

While the biology of nitrogen-fixing root nodules has been extensively studied, little is known about the evolutionary events that predisposed legume plants to form symbiosis with rhizobia. We have studied the presence and the expression of two pectic gene families in Medicago, polygalacturonases (PGs) and pectin methyl esterases (PMEs) during the early steps of the Sinorhizobium meliloti-Medicago interaction and compared them with related pollen-specific genes. First, we have compared the expression of MsPG3, a PG gene specifically expressed during the symbiotic interaction, with the expression of MsPG11, a highly homologous pollen-specific gene, using promoter-gus fusions in transgenic M. truncatula and tobacco plants. These results demonstrated that the symbiotic promoter functions as a pollen-specific promoter in the non-legume host. Second, we have identified the presence of a gene family of at least eight differentially expressed PMEs in Medicago. One subfamily is represented by one symbiotic gene (MtPER) and two pollen-expressed genes (MtPEF1 and MtPEF2) that are clustered in the M. truncatula genome. The promoter-gus studies presented in this work and the homology between plant PGs, together with the analysis of the PME locus structure and MtPER expression studies, suggest that the symbiotic MsPG3 and MtPER could have as ancestors pollen-expressed genes involved in polar tip growth processes during pollen tube elongation. Moreover, they could have been recruited after gene duplication in the symbiotic interaction to facilitate polar tip growth during infection thread formation.