The difficult question of sex: the mating game

Systematic Identification and Analysis of Lysine Succinylation in Strawberry Stigmata

The various post-translational modifications (PTMs) of plant proteins have important regulatory roles in development. We therefore examined various modified proteins from strawberry stigmata and found that succinylation of lysine residues was the most abundant type of modification. We then subjected proteins from strawberry stigmata to an efficient enrichment method for succinylated peptides and identified 200 uniquely succinylated lysines (Suks) in 116 proteins. A bioinformatics analysis revealed that these proteins are involved in important biological processes, including stress responses, vesicular transport, and energy metabolism. Proteomics, combined with immunoprecipitation and immunoblotting, revealed an obvious increase in succinylation of the assembly polypeptide 2 (AP2) and clathrin from 0.5 to 2 h after pollination, suggesting that succinylation is involved in the recognition of pollen-stigma signaling substances and vesicular transport. These results suggest that AP2/clathrin-mediated vesicular transport processes are regulated by lysine succinylation during pollen recognition.

Proteomic Identification of Differentially Expressed Proteins during Alfalfa (Medicago sativa L.) Flower Development

Flower development, pollination, and fertilization are important stages in the sexual reproduction process of plants; they are also critical steps in the control of seed formation and development. During alfalfa (Medicago sativa L.) seed production, some distinct phenomena such as a low seed setting ratio, serious flower falling, and seed abortion commonly occur. However, the causes of these phenomena are complicated and largely unknown. An understanding of the mechanisms that regulate alfalfa flowering is important in order to increase seed yield. Hence, proteomic technology was used to analyze changes in protein expression during the stages of alfalfa flower development. Flower samples were collected at pre-pollination (S1), pollination (S2), and the post-pollination senescence period (S3). Twenty-four differentially expressed proteins were successfully identified, including 17 down-regulated in pollinated flowers, one up-regulated in pollinated and senesced flowers, and six up-regulated in senesced flowers. The largest proportions of the identified proteins were involved in metabolism, signal transduction, defense response, oxidation reduction, cell death, and programmed cell death (PCD). Their expression profiles demonstrated that energy metabolism, carbohydrate metabolism, and amino acid metabolism provided the nutrient foundation for pollination in alfalfa. Furthermore, there were three proteins involved in multiple metabolic pathways: dual specificity kinase splA-like protein (kinase splALs), carbonic anhydrase, and NADPH: quinone oxidoreductase-like protein. Expression patterns of these proteins indicated that MAPK cascades regulated multiple processes, such as signal transduction, stress response, and cell death. PCD also played an important role in the alfalfa flower developmental process, and regulated both pollination and flower senescence. The current study sheds some light on protein expression profiles during alfalfa flower development and contributes to the understanding of the basic molecular mechanisms during the alfalfa flowering process. These results may offer insight into potential strategies for improving seed yield, quality, and stress tolerance in alfalfa.

Phytosulfokine peptide signaling controls pollen tube growth and funicular pollen tube guidance in Arabidopsis thaliana

Phytosulfokine (PSK) is a peptide growth factor that requires tyrosine sulfation carried out by tyrosylprotein sulfotransferase (TPST) for its activity. PSK is processed from precursor proteins encoded by five genes in Arabidopsis thaliana and perceived by receptor kinases encoded by two genes in Arabidopsis. pskr1-3 pskr2-1 and tpst-1 knockout mutants displayed reduced seed production, indicative of a requirement for PSK peptide signaling in sexual plant reproduction. Expression analysis revealed PSK precursor and PSK receptor gene activity in reproductive organs with strong expression of PSK2 in pollen. In support of a role for PSK signaling in pollen, in vitro pollen tube (PT) growth was enhanced by exogenously added PSK while PTs of pskr1-3 pskr2-1 and of tpst-1 were shorter. In planta, growth of wild-type pollen in pskr1-3 pskr2-1 and tpst-1 flowers appeared slower than growth in wild-type flowers. But PTs did eventually reach the base of the style, suggesting that PT elongation rate may not be responsible for the reduced fertility. Detailed analysis of anthers, style and ovules did not reveal obvious developmental defects. By contrast, a high percentage of unfertilized ovules in pskr1-3 pskr2-1 and in tpst-1 siliques displayed loss of funicular PT guidance, suggesting that PSK signaling is required to guide the PT from the transmitting tract to the embryo sac. Cross-pollination experiments with wild-type, pskr1-3 pskr2-1 and tpst-1 male and female parents revealed that both the PT and the female sporophytic tissue and/or female gametophyte contribute to successful PT guidance via PSK signaling and to fertilization success.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Comparative proteomic analysis reveals similar and distinct features of proteins in dry and wet stigmas

Angiosperm stigma supports compatible pollen germination and tube growth, resulting in fertilization and seed production. Stigmas are mainly divided into two types, dry and wet, according to the absence or presence of exudates on their surfaces. Here, we used 2DE and MS to identify proteins specifically and preferentially expressed in the stigmas of maize (Zea Mays, dry stigma) and tobacco (Nicotiana tabacum, wet stigma), as well as proteins rinsed from the surface of the tobacco stigma. We found that the specifically and preferentially expressed proteins in maize and tobacco stigmas share similar distributions in functional categories. However, these proteins showed important difference between dry and wet stigmas in a few aspects, such as protein homology in "signal transduction" and "lipid metabolism," relative expression levels of proteins containing signal peptides and proteins in "defense and stress response." These different features might be related to the specific structures and functions of dry and wet stigmas. The possible roles of some stigma-expressed proteins were discussed. Our results provide important information on functions of proteins in dry and wet stigmas and reveal aspects of conservation and divergence between dry and wet stigmas at the proteomic level.

Pollen tube growth and guidance: roles of small, secreted proteins

BACKGROUND

Pollination is a crucial step in angiosperm (flowering plant) reproduction. Highly orchestrated pollen-pistil interactions and signalling events enable plant species to avoid inbreeding and outcrossing as a species-specific barrier. In compatible pollination, pollen tubes carrying two sperm cells grow through the pistil transmitting tract and are precisely guided to the ovules, discharging the sperm cells to the embryo sac for fertilization.

SCOPE

In Lilium longiflorum pollination, growing pollen tubes utilize two critical mechanisms, adhesion and chemotropism, for directional growth to the ovules. Among several molecular factors discovered in the past decade, two small, secreted cysteine-rich proteins have been shown to play major roles in pollen tube adhesion and reorientation bioassays: stigma/style cysteine-rich adhesin (SCA, approx. 9·3 kDa) and chemocyanin (approx. 9·8 kDa). SCA, a lipid transfer protein (LTP) secreted from the stylar transmitting tract epidermis, functions in lily pollen tube tip growth as well as in forming the adhesive pectin matrix at the growing pollen tube wall back from the tip. Lily chemocyanin is a plantacyanin family member and acts as a directional cue for reorienting pollen tubes. Recent consecutive studies revealed that Arabidopsis thaliana homologues for SCA and chemocyanin play pivotal roles in tip polarity and directionality of pollen tube growth, respectively. This review outlines the biological roles of various secreted proteins in angiosperm pollination, focusing on plant LTPs and chemocyanin.

NtGNL1 plays an essential role in pollen tube tip growth and orientation likely via regulation of post-Golgi trafficking

BACKGROUND

Tobacco GNOM LIKE 1 (NtGNL1), a new member of the Big/GBF family, is characterized by a sec 7 domain. Thus, we proposed that NtGNL1 may function in regulating pollen tube growth for vesicle trafficking.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, we used an RNAi technique to down-regulate NtGNL1 expression and found that pollen tube growth and orientation were clearly inhibited. Cytological observations revealed that both timing and behavior of endocytosis was disrupted, and endosome trafficking to prevacuolar compartments (PVC) or multivesicular bodies (MVB) was altered in pollen tube tips. Moreover, NtGNL1 seemed to partially overlap with Golgi bodies, but clearly colocalized with putative late endosome compartments. We also observed that in such pollen tubes, the Golgi apparatus disassembled and fused with the endoplasmic reticulum, indicating abnormal post-Golgi trafficking. During this process, actin organization was also remodeled.

CONCLUSIONS/SIGNIFICANCE

Thus, we revealed that NtGNL1 is essential for pollen tube growth and orientation and it likely functions via stabilizing the structure of the Golgi apparatus and ensuring post-Golgi trafficking.

A gain-of-function mutation of Arabidopsis lipid transfer protein 5 disturbs pollen tube tip growth and fertilization

During compatible pollination of the angiosperms, pollen tubes grow in the pistil transmitting tract (TT) and are guided to the ovule for fertilization. Lily (Lilium longiflorum) stigma/style Cys-rich adhesin (SCA), a plant lipid transfer protein (LTP), is a small, secreted peptide involved in pollen tube adhesion-mediated guidance. Here, we used a reverse genetic approach to study biological roles of Arabidopsis thaliana LTP5, a SCA-like LTP. The T-DNA insertional gain-of-function mutant plant for LTP5 (ltp5-1) exhibited ballooned pollen tubes, delayed pollen tube growth, and decreased numbers of fertilized eggs. Our reciprocal cross-pollination study revealed that ltp5-1 results in both male and female partial sterility. RT-PCR and beta-glucuronidase analyses showed that LTP5 is present in pollen and the pistil TT in low levels. Pollen-targeted overexpression of either ltp5-1 or wild-type LTP5 resulted in defects in polar tip growth of pollen tubes and thereby decreased seed set, suggesting that mutant ltp5-1 acts as a dominant-active form of wild-type LTP5 in pollen tube growth. The ltp5-1 protein has additional hydrophobic C-terminal sequences, compared with LTP5. In our structural homology/molecular dynamics modeling, Tyr-91 in ltp5-1, replacing Val-91 in LTP5, was predicted to interact with Arg-45 and Tyr-81, which are known to interact with a lipid ligand in maize (Zea mays) LTP. Thus, Arabidopsis LTP5 plays a significant role in reproduction.

A novel C2-domain phospholipid-binding protein, OsPBP1, is required for pollen fertility in rice

Pollen fertility is a crucial factor for successful pollination and essential for seed formation. Recent studies have suggested that a diverse range of internal and external factors, signaling components and their related pathways are likely involved in pollen fertility. Here, we report a single C2-domain containing protein, OsPBP1, initially identified through cDNA microarray analysis. OsPBP1 is a single copy gene and preferentially expressed in pistil and pollen but down-regulated by pollination. OsPBP1 had a calcium concentration-dependent phospholipid-binding activity and was localized mainly in cytoplasm and nucleus, but translocated onto the plasma membrane in response to an intracellular Ca(2+) increase. Pollen grains of antisense OsPBP1 transgenic lines were largely nonviable, germinated poorly in vitro and of low fertility. OsPBP1 protein was localized in a region peripheral to pollen wall and vesicles of elongating pollen tube, and its repressed expression reduced substantially this association and led to alteration of microfilament polymerization during pollen germination. Taken together, these results indicate that OsPBP1 is a novel functional C2-domain phospholipids-binding protein that is required for pollen fertility likely by regulating Ca(2+) and phospholipid signaling pathways.

Targeting of pollen tubes to ovules is dependent on nitric oxide (NO) signaling

The guidance signals that drive pollen tube navigation inside the pistil and micropyle targeting are still, to a great extent, unknown. Previous studies in vitro showed that nitric oxide (NO) works as a negative chemotropic cue for pollen tube growth in lily (Lilium longiflorum). Furthermore, Arabidopsis thaliana Atnos1 mutant plants, which show defective NO production, have reduced fertility. Here, we focus in the role of NO in the process of pollen-pistil communication, using Arabidopsis in-vivo and lily semi-vivo assays. Cross-pollination between wild-type and Atnos1 plants shows that the mutation affects the pistil tissues in a way that is compatible with abnormal pollen tube guidance. Moreover, DAF-2DA staining for NO in kanadi floral mutants showed the presence of NO in an asymmetric restricted area around the micropyle. The pollen-pistil interaction transcriptome indicates a time-course-specific modulation of transcripts of AtNOS1 and two Nitrate Reductases (nr1 and nr2), which collectively are thought to trigger a putative NO signaling pathway. Semi-vivo assays with isolated ovules and lily pollen further showed that NO is necessary for micropyle targeting to occur. This evidence is supported by CPTIO treatment with subsequent formation of balloon tips in pollen tubes facing ovules. Activation of calcium influx in pollen tubes partially rescued normal pollen tube morphology, suggesting that this pathway is also dependent on Ca(2+) signaling. A role of NO in modulating Ca(2+) signaling was further substantiated by direct imaging the cytosolic free Ca(2+) concentration during NO-induced re-orientation, where two peaks of Ca(2+) occur-one during the slowdown/stop response, the second during re-orientation and growth resumption. Taken together, these results provide evidence for the participation of NO signaling events during pollen-pistil interaction. Of special relevance, NO seems to directly affect the targeting of pollen tubes to the ovule's micropyle by modulating the action of its diffusible factors.

Signaling on the Stigma: Potential New Roles for ROS and NO in Plant Cell Signaling

Reactive oxygen species (ROS) and reactive nitrogen species, particularly NO, are key components of diverse signaling networks in animals and plants. We have recently shown that epidermal cells of stigmas from a range of different angiosperms accumulate relatively large amounts of ROS, principally H(2)O(2), whereas pollen produces NO. Importantly, ROS/H(2)O(2) levels appeared reduced in stigma cells supporting developing pollen grains compared to cells without pollen grains attached. To explore a possible link between pollen NO production and reduced levels of stigmatic ROS/H(2)O(2), we supplied stigmas with NO and observed an overall reduction in levels of stigmatic ROS/H(2)O(2). These new and unexpected data suggest a potential new signaling role for ROS/H(2)O(2) and NO in pollen-stigma recognition processes.

Proteomic analyses ofOryza sativa mature pollen reveal novel proteins associated with pollen germination and tube growth

As a highly reduced organism, pollen performs specialized functions to generate and carry sperm into the ovule by its polarily growing pollen tube. Yet the molecular genetic basis of these functions is poorly understood. Here, we identified 322 unique proteins, most of which were not reported previously to be in pollen, from mature pollen of Oryza sativa L. ssp japonica using a proteomic approach, 23% of them having more than one isoform. Functional classification reveals that an overrepresentation of the proteins was related to signal transduction (10%), wall remodeling and metabolism (11%), and protein synthesis, assembly and degradation (14%), as well as carbohydrate and energy metabolism (25%). Further, 11% of the identified proteins are functionally unknown and do not contain any conserved domain associated with known activities. These analyses also identified 5 novel proteins by de novo sequencing and revealed several important proteins, mainly involved in signal transduction (such as protein kinases, receptor kinase-interacting proteins, guanosine 5'-diphosphate dissociation inhibitors, C2 domain-containing proteins, cyclophilins), protein synthesis, assembly and degradation (such as prohibitin, mitochondrial processing peptidase, putative UFD1, AAA+ ATPase), and wall remodeling and metabolism (such as reversibly glycosylated polypeptides, cellulose synthase-like OsCsLF7). The study is the first close investigation, to our knowledge, of protein complement in mature pollen, and presents useful molecular information at the protein level to further understand the mechanisms underlying pollen germination and tube growth.

Production of reactive oxygen species and reactive nitrogen species by angiosperm stigmas and pollen: potential signalling crosstalk?

Angiosperm stigmas exhibit high levels of peroxidase activity when receptive to pollen. To explore possible function(s) of this peroxidase activity we investigated amounts of reactive oxygen species (ROS), particularly hydrogen peroxide, in stigmas and pollen. Because nitric oxide (NO) was recently implicated in pollen tube growth, we also investigated amounts of NO in pollen and stigmas. Reactive oxygen species accumulation was assessed with confocal microscopy and light microscopy using ROS probes DCFH2-DA and TMB, respectively. NO was assayed using the NO probe DAF-2DA and confocal microscopy. Stigmas from various different angiosperms were found to accumulate ROS, predominantly H2O2, constitutively. In Senecio squalidus and Arabidopsis thaliana high amounts of ROS/H2O2 were localized to stigmatic papillae. ROS/H2O2 amounts appeared reduced in stigmatic papillae to which pollen grains had adhered. S. squalidus and A. thaliana pollen produced relatively high amounts of NO compared with stigmas; treating stigmas with NO resulted in reduced amounts of stigmatic ROS/H2O2. Constitutive accumulation of ROS/H2O2 appears to be a feature of angiosperm stigmas. This novel finding is discussed in terms of a possible role for stigmatic ROS/H2O2 and pollen-derived NO in pollen-stigma interactions and defence.

Maize ROP2 GTPase Provides a Competitive Advantage to the Male Gametophyte

Rop GTPases have been implicated in the regulation of plant signal transduction and cell morphogenesis. To explore ROP2 function in maize, we isolated five Mutator transposon insertions (rop2::Mu alleles). Transmission frequency through the male gametophyte, but not the female, was lower than expected in three of the rop2::Mu mutants. These three alleles formed an allelic series on the basis of the relative transmission rate of each when crossed as trans-heterozygotes. A dramatic reduction in the level of ROP2-mRNA in pollen was associated with the three alleles causing a transmission defect, whereas a rop2::Mu allele that did not result in a defect had wild-type transcript levels, thus confirming that mutation of rop2 causes the mutant phenotype. These data strongly support a role for rop2 in male gametophyte function, perhaps surprisingly, given the expression in pollen of the nearly identical duplicate gene rop9. However, the transmission defect was apparent only when a rop2::Mu heterozygote was used as the pollen donor or when a mixture of wild-type and homozygous mutant pollen was used. Thus, mutant pollen is at a competitive disadvantage compared to wild-type pollen, although mutant pollen grains lacked an obvious cellular defect. Our data demonstrate the importance in vivo of a specific Rop, rop2, in the male gametophyte.

Male and female synchrony and the regulation of mating in flowering plants

Successful mating clearly requires synchronous development of the male and female sexual organs. Evidence is accumulating that this synchrony of development also persists after pollination, with both pollen and pistil following complex, but highly integrated developmental pathways. The timing of the male-female interaction is crucial for the pistil, which, far from being a mature passive structure, is engaged in a continuing programme of development: only being receptive to the advances of the pollen for a relatively short window of time. This developmental programme is most conspicuous in the ovary, and this review focuses on the interaction between the male and female tissues in this structure. The review first considers pollen tube development in the ovary, concentrating of the mechanisms by which its growth is modulated at various control points associated with structures within the ovary. Second, alterations to this 'normal' developmental programme are reviewed and considered in the context of a breakdown of developmental synchrony. Finally, the consequences of male-female developmental synchrony and asynchrony are explored. Clearly, a synchronous male-female relationship leads to a successful fertilization. However, lack of synchrony also occurs, and could emerge as a powerful tool to investigate the regulation of mating.

Disruption of apyrases inhibits pollen germination in Arabidopsis

In Arabidopsis, we previously identified two highly similar apyrases, AtAPY1 and AtAPY2. Here, T-DNA knockout (KO) mutations of each gene were isolated in a reverse genetic approach. The single KO mutants lacked a discernible phenotype. The double KO mutants, however, exhibited a complete inhibition of pollen germination, and this correlated with positive beta-glucuronidase staining in the pollen of apyrase promoter:beta-glucuronidase fusion transgenic lines. The vast majority of the pollen grains of these mutants were identical to wild type in size, shape, and nuclear state and were viable as assayed by metabolic activity and plasma membrane integrity. Complementation with either AtAPY1 or AtAPY2 cDNA rescued pollen germination, confirming that the phenotype was apyrase specific. Despite the redundancy of the two apyrases in rescue potential, transmission analyses suggested a greater role for AtAPY2 in male gamete success. The effect of mutant apyrase on the transmission through the female gametophyte was only marginal, and embryo development appeared normal in the absence of apyrases. The male-specific double KO mutation is fully penetrant and shows that apyrases play a crucial role in pollen germination.

Topological incongruence between nuclear and chloroplast DNA trees suggesting hybridization in the urophyllum group of the genus Fagopyrum (Polygonaceae)

We performed phylogenetic analyses of Fagopyrum species in the urophyllum group based on nucleotide sequences of two nuclear genes, FLORICAULA/LEAFY (FLO/LFY) and AGAMOUS (AG), and three segments of chloroplast DNA (cpDNA), rbcL-accD, trnK intron, and trnC-rpoB spacer. The FLO/LFY and AG sequences turned out to be phylogenetically more informative at the intrageneric level than the cpDNA sequences. Congruence among these gene trees, inferred by a maximum-likelihood (ML) method, demonstrated that topologies were partially incongruent between the nuclear and chloroplast DNA phylogenies. The nuclear DNA sequence data supported a monophyletic relation of F. statice, F. gilesii, and F. jinshaense, whereas the former two species formed another monophyletic relation with the F. capillatum-F. gracilipes-F. gracilipedoides-F. rubifolium clade excluding F. jinshaense in the synthetic cpDNA phylogeny. In addition, two divergent sequences of FLO/LFY were found in F. rubifolium (tetraploid). One of these was sister to F. gracilipedoides and another was sister to F. statice, and a monophyletic relation of these two genes was rejected by a bootstrap analysis. These results suggest that hybridization may have occurred during diversification of Fagopyrum species in the urophyllum group, and that F. rubifolium is possibly allotetraploid species.

Pollen-tube guidance: beacons from the female gametophyte

The sperm cell of a flowering plant cannot migrate unaided and it must be transported by the pollen-tube cell before successful fertilization can occur. The pollen tube is precisely guided to the target female gametophyte, the embryo sac, which contains the egg cell. The mechanism that precisely directs the pollen tube through the pistil to the female gametophyte has been studied for more than a century. There has been controversy over whether a diffusible signal attracts the pollen tube or whether female tissues define its path. Emerging genetic and physiological data show that the female gametophyte produces at least two directional signals, and that at least one of these signals is diffusible and derived from the two synergid cells.

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).

ROP GTPase regulation of pollen tube growth through the dynamics of tip-localized F-actin

Pollen tubes expand by tip growth and extend directionally toward the ovule to deliver sperms during pollination. They provide an excellent model system for the study of cell polarity control and tip growth, because they grow into uniformly shaped cylindrical cells in culture. Mechanisms underlying tip growth are poorly understood in pollen tubes. It has been demonstrated that ROP1, a pollen-specific member of the plant-specific Rop subfamily of Rho GTPases, is a central regulator of pollen tube tip growth. Recent studies in pollen from Arabidopsis and other species have revealed a ROP-mediated signalling network that is localized to the apical PM region of pollen tubes. The results provide evidence that the localization of this signalling network establishes the site for tip growth and the localized activation of this signalling network regulates the dynamics of tip F-actin. These results have shown that the ROP1-mediated dynamics of tip F-actin is a key cellular mechanism behind tip growth in pollen tubes. Current understanding of the molecular basis for the regulation of the tip actin dynamics will be discussed.

Adhesion and guidance in compatible pollination

The mechanisms of compatible pollination are less studied than those of incompatible pollination and yet most of the angiosperms show self-compatibility. From the release of pollen from anthers to the penetration of the micropyle by the pollen tube tip, there are numerous steps where the interaction between pollen and the pistil can be regulated. Recent studies have documented some diverse ways in which pollen tubes carrying sperm cells are guided to the ovules through the pistil extracellular matrices of the transmitting tract. What is still missing is an understanding of pollen tube cell biology in vivo. A recent finding supports the role of the synergids in the crucial guidance cue for the pollen tube tip at the micropyle, but experimental evidence for other 'guidepost' cells in the pistil is still lacking. The fact that the pollen tube must first travel through the matrices of the stigma and style before it can respond to the cue from the ovule makes it likely that there is a hierarchy of signalling events in pollen-pistil interactions starting at the stigma and ending at the micropyle. On the pistil side, several model systems have been used in the discovery of molecules implicated in either physical or chemical guidance. In lily, which has a hollow style, adhesion molecules (pectin and SCA) are implicated in guidance. SCA alone is also capable of inducing pollen chemotropism in an in vitro assay, suggesting that this peptide plays a dual role in lily pollination: chemotactic in the stigma and haptotactic (adhesion mediated) in the style.