Introduction
Cells of multicellular organisms must communicate not only for development to occur after fertilization but also for successful survival and reproduction of the offspring. There are different forms of signals that cells can received and the signal being received is either from another cell or from change in the environment surrounding the cell. Communication between cells is therefore either chemical or environmental driven where plants usually use chemicals for signaling.

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Pollination and then fertilization process of flowering plants involves complex events with highly regulated cell-cell interactions and signaling between the pollen and the pistil. The success depends on series of events such as delivery of pollen to a stigma, pollen hydration, germination, growth through the pistil to the ovary, and, finally, fertilization [10, 11]. In a compatible pollination, pollen grains that have arrived on the stigmatic surface will easily undergo the three initial steps: adhesion, hydration and germination to the stigmatic cells form pollen tubes. The actively growing pollen tubes are often capable of penetrating the stigmatic cell walls to grow down through the style and finally reach the ovary to deliver sperm for fertilization. In self-incompatible pollination, these steps may be blocked having recognized as pollen from related plant source. In doing so out-crossing is favored between unrelated individual plants there by inbreeding related problems like inbreeding depression minimized. Therefore, understanding of pollen to stigma compatibility and self incompatibility is fundamental for pollination and fertilization in plants in general and for plant breeding in particular. To this end, the mechanism of pollen to stigma interaction which determines compatibility or self incompatibility is reviewed.
Compatibility and Self incompatibility
The first artificial pollination date back to third century though not recognized by scientists until 1682 when stamens of the flower as the male organ and the pollen as necessary for fruit production are known in science [9]. Later in 1824, an Italian scientist observed that pollen tube on stigma and he later anticipated that the pollen tube carried the sperm cells to the ovule for fertilization. After 17th century, the maternal involvement in embryology became clear and several basic researches have been undergoing in the last three decades which contributed a lot to science in the area of pollination and fertilization.
Commonly, pollen and stigma from distant species are not compatible while there is 30 to 50 % of flowering plant showed self incompatibility [19]. In a compatible pollen-pistil interaction, all steps from pollen capture and adhesion to the final entry of the pollen tube into the ovule and discharge of the pollen grain is complete, resulting in double fertilization to form zygote and endosperm. Self incompatibility occurs when pollen tube dies if at all germinates on stigma of the same plant while pollen of the same species from different plant can germinate and produce functional pollen tube on stigma
[2]. The self pollen rejection response can occur at any stage between pollen contact to stigma and pollen tube growth in the stigma which results no ovule fertilization and embryo development. This phenomenon enabled today’s flowering plant to exist successfully in different environment.
Genetically, such self incompatibility is classified as gametophytic and sporophytic [7, 10, 11]. Gametophytic self incompatibility is common in binucleate pollen which is characteristics of wet stigma whereas sporophytic self incompatibility is the case of trinucleat pollen and it is characteristics of dry stigma [1]. Nonetheless, examples of exceptions for this correlation are grasses with dry stigma and three nuclei exhibiting gametophytic; hetromorphic self incompatibility in some species which is rare in occurrence arises due to two or three flower morphs exist in population. Among these three, gametophytic is more common although detail study at molecular level is yet to be made. It is agreed that the stigma type and micro-ecology are influential to the nature of stigma-pollen interaction rather than the type of self incompatibility [6]. It is also reported that gametophytic self incompatibility occur when incompatible pollen tube is in side of style whereas sporophytic occur most of the time before a pollen tube penetrate the stigma [6]. Again exception is due to poppy and grasses in which gametophytic and stigmatic inhibition of incompatible pollen occur.
Stigma structure
Stigma in general is classified as dry and wet type which is correlated with pollen structure. Species with dry stigma possess pollen having three nuclei while those with wet stigma are with two nuclei pollen [6]. The surface cell of wet type releases secretion with proteins, polysaccharides, lipids, and pigment while the dry stigma type is with intact surface cells covered by cell wall, a waxy cuticle, and a proteinaceous pellicle [3]. The secretion is primarily lipidic or aqueous and carbohydrate-rich. Both secretions contain a wide range of protein and found to be available on stigma in later development internal from the reservoirs and vesicular activity at the stigma epidermis. In dry stigma where there is no stigmatic secretion, initial adhesion of pollen to stigma depends on the pollen wall. Tryphine, lipid-rich pollen coating, is found to be important in adhesion to the dry stigma.
Stigma-pollen specificity
The fate of pollen grain on stigma depends on the type of stigma (wet or dry) on which pollen lands. The very first step of pollination, adhesion of pollen on stigma, occurs on wet stigma fully with out discrimination because the availability of liquid surface is the only requirement. The surface of such stigma is with carbohydrate and lipid rich secretions that hydrate pollen none selectively as a result of this, the incompatible pollen is prevented at later stage of development. However, adhesion of pollen to dry surface stigma type is species specific in the way that only pollen from specific species is allowed to adhere and get hydration. Pollen type unrecognized by stigma is not allowed to deplete the female tissue resource for further development. Self incompatible specificity determinants were identified both in pollen and stigma. To pollen side, the gene called SP11/SCR (S-LOCUS CYSTEINE-RICH) is responsible for specificity which is completely expressed during pollen development and understood to encode cysteine-rich protein. The stigmatic s-locus linked molecule which is self incompatible determinant is called S-locus receptor Kinase (SRK) [18]. This female determinant of self incompatible is located in plasma membrane. Another determinant which does not directly affects specificity is S-locus glycoprotein. This soluble protein located in the cell wall only contributes to increase the strength of self incompatibility phenotype.
This first step of pollination is divided in to Pollen capture, pollen stigma cross linking and pollen hydration. Pollen capture requires nothing than exine wall and it is free from lipids and proteins [21]. In the cross linking phase lipids, carbohydrates, and proteins from each side (stigma and pollen) mix to generate the adhesive forces. In the pollen hydration stage pollen receive the nutrients and liquid required to activate metabolism and initiate pollen tube elongation.
In contrast to flowers with wet stigma where pollen grains are surrounded by stigmatic exudates flowing easily into pollen grains, fluid movement in dry stigma is not passive and usually depends on the conversion of pollen coat at the point of stigma contact. Hence, for successful pollen capture, diffusion of extracellular pollen coat along the stigma cells and then pollen hydration, pollen-stigma signaling is required.
Pollen-stigma signaling
Communication of plant cells are either molecules or physical (environmental) that one cell sensing on the neighboring cell may affected in different ways such as inhibiting the cell’s growth. There are different classes of molecules that are involved in signaling. These molecules are located either in the extracellular surface or within the underlying cell walls. The signal molecules can be ions or metabolites one cell supplies for another; can be hormones or they can be cell wall bound macromolecules. The source of these molecules is both from stigma and pollen grain. Pollen grains consists the inner layer cell wall called intine and the outer network cell wall called exine. The inner pollen cell wall is mainly cellulose while the outer pollen cell wall is rich in sporopollenin, a highly stable, mixed polymer containing long-chain fatty acids and phenolics [21].
Factors affecting adhesion of pollen to stigma is not known; however it was indicated that adhesion in Arabidopsis is exine dependant more than pollen coat [14, 15, 20]. Lipids, as medium of water up take in pollen hydration, is provided by the stigma and pollen coat. In some species where stigma exudate is mainly aqueous, the significance of lipids for pollen-stigma interaction is not determined but lipids are present in the pollen kit which still indicates the importance of lipids. In addition to lipid, glycine rich protein from pollen coat plays role in pollen hydration. Next to hydration is germination in which the pollen tube grows into the cell wall of the papillae toward the transmitting tissue. Germination is mainly regulated by Rop, a member of GTPase – large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate [10]. Research finding in Arabidopsis indicated that Rop1 over expression in wild type helps in pollen germination while rop1 mutant inhibits pollen germination [8]. Germination on the stigma means pollen tubes initiated in an Extra Cellular matrix (ECM) that is usually a combination of both pollen coat secretions and stigmatic exudates. In stigma type of Solanaceae, stigmatic lipids are found to be essential for pollen tube growth toward the style by creating a gradient of water. In other stigma type like that of lily, pollen tubes must be guided in the style. It was indicated that peptide stigmatic secretion of lily is involved in pollen tube guidance in the style [10]. Fertilization next to pollination is achieved after the tight control mechanisms operation that restricts mating of inter and intra species. As described above, fertilization in flowering plants needs to be in a well cellular coordination so that the sperm cells which are devoid of flagella go through flower tissues towards the ovules.
The internal signaling networks regulating pollen tube growth and development in flowering plants is highly conserved. However, no pollen-stigma signaling pathway reported so far that is common between species with wet and dry stigmas. Even with in these two broad divisions, there appeared to be differences in the ways that pollen and stigma interact to create compatibility. However, regardless of the type of stigma, pollen-stigma interactions in general requires lipid at the pollen-stigma interface, water as initial directional cue for pollen tube growth , and small cysteine-rich proteins are involved.
Molecular basis of self incompatibility
As described above self incompatibility is the means of preventing inbreeding and a tool for species diversity. Self-incompatibility is genetically controlled which depends on a number of complex cellular interactions between the self-incompatible pollen and stigma. The way Self-incompatibility works in rejecting self incompatible pollen varies greatly from species to species.
The self-incompatibility system in some plants is morphological in which flowering plants produce distinct flowers in such a way that the positions of the reproductive organs within a flower create an extra barrier in addition to the existing barrier within species [2]. In some species, the system is caused by the phenotype of the pollen not morphological character of the flower. The self incompatibility in flowers of the same morphological character is genetically controlled by S-locus and as mentioned earlier, classified as gametophytic and sporophytic. In the gametophytic systems, the self-incompatibility phenotype of the pollen is determined by is determined by its own haploid genotype where the developing pollen synthesizes its own pollen S product, whereas in sporophytic self-incompatibility systems, the self-incompatible behavior of the pollen is determined by the parental genotype.
Brassicaceae from mustard family is an example of Sporophytic type of self incompatibility. The important gene product called serinethreonine kinase (SRK) interact with peptid ligand cysteine-rich protein (SCR) which creates signals that lead to rejection of self pollen [10]. Molecular mechanisms are unknown but it was indicated that ARC1, a novel U-box protein that binds to the SRK kinase domain, found to be effector for SRK [16, 17]. It was indicated that a component of hetro-oligomeric protien complex, Exo70A1, interacts with ARC1 which determines the fate of pollen up on arrival on stigma. In transgenic Brassica and Arabidopsis thaliana, the loss of Exo70A1 resulted rejection of pollen by stigma which were compatible in non transgenic [13]. Gametophytic self incompatibility is widespread system of self incompatibility. The system in Papaveraceae (poppy family) is and the Solanaceae (tobacco family) are examples of gametophytic self incompatibility. In poppy, small S-proteins secreted by the stigma interact with the S-gene product in the pollen tube, causing the capture of incompatible pollen. Growing pollen tubes are known to have high concentration of Ca+2. This concentration is rapidly decreased in self incompatible pollen indicating the origin the first arrest of pollen tube [4]. The self incompatibility in Solanaceae is that the protein from S-gene is ribonucleases inhibitor that recognizes and inhibits S- ribonucleases in incompatible pollen.
Conclusion
For successful pollination and fertilization, pollen grain should travels all the way to ovule in which pistil acts as both facilitator and barrier. It facilitates by providing guidance and nutrients but it also creates barrier that rejects incompatible pollen. This biological function is highly regulated involving several gene products. Flowering plants are the most successful on earth in terms of the number of species and level of diversity of forms and ecological niches. Due to the fact that compatibility and self incompatibility system is the base for genetic variability and species boundary maintenance, studies in advanced level need to be continued. Studies at molecular level that address determinants of pollination and fertilization will contributor in understanding the basis of species barriers that maintain the enormous diversity seen in flowering plants.