Year: 2004
Type: review
Abstract: Pollination in Arabidopsis. (A) Diagram of one of the two ovary chambers after removal of the ovary wall (OW). Pollen lands on the stigma, where it hydrates, germinates, and extends a tube that enters the transmitting tract (TT). The tube emerges onto the septum (S), grows up the funiculus (f), and enters the micropyle opening of the ovule (O), where it fertilizes the egg and central cell. The position of the transmitting tract inside the septum is shown in the cross-section at the bottom of the pistil. (B) Transmission electron micrograph showing the point of contact between a pollen grain (P) and a stigma papillus (S), colorized to highlight the pollen coat (pink), intine (peach), exine (green), stigma cell wall (yellow), and stigma cuticle (blue). A foot of lipid-rich material (arrows) collects between the two surfaces. The thin stigma pellicle that covers the cuticle is not distinct here. (C) Diagram of early events on the stigma, including adhesion (1), pollen coat “foot” formation and pollen hydration (2), pollen tube emergence from the grain (3), and pollen tube invasion of the papillae cell wall and extension toward the style (4). (D) to (F) Sequence of early events during Arabidopsis pollination. (D) The pollen coat has mobilized to the site of contact between the pollen and the stigma, forming a foot between the two surfaces (arrow), as visualized with the lipid dye FM1-43. (E) The pollen tube (arrow) has emerged from the grain's exine wall and projects itself into the stigma papillus, as visualized with the cell wall dye Congo red. (F) Pollen tubes (arrow) wind their way toward the style, moving between the cell walls of the papillae, where they are protected from the Congo red dye. Angiosperm reproduction is highly selective. Female tissues are able to discriminate between pollen grains, recognizing pollen from the appropriate species while rejecting pollen from unrelated species (or from the same plant in self-incompatible species). This selectivity is accompanied by tremendous diversity in the cell surfaces of male and female reproductive structures. The uniquely rich fossil record of pollen wall structures has been of great benefit. Literally hundreds of years of scientific effort have focused on integrating the diversity of pollen form with angiosperm taxonomy. Linking these morphological differences to functional roles is more challenging, requiring molecular and genetic assays that reveal purpose not only within a species but across diverse taxa. Here, we first survey advances in the cellular and molecular understanding of angiosperm pollen and stigma biology, including pollen–stigma adhesion, pollen hydration and germination, and pollen tube emergence and invasion. By capitalizing on microscopy, molecular, genetic, and genomic resources, it is possible to magnify and dissect cellular functions at the pollen and stigma surfaces both before and after they make contact (Table 1 Pollen and Stigma Structural Features and Their Roles Pollen and Stigma Structural Features and Their Roles Mature angiosperm pollen grains are unusual vegetative cells that contain within themselves sperm cells, complete with cell walls and plasma membranes. This arrangement is accomplished soon after meiosis, when an asymmetric mitotic division produces a large cell that engulfs its diminutive sister, the generative cell (Twell et al., 1998; Yang and Sundaresan, 2000). Subsequently, the generative cell undergoes a second mitosis to form the second sperm cell required for double fertilization; “tricellular” pollen completes this division before it is released from the anther, whereas “bicellular” pollen undergoes this division only later, within the elongating pollen tube. These categories do not apply to gymnosperm pollen grains, which can contain a score of cells and differ from angiosperm pollen in several other respects (Pacini et al., 1999). Pollen cells are contained within a unique pollen wall, whose construction begins when the meiocyte is newly formed, with the surface layers elaborated over time. At maturity, the pollen surface can be divided into three principal strata, with the relative amount of each varying between species: (1) an outer exine wall, itself multilayered, composed of the chemically resistant polymer sporopollenin and interrupted by openings called apertures; (2) an inner intine, also sometimes multilayered, made primarily of cellulose; and (3) a pollen coat, composed of lipids, proteins, pigments, and aromatic compounds, that fills the sculptured cavities of the pollen exine (Figure 1B). Stigmas, the receptive portions of the female tissues, bind pollen and mediate tube migration into the style. Stigmas generally are classified into two groups: wet stigmas, which are covered with surface cells that often lyse to release a viscous surface secretion containing proteins, lipids, polysaccharides, and pigments; and dry stigmas, which have intact surface cells that typically protrude as papillae and are covered by a primary cell wall, a waxy cuticle, and a proteinaceous pellicle (Figure 1B). Stigmas have been studied extensively in plants that exhibit self-incompatibility, a process that restricts inbreeding. Self-incompatible stigmas reject self-pollen by inhibiting pollen hydration, germination, and tube invasion; these processes have been reviewed recently (Nasrallah, 2000; Silva and Goring, 2001; Wheeler et al., 2001; Hiscock and McInnis, 2003; Kao and Tsukamoto, 2004). To capture pollen grains, stigmas engage biotic and abiotic pollinators (such as insects and wind) and use rapid and strong adhesive interactions to retain pollen grains. The pollen–stigma interface can differ from species to species as a result of the wide variability in the morphology and content of stigma exudates, exine layers, and pollen coats. Several different methods have been devised to investigate and measure pollen–stigma adhesion (Stead et al., 1979; Luu et al., 1997a, 1997b; Zinkl et al., 1999), including chemical and detergent washes and centrifugation-based adhesion assays. Here, we focus on the characterization of pollen adhesion on the dry stigmas of the Brassicaceae, particularly Arabidopsis thaliana and Brassica oleracea. In both cases, stigmas adhere poorly to pollen grains from other botanical families, demonstrating a specificity that restricts inappropriate pollen access (Luu et al., 1998; Zinkl et al., 1999). Adhesion during downstream stages (such as between pollen tubes and the style) has been reviewed recently (Lord and Russell, 2002; Lord, 2003). In Arabidopsis, the nature of the pollen–stigma interface has been shown to change as pollination progresses, becoming considerably stronger over time, with different types of adhesive contacts supplementing and supplanting each other (Luu et al., 1997a, 1997b; Zinkl et al., 1999). Zinkl et al. (1999)measured a very rapid “initial” adhesion step in Arabidopsis that relies on the pollen exine but not on the pollen coat. This initial adhesion is not likely to be based on protein–protein interactions, because purified exine fragments retained their binding capacity even when washed with a range of compounds, including organic solvents, salts, and reducing and oxidizing agents. Thus, pollen capture in Arabidopsis most likely depends on biophysical and/or chemical interactions between the stigma surface and the polymers of the pollen exine; the nature of these contacts remains unknown. Identification of these selective adhesives requires further analysis of exine structure and characterization of stigma pellicle components. Given the challenge of purifying large quantities of these chemically complex materials, genetic dissection of the pathways required for their synthesis may prove most worthwhile. After exine-mediated adhesion, mobilization of the pollen coat occurs, leading to mixing of lipids and proteins to form a “foot” of contact on the stigma surface (Figures 1C and 1D). There is now extensive evidence that the proteins and lipids in the pollen coat, and proteins on the stigma surface, also contribute to adhesion, albeit most likely at a later stage than at the time of initial contact. This second stage was measured and shown to require protein–protein interactions, including interactions between pollen coat proteins and the Brassica S-locus–related protein (SLR1) (Luu et al., et al., 2000). is a to the a protein in et al., adhesion requires time to to the mixing of stigma and pollen the of initial pollen binding in plants is the same as in plants (Luu et al., 1999). to pollen coat This with the adhesion, can be by of the proteins (Luu et al., et al., 2000). pollen tubes from the grains, a stage of adhesion begins in which the pollen tube the foot into the stigma surface, the pollen grain to the stigma the tube et al., et al., 2000; et al., 2000). inappropriate pollen grains this further access is by inhibiting tube This is in plants with wet stigmas, in which pollen grains typically are and species selectivity et al., pollen grains are and highly from to when released from the et al., 2000). grains that on a wet stigma, but that on dry stigmas their lipid-rich pollen coat to form an interface between the two cell surfaces. This interface to a form to and et al., and other are into the grain from the stigma stigma papillae by that The of in the stigma has the that are in the rapid and release from the stigma to the pollen et al., the stigma cuticle may be uniquely to et al., 1998; et al., 2000). of the of pollen hydration often is both and hydration can have leading to within the and on the surface and et al., In plants with dry stigmas, pollen hydration an early to pollination. This is in self-incompatible et al., and in between species and et al., These processes are the stigma can a grain while the hydration of pollen on the same papillus understanding of the surface that this require both molecular dissection of stigma and pollen surface and genetic of their The genetic and molecular dissection of the lipid-rich in the pollen coat has considerably in In Arabidopsis, the pollen coat and lipids with a of proteins, including and proteins that contain a lipid binding et al., 2001; et al., 2004). The pollen of Brassica is and also has been shown to contain in et al., pollen coat lipids pollen coat proteins in species can pollen In that lipid and the of the pollen the hydration of Arabidopsis pollen and result in male et al., et al., contact can be to these grains by the of purified et al., that pollen coat proteins are because of functional The most protein in the Arabidopsis pollen coat is the proteins have but in the of pollen hydration and the of the pollen to with pollen and 2000). The of the proteins are across the Brassicaceae, a in which pollen coat may contribute to et al., 2001; et al., 2004). the proteins, the pollen coat lipids of most species are these for species specificity et al., that pollen hydration by the of lipids et al., may that in as the hydration of The lipid-rich stigma of plants with wet stigmas is to be in to the pollen coat that this female this can be by lipids et al., et al., The of is for hydration and germination, even on plants et al., These that lipids an in the of the stigma cuticle to et al., 1998; et al., 2000). In which is in a required for lipid cuticle and pollen hydration is on inappropriate cell surfaces. on these it is possible to a in which the of lipids, by the male the female surface, to while highly diverse proteins and mediate and pollen a pollen grain from a cell to a highly cell. tubes on a dry stigma surface, from a grain in stigma exudates, from pollen in the grain its and to the extension of a tube. These within after hydration and the formation of structures that the toward the site of tube emergence and and of the large vegetative that it enters the tube before the generative cells et al., and and of and at the site of the elongating tube et al., et al., and of the pollen plasma for and of at the site of tube emergence and is not the is and to a point for tube Several have been including lipids, and et al., et al., 1998; et al., for as a has from in with these grains in purified lipids in stigma exudates, and a in toward the in The pollen tubes from the to the that the of that et al., 1998; et al., only this it also a for lipids in for this from that the lipid-rich pollen et al., et al., grains are to on the grains do at but because they their pollen tubes often at relative to the stigma the of a binding protein in and the of at tube et al., 2003). with its binding to the of the pollen where they to focus et al., et al., et al., proteins that to the pollen tube may a in the initial of cell at the of elongating is a protein to be in exhibit binding to and et al., Pollen to be a particularly rich for integrating the and for and the cell has its relative to an the pollen tube the exine wall to from the on the species pollen tubes of the the exine In and tubes at the by intine layers, the thin sporopollenin wall, and the that these and et al., In Arabidopsis, whose pollen has three distinct pollen tubes often exine walls at the site of contact with the stigma surface (Figure of pollen tube requires (1) exine by from the inside of the wall, (2) exine by focused for the from of exine after contact with the stigma and and also but because in they be focused at the site of A of is most with the of the pollen grain to the of a of exine at the pollen–stigma There is evidence that pollen grains are to for pollen tube pollen grains are able to to the same as grains, and even tubes before the tubes The challenge of purifying that cell and tube emergence for the use of genetic that the between the two Arabidopsis sperm cells and the vegetative pollen have been These the of the male to the pollen tube by (1) the vegetative from the two sperm cells and (2) an intact male to the pollen wall and and have that tubes to from the grain the and of before pollen tube emergence and In for pollen have in germination, and tube emergence et al., 1998; et al., and 2001; et al., 2001; and 2002; and have not to This functional in the the other the of Arabidopsis with stigma cells et al., et al., into the for the After the exine wall, pollen tubes can only the style after the stigma The of this process considerably from species to In plants with the stigma is covered with an that is with the style, but in species with stigmas, pollen tubes the outer cuticle and cell wall of the stigma papillae to the style. by pollen have been to an in pollen tube invasion of the stigma surface and have been to pollen and tubes and particularly as are for the stigma they have been in the pollen of and other and et al., Hiscock et al., as as in the proteinaceous of dry stigma surfaces and Hiscock et al., the pellicle pollen tubes from the stigma, and and In the of pollen tubes to Brassica stigmas et al., of in the stigma cell wall also is for which pollen and et al., et al., A Brassica also has been at the of pollen tubes as they the stigma cell walls and of the stigma surface is to not the to inappropriate invasion. This likely requires between the pollen tube and the have been as of between the pollen tube and the of the large protein and to the pollen tube et al., the of these including cell proteins and proteins in in with is for pollen hydration and in and for tube in This that and are of an that may and pollen tube et al., of Angiosperm in Pollen and Stigma (A) pollen grain with Congo red. The intine several at the large the pollen on the surfaces of the exine (B) pollen of and with dye. (C) pollen of with pollen grains are (D) electron of several pollen including Arabidopsis, and which in exine and and arrangement of (E) A of the stigma of visualized with (F) Stigma papillae from Arabidopsis visualized by FM1-43. in (A) to (D) and in (E) and Pollen and (A) Pollen from the Arabidopsis complete pollen grains are by exine (B) pollen grains from the Arabidopsis not after meiosis, in a (C) pollen from events that exine and position also during soon after pollen typically produces a an early After wall cells this with their of sporopollenin material et al., construction and of the wall that pollen in et al., and the of wall in and and The initial of the requires between the pollen plasma and its and and and and et al., et al., the Arabidopsis a protein with to to with the plasma to sporopollenin et al., exine also has been in whose surface of is to a of sporopollenin with et al., 2000). in the Arabidopsis pollen grains with exine walls composed only of sporopollenin and 2000). the of in exine it possible to required for sporopollenin synthesis from required to in may be highly across in their in their site and of these are likely exine by and in one species may be in also in the and of the that it (Figure are diverse across within families, within and even within a plant et al., Pollen from has a a to be pollen grains have three in both have and during the of At the pollen from species whereas are exine with a very thin exine et al., The of the are during the when in the exine form at of between the plasma and the wall The of is but it has been to in the a in et al., et al., Yang et al., 2003). Several functions it has been to to position and functions for are (1) for and of during pollen and hydration, (2) that by grain and and (3) for the of the pollen tube. the of pollen grains are with et al., are to and on to grain and types of are with plant with wet dry the between and dry stigmas, that their in their to focus into the walls are for pollen grains that in the and on the stigma In this are to in and The of the sporopollenin wall itself also is for these et al., 2000). it has been that pollen tubes in this is by whereas as Arabidopsis have but do not use for tube emergence (Figure A of tube emergence across into the of the for different of tube an of between pollen and the they also are of to and invasion by et al., plants the with lipid-rich pollen that the of have been but it is not to use to these the for in and to types of the and to most other cell the intine is by the pollen cells structure often is with the exine pollen grains that have and are to bind quantities of pollen coat have complex that contain of at the intine exine at the can layers between the intine and the thin of of these intine layers contain and whereas to a in cell wall and tube emergence et al., within the intine wall toward the the grain's that are to the structure of the elongating pollen tube and and the pollen tube the intine with the cell wall of the with wall material at the The within the intine is into the tube a in invasion and To have been that intine in likely because of and to to the of intine Pollen grains are to the stigma by a of biotic and abiotic for which their structures are to be uniquely 2000; and 2000; 2000). In the for pollinators is strong that pollen grains are that are and that only for and grains are in whose pollen and the pollen can its tubes the style (Pacini and types of pollen that are in and but different in their to on and the stigma et al., these have for in the for a pollen between as surface and pollination by et al., 2000). The of other pollen to their by abiotic of angiosperm contain 2000). contribute to the of by wind and to the structures of and 2000). The structure of the pollen coat also is to different pollination is with an pollen coat (Pacini and the Brassica pollen coat can to of the of the pollen grain et al., 2000). By grains by the wind often have a more as that the lipids to be at the interface between pollen and stigma, with pollen may on lipid-rich stigma to the in et al., also differ in the and of their pollen with pollen often whereas more complex Brassica contain protein and lipid et al., 2000). Several to investigate the of pollen coat in pollen including (1) the of with pollen as the in Arabidopsis et al., et al., (2) the to proteins with and and (3) the to pollen coat et al., 2002; et al., 2004). The pollen coat is by the cells that pollen grains. These cells and proteins and the cavities of the exine with their with pollen are they can be and their can be purified in for et al., and et al., and 1998; et al., The first in in which pollen to be with as have and proteins, and et al., In the protein in the pollen coat is its release at stigma pollen tube invasion by the on the stigma surface et al., 1999). In pollen coat proteins, in from to have been on These proteins in pollen–stigma adhesion et al., and et al., and of molecular proteins from Arabidopsis a including binding proteins, and proteins with to the of the and 2000; et al., Pollen also contain lipids for as which are to be et al., of plant species from that the pollen of each species its of with the three of in and in pollen to different on the species and 2000). the of pollen coat proteins in Arabidopsis and Brassica is it only a of the proteins from the of the released likely before with the coat into structures that are for this is by Arabidopsis pollen coat in these cases, proteins are on the pollen surface but from the before the pollen is released et al., A survey of species has shown that stigma structure generally with and for the of pollen and stigma structures. angiosperm species that pollen typically have dry stigmas, whereas often with wet stigmas diverse as they may stigmas the same Stigmas capture hydration and germination, and and to pollen tubes to the Stigmas do not only discriminate between different pollen they also by the of their with pollen This has been for of have been and in which the male female are the first to and for stigma and are based on (1) the of (2) the to pollen germination, and (3) the to the most and Stigma is to the of other In Arabidopsis, stigmas to be able to pollen tube the is to to the et al., the the papillae to form cells receptive for pollination. that structure also stigma and have and two of stigma papillae, each at a have on stigma and 1999), as do in a binding protein in to the cell during and the plasma during cell et al., 2003). Transmission electron of in cell wall and plasma that result in the formation of papillae cells and et al., 2003). stigma and structures across that structures may highlight the very that have been At maturity, the receptive surface of a stigma and required for pollination. plants that male and female have been to retain their stigmas in the male but to the of these the stigma in their and stigma including of the cells and their in plants with wet stigmas, events release stigma from A great challenge in stigma is the of a likely to stigma with and is likely to and to a between of morphology and genetic of functional cell Here, we of pollen and stigma including (1) pollen adhesion, a process that is of protein–protein interactions but later pollen coat (2) pollen hydration, a step for which the of lipids, by the male the female surface, likely while diverse proteins mediate pollen (3) pollen and germination, processes on and the of for the focused of the exine and stigma a stage that the of also the great diversity in pollen and stigma structures across to including diversity in pollen wall pollen pollen coat and stigma of have and the stage for across taxa. in pollen wall the Arabidopsis are likely to be highly between in their in the and of their in is called for not only within the but between and even et al., in which and plasma are likely with wall and that roles in wall are the very to one species may be in as that exine is required to an pollen coat be Arabidopsis as and that exine The roles of pollen coat pollen in and pollen be of Arabidopsis as in pollen coat and the two the release of and be to these in as of genetic, and genomic of with of and pollen and stigma to be in the of pollination also are to reproductive and focused across a of species also to complete of the and diversity of are to of the and for their and of this also at the for with and This was in by from the the of the and the of and
Source: The Plant Cell
DOI: 10.1105/tpc.015800
Authors: A. F. Edlund, Robert Swanson, Daphne Preuss
Institutions: Howard Hughes Medical Institute, University of Chicago
Language: English
Topic: Plant Reproductive Biology
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2104408556