Pollen and Stigma Structure and Function:
The Role of Diversity in Pollination
Anna F. Edlund,1 Robert Swanson, and Daphne Preuss2
Howard Hughes Medical Institute, Molecular Genetics and Cell Biology Department, University of Chicago,
Chicago, Illinois 60637
INTRODUCTION
The ornate surfaces of male and female reproductive cells in
flowering plants have long attracted attention for their variety and
evolutionary significance. These structures, and the molecules
involved in sexual interactions, remain among the most rapidly
evolving and diverse characteristics known. As varied as they
may be, each element takes part in performing the same
functions, protecting pollen and stigma from the environment,
delivering and capturing pollen, promoting pollen hydration and
germination, allowing the entry of appropriate pollen tubes into
the stigma, and guiding the tubes to the ovary (Figure 1A). In this
review, we (1) survey recent discoveries of pollen and stigma
functions both before and after they make contact, and (2)
address the great diversity in pollen and stigma structures across
taxa, focusing on how they accomplish key tasks in pollination.
This system presents an exciting opportunity for the fruitful
unification of cell, genetic, and genomic studies of model organisms
with comparative studies of relationship and evolution.
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
pollen­stigma 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). Recent progress has revealed key molecules and
mechanisms and has poised the field for comparative studies
across taxa. In the second part of the review, we discuss
diversity in pollen and stigma structures, highlighting the
coadaptive evolutionary change that supports efficient pollination
within a species while restricting pollination between
species. Given its combination of facile, genetic model systems,
morphological analyses, and well-characterized phylogenies,
the study of angiosperm pollination provides an excellent opportunity
to unify cell and comparative biology and yields insight
beyond pollination to the very mechanisms of evolution and
speciation.
POLLEN AND STIGMA CELLULAR FUNCTIONS
Overview of Structures
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
1 Current address: Biology Department, Spelman College, Box 349, 350
Spelman Lane, Atlanta, GA 30314.
2 To whom correspondence should be addressed. E-mail dpreuss@
midway.uchicago.edu; fax 773-702-6648.
Article, publication date, and citation information can be found at
www.plantcell.org/cgi/doi/10.1105/tpc.015800.
The Plant Cell, Vol. 16, S84­S97, Supplement 2004, www.plantcell.org  2004 American Society of Plant Biologists
Figure 1. 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.
Pollen and Stigma Structure and Function S85
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. Selfincompatible
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).
Pollen Adhesion to the Stigma: First Contact
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 pollen­stigma
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 pollen­stigma 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 pollen­stigma 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 protein­protein 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,
Table 1. Pollen and Stigma Structural Features and Their Roles
Structural Feature Proposed Functional Significance
Pollen grain size Biotic and abiotic pollinator
preference and fluid dynamics
Number of pollen
grains per pollination
unit
May increase delivery efficiency
Pollen coat Protects pollen cells from excess
desiccation after dehiscence;
protects from UV radiation and
pathogen attack; stickiness,
color, and aroma may affect
interaction with pollination vectors;
protein components involved in
adhesion, signaling, and
compatibility; lipids and proteins
are necessary for hydration
Exine pattern Interacts with biotic and abiotic
pollination vectors; affects
the surface area of the stigma
interface; mediates stigma adhesion;
retains pollen coat; affects wall
strength and elasticity
Exine porosity Microchannels are sites of water
egress and ingress during
desiccation and hydration;
progressive desiccation limits
pollen viability and life expectancy
Aperture size,
number, and
complexity
Affect environmental vulnerability to
desiccation, fungal invasion, and
mechanical stress; accelerated
desiccation limits pollen viability
and life expectancy; sites of focused
water ingress during hydration;
allow extreme volume changes
accompanying desiccation and
hydration; serve as portals for
pollen tube exit during germination
Intine Thickness and complexity are inversely
coordinated with exine and pollen coat
characteristics; specialized layers and
inclusions at apertures are involved in
pollen tube emergence and invasion
of the stigma cell wall
Stigma coat Defines stigmas as uniquely
water-permeant sites on the plant;
proteins and lipids are involved in
adhesion, hydration, and germination;
dry stigmas are pollen compatibility
sites, with selective support of pollen
hydration and germination, whereas wet
stigmas often are covered in exudates
from apoptotic cells and block
inappropriate pollination only in later
steps; at the point of contact between
pollen and stigma, the two coatings may
mix, a process that mediates coat
conversion, hydration, germination, and
stigma invasion
S86 The Plant Cell
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 protein­protein interactions,
including interactions between pollen coat proteins and the
Brassica S-locus­related protein (SLR1) (Luu et al., 1999;
Takayama et al., 2000). SLR1 is a stigma-expressed gene
related to the S-locus glycoprotein, a protein involved in selfincompatibility
(Lalonde et al., 1989). SLR1-mediated adhesion
requires some time to establish itself, possibly to allow
the mixing of stigma and pollen matrices; consequently, the
strength of initial pollen binding in SLR1-deficient plants is
the same as in wild-type plants (Luu et al., 1999). SLR1 binds
to pollen coat proteins. This binding, along with the enhanced
cell-to-cell adhesion, can be blocked by antibody inhibition of
the SLR1 proteins (Luu et al., 1999; Takayama et al., 2000).
As pollen tubes germinate from the grains, a final stage of
adhesion begins in which the pollen tube passes through the
foot into the stigma surface, tethering the emptying pollen
grain to the stigma via the tube (Zinkl et al., 1999; Dickinson
et al., 2000; Heizmann et al., 2000). When inappropriate
pollen grains reach this stage, further access is blocked by
inhibiting tube growth. This mode is common in plants with
wet stigmas, in which pollen grains typically are bound and
hydrated without applying species selectivity (Wheeler et al.,
2001).
Pollen Hydration: Activating Metabolism
Most pollen grains are metabolically quiescent and highly
desiccated, ranging from 15 to 35% water content, when
released from the anthers (Heslop-Harrison, 1979a; Buitink
et al., 2000). Water immediately surrounds grains that land on
a wet stigma, but those that land on dry stigmas mobilize their
lipid-rich pollen coat to form an interface between the two cell
surfaces. This interface converts to a histochemically distinguishable
form thought to promote water flow (Elleman and
Dickinson, 1986; Elleman et al., 1992). Water, nutrients, and
other small molecules are transported rapidly into the grain
from the stigma exudate (wet stigmas) or stigma papillae (dry
stigmas) by mechanisms that remain unclear. The discovery of
aquaporin expression in the stigma has prompted the exciting
model that water channels are involved in the rapid and
regulated water release from the stigma to the pollen (Dixit
et al., 2001). Also, the stigma cuticle may be uniquely permeable
to water traffic (Lolle et al., 1997, 1998; Pruitt et al.,
2000). Regardless of the mechanism of transfer, pollen hydration
often is regulated, both temporally and spatially.
Inappropriate hydration can have disastrous consequences,
leading to premature germination within the anther (Johnson
and McCormick, 2001) or germination on the wrong surface
(Lolle and Cheung, 1993; Lolle et al., 1998). In plants with dry
stigmas, regulated pollen hydration provides an effective early
barrier to incompatible pollination. This mode is active in selfincompatible
crosses (Sarker et al., 1988) and in crosses
between species (Lewis and Crowe, 1958; Hu¨ lskamp et al.,
1995). These processes are remarkably localized: the stigma
can hydrate a compatible grain while restricting the hydration
of foreign or incompatible pollen on the same papillus
(Dickinson, 1995). An understanding of the precise surface
changes that provide this exquisite control will require both
molecular dissection of stigma and pollen surface components
and genetic analyses of their roles.
The genetic and molecular dissection of the lipid-rich
matrices found in the pollen coat has progressed considerably
in recent years. In Arabidopsis, the pollen coat contains longand
short-chain lipids along with a small set of proteins,
including six lipases and eight Gly-rich oleosin proteins that
contain a lipid binding domain (Mayfield et al., 2001; Fiebig
et al., 2004). The pollen coating of Brassica is structured
similarly and also has been shown to contain peptides involved
in self-incompatibility (Doughty et al., 1993). Disrupting pollen
coat lipids or pollen coat proteins in Brassicaceae species can
delay or block pollen hydration. In particular, mutations that
impair long-chain lipid synthesis, and consequently the proper
assembly of the pollen coating, severely reduce the hydration
of Arabidopsis pollen and result in male sterility (Preuss et al.,
1993; Hu¨ lskamp et al., 1995). Hydraulic contact can be
restored to these mutant grains by the addition of purified
triacylglycerides (Wolters-Arts et al., 1998). Mutations that
affect pollen coat proteins are less extreme, perhaps because
of partial functional redundancy. The most abundant protein in
the Arabidopsis pollen coat is the Gly-rich oleosin GRP17;
seven related proteins have similar oleosin domains but
variable hydrophilic tails. Mutations in GRP17 delay the onset
of pollen hydration and decrease the ability of the mutant
pollen to effectively compete with wild-type pollen (Mayfield
and Preuss, 2000). The sequences of the GRP proteins are
widely divergent across the Brassicaceae, supporting a model
in which pollen coat diversification may contribute to speciation
(Mayfield et al., 2001; Fiebig et al., 2004). Unlike the GRP
proteins, the pollen coat lipids of most Brassicaceae species
are quite similar, making these molecules poor candidates for
dictating species specificity (Piffanelli et al., 1998). Consequently,
experiments that promote pollen hydration by the
application of exogenous lipids (Wolters-Arts et al., 1998,
2002) may bypass crucial regulatory steps that normally occur
in vivo, such as those restricting the hydration of incompatible
or foreign pollen.
The lipid-rich stigma exudate of plants with wet stigmas is
thought to be functionally analogous, in part, to the pollen
coat (Dickinson, 1995). Mutations that eliminate this secreted
matrix cause female sterility; this defect can be bypassed by
adding exogenous lipids (Goldman et al., 1994; Wolters-Arts
et al., 1998). The addition of cis-unsaturated triacylglycerides
is sufficient for hydration and germination, even on stigmaless
tobacco plants (Wolters-Arts et al., 1998). These observations
suggest that lipids could play an important role in the permeability
of the stigma cuticle to water (Lolle et al., 1998; Pruitt et al., 2000).
In fiddlehead, which is defective in a b-ketoacyl-CoA synthase
required for long-chain lipid synthesis, leaf cuticle permeability
increases and pollen hydration is stimulated on inappropriate cell
surfaces. Based on these findings, it is possible to propose
a model in which the presence of lipids, whether provided by the
male or the female surface, modulates water transfer to
desiccated pollen, while highly diverse proteins and peptides
mediate self and foreign pollen recognition.
Pollen and Stigma Structure and Function S87
Pollen Polarization and Germination: Preparing
for Pollen Tube Growth
Hydration transforms a pollen grain from a nonpolar cell to
a highly polarized cell. Whether tubes emerge on a dry stigma
surface, from a grain submerged in stigma exudates, or from
pollen germinated in vitro, the grain organizes its cytoplasm
and cytoskeleton to support the extension of a single tube.
These changes occur within minutes after hydration and
include the formation of filamentous cytoskeletal structures
that wrap around the nuclei, actin cytoskeleton polarization
toward the site of tube emergence (Tiwari and Polito, 1988;
Heslop-Harrison and Heslop-Harrison, 1992), reorientation of
the large vegetative nucleus so that it enters the extending tube
before the generative cells (Heslop-Harrison et al., 1986a;
Heslop-Harrison and Heslop-Harrison, 1989; Lalanne and
Twell, 2002), assembly of mitochondria and polysaccharide
particles at the site of the elongating tube tip (Cresti et al.,
1977, 1985; Mazina et al., 2002), and selection of the pollen
plasma membrane for secretory vesicle targeting and deposition
of callose (b-1!3 glycan) at the site of tube emergence
(Johnson and McCormick, 2001).
It is not yet clear how the polarization signal is perceived and
subsequently transduced to select a single point for tube
emergence. Several candidate signals have been suggested,
including water, lipids, and ions (Feijo et al., 1995; Lush et al.,
1998; Wolters-Arts et al., 1998). Evidence for water as a polarity
signal has come from in vitro experiments with Nicotiana
pollen: immersing these grains in purified lipids or in stigma
exudates, and providing a nearby aqueous interface, results in
polarized growth toward the aqueous medium, mimicking
behavior in vivo. The pollen tubes emerge from the aperture
closest to the hydrophobic­aqueous interface, suggesting that
the source of water provides directional cues that establish
polarity (Lush et al., 1998; Wolters-Arts et al., 1998). Not only
does this example implicate water, it also suggests a role for
lipids in establishing polarity. Additional support for this idea
comes from mutants that lack the lipid-rich pollen coating
(Preuss et al., 1993; Hu¨ lskamp et al., 1995). As described
above, such grains are unable to hydrate or germinate on the
stigma. Coat-deficient grains do hydrate at increased ambient
humidities, but because they lack key axial information, their
pollen tubes often emerge at random orientations relative to
the stigma (Dickinson, 1995).
Polarization signals ultimately trigger the recruitment of RHO
OF PLANTS1 (ROP1), a GTP binding protein involved in F-actin
dynamics and the establishment of calcium gradients at tube tips
(Gu et al., 2003). ROP1, with its binding partner ROP INTERACTING
CRIB-CONTAINING1 protein, localizes to the tip of the
growing pollen tube, where they act to focus secretory vesicle
delivery (Kost et al., 1999; Li et al., 1999; Wu et al., 2001). Other
proteins that preferentially localize to the growing pollen tube
tip may play a role in the initial establishment of cell polarity.
One candidate, found at the tips of elongating fern rhizoids,
is annexin, a protein believed to be involved in tip-oriented
exocytosis events; annexins exhibit calcium-dependent binding
to phospholipids, affect cytoskeletal structure, and potentially
modulate voltage-dependent Ca21 channels (Clark et al., 1995).
Pollen activation promises to be a particularly rich subject for
studies integrating the external cues and internal signal
molecules necessary for global rearrangements and polarized
tip growth.
Once the cell has established its internal polarity relative to an
external signal, the pollen tube must breach the exine wall to
emerge from the grain. Depending on the species examined,
pollen tubes either grow out of the apertures or break directly
through the exine wall. In rye and eucalyptus, tubes emerge
strictly at the apertures, by dissolving apertural intine layers,
rupturing the thin sporopollenin wall, and displacing the opercula
that guard these sites (Heslop-Harrison, 1979b; Heslop-Harrison
and Heslop-Harrison, 1985; Heslop-Harrison et al., 1986b). In
Arabidopsis, whose pollen has three distinct apertures, pollen
tubes often break directly through interaperture exine walls
precisely at the site of contact with the stigma surface (Figure
1E). Regardless of exit site, pollen tube escape requires either (1)
exine weakening by enzymatic digestion from the inside or
outside of the wall, or (2) exine tearing by local gel-swelling forces
or focused turgor pressure. Evidence for the former possibility
comes from reports of significant exine remodeling after contact
with the stigma (Gherardini and Healey, 1969; Dickinson and
Lewis, 1974). Gel-swelling forces or turgor pressure also could
play important roles, but because such forces radiate in all
directions, they must be focused at the site of emergence. A
combination of mechanisms is most likely, with the increased
turgor pressure of the pollen grain contributing to the rupture of
a patch of partially degraded exine at the pollen­stigma interface.
There is evidence that pollen grains are preset to harness
biomechanical forces for pollen tube emergence; dead pollen
grains are able to hydrate to the same extent as living grains,
swell, alter apertural coverings, and even germinate short tubes
before the tubes ultimately rupture (Heslop-Harrison, 1979b).
The challenge of purifying factors that regulate cell polarity and
tube emergence calls for the use of complementary genetic
strategies. For example, mutations that upset the physical
association between the two Arabidopsis sperm cells and the
vegetative pollen nucleus have been identified. These mutations
disrupt the polarized transport of the male germ unit to the pollen
tube tip by (1) separating the vegetative nucleus from the two
sperm cells (germ unit malformed1 [gum1] and gum2) or (2)
mislocalizing an intact male germ unit to the pollen wall (mgu
displaced1 [mud1] and mud2) (Lalanne and Twell, 2002). Other
visual screens have identified mutations that allow tubes to grow
without emerging from the grain or disrupt the accumulation and
deposition of callose before pollen tube emergence (Johnson
and McCormick, 2001). In addition, screens for pollen gametophytic
mutations have yielded mutants altered in germination,
polarity, and tube emergence (Ryan et al., 1998; Grini et al., 1999;
Johnson and McCormick, 2001; Procissi et al., 2001; Johnson
and Preuss, 2002; Lalanne and Twell, 2002). Similar approaches
have not yet yielded stigma-specific mutations, despite numerous
screens to isolate sporophytic sterile mutants. This
deficiency suggests functional redundancy in the pollen-recognition
machinery involved. On the other hand, the fertility of
Arabidopsis flowers with genetically ablated stigma cells (Kandasamy
et al., 1993; Thorsness et al., 1993) calls into question
the absolute requirement for the tissue.
S88 The Plant Cell
Pollen Tube Invasion: Growing into the Stigma
After crossing the exine wall, pollen tubes can only enter the style
after transiting the stigma barrier. The details of this process vary
considerably from species to species. In plants with open styles,
the stigma is covered with an epidermis that is continuous with
the style, but in species with closed stigmas, pollen tubes grow
through the outer cuticle and cell wall of the stigma papillae to
enter the style. For >100 years, enzymes secreted by pollen have
been proposed to play an important role in pollen tube invasion of
the stigma surface (Green, 1894). Acid phosphatase, ribonuclease,
esterase, amylase, and protease activity have been
localized to pollen intines and tubes (Knox and Heslop-Harrison,
1970). Esterases, particularly those known as cutinases, are
important for breaching the stigma cuticle; they have been
identified in the pollen of Brassica, Tropaeolum, and many other
taxa (Knox and Heslop-Harrison, 1970; Shaykh et al., 1977;
Hiscock et al., 1994) as well as in the proteinaceous pellicles of
dry stigma surfaces (Heslop-Harrison and Shivanna, 1977;
Hiscock et al., 2002b). Removing or disrupting the pellicle
prevents compatible pollen tubes from entering the stigma,
despite normal germination (Heslop-Harrison and HeslopHarrison,
1975; Heslop-Harrison, 1977; Heslop-Harrison and
Shivanna, 1977). In addition, cutinase inhibitors significantly
reduce the ability of pollen tubes to penetrate Brassica stigmas
(Hiscock et al., 2002a). Hydrolysis of pectin in the stigma cell wall
also is necessary, for which pollen expresses genes encoding
pectin esterase and pectate lyase (Kim et al., 1996; Wu et al.,
1996). A pollen-specific Brassica polygalacturonase also has
been detected at the tip of pollen tubes as they enter the stigma
papillar cell walls (Dearnaley and Daggard, 2001).
Enzymatic penetration of the stigma surface is precisely
controlled to not expose the pistil to pathogenic or inappropriate
invasion. This control likely requires constant communication
between the pollen tube and the stigma. Recently, receptor
kinases have been identified as candidate mediators of
communication between the pollen tube and the stigma.
Members of the large Leu-rich repeat receptor kinase family
(Lycopersicon esculentum protein receptor kinase1 [LePRK1]
and LePRK2) localize to the growing pollen tube (Muschietti et al.,
1998). Yeast two-hybrid experiments using the extracellular
domains of these kinases yielded many candidate ligands,
including cell wall­remodeling proteins and Cys-rich proteins
implicated in extracellular signaling. One pollen-specific, Cysrich
protein, LATE ANTHER TOMATO52 (LAT52), interacts in vivo
with LePRK2; LAT52 is essential for pollen hydration and
germination in vitro and for normal tube growth in vivo. This
finding suggests that LAT52 and LePRK2 are members of an
autocrine signaling cascade that may regulate and maintain
pollen tube growth (Tang et al., 2002).
POLLEN AND STIGMA STRUCTURAL DIVERSITY
Although the processes discussed above are conserved across
angiosperms, they are accomplished by an astounding variety of
pollen and stigma structures. Male and female reproductive
tissues evolve rapidly; yet, to maintain their interactions, they
also evolve coordinately. This evolution is evident in angiosperm
pollen grains, which are remarkably diverse, varying in size (from
20 to 250 mm in diameter), in their pigments (Lunau, 2000), in their
aromas (Dobson and Bergstrom, 2000), in their nutritional
content for pollinators (Roulston and Cane, 2000), and in the
patterns of their ornate exine structures (Figures 2A to 2D). Many
of these characteristics are important for pollen delivery and for
modulating the adhesion, hydration, and emergence of pollen
tubes. Stigma cells are similarly diverse (Figures 2E and 2F); they
vary in form, are either unicellular or multicellular, and differ in
wall structure and cuticle pattern while remaining compatible
with their pollen counterparts. The taxonomic relationships of
a wealth of angiosperms are understood (Qiu et al., 1999). With
the availability of model systems that reveal gene functions, the
power of comparative genomics can now be harnessed to
investigate the role of genetic diversity in plant reproduction. This
system represents an opportunity with enormous potential: it is
now possible to merge an understanding of cell biology with the
immense evolutionary diversity seen in nature.
Pollen Walls: Generating Structural Diversity
Pollen wall patterns are so diverse and often so characteristic of
each species that they have long been used for taxonomic
classification and even for forensic identifications (Szibor et al.,
1998). Complete understanding of the evolution of diversity in
pollen wall characteristics calls for the study of the genes and
mechanisms behind their development. Species-specific pollen
patterns are established early in meiosis (Sheldon and Dickinson,
1983). Microsporogenesis can be divided into two broad
classes: (1) simultaneous, in which the first meiotic division is
followed immediately by the second, with a wall of callose that is
deposited subsequently around all four tetrahedrally arranged
microspores; and (2) successive, in which a callose wall is
deposited around the dyad formed by the first meiotic division,
before the second division occurs. Simultaneous microsporogenesis
usually is associated with eudicots, whereas successive
microsporogenesis is associated with monocots, although to
a lesser extent, because of several evolutionary reversals
(Furness et al., 2002). Microspore alignment within the callose
wall is important on several levels, including internal vegetative
and generative cell polarity (Twell et al., 1998), the release of
pollen as a monad, tetrad, or cluster, and exine sculpting and
aperture position. A primexine matrix, the precursor of exine,
typically is deposited between the callose wall and the plasma
membrane of the developing microspores. The callose wall later
is degraded, enabling the pollen grains to be released as monads
(Paxson-Sowders et al., 1997). In some species, including
a number of ancient flowering plants, pollen grains instead
remain in fused clusters, an arrangement thought to increase
pollination efficiency. Mutants defective in the early stages of
pollen development may provide insight into genes that are
modulated during evolution, resulting in the striking variation of
pollen form. For instance, mutations that mimic natural tetrad
clusters have been found in Arabidopsis. In quartet mutants
(Figure 3), callose deposition is disrupted during microspore
development, causing the exine layers of pollen tetrads to merge
(Preuss, 1994; Rhee and Somerville, 1998; Rhee et al., 2003).
quartet mutations have been useful for genetically dissecting
Pollen and Stigma Structure and Function S89
chromosome segregation during meiosis (Copenhaver et al.,
2000; Copenhaver, 2003). Other Arabidopsis mutations have
a more severe fusion defect. Pollen from tetraspore/stud (tes/
stud) mutants fail to undergo cytokinesis, resulting in the release
of a giant pollen grain that contains four vegetative nuclei and up
to eight sperm (Hulskamp et al., 1997; Spielman et al., 1997;
Yang et al., 2003). Although these mutations have led to
interesting insights, there are likely additional genes to be found;
screens for tes or quartet phenotypes were not performed to
saturation. Such genes would serve as appealing candidates for
comparative pollen development studies across taxa.
Patterning events that affect exine sculpting and aperture
position also occur during or soon after meiosis. Haploid pollen
typically produces a primexine, establishing an early pattern.
After callose wall degradation, diploid tapetal cells elaborate this
pattern with their deposition of sporopollenin material (PaxsonSowders
et al., 1997). Temporally regulated construction and
degradation of the callose wall that surrounds pollen occurs in
angiosperms (Heslop-Harrison, 1971; Munoz et al., 1995) and
resembles the patterns of spore wall development in ferns and
bryophytes (Pettitt and Jermy, 1974). The initial species-specific
pattern of the primexine requires coordination between the
pollen plasma membrane and its underlying cytoskeleton,
vesicles, and endoplasmic reticulum (Dickinson and Sheldon,
1986; Takahashi and Skvarla, 1991; Fitzgerald and Knox, 1995;
Perez-Munoz et al., 1995; Paxson-Sowders et al., 2001). For
example, the Arabidopsis DEFECTIVE IN EXINE1 gene product,
a predicted membrane-associated protein with limited similarity
to animal integrins, appears to interact with the plasma
membrane to nucleate sporopollenin deposition (Paxson-Sowders
et al., 2001). An exine pattern mutant also has been
described in Haplopappus gracilis, whose normal surface
pattern of densely packed spines is reduced to a random scatter
of sporopollenin deposits with few interspersed spines (Jackson
et al., 2000). Similarly, mutation in the Arabidopsis less adherent
pollen1 gene yields mutant pollen grains with discontinuous
exine walls composed only of globular sporopollenin deposits
(Zinkl and Preuss, 2000). With the identification of additional
mutants involved in exine development, it will become possible
to distinguish those genes required for sporopollenin synthesis
from those required to establish patterns. Patterning genes, in
particular, may be highly variable across species, either in their
coding sequence or in their site and timing of expression.
Because these genes are likely altered through evolution, exine
patterns discovered by mutation and considered aberrant in one
species may be normal in another species.
Exine also varies in the number, distribution, and architecture
of the apertures that interrupt it (Figure 2). Apertures are diverse
across taxa, within families, within species, and even within
a single plant (Mignot et al., 1994). Pollen from monocots
characteristically has a single aperture, a trait considered to be
ancestral. Most dicot pollen grains have three apertures,
although in both instances, aperture numbers have increased
or decreased repeatedly and independently during the course of
Figure 2. Examples of Angiosperm Diversity in Pollen and Stigma Structures.
(A) Hydrated Lilium longiflorum pollen grain stained with Congo red. The intine (red staining) contains several fissures at the single large aperture; the
pollen coating (green) forms droplets on the surfaces of the ornamented exine walls.
(B) Multiapertured pollen of Passiflora fanguinolenta, hydrated and stained with exine-specific Auramine O dye.
(C) Polyad pollen of Acacia retinoides, stained with Auramine O. Thirteen small, interconnected pollen grains are visible.
(D) Composite scanning electron micrographs of several pollen types, including Arabidopsis, Ambrosia (ragweed), maize, Plumbago, and Artemisia
douglasiana (mugwort), which vary in size, exine ornamentation, and number and arrangement of apertures.
(E) A portion of the stigma of Torenia fournieri, visualized with Auramine O.
(F) Stigma papillae from Arabidopsis visualized by FM1-43.
Bars  10 mm in (A) to (D) and 200 mm in (E) and (F).
S90 The Plant Cell
evolution. At the extremes, pollen exines from some species lack
apertures entirely, whereas others are ``omniaperturate,'' lacking
exine or coated everywhere with a very thin exine layer (Pacini,
1991; Rowley et al., 1997). The positions of the apertures are set
during the tetrad stage, when gaps in the exine form at sites of
close apposition between the plasma membrane and the
adjacent callose wall (Heslop-Harrison, 1968). The mechanism
of aperture placement is still unclear, but it has been linked to
microsporocyte cytokinesis. Indeed, in the tes/stud mutant,
a cytokinesis defect results in aberrant aperture positions
(Hulskamp et al., 1997; Spielman et al., 1997; Yang et al., 2003).
Several aperture functions seem likely, although it has been
difficult to link them experimentally to aperture position and
number. Three logical functions for apertures are (1) sites for
egress and ingress of water during pollen desiccation and
hydration, (2) structural components that distribute stresses
caused by grain shrinkage and swelling (termed harmomegathy),
and (3) portals for the exit of the pollen tube. First, although the
exines of some pollen grains are perforated with microchannels
(Rowley et al., 1959), others are impermeant to water and rely on
apertures to regulate grain desiccation and hydration. Consequently,
some types of apertures are associated with certain
plant types, especially with wet or dry stigmas. For instance,
Heslop-Harrison (1979) discussed the association between
multiple circular aperture (forate) exines and dry stigmas,
hypothesizing that their adaptive value lies in their ability to
focus water entry into the grain. Second, rigid walls are liabilities
for pollen grains that undergo enormous volume decreases in the
anther and subsequent increases on the stigma surface. In this
context, apertures are analogous to architectural spacers in
concrete, absorbing and permitting necessary shifts without
breakage. The elasticity of the sporopollenin wall itself also is
valuable for these deformations (Rowley et al., 2000). Third, it has
long been recognized that pollen tubes exit through apertures in
many pollens. Curiously, this behavior is by no means universal;
some pollens lack apertures altogether, whereas others, such as
Arabidopsis pollen, have apertures but do not regularly use them
for tube emergence (Figure 1E). A comparative study of tube
emergence across taxa could yield insight into the evolution of
the mechanisms necessary for different forms of tube emergence.
Although apertures provide an important means of
communication between pollen and the outside world, they also
are sites of vulnerability to excess desiccation and invasion by
fungal hyphae (Bih et al., 1999); consequently, some plants cover
Figure 3. Pollen Tetrads and Clusters.
(A) Pollen tetrads from the Arabidopsis mutant quartet. Four complete pollen grains are connected by exine bridges.
(B) Fused pollen grains from the Arabidopsis mutant tes/stud. Cytokinesis does not occur after meiosis, resulting in a multinucleate cytoplasm.
(C) Natural pollen tetrads from Drocera binata (sundew).
Bars  10 mm.
Pollen and Stigma Structure and Function S91
the apertures with specialized intines, lipid-rich pollen coatings,
or opercula. Mutations that disrupt the regular pattern of
apertures have been identified, but it is not currently feasible to
use genetics to remove or add these features. Such resources
would clarify the requirement for apertures in water transport,
communication, and adaptation to specific types of stigmas.
Unlike the exine, and similar to most other cell walls, the
cellulosic intine is constructed by the pollen cells themselves.
Intine structure often is coadapted with the overlying exine
structure. For example, pollen grains that have smooth or
reduced exines and are unable to bind significant quantities of
pollen coat have thickened or complex intines that frequently
contain inclusions of hydrolytic enzymes (Heslop-Harrison,
1975b, 1977). Moreover, at the apertures, intine thickening
complements exine thinning. Intine elaboration at the apertures
can include extra layers between the innermost intine proper and
the thin layer of aperture exine. Some of these intine layers
contain enzymes and potent allergens, whereas others appear to
play a role in cell wall swelling and tube emergence (Howlett et al.,
1973; Heslop-Harrison, 1979b). Cellulose fibers within the intine
wall radiate toward the aperture sites, generating hoops oriented
around the grain's circumference that are thought to strengthen
the polarized structure of the elongating pollen tube (Christen
and Horner, 1974; Heslop-Harrison, 1979b; Heslop-Harrison and
Heslop-Harrison, 1982). As the pollen tube emerges, the intine
layer becomes continuous with the cell wall of the tube, with new
wall material deposited at the growing tip. The enzymatic activity
localized within the intine then is incorporated into the lateral tube
walls, potentially playing a role in tissue invasion and selfincompatibility
reactions. To date, few mutants have been
identified that alter intine function in pollination, likely because
of multiple, redundant pathways. Genomic and biochemical
analyses promise to provide important clues to the identities of
key intine activities.
Pollen Structure and Delivery
Pollen grains are transported to the stigma by a variety of biotic
and abiotic mechanisms, for which their structures are believed
to be uniquely adapted (Ackerman, 2000; Dobson and
Bergstrom, 2000; Lunau, 2000). In some flowers, the selection
for attracting animal pollinators is so strong that pollen grains
are produced that are infertile and that function only for pollinator
attraction and reward. Such grains are seen in Lagerstroemia,
whose dimorphic anthers produce blue fertile pollen and yellow
feeding pollen; although the feeding pollen can germinate, its
tubes never reach the style (Pacini and Bellani, 1986). Similarly,
cashew trees produce four types of pollen that are comparable
in dimensions, structure, and staining characteristics but different
in their abilities to germinate on and penetrate the stigma
(Wunnachit et al., 1992). Beyond these extreme examples, many
studies have argued for pollinator preference in the evolutionary
selection for a given pollen characteristic, reporting correlations
between such characteristics as size or surface features and
pollination by beetles, flies, bees, moths, or birds (Skvarla et al.,
1978; Hesse, 2000). The evolutionary selection of other pollen
characteristics appears linked to their delivery by abiotic vectors;
20% of all angiosperm families contain wind- or waterpollinated
flowers (Ackerman, 2000). Most likely, biomechanical
principles contribute to the small size of many pollens delivered
by wind and to the filamentous structures of many aquatic
pollens (Cox and Knox, 1989; Ackerman, 2000).
Diversity in Pollen Coat Composition
The structure of the pollen coat also is adapted to different
delivery mechanisms. Insect pollination is correlated with an
abundant pollen coat (Pacini and Franchi, 1996). For example,
the Brassica pollen coat can constitute 10 to 15% of the mass of
the pollen grain (Dickinson et al., 2000). By contrast, grains
carried by the wind often have a more limited coating, such as
that seen among the grasses (Heslop-Harrison, 1979b). Because
lipids appear to be necessary at the interface between pollen and
stigma, those pollens with limited pollen coats may depend on
lipid-rich stigma exudates to assume the functionally equivalent
role in germination (Wolters-Arts et al., 1998). Pollens also differ
dramatically in the character and composition of their pollen
coats, with grass pollen coats (pollenkitts) often lacking
inclusions, whereas more complex Brassica coatings (tryphines)
contain protein and lipid bodies (Pacini, 1997; Dickinson et al.,
2000). Several resources provide exciting opportunities to experimentally
investigate the role of some pollen coat in pollen
delivery, including (1) the identification of mutants with globally
depleted pollen coats, such as the eceriferum defects in
Arabidopsis (Preuss et al., 1993; Hu¨ lskamp et al., 1995); (2) the
ability to engineer mutants lacking specific proteins with reverse
genetics (Mayfield and Preuss, 2000) or RNA interference
techniques; and (3) the opportunity to explore pollen coat
evolution through comparative genomics (Hall et al., 2002;
Fiebig et al., 2004).
The pollen coat is generated by the tapetal cells that surround
developing pollen grains. These cells secrete oil droplets and
proteins and subsequently degenerate, filling the cavities of the
exine with their cytoplasmic contents. Compared with sporopollenin-rich
exines, pollen coats are better characterized; they
can be extracted and their contents can be purified in sufficient
quantity for identification (Doughty et al., 1993; Ross and
Murphy, 1996; Ruiter et al., 1997; Murphy and Ross, 1998;
Mayfield et al., 2001). The first biochemical isolations were in
Liliacea, in which pollen coats were found to be largely lipidic,
with carotenoids as major pigments (Heslop-Harrison, 1968).
Further studies have identified medium-chain, long-chain, and
very-long-chain fatty acids, proteins, glycoconjugates, carotenoids,
and flavonoids (Piffanelli et al., 1998). In maize, the major
protein component in the pollen coat is xylanase, which, upon its
release at stigma binding, facilitates pollen tube invasion by
hydrolyzing the xylan on the stigma surface (Bih et al., 1999). In B.
oleracea, >100 pollen coat proteins, ranging in size from 5 to 50
kD, have been separated on denaturing gels. These include
proteins involved in pollen­stigma adhesion or self-incompatibility
responses (Preuss et al., 1993; Ross and Murphy, 1996;
Ruiter et al., 1997; Murphy and Ross, 1998). Analysis of higher
molecular mass proteins from Arabidopsis yielded a simpler
mixture, including lipases, Gly-rich oleosins, calcium binding
proteins, and proteins with similarity to the extracellular domains
of receptor kinases, although lacking the kinase domain
S92 The Plant Cell
(Mayfield and Preuss, 2000; Mayfield et al., 2001). Pollen coats
also contain volatile lipids for pollinator attraction, such as
polyunsaturated C18 free fatty acids, which are known to be potent
honeybee attractants (Hopkins et al., 1969). Chemical analyses
of 15 plant species from 10 families revealed that the pollen
of each species emits its own characteristic mixture of volatiles,
with the three major classes of volatiles seen in floral scents,
isoprenoids, fatty acids, and benzoids, present in pollen volatiles
to different degrees depending on the species (Dobson and
Bergstrom, 2000). Although the set of known pollen coat proteins
in Arabidopsis and Brassica is substantial, it represents only
a small subset of the proteins from the tapetal cytoplasm.
Extensive degradation of the released tapetum materials likely
occurs before dehiscence, with the surviving coat components
assembling into structures that are resistant. Support for this
idea is provided by Arabidopsis mutants lacking long-chain
pollen coat lipids; in these cases, proteins are deposited on the
pollen surface but disappear from the coating before the pollen is
released (Preuss et al., 1993).
Stigma Diversity and Development
A survey of 1000 species has shown that stigma structure
generally correlates with taxonomic subdivision and argues for
the coevolution of pollen and stigma structures. For example,
angiosperm species that produce trinucleate pollen typically
have dry stigmas, whereas binucleate pollens often interact with
wet stigmas (Heslop-Harrison, 1977). As diverse as they may be,
all stigmas perform the same functions. Stigmas capture pollen,
support hydration and germination, and offer entry points and
guidance to pollen tubes en route to the ovaries. Stigmas do not
only discriminate between different pollen grains; they also
promote outcrossing or self-fertilization by the coordinated
timing of their maturation with pollen release. This phenomenon,
termed dichogamy, has been investigated for >200 years. Two
forms of dichogamy have been recognized: protandry and
protogyny, in which the male or female structures, respectively,
are the first to mature (Bertin and Newman, 1993). Standard tests
for stigma maturation and receptivity are routinely conducted
based on (1) the presence of simple enzymatic activity, (2) the
ability to support pollen germination, and (3) the ability to support
fertilization (this third assay being the most stringent) (Dafni and
Maues, 1998). Stigma development is linked to the development
of other floral organs. In Arabidopsis, immature stigmas appear
to be able to promote pollen tube growth, although the immature
pistil then is unable to support navigation to the ovules (Nasrallah
et al., 1994). As the pistil matures, the papillae extend to form
elongated cells receptive for pollination. Mutations that alter pistil
structure also disrupt stigma structure; crabs claw, spatula, and
ettin mutants have unfused carpels, open styles, and two
clusters of stigma papillae, each at a carpel apex. spatula
mutations have additional effects on stigma papillar maturation
(Alvarez and Smyth, 1998, 1999), as do mutations in dynamin
(ALD1), a GTP binding protein involved in membrane trafficking.
ALD1 localizes to the developing cell plate during cytokinesis
and maintains the plasma membrane during cell expansion
(Kang et al., 2001, 2003). Transmission electron microscopy
analyses of ald1 mutants revealed defects in cell wall and plasma
membrane integrity that result in the formation of short papillae
cells and infertility (Kang et al., 2003). Because stigma and carpel
structures vary across taxa, mutants that superficially mimic
naturally occurring structures may highlight the very genes that
have been modified through evolution.
At maturity, the receptive surface of a stigma presents
secretions and enzymes required for pollination. Some plants
that separate male and female flowers (monoecious angiosperms)
have been found to retain their stigmas in the
functionally male flowers but to delay the maturation of these
stigmas. Likewise, some sterile cultivars lack the stigma
secretions visible in their fertile counterparts (Heslop-Harrison
and Shivanna, 1977). Other structural changes accompany late
stigma development, including modifications of the papillar cells
and their cuticles. Also, in plants with wet stigmas, apoptotic
events release stigma fluids from internal reservoirs (HeslopHarrison,
1977, 1981). A great challenge in investigating stigma
function is the dearth of informative mutants, a lack likely
attributable to pathway redundancy. Exploring stigma function
with reverse genetics and comparative genomics is likely to yield
valuable insights and to provide a necessary link between
evolutionary investigations of morphology and genetic analyses
of functional cell biology.
CONCLUSIONS
Here, we surveyed recent discoveries of pollen and stigma
functions, including (1) pollen adhesion, a multiphase process
that is initially independent of protein­protein interactions but
later involves pollen coat proteins; (2) pollen hydration, a step for
which the presence of lipids, whether provided by the male or the
female surface, likely modulates water transport while diverse
proteins mediate pollen incompatibility; (3) pollen activation
and germination, processes dependent on polarizing cues, cytoplasmic
rearrangements, and the harnessing of biomechanical
forces for the focused breaching of the exine wall; and (4) stigma
invasion, a stage that involves the localized activity of digestive
enzymes. We also addressed the great diversity in pollen and
stigma structures used across taxa to accomplish pollination,
including diversity in pollen wall development, pollen delivery,
pollen coat composition, and stigma development.
Studies of model organisms have revealed key molecules and
mechanisms, setting the stage for comparative studies across
taxa. For example, in pollen wall development, the patterning
genes identified through Arabidopsis screens are likely to be
highly variable between species, either in their coding sequences
or in the sites and timing of their expression. Work in comparative
genomics is called for not only within the angiosperms but
between pollen, fern spores, and even diatoms (Schmid et al.,
1996), in which similar cytoskeletal and plasma membrane shifts
are likely associated with wall construction, sculpting, and
aperture placement. Because genes that play roles in determining
wall morphologies are the very genes altered through
evolution, patterns considered aberrant or mutant to one species
may be considered normal in another. Suppositions such as that
sculpted exine is required to hold an abundant pollen coat could
Pollen and Stigma Structure and Function S93
be addressed using Arabidopsis mutations, such as lap1 (Zinkl
and Preuss, 2000), that affect exine patterning. The roles of
pollen coat or pollen dispersal unit size in pollinator preference
and pollen delivery could be addressed using knowledge of
Arabidopsis mutants, such as those in ECERIFERUM, TES/
STUD, or QUARTET genes (the former affects pollen coat
quantity and the latter two affect the release of monad pollen,
respectively). Comparative genomics and RNA interference
technology then could be exploited to knock out these genes
in insect-pollinated flowers, such as those of Brassica. Integrating
cell, genetic, and genomic studies of model organisms
with comparative studies of relationship and evolution allows
pollen and stigma sophistication to be viewed in the context of
coadaptive or mutual evolutionary adjustments. Because pollination
mechanisms also are key to reproductive isolation and
speciation, attention focused across a variety of species also will
help to complete any model of the success and diversity of
angiosperms.
ACKNOWLEDGMENTS
We are grateful to members of the Preuss laboratory, especially Kiera
Von Besser, Mark Johnson, and Ravishankar Palanivelu, for their helpful
discussion and review of this article. We also thank horticulturalist
Steven Meyer at the Lincoln Park Conservatory, Chicago Park District,
for flowers with tetrad and polyad pollens. This work was supported in
part by grants from the Howard Hughes Medical Institute (A.F.E.), the
U.S. Department of Agriculture (2002-35318-12560) (R.S.), the National
Science Foundation (MCB-0077854), and the University of Chicago
Materials Research Science and Engineering Center (National Science
Foundation DMR-0213745).
Received July 28, 2003; accepted January 15, 2004.
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