368
Cell–cell signaling in the self-incompatibility response
June B Nasrallah
Significant progress towards understanding the molecular
basis of self recognition in the self-incompatibility response of
Brassica has been made during the past two years. The highly
polymorphic molecules that determine the specificity of this
interaction in the pollen and stigma have been identified. The
structural features of these molecules suggest that a
ligand–receptor-kinase interaction triggers the initiation of a
signaling cascade within the stigma epidermis and the
subsequent arrest of self-pollination.
Addresses
Department of Plant Biology, Cornell University, Ithaca, New York
14853, USA; e-mail: jbn2@cornell.edu
Current Opinion in Plant Biology 2000, 3:368–373
1369-5266/00/$ - see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviations
ARC1
arm-repeat-containing protein 1
kb
kilobase
SCR
S-LOCUS CYSTEINE-RICH
SI
self-incompatibility
SLG
S-locus glycoprotein
S locus self-incompatibility locus
SRK
S-locus receptor kinase
Introduction
Cell–cell recognition and signaling phenomena ensure the
proper functioning of tissues and organs as well as allowing
individual organisms to distinguish between self and nonself. In plants, the pollen–pistil interactions that precede
fertilization have been recognized for some time as being a
useful model system for investigating cell–cell interactions [1]. Significant insight into the molecular and genetic
basis of pollen–pistil interactions and cell–cell signaling has
been provided by the study of predominately outbreeding
plants that possess a self-incompatibility (SI) system. SI is a
widely distributed genetic mechanism in flowering plants [2]
that allows plants with perfect flowers to avoid inbreeding.
This genetic barrier to self-fertilization is based on the ability of cells in the pistil to discriminate between self-related
and genetically unrelated pollen, and to arrest specifically
the development of self-related pollen. This article outlines
recent progress in the study of the SI system of the crucifer
genus Brassica, which is currently the only SI system for
which the determinants of SI specificity in both pistil and
pollen are known. The review focuses on the recognition
phase of SI and on the major advances, made largely during
the past two years, that culminated in the identification of
the molecules that determine the specificity of SI.
The SI response of Brassica as a cell–cell
recognition phenomenon
The arrest of self pollen in Brassica is a highly specific
cell–cell interaction that occurs at the surface of the epidermal
(i.e. papillar) cells of the stigma. The SI response is manifested within a few minutes of self-pollination by the
inhibition of pollen hydration, pollen germination, or pollentube invasion of the stigma epidermis [3]. Specificity in the
SI response is determined genetically by the 50 or so haplotypes (the classical ‘S alleles’ [i.e. self-incompatibility
alleles]) of a single locus, the S locus (self-incompatibility
locus) [4,5]. Pollen SI phenotype is determined by the two
S haplotypes carried by the parent plant (i.e. sporophytically) rather than by the single S haplotype inherited by
the pollen grain [4]. The ability of stigmas to recognize and
reject self pollen is only acquired at approximately one day
before flower opening, which allows the production and
maintenance of S-locus homozygotes by the forced
self-pollination of immature self-compatible stigmas.
Identification of the determinant of SI specificity
in stigma
The first S-locus-linked molecules to be identified were
two related polymorphic proteins expressed specifically
in the stigmatic papillar cells: the S-locus glycoprotein
(SLG) [6] is a soluble cell-wall-localized protein and the
S-locus receptor kinase (SRK) [7] is a plasma-membraneanchored signaling receptor [8–10], the extracellular
domain of which shares sequence similarity with SLG.
The involvement of SRK/SLG in the SI response of the
stigma (but not of the pollen) has been known for some
time because of the analysis of self-fertile plants that
occur spontaneously or are generated by a transgenic
approach [11–18]. Attempts at modifying SI specificity in
the stigma by the obvious transgenic experiment of
expressing SLG/SRK alleles isolated from one S haplotype in plants homozygous for another S haplotype were,
however, frustrated by homology-dependent gene silencing. In these experiments, expression of both the
transgene and the endogenous SLG/SRK gene pair was
dramatically reduced and resulted in a self-fertile transgenic phenotype. This phenotype resulted despite
efforts to minimize the likelihood of homology-dependent silencing by the use of transformation hosts with
SLG/SRK genes exhibiting only approximately 65%
sequence identity with the transgene [18].
This hurdle was overcome recently. In an important development, modification of SI specificity in the stigma was
achieved in B. campestris (syn. B. rapa) by transformation
with a SRK cDNA gene fused to a SLG promoter [19••],
and in B. napus by introduction of a 40-kilobase (kb) fragment containing both SLG and SRK [20•]. This success
may have been achieved in the former case because of the
use of B. campestris, which may lack some of the genetic
factors that effect homology-dependent suppression, as
transformation host. In the latter case, the use of a largeinsert fragment, which maintains the genomic context of
Cell–cell signaling in the self-incompatibility response Nasrallah
369
Figure 1
Structural heteromorphism of S haplotypes.
The organization of S haplotypes from
B. campestris (S8 [33,34], S9 [35] and S12
[39•]), B. oleracea (S6 [31,33]), and B. napus
(S910 and SA14 [36]) is shown. The
haplotypes are arranged according to the size
of the chromosomal segment spanning the
SLG–SRK–SCR complex, which varies from
6 kb to approximately 200 kb. The boxes that
represent the SLG, SRK, and SCR genes are
labeled above the S8 haplotype. These boxes
are not drawn to scale, and introns, which
occur in SRK and SCR but not in SLG, are
not shown. The three domains of the SRK
molecule are shown: the extracellular domain
as a cross-hatched box, the transmembrane
domain as a black box, and the kinase domain
as a gray box. Horizontal arrows indicate the
5′ to 3′ orientation of the genes. The small
boxes represent the location of other genes
that have been mapped in some, but not all,
haplotypes. Note that the positioning and
orientation of the SLG, SRK, and SCR genes
relative to each other is not conserved, and
that the cluster of SI-specificity genes can be
interrupted by other genes, as in the S910
haplotype. The location of SCR in the S910
and SA14 haplotypes was determined from
analysis of the sequence of these regions
([36], Genbank accession numbers
AJ245479 and AJ245480). In the S6
haplotype, the SLG and SCR genes are
SLG SCR
5kb
SRK
S8
13 kb
S9
13 kb
S910
~20 kb
6 kb
SA14
~32 kb
S12
~40 kb
S6
~200 kb
Current Opinion in Plant Biology
located within 10 kb of each other, but their
relative orientations are not known; the
orientation of SLG shown by the open arrow
is assumed on the basis of the conservation
of the haplotypic structure 3′ of SLG in all of
the other haplotypes analyzed. The vertical
arrows above the S8 haplotype depict the
the transgenes, is probably responsible for the success of
the transformation. Takasaki et al. [19••] introduced the
SRK28 cDNA (isolated from the S28 haplotype) fused to an
SLG promoter into a B. campestris S60 homozygote.
Significantly, expression of the SRK28 cDNA alone allowed
S60S60 stigmas to reject pollen from S28 homozygotes.
Nevertheless, unlike self-pollination of S28 plants, which
produces no seed, pollination of transgenic stigmas with
S28 pollen produced a small number of seed, which is
indicative of a somewhat weak SI response. Co-expression
of SRK28 and SLG28 was required to reproduce the strong
SI response that is characteristic of plants carrying the S28
haplotype [19••]. These results demonstrate that SRK represents the determinant of SI specificity in the stigma, and
that SLG contributes in some way to enhance the strength
of the SI phenotype.
What is the role of SLG?
Although SLG was the first S-locus gene to be isolated, its
role in SI is not well understood. The previously proposed
role of SLG as a determinant of SI specificity in the stigma
has now been ruled out, primarily by the results of the
transgenic experiments described in the previous paragraph. Additionally, sequence analysis of many different
SLG and SRK alleles isolated from plants having the same
or different SI specificities has established a more robust
correlation between SRK sequence divergence and SI
specificity than between SLG sequence divergence and SI
position of recombination breakpoints that
bracket a 50-kb region containing stigma and
pollen SI determinants [30•]. The
recombination breakpoint on the left
separates the largely colinear segment
downstream of SLG from the heteromorphic
segment containing SRK and SCR.
specificity [21•,22•]. Another proposed role for SLG is that
it functions in compatible pollination in the adhesion of
the pollen grain to the papillar cell [23]. This role was proposed on the basis of a study in which pretreatment of
stigmas with SLG antiserum reduced the force of pollen
adhesion. The biological significance of this hypothesis is
not evident, however, because mutant plants lacking an
SLG gene are fully fertile and support abundant pollentube germination and growth [24].
A more recent study suggests another role for SLG, at least
in plants that are homozygous for some S haplotypes. Selffertile mutants from which the SLG gene was deleted, and
which therefore produced no SLG transcripts and protein,
were found to lack SRK protein even though they accumulated normal levels of SRK transcripts [25]. In addition,
when expressed in tobacco leaves using the cauliflower
mosaic virus 35S promoter, the SRK6 protein was found to
form aberrant high molecular mass aggregates. This aggregation was prevented by the co-expression of SLG6 but
not by co-expression of S-locus related glycoprotein 1
(SLR1), which is located in the papillar cell wall but not
encoded at the S locus [25]. These results suggest that, at
least in plants homozygous for some S haplotypes, SLG
functions in the normal accumulation of SRK, possibly by
facilitating posttranslational maturation of the receptor.
This proposed function of SLG, as well as its possible role
in modulating the activity/specificity of SRK action, as
370
Cell signalling and gene regulation
Figure 2
SCR6
SCR13
SCR8
SCR9
SCR12
SCR52
MKSAIYALLCFIFLVSSHGQEVEANLKKN-- C VGKTRLP G P C GDSGASS CRD-LYNQTEKTMPVS C R C ----VPTGR C F C SL- C K
MKSAVYALLCFIFIVSGHIQEVEANLMMP-- C --GSFMF G N C RNIGARE CEK-LNSPGKRK-PSH C K C TDTQMGTYS C D C KL- C
MKSAVYALLCFIFIVSGHIQELEANLMKR-- C TRGFRKL G K C TTLEEEK C-KTLY------PRGQ C T C SDSKMNTHS C D C KS- C
MKSAIYALLCFIFIVSSHVQEVEANLRKT-- C VHRLNSG G S C GKSGQHD CEA-FYTNKTNQKAFY C N C TSPFR-TRY C D C AIK C KVR
MKSAIYALLCFIFIILSRSQELTEVGADKQQ C --KKVFP G H C ETSER-- CEN-TYKRLNKKVFD- C H C QPFGR--RL C T C K-- C
MKSVLYALLCFIFIVSSHAQDVEANLMNR-- C TRELPFP G K C GSSEDGG CIK-LYSSEKKLHPSR C E C EPRYKA-RF C R C KI- C
Current Opinion in Plant Biology
Sequence polymorphism of SCR proteins. The predicted amino-acid
sequences of SCR proteins encoded by six different S haplotypes are
aligned. The single-letter code for amino acids is used. The S6 and
S13 [29••] alleles are from B. oleracea and the S8 [29••], S9, S12, and
S52 [39•] alleles are from B. campestris. The predicted signal peptide
(underlined) is conserved among the various alleles. Only eight
cysteine residues (C; dark gray) and one glycine residue (G; light gray)
are strictly conserved in the predicted mature proteins.
suggested for ‘soluble receptors’ in animal systems [26],
are consistent with the SLG-induced enhancement of the
SI response observed in SRK transformants [19••].
Finally, an intriguing possibility is that SLG might act as
a repository for genetic variability from which new SRK
alleles might be generated via gene conversion and
unequal crossing-over; it has been suggested that similar
processes occur between related genes at plant disease
resistance loci [27,28].
studies clearly demonstrate that SCR is necessary and
sufficient for determining SI specificity in pollen.
Identification of the determinant of SI specificity
in pollen
A major breakthrough in SI research was the identification
of the long-sought-after pollen determinant of SI. This
gene, SCR (S-LOCUS CYSTEINE-RICH), is expressed
exclusively in anthers during pollen development and is
predicted to encode a small secreted and highly polymorphic cysteine-rich protein [29••]. The identification of this
gene resulted from detailed analyses of the S locus and the
construction of long-range maps of S haplotypes
(Figure 1). Recombinational analysis of the S8 haplotype
identified a 50-kb chromosomal segment in which both
pollen and stigma specificity genes must reside [30•]
(Figure 1). In parallel, physical, transcriptional, and
sequence analysis of the locus identified the presence of
several anther-expressed genes [29••,30•,31–38].
A self-fertile γ-ray-induced mutant strain that exhibited
a breakdown of SI in pollen, but not in the stigma, was
found to lack transcripts encoded by one of these antherexpressed genes, the SCR gene [29••]. A role for SCR6 in
SI was confirmed when its cDNA was placed under the
control of an SCR promoter and transformed into B. oleracea plants that were homozygous for the S2 haplotype.
Whereas pollen of untransformed S2 homozygotes is
compatible with S6 stigmas, the pollen of transgenic
plants that expressed SCR6 transcripts was rejected by S6
stigmas, and thus, was shown to have acquired S6 specificity [29••]. Subsequent analysis in progeny plants
derived by forced selfing of the primary transformants
demonstrated that this acquired S6 specificity is stably
inherited and cosegregates with the transgene
(M Wilson, JB Nasrallah, unpublished data). These
The SCR gene is expressed gametophytically in
microspores and sporophytically in the cells of the tapetum [29••,39•]. It is currently not known, however,
whether expression at both sites is physiologically relevant. The observed sporophytic control of the pollen SI
phenotype in S-locus heterozygotes may be a consequence
of the expression of the two SCR alleles from the tapetum
(as suggested by a classical model of sporophytic control [1]). Alternatively, SCR expression in haploid
microspores with subsequent mixing of their products in
the anther locule might produce an effect analogous to
sporophytic control (as has been proposed on the basis of
a study of other gametophytically expressed components
of the pollen coat [40,41]). In any case, the small
hydrophilic SCR protein is probably secreted and incorporated into the pollen coat. Such a localization would be
consistent with the speed of the SI response and with the
results of a bioassay in which a pollen-coat fraction was
reported to modify pollen SI phenotype, and thus was
inferred to contain the pollen determinant of SI [42].
Haplotypic diversity and the evolution of SI
specificities
The molecular analysis of various S haplotypes has demonstrated that the tremendous variability in SI specificities
reflects extensive molecular divergence in the organization
and sequence of S-locus genes. All of the S haplotypes that
have been mapped so far contain the SRK, SLG, and SCR
genes in close physical proximity to each other, but they can
vary significantly in overall physical size [29••,30•,33–36]
and in the relative orientation and position of their genes
(Figure 1). This evidence suggests that the S locus has a
complex evolutionary history. The structural heteromorphism of the S locus might also affect the frequency of
recombination in the region [30•,33], and possibly contribute to the maintenance of the S-locus complex in a
tightly linked genetic unit.
At the sequence level, SI-specificity genes have attained
some of the highest levels of allelic polymorphism known for
Cell–cell signaling in the self-incompatibility response Nasrallah
any locus, consistent with the expectation that S haplotypes
are less frequently rejected when rare and are subject to
diversifying selection [43]. Which domains or residues in the
molecules are likely to contribute to the unique SI specificity
of each variant is not known; however, SRK alleles can
diverge by as much as 35% in the amino-acid sequence that
they encode and, although SRK proteins exhibit relatively
conserved and highly variable regions, amino-acid substitutions occur over the length of the molecule [6,7,21•,22•].
Polymorphism in SCR alleles is extreme. As shown in
Figure 2, SCR alleles share a conserved signal sequence, but
the predicted mature proteins are highly variable, suggesting
strong positive selection for diversification. Only nine aminoacid residues are strictly conserved in all of the six alleles
shown in Figure 2, eight cysteine residues and a glycine
residue. In addition, the overall length of the predicted
mature protein varies from 50 to 59 amino acids because of a
variable number of residues between the cysteines and at the
carboxy-terminus. Indeed, in the alignment shown, a number of gaps were inserted to force alignment of the cysteines
on the assumption that the SCR variants share a conserved
pairing of cysteines and four intrachain disulfide bonds.
Specificity in the SI response would be established by the
divergence of the SRK ectodomain and the SCR protein,
which determine the distinctive molecular phenotype (i.e.
SI specificity) of the stigmatic papillar cell and pollen,
respectively. Thus, SRK and SCR must coevolve, and a
mutation that disrupts the specificity-determining
residues in one component must be answered by a complementary mutation in the other component. The
manner in which ‘matched’ allelic polymorphisms in
genes of the complex, and thus new SI specificities, can
be generated remains one of the most intriguing problems
in evolutionary biology [44].
A model of recognition and response in SI
With the pollen and stigma determinants of SI specificity
identified, a model of SI put forth several years ago upon
the isolation of the SRK gene [7] can now be refined.
Figure 3 depicts SRK as spanning the plasma membrane of
the stigma epidermal cell, and SCR as a diffusible signal
carried in the pollen coat that functions as a ligand for
SRK. Upon self-pollination, the SCR protein would be
delivered to the stigmatic surface via the pollen coat
(which is seen to flow onto the surface of the papillar cell
in electron micrographs [Figure 3]), translocate through
the cell wall, and interact with the SRK ectodomain presumably in an S-haplotype-specific manner, thus providing
the specificity inherent in self recognition.
The proposed SCR–SRK interaction would trigger a signaling cascade in the papillar cell that leads, ultimately,
to the inhibition of self-pollen tube development. The
binding of SCR to the SRK ectodomain might trigger
SRK oligomerization, transphosphorylation of the SRK
kinase domain, and the subsequent recruitment and
phosphorylation of cytoplasmic substrates, as does the
371
Figure 3
Pollen
SCR
Pollen
coat
SLG
SRK
Cell
wall
ARC1
?
?
Plasma
membrane
Stigma epidermal cell
Current Opinion in Plant Biology
A model of pollen–stigma recognition in self-incompatible plants. The
components of the model are overlaid on an electron micrograph of the
zone of contact between a stigma epidermal cell and the pollen. The
SRK receptor kinase spans the plasma membrane of the stigma
epidermal cell. SCR is predicted to be a component of the pollen coat
that is delivered to the stigma surface as this coating flows over the
papillar cell surface. Although other components of the pollen and
stigma surfaces have been discussed in relation to pollination and SI,
the figure only shows those molecules that are either encoded at the
S locus (e.g. SLG, which localizes in the wall of the stigma epidermal
cell [48]) or for which a role in SI has been demonstrated by a
transgenic approach (e.g. ARC1 [47•], which is predicted to localize to
the cytoplasm of the stigma epidermal cell). The model proposes that
SCR functions as a ligand for SRK, and that upon self-pollination the
SCR–SRK interaction triggers a signaling cascade within the stigma
epidermal cell that is poorly characterized but is likely to be complex
(arrows and question marks). This cascade would result in the
modification of the stigmatic cell surface at the site of pollen contact
and, ultimately, to the arrest of self pollen.
binding of ligands to receptor kinases in animal systems.
Alternatively, initiation of SRK signaling may involve
the binding of SCR to preformed SRK dimers and interaction with a coreceptor, as postulated on the basis of the
detection of SRK dimers in unpollinated stigmas (i.e. in
the absence of SCR) [10]. The ensuing SRK signaling
cascade has not been resolved but it is likely to be complex. Arrest of self pollen can occur at any one of several
stages, including pollen hydration, germination, or
pollen-tube ingress into the papillar cell wall [3]. Thus,
the SCR–SRK triggered signaling pathway may have
more than one molecular outcome in the papillar cell.
These might include the activation of a plasma-membrane-localized aquaporin-like protein that might result
in the localized depletion of water and other substances
required for pollen germination and tube growth from
the papillar cell wall [45], or the modification of the papillar cell wall so as to prevent its loosening and preclude
the ingrowth of the pollen tube.
372
Cell signalling and gene regulation
Molecular evidence for the complexity of the SI response
is provided by the analysis of arm-repeat-containing protein 1 (ARC1), a stigma-specific molecule that interacts
with the kinase domain of SRK in the yeast two-hybrid
system and is phosphorylated by SRK in vitro [46].
Downregulation of the ARC1 gene by expression of an
ARC1-antisense transgene resulted in appreciable seed set
upon self-pollination [47•], demonstrating that ARC1
functions in the SI response. However, the observed
breakdown of SI in these transgenic plants was only partial, suggesting that other, as yet unidentified, substrates of
SRK exist (Figure 3). Indeed, the involvement of additional components in the SRK signaling cascade is also
indicated by the occurrence of mutations at loci unlinked
to the S locus that cause the breakdown of SI despite normal expression of functional S-locus genes and ARC1 [24].
The molecular cloning of these modifier loci should be
feasible using an approach developed for the isolation of a
marker for a major modifier of SI in B. campestris [45].
Acknowledgements
Research in the author’s laboratory is supported by grants from the National
Science Foundation, the National Institutes of Health and the United
States Department of Agriculture.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
Heslop-Harrison J: Incompatibility and the pollen stigma
interaction. Annu Rev Plant Phys 1975, 26:403-425.
2.
de Nettancourt D: Incompatibility in Angiosperms. Monographs on
Theoretical and Applied Genetics 3. Berlin: Springer-Verlag; 1997.
3.
Dickinson HG: Dry stigmas, water and self-incompatibility. Sex
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4.
Bateman AJ: Self-incompatibility systems in angiosperms. III.
Cruciferae. Heredity 1955, 9:52-68.
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Ockendon DJ: Distribution of self-incompatibility alleles and
breeding structure of open-pollinated cultivars of Brussels
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6.
Nasrallah JB, Kao TH, Chen CH, Goldberg ML, Nasrallah ME: Aminoacid sequence of glycoproteins encoded by three alleles of the
S locus of Brassica oleracea. Nature 1987, 326:617-619.
7.
Stein JC, Howlett BH, Boyes DC, Nasrallah ME, Nasrallah JB:
Molecular cloning of a putative receptor protein kinase gene
encoded at the self-incompatibility locus of Brassica oleracea.
Proc Natl Acad Sci USA 1991, 88:8816-8820.
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Stein JC, Dixit R, Nasrallah ME, Nasrallah JB: SRK, the stigma-specific
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Conclusions and perspectives
Major advances have been made in the study of cell–cell
recognition in the SI system of Brassica that have led to
the identification of the stigma and pollen determinants
of SI specificity. The next phase of research will focus on
characterizing the molecular interactions between the
stigma and pollen determinants of SI as well as identifying components of the SRK-mediated signaling cascade.
To clarify the proposed interaction of SRK and SCR, it
will be important to determine if SCR is a ligand for SRK,
if this interaction occurs in a S-haplotype-specific manner, if it involves other non-S-haplotype-specific
molecules, and which of the residues in each protein are
involved in the interaction and contribute to the unique
SI specificity of each variant.
In these studies, it is likely that the production of SRK and
SCR in heterologous expression systems will be important,
if only because they might suggest hypotheses that will
then be tested in planta. The SCR protein has been produced in bacteria in an apparently functional form [39•]: in
a modification of a previously developed bioassay for
pollen-coat proteins [42], the recombinant SCR protein
was found to effect the inhibition of non-self pollen when
applied to papillar cells of the appropriate genotype [39•].
If this bioassay proves to be a reliable measure of SCR
activity, it would greatly facilitate analysis of this protein.
The SRK protein has been expressed in insect cells [9,10]
where it accumulates to high levels, is correctly modified
(at least in part), and localizes to the plasma membrane
where it assumes the expected topology [9]. Ideally, a
robust assay for the specific activation of SRK signaling
by addition of SCR is desired. Such an assay would expedite the analysis of structure–function relationships for
SCR and SRK, a prerequisite for understanding the
mechanism of signaling and response in cell–cell recognition and for tackling the challenging questions relating to
the concerted evolution of receptor–ligand pairs.
10. Giranton JL, Dumas C, Cock JM, Gaude T: The integral membrane
S-locus receptor kinase of Brassica has serine/threonine kinase
activity in a membranous environment and spontaneously forms
oligomers in planta. Proc Natl Acad Sci USA 2000, 97:3759-3764.
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12. Nasrallah ME, Kandasamy MK, Nasrallah JB: A genetically defined
trans-acting locus regulates S locus function in Brassica. Plant J
1992, 2:497-506.
13. Nasrallah JB, Rundle SJ, Nasrallah ME: Genetic evidence for the
requirement of the Brassica S-locus receptor kinase gene in the
self-incompatibility response. Plant J 1994, 5:373-384.
14. Toriyama K, Stein JC, Nasrallah ME, Nasrallah JB: Transformation of
Brassica oleracea with an S locus gene from B. campestris
changes the self-incompatibility phenotype. Theor App Genet
1991, 81:769-776.
15. Goring DR, Glavin TL, Schafer U, Rothstein SJ: An S receptor kinase
gene in self-compatible Brassica napus has a 1-bp deletion. Plant
Cell 1993, 5:531-539.
16. Stahl RJ, Arnoldo MA, Glavin TL, Goring DR, Rothstein SJ: The selfincompatibility phenotype in Brassica is altered by the transformation
of a mutant S locus receptor kinase. Plant Cell 1998, 10:209-218.
17.
Shiba H, Hinata K, Suzuki A, Isogai A: Breakdown of selfincompatibility in Brassica by the antisense RNA of the SLG gene.
Proc Jpn Acad Ser B 1995, 71:81-83.
18. Conner JA, Tantikanjana T, Stein JC, Kandasamy MK, Nasrallah JB,
Nasrallah ME: Transgene-induced silencing of S -locus genes and
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19. Takasaki T, Hatakeyama K, Suzuki G, Watanabe M, Isogai A, Hinata K:
•• The S receptor kinase determines self-incompatibility in Brassica
stigma. Nature 2000, 403:913-916.
The authors show that transformations of plants that are homozygous for an
S haplotype with an SRK allele isolated from another S haplotype results in
Cell–cell signaling in the self-incompatibility response Nasrallah
the acquisition of the introduced SI specificity in the stigma, thus demonstrating that SRK is the determinant of SI specificity in the stigma. The
authors also show that co-expression of SLG enhances the strength of the
SRK-mediated SI reaction.
20. Cui Y, Bi Y-M, Brugiere N, Arnoldo MA, Rothstein SJ: The S locus
•
glycoprotein and the S receptor kinase are sufficient for selfpollen rejection in Brassica. Proc Natl Acad Sci USA 2000,
97:3713-3717.
The modification of SI specificity in stigmas of B. napus plants transformed
with a 40-kb DNA fragment containing the SLG and SRK genes is
described. The success of this experiment predicts that it should be feasible
to transfer the entire S-locus complex, and thus SI specificity, in a single step
using a transgenic approach.
21. Kusaba M, Nishio T: Comparative analysis of S haplotypes with
•
very similar SLG alleles in Brassica rapa and Brassica oleracea.
Plant J 1999, 17:83-91.
The authors report on Brassica strains with different SI specificities that have
similar SLG genes and more diverged SRK genes. This paper supports the
role of SRK and not SLG as the primary determinant of SI specificity.
22. Kusaba M, Matsushita M, Okazaki K, Satta Y, Nishio T: Sequence
•
and structural diversity of the S locus genes from different lines
with the same self-recognition specificities in Brassica oleracea.
Genetics 2000, 154:413-420.
This paper describes two instances in which strains with the same SI specificity exhibit more sequence conservation in their SRK genes than in their
SLG genes. These results provide further support for the role of SRK as the
primary determinant of SI specificity.
23. Luu D-T, Marty-Mazars D, Trick M, Dumas C, Heizmann P:
Pollen–stigma adhesion in Brassica spp involves SLG and SLR1
glycoproteins. Plant Cell 1999, 11:251-262.
24. Nasrallah ME, Kandasamy MK, Chang M-C, Stadler Z, Lim S,
Nasrallah JB: Identifying genes for pollen–stigma recognition in
crucifers. Annals Botany 2000, 85(suppl A):125-132.
25. Dixit R, Nasrallah ME, Nasrallah JB: Post-transcriptional maturation
of the S receptor kinase of Brassica correlates with co-expression
of the S locus glycoprotein in Brassica stigmas and transgenic
tobacco plants. Plant Physiol 2000, in press.
26. Mohamed-Ali V, Goodrick S, Bulmer K, Holly JMP, Yudkin JS,
Coppack SW: Production of soluble tumor necrosis factor
receptors by human subcutaneous adipose tissue in vivo. Am J
Physiol — Endocrinol Metab 1999, 277:E971-E975.
27.
Kusaba M, Nishio T, Satta Y, Hinata K, Ockendon D: Striking
sequence similarity in inter- and intra-specific comparisons of
class I SLG alleles from Brassica oleracea and Brassica
campestris: implications for the evolution and recognition
mechanisms. Proc Natl Acad Sci USA 1997, 94:7673-7678.
28. Meyers BC, Chin DB, Shen KA, Sivaramakrishnan S, Lavelle DO,
Zhang Z, Michelmore RW: The major resistance gene cluster in
lettuce is highly duplicated and spans several megabases. Plant
Cell 1998, 10:1817-1832.
29. Schopfer CR, Nasrallah ME, Nasrallah JB: The male determinant of
•• self-incompatibility in Brassica. Science 1999, 286:1697-1700.
The authors report the identification of the pollen determinant of SI, the
S-LOCUS CYSTEINE-RICH (SCR) gene. The sequence of three SCR
alleles reveal extensive polymorphism for this gene. SCR is shown to be
expressed in anthers, and more specifically in microspores. The SCR
gene encodes a small protein that is predicted to be located in the pollen
coat. SCR is necessary and sufficient for determining SI specificity in
pollen, as demonstrated by a loss-of-function mutation that leads to the
loss of SI specificity in pollen, and by a gain-of-function transgenic experiment in which transformation with one SCR allele results in acquisition of
the corresponding SI specificity by pollen.
30. Casselman AL, Vrebalov J, Conner JA, Singhal A, Giovannoni J,
•
Nasrallah ME, Nasrallah JB: Determining the physical limits of the
Brassica S locus by recombinational analysis. Plant Cell 2000,
12:23-34.
A recombinational analysis of the S locus was carried out using a large F2
population segregating for two S haplotypes. A number of recombination
breakpoints were identified that defined an approximately 50-kb segment
spanning the SLG and SRK genes that contains both stigma- and pollenspecificity genes. No evidence for suppression of recombination across a
1000-kb region spanning the S locus proper and its flank was found with the
particular combination of S haplotypes analyzed in this study.
31. Boyes DC, Nasrallah JB: Physical linkage of the SLG and SRK
genes at the self-incompatibility locus of Brassica olearacea. Mol
Gen Genet 1993, 236:369-373.
373
32. Yu K, Schafer U, Glavin TL, Goring DR, Rothstein SJ: Molecular
characterization of the S locus in two self-incompatible Brassica
napus lines. Plant Cell 1996, 8:2369-2380.
33. Boyes DC, Nasrallah ME, Vrebalov J, Nasrallah JB: The selfincompatibility (S) haplotypes of Brassica contain highly
divergent and rearranged sequences of ancient origin. Plant Cell
1997, 9:1-12.
34. Conner JA, Conner P, Nasrallah ME, Nasrallah JB: Comparative
mapping of the Brassica S locus region and its homeolog in
Arabidopsis: implications for the evolution of mating systems in
the Brassicaceae. Plant Cell 1998, 10:801-812.
35. Suzuki G, Kai N, Hirose T, Fukui T, Nishio T, Takayama S, Isogai A,
Watanabe M, Hinata K: Genomic organization of the S locus:
identification and characterization of genes in SLG/SRK region of
S9 haplotype of Brassica campestris (syn rapa). Genetics 1999,
153:391-400.
36. Cui Y, Brugiere N, Jackman LM, Bi YM, Rothstein SJ: A structural and
transcriptional comparative analysis of the S locus regions in two
self-incompatible Brassica napus lines. Plant Cell 1999,
11:2217-2231.
37.
Boyes DC, Nasrallah JB: An anther-specific gene encoded by an
S locus haplotype of Brassica produces complementary and
differentially regulated transcripts. Plant Cell 1995, 7:1283-1294.
38. Casselman AL, Nasrallah ME, Nasrallah JB: Using S-locus deletions
to evaluate self-incompatibility candidate genes: an example for a
novel anther-expressed gene. Sex Plant Reprod 2000, 12:227-231.
39. Takayama S, Shiba H, Iwano M, Shimisato H, Che F-S, Kai N,
•
Watanabe M, Suzuki G, Hinata K, Isogai A: The pollen determinant
of self-incompatibility in Brassica campestris. Proc Natl Acad Sci
USA 2000, 97:1920-1925.
The authors report the production of SCR in bacteria in an apparently
functional form. When applied to papillar cells, the bacterially expressed
protein results in an S-haplotype-specific inhibition of pollen. This bioassay, if it proves to be reproducible, should facilitate future analysis of the
SCR protein. The paper also reports the sequences of three new SCR
alleles and describes the detection of SCR transcripts in microspores and
in the anther tapetum.
40. Doughty J, Dixon S, Hiscock SJ, Willis AC, Parkin IAP, Dickinson HG:
PCP-A1, a defensin-like Brassica pollen coat protein that binds
the S locus glycoprotein, is the product of gametophytic gene
expression. Plant Cell 1998, 10:1333-1347.
41. Doughty J, Wong HY, Dickinson HG: Cysteine-rich pollen coat
proteins (PCPs) and their interactions with stigmatic S
(incompatibility) and S-related proteins in Brassica: putative
roles in SI and pollination. Annals Botany 2000,
85(suppl A):161-169.
42. Stephenson AG, Doughty J, Dixon S, Elleman C, Hiscock S,
Dickinson HG: The male determinant of self-incompatibility in
Brassica oleracea is located in the pollen coating. Plant J 1997,
12:1351-1359.
43. Richman AD, Kohn JR: Learning from rejection: the evolutionary
biology of single-locus incompatibility. Trends Ecol Evol 1996,
11:497-502.
44. Charlesworth D: How can two-gene models of self-incompatibility
generate new specificities? Plant Cell 2000, 12:309-310.
45. Ikeda S, Nasrallah JB, Dixit R, Preiss S, Nasrallah ME: An aquaporinlike gene in the Brassica self-incompatibility response. Science
1997, 276:1564-1566.
46. Gu T, Mazzurco M, Sulaman W, Matias DD, Goring D: Binding of an
arm repeat protein to the kinase domain of the S-locus receptor
kinase. Proc Natl Acad Sci USA 1998, 95:382-387.
47.
•
Stone SL, Arnoldo MA, Goring DR: A breakdown of Brassica selfincompatibility in ARC1 antisense transgenic plants. Science
1999, 286:1729-1731.
This paper investigates the function of ARC1, an arm-repeat containing protein that interacts with, and is phosphorylated by, the SRK kinase domain. A
partial breakdown of SI is associated with the downregulation of the ARC1
gene in transgenic plants, demonstrating a role for the ARC1 protein in the
SRK signaling cascade.
48. Kandasamy MK, Paolillo D, Faraday C, Nasrallah JB, Nasrallah ME:
The S locus specific glycoproteins of Brassica accumulate in the
cell wall of developing stigma papillae. Dev Biol 1989,
134:462-472.