The Molecular and Genetic Bases of S-RNase-Based Self-Incompatibility

The majority of flowering plants produce perfect flowers that contain both the male and female reproductive organs in close proximity; consequently, they would have a strong tendency to self-fertilize if there were no mechanisms to prevent them from doing so. Because inbreeding can result in reduced fitness in the progeny, hermaphroditic plants have adopted a variety of reproductive strategies, including self-incompatibility (SI), by which inbreeding is prevented and outcrosses are promoted. SI allows the pistil of a flower to distinguish between genetically related (self) and unrelated (non-self) pollen. This self/non-self recognition results in the inhibition of germination of self-pollen on the stigmatic surface or the inhibition of growth of self-pollen tubes in the style. Thus, SI is a prezygotic reproductive barrier by which incompatible pollen/pollen tubes are prevented from delivering the sperm cells to the ovary to effect double fertilization. SI can be classified into homomorphic and heteromorphic types based on whether it is associated with floral polymorphism. In species that exhibit homomorphic SI, all individuals produce the same type of flower and the outcome of pollination depends only on the genetic identity of the male and female partners. In contrast, species that exhibit heteromorphic SI produce two or three different flower morphologies (e.g., a flower with short anthers and long style or a flower with long anthers and short style). For successful pollination, pollen must come from genetically unrelated individuals whose anthers are of the same height as the style of the flower being pollinated. To date, much of what we know about the molecular basis of SI has been deduced from studies of homomorphic SI, which will be the focus of this review. A monograph by de Nettancourt (2001)provides a comprehensive treatise on SI, including a discussion of the heteromorphic type. For homomorphic SI (hereafter referred to as SI), self/non-self discrimination between pollen and pistil is determined by one or more polymorphic loci, and this type of SI is further classified into gametophytic and sporophytic types based on the genetic control of pollen behavior. To date, four of the families that exhibit gametophytic SI (GSI), Solanaceae, Rosaceae, Scrophulariaceae, and Papaveraceae, and one of the families that exhibit sporophytic SI (SSI), Brassicaceae, have been studied extensively at the molecular level (Table 1 Summary of Three Types of SI Mechanisms Summary of Three Types of SI Mechanisms For the four GSI families, SI occurs when the S-haplotype of the pollen matches either of the two S-haplotypes carried by the pistil. That is, the SI phenotype of the pollen (gametophyte) is determined by its own S-genotype. For the SSI family, in the simplest case, SI occurs when the pollen-producing parent shares one or both S-haplotypes with the pistil. That is, the SI phenotype of the pollen is determined by the S-genotype of its diploid parent. For SSI, complex relationships often exist between the different S-haplotypes of the pollen and pistil parents. One S-haplotype could be dominant over or recessive to another, or it could interact with another to result in mutual weakening or in an entirely new S-haplotype specificity (Thompson and Taylor, 1966). During the past two decades, much progress has been made in identifying and characterizing the S-locus genes that control the specificity of the SI interaction in the five families mentioned above. Comparisons of the S-locus genes expressed in the pistil among the different families have revealed three biochemically distinct mechanisms (Table 1). The Solanaceae, Rosaceae, and Scrophulariaceae use the same mechanism, the Papaveraceae uses another, and the Brassicaceae uses a third. For the Solanaceae and Papaveraceae mechanisms, the gene that controls female specificity has been identified; these genes were named the S-RNase gene and the S-gene, respectively. Our understanding of the Solanaceae mechanism has progressed further, with the recent identification of a promising candidate for the male specificity gene. The Solanaceae mechanism involves S-RNase–mediated degradation of RNA in self-pollen tubes. The Papaveraceae mechanism is mediated by a signal transduction cascade in pollen that involves a number of known components of signal transduction (e.g., Ca2+, phosphoinositides, protein kinases, and phosphatases). For the SSI mechanism found in the Brassicaceae, both the gene that controls male specificity, S-locus cysteine-rich protein (SCR)/S-locus protein-11 (SP11), and the gene that controls female specificity, S-locus receptor kinase (SRK), have been identified. The SI response is mediated via a signal transduction cascade in the stigmatic papilla, which is elicited by the interaction of a pollen-borne ligand, SCR/SP11, and SRK, a receptor kinase in the stigmatic papilla. The discussion below focuses on the Solanaceae type of SI. For a recent review of the Brassicaceae type of SI, see Kachroo et al. (2002); for a recent review of the Papaveraceae type of SI, see Thomas et al. (2003). The Solanaceae type of SI was first discovered in Nicotiana sanderae (East and Mangelsdorf, 1925), and to date, this type of SI has been studied at the molecular level in four genera of the Solanaceae (Lycopersicon, Nicotiana, Petunia, and Solanum), three genera of the Rosaceae (Malus, Prunus, and Pyrus), and one genus of the Scrophulariaceae (Antirrhinum). The rejection of self-pollen occurs during pollen tube growth in the style, and the timing of the rejection coincides with the transition of pollen tube growth from the slow (“autotrophic”) growth phase to the accelerated (“heterotropic”) growth phase (Herrero and Hormaza, 1996). The increase in the growth rate is presumed to result from the increased acquisition of nutrients provided by the pistil tissue. Interestingly, several critical cellular events also occur around the time of this transition (e.g., mitotic division of the generative cell to give rise to two sperm nuclei). The search for the female determinant of SI was based on the prediction that the gene encoding it must exhibit allele-specific sequence differences and must be expressed in the pistil. Pistil-specific proteins that showed allele-specific differences in molecular mass and/or isoelectric point were first identified in Nicotiana alata (Bredemeijer and Blass, 1981), and the first sequence of such a protein was deduced from the cloning and sequencing of the corresponding cDNA (Anderson et al., 1986). These proteins were initially named S-allele–associated proteins or S-proteins, and the gene was named the pistil S-gene. Similar approaches were used to identify S-proteins and to isolate their cDNAs from other solanaceous species (Ai et al., 1990; Clark et al., 1990) and several rosaceous species (Sassa et al., 1993; Ishimizu et al., 1996). Sequence comparisons of solanaceous S-proteins have revealed five conserved and two et al., et al., The of distinct conserved has made it to and cDNA for S-proteins by This is for and studies of SI et al., et al., and to plants et al., For rosaceous the to identify by this is it the and pollination this et al. cDNAs for the of S-proteins in and to that this species the same type of SI mechanism as the solanaceous and rosaceous of S-proteins has that the mechanisms used by the species in these three related families a and that this mechanism be by the of of all and it is known whether the Solanaceae mechanism is by other This can be by to whether of S-proteins are in other The of S-proteins was revealed when the sequence of of et al., was determined and to sequence with S-proteins et al., This to the that S-proteins have in et al., et al., two of of which a are known to be in all S-proteins and to to two of the five conserved identified by et al. Thus, S-proteins have been and the gene has been the S-RNase gene. to have specificity in et al., and have been in a of named the family, which also and et al., exhibit sequence and they have been identified from both and species of the Solanaceae, Rosaceae, and Scrophulariaceae as as from species of several other have been identified from both and species of the Solanaceae, Rosaceae, and Scrophulariaceae, and they are more to to are to the pistil. exhibit sequence polymorphism. S-RNase genes could have been from the S-RNase gene as a result of gene by to other their sequence to and are to a in SI, and to date, the of of them The of the S-RNase gene in the SI interaction has been via et al., et al., These showed that of a new of the S-RNase gene into plants was to on the plants the to pollen the same as the S-RNase gene. of the of an of the S-RNase gene by the RNA the of the plants to the pollen the To the mechanism of S-RNase–mediated it is to know whether the of is an of their was used to the for one of the two of of with an and plants that to pollen et al., with this result is the that a of a S-RNase with one of the with et al., et al., Because the of is for pollen it is to that the degradation of RNA by self-pollen tubes results in growth et al. results with this showed that pollen tube was the of the the that the degradation of RNA is a the of growth inhibition of self-pollen tubes. have been made to identify the of that specificity that are in the interaction with the pollen Because are that in the number and of the determinant could a in the and/or the protein To this a of the for the only with was in and the SI of the pistil was The was found to have to that of the and to as as the in pollen et al., Thus, the determinant of in their protein One of is their of sequence For the two solanaceous only sequence identity et al., are a number of the the are in two named and These two were identified initially from of solanaceous et al., et al., were found to be the of as et al., and to to two of the four of rosaceous for which of has been found et al., The of a solanaceous S-RNase and a rosaceous S-RNase that both and are on the surface of the protein and to et al., et al., and are the for the determinant of have been to the if of and other of in specificity and et al., et al., For S-RNase the of the sequence was from one of the S-RNase with the sequence of the to be by another plants that produce S-RNase were for their to pollen of the two used in the of with a of sequence (e.g., identity between and of were used in the of these could on the protein its the specificity of the used as the of the S-RNase gene was and et al., Because all of these their to self-pollen was to the of the recognition two of and that sequence identity only were used for the of S-RNase it was found that the and were to on the the new specificity et al., That is, when the of and of were to of plants that this S-RNase pollen pollen. be that can only the of that between the two Thus, the results of et al. the of of and that are conserved between and et al., The that two and have sequence in and by only two in et al., that of these two are in the of genetic studies showed that the pollen and pistil in SI could to result in either or This that genes control these two In all of the to the of the S-RNase gene the pistil the pollen was by the of the S-RNase with the that the S-RNase gene control male et al. that the gene was in a of and that this the pistil the pollen During the past a of have been the identification of the gene that controls male specificity, the pollen S-gene. One to identifying the pollen is to search for genes that exhibit the pollen is to a of sequence A number of such genes have been identified in alata and by RNA and et al., et al., et al., has been to if of these genes is to the S-RNase gene. Because at the S-locus is as a result of its et al., a number of plants for S-haplotypes are to the to the S-RNase gene. of the genes of et al., and one of the genes of alata et al., were found to be to the S-RNase gene. The of different of of these genes were to the sequence The deduced of all of these genes exhibit sequence of the sequence of of alata has revealed no of which is of the S-locus genes in SI et al., In the of the has that the genes that are to the S-RNase gene are at or at from the S-RNase gene and could be as as and Thus, of the genes identified by this is to be the pollen S-gene, and their sequence result from their genetic to the polymorphic to identifying the pollen is based on the prediction that the pollen must be to the S-RNase gene. in the between these two genes would result in the of SI by different S-haplotype for pollen and such have been et al. a the gene of and identified and only 1 of the is expressed in the and This of the an protein and was named S-locus cDNA encoding a of was from a of and its deduced sequence is to that of it is from that whether and its named are et al. et al. to identify the pollen by sequencing the of and respectively. of that the gene is the of the S-locus et al., based on the showed that the sequence of this is between different the this are between different a with a this was in both pollen and pistil of this revealed in to the gene. with the of the two of the other are expressed in and both One of the named a level of sequence as the S-RNase gene. The sequence of and from to and of the same four of the S-RNase gene from to et al., For the other named S-locus the deduced of and are is a candidate for the pollen S-gene, it is to the S-RNase of the S-RNase gene in the four S-haplotypes it is expressed in and it a level of sequence contrast, its to the S-RNase gene and its pollen is to be the pollen of the of sequence be that was named to that it the same level of sequence as identified in et al., to that is an of In both and are to in their deduced et al. four genes in a of the S-locus of that the three of them also are in a of the S-locus the gene. The gene to the S-RNase gene was named it is expressed in pollen and a level of sequence The sequence of the and of from to Thus, of is the of of of in another rosaceous have been by and their deduced are et al., will be used to the gene that is the candidate for the pollen S-gene. The other three genes identified in were named and all of of much of sequence For the deduced of the and of are of of of Three The were by The two and were identified as in the The are with For the is on a in The of the proteins are by the of the species by the protein or and the identity of the of the first and in of sequence are at and respectively. proteins are in protein This uses and to the of on for degradation by the et al., 1996). protein is a of one of which also of and a protein and The with and a with The contain other interaction or et al., The contain interaction the two could be in The sequence of these are in Sequence of of Sequence of of Our has also identified a gene in close to the gene of and two genes that are at much to the S-RNase gene and For sequencing of a more and of S-locus of three other has revealed that this species also including at the S-locus et al., to the of sequence of the rosaceous the of four and of sequence by et al. has been to be the of from this Thus, all three families that use the SI mechanism have genes that are to the S-RNase gene. the only genes for that in close of the S-RNase gene that are to all three of these families are the is that these are the of in SI must be by in as has been for the S-RNase gene. The specificity of the SI interaction is determined by the S-RNase gene and the pollen and molecular genetic studies have revealed the of genes at other that are for the of the SI For to SI to species by of the S-locus from their have been successful et al., the S-RNase gene into species to on them the to self-pollen et al., et al., et al., et al., plants with in pistil were found to a S-RNase gene when into a (Ai et al., These which are of the S-locus are for the SI are One of the genes is to the of the S-locus For et al. found that several plants identified from a of carried a which produce of the gene. further showed that or for the were from of the plants that carried the Thus, the gene was and its was by a in the et al. that the gene could either a of the of the gene or an for the of an of the of the gene. Interestingly, the gene the of the or gene carried by these The identity of the gene is One that has been used to identify the genes is to search for or genes that are expressed in a species in related et al. a named that is expressed in the pistil of alata in that of of also have been identified in two other genera of the Solanaceae, and et al., et al., of the of by RNA and/or RNA to the of rejection of pollen in both alata and et al., et al., a of was found to a in to S-RNase et al., a protein that a of and the et al., have into the of Because the or protein level of the S-RNase gene was in the and RNA is for the of the S-RNase gene. interaction between and has been A is that in with other pistil is for the of into pollen tubes et al., is the first gene of GSI to be and the of its in SI will to a understanding of the mechanism of SI. to identifying the genes is to isolate pistil and pollen proteins that interact with A pollen protein of that with the of the and has been identified by the interaction and This named is a protein and a at its proteins also are in protein a in SI, it is more to be a one and as the is expressed in other it sequence and the interaction between its protein and is has been used to identify pistil proteins that interact with of alata et al., et al., Interestingly, three of the five proteins identified to are pistil that have been in pollen tube A first identified in has been to be into the of pollen tubes et al., 1996). first identified in has been to be associated with the pollen tube et al., 1993; et al., first identified in has been to be associated with the and of pollen tubes et al., Because these proteins are in the of the and their with are in they are to a in the of into pollen tubes et al., In to these three an protein with sequence to a of proteins as as S-RNase also were found to interact with is what the protein is it known whether or in be that of these proteins that interact with the S-RNase is to SI, they are to be for other as Thus, in their genes could be and from genetic of Because the of is for their in SI, it is that the degradation of pollen tube by the S-RNase results in the growth inhibition of self-pollen tubes in the style. different their have been to the degradation of self-pollen tube RNA (Thompson and and 1996). The receptor that the specificity in the of into a pollen the would be into the of a pollen Thus, this that the of pollen are or cell that as that only the (self) S-RNase is to a pollen The in its simplest that the of pollen are with the of all that of the One to the of these two is to if the of into a pollen tube is S-haplotype as by the receptor or if both and are by a pollen as by the et al. to of in that been with incompatible or pollen. found that was in the of both self-pollen tubes and pollen tubes of and This the and that if the of by pollen tubes a receptor a receptor it of for of and and are by an pollen only is in pollen RNA the pollen the of of to the pollen their of the of the pollen to the of between and the pollen is the of the and the of the This from the in several pollen are and they only contain the and a is for the inhibition of the of The inhibition of the of is in a to that for interaction based on the and which the of either or a the of both and are by the In all of these pistil proteins that are for the SI response are in when a pollen tube of is in a pistil of the pollen with and In the of the of the pollen would interact with the of by of the between the the of would be in such an in the of the of the pollen would interact with the of in the of the between their such an interaction would the of This is on the that the interaction between the of a pollen and its S-RNase is the interaction between the of the pollen and the of its to one of the S-RNase to the other Thus, of a pollen to the the S-RNase into its This can a which to a of pollen in SI by the of two of different in the pollen. This occurs when the or a critical of is in diploid plants that two different S-haplotypes or when diploid SI plants two different S-locus can be by with the as a or as a the pollen by such only whose S-locus is of a different S-haplotype from that of the S-locus to in SI. For if an an the only pollen that will be by the pistil are pollen that the et al. that in the the S-RNase gene that the with the that the pollen two different pollen be referred to as pollen et al., The that when two different pollen are expressed in the same pollen their the of all in the of SI. A was by et al. the SI of a S-RNase they et al., This named is a protein between and of that both and that produce the both and pollen. Interestingly, of plants that produce the were incompatible with pollen from plants of that the diploid pollen of by the was by the et al. showed that diploid pollen of was with and as would be from Thus, the from its two corresponding in its to and diploid pollen two different This also that both pollen are expressed in diploid pollen and the that the of SI by interaction is to of the of the two pollen The that the of pollen is a the pollen contain only the and a is for the inhibition of to this the would and all an S-RNase were to its pollen their a pollen two different of the pollen S-gene, the would which could either as a the would the of both In this also that the could other Thus, pollen tubes two different pollen would be with of S-genotype. In the of the it could the by the two pollen of the to the S-RNase would be This would the SI of the the could interact with its and that only are by the This would be with the that it is the and the of that is by the SI The degradation could be if the interaction between the of an and its S-RNase would prevent of the S-RNase and the of interaction between the of an and would result in of the of a to the of in SI. of an pistil with pollen from an that an The pollination will be if is the pollen S-gene. the pollen by the pollen the will be with the pistil of and pollen the will be by the pistil. will be in the progeny, and SI of the with the of be with an pistil pollen the will be by the pistil as a result of SI of the with the of For the same in such be with an pistil. pollen that the is to be the pollen S-gene, it would be of to what effect the of its has on SI behavior. both the and the as by the the of this protein would the pollen to For if the of were into of the pollen would the and the other would The be incompatible with of the in SI. Thus, if pollen from the plants were used to and only the plants would and of the would the the pollen in with plants that produce if only the as by the of the of would the pollen to the of the on a the pollen would be with of in the both and plants would with all of the the and of the the the of the S-RNase gene two much of what we have about the Solanaceae type of SI is to this female determinant of the SI The recent identification of the gene will the of in this type of SI. The in the short is to by in whether the determinant of SI. is to be the pollen S-gene, this will new of and to an understanding of the mechanism of inhibition of pollen tube can be whether as a protein in the degradation of all or whether it in the genes that both the male and female in we also could one of the about type of SI the male and female specificity genes to The that genes are to the S-RNase gene in all three families that exhibit the SI also about the of the genes that are to the S-locus and about the relationships among the it is to focus on S-haplotype specificity is we also in that proteins are for the of the SI Because of the candidate proteins identified to to be to the SI understanding they in SI will have for other The for the in and for in the of and for and and for in was by from the