Stamen Structure and Function

Stamens are the male reproductive organs of flowering plants. They consist of an anther, the site of pollen development, and in most species a stalk-like filament, which transmits water and nutrients to the anther and positions it to aid pollen dispersal. Within the anther, male sporogenous cells differentiate and undergo meiosis to produce microspores, which give rise to pollen grains, whereas other cell types contribute to pollen maturation, protection, or release. Pollen development involves an array of extraordinary events, including cell division and differentiation independent of a conventional meristem, the transition from sporophytic to gametophytic generation, and modifications of cell division to produce structures that are unusual in plant development, including coenocytic tissues (the tapetum and the microsporocyte mass), and subsequently free cells (microspores) that give rise to self-contained units for genome dispersal (pollen grains). Reviews of stamen development and gene expression include articles by Bhandari (1984), Scott et al. (1991a), Goldberg et al. (1993), and Irish (1999). A change in emphasis will be apparent since anther development was reviewed in the first Plant Reproduction special issue of The Plant Cell (Goldberg et al., 1993). The earlier review drew largely on studies of tobacco, but this article focuses more tightly on Arabidopsis, which has surpassed other species in the genetic tools publically available, including a sequenced genome, microarrays, well-characterized mutants associated with cloned genes, and knockout collections for reverse genetics. The advances of the last 10 years include an increased understanding of stamen specification, stamen-specific gene expression, internal patterning of the anther, regulation of male meiosis, and anther dehiscence. Stamen and pollen development are intimately linked. We discuss those aspects of pollen genesis that are essentially controlled by the sporophytic genome (differentiation of the male germline, meiosis, and pollen wall formation), but the biology of the male gametophyte, pollen tube growth, and fertilization are covered by other reviews in this volume (Edlund et al., 2004; Hanson and Bentolila, 2004; McCormick, 2004). Anther Development. (A) Scheme of a transverse section through an Arabidopsis floral bud showing the number, position, and orientation of the floral organs (after Hill and Lord, 1989). (B) Schemes of transverse sections through Arabidopsis anthers at different stages (after Sanders et al., 1999). Floral stages are as described by Smyth et al. (1990); anther stages are as described by Sanders et al. (1999). C, connective; E, epidermis; En, endothecium; ML, middle layer; S, septum; St, stomium; StR, stomium region; T, tapetum; Td, tetrads; TPG, tricellular pollen grains; V, vascular bundle. There are many recent reviews of floral organ specification (Jack, 2001, 2004; Theissen, 2001; Lohmann and Weigel, 2002); therefore, the regulation of stamen identity will be treated only briefly here. The identity of floral organ primordia is controlled by three classes (termed A, B, and C) of homeotic selector genes with overlapping areas of activity; stamens develop in the third whorl, where both B and C genes are expressed. It has been discovered that additional transcription factors, encoded by SEPALLATA (SEP) genes in Arabidopsis, are required to confer full activity on the homeotic genes in stamens (as well as petals and carpels). The ABC model was based on genetic studies in Arabidopsis thaliana and Antirrhinum majus (Coen and Meyerowitz, 1991). The B class genes are APETALA3 (AP3) and PISTILLATA (PI) in Arabidopsis and DEFICIENS (DEF) and GLOBOSA (GLO) in Antirrhinum; the C genes are AGAMOUS (AG) and PLENA, respectively. Mutations in any one of the B or C genes result in homeotic conversion of the third-whorl organs to a different type. Loss of B function causes transformation of stamens to carpels, loss of C function converts stamens to petals, and loss of both transforms stamens to sepals. Single-gene mutations of the SEP genes have very subtle phenotypes, but in sep1 sep2 sep3 triple mutants, all floral organs resemble sepals, suggesting that B and C function have been abolished (Pelaz et al., 2000). The signals that determine the number of floral organ primordia in each whorl are unknown (Irish, 1999). However, the number of stamens can be perturbed by various mechanisms: extra stamens develop in clavata1 mutants, which form enlarged meristems (Clark et al., 1993), and in superman mutants, which have an expanded BC domain (Bowman et al., 1992). By contrast, arrest of stamen primordia is a feature of normal development in some species. In the radially asymmetric flowers of Antirrhinum, five stamens initiate, but the dorsal stamen arrests early in development in response to the activity of the CYCLOIDEA gene (Luo et al., 1995). In some species that produce unisexual flowers, such as maize and white campion, stamens in female flowers initiate but later arrest or abort (Tanurdzik and Banks, 2004). The B class, C class, and SEP proteins all belong to the MADS family of transcription factors, which bind a target DNA sequence (the CArG box) as homodimers or heterodimers. In vitro, the B class proteins DEF and GLO (Antirrhinum) or AP3 and PI (Arabidopsis) bind CArG box sequences only as heterodimers (Jack, 2001; Thiessen, 2001). Yeast two-hybrid assays show that PI/AP3 interacts directly with SEP3 but not AG, and it has been proposed that SEP3 mediates interactions between PI/AP3 and AG dimers so that PI/AP3, AG, and SEP bind DNA in quaternary complexes; this effect would explain the combinatorial action of B and C class genes (along with SEP genes) in stamens (Honma and Goto, 2001). Ten years ago, one of the major unanswered questions about stamen development was the nature of the genes downstream of the B and C class transcription factors (Goldberg et al., 1993). After stamen specification, the B and C class genes as well as SEP genes continue to be expressed during stamen development (Bowman et al., 1991; Pelaz et al., 2000; Jack, 2001), so they could be directly responsible for activating many of the genes involved in stamen morphogenesis and function. Among the targets of B class genes in Arabidopsis are AP3 and PI themselves, because both genes are required for the continued expression of each in the developing flower (Lohmann and Weigel, 2002). Two main approaches have been used to identify additional targets of B and C class genes. These involve either transcriptional profiling methods, including subtractive hybridization, differential display of RNAs, and differential screening of cDNA or to genes with stamen-specific or expression, or screening for mutations that approaches have but is not to the that with homeotic gene Stamen expression studies have been in many and model including et al., et al., et al., Antirrhinum et al., et al., and maize et al., 1993), et al., et al., white et al., et al., and Arabidopsis et al., and Meyerowitz, et al., and of studies have B C class genes to identify are in more has been used to and identify proteins from anthers at different stages et al., The expression studies have genes involved in and cell wall and and These are with of stamen pollen development, such as growth, water associated with and and the of genes of unknown function of the genes have or to this include transcription factors in white and Arabidopsis et al., and and a of the of in to be in anthers at the suggesting a in signals to flowering with anther et al., In that genes in the expression studies are to a of stamen and cell including microspores, and A is the of in early the activity of this et al., In some of the to transcriptional profiling used in with or with function or expression of B and C class genes to aid the of differential of an cDNA was used to expressed in flowers but not mutants of Antirrhinum, which B function. expressed genes, including expressed in the tapetum and a and expressed in stamen and and a cell wall et al., 1991). et al. and flowers of Arabidopsis, which C subtractive hybridization, and a of from mutants have but not genes that are expressed in These genes to and a and gene expression in flowers with C function and B which treated with a to targets of by of B function in flowers in the of one of was in In to flowers expression in stamens and petals, both of B as well as in some organs the domain of PI/AP3 on and expression the proposed that in the transition between cell division and for in stamen and Irish to identify genes by the of AP3 and expression was in flowers from and from and with B function. genes to be directly or by PI/AP3 in stamen the used of the Arabidopsis genome, some genes be by in PI/AP3 genes to be expressed in or in stamens and petals, many genes in the cell that is a feature of and stamen transcription factors suggesting that AP3 and PI directly in gene of the genes one or more CArG in a that they could be directly by PI/AP3, sequences not targets for MADS many have a number of sporophytic genes required for male et al., et al., et al., Sanders et al., et al., 2001; et al., et al., Pollen development is by male gametophytic mutations and by mutations in the genome that result in of the tapetum and Bentolila, 2004). The sporophytic mutations a of in the such as or in meiosis, of the pollen wall and anther dehiscence. required for pollen development that first through include a required for et al., et al., a with a in pollen wall et al., and and both transcription factors expressed in the tapetum and in et al., 2001; et al., for male mutants that the differentiation of anther cell types et al., 1999). an for the loss of expression of a gene to early with the of mutations that development et al., 2002). three involved in patterning of the anther has been most et al., et al., 2002). this to be a gene in patterning the it is in more plant organs from of cells that to and cells for organ Anther development is unusual in that the from cells from stages in this are the of the specification of cell and the of the radially first in the anther with the of the for the the are the and are by the In Arabidopsis is by interactions genes that or with the the (Bowman et al., 2002). Mutations in either of genes result in either or and the of flowers in that anther is et al., et al., 1999). mutants for and or and produce radially the internal of which has to be with is expressed in the of the anther, the that stamens from on Development. (A) of and anther primordia with cells by extra cells in mutants et al., 2002). (B) for the differentiation of the cell in There has been in the last years genes involved in the first of In Arabidopsis, the transcription et al., et al., is required for specification, because anthers to form In male mutants produce but not a and a and are et al., the to the anther to because form on the the The number of cells that can in the anther to be by a to that used in the to of In the meristem, cell number is by the and 2001). In the anther, the number of cells is to one domain by a et al., et al., 2002). In mutants, which are only in male development, cells in the anther development with the result that an of is the first The for is but mutants in the gene have to those of et al., suggesting that a is involved at some in this or not as a for However, expression from that of and with a in cell this that a in has been in et al., and in with mutants in maize with mutants et al., show in both male and female The of the of cell division in Arabidopsis are very The a number of to the whereas the to form an cell to the and a cell The to a middle cell to the and a cell to the sporogenous have been described with in anther wall and in in which the an extra division but as tapetum et al., is cell are this is in for the middle cells are et al., et al., 2002). is required for specification, or the of extra sporogenous cells and middle is of the cell give rise to a array of different cell types However, this by the radially for cells of the to the have been to have a different from those on the in a number of species and Goldberg et al., 1993). The model for development most with the a on the sporogenous cells In this the from the a of signals which to be by the division of the division and development in the to the is to to form an cell to the of the The the division of the and middle However, signals are and by other cells to the sporogenous of which are the described model with some the cell described for Arabidopsis is not et al., not for all other species However, the division sequence by the sporogenous cells or are at an early in all and the wall subsequently develop radially There are many in in which is in the of a of cell division and A is a between the development of the and of the and However, from anthers that cells of the are for development with the be to to signals from the earlier other cells and those of the The nature of the is but mutants would have those of The that of a for the are in the et al., a first of a of signals on the developing of the different cell types at the and cells and display of gene expression, the through meiosis and of The middle of which is is by the of the and the After the of cell is between the and the tapetum as are by the of the microsporocyte wall the and the tapetum develop the meiosis, between to form to in through which These are to the microsporocyte in development, in such as Arabidopsis with between the and to form development involves between the by a this not meiosis, for to the in the of any and it is to be the of which the from the that causes arrest et al., 2002). The of and to all and the genes involved have been the of a number of reviews and 2000; et al., and are the of an However, in meiosis in at the not of but of the of The as a of male meiosis develop an independent for this plant meiosis, and some of the from those in other In plant events, including the and division of and and in and DNA during this in the and cDNA a in the of genes expressed et al., 1995). of sporophytic from the microsporocyte has been either as gametophytic development meiosis or the of including and and be by this in from the but is unusual in that it is to is to result from a for in by the and mutations in and genes that the have been in the development of male used in plant Hanson and Bentolila, 2004). in all during plant have been many of of for with in which in floral meiosis et al., 1999). The between and in is by the and and this of has been as of an of of et al., 1999). is of during in as in and et al., but any between types of has to be After DNA in the male is to that of other including the of and of the in most are by the of and at the genes involved in in are with but in gene number and expression et al., 2001; et al., from a of mutants that between and other et al., 2001), with at one from male development in Arabidopsis et al., 1999). However, mutations in the which to arrest at et al., that some of is in Cell wall during and is feature of male meiosis in plants. of meiosis, the of most species a wall of a between the and the the of meiosis, the the are of species undergo during male meiosis, in which a wall is between the cells meiosis and the of the meiosis in form By contrast, most including Arabidopsis, undergo microsporocyte that are the of meiosis In first as of from the wall the microsporocyte and the are and 2001; and 1995). of the division in sporophytic and in male In the site of cell is not by a each division in species with or meiosis in species with each by a array of that the and and 2001). the by the of the as and wall at the domain In mutants of Arabidopsis, is or of male so that all development in an et al., et al., The gene a that to be required for the of the the at the of meiosis, because are in mutants et al., The mutations are and is expressed the anther meiosis, that male is sporophytic In plant is by a et al. an Arabidopsis that a with and that a The gene is expressed in flowers and the responsible for microsporocyte of the of the wall have been has been proposed to as a or between the sporogenous cells and the of the that is for meiosis and It has been that as a wall that first the of meiosis to cell and and free or as a for the of the on pollen and These are not last function is in more mutants, and to the wall have more the of the of that the wall is not required for male both which a wall and and in which the wall is meiosis as a result of the expression of an et al., initiate and meiosis to produce a of The of the wall in et al., and maize to the of meiosis, the and of the are to by a of and by the In the anthers of and the expression and of activity is et al., and in the of expression, or to to of the which has been to be a of male in of including and to the wall of the because of the of the wall not the the normal of et al., 1992). for genes the of have been has been directly by knockout or The Arabidopsis gene a in sequence to and gene studies that gene expression is tapetum and with activity et al., 1993). of the which a to the of the site of show a of expression and one or more of the genes described in wall more is required to this and to this in pollen is The of expression array and gene knockout in Arabidopsis is to in this The and of the wall to be for the of that produce such as of the and develop or the of the and The but are the of the which and that the of is to the and of wall The of in the in some species that produce in in However, both and and produce in the of suggesting that in some species is not to that during wall the By contrast, mutants in Arabidopsis produce normal of wall and whereas are in the microsporocyte wall the that is for et al., and as a to the developing the tapetum only and for the of the pollen as between cells and the in and is to because between of cells are at an early However, and cells many and during pollen wall are of early in development and meiosis in the with and in many species. In can DNA of an of that is to the tapetum and with of early in development, an for and 1989). on the cells be either and at the of the development or and the and with the developing In other the tapetum at an early and of the the In to in pollen wall the tapetum a in many species. the tapetum of a of and (termed the and whereas the of the the of the pollen wall et al., is of the to the of and et al., 1991). These are to the pollen and a of and proteins for the development of the pollen on the et al., Arabidopsis mutants in can to et al., 1993), whereas in pollen proteins are involved in interactions with the and with the female of the et al., the male in species et al., belong to the pollen family of The pollen and are in this issue by et al. Pollen (A) Scheme of the main of a pollen The is to (B) of a pollen wall showing a by the of the The is through the of pollen wall development based on (after meiosis and to the wall is of genes in the microsporocyte of the by to produce a of of the areas to of to a through the activity of a from between the of more the in conversion and specification of are apparent as that of by the of on the of during which the or is to produce the The wall is are by is the of the is The wall is the first of at the is by the of the the and the and The is largely of and as a that the of the main of the pollen et al. that of the wall early in pollen wall in tobacco, the to only this that the wall a which the in in the of the wall not a to the of wall as proposed by and is from and by the the of are by the tapetum and the wall the of the by Scott et al., on the an of and to have in understanding both and of as the main of reviewed by However, the that a of to in et al., a of a of by that of and a of recent have this et al., et al., 1999). The are by of such as and which with and in a family of and In to pollen wall patterning involves the development of a number of and of The pollen wall the of and with the proposed for this is not in areas to that is by the of a of to the at the of the and the in by the the in pollen wall a feature by many species is the as of the to form a of of that the genes responsible for pollen wall patterning are the of the microsporocyte and that the is by the and There is for the of any in the However, with developing that the responsible for on the in the at the of meiosis and is the as meiosis and and These or the that the of the the it is to a of it could from and to that to all this is not and that with the and the would in a to the of on or the in the of a such as or more in the that such as the which by In the the the that with the the In the the a which the the are the There are very mutations to pollen wall However, in the described in of Arabidopsis, is and and pollen wall patterning is et al., 2001). The gene a that as a site for pollen is from the anther by a of cell in of the a each of anther Sanders et al. the major that during the in These with the of the middle and of the and of in and of the a anther, which is by stomium cell In tobacco, the is very only in the of of cell in that a stomium is for and associated with are with pollen development, anthers that pollen and tapetum undergo a normal (Goldberg et al., 1993), that not signals from cell Anther (A) stamens showing cells on the of the (B) for the regulation of pollen maturation, anther and flower by The areas that water and in response to from et al., 2001). the most recent in understanding the of has been the of the of a in the plant mutants in or a with of the anther and pollen in male mutants in have been that male and can be by the of and Sanders et al., 2000; and 2000; et al., et al., 2002). The of in male is by the which is male and to et al., et al., The gene a that the of et al., 2001). of anthers so that at the of flower and of the and cells the stomium in the and cells of anthers are expanded and the are with studies that activity is to the anther flower therefore, the the of the et al. a model in which in the water in the stamens and petals to about the of flowers, and anther In this is required for the expression of genes involved in water in a which is of to water in some of the cells the vascular during the stages of anther development et al., 1999). and are in flowers, suggesting that water well with the that the of anthers with the of the and with the that anther is by of the and that water is through the to the petals and model that cell in the anther as of the and 2000). et al. for the of in dehiscence. to normal but was and with flower in with in the of stomium cells and of the of anthers with in plants. and the in and Arabidopsis, or in both species. The that Arabidopsis mutants such as which the undergo in of Ten years ago, was about the genetic of stamen development the specification of stamen primordia by floral homeotic genes. have been in patterning of the regulation of meiosis, and anther dehiscence. such as the genesis of the pollen have because of a of questions to be those the of cell the the regulation of the very different in the and the between controlled and male development, the of pollen wall and the between and water during dehiscence. stamen development is to the of plant In a of and an for questions be a We are to all those in the Scott and have the years to on anther and pollen development, and review is to the of and