Control of Male Gametophyte Development
Year: 2004
Type: article
Abstract: In a previous review of male gametophyte development (McCormick, 1993), it was noted that the two areas posed by Mascarenhas (1975)as fruitful areas for future research were the following. What are the differences in the two cytoplasms that determine the different cell fates of the generative and vegetative cells? And what are the functions of pollen-specific proteins? Now, 10 years later, the genome sequences of Arabidopsis and rice have been completed. There are extensive EST databases for many plants and several data sets from microarray hybridizations. There are extensive resources for disrupting the functions of genes. The pollen research community has made significant progress toward a deeper understanding of pollen development using community resources as well as novel techniques developed specifically for the analysis of pollen. This review will provide an overview of these advances and prospects for the future, focusing on male gametophyte development, strictly defined as postmeiosis development, after the formation of the haploid microspores. Meiosis will not be discussed in detail. Anther development and the role of the tapetum in pollen development are discussed by Dickinson and Scott in this issue. Scheme of Microsporogenesis. Tetrads of Arabidopsis. Male meiosis is essentially synchronous within an anther. A common way to dissect a developmental pathway is to isolate mutants that disrupt the pathway. This approach has been used extensively to dissect male meiosis and subsequent stages of pollen development. Comprehensive reviews of meiotic stages and cytology in Arabidopsis (Ross et al., 1996; Armstrong and Jones, 2003) as well as a time course for meiotic stages (Armstrong et al., 2003) now provide the necessary foundation for the interpretation of phenotypes in mutants that disrupt particular phases of meiotic development. Caryl et al. (2003)comprehensively reviewed the mutants known to affect meiosis in Arabidopsis. Some mutants that disrupt pollen development affect the function of the tapetum. Because the tapetum is sporophytic, such male-sterile mutants generally are recessive and all of the pollen grains within an anther are affected. Mutants are termed gametophytic if they disrupt genes that act after meiosis, in the haploid phase of pollen development, and thus only the pollen grains carrying the mutant allele are affected. Male gametophytic mutants often exhibit segregation distortion (i.e., reduced transmission through the male) if the gene that is mutated is important for pollen development or function. Indeed, this feature was used successfully as a first-pass method in screens designed to identify gametophytic mutants (Feldmann et al., 1997; Bonhomme et al., 1998; Howden et al., 1998; Grini et al., 1999; Oh et al., 2003). The developmental stage affected in each mutant was determined later. In some cases, the phenotype was demonstrated to be attributable to a single gene disruption. For example, several of the male transmission–defective mutants described by Bonhomme et al. (1998)had similar phenotypes, and upon further mapping and cloning they were found to be alleles. The disrupted gene was renamed kinky pollen because pollen grains harboring a mutant allele exhibited aberrant tube growth (Procissi et al., 2003). However, frequently, mutations identified from T-DNA insertion screens were unlinked to the T-DNA insertion or exhibited complex molecular lesions. In such cases, it was difficult to identify the molecular basis for the transmission defect. For example, the T-DNA insertion mutant named halfman (Oh et al., 2003) was demonstrated to have a gametophytic defect with low penetrance. Molecular analysis showed that the T-DNA insertion in halfman had induced an adjacent deletion of 150 kb that encompassed 38 genes; the loss of 1 or more of these genes might have been responsible for the observed phenotype, in which ∼30% of the pollen aborted at approximately the bicellular stage of development. Assiduous screening for mutants based on morphology or staining characteristics of pollen from individual M1 or M2 plants also has yielded very interesting phenotypes (Chen and McCormick, 1996; Johnson and McCormick, 2001; Lalanne and Twell, 2002). As illustrated in Figure 1, pollen development undergoes stereotypical cell divisions to yield the three-celled male gametophyte. To test whether the asymmetric division that resulted in the formation of the vegetative cell and the generative cell could be disrupted mutationally, or if the number of cell divisions typical of pollen development could be disrupted mutationally, pollen of mutagenized populations was examined after 4′,6-diamidino-2-phenylindole staining. Such screens yielded, for example, the mutants sidecar pollen (Chen and McCormick, 1996) and gemini pollen (Park et al., 1998). At the mature pollen stage in sidecar pollen heterozygotes, ∼50% of the pollen was normal, ∼43% was aborted, and ∼7% showed the sidecar phenotype, namely an extra cell within the pollen exine. Analyses of earlier stages of pollen development revealed that microspores carrying the sidecar pollen mutant allele frequently underwent a premature and symmetric cell division. For unknown reasons, one of the resulting two cells then was able to undergo the normal mitotic divisions to form the generative cell and the two sperm cells. Despite the apparent equal sizes of the two cells, this finding suggests that an inherent asymmetry must persist. The molecular lesion in sidecar pollen is still unknown. In contrast to sidecar pollen, in gemini pollen, arrest occurs after the first division, so that at the mature pollen stage the affected pollen grains have two cells, each with decondensed chromatin and each able to express a vegetative cell–specific marker (Park et al., 1998). Gemini pollen encodes a microtubule-associated protein (MAP215 family), and the gemini pollen phenotype is now believed to be attributable to a defect in the correct positioning of the cytokinetic phragmoplast at pollen mitosis I (Twell et al., 2002). It was hypothesized that the aberrant partitioning of cytoplasmic factors led to the alterations in cell fate. In another mutant, duo, the first mitotic division occurs asymmetrically, yielding one larger cell with decondensed chromatin and a vegetative cell fate and a smaller generative cell with condensed chromatin. However, the second mitotic division of the generative cell, to form two sperm cells, fails. The molecular lesion in duo is not yet known. A gametophytic male-sterile mutant (gaMS-2) that affects this stage of development was described in maize (Sari-Gorla et al., 1997); in microspores carrying the mutant allele, the vegetative nucleus and the generative nucleus exhibit altered identities and sometimes undergo extra divisions. Mutants that affect these division patterns also were isolated from the T-DNA screens. For example, in limpet pollen (Howden et al., 1998), now named ingressus (http://www.le.ac.uk/biology/research/pollen/ing.html), the generative cell divides to form two sperm cells, but the sperm cells cannot migrate inward and instead remain near the pollen tube wall. More recently, Lalanne and Twell (2002)found that the association between the sperm cells and the vegetative nucleus could be disrupted mutationally. In gum (germ unit malformed), the vegetative nucleus stays near the pollen wall, although the sperm cell pair associates and migrates inward, as in the wild type. In mud (male germ unit displaced), the sperm cells associate with the vegetative nucleus, as in the wild type, but this group then remains near the pollen wall. The phenotypes in double mutants allowed these authors to infer that GUM acts earlier than MUD. It is usually the case that gametophytic mutants that affect pollen development or function must be maintained as heterozygotes. If the female gametophyte also is affected (Howden et al., 1998), the chance of obtaining homozygous lines is slight. However, it was possible (Chen and McCormick, 1996) to obtain a homozygous sidecar pollen line, albeit at a much lower than expected frequency, indicating that pollen grains carrying the sidecar pollen mutant allele could transmit the gene to the next generation. The gum and mud mutants (Lalanne and Twell, 2002) show only minor transmission defects through the male, and homozygous lines of these mutants were obtained readily. Consequently, visual screening facilitated the discovery of these mutants, because they would not have been detected readily in a segregation distortion screen. If homozygous lines can be generated, it will be possible to test for phenotypes in the sporophyte. The gum and mud homozygotes show no obvious vegetative phenotypes. Identifying additional alleles is a standard approach that facilitates the positional cloning of genes. For a pollen mutant whose phenotype is visible only after microscopic examination, this task is not easy. Additional visual screens of mutagenized populations will undoubtedly yield additional phenotypes of some sort, but there is no guarantee that the desired phenotype will be seen in the next 5000 plants screened. However, screening for new alleles can be simplified for certain mutants with striking phenotypes. For example, in anthers of raring-to-go (rtg) (Johnson and McCormick, 2001), some of the pollen stains with decolorized aniline blue, a stain specific for callose, a component of the pollen tube wall. Because pollen does not normally hydrate and begin it the finding the pollen phenotype in was To obtain additional mutagenized populations were by pollen from plants at a time and screening the pollen in after staining with decolorized aniline This yielded several alleles of as well as mutants pollen, pollen, and whose pollen grains exhibited staining the of these staining patterns could not have been (Johnson and McCormick, There are undoubtedly more mutants to from such visual screens. that protein can provide the normal of of unknown function et al., A for in that had been with a was at that are with et al., 2003). these resources not provide that can be used to for pollen et al. the of which is stages of male gametophyte development, et al. to the to it be to new lines using a In also might fruitful to lines harboring to pollen and then to these lines and for of the Such a was for mutants that a protein of the protein development et al., 2003). mutants in and have facilitated the analysis of male gametophytic is a recessive In all of the of a single meiosis are in a pollen development et al., but each pollen is normal and can analysis is used to test whether pollen phenotypes from a gametophytic or from a This test is important because many mutants that affect pollen phenotypes exhibit and an that ∼50% of the pollen has an altered phenotype is in and of of gametophytic To plants are as to homozygous whose pollen exhibit the mutant phenotype are and the are to identify the double If a is but has low it might be expected that the of normal and affected pollen resulting from meiosis would in each However, if a is the of normal to affected pollen resulting from meiosis be to an of such a after staining with aniline blue, two of the pollen grains in the are wild and the two show within the that are of the phenotype (Johnson and McCormick, Because pollen is it is not to determine if a is or To a mutant allele with a allele in one cell for a the mutant can be as with pollen from a However, the resulting are from plants have a of pollen that might with for the pollen phenotype of is to the mutant allele a and then the phenotype in the pollen by the This method has been used et al., 1996) in A in is to the pollen mutant of with Because plants that are homozygous for a mutant allele to undergo after meiosis, pollen grains are et al., 1997; et al., 1997; et al., 2003). This feature a way to determine the of male gametophytic genes. In a that also is for the gametophytic gene pollen grains carrying a mutant allele and a allele will with or for the of to Tetrads from a grains within an anther of a and pollen grains from a In to mutant there are several to determine the of genes that are in the male gametophyte. the analysis of T-DNA insertion lines et al., 2003) in particular genes and analysis of or lines for particular example, et al., et al., 2002). For example, because and were known to be important for pollen it was that and to the Arabidopsis of a pollen-specific protein pollen from There are in the Arabidopsis was only in pollen, the two genes were in pollen as well as in for the T-DNA insertion in could not be that there was a with transmission through the revealed that although was not pollen development, it was for pollen important role for of in pollen tube growth was by lines in which mutant of were in pollen by 2002). A from the analysis of a maize mutant that is disrupted in one of two very that are in maize pollen et al., 2003). The mutant allele is through the male in heterozygotes, that the mutant pollen is at a pollen tube growth in the mutant analysis might a role for of the pollen tube to the Such have allowed the of gene that must a but role in pollen development. For example, et al. that they could not identify homozygous T-DNA in a and that in heterozygotes, of the pollen grains were and to revealed that this defect was first and that the affected microspores to have will be to determine the of to pollen et al., 2003). are known to in they are important in several but in plants are not well In an to the of in gene were There are two genes in and of gene had no However, the double could not be because pollen grains carrying disrupted alleles could not further will be to determine what pollen are affected by the of In many only a are in pollen, and mutant phenotypes can sometimes be obtained the of one might in for the of another et al., and 2003). does to obtain a phenotype and thus infer a function for a For example, there are several that are pollen a homozygous T-DNA gene in one of showed no apparent defects in pollen or transmission through the male et al., 2003). this that genes whose mutants a are not and that gene be in to that important developmental be by several of which can the task in the of et al., and Twell, 2003) have used to gene patterns in mature pollen of Arabidopsis. The are not yet of the genes of Arabidopsis were but they can be to the The two and because of differences in and of data it is that the for the of genes in pollen and for the of genes that might be pollen specific are similar to from are in for further for genes as or of unknown function. such are only a and must be by have low and will not be by such as microarray Indeed, for several genes in the microarray et al., and Twell, are in pollen et al., et al., 2002) or in a maize sperm cell et al., 2003). A different analysis of gene was used by and obtain gene data from pollen from Arabidopsis plants normal or in an to plants exhibit male were not affected by the analysis a possible for pollen of some is not in in cells were in pollen In plants for which there are techniques such as have for genes for further For example, et al. this to gene patterns in anthers at different developmental What are the of the that are in on from with and it was that pollen would that are pollen development and for pollen However, many to such are in pollen, as was were et al., 1998; et al., 2002). The from the microarray et al., and Twell, 2003) that that and cell were and were analysis and 2003) that that cell were in pollen. The for one of genes in pollen were by Lalanne et al. the pollen phenotypes in T-DNA insertion lines that disrupted the and genes. and for the of to Lalanne et al. that the of to resulted in defects in pollen tube further now can be on which are important and et al., 2003) were used to show that were important for in A have to identify that are specific to the phase of pollen development. In are for et al. of this to the of in pollen. they a for that were or after the of to the identified such all were pollen and many to Despite such et al., also et al., it has been difficult to identify genes that are only upon pollen or to identify that are essentially not after pollen et al., that are pollen it is that only a have examined the for of gene pollen development. and the that pollen were For 10 they that the was very However, two were in pollen, and an that was very in cells was in pollen. et al. and plants to that the of the anther gene as a in pollen. significant in was seen with the were cells. In pollen, the was the stages of pollen development. can be detected at approximately the time of first mitosis et al., there might be in to significant of gene such as near pollen et al. that pollen-specific such as and might a role in this and the of the from in In this the was not pollen the in gene reduced gene of the gene in pollen and in cells. The of pollen-specific were with the et al., and Twell, and the et al., genes. and gene were by in pollen and in cells as well as in In this that could pollen to a were using with of these that the might for to a still not what factors to these identified so understanding of the for pollen-specific gene is It be to to identify all of the genes with similar in pollen and then et al., 2003) to for common However, the identified in the et al., are not well in the of and McCormick, This suggests that identities between might not be to the of genes. The of defined factors for pollen-specific does not that pollen-specific factors are not known. For example, et al. a pollen-specific in as et al. because of the maize gene pollen tube it was that this might be the phases of pollen tube et al. the pollen-specific of a that genes for function. et al. that different factors in were and at different stages of pollen development. that such factors might a and that each might have specific the in the The extensively (Twell et al., et al., is the for gene in pollen et al., et al., 1996; However, there is a for of and certain For example, of a gene not the as the gene whose function is in case the factors for are by the can be detected in microspores in Arabidopsis et al., and this gene at all developmental stages after the bicellular pollen after mature pollen (Twell et al., et al., If a of gene of a is the not be the For example, et al. to whether the pollen a role pollen because by the resulted in pollen at the have made progress in in the for gene at stages pollen development. For example, and a and showed that it gene in et al. the of the gene and showed that it will gene only in microspores. also the and showed that it was a later, bicellular pollen is The for the and are detected only in mature pollen et al., 1998; et al., 2002). not yet it is that these or the of Arabidopsis et al., might provide a way to express only at the stages of pollen development. for obvious as microarray et al., and Twell, 2003) are to stages of pollen development. Despite the of the anthers in which meiosis pollen cells can be from anthers at the It is to that molecular of isolated will be and that with such might provide that will be for gene at stages of meiotic development. Molecular of the generative cell were first with has a generative cell that is isolated of the vegetative cell, and generative cell–specific were and et al., et al., et al. a from generative cells to isolate and a generative cell–specific generative The protein was to the of the generative cell and sperm cells, that it might a role in et al. the in deletion they that generative cell–specific by the is attributable to a because the was the gene was more et al. a from sperm after screening of this and analysis of that two were sperm one was similar to a and one was similar to a protein of unknown function More recently, et al. from a from sperm of maize that had been from vegetative by cell and used and in to the patterns for some of these Arabidopsis from a a is a of the with a The is to more show that one of the two sperm cells has a cytoplasmic that to the vegetative cell that generative cells and sperm cells are to be it is sperm have et al., 2003). some of these are not they are to the cell or the In genes have been for from the and et al., but not all et al., 2001; et al., were not from the genome a after However, et al. a of maize harboring a to test whether there was from the allele after There were no in sperm of this line, but sperm carrying the were in with were within the first after and was It will be important to test such might using than to are et al., and Twell, 2003) for for it is important to determine the protein of the cells of the male gametophyte. To begin to the at different stages of pollen development, et al. and to the protein of rice This was not but it was able to identify protein that between different stages of development. Because for several of these were the authors that might a significant role pollen development. also are in plants whose are not yet because there are extensive EST databases from male of several example, et al., 2002). for many it will be difficult to of male or of component cells, so future will more et al., 2002) that not on the for Because had that many of the and molecular of are (McCormick, 1993), a and cell of in a was (McCormick, to provide a for future It also was that of the and from to more progress in understanding meiosis, cell fate and Such have pollen can a for the they are and then to to different And new to be developed or For example, et al. the of gene microspores of by in of these microspores to mature pollen. the resulting pollen grains were used for some plants were although the was low and this for and to If this can be it will be an way to if in of microspores can be developed for The of the is to the functions of all of the of Arabidopsis by the this for the in the male gene and the discussed in this have particular that are important for pollen development. much of is still is unknown. In some are to be by and 2002). screens have not yet been used extensively in pollen such screens have been et al., 2001; et al., it is that some identified from such screens also are pollen to be in cells might have different in the male and that are might have in the male gametophyte. et al., et al., 2003) for the for all of the developmental stages of the male and analysis of the male gametophyte et al. the to gene in two different cell found that of the genes were at such a low that they would not have been detected by standard techniques such as microarray the of was one cell and the genes. And there were identified that to no known (i.e., genes. used to gene on from many cells. et al. a method to gene of genes at the In this of are by in to genes can be at using for the the authors were able to have a identify each In to a gene from a microarray the of the the of the and the the course of the is that microarray et al., and Twell, 2003) are only a to a understanding of male gametophyte development. might the of differences in gene in different pollen grains in a The of sperm the of of the that can be for such In the 10 more and more has been on such as Arabidopsis. there is still a for Some very interesting are not known or analysis is not in the As one example, a of segregation distortion by genes is known in several and In lines that are for a gene that the allele undergo induced meiosis by with the et al. obtained an mutant at the mutant plants are because the not cloning of and molecular of this interesting be have genes for was no but that some were in cells was Now, et al. that one of these sperm in an similar to a found in et al. demonstrated that a protein that is from the of acts as a for pollen tube There is to that will with such for years to I all of the of for I for the in Figure to for Figure to Johnson for Figure to for the and the and to and and of for in and the in Figure I of and for on the I this to the of and The in is by of and by
Source: The Plant Cell
DOI: 10.1105/tpc.016659
Author: S. McCormick
Language: English
Topic: Plant Reproductive Biology
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2109522067