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The tree Cell, Volume 16, concern suppl_1, June 2004, Pages S46–S60, https://doi.org/10.1105/tpc.017012
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INTRODUCTION

Stamens space the masculine reproductive guts of flowering plants. Lock consist of an anther, the site of pollen development, and in most varieties a stalk-like filament, which transmits water and nutrients come the anther and also positions that to aid pollen dispersal. In ~ the anther, masculine sporogenous cells differentiate and undergo meiosis to produce microspores, which offer rise to pollen grains, whereas various other cell species contribute to pollen maturation, protection, or release. Pollen advance involves variety of extraordinary events, including cell department and differentiation live independence of a traditional meristem, the transition from sporophytic to gametophytic generation, and modifications the cell division to produce structures that space unusual in tree development, including coenocytic tissues (the tapetum and the microsporocyte mass), and subsequently totally free cells (microspores) that offer rise come self-contained units for genome dispersal (pollen grains). Reviews of stamen advance and gene expression include short articles by Bhandari (1984), Scott et al. (1991a), Goldberg et al. (1993), and also Irish (1999).

A adjust in focus will be evident since anther advance was the evaluation in the an initial Plant Reproduction special issue of The tree Cell (Goldberg et al., 1993). The previously review attracted largely on research studies of tobacco, however this post focuses more tightly on Arabidopsis, which has actually surpassed other types in the hereditary tools publically available, consisting of a sequenced genome, microarrays, well-characterized mutants linked with cloned genes, and knockout collections for reverse genetics. The advances of the last 10 years incorporate an raised understanding of stamen specification, stamen-specific gene expression, interior patterning the the anther, regulation of male meiosis, and also anther dehiscence.

Stamen and pollen development are intimately linked. We discuss those facets of pollen genesis that room essentially controlled by the sporophytic genome (differentiation of the masculine germline, meiosis, and also pollen wall surface formation), but the biological of the masculine gametophyte, pollen pipe growth, and fertilization space covered by other reviews in this volume (Edlund et al., 2004; Hanson and also Bentolila, 2004; McCormick, 2004).

SPECIFICATION of STAMENS


Flowers the eudicots are arranged into 4 concentric whorls of offal (sepals, petals, stamens, and carpels) the arise sequentially native the floral meristem (Figure 1
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Anther Development.

(A) system of a transverse section through one Arabidopsis floral bud mirroring the number, position, and also orientation of the floral organs (after Hill and also Lord, 1989).

(B) Schemes of transverse sections v Arabidopsis anthers at different stages (after Sanders et al., 1999). Floral stages are as defined by Smyth et al. (1990); anther stages are as defined by Sanders et al. (1999). C, connective; E, epidermis; En, endothecium; ML, center layer; S, septum; St, stomium; StR, stomium region; T, tapetum; Td, tetrads; TPG, tricellular pollen grains; V, vascular bundle.


). The 3rd whorl in Arabidopsis flowers consists of six stamens, four medial (long) and two lateral (short). Stamen primordia in Arabidopsis appear during floral stage 5 (defined through Smyth et al., 1990)/anther stage 1 (Sanders et al., 1999), through the lengthy stamens developing first. The stamen primordia distinguish a stalked basal region, which gives rise to the filament, and a more comprehensive upper region, which i do not care the anther. The anther locules, in which pollen develops, show up as convex protrusions on the adaxial surface ar (facing the carpels) in ~ floral stage 8/anther stage 4. Throughout this stage, the sporogenous cells, which offer rise to pollen, space visible in ~ locules the sectioned anthers. Concentric rings of various other cell species associated through pollen breakthrough and release differentiate around the sporogenous cells; collectively, this constitute the microsporangium. All anther cell types are existing by floral stage 9/anther stage 5. Sporogenous cells develop into microsporocytes (also known as pollen mom cells or male meiocytes), which experience meiosis to form tetrads the haploid microspores. These room released into the anther locule to begin male gametophyte development. By floral/anther phase 12, the anthers are almost at their last length and contain tricellular pollen, and the filaments start to elongate rapidly. In ~ floral/anther phase 13, the flower opens and anther dehiscence occurs to relax the pollen. The filaments proceed to prolong so that the anthers brush past the receptive stigma at floral stage 14.

There are numerous recent reviews of floral body organ specification (Jack, 2001, 2004; Theissen, 2001; Lohmann and Weigel, 2002); therefore, the regulation the stamen identity will be treated only briefly here. The identification of floral organ primordia is managed by 3 classes (termed A, B, and also C) of homeotic selector genes through overlapping locations of activity; stamens develop in the third whorl, wherein both B and also C genes room expressed. It has actually been uncovered that added transcription factors, encoded through SEPALLATA (SEP) gene in Arabidopsis, are required to confer full activity on the homeotic gene in stamens (as well together petals and also carpels). The ABC design was based upon genetic researches in Arabidopsis thaliana and also Antirrhinum majus (Coen and also Meyerowitz, 1991). The B course genes room APETALA3 (AP3) and PISTILLATA (PI) in Arabidopsis and DEFICIENS (DEF) and GLOBOSA (GLO) in Antirrhinum; the C genes room AGAMOUS (AG) and PLENA, respectively. Mutations in any kind of one that the B or C genes result in homeotic switch of the third-whorl organs to a different type. Lose of B function causes transformation of stamens come carpels, loss of C duty converts stamens to petals, and loss of both transforms stamens come sepals. Single-gene mutations of the SEP genes have an extremely subtle phenotypes, yet in sep1 sep2 sep3 triple mutants, every floral guts resemble sepals, saying that B and C role have to be abolished (Pelaz et al., 2000).

The signals that recognize the variety of floral body organ primordia in every whorl space unknown (Irish, 1999). However, the variety of stamens can be perturbed by various mechanisms: extra stamens build in clavata1 mutants, which form enlarged meristems (Clark et al., 1993), and also in superman mutants, which have actually an increased BC domain (Bowman et al., 1992). By contrast, arrest the stamen primordia is a function of normal advancement in some species. In the radially asymmetric flowers of Antirrhinum, 5 stamens initiate, yet the dorsal stamen arrests beforehand in advance in response to the activity of the CYCLOIDEA gene (Luo et al., 1995). In some species that produce unisexual flowers, such together 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 come the MADS family of transcription factors, which tie a target DNA sequence (the CArG box) as homodimers or heterodimers. In vitro, the B class proteins DEF and also GLO (Antirrhinum) or AP3 and also PI (Arabidopsis) tie CArG box sequences only as heterodimers (Jack, 2001; Thiessen, 2001). Yeast two-hybrid assays present that PI/AP3 interacts straight with SEP3 however not AG, and it has actually been proposed that SEP3 mediates interactions in between PI/AP3 and AG dimers so that PI/AP3, AG, and also SEP tie DNA in quaternary complexes; this impact would define the combinatorial action of B and also C class genes (along v SEP genes) in stamens (Honma and also Goto, 2001).

IDENTIFICATION the GENES associated IN STAMEN development AND FUNCTION

Ten year ago, among the major unanswered questions about stamen breakthrough was the nature of the gene downstream the the B and also C class transcription determinants (Goldberg et al., 1993). After ~ stamen specification, the B and C course genes and SEP genes continue to be expressed during stamen development (Bowman et al., 1991; Pelaz et al., 2000; Jack, 2001), for this reason they could be straight responsible because that activating countless of the genes connected in stamen morphogenesis and also function. Among the targets the B class genes in Arabidopsis are AP3 and also PI themselves, due to the fact that both gene are forced for the ongoing expression of each in the arising flower (Lohmann and Weigel, 2002). Two main approaches have been used to identify added targets that B and also C course genes. This involve either transcriptional profiling methods, consisting of subtractive hybridization, differential display of RNAs, and differential screening of cDNA libraries or arrays to uncover genes v stamen-specific or stamen-preferred expression, or screening for mutations that influence stamens. Both approaches have yielded amazing results, yet there is no yet enough information to rebuild the developmental pathways that start with homeotic gene expression.

Expression Studies

Stamen expression studies have been carried out in countless crop and also model species, including tomato (Ursin et al., 1989; Chmelnitsky et al., 2003), tobacco (Koltunow et al., 1990), Antirrhinum (Nacken et al., 1991), oilseed rape (Scott et al., 1991b; Shen and also Hsu, 1992), maize (Wright et al., 1993), rice (Tsuchiya et al., 1994), lily (Crossley et al., 1995), white campion (Barbacar et al., 1997), Lotus japonicus (Endo et al., 2002), and also Arabidopsis (Rubinelli et al., 1998; Sablowski and also Meyerowitz, 1998; Amagai et al., 2003; Zik and also Irish, 2003). Some of these studies have manipulated B and/or C class genes themselves to aid identify their targets; these experiments are debated in much more detail below. Proteomics analysis likewise has been used to compare and also identify proteins extracted native rice anthers at various developmental step (Kerim et al., 2003). The expression researches have identified genes affiliated in protein, starch, and also sucrose metabolism, osmoregulation, cell wall surface biosynthesis and expansion, sugar transport, lipid transfer, flavonoid synthesis, and also cytoskeleton structure. These are consistent with functions of stamen and/or pollen development, such as rapid growth, water movements linked with desiccation and also dehiscence, stress and anxiety response, and also the accumulation of warehouse compounds. Countless genes that unknown role also were recovered. Few of the gene have evident developmental or signaling roles. Exception to this include several candidate transcription determinants in white campion and Arabidopsis (Barbacar et al., 1997; Zik and Irish, 2003) and a subunit of the COP9 signalosome (a repressor that photomorphogenesis in plants) uncovered to it is in upregulated in rice anthers at the panicle heading stage, arguing a duty in mediating irradiate signals to name: coordinates flowering with anther dehiscence (Kerim et al., 2003).

In situ hybridization mirrors that genes figured out in the expression studies are specific to a variety of stamen regions and cell types, consisting of microsporocytes, microspores, tapetum, endothecium, connectives, epidermis, and also filaments. A typical theme is the advantage of tapetum-specific transcripts in early on anthers, mirroring the high metabolic task of this tissue (Scott et al., 1991a; check out below).

In several of the experiments described above, transcriptional profiling approaches were supplied in conjunction v mutant or transgenic plants through altered role or expression of B and C course genes to aid the to know of their targets. For example, differential hybridization of an inflorescence-enriched cDNA library was provided to isolate mRNAs expressed in wild-type flowers however not def mutants that Antirrhinum, which absence B function. This experiment surrendered 12 differentially expressed genes, consisting of tap1, express in the tapetum and encoding a suspect secreted protein, and also filamentous flower1 (fil1), expressed largely in stamen filaments and petal bases and encoding a candidate cell wall protein (Nacken et al., 1991). Rubinelli et al. (1998)compared wild-type and ag mutant flowers of Arabidopsis, which lack C function, utilizing subtractive hybridization, and also after a further display screen making usage of RNA native ap3 mutants (which have pistils yet not stamens) determined 13 gene that are expressed differentially in stamens. These consisted of genes predicted to encode hydrolytic enzymes and also a lipid transferase. Sablowski and Meyerowitz (1998)analyzed gene expression in flowers with no C function and inducible B function, which also were treated v a protein synthesis inhibitor, come discover direct targets of PI/AP3. Three mRNAs were upregulated repetitively by induction of B role in ap3-3 ag3 twin mutant flower in the absence of protein synthesis, one of which, NAC-LIKE, set off BY AP3/PI (NAP), was studied in detail. In situ hybridizations come wild-type flowers proved expression in stamens and also petals, both sites of B function, and also in part organs outside the domain the PI/AP3 activity. Based on ectopic and antisense expression studies, the writer proposed that NAP features in the shift between cell department and expansion, for circumstances in elongating stamen filaments.

Zik and also Irish (2003)used microarray analysis to determine genes influenced by the misexpression the AP3 and PI. Gene expression was compared in flower from wild-type plants and also from mutant and also transgenic tree with altered B function. This study figured out 47 genes most likely to be regulated directly or indirect by PI/AP3 in petal and/or stamen development. Due to the fact that the microarray supplied represented ∼25% of the Arabidopsis genome, part 200 genes might be affected by transforms in PI/AP3 activity. PI/AP3-responsive genes discovered to it is in expressed in stamens, or in stamens and also petals, consisted of many genes implicated in the quick cell growth that is a function of petal and stamen growth. Only two candidate transcription components were identified, saying that AP3 and also PI act relatively directly in regulation gene expression. Twenty-eight of the 47 genes had actually one or much more candidate CArG boxes in a 1-kb upstream region, indicating the they might be activated straight by PI/AP3, although these sequences do not provide specific targets for certain MADS proteins.

Mutant Analysis

Mutant screens performed over plenty of decades have revealed a big number the sporophytic genes required for masculine fertility (Kaul, 1988; Chaudhury, 1993; Dawson et al., 1993; Chaudhury et al., 1994; Taylor et al., 1998; Sanders et al., 1999; Bhatt et al., 2001; Sorensen et al., 2002; Caryl et al., 2003). Pollen breakthrough also is influenced by male gametophytic mutations (McCormick, 2004) and indirectly through mutations in the mitochondrial genome that result in degeneration of the tapetum (Hanson and also Bentolila, 2004). The reported sporophytic mutations annoy a range of procedures in the stamen, such together chromosome pairing or segregation in meiosis, viability of the tapetum, pollen wall surface formation, filament elongation, and also anther dehiscence. Genes compelled for pollen advance that were first identified v mutant evaluation include DETERMINATE, INFERTILE1 (DIF1)/SYN1, encoding a cohesin compelled for chromosome segregation (Bai et al., 1999; Bhatt et al., 1999); MALE STERILITY 2 (MS2), encoding a predicted fat acyl reductase with a potential function in pollen wall formation (Aarts et al., 1997); and also MS1 and ABORTED MICROSPORES, both candidate transcription factors expressed in the tapetum and also possibly in microspores (Wilson et al., 2001; Sorensen et al., 2003).

General displays for male sterility yield surprisingly ccivicpride-kusatsu.netle of mutants that impact the differentiation that anther cell varieties (Sanders et al., 1999). Therefore, we performed an extinction screen for the lose of expression the a β-glucuronidase reporter gene fused to beforehand tapetum-specific promoters, v the target of specifically detecting mutations that disrupt microsporangium advancement (Sorensen et al., 2002). This screen figured out three loci associated in patterning the the anther locule. That these, EXTRA SPOROGENOUS CELLS (EXS)/EXCESS MICROSPOROCYTES1 (EMS1) has been studied most thoroughly (Canales et al., 2002; Zhao et al., 2002). Due to the fact that this shows up to it is in a key gene in patterning the stamen, it is disputed in more detail below.

GENETIC manage OF ANTHER DEVELOPMENT

Most tree organs derive from meristems, undifferentiated populations of cell that divide to replenish themselves and also administer founder cell for organ primordia. Anther advance is inexplicable in the the microsporangia happen from single archesporial cells fairly than indigenous meristems. Crucial stages in this process are the establishment of adaxial-abaxial polarity, the specification of cabinet types, and the formation of the radially symmetry microsporangia.

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Asymmetry first becomes noticeable in the anther primordium with the development of the microsporangia, because that the two abaxial locules (facing the petals) are larger than the adaxial pair and also are additional separated through the connective tissue (Figure 1). In Arabidopsis leaves, asymmetry is established by cadastral interactions amongst genes the promote adaxial or abaxial identity, v the previous suppressing the latter (Bowman et al., 2002). Mutations in either collection of genes an outcome in either “adaxialized” or “abaxialized” filamentous leaves, and the research of flowers in these mutant lines suggests that anther polarity is regulated likewise (Sawa et al., 1999; Siegfried et al., 1999). For example, dual mutants for fil1 and yabby1 or kanadi1 (kan1) and kan2 produce radially symmetrical filamentous structures, the interior structure that which has yet to be determined. Similar to leaves, FIL is to express abaxially in the connective organization of the anther, supporting the watch that stamens developed from leaves bearing microsporangia ~ above their top surfaces.