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Female Gametophyte Development in Flowering Plants

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Female Gametophyte Development in Flowering Plants ANRV410-PP61-05 ARI 26 March 2010 21:40 Female Gametophyte Development in Flowering Plants Wei-Cai Yang,1 Dong-Qiao Shi,1 and Yan-Hong Chen2 1Key Laboratory of Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China; email: [email protected], [email protected] 2College of Life Sciences, Nantong University, Zhongxiu Campus, Nantong 226007, China; email: [email protected] by Universidad Veracruzana on 01/08/14. For personal use only. Annu. Rev. Plant Biol. 2010. 61:89–108 Key Words First published online as a Review in Advance on embryo sac, synergid cell, egg cell, central cell, cell fate, ovule February 24, 2010 Annu. Rev. Plant Biol. 2010.61:89-108. Downloaded from www.annualreviews.org The Annual Review of Plant Biology is online at Abstract plant.annualreviews.org The multicellular female gametophyte, a unique feature of higher This article’s doi: plants, provides us with an excellent experimental system to address 10.1146/annurev-arplant-042809-112203 fundamental questions in biology. During the past few years, we have Copyright c 2010 by Annual Reviews. gained significant insight into the mechanisms that control embryo sac All rights reserved polarity, gametophytic cell specification, and recognition between male 1543-5008/10/0602-0089$20.00 and female gametophytic cells. An auxin gradient has been shown for the first time to function in the female gametophyte to regulate gametic cell fate, and key genes that control gametic cell fate have also been iden- tified. This review provides an overview of these exciting discoveries with a focus on molecular and genetic data. 89 ANRV410-PP61-05 ARI 26 March 2010 21:40 Ovule development starts as a protrusion (pri- Contents mordium) on the edges of the septum of the gy- necium. As the ovule primordium elongates, a INTRODUCTION .................. 90 finger-like structure (nucellus) is formed. Then, OVULE DEVELOPMENT .......... 90 a hypodermal cell at the tip of the nucellus starts THE TRANSITION FROM to differentiate and forms an archesporial cell, SOMATIC TO GERMLINE which produces the germline. The archesporial FATE............................. 91 cell enters meiotic development to differentiate SPECIFICATION OF THE a megasporocyte, which becomes distinct by its FUNCTIONAL MEGASPORE . 93 large size and nuclear morphology (Figure 1a). PROGRESSION OF THE The megasporocyte then undergoes meiosis GAMETOPHYTIC MITOTIC to give rise to four haploid megaspores. In CYCLE ........................... 94 most flowering plants, which include the model The Initiation of Female species and rice, micropylar mega- Gametogenesis ................. 94 Arabidopsis spores undergo programmed cell death, and the Control of the Gametophytic chalazal-most megaspore becomes functional Cell Cycle ...................... 94 and ultimately forms the female gametophyte, CELLULARIZATION OF THE the embryo sac (Figure 1b). Concurrently, epi- EMBRYO SAC .................... 96 dermal cells at the proximal third of the nucel- EMBRYO SAC POLARITY AND lus divide parallel to the long axis and form two GAMETOPHYTIC CELL primodia, which become the inner and outer in- SPECIFICATION................. 96 teguments, respectively (Figure 1a). These en- THE FUNCTIONAL FEMALE close the functional megaspore, which becomes GERM UNIT ..................... 99 the embryo sac, forming a narrow opening at The Egg Cell ...................... 99 the micropyle where the pollen tube enters af- The Synergid Cell ................. 99 ter pollination. The Central Cell .................. 101 While nonfunctional megaspores undergo CONCLUSIONS .................... 102 cell death, the functional megaspore increases in size and undergoes a nuclear division with- INTRODUCTION out cytokinesis to produce a two-nucleate em- bryo sac (Figure 1c). The two daughter nuclei, A female gametophyte is a multicellular now separated to the poles by the formation haploid structure that develops into an embryo of a central vacuole (Figure 1d), proceed to by Universidad Veracruzana on 01/08/14. For personal use only. and endosperm after fertilization. In the a second karyokinesis to form a four-nucleate past decade, gametophyte development in embryo sac with nuclei in a 2n+2n configu- plants has emerged as an excellent system Annu. Rev. Plant Biol. 2010.61:89-108. Downloaded from www.annualreviews.org ration (Figure 1e). As the vacuole increases in to address fundamental questions in biology, size, the third karyokinesis takes place, which such as cell specification, cell-cell interaction, results in the formation of a huge coenocytic and the developmental role of basic cellular cell with eight nuclei that adopt a 4n+4n machinery. Significant progress has been made configuration—the eight-nucleate embryo sac to define the genetic components that govern (Figure 1f ). Thereafter, two polar nuclei, one gametogenesis. This review focuses on recent from each pole, migrate to the micropylar cyto- advances in defining genetic control of female plasm of the embryo sac and finally fuse to form gametophyte development. a diploid central nucleus. As polar nuclei mi- grate, cell walls are formed simultaneously, di- OVULE DEVELOPMENT viding the embryo sac into seven cells with four An ovule is a female organ within the carpel of cell types: three antipodal cells at the chalazal a flower that harbors the female gametophyte. end, a diploid central cell, two synergids, and 90 Yang · Shi · Chen ANRV410-PP61-05 ARI 26 March 2010 21:40 an egg cell at the micropylar end (Figure 1g). The archesporial cell first becomes morpho- Antipodal cells degenerate shortly before fer- logically distinguishable from its surrounding tilization in Arabidopsis (Figure 1h) or undergo nucellar cells by its larger size and pronounced further mitosis as seen in maize. Antipodal cells nucleus, and it is called a megasporocyte when are likely dispensable for fertilization. There- fore, the central cell, the egg, and two synergid cells form a female germ unit—a functional unit ab that is able to attract a pollen tube, interact with the tube to trigger sperm release, and complete double fertilization. As mentioned above, ovule development involves both sporophytic and ga- metophytic processes, and is an excellent system it to study the basic developmental mechanisms that control germline formation, cell growth and division, and gametic cell fate specification. ot 10µm 10µm THE TRANSITION FROM cd SOMATIC TO GERMLINE FATE In angiosperms, the initial cells of the germline, called archesporial cells, are formed de novo from the hypodermal L2 cell layer of the ovule primordium. Generally, in females, a single hy- it podermal L2 cell at the tip of the nucellus dif- fc ferentiates into the germline cell that enters the meiotic pathway, which ultimately gives rise to ot 10µm 10µm gametic cells: the egg and the central cell. −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→ ef Figure 1 Development of the female gametophyte in Arabidopsis. Confocal optical section showing (a) nucleus with MMC ( green) and primordia of by Universidad Veracruzana on 01/08/14. For personal use only. inner (it) and outer (ot) integuments; (b) one-nucleate embryo sac ( green) and degenerating megaspores ( yellow) close to the Annu. Rev. Plant Biol. 2010.61:89-108. Downloaded from www.annualreviews.org micropyle; (c) an early two-nucleate embryo sac ( green); (d ) a late two-nucleate embryo sac ( green), 10µm 10µm inner (it) and outer (ot) integuments, and funiculus (fc); (e) a four-nucleate embryo sac ( green); ( f )an gh early eight-nucleate embryo sac ( green), with antipodal nuclei (pink), polar nuclei (blue), egg nucleus (red ), and synergid nuclei ( yellow); ( g) a late eight-nucleate embryo sac ( green), with antipodal nuclei (pink), polar nuclei (blue), egg nucleus (red ), and synergid nuclei ( yellow); (h) a mature four-celled embryo sac ( green), with the secondary nucleus (blue), egg nucleus (red ), and synergid nuclei ( yellow). Scalebar: 10 μm. Confocal images were modified with PhotoShop to highlight the 10µm 10µm megaspore mother cell, embryo sac, and nuclei. www.annualreviews.org • Female Gametogenesis 91 ANRV410-PP61-05 ARI 26 March 2010 21:40 its nucleus enters the prophase of meiosis. that surround germline cells but not in the de- How the transition from a somatic cell fate veloping germline cells. This supports a role to a germline fate is controlled remains un- for MSP1 in determining germline cell fate known. However, several putative components by preventing the surrounding cells from be- that control this transition have been identi- coming germline cells, similar to MAC1 in fied through genetic approaches. In Arabidopsis, maize (64). MSP1 encodes a leucine-rich re- the SPOROCYTELESS gene (SPL/NOZZLE) peat containing receptor-like kinase (LRR- has been implicated in controlling germline RLK). This implies that signaling between cell fate. In spl mutants, archesporial cells are the archesporial cell and its neighboring cells formed in both anther and ovule primodia, but is controlled by a ligand-receptor signaling they fail to develop further, which results in a cascade, and this plays a critical role in fe- complete lack of germline in male and female male germline development. These findings organs (63, 75). This indicates that SPL plays an imply that a lateral inhibition mechanism may essential role in germline formation. A recent act in controlling the number of germline study revealed that the floral homeotic regu- cells. lator AGAMOUS (AG) can activate
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