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Plant Embryogenesis, Genetics of
Plant Embryogenesis, Genetics of M Elhiti and C Stasolla, University of Manitoba, Winnipeg, MB, Canada
© 2013 Elsevier Inc. All rights reserved.
This article is a revision of the previous edition article by PN Benfey, volume 3, pp 1474–1477, © 2001, Elsevier Inc.
Glossary Hypophysis Uppermost cell of the suspensor in contact with the embryo proper. Meristem Undifferentiated group of cells able to provide all cell types within the plant body. Quiescent cells Cells within the root meristem that never divide during embryogenesis. A function of these cells is to
The Unique Nature of Plant Embryogenesis Like animals, the initiation of plant embryogenesis is demarked by the fusion of the haploid sperm and egg cell, resulting in the formation of a diploid zygote, which then undergoes a series of divisions to produce a fully developed embryo. The uniqueness of plant embryogenesis resides on three characteristics that have tremendous implications for morphogenesis. First, plant cells do not move. Unlike the body of animal embryos that is reshaped by the movement of cells, the final form of the plant embryo is only dictated by the rate and plane of cell division as well as by cell expansion. Second, the plant embryo is not the ‘miniature’ version of the adult plant since many cell types and organs are elaborated during postembryonic development. This is in contrast to ani mal embryogenesis where the mature embryo contains most of the organs and features of the adult body. Finally, the later phases of plant embryogenesis are characterized by a desicca tion period required to terminate the developmental program and initiate germination. The duration of desiccation varies from species to species, and it is also associated to profound changes in gene expression. Studies on plant embryo morphogenesis have mainly been conducted by screening and characterizing mutants of Arabidopsis thaliana, which is used as a model system in plant biology. Arabidopsis is a diploid plant with a small and com pletely sequenced genome, able to produce a large number of seeds (up to 10 000 per plant) in a short period of time (2–3 months). These characteristics have been critical for rapidly advancing knowledge on the genetics of plant embryogenesis.
Establishing the Embryo Body Plan: Formation of the Apical–Basal Axis The establishment of the apical–basal axis of the embryo is responsible for the positioning of cotyledons surrounding the shoot apical meristem at the apical (top) end of the embryo, the hypocotyl in the central domain, and a root meristem in the basal (bottom) region of the embryo (Table 1). This distribu tion of organs along the apical–basal axis can be traced back to the polar organization displayed by the egg and the asymmetric division of the zygote delineating an apical and a basal cell.
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 5
maintain the surrounding cells in an undifferentiated state. Suspensor Smaller terminal cells supporting the embryo within the seed sac. Zygotic Derived from the fusion of the gametes, that is, sperm and egg.
These cells already have a differential gene expression pattern with PINFORMED7 preferentially localized in the basal cells and WOX2 in the apical cell. Mutations of either gene result in an abnormal division of the zygote. The asymmetric distribu tion of PINFORMED 7, an auxin efflux carrier, is in line with the role played by this hormone as the vectorial signal for the establishment of the apical–basal axis. The apical domain of the embryo, comprising the cotyle dons and the shoot apical meristem, is specified by the expression of GURKE and TOPLESS. GURKE encodes an acetyl-CoA carboxylase and GURKE mutant plants fail to develop cotyledons and a functional apical meristem. A similar phenotype is also observed in TOPLESS mutant plants, which sometimes form a root in the apical region of the embryo. This observation suggests that TOPLESS might be involved in inhi biting the development in the basal patterning within the apical domain. The central embryonic domain comprising the hypocotyl is defined by FACKEL, HYDRA1, and CEPHALOPOD. A mutation of these genes produces embryos in which the hypocotyl is absent and the apical domain is in contact with the embryonic root. The three genes encode enzymes participating in the biosynthesis of sterols, thereby suggesting the involvement of these molecules in hypocotyl specification. A similar pheno type, but with the additional deletion of the basal elements, was also observed in MONOPTEROS mutant plants. The MONOPTEROS gene encodes a transcription factor of the auxin-responsive factor family, which is implicated in confer ring auxin responsiveness to downstream genes. Of interest, the formation of the vascular tissue, which requires auxin for proper development, is also affected in the MONOPTEROS mutants. The screening of additional mutants including AUXIN-RESISTANT 6 and BONDELOS, both of which affect the sensitivity of the tissue to auxin, argues for the participating of this hormone in the establishment of the central domain of the embryo. The basal domain of the embryo includes the root meris tem, which is composed of quiescent cells surrounded by the stem cells. The origin of the quiescent cells can be traced back to the hypophysis, the uppermost cell of the suspensor. During the early phases of embryogenesis, the hypophysis is the site of auxin accumulation mediated by PINFORMED 1, 4, and 7, which encode auxin efflux facilitators. Downstream elements
doi:10.1016/B978-0-12-374984-0.01168-2
343
344
Table 1
Plant Embryogenesis, Genetics of
Key gene products with their respective function
Formation of apical–basal axis PINFORM – Efflux translocator of auxin that provides positional information within the embryo body GURKE – Acetyl-CoA Carboxylase Required For Normal Development of the apical region TOPLESS – Involved in cell fate acquisition in the apical region of the embryo FACKEL, HYDRA1, and CEPHALOPOD – Involved in the establishment of the central domain of the embryo MONOPTEROS, AUXIN-RESISTANT 6, and BONDELOS – Transcription factors participating in auxin response and responsible for the formation of central and basal domains PLETHORA and HOBBIT – Required for the proper division of the hypophysis and specification of quiescent cells SCARECROW and SHORT ROOT – Transcription factors needed to establish cell fate within the root cells Formation of the shoot apical meristem WUSCHEL – Transcription factor required for specification of stem cell niche in the central domain of the apical meristem CLAVATA – Proteins involved in cell differentiation through interaction with WUSCHEL CUP-SHAPED COTYLEDONS – Transcription factors demarking the boundaries between apical meristem and lateral organs Formation of the radial pettern ARABIDOPSIS THALIANA MERISTEM LAYER 1 and PROTODERMAL FACTOR 2 – Transcription factors implicated in the initial radial cell separations of the immature embryo KEULE and KNOLLE – Synthaxin-related proteins required for complete formation of cell wall during cytokinesis SCARECRPW and SHORT ROOT – Transcription factors needed to establish cell fate within the root cells
of the auxin signal are the PLETHORA genes, which are initially expressed in the hypophysis and later in the quiescent cells of the embryonic root. Mutation of these genes, which encode APETALA-2 domain transcription factors, causes the improper division of the hypophyseal cell and the subsequent misspeci fication of the quiescent cells. The effects of PLETHORA in the formation of the embryonic root are exercised through its interaction with SCARECROW and SHORT ROOT. Another factor mediating the proper division of the hypophysis and the development of the quiescent cells is HOBBIT, homolog of a subunit of the anaphase-promoting complex.
Establishment of the Shoot Apical Meristem The formation of the shoot apical meristem at the base of the cotyledons is a key event during embryogenesis. According to Laux, the formation of the shoot meristem comprises three steps: (1) the specification of the apical domain, described in the previous section; (2) the specification of the stem cell niche; and (3) the separation of the central and peripheral domains. The initial marker for the specification of the stem cell niche is WUSCHEL, a transcription factor that delineates the organiz ing center of the meristem. Expression of WUSCHEL is first observed in the subapical cells of the 16-cell embryo and is subsequently restricted to the inner cells of the apical pole. WUSCHEL maintains the adjacent stem cells in an undeter mined fate, thereby ensuring the proper maintenance of the apical region. WUSCHEL is regulated through a feedback mechanism involving the CLAVATA(CLV) genes, which are expressed at the heart stage of embryo development. WUSCHEL activates the expression of CLV3, which encodes a ligand produced by the apical cells of the embryo. CLV3 binds to receptor complexes composed of CLV2 and CLV3 and induces a signal cascade resulting in the repression of WUSCHEL. This feedback regulation is critical during post embryonic growth for the coordination of cell division and differentiation within the apical meristem. A later marker for the formation of the shoot is SHOOTMERISTEMLESS, which is
expressed in mature embryos and acts as a positive regulator of the meristematic cells. A mutation in this gene results in the failure to form a shoot meristem and in a fusion of the cotyledons. Within the apical pole of the embryo, a demarcation between the central and peripheral domain ensures the proper localization of the apical pole and cotyledons. Formation of the cotyledons in the central domain is repressed by CUP-SHAPED COTYLEDON (CUC) 1, 2, and 3, transcription factors homologous to the petunia NO APICAL MERISTEM. CUC genes are expressed at the boundary between cotyledons and shoot apical meristem, and mutant plants display a fusion of cotyledons and a loss of the meristem. The accumulation of CUC transcripts is mediated by the activity of microRNA 164, which delimits and restricts the expression to the specified domain. However, the fact that no phenotypes are observed in mutants for the microRNA 164 genes suggest that the microRNA regulation on CUC is not relevant for the specifica tion of central and peripheral domains.
Establishing the Embryo Body Plan: Formation of the Radial Pattern Radial patterning of growth is responsible for the formation of the vascular tissue, cortical tissue, and epidermis. The first hint of radial pattern formation coincides with the separation of the protoderm from the inner cells. The expression pattern of ARABIDOPSIS THALIANA MERISTEM LAYER 1 and PROTODERMAL FACTOR 2, which encode transcription fac tors containing the START domain implicated in the binding of lipids and sterols; this expression pattern suggests that signal originating from the cell wall might be implicated in this first radial separation. Screening of mutants identified KEULE and KNOLLE as additional genes involved in radial patterning. Mutants in both genes are unable to form a radial axis. The KNOLLE gene encodes a protein related to the syntaxin family, the members of which are involved in secretory processes. A mutation in this gene results in abnormal cytokinesis due to the
Plant Embryogenesis, Genetics of incomplete formation of the cell wall separating the two daughter cells. Cell identity in the root is under the control of SHORT ROOT and SCARECROW. SHORT ROOT mutant plants do not form an endodermal layer, whereas SCARECROW plants only have a single file of cells instead of cortex and endodermis. Cells within this file, however, have both cortex and endoder mis specifications. Both genes encode transcription factors of the GRAS family. SHORT ROOT is transcribed in the vascular tissue; however, the protein moves in the endodermal layer where it transcribes SCARECROW. Overexpression of SHORT ROOT driven by the promoter of SCARECROW results in the formation of extra layers of cells within the root.
Future Prospects The generation and screening of mutant plants with abnormal embryonic development has been crucial over the past years to elucidate the molecular mechanisms governing embryogenesis. A large part of this work, however, has not been followed by a
345
careful characterization of the mutated gene in order to deter mine its role and interaction with other genes. The advancement of molecular and genomic techniques will be beneficial to further advance the field of plant embryogenesis.
See also: Arabidopsis thaliana: The Premier Model Plant; Photomorphogenesis in Plants, Genetics of; Plant Development, Genetics of; Plant Hormones.
Further Reading Barton MK (2010) Twenty years on: The inner workings of the shoot apical meristem, a development dynamo. Developmental Biology 341: 95–113. Capron A, Chatfield S, Provart N, and Berleth T (2009) Embryogenesis: Pattern formation from a single cell. The Arabidopsis Book. http://www.bioone.org/doi/full/10.1199/ tab.0126 Laux T, Wurschun T, and Breuninger H (2004) Genetic regulation of embryonic pattern formation. Plant Cell 16: S190–S202. Park S and Harada JJ (2009) Arabidopsis embryogenesis. In: Suarez MF and Bozhkov PV (eds.) Methods in Molecular Biology, p. 427. Clifton, NJ: Humana Press.
© 2013 Elsevier Inc. All rights reserved.
This article is a revision of the previous edition article by PN Benfey, volume 3, pp 1474–1477, © 2001, Elsevier Inc.
Glossary Hypophysis Uppermost cell of the suspensor in contact with the embryo proper. Meristem Undifferentiated group of cells able to provide all cell types within the plant body. Quiescent cells Cells within the root meristem that never divide during embryogenesis. A function of these cells is to
The Unique Nature of Plant Embryogenesis Like animals, the initiation of plant embryogenesis is demarked by the fusion of the haploid sperm and egg cell, resulting in the formation of a diploid zygote, which then undergoes a series of divisions to produce a fully developed embryo. The uniqueness of plant embryogenesis resides on three characteristics that have tremendous implications for morphogenesis. First, plant cells do not move. Unlike the body of animal embryos that is reshaped by the movement of cells, the final form of the plant embryo is only dictated by the rate and plane of cell division as well as by cell expansion. Second, the plant embryo is not the ‘miniature’ version of the adult plant since many cell types and organs are elaborated during postembryonic development. This is in contrast to ani mal embryogenesis where the mature embryo contains most of the organs and features of the adult body. Finally, the later phases of plant embryogenesis are characterized by a desicca tion period required to terminate the developmental program and initiate germination. The duration of desiccation varies from species to species, and it is also associated to profound changes in gene expression. Studies on plant embryo morphogenesis have mainly been conducted by screening and characterizing mutants of Arabidopsis thaliana, which is used as a model system in plant biology. Arabidopsis is a diploid plant with a small and com pletely sequenced genome, able to produce a large number of seeds (up to 10 000 per plant) in a short period of time (2–3 months). These characteristics have been critical for rapidly advancing knowledge on the genetics of plant embryogenesis.
Establishing the Embryo Body Plan: Formation of the Apical–Basal Axis The establishment of the apical–basal axis of the embryo is responsible for the positioning of cotyledons surrounding the shoot apical meristem at the apical (top) end of the embryo, the hypocotyl in the central domain, and a root meristem in the basal (bottom) region of the embryo (Table 1). This distribu tion of organs along the apical–basal axis can be traced back to the polar organization displayed by the egg and the asymmetric division of the zygote delineating an apical and a basal cell.
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 5
maintain the surrounding cells in an undifferentiated state. Suspensor Smaller terminal cells supporting the embryo within the seed sac. Zygotic Derived from the fusion of the gametes, that is, sperm and egg.
These cells already have a differential gene expression pattern with PINFORMED7 preferentially localized in the basal cells and WOX2 in the apical cell. Mutations of either gene result in an abnormal division of the zygote. The asymmetric distribu tion of PINFORMED 7, an auxin efflux carrier, is in line with the role played by this hormone as the vectorial signal for the establishment of the apical–basal axis. The apical domain of the embryo, comprising the cotyle dons and the shoot apical meristem, is specified by the expression of GURKE and TOPLESS. GURKE encodes an acetyl-CoA carboxylase and GURKE mutant plants fail to develop cotyledons and a functional apical meristem. A similar phenotype is also observed in TOPLESS mutant plants, which sometimes form a root in the apical region of the embryo. This observation suggests that TOPLESS might be involved in inhi biting the development in the basal patterning within the apical domain. The central embryonic domain comprising the hypocotyl is defined by FACKEL, HYDRA1, and CEPHALOPOD. A mutation of these genes produces embryos in which the hypocotyl is absent and the apical domain is in contact with the embryonic root. The three genes encode enzymes participating in the biosynthesis of sterols, thereby suggesting the involvement of these molecules in hypocotyl specification. A similar pheno type, but with the additional deletion of the basal elements, was also observed in MONOPTEROS mutant plants. The MONOPTEROS gene encodes a transcription factor of the auxin-responsive factor family, which is implicated in confer ring auxin responsiveness to downstream genes. Of interest, the formation of the vascular tissue, which requires auxin for proper development, is also affected in the MONOPTEROS mutants. The screening of additional mutants including AUXIN-RESISTANT 6 and BONDELOS, both of which affect the sensitivity of the tissue to auxin, argues for the participating of this hormone in the establishment of the central domain of the embryo. The basal domain of the embryo includes the root meris tem, which is composed of quiescent cells surrounded by the stem cells. The origin of the quiescent cells can be traced back to the hypophysis, the uppermost cell of the suspensor. During the early phases of embryogenesis, the hypophysis is the site of auxin accumulation mediated by PINFORMED 1, 4, and 7, which encode auxin efflux facilitators. Downstream elements
doi:10.1016/B978-0-12-374984-0.01168-2
343
344
Table 1
Plant Embryogenesis, Genetics of
Key gene products with their respective function
Formation of apical–basal axis PINFORM – Efflux translocator of auxin that provides positional information within the embryo body GURKE – Acetyl-CoA Carboxylase Required For Normal Development of the apical region TOPLESS – Involved in cell fate acquisition in the apical region of the embryo FACKEL, HYDRA1, and CEPHALOPOD – Involved in the establishment of the central domain of the embryo MONOPTEROS, AUXIN-RESISTANT 6, and BONDELOS – Transcription factors participating in auxin response and responsible for the formation of central and basal domains PLETHORA and HOBBIT – Required for the proper division of the hypophysis and specification of quiescent cells SCARECROW and SHORT ROOT – Transcription factors needed to establish cell fate within the root cells Formation of the shoot apical meristem WUSCHEL – Transcription factor required for specification of stem cell niche in the central domain of the apical meristem CLAVATA – Proteins involved in cell differentiation through interaction with WUSCHEL CUP-SHAPED COTYLEDONS – Transcription factors demarking the boundaries between apical meristem and lateral organs Formation of the radial pettern ARABIDOPSIS THALIANA MERISTEM LAYER 1 and PROTODERMAL FACTOR 2 – Transcription factors implicated in the initial radial cell separations of the immature embryo KEULE and KNOLLE – Synthaxin-related proteins required for complete formation of cell wall during cytokinesis SCARECRPW and SHORT ROOT – Transcription factors needed to establish cell fate within the root cells
of the auxin signal are the PLETHORA genes, which are initially expressed in the hypophysis and later in the quiescent cells of the embryonic root. Mutation of these genes, which encode APETALA-2 domain transcription factors, causes the improper division of the hypophyseal cell and the subsequent misspeci fication of the quiescent cells. The effects of PLETHORA in the formation of the embryonic root are exercised through its interaction with SCARECROW and SHORT ROOT. Another factor mediating the proper division of the hypophysis and the development of the quiescent cells is HOBBIT, homolog of a subunit of the anaphase-promoting complex.
Establishment of the Shoot Apical Meristem The formation of the shoot apical meristem at the base of the cotyledons is a key event during embryogenesis. According to Laux, the formation of the shoot meristem comprises three steps: (1) the specification of the apical domain, described in the previous section; (2) the specification of the stem cell niche; and (3) the separation of the central and peripheral domains. The initial marker for the specification of the stem cell niche is WUSCHEL, a transcription factor that delineates the organiz ing center of the meristem. Expression of WUSCHEL is first observed in the subapical cells of the 16-cell embryo and is subsequently restricted to the inner cells of the apical pole. WUSCHEL maintains the adjacent stem cells in an undeter mined fate, thereby ensuring the proper maintenance of the apical region. WUSCHEL is regulated through a feedback mechanism involving the CLAVATA(CLV) genes, which are expressed at the heart stage of embryo development. WUSCHEL activates the expression of CLV3, which encodes a ligand produced by the apical cells of the embryo. CLV3 binds to receptor complexes composed of CLV2 and CLV3 and induces a signal cascade resulting in the repression of WUSCHEL. This feedback regulation is critical during post embryonic growth for the coordination of cell division and differentiation within the apical meristem. A later marker for the formation of the shoot is SHOOTMERISTEMLESS, which is
expressed in mature embryos and acts as a positive regulator of the meristematic cells. A mutation in this gene results in the failure to form a shoot meristem and in a fusion of the cotyledons. Within the apical pole of the embryo, a demarcation between the central and peripheral domain ensures the proper localization of the apical pole and cotyledons. Formation of the cotyledons in the central domain is repressed by CUP-SHAPED COTYLEDON (CUC) 1, 2, and 3, transcription factors homologous to the petunia NO APICAL MERISTEM. CUC genes are expressed at the boundary between cotyledons and shoot apical meristem, and mutant plants display a fusion of cotyledons and a loss of the meristem. The accumulation of CUC transcripts is mediated by the activity of microRNA 164, which delimits and restricts the expression to the specified domain. However, the fact that no phenotypes are observed in mutants for the microRNA 164 genes suggest that the microRNA regulation on CUC is not relevant for the specifica tion of central and peripheral domains.
Establishing the Embryo Body Plan: Formation of the Radial Pattern Radial patterning of growth is responsible for the formation of the vascular tissue, cortical tissue, and epidermis. The first hint of radial pattern formation coincides with the separation of the protoderm from the inner cells. The expression pattern of ARABIDOPSIS THALIANA MERISTEM LAYER 1 and PROTODERMAL FACTOR 2, which encode transcription fac tors containing the START domain implicated in the binding of lipids and sterols; this expression pattern suggests that signal originating from the cell wall might be implicated in this first radial separation. Screening of mutants identified KEULE and KNOLLE as additional genes involved in radial patterning. Mutants in both genes are unable to form a radial axis. The KNOLLE gene encodes a protein related to the syntaxin family, the members of which are involved in secretory processes. A mutation in this gene results in abnormal cytokinesis due to the
Plant Embryogenesis, Genetics of incomplete formation of the cell wall separating the two daughter cells. Cell identity in the root is under the control of SHORT ROOT and SCARECROW. SHORT ROOT mutant plants do not form an endodermal layer, whereas SCARECROW plants only have a single file of cells instead of cortex and endodermis. Cells within this file, however, have both cortex and endoder mis specifications. Both genes encode transcription factors of the GRAS family. SHORT ROOT is transcribed in the vascular tissue; however, the protein moves in the endodermal layer where it transcribes SCARECROW. Overexpression of SHORT ROOT driven by the promoter of SCARECROW results in the formation of extra layers of cells within the root.
Future Prospects The generation and screening of mutant plants with abnormal embryonic development has been crucial over the past years to elucidate the molecular mechanisms governing embryogenesis. A large part of this work, however, has not been followed by a
345
careful characterization of the mutated gene in order to deter mine its role and interaction with other genes. The advancement of molecular and genomic techniques will be beneficial to further advance the field of plant embryogenesis.
See also: Arabidopsis thaliana: The Premier Model Plant; Photomorphogenesis in Plants, Genetics of; Plant Development, Genetics of; Plant Hormones.
Further Reading Barton MK (2010) Twenty years on: The inner workings of the shoot apical meristem, a development dynamo. Developmental Biology 341: 95–113. Capron A, Chatfield S, Provart N, and Berleth T (2009) Embryogenesis: Pattern formation from a single cell. The Arabidopsis Book. http://www.bioone.org/doi/full/10.1199/ tab.0126 Laux T, Wurschun T, and Breuninger H (2004) Genetic regulation of embryonic pattern formation. Plant Cell 16: S190–S202. Park S and Harada JJ (2009) Arabidopsis embryogenesis. In: Suarez MF and Bozhkov PV (eds.) Methods in Molecular Biology, p. 427. Clifton, NJ: Humana Press.