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Abstract
The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.
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Affiliation(s)
- Tracey A Ruhlman
- Integrative Biology, University of Texas at Austin, Austin, TX, USA.
| | - Robert K Jansen
- Integrative Biology, University of Texas at Austin, Austin, TX, USA
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2
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Greiner S, Golczyk H, Malinova I, Pellizzer T, Bock R, Börner T, Herrmann RG. Chloroplast nucleoids are highly dynamic in ploidy, number, and structure during angiosperm leaf development. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2020; 102:730-746. [PMID: 31856320 DOI: 10.1111/tpj.14658] [Citation(s) in RCA: 28] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/26/2019] [Revised: 11/20/2019] [Accepted: 12/10/2019] [Indexed: 05/19/2023]
Abstract
Chloroplast nucleoids are large, compact nucleoprotein structures containing multiple copies of the plastid genome. Studies on structural and quantitative changes of plastid DNA (ptDNA) during leaf development are scarce and have produced controversial data. We have systematically investigated nucleoid dynamics and ptDNA quantities in the mesophyll of Arabidopsis, tobacco, sugar beet, and maize from the early post-meristematic stage until necrosis. DNA of individual nucleoids was quantified by DAPI-based supersensitive epifluorescence microscopy. Nucleoids occurred in scattered, stacked, or ring-shaped arrangements and in recurring patterns during leaf development that was remarkably similar between the species studied. Nucleoids per organelle varied from a few in meristematic plastids to >30 in mature chloroplasts (corresponding to about 20-750 nucleoids per cell). Nucleoid ploidies ranged from haploid to >20-fold even within individual organelles, with average values between 2.6-fold and 6.7-fold and little changes during leaf development. DNA quantities per organelle increased gradually from about a dozen plastome copies in tiny plastids of apex cells to 70-130 copies in chloroplasts of about 7 μm diameter in mature mesophyll tissue, and from about 80 plastome copies in meristematic cells to 2600-3300 copies in mature diploid mesophyll cells without conspicuous decline during leaf development. Pulsed-field electrophoresis, restriction of high-molecular-weight DNA from chloroplasts and gerontoplasts, and CsCl equilibrium centrifugation of single-stranded and double-stranded ptDNA revealed no noticeable fragmentation of the organelle DNA during leaf development, implying that plastid genomes in mesophyll tissues are remarkably stable until senescence.
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Affiliation(s)
- Stephan Greiner
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476, Potsdam-Golm, Germany
| | - Hieronim Golczyk
- Department of Molecular Biology, Institute of Biotechnology, John Paul II Catholic University of Lublin, Konstantynów 1i, 20-708, Lublin, Poland
| | - Irina Malinova
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476, Potsdam-Golm, Germany
| | - Tommaso Pellizzer
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476, Potsdam-Golm, Germany
| | - Ralph Bock
- Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, D-14476, Potsdam-Golm, Germany
| | - Thomas Börner
- Institut für Biologie/Molekulare Genetik, Humboldt-Universität zu Berlin, Rhoda Erdmann Haus, Philippstr. 13, D-10115, Berlin, Germany
| | - Reinhold G Herrmann
- Department für Biologie I, Ludwig-Maximilians-Universität München, Bereich Botanik, Menzinger Str. 67, D-80638, Munich, Germany
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Shen J, Shou W, Zhang Y, Yuan G, Zhao Y, Chen J, Havey MJ. Rare maternal and biparental transmission of the cucumber mitochondrial DNA reveals sorting of polymorphisms among progenies. TAG. THEORETICAL AND APPLIED GENETICS. THEORETISCHE UND ANGEWANDTE GENETIK 2019; 132:1223-1233. [PMID: 30758532 DOI: 10.1007/s00122-018-03274-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/11/2018] [Accepted: 12/22/2018] [Indexed: 05/28/2023]
Abstract
We used a mitochondrial (mt) mutant of cucumber to document rare maternal transmission of mt polymorphisms and demonstrate that polymorphisms can become more prevalent and sort to progenies to increase mt DNA diversity. The mitochondrial (mt) DNAs of most angiosperms show maternal inheritance, although relatively rare biparental or paternal transmission has been documented. The mt DNAs of plants in the genus Cucumis (family Cucurbitaceae) are paternally transmitted in intra- and interspecific crosses. MSC16 is an inbred line of cucumber (Cucumis sativus) with a mitochondrially associated mosaic (MSC) phenotype. MSC16 was crossed as the male parent to wild-type cultivar Calypso, and hybrid progenies were evaluated for the wild-type phenotype in order to screen for rare maternal or biparental transmission of the mt DNA. We then used standard and droplet digital (dd) PCR to study the transmission of polymorphic mt markers across three generations. We observed evidence for occasional maternal and biparental transmission of the mt DNA in cucumber. The transmission of specific regions of the maternal mt DNA could be as high as 17.8%, although the amounts of these maternal regions were often much lower relative to paternally transmitted regions. Different combinations of maternal and paternal mt polymorphisms were detected in progenies across generations, indicating that relatively rare maternal regions can be transmitted to progenies and become predominant to increase mt DNA diversity over generations.
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Affiliation(s)
- Jia Shen
- Institute of Vegetables, Zhejiang Academy of Agricultural Sciences, Hangzhou, 310021, China
- College of Horticulture, Nanjing Agricultural University, Nanjing, 210095, China
| | - Weisong Shou
- Institute of Vegetables, Zhejiang Academy of Agricultural Sciences, Hangzhou, 310021, China
| | - Yuejian Zhang
- Institute of Vegetables, Zhejiang Academy of Agricultural Sciences, Hangzhou, 310021, China
| | - Gaoya Yuan
- College of Horticulture, Nanjing Agricultural University, Nanjing, 210095, China
| | - Yu Zhao
- College of Horticulture, Nanjing Agricultural University, Nanjing, 210095, China
| | - Jinfeng Chen
- College of Horticulture, Nanjing Agricultural University, Nanjing, 210095, China
| | - Michael J Havey
- USDA-ARS and Department of Horticulture, University of Wisconsin, Madison, WI, 53706, USA.
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Shen J, Zhao J, Bartoszewski G, Malepszy S, Havey M, Chen J. Persistence and Protection of Mitochondrial DNA in the Generative Cell of Cucumber is Consistent with its Paternal Transmission. PLANT & CELL PHYSIOLOGY 2015; 56:2271-82. [PMID: 26412781 DOI: 10.1093/pcp/pcv140] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/16/2015] [Accepted: 09/18/2015] [Indexed: 05/25/2023]
Abstract
Plants predominantly show maternal transmission of mitochondrial DNA (mtDNA). One known exception is cucumber, in which the mtDNA is paternally inherited. However, the mechanisms regulating this unique mode of transmission are unclear. Here we monitored the amounts of mtDNA throughout the development of cucumber microspores into pollen and observed that mtDNA decreases in the vegetative cell, but persists in the generative cell that ultimately produces the sperm cells. We characterized the cucumber homolog (CsDPD1) of the Arabidopsis gene defective in pollen organelle DNA degradation 1 (AtDPD1), which plays a direct role in mtDNA degradation. CsDPD1 rescued an Arabidopsis AtDPD1 mutant, indicating the same function in both plants. Expression of CsDPD1 coincided with the decrease of mtDNA in pollen, except in the generative cell where both the expression of CsDPD1 and mtDNA levels remained high. Our cytological results confirmed that the persistence of mtDNA in the cucumber generative cell is consistent with its paternal transmission. Our molecular analyses suggest that protection of mtDNA in the generative cell may be the critical factor for paternal mtDNA transmission, rather than mtDNA degradation mediated by CsDPD1. Taken together, these findings indicate that a mechanism may protect paternal mtDNA from degradation and is likely to be the genetic basis of paternal mtDNA transmission.
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Affiliation(s)
- Jia Shen
- State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, PR China Department of Horticulture, University of Wisconsin, Madison, WI 53706, USA
| | - Juan Zhao
- State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, PR China
| | - Grzegorz Bartoszewski
- Department of Plant Genetics, Breeding and Biotechnology, Faculty of Horticulture, Biotechnology and Lands-ape Architecture, Warsaw University of Life Sciences, ul. Nowoursynowska 159, 02-776 Warsaw, Poland
| | - Stefan Malepszy
- Department of Plant Genetics, Breeding and Biotechnology, Faculty of Horticulture, Biotechnology and Lands-ape Architecture, Warsaw University of Life Sciences, ul. Nowoursynowska 159, 02-776 Warsaw, Poland
| | - Michael Havey
- USDA-ARS and Department of Horticulture, University of Wisconsin, Madison, WI 53706, USA
| | - Jinfeng Chen
- State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, PR China
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Kumar RA, Oldenburg DJ, Bendich AJ. Changes in DNA damage, molecular integrity, and copy number for plastid DNA and mitochondrial DNA during maize development. JOURNAL OF EXPERIMENTAL BOTANY 2014; 65:6425-39. [PMID: 25261192 PMCID: PMC4246179 DOI: 10.1093/jxb/eru359] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
The amount and structural integrity of organellar DNAs change during plant development, although the mechanisms of change are poorly understood. Using PCR-based methods, we quantified DNA damage, molecular integrity, and genome copy number for plastid and mitochondrial DNAs of maize seedlings. A DNA repair assay was also used to assess DNA impediments. During development, DNA damage increased and molecules with impediments that prevented amplification by Taq DNA polymerase increased, with light causing the greatest change. DNA copy number values depended on the assay method, with standard real-time quantitative PCR (qPCR) values exceeding those determined by long-PCR by 100- to 1000-fold. As the organelles develop, their DNAs may be damaged in oxidative environments created by photo-oxidative reactions and photosynthetic/respiratory electron transfer. Some molecules may be repaired, while molecules with unrepaired damage may be degraded to non-functional fragments measured by standard qPCR but not by long-PCR.
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Affiliation(s)
- Rachana A Kumar
- Department of Biology, University of Washington, Seattle, WA 98195-5325, USA
| | - Delene J Oldenburg
- Department of Biology, University of Washington, Seattle, WA 98195-5325, USA
| | - Arnold J Bendich
- Department of Biology, University of Washington, Seattle, WA 98195-5325, USA
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Candaele J, Demuynck K, Mosoti D, Beemster GT, Inzé D, Nelissen H. Differential methylation during maize leaf growth targets developmentally regulated genes. PLANT PHYSIOLOGY 2014; 164:1350-64. [PMID: 24488968 PMCID: PMC3938625 DOI: 10.1104/pp.113.233312] [Citation(s) in RCA: 61] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2013] [Accepted: 01/28/2014] [Indexed: 05/20/2023]
Abstract
DNA methylation is an important and widespread epigenetic modification in plant genomes, mediated by DNA methyltransferases (DMTs). DNA methylation is known to play a role in genome protection, regulation of gene expression, and splicing and was previously associated with major developmental reprogramming in plants, such as vernalization and transition to flowering. Here, we show that DNA methylation also controls the growth processes of cell division and cell expansion within a growing organ. The maize (Zea mays) leaf offers a great tool to study growth processes, as the cells progressively move through the spatial gradient encompassing the division zone, transition zone, elongation zone, and mature zone. Opposite to de novo DMTs, the maintenance DMTs were transcriptionally regulated throughout the growth zone of the maize leaf, concomitant with differential CCGG methylation levels in the four zones. Surprisingly, the majority of differentially methylated sequences mapped on or close to gene bodies and not to repeat-rich loci. Moreover, especially the 5' and 3' regions of genes, which show overall low methylation levels, underwent differential methylation in a developmental context. Genes involved in processes such as chromatin remodeling, cell cycle progression, and growth regulation, were differentially methylated. The presence of differential methylation located upstream of the gene anticorrelated with transcript expression, while gene body differential methylation was unrelated to the expression level. These data indicate that DNA methylation is correlated with the decision to exit mitotic cell division and to enter cell expansion, which adds a new epigenetic level to the regulation of growth processes.
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Golczyk H, Greiner S, Wanner G, Weihe A, Bock R, Börner T, Herrmann RG. Chloroplast DNA in mature and senescing leaves: a reappraisal. THE PLANT CELL 2014; 26:847-54. [PMID: 24668747 PMCID: PMC4001396 DOI: 10.1105/tpc.113.117465] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2013] [Revised: 10/07/2013] [Accepted: 03/11/2014] [Indexed: 05/04/2023]
Abstract
The fate of plastid DNA (ptDNA) during leaf development has become a matter of contention. Reports on little change in ptDNA copy number per cell contrast with claims of complete or nearly complete DNA loss already in mature leaves. We employed high-resolution fluorescence microscopy, transmission electron microscopy, semithin sectioning of leaf tissue, and real-time quantitative PCR to study structural and quantitative aspects of ptDNA during leaf development in four higher plant species (Arabidopsis thaliana, sugar beet [Beta vulgaris], tobacco [Nicotiana tabacum], and maize [Zea mays]) for which controversial findings have been reported. Our data demonstrate the retention of substantial amounts of ptDNA in mesophyll cells until leaf necrosis. In ageing and senescent leaves of Arabidopsis, tobacco, and maize, ptDNA amounts remain largely unchanged and nucleoids visible, in spite of marked structural changes during chloroplast-to-gerontoplast transition. This excludes the possibility that ptDNA degradation triggers senescence. In senescent sugar beet leaves, reduction of ptDNA per cell to ∼30% was observed reflecting primarily a decrease in plastid number per cell rather than a decline in DNA per organelle, as reported previously. Our findings are at variance with reports claiming loss of ptDNA at or after leaf maturation.
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Affiliation(s)
- Hieronim Golczyk
- Department of Molecular Biology, Institute of
Biotechnology, John Paul II Catholic University of Lublin, 20-708 Lublin,
Poland
| | - Stephan Greiner
- Max-Planck-Institut für Molekulare
Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Gerhard Wanner
- Department für Biologie I, Bereich Botanik,
Biozentrum der Ludwig-Maximilians–Universität München, D-82152
Planegg-Martinsried, Germany
| | - Andreas Weihe
- Institut für Biologie/Genetik,
Humboldt-Universität zu Berlin, D-10115 Berlin, Germany
| | - Ralph Bock
- Max-Planck-Institut für Molekulare
Pflanzenphysiologie, D-14476 Potsdam-Golm, Germany
| | - Thomas Börner
- Institut für Biologie/Genetik,
Humboldt-Universität zu Berlin, D-10115 Berlin, Germany
| | - Reinhold G. Herrmann
- Department für Biologie I, Bereich Botanik,
Biozentrum der Ludwig-Maximilians–Universität München, D-82152
Planegg-Martinsried, Germany
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8
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Abstract
The plastid genome (plastome) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations are allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and tools of plastid genetic engineering.
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Affiliation(s)
- Tracey A Ruhlman
- Integrative Biology, University of Texas at Austin, Austin, TX, USA
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Cupp JD, Nielsen BL. Arabidopsis thaliana organellar DNA polymerase IB mutants exhibit reduced mtDNA levels with a decrease in mitochondrial area density. PHYSIOLOGIA PLANTARUM 2013; 149:91-103. [PMID: 23167278 DOI: 10.1111/ppl.12009] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2012] [Revised: 10/31/2012] [Accepted: 10/31/2012] [Indexed: 05/04/2023]
Abstract
Plant organelle genomes are complex and the mechanisms for their replication and maintenance remain unclear. Arabidopsis thaliana has two DNA polymerase genes, DNA polymerase IA (polIA) and polIB, that are dual targeted to mitochondria and chloroplasts and are differentially expressed in primary plant tissues. PolIB gene expression occurs at higher levels in tissues not primary for photosynthesis. Arabidopsis T-DNA polIB mutants have a 30% reduction in relative mitochondrial DNA (mtDNA) levels, but also exhibit a 70% increase in polIA gene expression. The polIB mutant shows an increase in mitochondrial numbers but a significant decrease in mitochondrial area density within the hypocotyl epidermis, shoot apex and root tips. Chloroplast numbers are not significantly different in mesophyll protoplasts. These mutants do not have a significant difference in total dark mitorespiration levels but exhibit a difference in light respiration levels and photosynthesis capacity. Organelle-encoded genes for components of respiration and photosynthesis are upregulated in polIB mutants. The mutants exhibited slow growth in conjunction with a decreased rate of cell expansion and other secondary phenotypic effects. Evidence suggests that early plastid development and DNA levels are directly affected by a polIB mutation but are resolved to wild-type levels over time. However, mitochondria numbers and DNA levels never reach wild-type levels in the polIB mutant. We propose that both polIA and polIB are required for mtDNA replication. The results suggest that polIB mutants undergo an adjustment in cell homeostasis, enabling them to maintain functional mitochondria at the cost of normal cell expansion and plant growth.
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Affiliation(s)
- John D Cupp
- Department of Microbiology and Molecular Biology, Brigham Young University, Provo, UT 84602, USA
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Oldenburg DJ, Kumar RA, Bendich AJ. The amount and integrity of mtDNA in maize decline with development. PLANTA 2013; 237:603-17. [PMID: 23229060 DOI: 10.1007/s00425-012-1802-z] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2012] [Accepted: 10/26/2012] [Indexed: 05/10/2023]
Abstract
In maize and other grasses there is a developmental gradient from the meristematic cells at the base of the stalk to the differentiated cells at the leaf tip. This gradient presents an opportunity to investigate changes in mitochondrial DNA (mtDNA) that accompany growth under light and dark conditions, as done previously for plastid DNA. Maize mtDNA was analyzed by DAPI-DNA staining of individual mitochondria, gel electrophoresis/blot hybridization, and real-time qPCR. Both the amount and integrity of the mtDNA were found to decline with development. There was a 20-fold decline in mtDNA copy number per cell from the embryo to the light-grown leaf blade. The amount of DNA per mitochondrial particle was greater in dark-grown leaf blade (24 copies, on average) than in the light (2 copies), with some mitochondria lacking any detectable DNA. Three factors that influence the demise of mtDNA during development are considered: (1) the decision to either repair or degrade mtDNA molecules that are damaged by the reactive oxygen species produced as byproducts of respiration; (2) the generation of ATP by photophosphorylation in chloroplasts, reducing the need for respiratory-competent mitochondria; and (3) the shift in mitochondrial function from energy-generating respiration to photorespiration during the transition from non-green to green tissue.
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Affiliation(s)
- Delene J Oldenburg
- Department of Biology, University of Washington, Seattle, WA 98195-5325, USA.
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Udy DB, Belcher S, Williams-Carrier R, Gualberto JM, Barkan A. Effects of reduced chloroplast gene copy number on chloroplast gene expression in maize. PLANT PHYSIOLOGY 2012; 160:1420-31. [PMID: 22977281 PMCID: PMC3490597 DOI: 10.1104/pp.112.204198] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2012] [Accepted: 09/12/2012] [Indexed: 05/18/2023]
Abstract
Chloroplasts and other members of the plastid organelle family contain a small genome of bacterial ancestry. Young chloroplasts contain hundreds of genome copies, but the functional significance of this high genome copy number has been unclear. We describe molecular phenotypes associated with mutations in a nuclear gene in maize (Zea mays), white2 (w2), encoding a predicted organellar DNA polymerase. Weak and strong mutant alleles cause a moderate (approximately 5-fold) and severe (approximately 100-fold) decrease in plastid DNA copy number, respectively, as assayed by quantitative PCR and Southern-blot hybridization of leaf DNA. Both alleles condition a decrease in most chloroplast RNAs, with the magnitude of the RNA deficiencies roughly paralleling that of the DNA deficiency. However, some RNAs are more sensitive to a decrease in genome copy number than others. The rpoB messenger RNA (mRNA) exhibited a unique response, accumulating to dramatically elevated levels in response to a moderate reduction in plastid DNA. Subunits of photosynthetic enzyme complexes were reduced more severely than were plastid mRNAs, possibly because of impaired translation resulting from limiting ribosomal RNA, transfer RNA, and ribosomal protein mRNA. These results indicate that chloroplast genome copy number is a limiting factor for the expression of a subset of chloroplast genes in maize. Whereas in Arabidopsis (Arabidopsis thaliana) a pair of orthologous genes function redundantly to catalyze DNA replication in both mitochondria and chloroplasts, the w2 gene is responsible for virtually all chloroplast DNA replication in maize. Mitochondrial DNA copy number was reduced approximately 2-fold in mutants harboring strong w2 alleles, suggesting that w2 also contributes to mitochondrial DNA replication.
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Kayal E, Bentlage B, Collins AG, Kayal M, Pirro S, Lavrov DV. Evolution of linear mitochondrial genomes in medusozoan cnidarians. Genome Biol Evol 2011; 4:1-12. [PMID: 22113796 PMCID: PMC3267393 DOI: 10.1093/gbe/evr123] [Citation(s) in RCA: 101] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
In nearly all animals, mitochondrial DNA (mtDNA) consists of a single circular molecule that encodes several subunits of the protein complexes involved in oxidative phosphorylation as well as part of the machinery for their expression. By contrast, mtDNA in species belonging to Medusozoa (one of the two major lineages in the phylum Cnidaria) comprises one to several linear molecules. Many questions remain on the ubiquity of linear mtDNA in medusozoans and the mechanisms responsible for its evolution, replication, and transcription. To address some of these questions, we determined the sequences of nearly complete linear mtDNA from 24 species representing all four medusozoan classes: Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa. All newly determined medusozoan mitochondrial genomes harbor the 17 genes typical for cnidarians and map as linear molecules with a high degree of gene order conservation relative to the anthozoans. In addition, two open reading frames (ORFs), polB and ORF314, are identified in cubozoan, schyphozoan, staurozoan, and trachyline hydrozoan mtDNA. polB belongs to the B-type DNA polymerase gene family, while the product of ORF314 may act as a terminal protein that binds telomeres. We posit that these two ORFs are remnants of a linear plasmid that invaded the mitochondrial genomes of the last common ancestor of Medusozoa and are responsible for its linearity. Hydroidolinan hydrozoans have lost the two ORFs and instead have duplicated cox1 at each end of their mitochondrial chromosome(s). Fragmentation of mtDNA occurred independently in Cubozoa and Hydridae (Hydrozoa, Hydroidolina). Our broad sampling allows us to reconstruct the evolutionary history of linear mtDNA in medusozoans.
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Affiliation(s)
- Ehsan Kayal
- Department of Ecology, Evolution, and Organismal Biology, Iowa State University, USA.
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