Anna M G Koltunow

The Commonwealth Scientific and Industrial Research Organisation, Canberra, Australian Capital Territory, Australia

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Publications (29)178.66 Total impact

  • Melanie L Hand, Anna M G Koltunow
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    ABSTRACT: Apomixis (asexual seed formation) is the result of a plant gaining the ability to bypass the most fundamental aspects of sexual reproduction: meiosis and fertilization. Without the need for male fertilization, the resulting seed germinates a plant that develops as a maternal clone. This dramatic shift in reproductive process has been documented in many flowering plant species, although no major seed crops have been shown to be capable of apomixis. The ability to generate maternal clones and therefore rapidly fix desirable genotypes in crop species could accelerate agricultural breeding strategies. The potential of apomixis as a next-generation breeding technology has contributed to increasing interest in the mechanisms controlling apomixis. In this review, we discuss the progress made toward understanding the genetic and molecular control of apomixis. Research is currently focused on two fronts. One aims to identify and characterize genes causing apomixis in apomictic species that have been developed as model species. The other aims to engineer or switch the sexual seed formation pathway in non-apomictic species, to one that mimics apomixis. Here we describe the major apomictic mechanisms and update knowledge concerning the loci that control them, in addition to presenting candidate genes that may be used as tools for switching the sexual pathway to an apomictic mode of reproduction in crops.
    Genetics 06/2014; 197(2):441-450. · 4.39 Impact Factor
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    ABSTRACT: Over 200 imprinted genes in rice endosperm are known, but the mechanisms modulating their parental allele-specific expression are poorly understood. Here we use three imprinted genes, OsYUCCA11, yellow2-like and ubiquitin hydrolase, to show that differential DNA methylation and trimethylation of histone H3 lysine 27 (H3K27me3) in the promoter and/or gene body influences allele-specific expression or the site of transcript initiation. Paternal expression of OsYUCCA11 required DNA methylation in the gene body whereas the gene body of the silenced maternal allele was hypomethylated and marked with H3K27me3. These differential markings mirror those proposed to modulate paternal expression of two Arabidopsis genes, PHERES1 and a YUCCA homolog, indicating conservation of imprinting mechanisms. At yellow2-like, DNA hypomethylation in the upstream flanking region resulted in maternal transcripts that were longer than paternal transcripts; the maternal transcript initiation site was marked by DNA methylation in the paternal allele, and transcription initiated ~700bp downstream. The paternal allele of an ubiquitin hydrolase gene exhibited gene body DNA methylation and produced full-length transcripts, while the maternal allele was hypomethylated in the 5′ gene body and transcripts initiated from a downstream promoter. Inhibition of DNA methylation by 5-azacytidine or zebularine activated the long transcripts from yellow2-like and enhanced expression of the short transcripts from the ubiquitin hydrolase in seedlings, indicating that DNA methylation prevents transcript initiation from cryptic promoters. These observations suggest a paradigm whereby maternal genome hypomethylation is associated with the production of distinct transcripts, potentially diversifying the gene products from the two alleles.This article is protected by copyright. All rights reserved.
    The Plant Journal 05/2014; · 6.58 Impact Factor
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    ABSTRACT: Apomixis or asexual seed formation in Hieracium praealtum (Asteraceae) is controlled by two independent dominant loci. One of these, the LOSS OF APOMEIOSIS (LOA) locus, controls apomixis initiation, mitotic embryo sac formation (apospory) and suppression of the sexual pathway. The LOA locus is found near the end of a hemizygous chromosome surrounded by extensive repeats extending along the chromosome arm. Similar apomixis-carrying chromosome structures have been found in some apomictic grasses, suggesting that the extensive repetitive sequences may be functionally relevant to apomixis. Fluorescence in situ hybridization (FISH) was used to examine chromosomes of apomeiosis deletion mutants and rare recombinants in the critical LOA region arising from a cross between sexual Hieracium pilosella and apomictic H. praealtum. The combined analyses of aposporous and nonaposporous recombinant progeny and chromosomal karyotypes were used to determine that the functional LOA locus can be genetically separated from the very extensive repeat regions found on the LOA-carrying chromosome. The large-scale repetitive sequences associated with the LOA locus in H. praealtum are not essential for apospory or suppression of sexual megasporogenesis (female meiosis).
    New Phytologist 02/2014; 201(3):973-981. · 6.74 Impact Factor
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    ABSTRACT: In apomictic Hieracium subgenus Pilosella species, embryo sacs develop in ovules without meiosis. Embryo and endosperm formation then occur without fertilization, producing seeds with a maternal genotype encased in a fruit (achene). Genetic analyses in H. praealtum indicate a dominant locus (LOA) controls meiotic avoidance, and another dominant locus (LOP) controls both fertilization-independent embryogenesis and endosperm formation. While cytologically examining developmental events in ovules of progeny from crosses between different wild-type and mutant Hieracium apomicts, and a sexual Hieracium species, we identified two plants, AutE196 and AutE24, which have lost the capacity for meiotic avoidance and fertilization-independent embryo formation. AutE196 and AutE24 exhibit autonomous endosperm formation and set parthenocarpic, seedless achenes at a penetrance of 18 %. Viable seed form after pollination. Cytological examination of 102 progeny from a backcross of AutE196 with sexual H. pilosella showed that autonomous endosperm formation is a heritable, dominant, qualitative trait, detected in 51 % of progeny. Variation in quantitative trait penetrance indicates other factors influence its expression. The correlation between autonomous endosperm development and mature parthenocarpic achene formation suggests the former is sufficient to trigger fruit maturation in Hieracium. The developmental component of autonomous endosperm formation is therefore genetically separable from those controlling meiotic avoidance and autonomous embryogenesis in Hieracium and has been denoted as AutE. We postulate that tight linkage of AutE and genes controlling autonomous embryogenesis at the LOP locus in H. praealtum may explain why inheritance of autonomous seed formation is typically observed as a single component.
    Plant reproduction. 03/2013;
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    ABSTRACT: Female gamete development in Arabidopsis ovules comprises two phases. During megasporogenesis, a somatic ovule cell differentiates into a megaspore mother cell and undergoes meiosis to produce four haploid megaspores, three of which degrade. The surviving functional megaspore participates in megagametogenesis, undergoing syncytial mitosis and cellular differentiation to produce a multicellular female gametophyte containing the egg and central cell, progenitors of the embryo and endosperm of the seed. The transition between megasporogenesis and megagametogenesis is poorly characterised, partly owing to the inaccessibility of reproductive cells within the ovule. Here, laser capture microdissection was used to identify genes expressed in and/or around developing megaspores during the transition to megagametogenesis. ARGONAUTE5 (AGO5), a putative effector of small RNA (sRNA) silencing pathways, was found to be expressed around reproductive cells during megasporogenesis, and a novel semi-dominant ago5-4 insertion allele showed defects in the initiation of megagametogenesis. Expression of a viral RNAi suppressor, P1/Hc-Pro, driven by the WUSCHEL and AGO5 promoters in somatic cells flanking the megaspores resulted in a similar phenotype. This indicates that sRNA-dependent pathways acting in somatic ovule tissues promote the initiation of megagametogenesis in the functional megaspore. Notably, these pathways are independent of AGO9, which functions in somatic epidermal ovule cells to inhibit the formation of multiple megaspore-like cells. Therefore, one somatic sRNA pathway involving AGO9 restricts reproductive development to the functional megaspore and a second pathway, inhibited by ago5-4 and P1/Hc-Pro, promotes megagametogenesis.
    Development 03/2012; 139(8):1399-404. · 6.60 Impact Factor
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    ABSTRACT: Apomixis in Hieracium subgenus Pilosella initiates in ovules when sporophytic cells termed aposporous initial (AI) cells enlarge near sexual cells undergoing meiosis. AI cells displace the sexual structures and divide by mitosis to form unreduced embryo sac(s) without meiosis (apomeiosis) that initiate fertilization-independent embryo and endosperm development. In some Hieracium subgenus Pilosella species, these events are controlled by the dominant LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP) loci. In H. praealtum and H. piloselloides, which both contain the same core LOA locus, the timing and frequency of AI cell formation is altered in derived mutants exhibiting abnormal funiculus growth and in transgenic plants expressing rolB which alters cellular sensitivity to auxin. The impact on apomictic and sexual reproduction was examined here when a chimeric RNAse gene was targeted to the funiculus and basal portions of the ovule, and also when polar auxin transport was inhibited during ovule development following N-1-naphthylphthalamic acid (NPA) application. Both treatments led to ovule deformity in the funiculus and distal parts of the ovule and LOA-dependent alterations in the timing, position, and frequency of AI cell formation. In the case of NPA treatment, this correlated with increased expression of DR5:GFP in the ovule, which marks the accumulation of the plant hormone auxin. Our results show that sporophytic information potentiated by funiculus growth and polar auxin transport influences ovule development, the initiation of apomixis, and the progression of embryo sac development in Hieracium. Signals associated with ovule pattern formation and auxin distribution or perception may influence the capacity of sporophytic ovule cells to respond to LOA.
    Journal of Experimental Botany 02/2012; 63(8):3229-41. · 5.79 Impact Factor
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    ABSTRACT: The LOSS OF APOMEIOSIS (LOA) locus is one of two dominant loci known to control apomixis in the eudicot Hieracium praealtum. LOA stimulates the differentiation of somatic aposporous initial cells after the initiation of meiosis in ovules. Aposporous initial cells undergo nuclear proliferation close to sexual megaspores, forming unreduced aposporous embryo sacs, and the sexual program ceases. LOA-linked genetic markers were used to isolate 1.2 Mb of LOA-associated DNAs from H. praealtum. Physical mapping defined the genomic region essential for LOA function between two markers, flanking 400 kb of identified sequence and central unknown sequences. Cytogenetic and sequence analyses revealed that the LOA locus is located on a single chromosome near the tip of the long arm and surrounded by extensive, abundant complex repeat and transposon sequences. Chromosomal features and LOA-linked markers are conserved in aposporous Hieracium caespitosum and Hieracium piloselloides but absent in sexual Hieracium pilosella. Their absence in apomictic Hieracium aurantiacum suggests that meiotic avoidance may have evolved independently in aposporous subgenus Pilosella species. The structure of the hemizygous chromosomal region containing the LOA locus in the three Hieracium subgenus Pilosella species resembles that of the hemizygous apospory-specific genomic regions in monocot Pennisetum squamulatum and Cenchrus ciliaris. Analyses of partial DNA sequences at these loci show no obvious conservation, indicating that they are unlikely to share a common ancestral origin. This suggests convergent evolution of repeat-rich hemizygous chromosomal regions containing apospory loci in these monocot and eudicot species, which may be required for the function and maintenance of the trait.
    Plant physiology 09/2011; 157(3):1327-41. · 6.56 Impact Factor
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    ABSTRACT: Genomic imprinting causes the expression of an allele depending on its parental origin. In plants, most imprinted genes have been identified in Arabidopsis endosperm, a transient structure consumed by the embryo during seed formation. We identified imprinted genes in rice seed where both the endosperm and embryo are present at seed maturity. RNA was extracted from embryos and endosperm of seeds obtained from reciprocal crosses between two subspecies Nipponbare (Japonica rice) and 93-11 (Indica rice). Sequenced reads from cDNA libraries were aligned to their respective parental genomes using single-nucleotide polymorphisms (SNPs). Reads across SNPs enabled derivation of parental expression bias ratios. A continuum of parental expression bias states was observed. Statistical analyses indicated 262 candidate imprinted loci in the endosperm and three in the embryo (168 genic and 97 non-genic). Fifty-six of the 67 loci investigated were confirmed to be imprinted in the seed. Imprinted loci are not clustered in the rice genome as found in mammals. All of these imprinted loci were expressed in the endosperm, and one of these was also imprinted in the embryo, confirming that in both rice and Arabidopsis imprinted expression is primarily confined to the endosperm. Some rice imprinted genes were also expressed in vegetative tissues, indicating that they have additional roles in plant growth. Comparison of candidate imprinted genes found in rice with imprinted candidate loci obtained from genome-wide surveys of imprinted genes in Arabidopsis to date shows a low degree of conservation, suggesting that imprinting has evolved independently in eudicots and monocots.
    PLoS Genetics 06/2011; 7(6):e1002125. · 8.52 Impact Factor
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    ABSTRACT: Asexual seed formation, or apomixis, in the Hieracium subgenus Pilosella is controlled by two dominant independent genetic loci, LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP). We examined apomixis mutants that had lost function in one or both loci to establish their developmental roles during seed formation. In apomicts, sexual reproduction is initiated first. Somatic aposporous initial (AI) cells differentiate near meiotic cells, and the sexual pathway is terminated as AI cells undergo mitotic embryo sac formation. Seed initiation is fertilization-independent. Using a partially penetrant cytotoxic reporter to inhibit meioisis, we showed that developmental events leading to the completion of meiotic tetrad formation are required for AI cell formation. Sexual initiation may therefore stimulate activity of the LOA locus, which was found to be required for AI cell formation and subsequent suppression of the sexual pathway. AI cells undergo nuclear division to form embryo sacs, in which LOP functions gametophytically to stimulate fertilization-independent embryo and endosperm formation. Loss of function in either locus results in partial reversion to sexual reproduction, and loss of function in both loci results in total reversion to sexual reproduction. Therefore, in these apomicts, sexual reproduction is the default reproductive mode upon which apomixis is superimposed. These loci are unlikely to encode genes essential for sexual reproduction, but may function to recruit the sexual machinery at specific time points to enable apomixis.
    The Plant Journal 03/2011; 66(5):890-902. · 6.58 Impact Factor
  • Anna M G Koltunow, Susan D Johnson, Takashi Okada
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    ABSTRACT: Mendel used hawkweeds and other plants to verify the laws of inheritance he discovered using Pisum. Trait segregation was not evident in hawkweeds because many form seeds asexually by apomixis. Meiosis does not occur during female gametophyte formation and the mitotically formed embryo sacs do not require fertilization for seed development. The resulting progeny retain a maternal genotype. Hawkweeds in Hieracium subgenus Pilosella form mitotic embryo sacs by apospory. The initiation of sexual reproduction is required to stimulate apospory in ovules and to promote the function of the dominant locus, LOSS OF APOMEIOSIS, which stimulates the differentiation of somatic aposporous initial (AI) cells near sexually programmed cells. As AI cells undergo nuclear mitosis the sexual pathway terminates. The function of the dominant locus LOSS OF PARTHENOGENESIS in aposporous embryo sacs enables fertilization-independent embryo and endosperm development. Deletion of either locus results in partial reversion to sexual reproduction, and loss of function in both loci results in reversion to sexual development. In these apomicts, sexual reproduction is therefore the default reproductive mode upon which apomixis is superimposed. These loci are unlikely to encode factors critical for sexual reproduction but might recruit the sexual pathway to enable apomixis. Incomplete functional penetrance of these dominant loci is likely to lead to the generation of rare sexual progeny also derived from these facultative apomicts.
    Journal of Experimental Botany 02/2011; 62(5):1699-707. · 5.79 Impact Factor
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    Gary N Drews, Anna M G Koltunow
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    ABSTRACT: The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins early in ovule development with the formation of a diploid megaspore mother cell that undergoes meiosis. One resulting haploid megaspore then develops into the female gametophyte. Genetic and epigenetic processes mediate specification of megaspore mother cell identity and limit megaspore mother cell formation to a single cell per ovule. Auxin gradients influence female gametophyte polarity and a battery of transcription factors mediate female gametophyte cell specification and differentiation. The mature female gametophyte secretes peptides that guide the pollen tube to the embryo sac and contains protein complexes that prevent seed development before fertilization. Post-fertilization, the female gametophyte influences seed development through maternal-effect genes and by regulating parental contributions. Female gametophytes can form by an asexual process called gametophytic apomixis, which involves formation of a diploid female gametophyte and fertilization-independent development of the egg into the embryo. These functions collectively underscore the important role of the female gametophyte in seed and food production.
    The Arabidopsis Book 01/2011; 9:e0155.
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    ABSTRACT: In Hieracium, the formation of seeds through sexual and apomictic processes differ in two aspects: meiosis is avoided during apomictic female gametophyte formation (apomeiosis), which is controlled by the loss of apomeiosis (LOA) locus; and embryo and endosperm develop autonomously, a process controlled by an independent locus, loss of parthenogenesis (LOP). In sexual Arabidopsis, seed initiation is inhibited by the Fertilization Independent Seed (FIS) protein complex, which includes MULTICOPY SUPPRESSOR OF IRA1 (MSI1) and a distinct complex involving MSI1 and RETINOBLASTOMA-RELATED PROTEIN (RBR). msi1 mutants initiate autonomous egg cell proliferation forming a non-viable parthenogenetic embryo. Here we examined the association of HMSI1 orthologue with the initiation of seed development in Hieracium. HMSI1 is expressed in apomictic and sexual female gametes and in ovaries containing developing seeds. HMSI1 interacted with HRBR in yeast two-hybrid analyses as found in Arabidopsis. HMSI1 was not commonly altered in the genomes of gamma radiation lop mutants with lesions at the LOP locus and HMSI1 expression was down-regulated relative to the wild type in an unpollinated lop mutant that requires fertilization to develop functional embryos. The data suggest that HMSI1 is not linked to LOP and is thus unlikely to trigger the switch to autonomous seed development in apomictic Hieracium but more likely to be involved in seed formation events downstream of LOP activity.
    Plant Science 12/2010; · 4.11 Impact Factor
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    ABSTRACT: In sexually reproducing angiosperms, double fertilization initiates seed development, giving rise to two fertilization products, the embryo and the endosperm. In the endosperm, a terminal nutritive tissue that supports embryo growth, certain genes are expressed differentially depending on their parental origin, and this genomic imbalance is required for proper seed formation. This parent-of-origin effect on gene expression, called genomic imprinting, is controlled epigenetically through histone modifications and DNA methylation. In the sexual model plant Arabidopsis, the Polycomb group (PcG) genes of the plant Fertilization Independent Seed (FIS)-class control genomic imprinting by specifically silencing maternal or paternal target alleles through histone modifications. Mutations in FIS genes can lead to a bypass in the requirement of fertilization for the initiation of endosperm development and seed abortion. In this review, we discuss the role of the FIS complex in establishing and maintaining genomic imprinting, focusing on recent advances in elucidating the expression and function of FIS-related genes in maize, rice, and Hieracium, and particularly including apomictic Hieracium species that do not require paternal contribution and thus form seeds asexually. Surprisingly, not all FIS-mediated functions described in Arabidopsis are conserved. However, the function of some PcG components are required for viable seed formation in seeds formed via sexual and asexual processes (apomixis) in Hieracium, suggesting a conservation of the seed viability function in some eudicots.
    Sexual Plant Reproduction 06/2010; 23(2):123-33. · 2.07 Impact Factor
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    ABSTRACT: The Arabidopsis thaliana MYB5 gene is expressed in trichomes and seeds, including the seed coat. Constitutive expression of MYB5 resulted in the formation of more small trichomes and ectopic trichomes and a reduction in total leaf trichome numbers and branching. A myb5 mutant displayed minimal changes in trichome morphology, while a myb23 mutant produced increased numbers of small trichomes and two-branched trichomes. A myb5 myb23 double mutant developed more small rosette trichomes and two-branched trichomes than the single mutants. These results indicate that MYB5 and MYB23 regulate trichome extension and branching. The seed coat epidermal cells of myb5 and myb5 myb23 were irregular in shape, developed flattened columellae, and produced less mucilage than those of the wild type. Among the downregulated genes identified in the myb5 seeds using microarray analysis were ABE1 and ABE4 (alpha/beta fold hydrolase/esterase genes), MYBL2, and GLABRA2. The same genes were also downregulated in transparent testa glabra1 (ttg1) seeds, suggesting that MYB5 collaborates with TTG1 in seed coat development. These genes were upregulated in leaves and roots by ectopically expressed MYB5. The MYBL2, ABE1, and ABE4 promoters were active in seeds, including seed coats, and the latter two also in trichomes. Models of the MYB5 regulatory networks involved in seed coat and trichome development are presented.
    The Plant Cell 02/2009; 21(1):72-89. · 9.25 Impact Factor
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    Anna M G Koltunow, Matthew R Tucker
    Journal of Biosciences 10/2008; 33(3):309-11. · 1.76 Impact Factor
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    ABSTRACT: A Polycomb-Group (PcG) complex, FERTILIZATION INDEPENDENT SEED (FIS), represses endosperm development in Arabidopsis thaliana until fertilization occurs. The Hieracium genus contains apomictic species that form viable seeds asexually. To investigate FIS function during apomictic seed formation, FERTILIZATION INDEPENDENT ENDOSPERM (FIE), encoding a WD-repeat member of the FIS complex, was isolated and downregulated in sexual and apomictic Hieracium species. General downregulation led to defects in leaf and seed development, consistent with a role in developmental transitions and cell fate. PcG-like activity of Hieracium FIE was also supported by its interaction in vitro with the Arabidopsis CURLY LEAF PcG protein. By contrast, specific downregulation of FIE in developing seeds of sexual Hieracium did not result in autonomous endosperm proliferation but led to seed abortion after cross-pollination. Furthermore, in apomictic Hieracium, specific FIE downregulation inhibited autonomous embryo and endosperm initiation, and most autonomous seeds displayed defective embryo and endosperm growth. Therefore, FIE is required for both apomictic and fertilization-induced seed initiation in Hieracium. Since Hieracium FIE failed to interact with FIS class proteins in vitro, its partner proteins might differ from those in the FIS complex of Arabidopsis. These differences in protein interaction were attributed to structural modifications predicted from comparisons of Arabidopsis and Hieracium FIE molecular models.
    The Plant Cell 10/2008; 20(9):2372-86. · 9.25 Impact Factor
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    ABSTRACT: Asexual seed formation (apomixis) in Hieracium aurantiacum occurs by mitotic embryo sac formation without prior meiosis in ovules (apomeiosis), followed by fertilization-independent embryo and endosperm development. Sexual reproduction begins first in Hieracium ovules with megaspore mother cell (MMC) formation. Apomixis initiates with the enlargement of somatic cells, termed aposporous initial (AI) cells, near sexual cells. AI cells grow towards sexually programmed cells undergoing meiosis, which degrade as the dividing nuclei of AIs obscure and displace them. Following Agrobacterium-mediated transformation of an aneuploid Hieracium aurantiacum apomict, a somaclonal mutant designated “loss of apomeiosis 1” (loa1) was recovered, which had significantly lost the ability to form apomictic seed. Maternal apomictic progeny were rare and low levels of germinable seedlings were primarily derived from meiotically derived eggs. Cytological analysis revealed defects in AI formation and function in loa1. Somatic cells enlarged some distance away from sexual cells and unlike AI cells, these expanded away from sexual cells without nuclear division. Surprisingly, many accumulated callose in the walls, a marker associated with meiotically specified cells. These defective AI (DAI) cells only had partial sexual identity as they failed to express a marker reflecting entry to meiosis that was easily detected in MMCs and they ultimately degraded. DAI cell formation did not lead to a compensatory increase in functional sexual embryo sacs, as collapse of meiotic embryo sacs was prevalent in the aneuploid somaclonal mutant. Positional cues that are important for AI cell differentiation, growth and fate may have been disrupted in the loa1 mutant and this is discussed.
    Sexual Plant Reproduction 11/2007; 20(4):199-211. · 2.07 Impact Factor
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    ABSTRACT: Fruit initiation in Arabidopsis (Arabidopsis thaliana) is generally repressed until fertilization occurs. However, mutations in AUXIN RESPONSE FACTOR8 (ARF8) uncouple fruit initiation from fertilization, resulting in the formation of seedless, parthenocarpic fruit. Here we induced parthenocarpy in wild-type Arabidopsis by introducing either the mutant genomic (g) Atarf8-4 sequence or gAtARF8:beta-glucuronidase translational fusion constructs by plant transformation. Silencing of endogenous AtARF8 transcription was not observed, indicating that the introduced, aberrant ARF8 transcripts were compromising the function of endogenous ARF8 and/or associated factors involved in suppressing fruit initiation. To analyze the role of ARF8 in tomato (Solanum lycopersicum) we initially emasculated 23 tomato cultivars to test for background parthenocarpy. Surprisingly, all had a predisposition to initiate fertilization-independent fruit growth. Expression of gAtarf8-4 in transgenic tomato ('Monalbo') resulted in a significant increase in the number and size of parthenocarpic fruit. Isolation of tomato ARF8 cDNA indicated significant sequence conservation with AtARF8. SlARF8 may therefore control tomato fruit initiation in a similar manner as AtARF8 does in Arabidopsis. Two SlARF8 cDNAs differing in size by 5 bp were found, both arising from the same gene. The smaller cDNA is a splice variant and is also present in Arabidopsis. We propose that low endogenous levels of the splice variant products might interfere with efficient formation/function of a complex repressing fruit initiation, thereby providing an explanation for the observed ovary expansion in tomato and also Arabidopsis after emasculation. Increasing the levels of aberrant Atarf8-4 transcripts may further destabilize formation/function of the complex in a dosage-dependent manner enhancing tomato parthenocarpic fruit initiation frequency and size and mimicking the parthenocarpic dehiscent silique phenotype found in homozygous Atarf8-4 mutants. Collectively these data suggest that similar mechanisms involving auxin signaling exist to inhibit parthenocarpic fruit set in tomato and Arabidopsis.
    Plant physiology 11/2007; 145(2):351-66. · 6.56 Impact Factor
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    ABSTRACT: Fruit and seed formation in plants is normally initiated after pollination and fertilization, and, in the absence of fertilization, flowers senesce. In the Arabidopsis thaliana mutant fruit without fertilization, a mutation in AUXIN RESPONSE FACTOR8 (ARF8) results in the uncoupling of fruit development from pollination and fertilization and gives rise to seedless (parthenocarpic) fruit. Parthenocarpy was confirmed in two additional recessive alleles and was caused by mutations within the coding region of ARF8. Genetic experiments indicate that ARF8 acts as an inhibitor to stop further carpel development in the absence of fertilization and the generation of signals required to initiate fruit and seed development. Expression of ARF8 was found to be regulated at multiple levels, and transcriptional autoregulation of ARF8 was observed. Analysis of plants transformed with a transcriptional P(ARF8):beta-glucuronidase (GUS) construct or a translational ARF8:GUS fusion construct displayed distinct developmental regulation of the reporter in floral tissues involved in pollination and fertilization and in the carpel wall. After fertilization, the level of GUS activity declined in the developing seed, while in unfertilized ovules that are destined to senesce, ARF8:GUS expression spread throughout the ovule. This is consistent with a proposed role for ARF8 in restricting signal transduction processes in ovules and growth in pistils until the fruit initiation cue.
    The Plant Cell 09/2006; 18(8):1873-86. · 9.25 Impact Factor
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    ABSTRACT: Geminiviruses have been reported to replicate in, and localize to, the nuclei of host plant cells. We have investigated the tissue and intracellular distribution of the monopartite Tomato leaf curl virus (TLCV) by in situ hybridization. Contrary to the current understanding of geminiviral localization, single-stranded (ss) DNA of TLCV accumulated in the cytoplasm. TLCV ssDNA was also found in the nucleus, as was lower levels of replicative form double-stranded (ds) DNA. Under the same conditions, Tomato golden mosaic virus (TGMV) ssDNA and dsDNA were found in nuclei. ssDNA of TLCV, TGMV, and Tomato yellow leaf curl Sardinia virus (TYLCSV) was detected in some xylem vessels under specific hybridization conditions. Tissue specificity of TLCV was partially released by co-infection with TGMV. Our observations suggest that the mechanism of TLCV movement may differ from that of bipartite begomoviruses.
    Virology 05/2006; 348(1):120-32. · 3.37 Impact Factor

Publication Stats

837 Citations
178.66 Total Impact Points

Institutions

  • 2002–2011
    • The Commonwealth Scientific and Industrial Research Organisation
      Canberra, Australian Capital Territory, Australia
  • 2010
    • Brazilian Agricultural Research Corporation (EMBRAPA)
      • Embrapa Recursos Genéticos e Biotecnologia
      Brasília, Distrito Federal, Brazil
    • CEP America
      Emeryville, California, United States
  • 1998–2003
    • University of Adelaide
      Tarndarnya, South Australia, Australia