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Citrus unshiu Marc. cv. Miyagawa-wase is the most widely cultivated citrus variety in Korea. To determine whether the C. unshiu genome used in this study shows genetic variation compared to the published reference genome (C. unshiu Marc. Miyagawa-wase), we conducted genome re-sequencing of two C. unshiu cultivars (Miyagawa-1 and Miyagawa-2) cultivated on Jeju in Korea. Compared with the reference genome, 1,198,650 and 1,207,084 single-nucleotide polymorphisms (SNPs) and 172,259 and 172,391 insertion/deletion polymorphisms (InDels) were detected in the Miyagawa-1 and -2 genomes, respectively. In SNP and InDel classifications by genome annotation, 367,591 and 369,068 SNPs and 45,362 and 45,464 InDels were located in the genic regions of Miyagawa-1 and -2, respectively. Among the SNPs of Miyagawa-1 and -2, transitions were more frequent than transversions. The majority of InDels was distributed in 1-bp InDels in both cultivars. The comparative number of total SNPs between Miyagawa-1 and -2 was smaller than the number of SNPs between the reference genome and Miyagawa-1 or -2. Gene ontology (GO) analysis showed that 23,164 and 23,049 genes with SNPs and 16,830 and 16,774 genes with InDels were annotated in the GO database. Taken together, Miyagawa-1 and -2 show genome-wide variation, including SNPs and InDels, compared to the published C. unshiu Marc. cv. Miyagawa-wase genome. This study suggests it would be more accurate to use the Miyagawa-1 and -2 genome sequences as a reference when conducting research using C. unshiu cultivated in Korea.

Dart1-24, one of the 37 autonomous DNA transposon Dart1s, was heritably activated by the demethylation of the 5' region following 5-azaC treatment of rice seeds. Transposons are controlled by epigenetic regulations. To obtain newly activated autonomous elements of Dart1, a DNA transposon, in rice, seeds of a stable pale yellow leaf (pyl-stb) mutant caused by the insertion of nDart1-0, a nonautonomous element in OsClpP5, were treated with 5-azaC, a demethylating agent. In the 5-azaC-treated M1 plants, 60-70% of the plants displayed variegated pale yellow leaf (pyl-v) phenotype, depending on the concentration of 5-azaC used, suggesting that inactivated Dart1 might become highly activated by 5-azaC treatment and nDart1-0 was excised from OsClpP5 by the activated Dart1s. Although the M2 plants derived from most of these pyl-v plants showed stable pyl phenotypes, some variegated M1 plants generated pyl-v M2 progeny. These results indicated that most M1 pyl-v phenotypes at M1 were not heritable. Dart1-24, 1-27 and 1-28 were expressed in the M2 pyl-v plants, and mapping analysis confirmed that Dart1-24 was newly activated. Further, the transgenerational activation of Dart1-24 was demonstrated to be caused by the demethylation of nucleotides in its 5' region.

Recent methylome analyses of the entire Arabidopsis thaliana genome using various mutants have provided detailed information about the DNA methylation pattern and its function. However, information about DNA methylation in other plants is limited, partly because of the lack of mutants. To study DNA methylation in rice (Oryza sativa) we applied homologous recombination-mediated gene targeting to generate targeted disruptants of OsDRM2, a rice orthologue of DOMAINS REARRANGED METHYLASE 1 and 2 (DRM1/2), which encode DNA methyltransferases responsible for de novo and non-CG methylation in Arabidopsis. Whereas Arabidopsis drm1 drm2 double mutants showed no morphological alterations, targeted disruptants of rice OsDRM2 displayed pleiotropic developmental phenotypes in both vegetative and reproductive stages, including growth defects, semi-dwarfed stature, reductions in tiller number, delayed heading or no heading, abnormal panicle and spikelet morphology, and complete sterility. In these osdrm2 disruptants, a 13.9% decrease in 5-methylcytosine was observed by HPLC analysis. The CG and non-CG methylation levels were reduced in RIRE7/CRR1 retrotransposons, and in 5S rDNA repeats. Associated transcriptional activation was detected in RIRE7/CRR1. Furthermore, de novo methylation by an RNA-directed DNA methylation (RdDM) process involving transgene-derived exogenous small interfering RNA (siRNA) was deficient in osdrm2-disrupted cells. Impaired growth and abnormal DNA methylation of osdrm2 disruptants were restored by the complementation of wild-type OsDRM2 cDNA. Our results suggest that OsDRM2 is responsible for de novo, CG and non-CG methylation in rice genomic sequences, and that DNA methylation regulated by OsDRM2 is essential for proper rice development in both vegetative and reproductive stages.

A large part of the rice genome is composed of transposons. Since active excision/reintegration of these mobile elements may result in harmful genetic changes, many transposons are maintained in a genetically or epigenetically inactivated state. However, some non-autonomous DNA transposons of the nDart1-3 subgroup, including nDart1-0, actively transpose in specific rice lines, such as pyl-v which carries an active autonomous element, aDart1-27, on chromosome 6. Although nDart1-3 subgroup elements show considerable sequence identity, they display different excision frequencies. The most active element, nDart1-0, had a low cytosine methylation status. The aDart1-27 sequence showed conservation between pyl-stb (pyl-v derivative line) and Nipponbare, which both lack autonomous activity for transposition of nDart1-3 subgroup elements. In pyl-v plants, the promoter region of the aDart1-27 transposase gene was more hypomethylated than in other rice lines. Treatment with the methylation inhibitor 5-azacytidine (5-azaC) induced transposition of nDart1-3 subgroup elements in both pyl-stb and Nipponbare plants; the new insertion sites were frequently located in genic regions. 5-AzaC treatment principally induced expression of Dart1-34 transposase rather than the other 38 aDart1-related elements in both pyl-stb and Nipponbare treatment groups. Our observations show that transposition of nDart1-3 subgroup elements in the nDart1/aDart1 tagging system is correlated with the level of DNA methylation. Our system does not cause somaclonal variation due to an absence of transformed plants, offers the possibility of large-scale screening in the field and can identify dominant mutants. We therefore propose that this tagging system provides a valuable addition to the tools available for rice functional genomics.

An endogenous 0.6-kb rice DNA transposon, nDart1-0, was found as an active nonautonomous element in a mutable virescent line, pyl-v, displaying leaf variegations. Here, we demonstrated that the active autonomous element aDart in pyl-v corresponds to Dart1-27 on chromosome 6 in Nipponbare, which carries no active aDart elements, and that aDart and Dart1-27 are identical in their sequences and chromosomal locations, indicating that Dart1-27 is epigenetically silenced in Nipponbare. The identification of aDart in pyl-v was first performed by map-based cloning and by detection of the accumulated transposase transcripts. Subsequently, various transposition activities of the cloned Dart1-27 element from Nipponbare were demonstrated in Arabidopsis. Dart1-27 in Arabidopsis was able to excise nDart1-0 and Dart1-27 from cloned sites, generating footprints, and to integrate into new sites, generating 8-bp target site duplications. In addition to Dart1-27, Nipponbare contains 37 putative autonomous Dart1 elements because their putative transposase genes carry no apparent nonsense or frameshift mutations. Of these, at least four elements were shown to become active aDart elements in transgenic Arabidopsis plants, even though considerable sequence divergence arose among their transposases. Thus, these four Dart1 elements and Dart1-27 in Nipponbare must be potential autonomous elements silenced epigenetically. The regulatory and evolutionary implications of the autonomous Dart1 elements and the development of an efficient transposon-tagging system in rice are discussed. Electronic supplementary material The online version of this article (doi:10.1007/s00438-008-0410-x) contains supplementary material, which is available to authorized users.

Carotenoid isomerase (CRTISO) catalyzes the isomerization of prolycopene to all-trans-lycopene in the carotenoid biosynthetic pathway. We isolated a promoter region of CuCRTISO from Citrus unshiu. We determined whether the promoter encoded organ-specific or developmental-specific expression and identified possible cis-acting promoter elements. The promoter and two truncated versions were fused to the β-glucuronidase (GUS) gene and transformed into Arabidopsis thaliana. Transgenic lines expressing the 2501-bp promoter (pCiso-Prom1) and truncated promoters (pCiso-Prom2 and pCiso-Prom3) showed the same developmental and organ-specific activity. GUS expression was detected in the cotyledon and root at 5 and 10 days after germination, mature leaf, and anther. The CuCRTISO promoter contained several cis-acting elements involved in hormonal and environmental stress. Drought stress or abscisic acid treatment did not induce GUS expression in any transgenic lines. Heat stress induced GUS expression in the pCiso-Prom1 line; this promoter construct contains the heat stress-responsive element. Ethylene and cold stress treatments induced GUS expression only in the pCiso-Prom3 line, although all transgenic lines contained the same cis-acting ethylene and low-temperature response elements, which could indicate the existence of unknown repressor element(s) in the CuCRTISO promoter. The CuCRTISO expression of Citrus unshiu during fruit maturation showed that in the green stage, it was highly expressed and then subsequently decreased after the stage. These studies indicate that CuCRTISO promoter activity is regulated in a developmental and organ-specific manner that responds to heat, cold, and ethylene. These results provide new insights into the role of cis-acting element(s) in CuCRTISO promoter activity.

While characterized mutable alleles caused by DNA transposons have been abundant in maize since the discovery of Dissociation conferring variegation by Barbara McClintock, only a few mutable alleles have been described in rice even though the rice genome contains various transposons. Here, we show that a spontaneous mutable virescent allele, pyl-v, is caused by the disruption of the nuclear-coded essential chloroplast protease gene, OsClpP5, due to insertion of a 607-bp non-autonomous DNA transposon, non-autonomous DNA-based active rice transposon one (nDart1), belonging to the hAT superfamily. The transposition of nDart1 can be induced by crossing with a line containing an autonomous element, aDart, and stabilized by segregating out of aDart. We also identified a novel mutable dwarf allele thl-m caused by an insertion of nDart1. The japonica cultivar Nipponbare carries no aDart, although it contains epigenetically silenced Dart element(s), which can be activated by 5-azacytidine. Nipponbare bears four subgroups of about 3.6-kb Dart-like sequences, three of which contain potential transposase genes, and around 3.6-kb elements without an apparent transposase gene, as well as three subgroups of about 0.6-kb nDart1-related elements that are all internal deletions of the Dart-like sequences. Both nDart1 and 3.6-kb Dart-like elements were also present in indica varieties 93-11 and Kasalath. nDart1 appears to be the most active mutagen among nDart1-related elements contributing to generating natural variations. A candidate for an autonomous element, aDart, and a possible application of nDart1 for transposon tagging are discussed.

In contrast to angiosperms, some gymnosperms form well-development suspensors during somatic embryogenesis. This feature is highly useful to studies of suspensor biology. In cell cultures of Japanese larch ( Larix leptolepis Gordon), somatic embryos rarely formed when the initial cell density was lower than 0.1 ml packed cell volume (PCV) l −1 . However, in the presence of phytosulfokine (PSK), a peptidyl plant growth factor, mitotic activity during somatic embryogenesis was stimulated, even when the cell density was lower than 0.1 ml PCV l −1 , particularly, the development of the suspensor. Manual separation of somatic embryos into the embryo proper and the suspensor arrested further development, but when embryos proper lacking a suspensor were treated with PSK, the suspensor regenerated and development proceeded to maturity. Because no division of the suspensor cells was observed, the cells of the regenerated suspensor probably originated from basal cells of the embryo proper. In contrast, PSK treatment of suspensors lacking the embryo proper did not stimulate further development. These results indicate that PSK stimulates not only cell division of embryo proper but also development of the suspensor.

Phytosulfokine-α (PSK-α), a peptidyl plant growth factor that was originally isolated from conditioned medium derived from asparagus mesophyll cell culture, stimulates cell proliferation in some plants in suspension culture. However, the physiological relationship between phytohormones and this growth factor in plants is not well understood. Using carrot ( Daucus carota L.) non-embryogenic cell culture, we investigated the stimulatory effects of PSK-α on the cell proliferation induced by different combinations and concentrations of auxin [2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), or naphthaleneacetic acid (NAA)] and PSK-α. Cell proliferation was not stimulated by PSK-α at high auxin concentrations, but was promoted at low auxin concentrations. There was no proliferation in the absence of auxin. These results indicate that PSK-α stimulates cell proliferation when cell division activity is low, and that auxin is required for this process. We also evaluated PSK-α production using medium derived from non-embryogenic cell culture with high or low concentrations of auxin (2,4-D, IAA or NAA) in a competition ELISA system. More PSK-α was produced at high auxin concentrations than at low auxin concentrations. These results suggest that auxin participates in PSK-α production.

Rice is an important staple food for more than half the world’s population, and it has become the first crop plant to have its 389-Mb genome sequenced (IRGSP 2005). Even though various functional genomic tools for elucidating the function of rice genes are available (Hirochika et al. 2004; Leung and An 2004; Sasaki et al. 2005; Upadhyaya 2007), developing new methods for characterizing genes of interest by both forward and reverse genetic approaches has become particularly important. This chapter contains a description of the current state of gene targeting mediated by homologous recombination (HR) and gene tagging promoted by a nonautonomous DNA-based active rice transposon, nDart, for rice functional genomics, both of which are being developed (Terada et al. 2002, 2007; Tsugane et al. 2006; Takagi et al. 2007). Gene targeting refers to the alteration of a specific DNA sequence in an endogenous gene at its original locus in the genome and, often, to