Establishing an efficient Ac/Ds tagging system in rice: large-scale analysis of Ds flanking sequences

Transposon tagging in diploid strawberry

Fragaria vesca was transformed with a transposon tagging construct harbouring amino terminally deleted maize transposase and EGFP (Ac element), NPTII, CaMV 35S promoter (P35S) driving transposase and mannopine synthase promoter (Pmas) driving EGFP (Ds element). Of 180 primary transgenics, 48 were potential launch pads, 72 were multiple insertions or chimaeras, and 60 exhibited somatic transposition. T₁ progeny of 32 putative launch pads were screened by multiplex PCR for transposition. Evidence of germ-line transposition occurred in 13 putative launch pads; however, the transposition frequency was too low in three for efficient recovery of transposants. The transposition frequency in the remaining launch pads ranged from 16% to 40%. After self-pollination of the T₀ launch pads, putative transposants in the T₁ generation were identified by multiplex PCR. Sequencing of hiTAIL-PCR products derived from nested primers within the Ds end sequences (either P35S at the left border or the inverted repeat at the right border) of T₁ plants revealed transposition of the Ds element to distant sites in the strawberry genome. From more than 2400 T₁ plants screened, 103 unique transposants have been identified, among which 17 were somatic transpositions observed in the T₀ generation. Ds insertion sites were dispersed among various gene elements [exons (15%), introns (23%), promoters (30%), 3' UTRs (17%) as well as intergenically (15%)]. Three-primer (one on either side of the Ds insertion and one within the Ds T-DNA) PCR could be used to identify homozygous T₂ transposon-tagged plants. The mutant collection has been catalogued in an on-line database.

In-depth molecular and phenotypic characterization in a rice insertion line library facilitates gene identification through reverse and forward genetics approaches

We report here the molecular and phenotypic features of a library of 31,562 insertion lines generated in the model japonica cultivar Nipponbare of rice (Oryza sativa L.), called Oryza Tag Line (OTL). Sixteen thousand eight hundred and fourteen T-DNA and 12,410 Tos17 discrete insertion sites have been characterized in these lines. We estimate that 8686 predicted gene intervals--i.e. one-fourth to one-fifth of the estimated rice nontransposable element gene complement--are interrupted by sequence-indexed T-DNA (6563 genes) and/or Tos17 (2755 genes) inserts. Six hundred and forty-three genes are interrupted by both T-DNA and Tos17 inserts. High quality of the sequence indexation of the T2 seed samples was ascertained by several approaches. Field evaluation under agronomic conditions of 27,832 OTL has revealed that 18.2% exhibit at least one morphophysiological alteration in the T1 progeny plants. Screening 10,000 lines for altered response to inoculation by the fungal pathogen Magnaporthe oryzae allowed to observe 71 lines (0.7%) developing spontaneous lesions simulating disease mutants and 43 lines (0.4%) exhibiting an enhanced disease resistance or susceptibility. We show here that at least 3.5% (four of 114) of these alterations are tagged by the mutagens. The presence of allelic series of sequence-indexed mutations in a gene among OTL that exhibit a convergent phenotype clearly increases the chance of establishing a linkage between alterations and inserts. This convergence approach is illustrated by the identification of the rice ortholog of AtPHO2, the disruption of which causes a lesion-mimic phenotype owing to an over-accumulation of phosphate, in nine lines bearing allelic insertions.

Ac/Ds-transposon activation tagging in poplar: a powerful tool for gene discovery

BACKGROUND

Rapid improvements in the development of new sequencing technologies have led to the availability of genome sequences of more than 300 organisms today. Thanks to bioinformatic analyses, prediction of gene models and protein-coding transcripts has become feasible. Various reverse and forward genetics strategies have been followed to determine the functions of these gene models and regulatory sequences. Using T-DNA or transposons as tags, significant progress has been made by using "Knock-in" approaches ("gain-of-function" or "activation tagging") in different plant species but not in perennial plants species, e.g. long-lived trees. Here, large scale gene tagging resources are still lacking.

RESULTS

We describe the first application of an inducible transposon-based activation tagging system for a perennial plant species, as example a poplar hybrid (P. tremula L. × P. tremuloides Michx.). Four activation-tagged populations comprising a total of 12,083 individuals derived from 23 independent "Activation Tagging Ds" (ATDs) transgenic lines were produced and phenotyped. To date, 29 putative variants have been isolated and new ATDs genomic positions were successfully determined for 24 of those. Sequences obtained were blasted against the publicly available genome sequence of P. trichocarpa v2.0 (Phytozome v7.0; http://www.phytozome.net/poplar) revealing possible transcripts for 17 variants.In a second approach, 300 randomly selected individuals without any obvious phenotypic alterations were screened for ATDs excision. For one third of those transposition of ATDs was confirmed and in about 5% of these cases genes were tagged.

CONCLUSIONS

The novel strategy of first genotyping and then phenotyping a tagging population as proposed here is, in particular, applicable for long-lived, difficult to transform plant species. We could demonstrate the power of the ATDs transposon approach and the simplicity to induce ATDs transposition in vitro. Since a transposon is able to pass chromosomal boundaries, only very few primary transposon-carrying transgenic lines are required for the establishment of large transposon tagging populations. In contrast to T-DNA-based activation tagging, which is plagued by a lack of transformation efficiency and its time consuming nature, this for the first time, makes it feasible one day to tag (similarly to Arabidopsis) every gene within a perennial plant genome.

Ac-Ds solutions for rice insertion mutagenesis

Rice is the model plant for monocotyledons. Since the completion of the high-quality sequence of its genome, the international community is deploying efforts to identify the function of the 30-40,000 nontransposable element genes of rice. These efforts comprise the creation of large collections of rice mutants accessible to the international scientific community. In addition to loss of function mutants, insertion mutagenesis using Agrobacterium-mediated transformation and engineered mobile elements allows the identification of genes through enhancer or gene trapping or activation tagging. The maize transposable element Ac-Ds is known to be active in rice since the early 1990s and it does not interfere with endogenous rice transposons. This is the guaranty that induced mutation obtained with the mobility of the Ds element will be stable when the source of Ac transposase is removed from the mutated genome. In this paper, we describe single- or double-component T-DNA constructs that have been used to introduce a functional Ac-Ds system in rice and allowed the generation and selection of different type of Ds insertion mutations in the rice genome.

Manipulation of plant architecture to enhance lignocellulosic biomass

BACKGROUND

Biofuels hold the promise to replace an appreciable proportion of fossil fuels. Not only do they emit significantly lower amounts of greenhouse gases, they are much closer to being 'carbon neutral', since the source plants utilize carbon dioxide for their growth. In particular, second-generation lignocellulosic biofuels from agricultural wastes and non-food crops such as switchgrass promise sustainability and avoid diverting food crops to fuel. Currently, available lignocellulosic biomass could yield sufficient bioethanol to replace ∼10 % of worldwide petroleum use. Increasing the biomass used for biofuel production and the yield of bioethanol will thus help meet global energy demands while significantly reducing greenhouse gas emissions.

SCOPE

We discuss the advantages of various biotechnological approaches to improve crops and highlight the contribution of genomics and functional genomics in this field. Current knowledge concerning plant hormones and their intermediates involved in the regulation of plant architecture is presented with a special focus on gibberellins and cytokinins, and their signalling intermediates. We highlight the potential of information gained from model plants such as Arabidopsis thaliana and rice (Oryza sativa) to accelerate improvement of fuel crops.

Oryza sativa cytochrome P450 family member OsCYP96B4 reduces plant height in a transcript dosage dependent manner

Background

Plant cytochromes P450 are involved in a wide range of biosynthetic reactions and play various roles in plant development. However, little is known about the biological functions of the subfamily CYP96 in plants.

Methodology/Principal Findings

Here, we report a novel semi-dwarf rice mutant, in which a single copy of transposon dissociator (Ds) was inserted into the gene OsCYP96B4 (Oryza sativa Cytochrome P450 96B4). The mutant exhibits the defects in cell elongation and pollen germination, which can be complemented by the wild type OsCYP96B4 and be rescued by remobilization of the Ds element with the presence of the transposase Activator (Ac). Transgenic plants harboring OsCYP96B4 double-stranded RNA interference construct mimicked the mutant phenotype. The oscyp96b4 mutant phenotype could not be rescued by all the tested phytohormones and it was found that OsCYP96B4 reduced plant height in a transcript dosage dependent manner. Heterologous expression of OsCYP96B4 in Schizosaccharomyces pombe resulted in missegregation and wider cells. Further investigation showed that the mutant exhibited the defects in the metabolism of some lipid molecular species when compared with the wild type.

Conclusions/Significance

The oscyp96b4 mutant is a novel rice semi-dwarf mutant. Our data suggest that OsCYP96B4 might be involved in lipid metabolism and regulate cell elongation.

Rice functional genomics research: progress and implications for crop genetic improvement

Rice is a staple food crop and has become a reference of monocot plant for functional genomic research. With the availability of high quality rice genome sequence, there has been rapid accumulation of functional genomic resources, including: large mutant libraries by T-DNA insertion, transposon tagging, and chemical mutagenesis; global expression profiles of the genes in the entire life cycle of rice growth and development; full-length cDNAs for both indica and japonica rice; sequences from resequencing large numbers of diverse germplasm accessions. Such resource development has greatly accelerated gene cloning. By the end of 2010, over 600 genes had been cloned using various methods. Many of the genes control agriculturally useful traits such as yield, grain quality, resistances to biotic and abiotic stresses, and nutrient-use efficiency, thus have potential utility in crop genetic improvement. This review was aimed to provide a comprehensive summary of such progress. We also presented our perspective for future studies.

In silico analysis of transcription factor repertoires and prediction of stress-responsive transcription factors from six major gramineae plants

The interactions between transcription factors (TFs) and cis-regulatory DNA sequences control gene expression, constituting the essential functional linkages of gene regulatory networks. The aim of this study is to identify and integrate all putative TFs from six grass species: Brachypodium distachyon, maize, rice, sorghum, barley, and wheat with significant information into an integrative database (GramineaeTFDB) for comparative genomics and functional genomics. For each TF, sequence features, promoter regions, domain alignments, GO assignment, FL-cDNA information, if available, and cross-references to various public databases and genetic resources are provided. Additionally, GramineaeTFDB possesses a tool which aids the users to search for putative cis-elements located in the promoter regions of TFs and predict the functions of the TFs using cis-element-based functional prediction approach. We also supplied hyperlinks to expression profiles of those TF genes of maize, rice, and barley, for which data are available. Furthermore, information about the availability of FOX and Ds mutant lines for rice and maize TFs, respectively, are also accessible through hyperlinks. Our study provides an important user-friendly public resource for functional analyses and comparative genomics of grass TFs, and understanding of the architecture of transcriptional regulatory networks and evolution of the TFs in agriculturally important cereal crops.

The activator/dissociation transposable elements comprise a two-component gene regulatory switch that controls endogenous gene expression in maize

The maize Activator/Dissociation (Ac/Ds) elements are able to replicate and transpose throughout the maize genome. Both elements preferentially insert into gene-rich regions altering the maize genome by creating unstable insertion alleles, stable derivative or excision alleles, or by altering the spatial or temporal regulation of gene expression. Here, we characterize an Ac insertion in the 5'-UTR of the Pink Scutellum1 (Ps1) gene and five Ds derivatives generated through abortive transposition events. Characterization of Ps1 transcription initiation sites in this allelic series revealed several that began within the terminus of the Ac and Ds elements. Transcripts originating within Ds or Ac accumulated to lower levels than the wild-type Ps1 allele, but were often sufficient to rescue the seedling lethal phenotype associated with severe loss-of-function alleles. Transcription initiation sites were similar in Ac and Ds derivatives, suggesting that Ac transposase does not influence transcript initiation site selection. However, we show that Ac transposase can negatively regulate Ps1 transcript accumulation in a subset of Ds-insertion alleles resulting in a severe mutant phenotype. The role of maize transposons in gene evolution is discussed.

Genome rearrangements in maize induced by alternative transposition of reversed ac/ds termini

Alternative transposition can induce genome rearrangements, including deletions, inverted duplications, inversions, and translocations. To investigate the types and frequency of the rearrangements elicited by a pair of reversed Ac/Ds termini, we isolated and analyzed 100 new mutant alleles derived from two parental alleles that both contain an intact Ac and a fractured Ac (fAc) structure at the maize p1 locus. Mutants were characterized by PCR and sequencing; the results show that nearly 90% (89/100) of the mutant alleles represent structural rearrangements including deletions, inversions, translocations, or rearrangement of the intertransposon sequence (ITS). Among 37 deletions obtained, 20 extend into the external flanking sequences, while 17 delete portions of the intertransposon sequence. Interestingly, one deletion allele that contains only a single nucleotide between the retained Ac and fAc termini is not competent for further alternative transposition events. We propose a new model for the formation of intertransposon deletions through insertion of reversed transposon termini into sister-chromatid sequences. These results document the types and frequencies of genome rearrangements induced by alternative transposition of reversed Ac/Ds termini in maize.

RiceFOX: a database of Arabidopsis mutant lines overexpressing rice full-length cDNA that contains a wide range of trait information to facilitate analysis of gene function

Identification of gene function is important not only for basic research but also for applied science, especially with regard to improvements in crop production. For rapid and efficient elucidation of useful traits, we developed a system named FOX hunting (Full-length cDNA Over-eXpressor gene hunting) using full-length cDNAs (fl-cDNAs). A heterologous expression approach provides a solution for the high-throughput characterization of gene functions in agricultural plant species. Since fl-cDNAs contain all the information of functional mRNAs and proteins, we introduced rice fl-cDNAs into Arabidopsis plants for systematic gain-of-function mutation. We generated >30,000 independent Arabidopsis transgenic lines expressing rice fl-cDNAs (rice FOX Arabidopsis mutant lines). These rice FOX Arabidopsis lines were screened systematically for various criteria such as morphology, photosynthesis, UV resistance, element composition, plant hormone profile, metabolite profile/fingerprinting, bacterial resistance, and heat and salt tolerance. The information obtained from these screenings was compiled into a database named 'RiceFOX'. This database contains around 18,000 records of rice FOX Arabidopsis lines and allows users to search against all the observed results, ranging from morphological to invisible traits. The number of searchable items is approximately 100; moreover, the rice FOX Arabidopsis lines can be searched by rice and Arabidopsis gene/protein identifiers, sequence similarity to the introduced rice fl-cDNA and traits. The RiceFOX database is available at http://ricefox.psc.riken.jp/.

FOX-superroots of Lotus corniculatus, overexpressing Arabidopsis full-length cDNA, show stable variations in morphological traits

Using the full-length cDNA overexpressor (FOX) gene-hunting system, we have generated 130 Arabidopsis FOX-superroot lines in bird's-foot trefoil (Lotus corniculatus) for the systematic functional analysis of genes expressed in roots and for the selection of induced mutants with interesting root growth characteristics. We used the Arabidopsis-FOX Agrobacterium library (constructed by ligating pBIG2113SF) for the Agrobacterium-mediated transformation of superroots (SR) and the subsequent selection of gain-of-function mutants with ectopically expressed Arabidopsis genes. The original superroot culture of L. corniculatus is a unique host system displaying fast root growth in vitro, allowing continuous root cloning, direct somatic embryogenesis and mass regeneration of plants under entirely hormone-free culture conditions. Several of the Arabidopsis FOX-superroot lines show interesting deviations from normal growth and morphology of roots from SR-plants, such as differences in pigmentation, growth rate, length or diameter. Some of these mutations are of potential agricultural interest. Genomic PCR analysis revealed that 100 (76.9%) out of the 130 transgenic lines showed the amplification of single fragments. Sequence analysis of the PCR fragments from these 100 lines identified full-length cDNA in 74 of them. Forty-three out of 74 full-length cDNA carried known genes. The Arabidopsis FOX-superroot lines of L. corniculatus, produced in this study, expand the FOX hunting system and provide a new tool for the genetic analysis and control of root growth in a leguminous forage plant.

Development of an efficient inverse PCR method for isolating gene tags from T-DNA insertional mutants in rice

The central goal of current genomics research in plants, as in other organisms, is to elucidate the functions of every gene. Insertional mutagenesis using known DNA sequences such as T-DNA is a powerful tool in functional genomics. Development of efficient methods for isolating the genomic sequences flanking insertion elements accelerates the systematic cataloging of insertional mutants, and thus allows functions to be assigned to uncharacterized genes via reverse genetic approaches. In our current study, we report a rapid and efficient inverse PCR (iPCR) method for the isolation of gene tags in T-DNA mutant lines of rice (Oryza sativa), a model monocot plant.

Enhancer trapping in plants

Advances in sequencing technology have led to the availability of complete genome sequences of many different plant species. In order to make sense of this deluge of information, functional genomics efforts have been intensified on many fronts. With improvements in plant transformation technologies, T-DNA and/or transposon-based gene and enhancer-tagged populations in various crop species are being developed to augment functional annotation of genes and also to help clone important genes. State-of-the-art cloning and sequencing technologies, which would help identify T-DNA or transposon junction sequences in large genomes, have also been initiated. This chapter gives a brief history of enhancer trapping and then proceeds to describe gene and enhancer tagging in plants. The significance of reporter gene fusion populations in plant genomics, especially in important cereal crops, is discussed.

Activation tagging and insertional mutagenesis in barley

The process of activation tagging in plants involves the random distribution of plant regulatory sequences throughout the genome. The insertion of a regulatory sequence in the vicinity of an endogenous gene can alter the transcriptional pattern of this gene resulting in a mutant phenotype that arises from excess functional gene product. Activation tagging has been undertaken extensively in a number of dicot plants and also in rice. This has been achieved primarily by high-throughput plant transformation using T-DNA sequences that encode regulatory elements. Apart from rice, most cereals do not have a suitably efficient transformation system for high-throughput transformation. In this article, we detail an activation tagging system in barley that exploits the mobility of the maize Ac/Ds transposable element system to distribute a highly expressed promoter throughout the barley genome. The advantage of this approach in this species is that a relatively small number of primary transgenics are required to generate an activation tagging population. Insertion of this transposable element into genes can also generate insertional inactivation mutants enabling both gene overexpression and gene knockout mutants to be identified in the same population.

Transposon insertional mutagenesis in rice

Insertion mutants offer one of the direct ways to relate a gene to its function by employing forward or reverse genetics approaches. Both T-DNA and transposon insertional mutants are being produced in several crops, including rice, the first cereal with its complete genome sequenced. Transposons have several advantages over T-DNA including the ability to produce multiple independent insertion lines from individual starter lines, as well as producing revertants by remobilization. With our new gene constructs, and a two-component transposon iAc/Ds mutagenesis protocol, we have improved both gene trapping and screening efficiencies in rice.

Functional genomics of rice pollen and seed development by genome-wide transcript profiling and Ds insertion mutagenesis

Rice pollen and seed development are directly related to grain yield. To further improve rice yield, it is important for us to functionally annotate the genes controlling pollen/seed development and to use them for rice breeding. Here we first carried out a genome-wide expression analysis with an emphasis on genes being involved in rice pollen and seed development. Based on the transcript profiling, we have identified and functionally classified 82 highly expressed pollen-specific, 12 developing seed-specific and 19 germinating seed-specific genes. We then presented the utilization of the maize transposon Dissociation (Ds) insertion lines for functional genomics of rice pollen and seed development and as alternative germplasm resources for rice breeding. We have established a two-element Activator/Dissociation (Ac/Ds) gene trap tagging system and generated around 20,000 Ds insertion lines. We have subjected these lines for screens to obtain high and low yield Ds insertion lines. Some interesting lines have been obtained with higher yield or male sterility. Flanking Sequence Tags (FSTs) analyses showed that these Ds-tagged genes encoded various proteins including transcription factors, transport proteins, unknown functional proteins and so on. They exhibited diversified expression patterns. Our results suggested that rice could be improved not only by introducing foreign genes but also by knocking out its endogenous genes. This finding might provide a new way for rice breeder to further improve rice varieties.

Rice Snl6, a cinnamoyl-CoA reductase-like gene family member, is required for NH1-mediated immunity to Xanthomonas oryzae pv. oryzae

Rice NH1 (NPR1 homolog 1) is a key mediator of innate immunity. In both plants and animals, the innate immune response is often accompanied by rapid cell death at the site of pathogen infection. Over-expression of NH1 in rice results in resistance to the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo), constitutive expression of defense related genes and enhanced benzothiadiazole (BTH)- mediated cell death. Here we describe a forward genetic screen that identified a suppressor of NH1-mediated lesion formation and resistance, snl6. Comparative genome hybridization and fine mapping rapidly identified the genomic location of the Snl6 gene. Snl6 is a member of the cinnamoyl-CoA reductase (CCR)-like gene family. We show that Snl6 is required for NH1-mediated resistance to Xoo. Further, we show that Snl6 is required for pathogenesis-related gene expression. In contrast to previously described CCR family members, disruption of Snl6 does not result in an obvious morphologic phenotype. Snl6 mutants have reduced lignin content and increased sugar extractability, an important trait for the production of cellulosic biofuels. These results suggest the existence of a conserved group of CCR-like genes involved in the defense response, and with the potential to alter lignin content without affecting development.

Transposition and target preferences of an active nonautonomous DNA transposon nDart1 and its relatives belonging to the hAT superfamily in rice

The nonautonomous nDart1 element in the hAT superfamily is one of a few active DNA transposons in rice. Its transposition can be induced by crossing with a line containing an active autonomous element, aDart1, and stabilized by segregating aDart1. No somaclonal variation should occur in nDart1-promoted gene tagging because no tissue culture is involved in nDart1 activation. By transposon display analysis, we examined the activities of nDart1-related elements in the selfed progeny of a mutable virescent pyl-v plant containing aDart1. Although various nDart1-related elements are present in the rice genome, only nDart1-3 subgroup elements, nDart1-0 and nDart1-3 in particular, were found to be transposed frequently and integrated into various sites almost all over the genome, and a fraction of the transposed elements were found to be transmitted to the next generation. More than half of the newly integrated elements were identified as nDart1-0. Analysis of the newly inserted sites revealed that the nDart1-3 subgroup elements were predominantly integrated into single-copy regions. More than 60% of the transposed elements were inserted into the genic regions that comprise putative coding regions and their 0.5-kb flanking segments, and approximately two-thirds of them were within the 0.5-kb area in front of the putative initiation codons, i.e., promoter-proximal genic regions. These characteristic features of nDart1-3 subgroup elements seem to be suitable for developing an efficient and somaclonal variation-free gene tagging system for rice functional genomics.

Assigning biological functions to rice genes by genome annotation, expression analysis and mutagenesis

Rice is the first cereal genome to be completely sequenced. Since the completion of its genome sequencing, considerable progress has been made in multiple areas including the whole genome annotation, gene expression profiling, mutant collection, etc. Here, we summarize the current status of rice genome annotation and review the methodology of assigning biological functions to hundreds of thousands of rice genes as well as discuss the major limitations and the future perspective in rice functional genomics. Available data analysis shows that the rice genome encodes around 32,000 protein-coding genes. Expression analysis revealed at least 31,000 genes with expression evidence from full-length cDNA/EST collection or other transcript profiling. In addition, we have summarized various strategies to generate mutant population including natural, physical, chemical, T-DNA, transposon/retrotransposon or gene silencing based mutagenesis. Currently, more than 1 million of mutants have been generated and 27,551 of them have their flanking sequence tags. To assign biological functions to hundreds of thousands of rice genes, global co-operations are required, various genetic resources should be more easily accessible and diverse data from transcriptomics, proteomics, epigenetics, comparative genomics and bioinformatics should be integrated to better understand the functions of these genes and their regulatory mechanisms.

SNP discovery and genetic mapping of T-DNA insertional mutants in Fragaria vesca L

As part of a program to develop forward and reverse genetics platforms in the diploid strawberry [Fragaria vesca L.; (2n = 2x = 14)] we have generated insertional mutant lines by T-DNA mutagenesis using pCAMBIA vectors. To characterize the T-DNA insertion sites of a population of 108 unique single copy mutants, we utilized thermal asymmetric interlaced PCR (hiTAIL-PCR) to amplify the flanking region surrounding either the left or right border of the T-DNA. Bioinformatics analysis of flanking sequences revealed little preference for insertion site with regard to G/C content; left borders tended to retain more of the plasmid backbone than right borders. Primers were developed from F. vesca flanking sequences to attempt to amplify products from both parents of the reference F. vesca 815 x F. bucharica 601 mapping population. Polymorphism occurred as: presence/absence of an amplification product for 16 primer pairs and different size products for 12 primer pairs, For 46 mutants, where polymorphism was not found by PCR, the amplification products were sequenced to reveal SNP polymorphism. A cleaved amplified polymorphic sequence/derived cleaved amplified polymorphism sequence (CAPS/dCAPS) strategy was then applied to find restriction endonuclease recognition sites in one of the parental lines to map the SNP position of 74 of the T-DNA insertion lines. BLAST search of flanking regions against GenBank revealed that 46 of 108 flanking sequences were close to presumed strawberry genes related to annotated genes from other plants.

Genome-wide distribution of transposed Dissociation elements in maize

The maize (Zea mays) transposable element Dissociation (Ds) was mobilized for large-scale genome mutagenesis and to study its endogenous biology. Starting from a single donor locus on chromosome 10, over 1500 elements were distributed throughout the genome and positioned on the maize physical map. Genetic strategies to enrich for both local and unlinked insertions were used to distribute Ds insertions. Global, regional, and local insertion site trends were examined. We show that Ds transposed to both linked and unlinked sites and displayed a nonuniform distribution on the genetic map around the donor r1-sc:m3 locus. Comparison of Ds and Mutator insertions reveals distinct target preferences, which provide functional complementarity of the two elements for gene tagging in maize. In particular, Ds displays a stronger preference for insertions within exons and introns, whereas Mutator insertions are more enriched in promoters and 5'-untranslated regions. Ds has no strong target site consensus sequence, but we identified properties of the DNA molecule inherent to its local structure that may influence Ds target site selection. We discuss the utility of Ds for forward and reverse genetics in maize and provide evidence that genes within a 2- to 3-centimorgan region flanking Ds insertions will serve as optimal targets for regional mutagenesis.

Natural and artificial mutants as valuable resources for functional genomics and molecular breeding

With the completion of rice genome sequencing, large collection of expression data and the great efforts in annotating rice genomes, the next challenge is to systematically assign functions to all predicted genes in the genome. The generations and collections of mutants at the genome-wide level form technological platform of functional genomics. In this study, we have reviewed currently employed tools to generate such mutant populations. These tools include natural, physical, chemical, tissue culture, T-DNA, transposon or gene silencing based mutagenesis. We also reviewed how these tools were used to generate a large collection of mutants and how these mutants can be screened and detected for functional analysis of a gene. The data suggested that the current population of mutants might be large enough to tag all predicted genes. However, the collection of flanking sequencing tags (FSTs) is limited due to the relatively higher cost. Thus, we have proposed a new strategy to generate gene-silencing mutants at the genome-wide level. Due to the large collection of insertion mutants, the next step to rice functional genomics should be focusing on functional characterization of tagged genes by detailed survey of corresponding mutants. Additionally, we also evaluated the utilization of these mutants as valuable resources for molecular breeding.

Genomics and bioinformatics resources for crop improvement

Recent remarkable innovations in platforms for omics-based research and application development provide crucial resources to promote research in model and applied plant species. A combinatorial approach using multiple omics platforms and integration of their outcomes is now an effective strategy for clarifying molecular systems integral to improving plant productivity. Furthermore, promotion of comparative genomics among model and applied plants allows us to grasp the biological properties of each species and to accelerate gene discovery and functional analyses of genes. Bioinformatics platforms and their associated databases are also essential for the effective design of approaches making the best use of genomic resources, including resource integration. We review recent advances in research platforms and resources in plant omics together with related databases and advances in technology.

Unexpected consequences of a sudden and massive transposon amplification on rice gene expression

High-copy-number transposable elements comprise the majority of eukaryotic genomes where they are major contributors to gene and genome evolution. However, it remains unclear how a host genome can survive a rapid burst of hundreds or thousands of insertions because such bursts are exceedingly rare in nature and therefore difficult to observe in real time. In a previous study we reported that in a few rice strains the DNA transposon mPing was increasing its copy number by approximately 40 per plant per generation. Here we exploit the completely sequenced rice genome to determine 1,664 insertion sites using high-throughput sequencing of 24 individual rice plants and assess the impact of insertion on the expression of 710 genes by comparative microarray analysis. We find that the vast majority of transposable element insertions either upregulate or have no detectable effect on gene transcription. This modest impact reflects a surprising avoidance of exon insertions by mPing and a preference for insertion into 5' flanking sequences of genes. Furthermore, we document the generation of new regulatory networks by a subset of mPing insertions that render adjacent genes stress inducible. As such, this study provides evidence for models first proposed previously for the involvement of transposable elements and other repetitive sequences in genome restructuring and gene regulation.

Construction and application of efficient Ac-Ds transposon tagging vectors in rice

Transposons are effective mutagens alternative to T-DNA for the generation of insertional mutants in many plant species including those whose transformation is inefficient. The current strategies of transposon tagging are usually slow and labor-intensive and yield low frequency of tagged lines. We have constructed a series of transposon tagging vectors based on three approaches: (i) AcTPase controlled by glucocorticoid binding domain/VP16 acidic activation domain/Gal4 DNA-binding domain (GVG) chemical-inducible expression system; (ii) deletion of AcTPase via Cre-lox site-specific recombination that was initially triggered by Ds excision; and (iii) suppression of early transposition events in transformed rice callus through a dual-functional hygromycin resistance gene in a novel Ds element (HPT-Ds). We tested these vectors in transgenic rice and characterized the transposition events. Our results showed that these vectors are useful resources for functional genomics of rice and other crop plants. The vectors are freely available for the community.

Transposon-based activation tagging in cereals

Advances in DNA sequencing technologies have produced an ever increasing number of sequenced genomes. However, many of the genes identified in these sequencing efforts have unknown functions or functions inferred based upon sequence homology, highlighting the necessity for functional gene analysis. Mutagenesis combined with phenotypic analyses remains a key mechanism for identifying and establishing gene function. Activation tagging is a mutagenic process that uses altered gene expression, usually gene overexpression, to generate mutant phenotypes. We have developed an activation tagging system in barley (Hordeum vulgare L.) based upon a maize (Zea mays L.) transposable element that carries two highly expressed cereal promoters. Insertion of this mobile genetic element in the genome can lead to insertional gene inactivation, gene overexpression and gene silencing through the production of antisense transcripts. This transposable element system has also been introduced into both wheat (Triticum aestivum L.) and maize and transposon mobility observed.

DLA-based strategies for cloning insertion mutants: cloning the gl4 locus of maize using Mu transposon tagged alleles

Digestion-ligation-amplification (DLA), a novel adaptor-mediated PCR-based method that uses a single-stranded oligo as the adaptor, was developed to overcome difficulties of amplifying unknown sequences flanking known DNA sequences in large genomes. DLA specifically overcomes the problems associated with existing methods for amplifying genomic sequences flanking Mu transposons, including high levels of nonspecific amplification. Two DLA-based strategies, MuClone and DLA-454, were developed to isolate Mu-tagged alleles. MuClone allows for the amplification of subsets of the numerous Mu transposons in the genome, using unique three-nucleotide tags at the 3' ends of primers, simplifying the identification of flanking sequences that cosegregate with mutant phenotypes caused by Mu insertions. DLA-454, which combines DLA with 454 pyrosequencing, permits the efficient cloning of genes for which multiple independent insertion alleles are available without the need to develop segregating populations. The utility of each approach was validated by independently cloning the gl4 (glossy4) gene. Mutants of gl4 lack the normal accumulation of epicuticular waxes. The gl4 gene is a homolog of the Arabidopsis CUT1 gene, which encodes a condensing enzyme involved in the synthesis of very-long-chain fatty acids, which are precursors of epicuticular waxes.

Phenome analysis in plant species using loss-of-function and gain-of-function mutants

Analysis of genetic mutations is one of the most effective ways to investigate gene function. We now have methods that allow for mass production of mutant lines and cells in a variety of model species. Recently, large numbers of mutant lines have been generated by both 'loss-of-function' and 'gain-of-function' techniques. In parallel, phenotypic information covering various mutant resources has been acquired and released in web-based databases. As a result, significant progress in comprehensive phenotype analysis is being made through the use of these tools. Arabidopsis and rice are two major model plant species in which genome sequencing projects have been completed. Arabidopsis is the most widely used experimental plant, with a large number of mutant resources and several examples of systematic phenotype analysis. Rice is a major crop species and is used as a model plant, with an increasing number of mutant resources. Other plant species are also being employed in functional genetics research. In this review, the present status of mutant resources for large-scale studies of gene function in plant research and the current perspective on using loss-of-function and gain-of-function mutants in phenome research will be discussed.

Mapping barleyDsinsertions using wheat deletion lines reveals high insertion frequencies in gene-rich regions with high to moderate recombination rates

Gene distribution is highly uneven in the large genomes of barley and wheat; however, location, order, and gene density of gene-containing regions are very similar between the two genomes. Flanking sequences from 35 unique, single-copy, barley Ds insertion events were physically mapped using wheat nullisomic-tetrasomic, ditelosomic, and deletion lines. Of the 35 sequences, 23 (66%) detected 34 loci mapping on all 7 homoeologous wheat groups. Seven sequences were not mapped owing to lack of polymorphism and the remaining 5 (14%) were barley-specific. All 34 loci physically mapped to the previously identified gene-rich regions (GRRs) of wheat, making the contained genes candidates for targeted mutagenesis by remobilization. Transpositions occurred preferentially into GRRs with higher recombination rates. The GRRs containing 17 of the 23 Ds insertions accounted for 60%–89% of the respective arm’s recombination. The remaining 6 (17%) insertions mapped to GRRs with <15% of the arm’s recombination. Overall, kb/cM estimates for the Ds-containing GRRs were twofold higher than those for regions without insertions. These results suggest that all genes may be targeted by transposon-based gene cloning, although the transposition frequency for genes present in recombination-poor regions is significantly less than that present in highly recombinogenic regions.

Systematic approaches to using the FOX hunting system to identify useful rice genes

Ectopic gene expression, or the gain-of-function approach, has the advantage that once the function of a gene is known the gene can be transferred to many different plants by transformation. We previously reported a method, called FOX hunting, that involves ectopic expression of Arabidopsis full-length cDNAs in Arabidopsis to systematically generate gain-of-function mutants. This technology is most beneficial for generating a heterologous gene resource for analysis of useful plant gene functions. As an initial model we generated more than 23,000 independent Arabidopsis transgenic lines that expressed rice fl-cDNAs (Rice FOX Arabidopsis lines). The short generation time and rapid and efficient transformation frequency of Arabidopsis enabled the functions of the rice genes to be analyzed rapidly. We screened rice FOX Arabidopsis lines for alterations in morphology, photosynthesis, element accumulation, pigment accumulation, hormone profiles, secondary metabolites, pathogen resistance, salt tolerance, UV signaling, high light tolerance, and heat stress tolerance. Some of the mutant phenotypes displayed by rice FOX Arabidopsis lines resulted from the expression of rice genes that had no homologs in Arabidopsis. This result demonstrated that rice fl-cDNAs could be used to introduce new gene functions in Arabidopsis. Furthermore, these findings showed that rice gene function could be analyzed by employing Arabidopsis as a heterologous host. This technology provides a framework for the analysis of plant gene function in a heterologous host and of plant improvement by using heterologous gene resources.

Suppression of the barley uroporphyrinogen III synthase gene by a Ds activation tagging element generates developmental photosensitivity

Chlorophyll production involves the synthesis of photoreactive intermediates that, when in excess, are toxic due to the production of reactive oxygen species (ROS). A novel, activation-tagged barley (Hordeum vulgare) mutant is described that results from antisense suppression of a uroporphyrinogen III synthase (Uros) gene, the product of which catalyzes the sixth step in the synthesis of chlorophyll and heme. In homozygous mutant plants, uroporphyrin(ogen) I accumulates by spontaneous cyclization of hydroxyl methylbilane, the substrate of Uros. Accumulation of this tetrapyrrole intermediate results in photosensitive cell death due to the production of ROS. The efficiency of Uros gene suppression is developmentally regulated, being most effective in mature seedling leaves compared with newly emergent leaves. Reduced transcript accumulation of a number of nuclear-encoded photosynthesis genes occurs in the mutant, even under 3% light conditions, consistent with a retrograde plastid-nuclear signaling mechanism arising from Uros gene suppression. A similar set of nuclear genes was repressed in wild-type barley following treatment with a singlet oxygen-generating herbicide, but not by a superoxide generating herbicide, suggesting that the retrograde signaling apparent in the mutant is specific to singlet oxygen.

An Ac/Ds-mediated gene trap system for functional genomics in barley

Background

Gene trapping is a powerful tool for gene discovery and functional genomics in both animals and plants. Upon insertion of the gene trap construct into an expressed gene, splice donor and acceptor sites facilitate the generation of transcriptional fusions between the flanking sequence and the reporter. Consequently, detection of reporter gene expression allows the identification of genes based on their expression pattern. Up to now rice is the only cereal crop for which gene trap approaches exist. In this study we describe a gene trap system in barley (Hordeum vulgare L.) based on the maize transposable elements Ac/Ds.

Results

We generated gene trap barley lines by crossing Ac transposase expressing plants with multiple independent transformants carrying the Ds based gene trap construct GTDsB. Upstream of the β-Glucuronidase start codon GTDsB carries splice donor and acceptor sites optimized for monocotyledonous plants. DNA blot analysis revealed GTDsB transposition frequencies of 11% and 26% in the F₁ and F₂ generation of gene trap lines and perpetuation of transposition activity in later generations. Furthermore, analysis of sequences flanking transposed GTDsB elements evidenced preferential insertion into expressed regions of the barley genome. We screened leaves, nodes, immature florets, pollinated florets, immature grains and seedlings of F₂ plants and detected GUS expression in 51% (72/141) of the plants. Thus, reporter gene expression was found in 24 of the 28 F₁ lines tested and in progeny of all GTDsB parental lines.

Conclusion

Due to the frequent transposition of GTDsB and the efficient expression of the GUS reporter gene, we conclude that this Ac/Ds-based gene trap system is an applicable approach for gene discovery in barley. The successful introduction of a gene trap construct optimized for monocots in barley contributes a novel functional genomics tool for this cereal crop.

Establishment of a soybean (Glycine max Merr. L) transposon-based mutagenesis repository

Soybean is a major crop species providing valuable feedstock for food, feed and biofuel. In recent years, considerable progress has been made in developing genomic resources for soybean, including on-going efforts to sequence the genome. These efforts have identified a large number of soybean genes, most with unknown function. Therefore, a major research priority is determining the function of these genes, especially those involved in agronomic performance and seed traits. One means to study gene function is through mutagenesis and the study of the resulting phenotypes. Transposon-tagging has been used successfully in both model and crop plants to support studies of gene function. In this report, we describe efforts to generate a transposon-based mutant collection of soybean. The Ds transposon system was used to create activation-tagging, gene and enhancer trap elements. Currently, the repository houses approximately 900 soybean events, with flanking sequence data derived from 200 of these events. Analysis of the insertions revealed approximately 70% disrupted known genes, with the majority matching sequences derived from either Glycine max or Medicago truncatula sequences. Among the mutants generated, one resulted in male-sterility and was shown to disrupt the strictosidine synthase gene. This example clearly demonstrates that it is possible to disrupt soybean gene function by insertional mutagenesis and to derive useful mutants by this approach in spite of the tetraploid nature of the soybean genome.

Mutation discovery for crop improvement

Increasing crop yields to ensure food security is a major challenge. Mutagenesis is an important tool in crop improvement and is free of the regulatory restrictions imposed on genetically modified organisms. The forward genetic approach enables the identification of improved or novel phenotypes that can be exploited in conventional breeding programmes. Powerful reverse genetic strategies that allow the detection of induced point mutations in individuals of the mutagenized populations can address the major challenge of linking sequence information to the biological function of genes and can also identify novel variation for plant breeding. This review briefly discusses recent advances in the detection of mutants and the potential of mutagenesis for crop improvement.

XA27 depends on an amino-terminal signal-anchor-like sequence to localize to the apoplast for resistance to Xanthomonas oryzae pv oryzae

The rice (Oryza sativa) gene Xa27 confers resistance to Xanthomonas oryzae pv oryzae, the causal agent of bacterial blight disease in rice. Sequence analysis of the deduced XA27 protein provides little or no clue to its mode of action, except that a signal-anchor-like sequence is predicted at the amino (N)-terminal region of XA27. As part of an effort to characterize the biochemical function of XA27, we decided to determine its subcellular localization. Initial studies showed that a functional XA27-green fluorescent protein fusion protein accumulated in vascular elements, the host sites where the bacterial blight pathogens multiply. The localization of XA27-green fluorescent protein to the apoplast was verified by detection of the protein on cell walls of leaf sheath and root cells after plasmolysis. Similarly, XA27-FLAG localizes to xylem vessels and cell walls of xylem parenchyma cells, revealed by immunogold electron microscopy. XA27-FLAG could be secreted from electron-dense vesicles in cytoplasm to the apoplast via exocytosis. The signal-anchor-like sequence has an N-terminal positively charged region including a triple arginine motif followed by a hydrophobic region. Deletion of the hydrophobic region or substitution of the triple arginine motif with glycine or lysine residues abolished the localization of the mutated proteins to the cell wall and impaired the plant's resistance to X. oryzae pv oryzae. These results indicate that XA27 depends on the N-terminal signal-anchor-like sequence to localize to the apoplast and that this localization is important for resistance to X. oryzae pv oryzae.

Construction of a rice glycosyltransferase phylogenomic database and identification of rice-diverged glycosyltransferases

Glycosyltransferases (GTs; EC 2.4.x.y) constitute a large group of enzymes that form glycosidic bonds through transfer of sugars from activated donor molecules to acceptor molecules. GTs are critical to the biosynthesis of plant cell walls, among other diverse functions. Based on the Carbohydrate-Active enZymes (CAZy) database and sequence similarity searches, we have identified 609 potential GT genes (loci) corresponding to 769 transcripts (gene models) in rice (Oryza sativa), the reference monocotyledonous species. Using domain composition and sequence similarity, these rice GTs were classified into 40 CAZy families plus an additional unknown class. We found that two Pfam domains of unknown function, PF04577 and PF04646, are associated with GT families GT61 and GT31, respectively. To facilitate functional analysis of this important and large gene family, we created a phylogenomic Rice GT Database (http://ricephylogenomics.ucdavis.edu/cellwalls/gt/). Through the database, several classes of functional genomic data, including mutant lines and gene expression data, can be displayed for each rice GT in the context of a phylogenetic tree, allowing for comparative analysis both within and between GT families. Comprehensive digital expression analysis of public gene expression data revealed that most ( approximately 80%) rice GTs are expressed. Based on analysis with Inparanoid, we identified 282 'rice-diverged' GTs that lack orthologs in sequenced dicots (Arabidopsis thaliana, Populus tricocarpa, Medicago truncatula, and Ricinus communis). Combining these analyses, we identified 33 rice-diverged GT genes (45 gene models) that are highly expressed in above-ground, vegetative tissues. From the literature and this analysis, 21 of these loci are excellent targets for functional examination toward understanding and manipulating grass cell wall qualities. Study of the remainder may reveal aspects of hormone and protein metabolism that are critical for rice biology. This list of 33 genes and the Rice GT Database will facilitate the study of GTs and cell wall synthesis in rice and other plants.

Generation of a 3D indexed Petunia insertion database for reverse genetics

BLAST searchable databases containing insertion flanking sequences have revolutionized reverse genetics in plant research. The development of such databases has so far been limited to a small number of model species and normally requires extensive labour input. Here we describe a highly efficient and widely applicable method that we adapted to identify unique transposon-flanking genomic sequences in Petunia. The procedure is based on a multi-dimensional pooling strategy for the collection of DNA samples; up to thousands of different templates are amplified from each of the DNA pools separately, and knowledge of their source is safeguarded by the use of pool-specific (sample) identification tags in one of the amplification primers. All products are combined into a single sample that is subsequently used as a template for unidirectional pyrosequencing. Computational analysis of the clustered sequence output allows automatic assignment of sequences to individual DNA sources. We have amplified and analysed transposon-flanking sequences from a Petunia transposon insertion library of 1000 individuals. Using 30 DNA isolations, 70 PCR reactions and two GS20 sequencing runs, we were able to allocate around 10 000 transposon flanking sequences to specific plants in the library. These sequences have been organized in a database that can be BLAST-searched for insertions into genes of interest. As a proof of concept, we have performed an in silico screen for insertions into members of the NAM/NAC transcription factor family. All in silico-predicted transposon insertions into members of this family could be confirmed in planta.

A versatile transposon-based activation tag vector system for functional genomics in cereals and other monocot plants

Transposon insertional mutagenesis is an effective alternative to T-DNA mutagenesis when transformation through tissue culture is inefficient as is the case for many crop species. When used as activation tags, transposons can be exploited to generate novel gain-of-function phenotypes without transformation and are of particular value in the study of polyploid plants where gene knockouts will not have phenotypes. We have developed an in cis-activation-tagging Ac-Ds transposon system in which a T-DNA vector carries a Dissociation (Ds) element containing 4x cauliflower mosaic virus enhancers along with the Activator (Ac) transposase gene. Stable Ds insertions were selected using green fluorescent protein and red fluorescent protein genes driven by promoters that are functional in maize (Zea mays) and rice (Oryza sativa). The system has been tested in rice, where 638 stable Ds insertions were selected from an initial set of 26 primary transformants. By analysis of 311 flanking sequences mapped to the rice genome, we could demonstrate the wide distribution of the elements over the rice chromosomes. Enhanced expression of rice genes adjacent to Ds insertions was detected in the insertion lines using semiquantitative reverse transcription-PCR method. The in cis-two-element vector system requires minimal number of primary transformants and eliminates the need for crossing, while the use of fluorescent markers instead of antibiotic or herbicide resistance increases the applicability to other plants and eliminates problems with escapes. Because Ac-Ds has been shown to transpose widely in the plant kingdom, the activation vector system developed in this study should be of utility more generally to other monocots.

An efficient field screening procedure for identifying transposants for constructing an Ac/Ds-based insertional-mutant library of rice

An efficient system was developed, and several variables tested, for generating a large-scale insertional-mutagenesis population of rice. The most important feature in this improved Ac/Ds tagging system is that one can conveniently carry out large-scale screening in the field and select transposants at the seedling stage. Rice was transformed with a plasmid that includes a Basta-resistance gene (bar). After the Ds element is excised during transposition, bar becomes adjacent to the ubiquitin promoter, and the rice plant becomes resistant to the herbicide Basta. In principle, one can plant up to one million plants in the field and select those plants that survive after spraying with Basta. To test the utility of this system, 4 Ds starter lines were crossed with 14 different Ac plants, and many transposants were successfully identified after planting 134 285 F2 plants in the field. Over 2 800 of these transposants were randomly chosen for PCR analysis, and the results fully confirmed the reliability of the field screening procedure.

The Rice Annotation Project Database (RAP-DB): 2008 update

The Rice Annotation Project Database (RAP-DB) was created to provide the genome sequence assembly of the International Rice Genome Sequencing Project (IRGSP), manually curated annotation of the sequence, and other genomics information that could be useful for comprehensive understanding of the rice biology. Since the last publication of the RAP-DB, the IRGSP genome has been revised and reassembled. In addition, a large number of rice-expressed sequence tags have been released, and functional genomics resources have been produced worldwide. Thus, we have thoroughly updated our genome annotation by manual curation of all the functional descriptions of rice genes. The latest version of the RAP-DB contains a variety of annotation data as follows: clone positions, structures and functions of 31 439 genes validated by cDNAs, RNA genes detected by massively parallel signature sequencing (MPSS) technology and sequence similarity, flanking sequences of mutant lines, transposable elements, etc. Other annotation data such as Gnomon can be displayed along with those of RAP for comparison. We have also developed a new keyword search system to allow the user to access useful information. The RAP-DB is available at: http://rapdb.dna.affrc.go.jp/ and http://rapdb.lab.nig.ac.jp/.

Analysis of gene-trap Ds rice populations in Korea

Insertional mutagen-mediated gene tagging populations have been essential resources for analyzing the function of plant genes. In rice, maize transposable elements have been successfully utilized to produce transposant populations. However, many generations and substantial field space are required to obtain a sufficiently sized transposant population. In rice, the japonica and indica subspecies are phenotypically and genetically divergent. Here, callus cultures with seeds carrying Ac and Ds were used to produce 89,700 lines of Dongjin, a japonica cultivar, and 6,200 lines of MGRI079, whose genome is composed of a mixture of the genetic backgrounds of japonica and indica. Of the more than 3,000 lines examined, 67% had Ds elements. Among the Ds-carrying lines, 81% of Dongjin and 63% of MGRI079 contained transposed Ds, with an average of around 2.0 copies. By examining more than 15,000 lines, it was found that 12% expressed the reporter gene GUS during the early-seedling stage. GUS was expressed in root hairs and crown root initials at estimated frequencies of 0.78% and 0.34%, respectively. The 5,271 analyzed Ds loci were found to be randomly distributed over all of the rice chromosomes.

Ds insertion mutagenesis as an efficient tool to produce diverse variations for rice breeding

The availability of diversified germplasm resources is the most important for developing improved rice varieties with higher seed yield or tolerance to various biotic or abiotic stresses. Here we report an efficient tool to create increased variations in rice by maize Ac/Ds transposon (a gene trap system) insertion mutagenesis. We have generated around 20,000 Ds insertion rice lines of which majority are homozygous for Ds element. We subjected these lines to phenotypic and abiotic stress screens and evaluated these lines with respect to their seed yields and other agronomic traits as well as their tolerance to drought, salinity and cold. Based on this evaluation, we observed that random Ds insertions into rice genome have led to diverse variations including a range of morphological and conditional phenotypes. Such differences in phenotype among these lines were accompanied by differential gene expression revealed by GUS histochemical staining of gene trapped lines. Among the various phenotypes identified, some Ds lines showed significantly higher grain yield compared to wild-type plants under normal growth conditions indicating that rice could be improved in grain yield by disrupting certain endogenous genes. In addition, several 1,000s of Ds lines were subjected to abiotic stresses to identify conditional mutants. Subsequent to these screens, over 800 lines responsive to drought, salinity or cold stress were obtained, suggesting that rice has the genetic potential to survive under abiotic stresses when appropriate endogenous genes were suppressed. The mutant lines that have higher seed yielding potential or display higher tolerance to abiotic stresses may be used for rice breeding by conventional backcrossing combining with molecular marker-assisted selection. In addition, by exploiting the behavior of Ds to leave footprints upon remobilization, we have shown an alternative strategy to develop new rice varieties without foreign DNA sequences in their genome.

A genome-wide gain-of function analysis of rice genes using the FOX-hunting system

The latest report has estimated the number of rice genes to be approximately 32,000. To elucidate the functions of a large population of rice genes and to search efficiently for agriculturally useful genes, we have been taking advantage of the Full-length cDNA Over-eXpresser (FOX) gene-hunting system. This system is very useful for analyzing various gain-of-function phenotypes from large populations of transgenic plants overexpressing cDNAs of interest and others with unknown or important functions. We collected the plasmid DNAs of 13,980 independent full-length cDNA (FL-cDNA) clones to produce a FOX library by placing individual cDNAs under the control of the maize Ubiquitin-1 promoter. The FOX library was transformed into rice by Agrobacterium-mediated high-speed transformation. So far, we have generated approximately 12,000 FOX-rice lines. Genomic PCR analysis indicated that the average number of FL-cDNAs introduced into individual lines was 1.04. Sequencing analysis of the PCR fragments carrying FL-cDNAs from 8615 FOX-rice lines identified FL-cDNAs in 8225 lines, and a database search classified the cDNAs into 5462 independent ones. Approximately 16.6% of FOX-rice lines examined showed altered growth or morphological characteristics. Three super-dwarf mutants overexpressed a novel gibberellin 2-oxidase gene,confirming the importance of this system. We also show here the other morphological alterations caused by individual FL-cDNA expression. These dominant phenotypes should be valuable indicators for gene discovery and functional analysis.