Gene Trapping in Arabidopsis Reveals Genes Involved in Vascular Development

Distinct and redundant roles of the Arabidopsis OCTOPUS gene family in plant growth beyond phloem development

The Arabidopsis root apical meristem is an excellent model for studying plant organ growth. This involves a coordinated process of cell division, elongation, and differentiation, with each tissue type developing according to its own schedule. Among these tissues, the protophloem is particularly important, differentiating early to supply nutrients and signalling molecules to the growing root tip. The OCTOPUS (OPS) protein and its homologue OPS-LIKE 2 (OPL2) are essential for proper root protophloem differentiation and, probably through this role, indirectly promote root growth. Here, we explored the roles of the other three OPS homologues in Arabidopsis, OPL1, OPL3, and OPL4. OPS/OPL genes exhibited overlapping expression patterns and functions, with a high degree of redundancy among them. Although higher order mutants did not display more severe phloem defects, they exhibited significantly reduced root growth compared with the ops opl2 mutant. These results indicate a direct contribution of the investigated OPL genes to meristematic activity. While our focus was on root growth, the OPS/OPL gene family also plays a positive role in regulating shoot growth, emphasizing its broader impact on plant development. Furthermore, our analyses reiterate the central role of OPS and the phloem domain in controlling overall plant growth.

Hormonal control of the molecular networks guiding vascular tissue development in the primary root meristem of Arabidopsis

Vascular tissues serve a dual function in plants, both providing physical support and controlling the transport of nutrients, water, hormones, and other small signaling molecules. Xylem tissues transport water from root to shoot; phloem tissues transfer photosynthates from shoot to root; while divisions of the (pro)cambium increase the number of xylem and phloem cells. Although vascular development constitutes a continuous process from primary growth in the early embryo and meristem regions to secondary growth in the mature plant organs, it can be artificially separated into distinct processes including cell type specification, proliferation, patterning, and differentiation. In this review, we focus on how hormonal signals orchestrate the molecular regulation of vascular development in the Arabidopsis primary root meristem. Although auxin and cytokinin have taken center stage in this aspect since their discovery, other hormones including brassinosteroids, abscisic acid, and jasmonic acid also take leading roles during vascular development. All these hormonal cues synergistically or antagonistically participate in the development of vascular tissues, forming a complex hormonal control network.

Glucose-6-P/phosphate translocator2 mediates the phosphoglucose-isomerase1-independent response to microbial volatiles

In Arabidopsis (Arabidopsis thaliana), the plastidial isoform of phosphoglucose isomerase (PGI1) mediates photosynthesis, metabolism, and development, probably due to its involvement in the synthesis of isoprenoid-derived signals in vascular tissues. Microbial volatile compounds (VCs) with molecular masses of <45 Da promote photosynthesis, growth, and starch overaccumulation in leaves through PGI1-independent mechanisms. Exposure to these compounds in leaves enhances the levels of GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR2 (GPT2) transcripts. We hypothesized that the PGI1-independent response to microbial volatile emissions involves GPT2 action. To test this hypothesis, we characterized the responses of wild-type (WT), GPT2-null gpt2-1, PGI1-null pgi1-2, and pgi1-2gpt2-1 plants to small fungal VCs. In addition, we characterized the responses of pgi1-2gpt2-1 plants expressing GPT2 under the control of a vascular tissue- and root tip-specific promoter to small fungal VCs. Fungal VCs promoted increases in growth, starch content, and photosynthesis in WT and gpt2-1 plants. These changes were substantially weaker in VC-exposed pgi1-2gpt2-1 plants but reverted to WT levels with vascular and root tip-specific GPT2 expression. Proteomic analyses did not detect enhanced levels of GPT2 protein in VC-exposed leaves and showed that knocking out GPT2 reduced the expression of photosynthesis-related proteins in pgi1-2 plants. Histochemical analyses of GUS activity in plants expressing GPT2-GUS under the control of the GPT2 promoter showed that GPT2 is mainly expressed in root tips and vascular tissues around hydathodes. Overall, the data indicated that the PGI1-independent response to microbial VCs involves resetting of the photosynthesis-related proteome in leaves through long-distance GPT2 action.

OCTOPUS regulates BIN2 to control leaf curvature in Chinese cabbage

Heading is one of the most important agronomic traits for Chinese cabbage crops. During the heading stage, leaf axial growth is an essential process. In the past, most genes predicted to be involved in the heading process have been based on leaf development studies in Arabidopsis. No genes that control leaf axial growth have been mapped and cloned via forward genetics in Chinese cabbage. In this study, we characterize the inward curling mutant ic1 in Brassica rapa ssp. pekinensis and identify a mutation in the OCTOPUS (BrOPS) gene by map-based cloning. OPS is involved in phloem differentiation in Arabidopsis, a functionalization of regulating leaf curvature that is differentiated in Chinese cabbage. In the presence of brassinosteroid (BR) at the early heading stage in ic1, the mutation of BrOPS fails to sequester brassinosteroid insensitive 2 (BrBIN2) from the nucleus, allowing BrBIN2 to phosphorylate and inactivate BrBES1, which in turn relieves the repression of BrAS1 and results in leaf inward curving. Taken together, the results of our findings indicate that BrOPS positively regulates BR signaling by antagonizing BrBIN2 to promote leaf epinastic growth at the early heading stage in Chinese cabbage.

Phloem function and development-biophysics meets genetics

Evolution of the vascular tissues allowed plants to efficiently settle land, occupy new ecological niches, and thereby crucially shape earth's biosphere. Of the two conducting cell types in the plant vasculature, the tubular network of phloem sieve elements transports phloem sap from source to sink organs. Recent years have witnessed the identification of ever more regulators of sieve element differentiation, as well as a more detailed understanding of phloem physiology and function. From molecular regulators of the commitment to sieve element fate, to enzymatic executors of the differentiation process, the toolbox to investigate sieve element formation has been greatly enlarged. To connect the various players in different genetic layers, and thus to ultimately attain a comprehensive description and understanding of sieve element development at the molecular level, appears to be within reach.

OCTOPUS-LIKE 2, a novel player in Arabidopsis root and vascular development, reveals a key role for OCTOPUS family genes in root metaphloem sieve tube differentiation

Protophloem and metaphloem sieve tubes are essential for transporting carbohydrates and signalling molecules towards sink tissues. OCTOPUS (OPS) was previously identified as an important regulator of protophloem differentiation in Arabidopsis roots. Here, we investigated the role of OCTOPUS-LIKE 2 (OPL2), a gene homologous to OPS. OPL2 expression patterns were analysed, and functional equivalence of OPS and OPL2 was tested. Mutant and double mutant phenotypes were investigated. OPS and OPL2 displayed overlapping expression patterns and a high degree of functional overlap. A mutation in OPL2 revealed redundant functions of OPS and OPL2 in developmental processes in which OPS was known to play a role, notably cotyledon vascular patterning and protophloem development. Moreover, we also uncovered redundant roles for OPS and OPL2 in leaf vascular patterning and, most interestingly, metaphloem sieve tube differentiation. Our results reveal a novel OPS-like protein that, together with OPS, is an important regulator of vascular patterning, root growth and phloem development. OPS and OPL2 are the first genes identified that play a role in metaphloem sieve tube differentiation.

Katanin: A Sword Cutting Microtubules for Cellular, Developmental, and Physiological Purposes

KATANIN is a well-studied microtubule severing protein affecting microtubule organization and dynamic properties in higher plants. By regulating mitotic and cytokinetic and cortical microtubule arrays it is involved in the progression of cell division and cell division plane orientation. KATANIN is also involved in cell elongation and morphogenesis during plant growth. In this way KATANIN plays critical roles in diverse plant developmental processes including the development of pollen, embryo, seed, meristem, root, hypocotyl, cotyledon, leaf, shoot, and silique. KATANIN-dependent microtubule regulation seems to be under the control of plant hormones. This minireview provides an overview on available KATANIN mutants and discusses advances in our understanding of KATANIN biological roles in plants.

A novel system for xylem cell differentiation in Arabidopsis thaliana

During vascular development, procambial and cambial cells give rise to xylem and phloem cells. Because the vascular tissue is deeply embedded, it has been difficult to analyze the processes of vascular development in detail. Here, we establish a novel in vitro experimental system in which vascular development is induced in Arabidopsis thaliana leaf-disk cultures using bikinin, an inhibitor of glycogen synthase kinase 3 proteins. Transcriptome analysis reveals that mesophyll cells in leaf disks synchronously turn into procambial cells and then differentiate into tracheary elements. Leaf-disk cultures from plants expressing the procambial cell markers TDR(pro):GUS and TDR(pro):YFP can be used for spatiotemporal visualization of procambial cell formation. Further analysis with the tdr mutant and TDIF (tracheary element differentiation inhibitory factor) indicates that the key signaling TDIF-TDR-GSK3s regulates xylem differentiation in leaf-disk cultures. This new culture system can be combined with analysis using the rich material resources for Arabidopsis including cell-marker lines and mutants, thus offering a powerful tool for analyzing xylem cell differentiation.

Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching

Protophloem is a specialized vascular tissue in growing plant organs, such as root meristems. In Arabidopsis mutants with impaired primary root protophloem differentiation, brevis radix (brx) and octopus (ops), meristematic activity and consequently overall root growth are strongly reduced. Second site mutation in the protophloem-specific presumed phosphoinositide 5-phosphatase cotyledon vascular pattern 2 (CVP2), but not in its homolog CVP2-like 1 (CVL1), partially rescues brx defects. Consistent with this finding, CVP2 hyperactivity in a wild-type background recreates a brx phenotype. Paradoxically, however, while cvp2 or cvl1 single mutants display no apparent root defects, the root phenotype of cvp2 cvl1 double mutants is similar to brx or ops, although, as expected, cvp2 cvl1 seedlings contain more phosphatidylinositol-4,5-biphosphate. Thus, tightly balanced phosphatidylinositol-4,5-biphosphate levels appear essential for proper protophloem differentiation. Genetically, OPS acts downstream of phosphatidylinositol-4,5-biphosphate levels, as cvp2 mutation cannot rescue ops defects, whereas increased OPS dose rescues cvp2 cvl1 defects. Finally, all three mutants display higher density and accelerated emergence of lateral roots, which correlates with increased auxin response in the root differentiation zone. This phenotype is also created by application of peptides that suppress protophloem differentiation, clavata3/embryo surrounding region 26 (CLE26) and CLE45. Thus, local changes in the primary root protophloem systemically shape overall root system architecture.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Tissue-specific expression of SMALL AUXIN UP RNA41 differentially regulates cell expansion and root meristem patterning in Arabidopsis

Among the three primary auxin-induced gene families, Auxin/Indole-3-Acetic Acid (Aux/IAA), Gretchen Hagen3 (GH3) and SMALL AUXIN UP RNA (SAUR), the function of SAUR genes remains unclear. Arabidopsis SAUR genes have been phylogenetically classified into three clades. Recent work has suggested that SAUR19 (clade II) and SAUR63 (clade I) promote cell expansion through the modulation of auxin transport. Herein, we present our work on SAUR41, a clade III SAUR gene with a distinctive expression pattern in root meristems. SAUR41 was normally expressed in the quiescent center and cortex/endodermis initials; upon auxin stimulation, the expression was provoked in the endodermal layer. During lateral root development, SAUR41 was expressed in prospective stem cell niches of lateral root primordia and in expanding endodermal cells surrounding the primordia. SAUR41-EGFP (enhanced green fluorescent protein) fusion proteins localized to the cytoplasm. Overexpression of SAUR41 from the Cauliflower mosaic virus 35S promoter led to pleiotropic auxin-related phenotypes, including long hypocotyls, increased vegetative biomass and lateral root development, expanded petals and twisted inflorescence stems. Ectopic SAUR41 proteins were able to promote auxin transport in hypocotyls. Tissue-specific expression of SAUR41 from the PIN1, WOX5, PLT2 and ACR4 promoters induced the formation of new auxin accumulation/signaling peaks above the quiescent centers, whereas tissue-specific expression of SAUR41 from the PIN2 and PLT2 promoters enhanced root gravitropic growth. Cells in the root stem cell niches of these transgenic seedlings were differentially enlarged. The distinctive expression pattern of the SAUR41 gene and the explicit function of SAUR41 proteins implied that further investigations on the loss-of-function phenotypes of this gene in root development and environmental responses are of great interest.

Genetic and hormonal regulation of cambial development

The stems and roots of most dicot plants increase in diameter by radial growth, due to the activity of secondary meristems. Two types of meristems function in secondary plant body formation: the vascular cambium, which gives rise to secondary xylem and phloem, and the cork cambium, which produces a bark layer that replaces the epidermis and protects the plant stem from mechanical damage and pathogens. Cambial development, the initiation and activity of the vascular cambium, leads to an accumulation of wood, the secondary xylem tissue. The thick, cellulose-rich cell walls of wood provide a source of cellulose and have the potential to be used as a raw material for sustainable and renewable energy production. In this review, we will discuss what is known about the mechanisms regulating the cambium and secondary tissue development.

OCTOPUS, a polarly localised membrane-associated protein, regulates phloem differentiation entry in Arabidopsis thaliana

Vascular development is embedded into the developmental context of plant organ differentiation and can be divided into the consecutive phases of vascular patterning and differentiation of specific vascular cell types (phloem and xylem). To date, only very few genetic determinants of phloem development are known. Here, we identify OCTOPUS (OPS) as a potentiator of phloem differentiation. OPS is a polarly localised membrane-associated protein that is initially expressed in provascular cells, and upon vascular cell type specification becomes restricted to the phloem cell lineage. OPS mutants display a reduction of cotyledon vascular pattern complexity and discontinuous phloem differentiation, whereas OPS overexpressers show accelerated progress of cotyledon vascular patterning and phloem differentiation. We propose that OPS participates in vascular differentiation by interpreting longitudinal signals that lead to the transformation of vascular initials into differentiating protophloem cells.

PF19 encodes the p60 catalytic subunit of katanin and is required for assembly of the flagellar central apparatus in Chlamydomonas

For all eukaryotic cilia the basal bodies provide a template for the assembly of the doublet microtubules, and intraflagellar transport provides a mechanism for transport of axonemal components into the growing cilium. What is not known is how the central pair of microtubules is nucleated or how their associated polypeptides are assembled. Here we report that the Chlamydomonas pf19 mutation results in a single amino acid change within the p60 catalytic subunit of katanin, and that this mutation prevents microtubule severing activity. The pf19 mutant has paralyzed flagella that lack the central apparatus. Using a combination of mutant analysis, RNAi-mediated reduction of protein expression and in vitro assays, we demonstrate that the p60 catalytic subunit of the microtubule severing protein katanin is required for central apparatus assembly in Chlamydomonas. In addition, we show that in Chlamydomonas the microtubule severing activity of p60 katanin is not required for stress-induced deflagellation or cell cycle progression as has been previously reported.

Double-filter identification of vascular-expressed genes using Arabidopsis plants with vascular hypertrophy and hypotrophy

Genes expressed in vascular tissues have been identified by several strategies, usually with a focus on mature vascular cells. In this study, we explored the possibility of using two opposite types of altered tissue compositions in combination with a double-filter selection to identify genes with a high probability of vascular expression in early organ primordia. Specifically, we generated full-transcriptome microarray profiles of plants with (a) genetically strongly reduced and (b) pharmacologically vastly increased vascular tissues and identified a reproducible cohort of 158 transcripts that fulfilled the dual requirement of being underrepresented in (a) and overrepresented in (b). In order to assess the predictive value of our identification scheme for vascular gene expression, we determined the expression patterns of genes in two unbiased subsamples. First, we assessed the expression patterns of all twenty annotated transcription factor genes from the cohort of 158 genes and found that seventeen of the twenty genes were preferentially expressed in leaf vascular cells. Remarkably, fifteen of these seventeen vascular genes were clearly expressed already very early in leaf vein development. Twelve genes with published leaf expression patterns served as a second subsample to monitor the representation of vascular genes in our cohort. Of those twelve genes, eleven were preferentially expressed in leaf vascular tissues. Based on these results we propose that our compendium of 158 genes represents a sample that is highly enriched for genes expressed in vascular tissues and that our approach is particularly suited to detect genes expressed in vascular cell lineages at early stages of their inception.

CLE peptides can negatively regulate protoxylem vessel formation via cytokinin signaling

Cell-cell communication is critical for tissue and organ development. In plants, secretory CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) peptides function as intercellular signaling molecules in various aspects of tissue development including vascular development. However, little is known about intracellular signaling pathways functioning in vascular development downstream of the CLE ligands. We show that CLE peptides including CLE10, which is preferentially expressed in the root vascular system, inhibit protoxylem vessel formation in Arabidopsis roots. GeneChip analysis displayed that CLE10 peptides repressed specifically the expression of two type-A Arabidopsis Response Regulators (ARRs), ARR5 and ARR6, whose products act as negative regulators of cytokinin signaling. The arr5 arr6 roots exhibited defective protoxylem vessel formation. These results indicate that CLE10 inhibits protoxylem vessel formation by suppressing the expression of type-A ARR genes including ARR5 and ARR6. This was supported by the finding that CLE10 did not suppress protoxylem vessel formation in a background of arr10 arr12, a double mutant of type-B ARR genes. Thus, our results revealed cross-talk between CLE signaling and cytokinin signaling in protoxylem vessel formation in roots. Taken together with the indication that cytokinin signaling functions downstream of the CLV3/WUS signaling pathway in the shoot apical meristem, the cross-talk between CLE and cytokinin signaling pathways may be a common feature in plant development.

Identification of Arabidopsis subtilisin-like serine protease specifically expressed in root stele by gene trapping

Xylem plays a role not only in the transport of water and nutrients but also in the regulation of growth and development through the transport of biologically active substances. In addition to mineral salts, xylem sap contains hormones, organic nutrients and proteins. However, the physiological functions of most of those substances remain unclear. To explore genes involved in xylem sap production, we identified Arabidopsis genes expressed in the root stele of the root hair zone from gene-trap lines by randomly inserting the beta-glucuronidase gene into the genome. Among 26 000 gene-trap lines, we found that 10 lines had beta-glucuronidase (GUS) staining predominantly in the root stele of the root hair zone and no GUS staining in the shoots. Of these 10 lines, 2 lines showed that gene-trap tags inserted into the promoter region of the same gene, denoted Arabidopsis thaliana subtilase 4.12(AtSBT4.12). Analysis of AtSBT4.12 promoter using an pAtSBT4.12::beta-glucuronidase transgenic line showed that the AtSBT4.12 gene was expressed only in the root stele of the root hair zone. AtSBT4.12 expression in roots was increased by application of methyl jasmonate. Subtilase proteins are commonly detected in proteomic analyses of xylem sap from various plant species, including Brassica napus, a relative of Arabidopsis. These results suggest that AtSBT4.12 may be a protein localized in the apoplast of root stele including xylem vessel and involved in stress responses in Arabidopsis roots.

A DNA element between At4g28630 and At4g28640 confers companion-cell specific expression following the sink-to-source transition in mature minor vein phloem

The collection phloem in minor veins is distinct from other vein classes in that the minor veins mature during the sink to source transition and are the primary sites of phloem loading. After maturation, minor vein phloem maintains its character in part through minor-vein specific regulatory cascades; however despite its physiological significance, little of these developmental programs is understood. From an Arabidopsis enhancer trap screen, we identified MATURE MINOR VEIN ELEMENT1 (MMVE1) in the intergenic region between two oppositely oriented genes, the ABC transporter ATM1 (At4g28630) and IAA11 (At4g28640). MMVE1 promotes reporter gene activity in minor vein phloem in a pattern resembling the sink to source transition. Promoter truncation experiments and phylogenetic footprinting demonstrate sequences proximal to ATM1 promote minor vein expression whereas sequences closer to IAA11 repress it. Both orientations of the promoter were used to drive expression of CONSTANS to generate a phloem mobile signal conferring early flowering under non-inductive conditions. Tandem copies of MMVE1 increase minor vein expression strength and specificity. MMVE1 is the first minor vein enhancer characterized from a species that loads from the apoplast, and supports the presence of unique regulatory cascades operating in minor vein phloem.

Gene trap-based identification of a guard cell promoter in Arabidopsis

Preserving crop yield under drought stress is a major challenge for modern agriculture. To cope with the detrimental effects of water scarcity on crop productivity it is important to develop new plants with a more sustainable use of water and capable of higher performance under stress conditions. Transpiration through stomatal pores accounts for over 90% of water loss in land plants. Recent studies have increased our understanding of the networks that control stomatal activity and have led to practical approaches for enhancing drought tolerance. Genetic engineering of target genes in stomata requires effective expression systems, including suitable promoters, because constitutive promoters (i.e., CaMV35S) are not always functional or can have negative effects on plant growth and productivity. Here we describe the identification of the CYP86A2 guard cell promoter and discuss its potential for gene expression in stomata.

Gene trap lines identify Arabidopsis genes expressed in stomatal guard cells

We employed a gene trap approach to identify genes expressed in stomatal guard cells of Arabidopsis thaliana. We examined patterns of reporter gene expression in approximately 20,000 gene trap lines, and recovered five lines with exclusive or preferential expression in stomata. The screen yielded two insertions in annotated genes, encoding the CYTOCHROME P450 86A2 (CYP86A2) mono-oxygenase, and the PLEIOTROPIC DRUG RESISTANCE 3 (AtPDR3) transporter. Expression of the trapped genes in guard cells was confirmed by RT-PCR experiments in purified stomata. Examination of homozygous mutant lines revealed that abscisic acid (ABA)-induced stomatal closure was impaired in the atpdr3 mutant. In three lines, insertions occurred outside transcribed units. Expression analysis of the genes surrounding the trapping inserts identified two genes selectively expressed in guard cells, corresponding to a PP2C PROTEIN PHOSPHATASE and an unknown expressed protein gene. Statistical analyses of the chromosomal regions tagged by the gene trap insertions revealed an over-represented [A/T]AAAG motif, previously described as an essential cis-active element for gene expression in stomata. The lines described in this work identify novel genes involved in the modulation of stomatal activity, provide useful markers for the study of developmental pathways in guard cells, and are a valuable source of guard cell-specific promoters.

Gene trap mutagenesis: a functional genomics approach towards reproductive research

We have entered a new era of genomics in biomedical research with the availability of genome-wide sequences and expression data, resulting in the identification of a huge number of novel reproductive genes. The challenge we are facing today is how to determine the function of those novel and known genes and their roles in normal reproductive physiology, such as gamete production, pregnancy and fertilization, and the disease physiology such as infertility, spontaneous abortion and gynecological cancers. Mouse genetics has contributed tremendously to our understanding of the genetic causes of human diseases in the past decades. The establishment of mouse mutations is an effective way to understand the function of many reproductive proteins. One of the fast-growing mouse mutagenesis technologies-gene trap mutagenesis-represents a cost-effective way to generate mutations because of the public availability of mouse embryonic stem (ES) cell lines carrying insertional mutations and the continuing expansion of those ES gene trap cell lines. We review here the gene trapping technology and in particular examine its efficacy in generating mouse mutations for reproductive research. Even with the existing gene trap cell lines, many of the genes important for reproductive function through traditional knockout and chemical mutagenesis have been trapped, demonstrating gene trapping's efficacy in mutating genes involved in reproductive development. Comparing genes expressed in specific reproductive sub-cellular organelles and in the entire testis and ovary with gene trap lines in the International Gene Trap Consortium (IGTC) database, we could identify a significant portion of those genes as having been trapped, representing a great resource for establishing mouse models for reproductive research. Establishment and analysis of these mouse models, for example, could help with identifying genetic abnormalities underlying male infertility and other reproductive diseases.

A GUS/Luciferase Fusion Reporter for Plant Gene Trapping and for Assay of Promoter Activity with Luciferin-Dependent Control of the Reporter Protein Stability

A gene-trapping vector carrying a GUS/Luciferase dual reporter gene was developed to establish an efficient and convenient screening system for T-DNA-based gene trapping in plants. A key feature of this gene trap scheme is to place two different types of reporters, luciferase (Luc) and beta-glucuronidase (GUS), as a fusion protein within a trapped gene to probe the activity of the gene. Luc is then utilized as a non-invasive, vital and highly sensitive screening reporter to identify trapped lines, including direct screening of the trapped lines from the primary T-DNA mutant pools. GUS is utilized as a histochemical assay reporter to analyze detailed cellular expression patterns. Transgenic expression studies in Arabidopsis showed that this fusion reporter protein retains functional enzyme activity for both GUS and Luc. Using this system in Arabidopsis, we were able to identify 3,737 trapped lines from 26,900 individual T-DNA insertion lines. Sequence determination of the T-DNA insertion loci in the genome of 78 trapped lines identified GUS/Luc fusions with 27 annotated Arabidopsis genes which included a subset of transcription factors, protein kinases, regulatory proteins and metabolic enzymes. Of these, particular expression patterns of four tagged genes were further confirmed by analyzing putative promoter regions of the corresponding wild-type genes. Furthermore, the protein stability of the GUS/Luc fusion reporter was controlled by application of luciferase substrate (luciferin), overcoming the excessive stability problem of GUS that causes misrepresentation of the transcriptional activity of a promoter. These results demonstrate the utility of the GUS/Luc dual reporter system as a gene trap reporter for studying plant genome function and also as a convenient dual reporter system for study of gene expression.