Specificity of a promoter from the rice tungro bacilliform virus for expression in phloem tissues

Harnessing Viral Promoters for Targeted RNAi: Engineering Phloem-Specific Resistance Against Tomato Yellow Leaf Curl Thailand Virus

Phloem-specific promoter efficiently triggers graft-transmissible RNA interference (gtRNAi). We leveraged a phloem-specific promoter derived from the Rice tungro bacilliform virus, optimizing the RNAi mechanism's efficiency and specificity. Here, we detail the construction of phloem-specific promoter-based gtRNAi system and its application through grafting experiments, demonstrating its effectiveness in inducing tomato yellow leaf curl Thailand virus (TYLCHTV) resistance in non-transgenic scions. This strategy presents a practical application for protecting crops against viruses without genetically modifying the entire plant.

Targeted suppression of gibberellin biosynthetic genes ZmGA20ox3 and ZmGA20ox5 produces a short stature maize ideotype

Maize is one of the world's most widely cultivated crops. As future demands for maize will continue to rise, fields will face ever more frequent and extreme weather patterns that directly affect crop productivity. Development of environmentally resilient crops with improved standability in the field, like wheat and rice, was enabled by shifting the architecture of plants to a short stature ideotype. However, such architectural change has not been implemented in maize due to the unique interactions between gibberellin (GA) and floral morphology which limited the use of the same type of mutations as in rice and wheat. Here, we report the development of a short stature maize ideotype in commercial hybrid germplasm, which was generated by targeted suppression of the biosynthetic pathway for GA. To accomplish this, we utilized a dominant, miRNA-based construct expressed in a hemizygous state to selectively reduce expression of the ZmGA20ox3 and ZmGA20ox5 genes that control GA biosynthesis primarily in vegetative tissues. Suppression of both genes resulted in the reduction of GA levels leading to inhibition of cell elongation in internodal tissues, which reduced plant height. Expression of the miRNA did not alter GA levels in reproductive tissues, and thus, the reproductive potential of the plants remained unchanged. As a result, we developed a dominant, short-stature maize ideotype that is conducive for the commercial production of hybrid maize. We expect that the new maize ideotype would enable more efficient and more sustainable maize farming for a growing world population.

Characterization of a strong constitutive promoter from paper mulberry vein banding virus

Paper mulberry vein banding virus (PMVBV), a member of the genus Badnavirus in the family Caulimoviridae, infects paper mulberry (Broussonetia papyrifera), a dicotyledonous plant. Putative promoter regions in the PMVBV genome were tested using recombinant plant expression vectors, revealing that the promoter activity of three genome fragments was about 1.5-fold higher than that of the 35S promoter of cauliflower mosaic virus in Nicotiana benthamiana. In transformed transgenic Arabidopsis thaliana plants, these promoter constructs showed constitutive expression. Based on the activity and gene expression patterns of these three promoter constructs, a fragment of 384 bp (named PmVP) was deduced to contain the full-length promoter of the PMVBV genome. The results suggest that the PMVBV-derived promoter can be used for the constitutive expression of transgenes in dicotyledonous plants.

Developing specific leaf promoters tools for genetic use in transgenic plants towards food security

Significant yields enrichments are necessitated for meeting the rapid global growth population together with the expected demanding for food, particularly major crops. Photosynthesis improvement is an unexploited opportunity in research on improving crop yields. However, the lack of sufficient molecular promoters tools leads to the need to explore and analyze native leaf-specified promoters for manipulating photosynthesis activities in plants. Two B. distachyon promoters, sedoheptulose-1, 7-bisphosphatase (SBPase) and fructose-1, 6-bisphosphate aldolase (FBPA), were isolated and cloned into an expression vector upstream of the eYFP reporter gene. The results demonstrate that both promoters actively function in N. benthamiana leaves in both agro-transiently assays, successfully regulating expression specifically to leaf-tissues. Exploring these active promoters could potentially provide new well genetic tools for any transgene expression in plants or leaves to genetically manipulate photosynthesis for yield improvement.

Recent Advances of In Vitro Culture for the Application of New Breeding Techniques in Citrus

Citrus is one of the most important fruit crops in the world. This review will discuss the recent findings related to citrus transformation and regeneration protocols of juvenile and adult explants. Despite the many advances that have been made in the last years (including the use of inducible promoters and site-specific recombination systems), transformation efficiency, and regeneration potential still represent a bottleneck in the application of the new breeding techniques in commercial citrus varieties. The influence of genotype, explant type, and other factors affecting the regeneration and transformation of the most used citrus varieties will be described, as well as some examples of how these processes can be applied to improve fruit quality and resistance to various pathogens and pests, including the potential of using genome editing in citrus. The availability of efficient regeneration and transformation protocols, together with the availability of the source of resistance, is made even more important in light of the fast diffusion of emerging diseases, such as Huanglongbing (HLB), which is seriously challenging citriculture worldwide.

Simultaneous resistance against the two viruses causing rice tungro disease using RNA interference

Rice tungro is the most important viral disease affecting rice in South and Southeast Asia, caused by two viruses rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). Transgenic resistance using RNA-interference (RNAi) has been reported individually against RTBV and RTSV earlier. Here we report the development of transgenic rice plants expressing RNAi against both RTBV and RTSV simultaneously. A DNA construct carrying 300 bp of RTBV DNA and 300 bp of RTSV cDNA were cloned as the two arms in hairpin orientation in a binary plasmid background to generate RNAi against both viruses simultaneously. Transgenic rice plants were raised using the above construct and their resistance against RTBV and RTSV was quantified at the T₁ plants. Levels of both the viral nucleic acids showed a fall of 100- to 500-fold in the above plants, compared with the non-transgenic controls, coupled with the amelioration of stunting. The transgenic plants also retained higher chlorophyll levels than the control non-transgenic plants after infection with RTBV and RTSV. Small RNA analysis of virus inoculated transgenic plants indicated the presence of 21 nt and 22 nt siRNAs specific to RTBV and RTSV. The evidence points towards an active RNAi mechanism leading to resistance against the tungro viruses in the plants analysed.

Identification of Leaf Promoters for Use in Transgenic Wheat

Wheat yields have plateaued in recent years and given the growing global population there is a pressing need to develop higher yielding varieties to meet future demand. Genetic manipulation of photosynthesis in elite wheat varieties offers the opportunity to significantly increase yields. However, the absence of a well-defined molecular tool-box of promoters to manipulate leaf processes in wheat hinders advancements in this area. Two promoters, one driving the expression of sedoheptulose-1,7-bisphosphatase (SBPase) and the other fructose-1,6-bisphosphate aldolase (FBPA) from Brachypodium distachyon were identified and cloned into a vector in front of the GUS reporter gene. Both promoters were shown to be functionally active in wheat in both transient assays and in stably transformed wheat plants. Analysis of the stable transformants of wheat (cv. Cadenza) showed that both promoters controlled gus expression throughout leaf development as well as in other green tissues. The availability of these promoters provides new tools for the expression of genes in transgenic wheat leaves and also paves the way for multigene manipulation of photosynthesis to improve yields.

Interaction of Arabidopsis TGA3 and WRKY53 transcription factors on Cestrum yellow leaf curling virus (CmYLCV) promoter mediates salicylic acid-dependent gene expression in planta

This paper highlighted a salicylic acid-inducible Caulimoviral promoter fragment from Cestrum yellow leaf curling virus (CmYLCV). Interaction of Arabidopsis transcription factors TGA3 and WRKY53 on CmYLCV promoter resulted in the enhancement of the promoter activity via NPR1-dependent salicylic acid signaling. Several transcriptional promoters isolated from plant-infecting Caulimoviruses are being presently used worldwide as efficient tools for plant gene expression. The CmYLCV promoter has been isolated from the Cestrum yellow leaf curling virus (Caulimoviruses) and characterized more than 12 years ago; also we have earlier reported a near-constitutive, pathogen-inducible CmYLCV promoter fragment (-329 to +137 from transcription start site; TSS) that enhances stronger (3×) expression than the previously reported fragments; all these fragments are highly efficient in monocot and dicot plants (Sahoo et al. Planta 240: 855-875, 2014). Here, we have shown that the full-length CmYLCV promoter fragment (-729 to +137 from TSS) is salicylic acid (SA) inducible. In this context, we have performed an in-depth study to elucidate the factors responsible for SA-inducibility of the CmYLCV promoter. We found that the as-1 ₁ and W-box₁ elements (located at -649 and -640 from the TSS) of the CmYLCV promoter are required for SA-induced activation by recruiting Arabidopsis TGA3 and WRKY53 transcription factors. Consequently, as a nascent observation, we established the physical interaction between TGA3 and WYKY53; also demonstrated that the N-terminal domain of TGA3 is sufficient for the interaction with the full-length WRKY53. Such interaction synergistically activates the CmYLCV promoter activity in planta. Further, we found that activation of the CmYLCV promoter by SA through TGA3 and WRKY53 interaction depends on NPR1. Finally, the findings presented here provide strong support for the direct regulatory roles of TGA3 and WRKY53 in the SA and NPR1-dependent activation of a Caulimoviral promoter (CmYLCV).

A novel Sugarcane bacilliform virus promoter confers gene expression preferentially in the vascular bundle and storage parenchyma of the sugarcane culm

BACKGROUND

Saccharum species such as sugarcane and energy cane are key players in the expanding bioeconomy for sugars, bioenergy, and production of high-value proteins. Genomic tools such as culm-regulated promoters would be of great value in terms of improving biomass characteristics through enhanced carbon metabolism for sugar accumulation and/or fiber content for biofuel feedstock. Unlike the situation in dicots, monocot promoters currently used are limited and mostly derived from highly expressed constitutive plant genes and viruses. In this study, a novel promoter region of Sugarcane bacilliform virus (SCBV; genus Badnavirus, family Caulimoviridae), SCBV21 was cloned and mapped by deletion analysis and functionally characterized transiently in monocot and dicot species and stably in sugarcane.

RESULTS

In silico analysis of SCBV21 [1816 base pair (bp)] identified two putative promoter regions (PPR1 and PPR2) with transcription start sites (TSS1 and TSS2) and two TATA-boxes (TATAAAT and ATATAA), and several vascular-specific and regulatory elements. Deletion analysis revealed that the 710 bp region spanning PPR2 (with TSS2 and ATATAA) at the 3' end of SCBV21 retained the full promoter activity in both dicots and monocots, as shown by transient expression of the enhanced yellow fluorescent protein (EYFP) gene. In sugarcane young leaf segments, SCBV21 directed a 1.8- and 2.4-fold higher transient EYFP expression than the common maize ubiquitin 1 (Ubi1) and Cauliflower mosaic virus 35S promoters, respectively. In transgenic sugarcane, SCBV21 conferred a preferential expression of the β-glucuronidase (GUS) gene in leaves and culms and specifically in the culm storage parenchyma surrounding the vascular bundle and in vascular phloem cells. Among the transgenic events and tissues characterized in this study, the SCBV21 promoter frequently produced higher GUS activity than the Ubi1 or 35S promoters in a manner that was not obviously correlated with the transgene copy number.

CONCLUSIONS

The newly developed plant viral SCBV21 promoter is distinct from the few existing SCBV promoters in its sequence and expression pattern. The potential of SCBV21 as a tissue-regulated promoter with a strong activity in the culm vascular bundle and its storage parenchyma makes it useful in sugarcane engineering for improved carbon metabolism, increased bioenergy production, and enhanced stress tolerance.

Strength, Stability, and cis-Motifs of In silico Identified Phloem-Specific Promoters in Brassica juncea (L.)

Aphids, a hemipteran group of insects pose a serious threat to many of the major crop species including Brassica oilseeds. Transgenic strategies for developing aphid-resistant plant types necessitate phloem-bound expression of the insecticidal genes. A few known phloem-specific promoters, in spite of tissue-specific activity fail to confer high level gene-expression. Here, we identified seven orthologues of phloem-specific promoters in B. juncea (Indian mustard), and experimentally validated their strength of expression in phloem exudates. Significant cis-motifs, globally occurring in phloem-specific promoters showed variable distribution frequencies in these putative phloem-specific promoters of B. juncea. In RT-qPCR based gene-expression study promoter of Glutamine synthetase 3A (GS3A) showed multifold higher activity compared to others, across the different growth stages of B. juncea plants. A statistical method employing four softwares was devised for rapidly analysing stability of the promoter-activities across the plant developmental stages. Different statistical softwares ranked these B. juncea promoters differently in terms of their stability in promoter-activity. Nevertheless, the consensus in output empirically suggested consistency in promoter-activity of the six B. juncea phloem- specific promoters including GS3A. The study identified suitable endogenous promoters for high level and consistent gene-expression in B. juncea phloem exudate. The study also demonstrated a rapid method of assessing species-specific strength and stability in expression of the endogenous promoters.

Overexpression of a NAC transcription factor delays leaf senescence and increases grain nitrogen concentration in wheat

Increasing the duration of leaf photosynthesis during grain filling using slow-senescing functional stay-green phenotypes is a possible route for increasing grain yields in wheat (Triticum aestivum L.). However, delayed senescence may negatively affect nutrient remobilisation and hence reduce grain protein concentrations and grain quality. A novel NAC1-type transcription factor (hereafter TaNAC-S) was identified in wheat, with gene expression located primarily in leaf/sheath tissues, which decreased during post-anthesis leaf senescence. Expression of TaNAC-S in the second leaf correlated with delayed senescence in two doubled-haploid lines of an Avalon × Cadenza population (lines 112 and 181), which were distinct for leaf senescence. Transgenic wheat plants overexpressing TaNAC-S resulted in delayed leaf senescence (stay-green phenotype). Grain yield, aboveground biomass, harvest index and total grain N content were unaffected, but NAC over-expressing lines had higher grain N concentrations at similar grain yields compared to non-transgenic controls. These results indicate that TaNAC-S is a negative regulator of leaf senescence, and that delayed leaf senescence may lead not only to increased grain yields but also to increased grain protein concentrations.

Efficient chimeric plant promoters derived from plant infecting viral promoter sequences

In the present study, we developed a set of three chimeric/hybrid promoters namely FSgt-PFlt, PFlt-UAS-2X and MSgt-PFlt incorporating different important domains of Figwort Mosaic Virus sub-genomic transcript promoter (FSgt, -270 to -60), Mirabilis Mosaic Virus sub-genomic transcript promoter (MSgt, -306 to -125) and Peanut Chlorotic Streak Caulimovirus full-length transcript promoter (PFlt-, -353 to +24 and PFlt-UAS, -353 to -49). We demonstrated that these chimeric/hybrid promoters can drive the expression of reporter genes in different plant species including tobacco, Arabidopsis, petunia, tomato and spinach. FSgt-PFlt, PFlt-UAS-2X and MSgt-PFlt promoters showed 4.2, 1.5 and 1.2 times stronger GUS activities compared to the activity of the CaMV35S promoter, respectively, in tobacco protoplasts. Protoplast-derived recombinant promoter driven GFP showed enhanced accumulation compared to that obtained under the CaMV35S promoter. FSgt-PFlt, PFlt-UAS-2X and MSgt-PFlt promoters showed 3.0, 1.3 and 1.0 times stronger activities than the activity of the CaMV35S² (a modified version of the CaMV35S promoter with double enhancer domain) promoter, respectively, in tobacco (Nicotiana tabacum, var. Samsun NN). Alongside, we observed a fair correlation between recombinant promoter-driven GUS accumulation with the corresponding uidA-mRNA level in transgenic tobacco. Histochemical (X-gluc) staining of whole transgenic seedlings and fluorescence images of ImaGene Green™ treated floral parts expressing the GUS under the control of recombinant promoters also support above findings. Furthermore, we confirmed that these chimeric promoters are inducible in the presence of 150 μM salicylic acid (SA) and abscisic acid (ABA). Taken altogether, we propose that SA/ABA inducible chimeric/recombinant promoters could be used for strong expression of gene(s) of interest in crop plants.

The plant-specific dof transcription factors family: new players involved in vascular system development and functioning in Arabidopsis

In higher plants phloem and xylem are responsible for long-distance transport of water, nutrients, and signals that act systemically at short or long-distance to coordinate developmental processes. The formation of the plant vascular system is a complex process that integrates signaling events and gene regulation at transcriptional and posttranscriptional levels. Thanks to transcriptomic and proteomic analysis we start to better understand the mechanisms underlying the formation and the functioning of the vascular system. The role of the DNA-binding with one finger (Dof TFs), a group of plant-specific transcription factors, recently emerged as part of the transcriptional regulatory networks acting on the formation and functioning of the vascular tissues. More than half of the members of this TF family are expressed in the vascular system. In addition some of them have been proposed to be mobile proteins, suggesting a possible role in the control of short- or long-distance signaling as well. This review summarizes the current knowledge on Dof TFs family in Arabidopsis with a special focus on their role in vascular development and functioning.

Development of vascular tissue and stress inducible hybrid-synthetic promoters through dof-1 motifs rearrangement

A Caulimovirus-based hybrid-promoter, EFCFS, was derived by fusing the distal region (-227 to -54, FUAS) of Figwort mosaic virus full-length transcript promoter (F20) with the core promoter (-151 to +12, FS3CP) domain of Figwort mosaic virus sub-genomic transcript promoter (FS3). The hybrid-promoter (EFCFS) showed enhanced activity compared to the CaMV35S, F20 and FS3 promoters; while it showed equivalent activity with that of the CAMV35S(2) promoter in both transient protoplast (Nicotiana tabacum cv. Xanthi Brad) and transgenic plants (Nicotiana tabacum; Samsun NN). Further, we have engineered the EFCFS promoter sequence by inserting additional copies of the stress-inducible 'AAAG' cis-motif (Dof-1) to generate a set of three hybrid-synthetic promoters namely; EFCFS-HS-1, EFCFS-HS-2 and EFCFS-HS-3-containing 10, 11 and 13 'AAAG' motif, respectively. Transgenic plants expressing these hybrid synthetic promoters coupled to the GUS reporter were developed and their transcriptional activities were compared with F20, FS3, 35S and 35S(2) promoters, respectively. The relative levels of uidA-mRNA accumulation in transgenic plants driven by above promoters individually were compared by qRT-PCR. Localization of GUS reporter activity in plant tissue was assayed by histochemical approach. CLSM-based study revealed that hybrid-synthetic promoters namely; EFCFS-HS-1, EFCFS-HS-2 and EFCFS-HS-3 showed enhanced activity in vascular tissue compared to the CaMV35S promoter. In the presence of abiotic stress elicitors, salicylic acid and jasmonic acid, the EFCFS-HS-1 promoters showed enhanced activity compared to the 35S promoter. Newly derived hybrid-synthetic promoter/s with enhanced activity and stress inducibility could become efficient tools for advancement of plant biotechnology.

Evaluation of four phloem-specific promoters in vegetative tissues of transgenic citrus plants

'Mexican' lime (Citrus aurantifolia Swingle) was transformed with constructs that contained chimeric promoter-gus gene fusions of phloem-specific rolC promoter of Agrobacterium rhizogenes, Arabidopsis thaliana sucrose-H(+) symporter (AtSUC2) gene promoter of Arabidopsis thaliana, rice tungro bacilliform virus (RTBV) promoter and sucrose synthase l (RSs1) gene promoter of Oryza sativa (rice). Histochemical β-glucuronidase (GUS) analysis revealed vascular-specific expression of the GUS protein in citrus. The RTBV promoter was the most efficient promoter in this study while the RSs1 promoter could drive low levels of gus gene expression in citrus. These results were further validated by reverse transcription real-time polymerase chain reaction and northern blotting. Southern blot analysis confirmed stable transgene integration, which ranged from a single insertion to four copies per genome. The use of phloem-specific promoters in citrus will allow targeted transgene expression of antibacterial constructs designed to battle huanglongbing disease (HLB or citrus greening disease), associated with a phloem-limited Gram-negative bacterium.

The large intergenic region of Rice tungro bacilliform virus evolved differentially among geographically distinguished isolates

Rice tungro bacilliform virus (RTBV) is a plant pararetrovirus. The large intergenic region (LIGR) of RTBV having a single transcriptional promoter produces more than genome length pregenomic RNA (pgRNA) which directs synthesis of circular double-stranded viral DNA and serves as a polycistronic mRNA. By computer-aided analysis of LIGR, the 11 RTBV isolates sequenced so far were compared with respect to structural organization of promoter and pgRNA 5'-leader. The results revealed only 74.90% identity at LIGR between 'Southeast Asian' (SEA) and 'South Asian' (SA) isolates of RTBV indicating considerable variation between two groups which was also reflected during analysis of promoter and leader sequence. The predicted promoter region of SA isolates exhibited major variations in terms of transcription start site and consensus sequences of cis motifs expecting further exploitation of promoter region of SA isolates. The reduced length of leader sequence along with less numbers and different arrangements of small open reading frames (sORFs) in case of SA isolates might have some alterations in the control of expression of ORF II and III between the two groups. In spite of these variations, the leader sequence of both SEA and SA type isolates showed formation of stable secondary or stem-loop structure having identical features for efficient translation. The conservation of sORF1 at seven nucleotides upstream of stable stem-loop, CU-rich sequence following the sORF1 stop codon and AU-rich shunt landing sequence immediately downstream of the secondary structure suggested conservation of ribosomal shunt mechanism in all RTBV isolates irrespective of their geographical distribution.

A negative element in the downstream region of the Rice tungro bacilliform virus promoter is orientation- and position-independent and is active with heterologous promoters

The promoter of an Indian isolate of the pararetrovirus Rice tungro bacilliform virus (RTBV-WB) contains a negative element downstream of the transcription start site (TSS), between nucleotide residues +58 and +195 (Mathur and Dasgupta, 2007). To further characterize the element, we show, by using transient gus reporter gene assays in the cells of onion peel, rice calli and Arabidopsis leaves, that it down-regulates heterologous promoters CaMV35S and Maize ubiquitin. Quantitative measurements of transient GUS activity indicated more than 90% inhibition of reporter gene expression by the negative element. We also show, by reversing the orientation of the element downstream and by placing it in a position upstream to a constitutively expressing RTBV promoter, that the negative element is orientation- and position-independent, pointing towards its activity at the transcriptional and not post-transcriptional level.

Promoter sequences for defining transgene expression

The design of reverse genetic experiments that utilize transgenic approaches often requires transgenes to be expressed in a predefined pattern and there is limited information regarding the gene expression profile for specific promoters. It is important that expression patterns are predetermined in the specific genotype targeted for transformation because the same promoter-transgene construct can produce different expression patterns in different host species. This chapter compares constitutive, targeted, or inducible promoters that have been characterized in specific cereal species.

Functional analysis of bipartite begomovirus coat protein promoter sequences

We demonstrate that the AL2 gene of Cabbage leaf curl virus (CaLCuV) activates the CP promoter in mesophyll and acts to derepress the promoter in vascular tissue, similar to that observed for Tomato golden mosaic virus (TGMV). Binding studies indicate that sequences mediating repression and activation of the TGMV and CaLCuV CP promoter specifically bind different nuclear factors common to Nicotiana benthamiana, spinach and tomato. However, chromatin immunoprecipitation demonstrates that TGMV AL2 can interact with both sequences independently. Binding of nuclear protein(s) from different crop species to viral sequences conserved in both bipartite and monopartite begomoviruses, including TGMV, CaLCuV, Pepper golden mosaic virus and Tomato yellow leaf curl virus suggests that bipartite begomoviruses bind common host factors to regulate the CP promoter. This is consistent with a model in which AL2 interacts with different components of the cellular transcription machinery that bind viral sequences important for repression and activation of begomovirus CP promoters.

Downstream promoter sequence of an Indian isolate of Rice tungro bacilliform virus alters tissue-specific expression in host rice and acts differentially in heterologous system

An Indian isolate of Rice tungro bacilliform virus from West Bengal (RTBV-WB) showed significant nucleotide differences in its putative promoter region when compared with a previously characterized isolate from Philippines. The transcription start site of RTBV-WB was mapped followed by assessing the activity and tissue-specificity of the full-length (FL) promoter (-231 to +645) and several of its upstream and downstream deletions by studying the expression of beta-Glucuronidase (GUS) reporter gene in transgenic rice (Oryza sativa L. subsp. indica) plants at various stages of development. In addition to the expected vascular-specific expression pattern, studied by histochemical staining, GUS enzymatic assay and northern and RT-PCR analysis, two novel patterns were revealed in some of the downstream deleted versions; a non-expressing type, representing no expression at any stage in any tissue and constitutive type, representing constitutive expression at all stages in most tissues. This indicated the presence of previously unreported positive and negative cis-regulatory elements in the downstream region. The negative element and a putative enhancer region in the upstream region specifically bound to rice nuclear proteins in vitro. The FL and its deletion derivatives were also active in heterologous systems like tobacco (Nicotiana tabacum) and wheat (Triticum durum). Expression patterns in tobacco were different from those observed in rice suggesting the importance of upstream elements in those systems and host-specific regulation of the promoter in diverse organisms. Thus, the RTBV-WB FL promoter and its derivatives contain an array of cis-elements, which control constitutive or tissue- and development-specific gene expression in a combinatorial fashion.

Role of the C-terminal domains of rice (Oryza sativa L.) bZIP proteins RF2a and RF2b in regulating transcription

Rice (Oryza sativa L.) transcription factors RF2a and RF2b are bZIP (basic leucine zipper) proteins that interact with, and activate transcription from the RTBV (rice tungro bacilliform virus) promoter. Here we characterize the C-terminal domains of RF2a and RF2b: these domains are rich in glutamine and proline/glutamine, respectively. Affinity pull-down assays demonstrated that the C-terminal domains of RF2a and RF2b can associate to form either homodimers or heterodimers; however, they do not interact with other domains of RF2a or RF2b. Results of in vitro transcription assays using a rice whole-cell extract demonstrate that the C-terminal domains of both RF2a and RF2b activate transcription from the RTBV promoter. In addition, dimerization of the RF2a C-terminal domain is involved in regulating the transcription activation function of RF2a. The predicted helical region within the RF2a C-terminal glutamine-rich domain was determined to be involved in inter-molecular dimerization, and contributed to the regulatory functions of RF2a in these assays.

Essential role of the Box II cis element and cognate host factors in regulating the promoter of Rice tungro bacilliform virus

Rice tungro bacilliform virus (RTBV) is a double-stranded DNA virus with a single, tissue-specific promoter that is expressed primarily in phloem tissues. Rice transcription factors RF2a and RF2b bind to Box II, a cis element adjacent to the TATA box, and control gene expression from the promoter. Mutations were made in the promoter to delete or mutate Box II and the mutated promoters were fused to a reporter gene; the chimeric genes were expressed in transient BY-2 protoplast assays and in transgenic Arabidopsis plants. The results of these studies showed that Box II is essential to the activity of the RTBV promoter. A chimeric beta-glucuronidase (GUS) reporter gene containing the Box II sequence and a minimal promoter derived from the Cauliflower mosaic virus 35S promoter were co-transfected into protoplasts with gene constructs that encoded RF2a or RF2b. The reporter gene produced threefold higher GUS activity when co-transfected with RF2a, and 11-fold higher activity when co-transfected with RF2b, than in the absence of added transcription factors. Moreover, chimeric reporter genes were activated by approximately threefold following induction of expression of the RF2a gene in transgenic Arabidopsis plants. The work presented here and earlier findings show that Box II and its interactions with cognate rice transcription factors, including RF2a and RF2b, are essential to the activity of the RTBV promoter and are probably involved in expression of the RTBV genome during virus replication.

A variable region of the sugarcane bacilliform virus (SCBV) genome can be used to generate promoters for transgene expression in sugarcane

Four promoters derived from sugarcane bacilliform virus (SCBV) were compared and characterised. Three were obtained by PCR amplification of purified virion DNA extracted from three sugarcane cultivars. The fourth promoter was obtained by subcloning from an almost genome-length clone of SCBV. All promoters were able to drive stable expression of beta-glucuronidase in sugarcane. The PCR-derived promoter sequences shared more DNA homology with banana streak virus than to the subcloned SCBV. The subcloned promoter was the strongest expressing and was able to drive reporter gene expression in vitro and in the leaves, meristems and roots of glasshouse-grown sugarcane. Expression levels were at least equal to or higher than those measured for the maize polyubiquitin promoter.

RF2b, a rice bZIP transcription activator, interacts with RF2a and is involved in symptom development of rice tungro disease

The phloem-specific promoter of rice tungro bacilliform virus (RTBV) is regulated in part by sequence-specific DNA-binding proteins that bind to Box II, an essential cis element. Previous studies demonstrated that the bZIP protein RF2a is involved in transcriptional regulation of the RTBV promoter. Here we report the identification and functional characterization of a second bZIP protein, RF2b. RF2b, identified by its interaction with RF2a, binds to Box II in in vitro assays as a homodimer and as RF2a/RF2b heterodimers. Like RF2a, RF2b activates the RTBV promoter in transient assays and in transgenic tobacco plants. Both RF2a and RF2b are predominantly expressed in vascular tissues. However, RF2a and RF2b have different DNA-binding affinities to Box II, show distinctive expression patterns in different rice organs, and exhibit different patterns of subcellular localization. Furthermore, transgenic rice plants with reduced levels of RF2b exhibit a disease-like phenotype. We propose that the regulation of phloem-specific expression of the RTBV promoter and potentially the control of RTBV replication are mainly achieved via interactions of the Box II cis element with multiple host factors, including RF2a and RF2b. We also propose that quenching/titration of these and perhaps other transcription factors by RTBV is involved in the development of the symptoms of rice tungro disease.

Functional analysis of RF2a, a rice transcription factor

RF2a is a bZIP transcription factor that regulates expression of the promoter of rice tungro bacilliform badnavirus. RF2a is predicted to include three domains that contribute to its function. The results of transient assays with mutants of RF2a from which one or more domains were removed demonstrated that the acidic domain was essential for the activation of gene expression, although the proline-rich and glutamine-rich domains each played a role in this function. Studies using fusion proteins of different functional domains of RF2a with the 2C7 synthetic zinc finger DNA-binding domain showed that the acidic region is a relatively strong activation domain, the function of which is dependent on the context in which the domain is placed. Data from transgenic plants further supported the conclusion that the acidic domain was important for maintaining the biological function of RF2a. RF2a and TBP (TATA-binding protein) synergistically activate transcription in vitro (Zhu, Q., Ordiz, M. I., Dabi, T., Beachy, R. N., and Lamb, C. (2002) Plant Cell 14, 795-803). In vitro and in vivo assays showed that RF2a interacts with TBP through the glutamine-rich domain but not the acidic domain. Functional analysis of such interactions indicates that the acidic domain activates transcription through mechanisms other than via the direct recruitment of TBP.

Analysis of cis-sequence of subgenomic transcript promoter from the Figwort mosaic virus and comparison of promoter activity with the cauliflower mosaic virus promoters in monocot and dicot cells

A sub-genomic transcript (Sgt) promoter was isolated from the Figwort mosaic virus (FMV) genomic clone. The FMV Sgt promoter was linked to heterologous coding sequences to form a chimeric gene construct. The 5'-3'-boundaries required for maximal activity and involvement of cis-sequences for optimal expression in plants were defined by 5'-, 3'-end deletion and internal deletion analysis of FMV Sgt promoter fragments coupled with a beta-glucuronidase reporter gene in both transient protoplast expression experiments and in transgenic plants. A 301 bp FMV Sgt promoter fragment (sequence -270 to +31 from the transcription start site; TSS) provided maximum promoter activity. The TSS of the FMV Sgt promoter was determined by primer extension analysis using total RNA from transgenic plants developed for FMV Sgt promoter: uidA fusion gene. An activator domain located upstream of the TATA box at -70 to -100 from TSS is absolutely required for promoter activity and its function is critically position-dependent with respect to TATA box. Two sequence motifs AGATTTTAAT (coordinates -100 to -91) and GTAAGCGC (coordinates -80 to -73) were found to be essential for promoter activity. The FMV Sgt promoter is less active in monocot cells; FMV Sgt promoter expression level was about 27.5-fold higher in tobacco cells compared to that in maize cells. Comparative expression analysis of FMV Sgt promoter with cauliflower mosaic virus (CaMV) 35S promoter showed that the FMV Sgt promoter is about 2-fold stronger than the CaMV 35S promoter. The FMV Sgt promoter is a constitutive promoter; expression level in seedlings was in the order: root>leaf>stem.

Regulation of transgene expression in plants with polydactyl zinc finger transcription factors

Designer zinc finger transcription factors (TFs(ZF)) have been developed to control the expression of transgenes and endogenous genes in mammalian cells. Application of TFs(ZF) technology in plants would enable a wide range of both basic and applied studies. In this paper, we report the use of TFs(ZF) to target a defined 18-bp DNA sequence to control gene expression in plant cells and in transgenic plants. A beta-glucuronidase reporter gene was activated by using the designed six-zinc finger protein 2C7 expressed as a fusion with the herpes simplex virus VP16 transcription factor activation domain. Reporter gene expression was activated 5- to 30-fold by using TFs(ZF) in BY-2 protoplasts, whereas expression was increased as much as 450 times in transgenic tobacco plants. Use of a phloem-specific promoter to drive expression of the TFs(ZF) resulted in activation of the reporter gene in vascular tissues. Transgenic tobacco plants that produce 2C7 transcription factors were phenotypically normal through two generations, suggesting that the factors exerted no adverse effects. This study demonstrates the utility of zinc finger technology in plants, setting the stage for its application in basic and applied agricultural biotechnology.

Expression of a bifunctional green fluorescent protein (GFP) fusion marker under the control of three constitutive promoters and enhanced derivatives in transgenic grape (Vitis vinifera)

Activity of three constitutive promoters and enhanced derivatives in transgenic grape (Vitis vinifera L. cv. Thompson Seedless) was characterized using a bifunctional fusion marker containing the enhanced green fluorescent protein (EGFP) and neomycin phosphotransferase (NPTII) genes. Relative differences in transient GFP expression and stable transformation efficiencies were used to compare promoter activity. Expression patterns in transformed somatic embryos revealed that the ACT2 promoter from Arabidopsis thaliana, previously shown to be a strong constitutive promoter in A. thaliana and other species, failed to promote strong expression in grape. In contrast, a promoter isolated from cassava vein mosaic virus (CsVMV) supported high levels of transgene expression equivalent to those achieved using an enhanced double cauliflower mosaic virus (CaMV) 35S promoter. Duplication of the 5'-upstream enhancer region of the CsVMV promoter further enhanced its ability to increase transgene expression. However, the pattern of transgene expression driven by these two viral promoters was significantly different at the whole plant level. The enhanced double CaMV 35S promoter was highly active in most tissues and organs including roots, mature leaves, shoot apices and lateral buds. In contrast, the CsVMV promoter and its double enhancer derivative induced relatively weak expression in these tissues. Our results suggest that activity of the CsVMV promoter, in contrast to the CaMV 35S promoter, was under developmental regulation in transgenic grape plants as compared with the CaMV 35S promoter.

Rice transformation for crop improvement and functional genomics

Although several japonica and some indica varieties of rice have already been transformed, there is significant scope for improvement in the technology for transformation of economically important indica varieties. Successful transformation of rice employing Agrobacterium and recent advances in direct gene transfer by biolistics, evidenced by transfer of multiple genes, have removed some of the serious impediments in the area of gene engineering. The transfer of genes for nutritionally important biosynthetic pathway has provided many opportunities for performing metabolic engineering. Other useful genes for resistance against pests, diseases and abiotic stresses have also been transferred to rice. But the limited knowledge about important target genes requires rapid progress in the field of functional genomics. Transgenic rice system can be applied to isolate new genes, promoters, and enhancers and their functions could be unravelled. The combination of novel regulatory systems for targeted expression and useful new genes should pave the way for improvement of rice and other cereals.

The maize streak virus coat protein transcription unit exhibits tissue-specific expression in transgenic rice

Maize streak geminivirus (MSV) is a single-stranded DNA virus that infects cereals and other grasses. A promoter region incorporating the MSV large intergenic region and movement protein gene sequence was ligated to the gus (beta-glucuronidase) reporter gene which replaced the virus coat protein (CP) gene. The CP promoter activity was analysed in transgenic rice plants (Oryza sativa L.) and was compared with that obtained in plants transformed with the gus gene downstream of the cauliflower mosaic virus (CaMV) 35S promoter. The MSV CP promoter activity varied in the five plant lines tested, but was always less than that of the CaMV promoter. Histochemistry showed that the MSV CP promoter was active in cells of regenerating callus but in regenerated plants it provided an expression pattern restricted to the vascular tissues of the root, stem, leaf and floral organs. Expression was highest in phloem-associated tissues of the vegetative organs and was absent from the tip and elongation region of seedling roots. Thus, the MSV CP promoter shows a degree of developmental regulation and can be used to confer tissue-specific expression in transgenic rice plants.

Transcriptional activation of the rice tungro bacilliform virus gene is critically dependent on an activator element located immediately upstream of the TATA box

To investigate the transcriptional mechanisms of rice tungro bacilliform virus, we have systematically analyzed an activator element located immediately upstream of the TATA box in the rice tungro bacilliform virus promoter and its cognate trans-acting factors. Using electrophoretic mobility shift assays, we showed that rice nuclear proteins bind to the activator element, forming multiple specific DNA-protein complexes via protein-protein interactions. Copper-phenanthroline footprinting and DNA methylation interference analysis indicated that multiple DNA-protein complexes share a common binding site located between positions -60 to -39, and the proteins contact the activator element in the major groove. DNA UV cross-linking assays further showed that two nuclear proteins (36 and 33 kDa), found in rice cell suspension and shoot nuclear extracts, and one (27 kDa), present in root nuclear extracts, bind to this activator element. In protoplasts derived from a rice (Oryza sativa) suspension culture, the activator element is a prerequisite for promoter activity and its function is critically dependent on its position relative to the TATA box. Thus, transcriptional activation may function via interactions with the basal transcriptional machinery, and we propose that this activation is mediated by protein-protein interactions in a position-dependent mechanism.

A short open reading frame terminating in front of a stable hairpin is the conserved feature in pregenomic RNA leaders of plant pararetroviruses

In plant pararetroviruses, pregenomic RNA (pgRNA) directs synthesis of circular double-stranded viral DNA and serves as a polycistronic mRNA. By computer-aided analysis, the 14 plant pararetroviruses sequenced so far were compared with respect to structural organization of their pgRNA 5'-leader. The results revealed that the pgRNA of all these viruses carries a long leader sequence containing several short ORFs and having the potential to form a large stem-loop structure; both features are known to be inhibitory for downstream translation. Formation of the structure brings the first long ORF into the close spatial vicinity of a 5'-proximal short ORF that terminates 5 to 10 nt upstream of the stable structural element. The first long ORF on the pgRNA is translated by a ribosome shunt mechanism discovered in cauliflower mosaic (CaMV) and rice tungro bacilliform viruses, representing the two major groups of plant pararetroviruses. Both the short ORF and the structure have been implicated in the shunt process for CaMV pgRNA translation. The conservation of these elements among all plant pararetroviruses suggests conservation of the ribosome shunt mechanism. For some of the less well-studied viruses, the localization of the conserved elements also allowed predictions of the pgRNA promoter region and the translation start site of the first long ORF.

Isolation and expression analysis of peanut chlorotic streak caulimovirus (PClSV) full-length transcript (FLt) promoter in transgenic plants

A promoter fragment from peanut chlorotic streak caulimovirus (PClSV) full-length transcript (FLt) was identified and later modified to have duplicated enhancer domain. The FLt promoter with its single or double enhancer domains, fused with the GUS reporter gene to form chimeric gene constructs, showed a high level of expression of these genes in cells and transgenic plants. The FLt promoter with its double enhancer domain gives an average threefold greater expression of genes compared to the FLt promoter with its single enhancer domain in transgenic plants. In young seedlings the expression was in the order root > leaf > stem. The histochemical GUS assay in young seedlings showed more activity in root tips and leaf midribs, veins, and other vascular tissues. The expression from the PClSV FLt promoter was compared with that from the figwort mosaic virus promoter in transgenic plants. These constitutive promoters were comparable in respect to GUS expression level.

RF2a, a bZIP transcriptional activator of the phloem-specific rice tungro bacilliform virus promoter, functions in vascular development

Rice tungro bacilliform virus (RTBV) replicates only in phloem cells in infected rice plants and its promoter drives strong phloem-specific reporter gene expression in transgenic rice plants. We isolated a cDNA encoding a basic leucine zipper (bZIP) protein, RF2a, which binds to the Box II cis element that is important for expression from the promoter. RF2a, which stimulates Box II-dependent transcription in a homologous in vitro transcription system, accumulates in nuclei of phloem and certain other cell types in shoots, but is found at only very low levels in roots. Transgenic antisense plants in which RF2a accumulation was suppressed had normal roots but stunted, twisted leaves with small, disorganized vascular bundles, an enlarged sclerenchyma and large air spaces. We propose that the RTBV promoter exploits a host transcription factor that is critical for leaf tissue differentiation and vascular development for its expression.

Regulation of a geminivirus coat protein promoter by AL2 protein (TrAP): evidence for activation and derepression mechanisms

Tomato golden mosaic virus (TGMV) is a bipartite member of the subgroup III Geminiviridae. Like all geminiviruses, TGMV replicates in the nucleus of susceptible cells by rolling circle replication (RCR). Double-stranded replicative form DNA generated during RCR serves as template for the transcription of viral genes by RNA polymerase II and the associated cellular transcription machinery. Previous studies in tobacco protoplasts and Nicotiana benthamiana leaf discs have shown that the viral AL2 gene product transactivates expression of the coat protein (CP) and BR1 movement protein genes, and that activation occurs at the level of transcription. Because of its function and properties, we propose the name TrAP, transcriptional activator protein, for the AL2 gene product. Using transgenes consisting of complete and truncated versions of the CP promoter fused to the GUS reporter gene, we show in the studies presented here that TrAP is required for CP gene expression in both mesophyll and phloem tissues. Surprisingly, TrAP appears to induce CP expression by different mechanisms in different cell types: it may activate the CP promoter in mesophyll cells, and acts to derepress the promoter in phloem tissue. In addition, TrAP is clearly capable of inducing the expression of responsive chromosomal promoters and could, in principle, activate host genes. Distinct viral sequence elements mediate expression and derepression in phloem and activation in mesophyll, suggesting that TrAP interacts with different components of the cellular transcription machinery to accomplish CP gene expression in different cell types, and underscoring the intricacy and complexity of virus-host interactions.

Efficient transcription from the rice tungro bacilliform virus promoter requires elements downstream of the transcription start site

Elements downstream of the transcription start site enhance the activity of the rice tungro bacilliform virus (RTBV) promoter in protoplasts derived from cultured rice cells. This enhancer region was located to the first 90 nucleotides of the RTBV leader sequence. Within this region, at least two components which act together to enhance expression from the RTBV promoter could be identified. One is a position- and orientation-independent DNA element within a CT-rich region, and the other is a position-dependent element. Either element was found to be capable of acting independently on a heterologous promoter. The enhancer activity of the DNA element correlates with specific binding of nuclear proteins. Nuclear proteins also recognize an RNA transcript covering the first 90 nucleotides of the RTBV leader.

Isolation and expression in transgenic tobacco and rice plants, of the cassava vein mosaic virus (CVMV) promoter

The cassava vein mosaic virus (CVMV) is a double stranded DNA virus which infects cassava plants (Manihot esculenta Crantz) and has been characterized as a plant pararetrovirus belonging to the caulimovirus subgroup. Two DNA fragments, CVP1 of 388 nucleotides from position -368 to +20 and CVP2 of 511 nucleotides from position -443 to +72, were isolated from the viral genome and fused to the uidA reporter gene to test promoter expression. The transcription start site of the viral promoter was determined using RNA isolated from transgenic plants containing the CVMV promoter:uidA fusion gene. Both promoter fragments were able to cause high levels of gene expression in protoplasts isolated from cassava and tobacco cell suspensions. The expression pattern of the CVMV promoters was analyzed in transgenic tobacco and rice plants, and revealed that the GUS staining pattern was similar for each construct and in both plants. The two promoter fragments were active in all plant organs tested and in a variety of cell types, suggesting a near constitutive pattern of expression. In both tobacco and rice plants, GUS activity was highest in vascular elements, in leaf mesophyll cells, and in root tips.

Molecular biology of rice tungro viruses

Rice tungro, the most important virus disease of rice in South and Southeast Asia, is caused by a complex of two viruses, rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV). RTBV is a plant pararetrovirus with bacilliform particles, the structure of which is based on T = 3 icosahedral symmetry cut across the threefold axis.The particles encapsidate a circular double-stranded DNA of 8 kbp that encodes four proteins. The current information on the properties, functions, and expression of these proteins is discussed, as is the evidence for replication by reverse transcription. Two major strains of RTBV have been recognized, one from the Indian subcontinent and the other from Southeast Asia. RTSV particles contain a single-stranded RNA genome of 12 kb that encodes a large polyprotein and possibly one or two smaller proteins. The properties and processing of the polyprotein are described and the resemblance to picornaviruses noted.

Genetic elements of plant viruses as tools for genetic engineering

Viruses have developed successful strategies for propagation at the expense of their host cells. Efficient gene expression, genome multiplication, and invasion of the host are enabled by virus-encoded genetic elements, many of which are well characterized. Sequences derived from plant DNA and RNA viruses can be used to control expression of other genes in vivo. The main groups of plant virus genetic elements useful in genetic engineering are reviewed, including the signals for DNA-dependent and RNA-dependent RNA synthesis, sequences on the virus mRNAs that enable translational control, and sequences that control processing and intracellular sorting of virus proteins. Use of plant viruses as extrachromosomal expression vectors is also discussed, along with the issue of their stability.

Inheritance of gusA and neo genes in transgenic rice

Inheritance of foreign genes neo and gusA in rice (Oryza sativa L. cv. IR54 and Radon) has been investigated in three different primary (T0) transformants and their progeny plants. T0 plants were obtained by co-transforming protoplasts from two different rice suspension cultures with the neomycin phosphotransferase II gene [neo or aph (3') II] and the beta-glucuronidase gene (uidA or gusA) residing on separate chimeric plasmid constructs. The suspension cultures were derived from callus of immature embryos of indica variety IR54 and japonica variety Radon. One transgenic line of Radon (AR2) contained neo driven by the CaMV 35S promoter and gusA driven by the rice actin promoter. A second Radon line (R3) contained neo driven by the CaMV 35S promoter and gusA driven by a promoter of the rice tungro bacilliform virus. The third transgenic line, IR54-1, contained neo driven by the CaMV 35S promoter and gusA driven by the CaMV 35S. Inheritance of the transgenes in progeny of the transgenic rice was investigated by Southern blot analysis and enzyme assays. Southern blot analysis of genomic DNA showed that, regardless of copy numbers of the transgenes in the plant genome and the fact that the two transgenes resided on two different plasmids before transformation, the introduced gusA and neo genes were stably transmitted from one generation to another and co-inherited together in transgenic rice progeny plants derived from self-pollination. Analysis of GUS and NPT II activities in T1 to T2 plants provided evidence that inheritance of the gusA and neo genes was in a Mendelian fashion in one plant line (AR2), and in an irregular fashion in the two other plant lines (R3 and IR54-1). Homozygous progeny plants expressing the gusA and neo genes were obtained in the T2 generation of AR2, but the homozygous state was not found in the other two lines of transgenic rice.

Rice tungro bacilliform virus encodes reverse transcriptase, DNA polymerase, and ribonuclease H activities

Rice tungro bacilliform virus (RTBV) is a newly described badnavirus and proposed member of the plant pararetrovirus group. RTBV open reading frame 3 is predicted to encode a capsid protein, protease (PR), and reverse transcriptase (RT) and has the capacity to encode other proteins of as yet unknown function. To study the possible enzymatic activities encoded by open reading frame 3, a DNA fragment containing the putative PR and RT domains was used to construct the recombinant baculovirus PR/RT-BBac. Trichoplusia ni insect cells infected with PR/RT-BBac were used in pulse-labeling experiments and demonstrated synthesis of an 87-kDa polyprotein that corresponds in molecular mass to that predicted from the PR/RT DNA coding sequence. The 87-kDa polyprotein was processed with concomitant accumulation of 62-kDa (p62) and 55-kDa (p55) proteins. Amino-terminal sequencing of p62 and p55 determined that they mapped to the PR/RT domain and shared common amino termini. p62 and p55 were purified and exhibited both RT and DNA polymerase activities using synthetic primer/template substrates. Only p55 had detectable ribonuclease H activity, an activity intrinsic to all reverse transcriptases studied to date. Characterization of the RTBV RT provides a biochemical basis for classifying RTBV as a pararetrovirus and will lead to further studies of these proteins and their role in virus replication.