Engineered GFP as a vital reporter in plants

Novel aspects of tomato root colonization and infection by Fusarium oxysporum f. sp. radicis-lycopersici revealed by confocal laser scanning microscopic analysis using the green fluorescent protein as a marker

The fungus Fusarium oxysporum f. sp. radicis-lycopersici is the causal agent of tomato foot and root rot disease. The green fluorescent protein (GFP) was used to mark this fungus in order to visualize and analyze the colonization and infection processes in vivo. Transformation of F oxysporum f. sp. radicis-lycopersici was very efficient and gfp expression was stable for at least nine subcultures. Microscopic analysis of the transformants revealed homogeneity of the fluorescent signal, which was clearly visible in the hyphae as well as in the chlamydospores and conidia. To our knowledge, this is the first report in which this is shown. The transformation did not affect the pathogenicity. Using confocal laser scanning microscopy, colonization, infection, and disease development on tomato roots were visualized in detail and several new aspects of these processes were observed, such as (i) the complete colonization pattern of the tomato root system; (ii) the very first steps of contact between the fungus and the host, which takes place at the root hair zone by mingling and by the attachment of hyphae to the root hairs; (iii) the preferential colonization sites on the root surface, which are the grooves along the junctions of the epidermal cells; and (iv) the absence of specific infection sites, such as sites of emergence of secondary roots, root tips, or wounded tissue, and the absence of specific infection structures, such as appressoria. The results of this work prove that the use of GFP as a marker for F. oxysporum f. sp. radicis-lycopersici is a convenient, fast, and effective approach for studying plant-fungus interactions.

Two distinct high-affinity sulfate transporters with different inducibilities mediate uptake of sulfate in Arabidopsis roots

Sulfate transporters present at the root surface facilitate uptake of sulfate from the environment. Here we report that uptake of sulfate at the outermost cell layers of Arabidopsis root is associated with the functions of highly and low-inducible sulfate transporters, Sultr1;1 and Sultr1;2, respectively. We have previously reported that Sultr1;1 is a high-affinity sulfate transporter expressed in root hairs, epidermal and cortical cells of Arabidopsis roots, and its expression is strongly upregulated in plants deprived of external sulfate. A novel sulfate transporter gene, Sultr1;2, identified on the BAC clone F28K19 of Arabidopsis, encoded a polypeptide of 653 amino acids that is 72.6% identical to Sultr1;1 and was able to restore sulfate uptake capacity of a yeast mutant lacking sulfate transporter genes (K(m) for sulfate = 6.9 +/- 1.0 microm). Transgenic Arabidopsis plants expressing the fusion gene construct of the Sultr1;2 promoter and green fluorescent protein (GFP) showed specific localization of GFP in the root hairs, epidermal and cortical cells of roots, and in the guard cells of leaves, suggesting that Sultr1;2 may co-localize with Sultr1;1 in the same cell layers at the root surface. Sultr1;1 mRNA was abundantly expressed under low-sulfur conditions (50-100 microm sulfate), whereas Sultr1;2 mRNA accumulated constitutively at high levels under a wide range of sulfur conditions (50-1500 microm sulfate), indicating that Sultr1;2 is less responsive to changes in sulfur conditions. Addition of selenate to the medium increased the level of Sultr1;1 mRNA in parallel with a decrease in the internal sulfate pool in roots. The level of Sultr1;2 mRNA was not influenced under these conditions. Antisense plants of Sultr1;1 showed reduced accumulation of sulfate in roots, particularly in plants treated with selenate, suggesting that the inducible transporter Sultr1;1 contributes to the uptake of sulfate under stressed conditions.

E2F sites that can interact with E2F proteins cloned from rice are required for meristematic tissue-specific expression of rice and tobacco proliferating cell nuclear antigen promoters

Plants have recently been found to have E2F-like and Rb-like proteins, regulators responsible for the G1(G0)-S phase transition of the cell cycle in animals. Here we show that E2F is involved in transcription of plant genes for proliferating cell nuclear antigen (PCNA), which is required for DNA replication. Potential E2F binding sites found in the rice PCNA promoters mediated transcriptional activation in actively dividing cells and tissues of tobacco, but not transcriptional repression in terminally differentiated tissues, as also observed for the PCF binding sites previously found in the rice promoter. Similar results were obtained from analyses for a PCNA promoter isolated from tobacco, which contained two E2F-like sites, each with a different degree of contribution to the promoter activation. These E2F-like sites except for a rice site were indeed bound specifically by recombinant proteins of rice E2F, OsE2F1 and OsE2F2, and complexes of OsE2F1 with Arabidopsis DP proteins. Furthermore, OsE2F1 had the ability to transactivate an E2F-reporter gene containing the tobacco E2F site on co-expression with an Arabidopsis DP, and the transactivation was greatly enhanced by tagging a canonical nuclear localization signal to OsE2F1, suggesting a nuclear import-mediated regulation of the OsE2F1 function. In addition, we found that a large number of replication- and mismatch repair-associated genes in Arabidopsis contain E2F binding sequences conserved in their predicted promoter regions.

Dual targeting of phage-type RNA polymerase to both mitochondria and plastids is due to alternative translation initiation in single transcripts

We isolated and sequenced a nuclear gene and cDNA encoding a bacteriophage T7-type RNA polymerase, NsRpoT-B, from Nicotiana sylvestris. The gene, NsRpoT-B, consists of 19 exons and 18 introns and encodes a polypeptide of 1020 amino acid residues. The predicted NsRpoT-B protein shows 71% amino acid identity with NsRpoT-A which is a mitochondrial protein. Quantitative RT-PCR revealed that steady-state NsRpoT-B mRNA accumulation is highest in the mature leaves and lowest in the cotyledons. Transient expression assays in protoplasts from N. sylvestris leaves demonstrated that the putative N-terminal transit peptide of NsRpoT-B encodes dual targeting signals directing the protein into mitochondria and plastids. This strongly suggests that NsRpoT-B functions as an RNA polymerase transcribing genes from two different plant organelle genomes. NsRpoT-B transcripts have two potential translation initiation codons. An in vitro translation assay indicated that a chimeric mRNA encoding the N-terminal NsRpoT-B fused to an sGFP produced two polypeptides translated from the first and second initiation codons. This implies that the dual targeting of NsRpoT-B protein is regulated, in part, at the level of translation. We have designated this protein NsRpoTpm.

A chloroplast protein homologous to the eubacterial topological specificity factor minE plays a role in chloroplast division

We report the identification of a nucleus-encoded minE gene, designated AtMinE1, of Arabidopsis. The encoded AtMinE1 protein possesses both N- and C-terminal extensions, relative to the eubacterial and algal chloroplast-encoded MinE proteins. The N-terminal extension functioned as a chloroplast-targeting transit peptide, as revealed by a transient expression assay using an N terminus:green fluorescent protein fusion. Histochemical beta-glucuronidase staining of transgenic Arabidopsis lines harboring an AtMinE1 promoter::uidA reporter fusion unveiled specific activation of the promoter in green tissues, especially at the shoot apex, which suggests a requirement for cell division-associated AtMinE1 expression for proplastid division in green tissues. In addition, we generated transgenic plants overexpressing a full-length AtMinE1 cDNA and examined the subcellular structures of those plants. Giant heteromorphic chloroplasts were observed in transgenic plants, with a reduced number per cell, whereas mitochondrial morphology remained similar to that of wild-type plants. Taken together, these observations suggest that MinE is the third conserved component involved in chloroplast division.

Signal transduction in maize and Arabidopsis mesophyll protoplasts

Plant protoplasts show physiological perceptions and responses to hormones, metabolites, environmental cues, and pathogen-derived elicitors, similar to cell-autonomous responses in intact tissues and plants. The development of defined protoplast transient expression systems for high-throughput screening and systematic characterization of gene functions has greatly contributed to elucidating plant signal transduction pathways, in combination with genetic, genomic, and transgenic approaches.

Genomic organization and organ-specific expression of a nuclear gene encoding phage-type RNA polymerase in Nicotiana sylvestris

We have isolated and sequenced a nuclear gene and cDNA encoding bacteriophage T7-type single subunit RNA polymerase, NsRpoT-A, from Nicotiana sylvestris. NsRpoT-A consists of 19 exons and 18 introns; the first intron is 17 kb, the longest yet identified in a plant gene. Genomic Southern analysis indicated that N. sylvestris contains a small family of NsRpoT genes. Quantitative RT-PCR revealed that steady-state mRNA levels are highest in the leaves and lowest in the cotyledons. Phylogenetic analysis of NsRpoT-A and the RpoT proteins of other plant species suggested that NsRpoT-A is a mitochondrial protein. The TargetP program predicted localization of the NsRpoT-A gene product to the mitochondria. Using a transient expression assay and protoplasts from N. sylvestris mesophyll cells, we clearly demonstrated that the N-terminal sequence of NsRpoT-A targets the protein to the mitochondria. We therefore named this protein NsRpoTm.

Molecular characterization of two Arabidopsis Ire1 homologs, endoplasmic reticulum-located transmembrane protein kinases

A major response of eukaryotic cells to the presence of unfolded proteins in the lumen of the endoplasmic reticulum (ER) is to activate genes that encode ER-located molecular chaperones, such as the binding protein. This response, called the unfolded protein response, requires the transduction of a signal from the ER to the nucleus. In yeast (Saccharomyces cerevisiae) and mammalian cells, an ER-located transmembrane receptor protein kinase/ribonuclease called Ire1, with a sensor domain in the lumen of the ER, is the first component of this pathway. Here, we report the cloning and derived amino acid sequences of AtIre1-1 and AtIre1-2, two Arabidopsis homologs of Ire1. The two proteins are located in the perinuclear ER (based on heterologous expression of fusions with green fluorescent protein). The expression patterns of the two genes (using beta-glucuronidase fusions) are nearly nonoverlapping. We also demonstrate functional complementation of the sensor domains of the two proteins in yeast and show that the Ire1-2 protein is capable of autotransphosphorylation. These and other findings are discussed in relation to the involvement of these genes in unfolded protein response signaling in plants.

Use of red fluorescent protein from Discosoma sp. (dsRED) as a reporter for plant gene expression

The suitability of the recently described red fluorescent protein dsRED from reef corals for use as a reporter in plant molecular biology was investigated. Based on the clone pDSRED (Clontech), plant expression vectors were constructed for constitutive dsRED expression in the cytosol, the endoplasmic reticulum and the vacuole. Fluorescence microscopy of tobacco BY2 suspension culture cells transiently expressing the plant vectors generated proved that cytosolic expression of the dsRED gives rise to readily detectable levels of red fluorescence, whereas expression in the ER was poor. Vacuolar dsRED expression did not result in any significant fluorescence. dsRED transgenic tobacco SR1 plants were generated to test the sensitivity of dsRED as a reporter in an autofluorescent background, and to identify the possible impact of the introduced fluorescent protein on morphogenesis, plant development and fertility. During the transformation and regeneration phase plants did not show any abnormalities, indicating that dsRED is not interfering with plant development and morphogenesis. Regenerated plants were analysed by PCR, Western blot and fluorescence microscopy for the presence and expression of the transferred genes. The filter sets chosen for fluorescence microscopy proved to be able to block the red chlorophyll fluorescence completely, allowing specific dsRED detection. Best expression levels were obtained with dsRED targeted to the cytosol or chloroplasts. ER-targeted expression of dsRED also gave rise to readily detectable fluorescence levels, whereas vacuolar expression yielded no fluorescence. dsRED transgenic plant lines expressing the protein in the cytosol, ER or chloroplast proved to be fertile. Seed set and germination were normal, except that the seeds and seedlings maintained the red fluorescence phenotype.

Quantitative analysis by flow cytometry of abscisic acid-inducible gene expression in transiently transformed rice protoplasts

BACKGROUND

Quantifying plant gene expression by flow cytometry (FCM) would allow multidimensional cell-parameter analysis on a per-cell basis, thereby providing insight into the cellular mechanisms of plant gene regulation. Here we sought to establish quantitation by FCM of plant hormone (abscisic acid, ABA)-inducible green fluorescent protein (GFP) expression and to compare the method directly with traditional reporter enzyme assays.

MATERIALS AND METHODS

GFP, beta-glucuronidase, and luciferase reporter genes driven by ABA-inducible or constitutive promoter constructs were expressed in transiently cotransformed rice protoplasts and reporter activities quantified by FCM (for GFP) or traditional enzyme assays. Treatments included cotransformations with specific ABA signaling effector cDNA constructs (encoding VIVIPAROUS-1, an ABA transcription factor, and ABA-INSENSITIVE1-1, a dominant-negative protein phosphatase regulator) and the ABA agonist lanthanum chloride. Dual-color FCM was also performed on GFP-expressing cells immunodecorated with an mAb recognizing a rice cell surface epitope.

RESULTS

Quantitative analysis of ABA-inducible gene expression by FCM using GFP as reporter gave comparable results to traditional reporter enzyme assays, although the signal-to-noise ratio was less for FCM, which can be a limitation of the method at low promoter strengths. Multiparameter-correlated analysis of ABA-inducible GFP expression with a plasma membrane marker showed no apparent correlation between ABA sensitivity, marked by GFP, and presence of a cell surface arabinogalactan glycoprotein.

CONCLUSIONS

Quantitative FCM of GFP-expressing plant cells is a rapid, robust, reproducible, and value-added method relative to traditional enzymatic reporter gene assays.

Identification of genes in the bW/bE regulatory cascade in Ustilago maydis

In the phytopathogenic fungus Ustilago maydis, the switch to filamentous growth and pathogenic development is controlled by a heterodimeric transcription factor consisting of the bW and bE homeodomain proteins. To identify genes in the regulatory cascade triggered by the bW/bE heterodimer, we have constructed strains in which transcription of the b genes is inducible by either arabinose or nitrate. At different time-points after induction, genes that are switched on or off were identified through a modified, non-radioactive RNA fingerprint procedure. From 348 gene fragments isolated initially, 48 fragments representing 34 different genes were characterized in more detail. After eliminating known genes, false positives and genes influenced in their expression profile by media conditions, 10 new b-regulated genes were identified. Of these, five are upregulated and five are downregulated in presence of the b heterodimer. Two do not share significant similarity to database entries, whereas the other eight show similarity to disulphide isomerases, exochitinases, cation antiporters, plasma membrane (H+)-ATPases, acyl transferases, a capsular associated protein of Cryptococcus neoformans, DNA polymerases X, as well as to a potential protein of Neurospora crassa. We demonstrate that in one of the early upregulated genes, the promoter can be bound by a bW/bE fusion protein in vitro. Interestingly, three out of the four genes that are downregulated by the b heterodimer appear upregulated after pheromone stimulation, suggesting a connection to the mating process.

Characterization of an Arabidopsis cDNA encoding a subunit of serine palmitoyltransferase, the initial enzyme in sphingolipid biosynthesis

Serine palmitoyltransferase (SPT; EC 2.3.1.50) catalyzes the condensation of serine with palmitoyl-CoA to form 3-ketosphinganine in the first step of de novo sphingolipid biosynthesis. In this study, we describe the cloning and functional characterization of a cDNA from Arabidopsis thaliana encoding the LCB2 subunit of SPT. The Arabidopsis LCB2 (AtLCB2) cDNA contains an open reading frame of 1,467 nucleotides, encoding 489 amino acids. The predicted polypeptide contains three transmembrane helices and a highly conserved motif involved in pyridoxal phosphate binding. Expression of this open reading frame in the Saccharomyces cerevisiae mutant strains defective in SPT activity resulted in the expression of a significant level of sphinganine, suggesting that AtLCB2 cDNA encodes SPT. Southern blot analysis and inspection of the complete Arabidopsis genome sequence database suggest that there is a second LCB2-like gene in Arabidopsis. Expression of a green fluorescent protein (GFP) fusion product in suspension-cultured tobacco BY-2 cells showed that AtLCB2 is localized to the endoplasmic reticulum. AtLCB2 cDNA may be used to study how sphingolipid synthesis is regulated in higher plants.

Two distinct isopentenyl diphosphate isomerases in cytosol and plastid are differentially induced by environmental stresses in tobacco

Two distinct cDNA clones (IPI1 and IPI2) encoding IPI were isolated from Nicotiana tabacum. In situ expression of isopentenyl diphosphate isomerase-1 (IPI1)- and IPI2-green fluorescent protein fusion constructs revealed that IPI1 and IPI2 were localized in chloroplast and cytosol, respectively. The level of IPI1 mRNA was increased under high-salt and high-light stress conditions, while that of IPI2 mRNA was increased under high-salt and cold stress conditions. Both IPI transcripts were increased in an abscisic acid-independent manner. This is the first report of a cytosolic IPI. The results indicated that two distinct IPIs were differentially induced in response to stress.

Two types of MGDG synthase genes, found widely in both 16:3 and 18:3 plants, differentially mediate galactolipid syntheses in photosynthetic and nonphotosynthetic tissues in Arabidopsis thaliana

In Arabidopsis, monogalactosyldiacylglycerol (MGDG) is synthesized by a multigenic family of MGDG synthases consisting of two types of enzymes differing in their N-terminal portion: type A (atMGD1) and type B (atMGD2 and atMGD3). The present paper compares type B isoforms with the enzymes of type A that are known to sit in the inner membrane of plastid envelope. The occurrence of types A and B in 16:3 and 18:3 plants shows that both types are not specialized isoforms for the prokaryotic and eukaryotic glycerolipid biosynthetic pathways. Type A atMGD1 gene is abundantly expressed in green tissues and along plant development and encodes the most active enzyme. Its mature polypeptide is immunodetected in the envelope of chloroplasts from Arabidopsis leaves after cleavage of its transit peptide. atMGD1 is therefore likely devoted to the massive production of MGDG required to expand the inner envelope membrane and build up the thylakoids network. Transient expression of green fluorescent protein fusions in Arabidopsis leaves and in vitro import experiments show that type B precursors are targeted to plastids, owing to a different mechanism. Noncanonical addressing peptides, whose processing could not be assessed, are involved in the targeting of type B precursors, possibly to the outer envelope membrane where they might contribute to membrane expansion. Expression of type B enzymes was higher in nongreen tissues, i.e., in inflorescence (atMGD2) and roots (atMGD3), where they conceivably influence the eukaryotic structure prominence in MGDG. In addition, their expression of type B enzymes is enhanced under phosphate deprivation.

A proteinase-storing body that prepares for cell death or stresses in the epidermal cells of Arabidopsis

Plants degrade cellular materials during senescence and under various stresses. We report that the precursors of two stress-inducible cysteine proteinases, RD21 and a vacuolar processing enzyme (VPE), are specifically accumulated in approximately 0.5 microm diameter x approximately 5 microm long bodies in Arabidopsis thaliana. Such bodies have previously been observed in Arabidopsis but their function was not known. They are surrounded with ribosomes and thus are assumed to be directly derived from the endoplasmic reticulum (ER). Therefore, we propose to call them the ER bodies. The ER bodies are observed specifically in the epidermal cells of healthy seedlings. These cells are easily wounded and stressed by the external environment. When the seedlings are stressed with a concentrated salt solution, leading to death of the epidermal cells, the ER bodies start to fuse with each other and with the vacuoles, thereby mediating the delivery of the precursors directly to the vacuoles. This regulated, direct pathway differs from the usual case in which proteinases are transported constitutively from the ER to the Golgi complex and then to vacuoles, with intervention of vesicle-transport machinery, such as a vacuolar-sorting receptor or a syntaxin of the SNARE family. Thus, the ER bodies appear to be a novel proteinase-storing system that assists in cell death under stressed conditions.

Plant growth homeostasis is controlled by the Arabidopsis BON1 and BAP1 genes

Wild-type Arabidopsis plants maintain a relatively constant size over a wide range of temperatures. Here we show that this homeostasis requires the BONZAI1 (BON1) gene because bon1 null mutants make miniature fertile plants at 22 degrees C but have wild-type appearance at 28 degrees C. The expression of BON1 and a BON1-associated protein (BAP1) is modulated by temperature. Thus BON1 and BAP1 may have a direct role in regulating cell expansion and cell division at lower temperatures. BON1 contains a Ca(2+)-dependent phospholipid-binding domain and is associated with the plasma membrane. It belongs to the copine gene family, which is conserved from protozoa to humans. Our data suggest that this gene family may function in the pathway of membrane trafficking in response to external conditions.

An essential role of a TatC homologue of a Delta pH- dependent protein transporter in thylakoid membrane formation during chloroplast development in Arabidopsis thaliana

At least three transport systems function in targeting nuclear-encoded chloroplast proteins to the chloroplast thylakoid membrane. One of these systems requires a thylakoid pH gradient and is named the DeltapH-dependent protein transport system. A similar DeltapH export system of Escherichia coli contains four components, twin arginine translocation A (TatA), TatB, TatC, and TatE. TatC is a major component of the DeltapH-dependent protein transporter in E. coli and functions in the translocation of tightly folded proteins across membranes. We have isolated four transposon-inserted albino mutants named albino and pale green 2 (apg2) from Arabidopsis thaliana and showed that the transposons were inserted into different sites of a single gene. The APG2 gene product (named cpTatC) has sequence similarity with bacterial TatC and contains six putative transmembrane domains, including bacterial TatC proteins and a transit peptide in its N terminus. apg2 mutants showed albino phenotypes and could not grow in soil. The apg2 plastids were highly vacuolated, lacked internal membrane structures and lamellae of the thylakoid membrane, and contained many densely stained globule structures, like undifferentiated proplastids. Immunoblot analysis detected no thylakoid membrane proteins such as D1, light-harvesting complex, and OE23 in apg2 plastids, whereas soluble proteins such as rubisco large and small subunits were not decreased. These results indicate an essential role of cpTatC in chloroplast development, especially in thylakoid membrane formation.

Molecular identification of phenylalanine ammonia-lyase as a substrate of a specific constitutively active Arabidopsis CDPK expressed in maize protoplasts

Phenylalanine ammonia-lyase (PAL) is a key enzyme in pathogen defence, stress response and secondary metabolism and is subject to post-translational phosphorylation. In order to address the significance of this phenomenon it is necessary to identify the protein kinase (PK) responsible and place it in its regulatory circuit. Using protoplast transient expression of Arabidopsis kinase genes coupled to immunocomplex kinase assay, it has been possible to screen for specific PAL kinase. We show here that AtCPK1 (calcium dependent PK), but not other closely related PKs could phosphorylate both a recombinant PAL protein and a peptide (SRVAKTRTLTTA) that is a site phosphorylated in vivo. Identification of the specific CDPK as a PAL kinase now opens up the possibility of exploring the calcium link in biotic stress signalling, salicylate and phytoalexin production as well as the significance of PAL phosphorylation. The protoplast transient expression system is a potentially powerful method to determine and screen for plant gene functions utilising genomic and proteomic data.

Novel anther-specific myb genes from tobacco as putative regulators of phenylalanine ammonia-lyase expression

Two cDNA clones (NtmybAS1 and NtmybAS2) encoding MYB-related proteins with strong sequence similarity to petunia (Petunia hybrida) PhMYB3 were isolated from a tobacco (Nicotiana tabacum cv Samsun) pollen cDNA library. Northern blot and in situ hybridization revealed that NtmybAS transcripts are specifically expressed in both sporophytic and gametophytic tissues of the anther including tapetum, stomium, vascular tissue, and developing pollen. Random binding site selection assays revealed that NtMYBAS1 bound to DNA sequences closely resembling consensus MYB binding sites MBSI and MBSIIG, with a higher affinity for MBSI. Transient expression analyses of the N-terminal MYB domain demonstrated the presence of functional nuclear localization signals, and full-length NtMYBAS1 was able to activate two different phenylalanine ammonia-lyase promoters (PALA and gPAL1) in tobacco leaf protoplasts. Similar analysis of truncated NtmybAS1 cDNAs identified an essential, C-terminal trans-activation domain. Further in situ hybridization analyses demonstrated strict co-expression of NtmybAS and gPAL1 in the tapetum and stomium. Despite abundant expression of NtmybAS transcripts in mature pollen, gPAL1 transcripts were not detectable in pollen. Our data demonstrate that NtMYBAS1 is a functional anther-specific transcription factor, which is likely to be a positive regulator of gPAL1 expression and phenylpropanoid synthesis in sporophytic, but not in gametophytic, tissues of the anther.

KNOX homeobox genes are sufficient in maintaining cultured cells in an undifferentiated state in rice

We produced transgenic rice calli, which constitutively express each of four KNOX family class 1 homeobox genes of rice, OSH1, OSH16, OSH15, and OSH71, and found that constitutive and ectopic expression of such genes inhibits normal regeneration from transformed calli, which showed continuous growth around their shoot-regenerating stages. Transgenic calli transferred onto regeneration medium began to display green spots, a sign of regeneration, but most of the transformants continued to propagate green spots at given stages. In the normal shoot-regeneration process of calli, expression of endogenous OSH1 was restricted in presumptive shoot-regenerating regions of calli and not observed in other areas. This restricted expression pattern should be required for further differentiation of the regenerating shoots. Thus our present results support the proposed function that KNOX family class 1 homeobox genes play a role in the formation and maintenance of the undetermined meristematic state of cells.

High-level expression of a synthetic red-shifted GFP coding region incorporated into transgenic chloroplasts

We describe here a synthetic red-shifted variant of GFP that can be introduced into tobacco plastid genomes and is highly expressed in regenerated plants that appear normal and fertile. The variant contains the S65G and S72A mutations which shift the absorption maximum from the 395 nm of wild-type GFP closer to 488 nm, a wavelength emitted by a laser commonly used in confocal microscopy. In addition to enhanced fluorescence, the removal of significant absorption below 450 nm will potentially facilitate double-labelling experiments. The variant GFP encoded by the synthetic gene can be expressed at a high level, forming approximately 5% of total leaf protein.

The transcriptional activation domain of the plant-specific Dof1 factor functions in plant, animal, and yeast cells

Maize Dof1, one of the plant-specific Dof transcription factors, is involved in light-regulated gene expression. To elucidate the molecular mechanism underlying the activity of Dof1, in vivo functional analyses were carried out using transient expression assays with maize mesophyll protoplasts. The results suggest that the Dof domain alone, the region conserved among Dof factors, can mediate interaction with DNA in vivo and distinct Dof1 activities in greening and etiolated protoplasts. A region rich in basic amino acids was identified as a nuclear localization signal. Deletion analysis defined the transcriptional activation domain of 48 amino acids located in the C-terminus of Dof1. This activation domain was also found to be functional in both human cells and yeast, implying that Dof1 may stimulate transcription through a mechanism evolutionarily conserved among eukaryotes. A computer homology search with known transcription factors revealed that the activation domain of Dof1 displayed only a limited similarity to Activation domain II of animal transcription factor GATA-4. Mutational analysis revealed the critical role of a tryptophan residue within the activation domain of Dof1, as had been shown in Activation domain II of GATA-4.

Agrobacterium-mediated transformation of creeping bentgrass using GFP as a reporter gene

Creeping bentgrass (Agrostis palustris Huds.) is a cool season grass widely used on putting greens in golf courses. Transformation of creeping bentgrass has been conducted using microprojectile bombardment and protoplast electroporation. The objective of our study is to develop an alternative and more efficient approach in transforming the grass using Agrobacterium (strain EHA 101). This technique was effective in transforming 40-day old calli derived from mature seeds cultured on MS medium supplemented with 2,4-D, kinetin, and sucrose. Dozens of transgenic plants have been produced from two independent transformed calli. Presence of functional green fluorescence protein (GFP) was detected in leaves, stems, and roots of transgenic seedlings. Four putative transgenic plants and two control plants were randomly chosen and analyzed by Southern blot analysis. Bands corresponding to the GFP gene were clearly shown in transgenic plants. These results indicated that Agrobacterium transformation can successfully be applied to creeping bentgrass.

A cDNA for nuclear-encoded chloroplast translational initiation factor 2 from a higher plant is able to complement an infB Escherichia coli null mutant

Formation of the initiation translation complex containing the three initiation factors, IF1, IF2, and IF3, tRNA(fMet), and GTP constitutes the earliest event in the protein synthesis. IF2, a GTP-binding protein, is the principal factor involved in selecting and binding fMet-tRNA(fMet) to the 30 S ribosomal subunit. Although some chloroplast initiation translational factors have been identified and purified from algae, none of these factors have been characterized from plants. In this work, we report the molecular characterization of a nuclear-encoded chloroplastic IF2 gene from common bean (PvIF2cp). We show that the PvIF2cp gene encodes a protein containing a chloroplast translocation signal peptide, able to target a green fluorescent protein fusion protein to chloroplasts. A high accumulation of PvIF2cp transcript was found in photosynthetic tissues, whereas low mRNA levels were detected in etiolated plants and in nonphotosynthetic organs. Additional data indicate that the PvIF2cp transcript accumulation is modulated by light. The PvIF2cp gene encodes a functional factor, since the PvIF2cp conserved region, containing the G-domain and the C-terminal end, complements an Escherichia coli infB null mutation. Phylogenetic analysis using the PvIF2cp conserved region suggests that the PvIF2cp gene originated via endosymbiotic gene transfer to the nucleus and that it may be a useful marker for phylogeny reconstruction.

Functional expression and cellular localization of a green fluorescent protein-tagged proline transporter in Aspergillus nidulans

The PrnB protein is a highly specific proline transporter that belongs to an amino acid transporter family conserved in both prokaryotes and eukaryotes. In this work, we detected and analyzed the cellular localization of PrnB in vivo by means of green fluorescent protein (GFP) fusions. Several prnB-gfp gene fusions, driven by prnB native promoter sequences, were constructed and targeted to the genomic locus of a prnB null mutant. Chimeric proteins containing GFP fused to the C terminus of PrnB through a linker of two, four, or eight amino acids, with low potential to form secondary structure elements, were shown to be functional in vivo. A two-linker fusion results in partial complementation at both 25 and 37 degrees C. A four-linker fusion affords almost full complementation at 25 degrees C but partial complementation at 37 degrees C, whereas the eight-linker fusion results in partial complementation at both temperatures but in no GFP fluorescence. These results show that the number of linker amino acids is critical for the correct expression and/or translocation of PrnB-GFP fused proteins to the plasma membrane and for the fluorescence of the GFP. The expression of the four-linker PrnB-GFP transporter was detected and analyzed in vivo by both conventional fluorescence and confocal laser microscopy. This chimeric protein is localized in the plasma membrane, secondarily in large vacuoles found in the swollen conidial end of the germlings, and in other small intracellular structures observed as fluorescent granules. A strong correlation between known patterns of PrnB expression and intensity of PrnB-GFP fluorescence was observed. This work also demonstrates that the GFP fusion technology is a unique tool with which to study the expression and cellular localization of low-abundance transmembrane transporters expressed from their native promoters.

Trophic conversion of an obligate photoautotrophic organism through metabolic engineering

Most microalgae are obligate photoautotrophs and their growth is strictly dependent on the generation of photosynthetically derived energy. We show that the microalga Phaeodactylum tricornutum can be genetically engineered to thrive on exogenous glucose in the absence of light through the introduction of a gene encoding a glucose transporter (glut1 or hup1). This demonstrates that a fundamental change in the metabolism of an organism can be accomplished through the introduction of a single gene. This also represents progress toward the use of fermentation technology for large-scale commercial exploitation of algae by reducing limitations associated with light-dependent growth.

Characterization of two genes, Sig1 and Sig2, encoding distinct plastid sigma factors(1) in the moss Physcomitrella patens: phylogenetic relationships to plastid sigma factors in higher plants

We isolated the cDNA for a sigma factor from the moss Physcomitrella patens, which possesses unusually large N-terminal extension and the conserved subdomains 1.2-4.2. Phylogenetic analyses indicated that this novel sigma factor and PpSIG1*(2), a plastid sigma factor previously identified from Physcomitrella, were classified into SigA and SigB groups, two major classes of higher plant plastid sigma factors, respectively. According to the nomenclature recently proposed, we renamed PpSIG1* into PpSIG2, and named the novel sigma factor PpSIG1. A transient expression assay using a green fluorescent protein showed that the N-terminal region of PpSIG1 acts as a chloroplast-targeting signal. Reverse transcription-PCR experiments showed that light induces the expression of the Sig1 and Sig2 genes encoding PpSIG1 and PpSIG2, respectively. Thus, PpSIG1 and PpSIG2 are likely plastid sigma factors regulating plastid gene expression in response to light signals.

Serine acetyltransferase involved in cysteine biosynthesis from spinach: molecular cloning, characterization and expression analysis of cDNA encoding a plastidic isoform

A cDNA clone that encodes a chloroplast-localizing isoform of serine acetyltransferase (SATase) (EC 2.3.1.30) was isolated from spinach (Spinacia oleracea L.). The cDNA encodes a polypeptide of 347 amino acids containing a putative transit peptide of ca. 60-70 amino acids at the N-terminal. Deduced amino acid sequence of SATase from spinach exhibited homology with other SATases from plants. DNA blot hybridization analysis showed the presence of 2-3 copies of Sat gene in the genome of spinach. RNA blot hybridization analysis indicated the constitutive expression of Sat gene in green and etiolated seedlings of spinach. Bacterial expression of the cDNA could directly rescue the cysteine auxotrophy of Escherchia coli caused by a lack of SATase locus (cysE). Catalytically active SATase protein was produced in E. coli cells. L-Cysteine, an end product of the cysteine biosynthetic pathway, inhibited the activity of recombinant spinach SATase, indicating the regulatory function of SATase in this metabolic pathway. A chloroplastic localization of this spinach SATase was revealed by the analyses of transgenic plant expressing transit peptide of SATase-beta-glucuronidase (GUS) fusion protein, and transient expression using the transit peptide-green fluorescent protein (GFP) fusion protein. The result from in vitro translation analysis suggests that this cDNA may encode both plastidic and cytosolic SATases.

The gene for alternative oxidase-2 (AOX2) from Arabidopsis thaliana consists of five exons unlike other AOX genes and is transcribed at an early stage during germination

We investigated the expressions of genes for alternative oxidase (AOX1a, AOX1b, AOX1c and AOX2) and genes for cytochrome c oxidase (COX5b and COX6b) during germination of Arabidopsis thaliana, and examined oxygen uptakes of the alternative respiration and the cytochrome respiration in imbibed Arabidopsis seeds. A Northern blot analysis showed that AOX2 mRNA has already accumulated in dry seeds and subsequently decreased, whereas accumulation ofAOX1a mRNA was less abundant from 0 hours to 48 hours after imbibition and then increased. The increase of the capacity of the alternative pathway appeared to be dependent on the expressions of both AOX2 and AOX1a. On the other hand, steady-state mRNA levels of COX5b and COX6b were gradually increased during germination, and the capacity of the cytochrome pathway was correlated with the increase of expressions of the COX genes. Antimycin A, the respiratory inhibitor, strongly increased the expression of AOX1a but had no effect on the expression of AOX2. A 5'RACE analysis showed that AOX2 consists of five exons, which is different from the case of most AOX genes identified so far. Analysis of subcellular localization of AOX2 using green fluorescent protein indicated that the AOX2 protein is imported into the mitochondria.

GFP imaging: methodology and application to investigate cellular compartmentation in plants

The cloning of the jellyfish gfp (green fluorescent protein) gene and its alteration for expression in subcellular locations in transformed plant cells have resulted in new views of intracellular organization and dynamics. Fusions of GFP with entire proteins of known or unknown function have shown where the proteins are located and whether the proteins move from one compartment to another. GFP and variants with different spectral properties have been deliberately targeted to separate compartments to determine their size, shape, mobility, and dynamic changes during development or environmental response. Fluorescence Resonance Energy Transfer (FRET) between GFP variants can discern protein/ protein interactions. GFP has been used as a sensor to detect changes or differences in calcium, pH, voltage, metal, and enzyme activity. Photobleaching and photoactivation of GFP as well as fluorescence correlation spectroscopy can measure rates of diffusion and movement of GFP within or between compartments. This review covers past applications of these methods as well as promising developments in GFP imaging for understanding the functional organization of plant cells.

Functional interactions of lanthanum and phospholipase D with the abscisic acid signaling effectors VP1 and ABI1-1 in rice protoplasts

cis,trans-Abscisic acid (ABA) plays an important role in plant growth and development, regulation of seed maturation, germination, and adaptation to environmental stresses. Knowledge of ABA mechanisms of action and the interactions of components required for ABA signal transduction is far from complete. Using transient gene expression in rice protoplasts, we observed additive and inhibitory effects between maize VP1 (Viviparous-1, a transcriptional activator) and a dominant-negative mutant protein phosphatase, ABI1-1 (ABA-insensitive-1-1), from Arabidopsis. Lanthanide ions were shown to be specific agonists of ABA-inducible gene expression and to interact synergistically with ABA and overexpressed VP1. Both VP1 and lanthanum activities could be antagonized by coexpression of ABI1-1, which demonstrates the specific ABA dependence of these effectors on ABA-regulated gene expression. We obtained pharmacological evidence that phospholipase D (PLD) functions in ABA-inducible gene expression in rice. Antagonism of ABA, VP1, and lanthanum synergy by 1-butanol, a specific inhibitor of PLD, was similar to the inhibition by coexpression of ABI1-1. These results demonstrate that ABA, VP1, lanthanum, PLD, and ABI1 are all involved in ABA-regulated gene expression and are consistent with an integrated model whereby La(3+) acts upstream of PLD.

Molecular cloning of a novel importin alpha homologue from rice, by which constitutive photomorphogenic 1 (COP1) nuclear localization signal (NLS)-protein is preferentially nuclear imported

Nuclear import of proteins that contain classical nuclear localization signals (NLS) is initiated by importin alpha, a protein that recognizes and binds to the NLS in the cytoplasm. In this paper, we have cloned a cDNA for a novel importin alpha homologue from rice which is in addition to our previously isolated rice importin alpha1a and alpha2, and we have named it rice importin alpha1b. In vitro binding and nuclear import assays using recombinant importin alpha1b protein demonstrate that rice importin alpha1b functions as a component of the NLS-receptor in plant cells. Analysis of the transcript levels for all three rice importin alpha genes revealed that the genes were not only differentially expressed but that they also responded to dark-adaptation in green leaves. Furthermore, we also show that the COP1 protein bears a bipartite-type NLS and its nuclear import is mediated preferentially by the rice importin alpha1b. These data suggest that each of the different rice importin alpha proteins carry distinct groups of nuclear proteins, such that multiple isoforms of importin alpha contribute to the regulation of plant nuclear protein transport.

Stable genetic transformation of Eschscholzia californica expressing synthetic green fluorescent proteins

An efficient protocol is described for the stable genetic transformation of Eschscholzia californica (California poppy) using Agrobacterium tumefaciens as a vector. We have employed the disarmed A. tumefaciens LBA4404 encoding a synthetic green fluorescent protein reporter gene that is further controlled by an enhanced cauliflower mosaic virus 35S promoter. Stably transformed E. californica cells appear 3 weeks after initial cocultivation of A. tumefaciens with poppy leaves, stems, or roots. Transformed poppy calli were visualized by exposure to long-wave UV or blue light and analyzed in detail by fluorescent microscopy and laser-scanning microscopy. Moreover, green fluorescent calli have been maintained through continual subculture and grow well either on Gamborg's B5 agarose or liquid medium.

Endoxyloglucan transferase is localized both in the cell plate and in the secretory pathway destined for the apoplast in tobacco cells

Intracellular trafficking of enzymes responsible for constructing and modifying the cell wall architecture in plants is mostly unknown. To examine their translocation pathways, we employed an endoxyloglucan transferase (EXGT), a key enzyme responsible for forming and rearranging the cellulose/xyloglucan network of the cell wall. We traced its intracellular localization in suspension-cultured cells of tobacco bright yellow-2 by means of green fluorescent protein-fusion gene procedures as well as by indirect immunofluorescence. During interphase the protein was extensively secreted into the apoplast via the endoplasmic reticulum-Golgi apparatus network, whereas during cytokinesis, the protein was exclusively located in the phragmoplast and eventually transported to the cell plate. These results clearly indicate commitment of EXGT protein to the construction of both the cell plate and the cell wall. This study also visualized the process of phragmoplast development at a level of vesicle translocation in the living cell.

Ambient pH signaling regulates nuclear localization of the Aspergillus nidulans PacC transcription factor

The Aspergillus nidulans zinc finger transcription factor PacC is activated by proteolytic processing in response to ambient alkaline pH. The pH-regulated step is the transition of full-length PacC from a closed to an open, protease-accessible conformation. Here we show that in the absence of ambient pH signaling, the C-terminal negative-acting domain prevents the nuclear localization of full-length closed PacC. In contrast, the processed PacC form is almost exclusively nuclear at any ambient pH. In the presence of ambient pH signaling, the fraction of PacC that is in the open conformation but has not yet been processed localizes to the nucleus. Therefore, ambient alkaline pH leads to an increase in nuclear PacC by promoting the proteolytic elimination of the negative-acting domain to yield the processed form and by increasing the proportion of full-length protein that is in the open conformation. These findings explain why mutations resulting in commitment of PacC to processing irrespective of ambient pH lead to permanent PacC activation and alkalinity mimicry. A nuclear import signal that targets Escherichia coli beta-galactosidase to the nucleus has been located to the PacC zinc finger region. A mutation abolishing DNA binding does not prevent nuclear localization of the processed form, showing that PacC processing does not lead to nuclear localization by passive diffusion of the protein made possible by the reduction in size, followed by retention in the nucleus after DNA binding.

Development of a transformation system for Mycosphaerella pathogens of banana: a tool for the study of host/pathogen interactions

A genetic transformation system has been developed for three Mycosphaerella pathogens of banana and plantain (Musa spp.). Mycosphaerella fijiensis and Mycosphaerella musicola, the causal agents of black and yellow Sigatoka, respectively, and Mycosphaerella eumusae, which causes Septoria leaf spot of banana, were transformed with a construct carrying a synthetic gene encoding green fluorescent protein (GFP). Most single-spored transformants that expressed GFP constitutively were mitotically stable in the absence of selection for hygromycin B resistance. Transformants of all three species were pathogenic on the susceptible banana cultivar Grand Nain, and growth in planta was comparable to wild-type strains. GFP expression by transformants allowed us to observe extensive fungal growth within leaf tissue that eventually turned necrotic, at which point the fungi grew saprophytically on the dead tissue. Leaf chlorosis and necrosis were often observed in advance of saprophytic growth of the mycelium on necrotic tissue, which supports previous reports suggesting secretion of a phytotoxin.

Use of the GFP reporter as a vital marker for Agrobacterium-mediated transformation of sugar beet (Beta vulgaris L.)

Molecular approaches to sugar beet improvement will benefit from an efficient transformation procedure that does not rely upon exploitation of selectable marker genes such as those which confer antibiotic or herbicide resistance upon the transgenic plants. The expression of the green fluorescent protein (GFP) signal has been investigated during a program of research that was designed to address the need to increase the speed and efficiency of selection of sugar beet transformants. It was envisaged that the GFP reporter could be used initially as a supplement to current selection regimes in order to help eliminate "escapes" and perhaps eventually as a replacement marker in order to avoid the public disquiet associated with antibiotic/herbicide-resistance genes in field-released crops. The sgfp-S65T gene has been modified to have a plant-compatible codon usage, and a serine to threonine mutation at position 65 for enhanced fluorescence under blue light. This gene, under the control of the CaMV 35S promoter, was introduced into sugar beet via Agrobacterium-mediated transformation. Early gene expression in cocultivated sugar beet cultures was signified by green fluorescence several days after cocultivation. Stably transformed calli, which showed green fluorescence at a range of densities, were obtained at frequencies of 3-11% after transferring the inoculated cultures to selection media. Cocultivated shoot explants or embryogenic calli were regularly monitored under the microscope with blue light when they were transferred to media without selective agents. Green fluorescent shoots were obtained at frequencies of 2-5%. It was concluded that the sgfp-S65T gene can be used as a vital marker for noninvasive screening of cells and shoots for transformation, and that it has potential for the development of selectable marker-free transgenic sugar beet.

Transcriptional expression characteristics and subcellular localization of ADP-glucose pyrophosphorylase in the oil plant Perilla frutescens

Three ADP-glucose pyrophosphorylase clones were isolated from the cotyledon cDNA library of the oil plant, Perilla frutescens, and their intracellular localization investigated. Two of three cDNAs (PfagpS1 and PfagpS2) were homologous to the catalytic small subunit of AGPases found in other plants, while the third clone (PfagpL) was highly similar to the large subunit type. Transcripts for PfagpS1 and PfagpS2 were observed in both photosynthetic and non-photosynthetic tissue, showing the highest expression in the stem, while PfagpL transcripts were abundantly expressed in stem and cotyledon. To evaluate the subcellular localization of PfagpS2 and PfagpL as well as the maize BT2, N-terminus-GFP DNA fusion were constructed and transformed into tobacco plants. Immunoblot analysis showed that the expressed PfagpS2- and PfagpL-GFP fusions were targeted to the plastid in the heterologous tobacco system whereas the BT2-GFP remained intact, suggesting a cytoplasmic location. These intracellular assignments were confirmed by direct confocal microscopic examination. GFP signals were localized to the cytoplasm as well as in the nucleus in BT2-GFP plants, and to the plastids in PfagpS2- and PfagpL-GFP plants. Our results indicate that Perilla cotyledons contain multiple AGPase subunits, of which at least two isoforms and very likely the third, are plastidial in nature.

The NPK1 mitogen-activated protein kinase kinase kinase is a regulator of cell-plate formation in plant cytokinesis

Mitogen-activated protein kinase (MAPK) cascades play important roles not only in the transduction of extracellular signals but in the progression of the cell cycle. However, evidence for their role in cytokinesis is limited. Here, we show that a tobacco MAPK kinase kinase (MAPKKK), designated NPK1, is required for cytokinesis. The activity of NPK1 increases in the late M phase of the tobacco cell cycle. During expansion of a new cross-wall (cell plate) toward the cell cortex, NPK1 is consistently localized to the equatorial zone of the phragmoplast, the cytokinetic apparatus where the cell plate is formed. Expression of a kinase-negative mutant of NPK1 results in the generation of multinucleate cells with incomplete cell plates. Phragmoplasts can be formed, but its expansion toward the cell cortex is also blocked. Thus, our results indicate that the NPK1 MAPKKK is essential for the formation of the cell plate, especially for its lateral growth.

Involvement of a nuclear-encoded basic helix-loop-helix protein in transcription of the light-responsive promoter of psbD

In the chloroplast psbD light-responsive promoter (LRP), a highly conserved sequence exists upstream from the bacterial -10/-35 elements. Multiple sequence-specific DNA binding proteins are predicted to bind to the conserved sequence as transcription factors. Using yeast one-hybrid screening of an Arabidopsis cDNA library, a possible DNA binding protein of the psbD LRP upstream sequence was identified. The protein, designated PTF1, is a novel protein of 355 amino acids (estimated molecular weight of 39.6) that contains a basic helix-loop-helix DNA binding motif in the predicted N-terminal region of the mature protein. Transient expression assay of PTF1-GFP fusion protein showed that PTF1 was localized in chloroplasts. Using the modified DNA sequence in the one-hybrid system, the ACC repeat was shown to be essential for PTF1 binding. The rate of psbD LRP mRNA accumulation was reduced in a T-DNA-inserted Arabidopsis ptf1 mutant. Compared with wild-type plants, the mutant had pale green cotyledons and its growth was inhibited under short-day conditions. These results suggest that PTF1 is a trans-acting factor of the psbD LRP.

Geranylgeranyl diphosphate synthase from Scoparia dulcis and Croton sublyratus. Plastid localization and conversion to a farnesyl diphosphate synthase by mutagenesis

cDNAs encoding geranylgeranyl diphosphate synthase (GGPPS) of two diterpene-producing plants, Scoparia dulcis and Croton sublyratus, have been isolated using the homology-based polymerase chain reaction (PCR) method. Both clones contained highly conserved aspartate-rich motifs (DDXX(XX)D) and their N-terminal residues exhibited the characteristics of chloroplast targeting sequence. When expressed in Escherichia coli, both the full-length and truncated proteins in which the putative targeting sequence was deleted catalyzed the condensation of farnesyl diphosphate and isopentenyl diphosphate to produce geranylgeranyl diphosphate (GGPP). The structural factors determining the product length in plant GGPPSs were investigated by constructing S. dulcis GGPPS mutants on the basis of sequence comparison with the first aspartate-rich motif (FARM) of plant farnesyl diphosphate synthase. The result indicated that in plant GGPPSs small amino acids, Met and Ser, at the fourth and fifth positions before FARM and Pro and Cys insertion in FARM play essential roles in determination of product length. Further, when a chimeric gene comprised of the putative transit peptide of the S. dulcis GGPPS gene and a green fluorescent protein was introduced into Arabidopsis leaves by particle gun bombardment, the chimeric protein was localized in chloroplasts, indicating that the cloned S. dulcis GGPPS is a chloroplast protein.

Cloning and characterization of the cDNA for a plastid sigma factor from the moss Physcomitrella patens

We isolated a cDNA PpSig1 encoding a plastid sigma factor from the moss Physcomitrella patens. The PpSIG1 protein is composed of the conserved subdomains for recognition of -10 and -35 promoter elements, core complex binding and DNA melting. Southern blot analysis showed that the moss sig1 gene is likely a member of a small gene family. Transient expression assay using green fluorescent protein demonstrated that the N-terminal region of PpSIG1 functions as a chloroplast-targeting signal peptide. These observations suggest that multiple nuclear-encoded sigma factors regulate chloroplast gene expression in P. patens.

Transgenic citrus plants expressing the citrus tristeza virus p23 protein exhibit viral-like symptoms

Summary The 23 kDa protein (p23) coded by the 3'-terminal gene of Citrus tristeza virus (CTV), a member of the genus Closterovirus with the largest genome among plant RNA viruses, is an RNA-binding protein that contains a motif rich in cysteine and histidine residues in the core of a putative zinc-finger domain. On this basis, a regulatory role for CTV replication or gene expression has been suggested for p23. To explore whether over-expression of this protein in transgenic plants could affect the normal CTV infection process, transgenic Mexican lime plants were generated carrying the p23 transgene, or a truncated version thereof, under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Constitutive expression of p23 induced phenotypic aberrations that resembled symptoms incited by CTV in non-transgenic lime plants, whereas transgenic plants expressing the p23 truncated version were normal. The onset of CTV-like symptoms in p23-transgenic plants was associated with the expression of p23, and its accumulation level paralleled the intensity of the symptoms. This demonstrates that p23 is involved in symptom development and that it most likely plays a key role in CTV pathogenesis. This is the first case in which a protein encoded by a woody plant-infecting RNA virus has been identified as being directly involved in pathogenesis in its natural host. This finding also delimits a small region of the large CTV genome for the future mapping of specific pathogenic determinants.

Spontaneous deletion enhances movement of a cucumber necrosis virus based chimera expressing the red clover necrotic mosaic virus movement protein genedagger

Summary The 35-kDa movement protein (MP) gene of red clover necrotic mosaic virus (RCNMV) and 3' flanking sequence were inserted in a cucumber necrosis virus (CNV) deletion mutant lacking a large portion of the coding region for the MP. Nicotiana benthamiana plants inoculated with chimeric synthetic transcripts of the resulting hybrid cDNA clone (M5/RM2) developed both local and systemic symptoms and accumulated high levels of chimeric viral RNA. Reverse transcriptase polymerase chain reaction (RT-PCR) and sequence analysis of viral RNA extracted from systemically infected leaves of four different plants revealed that in each plant a large portion (305, 308, 315 or 127 nts) of the 3' terminus of the inserted sequence spontaneously deleted during infection. In three of the deletion derivatives, the truncated RCNMV MP open reading frame (ORF) was fused in-frame with the remaining portion of the 3' terminal region of CNV MP ORF. The movement efficiencies of M5/RM2, a cloned copy of one of the deletion derivatives (ClM5/RM2dd1), and a stop codon mutant of ClM5/RM2dd1 (ClM5/RM2dd1stop), which prevents translational fusion to the CNV MP, were compared and it was determined that deletion of RCNMV MP sequences in conjunction with fusion to CNV MP sequences increases the movement efficiency of the chimeric virus genome. Absence of the C-terminal region of the RCNMV MP in RCNMV RNA-2 abolished RCNMV movement. However, movement could be complemented in trans if cells were coinoculated with ClM5/RM2dd1. Complementation of RCNMV movement did not occur using ClM5/RM2dd1stop, suggesting a role for appended CNV MP sequences in movement of the RCNMV genome. The ability of the CNV replicase to delete unnecessary or deleterious RCNMV sequences and to append the required CNV MP sequences reinforces the role of RNA recombination in the adaptation and evolution of viral genomes.

Dead cells don't dance: insights from live-cell imaging in plants

Live-cell imaging has yielded surprising pictures of subcellular structures and dynamics in living plant cells. Recent studies illustrate the power of live-cell observation for revealing new biological phenomena and for generating new questions about plant cell structure and function.

A stress-induced calcium-dependent protein kinase from Mesembryanthemum crystallinum phosphorylates a two-component pseudo-response regulator

McCDPK1 is a salinity- and drought-induced calcium-dependent protein kinase (CDPK) isolated from the common ice plant, Mesembryanthemum crystallinum. A yeast two-hybrid experiment was performed, using full-length McCDPK1 and truncated forms of McCDPK1 as baits, to identify interacting proteins. A catalytically impaired bait isolated a cDNA clone encoding a novel protein, CDPK substrate protein 1 (CSP1). CSP1 interacted with McCDPK1 in a substrate-like fashion in both yeast two-hybrid assays and wheat germ interaction assays. Furthermore, McCDPK1 was capable of phosphorylating CSP1 in vitro in a calcium-dependent manner. Our results demonstrate that the use of catalytically impaired and unregulated CDPKs with the yeast two-hybrid system can accelerate the discovery of CDPK substrates. The deduced CSP1 amino acid sequence indicated that it is a novel member of a class of pseudo-response regulator-like proteins that have a highly conserved helix-loop-helix DNA binding domain and a C-terminal activation domain. McCDPK1 and CSP1 co-localized to nuclei of NaCl-stressed ice plants. Csp1 transcript accumulation was not regulated by NaCl or dehydration stress. Our results strongly suggest that McCDPK1 may regulate the function of CSP1 by reversible phosphorylation.

Identification of eukaryotic peptide deformylases reveals universality of N-terminal protein processing mechanisms

The N-terminal protein processing pathway is an essential mechanism found in all organisms. However, it is widely believed that deformylase, a key enzyme involved in this process in bacteria, does not exist in eukaryotes, thus making it a target for antibacterial agents such as actinonin. In an attempt to define this process in higher eukaryotes we have used Arabidopsis thaliana as a model organism. Two deformylase cDNAs, the first identified in any eukaryotic system, and six distinct methionine aminopeptidase cDNAs were cloned. The corresponding proteins were characterized in vivo and in vitro. Methionine aminopeptidases were found in the cytoplasm and in the organelles, while deformylases were localized in the organelles only. Our work shows that higher plants have a much more complex machinery for methionine removal than previously suspected. We were also able to identify deformylase homologues from several animals and clone the corresponding cDNA from human cells. Our data provide the first evidence that lower and higher eukaryotes, as well as bacteria, share a similar N-terminal protein processing machinery, indicating universality of this system.