Plant Cell Division Analyzed by Transient Agrobacterium-Mediated Transformation of Tobacco BY-2 Cells

The continuing analysis of plant cell division will require additional protein localization studies. This is greatly aided by GFP-technology, but plant transformation and the maintenance of transgenic lines can present a significant technical bottleneck. In this chapter I describe a method for the Agrobacterium-mediated genetic transformation of tobacco BY-2 cells. The method allows for the microscopic analysis of fluorescence-tagged proteins in dividing cells in within 2 days after starting a coculture. This transient transformation procedure requires only standard laboratory equipment. It is hoped that this rapid method would aid researchers conducting live-cell localization studies in plant mitosis and cytokinesis.

Identification and characterization of plant-specific NAC gene family in canola (Brassica napus L.) reveal novel members involved in cell death

NAC transcription factors are plant-specific and play important roles in plant development processes, response to biotic and abiotic cues and hormone signaling. However, to date, little is known about the NAC genes in canola (or oilseed rape, Brassica napus L.). In this study, a total of 60 NAC genes were identified from canola through a systematical analysis and mining of expressed sequence tags. Among these, the cDNA sequences of 41 NAC genes were successfully cloned. The translated protein sequences of canola NAC genes with the NAC genes from representative species were phylogenetically clustered into three major groups and multiple subgroups. The transcriptional activities of these BnaNAC proteins were assayed in yeast. In addition, by quantitative real-time RT-PCR, we further observed that some of these BnaNACs were regulated by different hormone stimuli or abiotic stresses. Interestingly, we successfully identified two novel BnaNACs, BnaNAC19 and BnaNAC82, which could elicit hypersensitive response-like cell death when expressed in Nicotiana benthamiana leaves, which was mediated by accumulation of reactive oxygen species. Overall, our work has laid a solid foundation for further characterization of this important NAC gene family in canola.

Stress-induced chloroplast degradation in Arabidopsis is regulated via a process independent of autophagy and senescence-associated vacuoles

Two well-known pathways for the degradation of chloroplast proteins are via autophagy and senescence-associated vacuoles. Here, we describe a third pathway that was activated by senescence- and abiotic stress-induced expression of Arabidopsis thaliana CV (for chloroplast vesiculation). After targeting to the chloroplast, CV destabilized the chloroplast, inducing the formation of vesicles. CV-containing vesicles carrying stromal proteins, envelope membrane proteins, and thylakoid membrane proteins were released from the chloroplasts and mobilized to the vacuole for proteolysis. Overexpression of CV caused chloroplast degradation and premature leaf senescence, whereas silencing CV delayed chloroplast turnover and senescence induced by abiotic stress. Transgenic CV-silenced plants displayed enhanced tolerance to drought, salinity, and oxidative stress. Immunoprecipitation and bimolecular fluorescence complementation assays demonstrated that CV interacted with photosystem II subunit PsbO1 in vivo through a C-terminal domain that is highly conserved in the plant kingdom. Collectively, our work indicated that CV plays a crucial role in stress-induced chloroplast disruption and mediates a third pathway for chloroplast degradation. From a biotechnological perspective, silencing of CV offers a suitable strategy for the generation of transgenic crops with increased tolerance to abiotic stress.

Putative signal peptides of two BURP proteins can direct proteins to their destinations in tobacco cell system

Plant-specific BURP family proteins have a diverse subcellular localization with different functions. However, only limited studies have investigated the functions of their different domains. In the present study, the role of the N-terminal putative signal peptide in protein subcellular localization was investigated using a tobacco cell system. The results showed that SALI3-2 was present in vacuoles, whereas AtRD22 was directed to the apoplast. The N-terminal putative signal peptides of both proteins were confirmed to be the essential and critical domains for targeting the proteins to their destinations. We also demonstrate that the expression and accumulation of mGFP in tobacco cells was increased when mGFP was fused to the putative signal peptide of SALI3-2. The findings offer the potential application of this short peptide in protein production in plants.

A faster Rubisco with potential to increase photosynthesis in crops

In photosynthetic organisms, D-ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the major enzyme assimilating atmospheric CO2 into the biosphere. Owing to the wasteful oxygenase activity and slow turnover of Rubisco, the enzyme is among the most important targets for improving the photosynthetic efficiency of vascular plants. It has been anticipated that introducing the CO2-concentrating mechanism (CCM) from cyanobacteria into plants could enhance crop yield. However, the complex nature of Rubisco's assembly has made manipulation of the enzyme extremely challenging, and attempts to replace it in plants with the enzymes from cyanobacteria and red algae have not been successful. Here we report two transplastomic tobacco lines with functional Rubisco from the cyanobacterium Synechococcus elongatus PCC7942 (Se7942). We knocked out the native tobacco gene encoding the large subunit of Rubisco by inserting the large and small subunit genes of the Se7942 enzyme, in combination with either the corresponding Se7942 assembly chaperone, RbcX, or an internal carboxysomal protein, CcmM35, which incorporates three small subunit-like domains. Se7942 Rubisco and CcmM35 formed macromolecular complexes within the chloroplast stroma, mirroring an early step in the biogenesis of cyanobacterial β-carboxysomes. Both transformed lines were photosynthetically competent, supporting autotrophic growth, and their respective forms of Rubisco had higher rates of CO2 fixation per unit of enzyme than the tobacco control. These transplastomic tobacco lines represent an important step towards improved photosynthesis in plants and will be valuable hosts for future addition of the remaining components of the cyanobacterial CCM, such as inorganic carbon transporters and the β-carboxysome shell proteins.

Enhanced heterologous expression of biologically active human granulocyte colony stimulating factor in transgenic tobacco BY-2 cells by localization to endoplasmic reticulum

Tobacco Bright Yellow-2 (BY-2) cells, one of the best characterized cell lines is an attractive expression system for heterologous protein expression. However, the expression of foreign proteins is currently hampered by their low yield, which is partially the result of proteolytic degradation. Human granulocyte colony stimulating factor (hG-CSF) is a hematopoietic cytokine. Recombinant hG-CSF is successfully being used for the treatment of chemotherapy-induced neutropenia in cancer patients. Here, we describe a simple strategy for producing biologically active hG-CSF in tobacco BY-2 cells, localized in the apoplast of BY-2 cells, as well as targeted to the endoplasmic reticulum (ER). ER targeting significantly enhanced recombinant production which scaled to 17.89 mg/l from 4.19 mg/l when expressed in the apoplasts. Southern blotting confirmed the stable integration of hG-CSF in the BY-2 nuclear genome, and the expression of hG-CSF was analysed by Western blotting. Total soluble protein containing hG-CSF isolated from positive calli showed proliferative potential when tested on HL-60 cell lines by MTT assay. We also report the potential of a Fluorescence-activated cell sorting approach for an efficient sorting of the hG-CSF-expressing cell lines, which will enable the generation of homogenous high-producing cell lines.

Wound healing response and xylem differentiation in tobacco plants over-expressing a fungal endopolygalacturonase is mediated by copper amine oxidase activity

In this work, we have investigated the involvement of copper amine oxidase (CuAO; EC 1.4.3.21) in wound healing and xylem differentiation of Nicotiana tabacum plants over-expressing a fungal endopolygalacturonase (PG plants), which show constitutively activated defence responses. In petioles and stems of PG plants, we found higher CuAO activity and lower polyamine (PA) levels, particularly putrescine (Put), with respect to wild-type (WT) plants. Upon wounding, a more intense autofluorescence of cell wall phenolics was observed in correspondence of wound surface, extending to epidermis and cortical parenchima only in PG plants. This response was mostly dependent on CuAO activity, as suggested by the reversion of autofluorescence upon supply of 2-bromoethylamine (2-BrEt), a CuAO specific inhibitor. Moreover, in unwounded plants, histochemical analysis revealed a tissue-specific expression of the enzyme in the vascular cambium and neighboring derivative cells of both petioles and stems of PG plants, whereas the corresponding WT tissues appeared unstained or faintly stained. A higher histochemical CuAO activity was also observed in xylem cells of PG plants as compared to WT xylem tissues suggesting a peculiar role of CuAO activity in xylem differentiation in PG plants. Indeed, roots of PG plants exhibited early xylem differentiation, a phenotype consistent with both the higher CuAO and the lower Put levels observed and supported by the 2-BrEt-mediated reversion of early root xylem differentiation and H2O2 accumulation. These results strongly support the relevance of PA-catabolism derived H2O2 in defence responses, such as those signaled by a compromised status of cell wall pectin integrity.

SPX1 is an important component in the phosphorus signalling network of common bean regulating root growth and phosphorus homeostasis

Proteins containing the SPX domain are believed to play vital roles in the phosphorus (P) signalling network in plants. However, the functions of SPX proteins in legumes remain largely unknown. In this study, three SPX members, PvSPX1-PvSPX3 were cloned from common bean (Phaseolus vulgaris L.). It was found that the transcripts of all three PvSPX members were significantly enhanced in both bean leaves and roots by phosphate (Pi) starvation. Among them, the expression of nuclear localized PvSPX1 showed more sensitive and rapid responses to Pi starvation. Consistently, only overexpression of PvSPX1 resulted in increased root P concentration and modified morphology of transgenic bean hairy roots, such as inhibited root growth and an enlarged root hair zone. It was further demonstrated that PvSPX1 transcripts were up-regulated by overexpressing PvPHR1, and overexpressing PvSPX1 led to increased transcripts of 10 Pi starvation-responsive genes in transgenic bean hairy roots. Taken together, it is suggested that PvSPX1 is a positive regulator in the P signalling network of common bean, and is downstream of PvPHR1.

HopW1 from Pseudomonas syringae disrupts the actin cytoskeleton to promote virulence in Arabidopsis

A central mechanism of virulence of extracellular bacterial pathogens is the injection into host cells of effector proteins that modify host cellular functions. HopW1 is an effector injected by the type III secretion system that increases the growth of the plant pathogen Pseudomonas syringae on the Columbia accession of Arabidopsis. When delivered by P. syringae into plant cells, HopW1 causes a reduction in the filamentous actin (F-actin) network and the inhibition of endocytosis, a known actin-dependent process. When directly produced in plants, HopW1 forms complexes with actin, disrupts the actin cytoskeleton and inhibits endocytosis as well as the trafficking of certain proteins to vacuoles. The C-terminal region of HopW1 can reduce the length of actin filaments and therefore solubilize F-actin in vitro. Thus, HopW1 acts by disrupting the actin cytoskeleton and the cell biological processes that depend on actin, which in turn are needed for restricting P. syringae growth in Arabidopsis.

Efficient transformation of oil palm protoplasts by PEG-mediated transfection and DNA microinjection

Background

Genetic engineering remains a major challenge in oil palm (Elaeis guineensis) because particle bombardment and Agrobacterium-mediated transformation are laborious and/or inefficient in this species, often producing chimeric plants and escapes. Protoplasts are beneficial as a starting material for genetic engineering because they are totipotent, and chimeras are avoided by regenerating transgenic plants from single cells. Novel approaches for the transformation of oil palm protoplasts could therefore offer a new and efficient strategy for the development of transgenic oil palm plants.

Methodology/Principal Findings

We recently achieved the regeneration of healthy and fertile oil palms from protoplasts. Therefore, we focused on the development of a reliable PEG-mediated transformation protocol for oil palm protoplasts by establishing and validating optimal heat shock conditions, concentrations of DNA, PEG and magnesium chloride, and the transfection procedure. We also investigated the transformation of oil palm protoplasts by DNA microinjection and successfully regenerated transgenic microcalli expressing green fluorescent protein as a visible marker to determine the efficiency of transformation.

Conclusions/Significance

We have established the first successful protocols for the transformation of oil palm protoplasts by PEG-mediated transfection and DNA microinjection. These novel protocols allow the rapid and efficient generation of non-chimeric transgenic callus and represent a significant milestone in the use of protoplasts as a starting material for the development of genetically-engineered oil palm plants.

Genome-wide expressional and functional analysis of calcium transport elements during abiotic stress and development in rice

Ca²⁺ homeostasis is required to maintain a delicate balance of cytosolic Ca²⁺ during normal and adverse growth conditions. Various Ca²⁺ transporters actively participate to maintain this delicate balance especially during abiotic stresses and developmental events in plants. In this study, we present a genome-wide account, detailing expression profiles, subcellular localization and functional analysis of rice Ca²⁺ transport elements. Exhaustive in silico data mining and analysis resulted in the identification of 81 Ca²⁺ transport element genes, which belong to various groups such as Ca²⁺-ATPases (pumps), exchangers, channels, glutamate receptor homologs and annexins. Phylogenetic analysis revealed that different Ca²⁺ transporters are evolutionarily conserved across different plant species. Comprehensive expression analysis by gene chip microarray and quantitative RT-PCR revealed that a substantial proportion of Ca²⁺ transporter genes were expressed differentially under abiotic stresses (salt, cold and drought) and reproductive developmental stages (panicle and seed) in rice. These findings suggest a possible role of rice Ca²⁺ transporters in abiotic stress and development triggered signaling pathways. Subcellular localization of Ca²⁺ transporters from different groups in Nicotiana benthamiana revealed their variable localization to different compartments, which could be their possible sites of action. Complementation of Ca²⁺ transport activity of K616 yeast mutant by Ca²⁺-ATPase OsACA7 and involvement in salt tolerance verified its functional behavior. This study will encourage detailed characterization of potential candidate Ca²⁺ transporters for their functional role in planta.

[Abnormal floral meristem development in transgenic tomato plants do not depend on the expression of genes encoding defense-related PR-proteins and antimicrobial peptides]

In this study, the morphological and cytoembryological analyses of the tomato plants transformed with the genes encoding chitin-binding proteins (ac and RS-intron-Shir) from Amaranthus caudatus L. andA. retroflexus L., respectively, as well as the gene amp2 encoding hevein-like antimicrobial peptides from Stellaria media L., have been performed. The transgenic lines were adapted to soil and grown the greenhouse. The analysis of putative transgenic tomato plants revealed several lines that did not differ phenotypically from the wild type plants and three lines with disruption in differentiation of the inflorescence shoot and the flower, as well as the fruit formation (modified plants of each line were transformed with a single gene as noted before). Abnormalities in the development of the generative organs were maintained for at least six vegetative generations. These transgenic plants were shown to be defective in the mail gametophyte formation, fertilization, and, consequently, led to parthenocarpic fruits. The detailed analysis of growing ovules in the abnormal transgenic plants showed that the replacement tissue was formed and proliferated instead of unfertilized embryo sac. The structure of the replacement tissue differed from both embryonic and endosperm tissue of the normal ovule. The formation of the replacement tissue occurred due to continuing proliferation of the endothelial cells that lost their ability for differentiation. The final step in the development of the replacement tissue was its death, which resulted in the cell lysis. The expression of the genes used was confirmed by RT-PCR in all three lines with abnormal phenotype, as well as in several lines that did not phenotypically differ from the untransformed control. This suggests that abnormalities in the organs of the generative sphere in the transgenic plants do not depend on the expression of the foreign genes that were introduced in the tomato genome. Here, we argue that agrobacterial transformation affects, directly or indirectly, expression of genes encoding for transcription factors that can activate a gene cascade responsible for the normal plant development.

Cosuppression of NtmybA1 and NtmybA2 causes downregulation of G2/M phaseexpressed genes and negatively affects both cell division and expansion in tobacco

During the plant cell cycle, genes preferentially expressed at the G2/M phase are regulated by R1R2R3-type Myb transcription factors. To address the function of 2 tobacco R1R2R3-Myb proteins, NtmybA1 and NtmybA2, we generated transgenic tobacco plants in which endogenous NtmybA2 transcripts were significantly decreased, presumably due to cosuppression triggered by the presence of the NtmybA2 transgene. These lines showed a concomitant downregulation of structurally related NtmybA1 and many G2/M-expressed genes. In the cosuppression plants, we found a dwarf phenotype due to both reduced cell size and decreased cell number. Our results provide evidence confirming our previous view that NtmybA1 and NtmybA2 may regulate cell expansion as well as cell division by transcriptionally activating many G2/M-expressed genes in tobacco.

Cosuppression of NtmybA1 and NtmybA2 causes downregulation of G2/M phaseexpressed genes and negatively affects both cell division and expansion in tobacco

During the plant cell cycle, genes preferentially expressed at the G2/M phase are regulated by R1R2R3-type Myb transcription factors. To address the function of 2 tobacco R1R2R3-Myb proteins, NtmybA1 and NtmybA2, we generated transgenic tobacco plants in which endogenous NtmybA2 transcripts were significantly decreased, presumably due to cosuppression triggered by the presence of the NtmybA2 transgene. These lines showed a concomitant downregulation of structurally related NtmybA1 and many G2/M-expressed genes. In the cosuppression plants, we found a dwarf phenotype due to both reduced cell size and decreased cell number. Our results provide evidence confirming our previous view that NtmybA1 and NtmybA2 may regulate cell expansion as well as cell division by transcriptionally activating many G2/M-expressed genes in tobacco.

Replication protein A2c coupled with replication protein A1c regulates crossover formation during meiosis in rice

Replication protein A (RPA) is a conserved heterotrimeric protein complex comprising RPA1, RPA2, and RPA3 subunits involved in multiple DNA metabolism pathways attributable to its single-stranded DNA binding property. Unlike other species possessing a single RPA2 gene, rice (Oryza sativa) possesses three RPA2 paralogs, but their functions remain unclear. In this study, we identified RPA2c, a rice gene preferentially expressed during meiosis. A T-DNA insertional mutant (rpa2c) exhibited reduced bivalent formation, leading to chromosome nondisjunction. In rpa2c, chiasma frequency is reduced by ~78% compared with the wild type and is accompanied by loss of the obligate chiasma. The residual ~22% chiasmata fit a Poisson distribution, suggesting loss of crossover control. RPA2c colocalized with the meiotic cohesion subunit REC8 and the axis-associated protein PAIR2. Localization of REC8 was necessary for loading of RPA2c to the chromosomes. In addition, RPA2c partially colocalized with MER3 during late leptotene, thus indicating that RPA2c is required for class I crossover formation at a late stage of homologous recombination. Furthermore, we identified RPA1c, an RPA1 subunit with nearly overlapping distribution to RPA2c, required for ~79% of chiasmata formation. Our results demonstrate that an RPA complex comprising RPA2c and RPA1c is required to promote meiotic crossovers in rice.

Neighboring parenchyma cells contribute to Arabidopsis xylem lignification, while lignification of interfascicular fibers is cell autonomous

Lignin is a critical structural component of plants, providing vascular integrity and mechanical strength. Lignin precursors (monolignols) must be exported to the extracellular matrix where random oxidative coupling produces a complex lignin polymer. The objectives of this study were twofold: to determine the timing of lignification with respect to programmed cell death and to test if nonlignifying xylary parenchyma cells can contribute to the lignification of tracheary elements and fibers. This study demonstrates that lignin deposition is not exclusively a postmortem event, but also occurs prior to programmed cell death. Radiolabeled monolignols were not detected in the cytoplasm or vacuoles of tracheary elements or neighbors. To experimentally define which cells in lignifying tissues contribute to lignification in intact plants, a microRNA against cinnamoyl CoA-reductase1 driven by the promoter from cellulose synthase7 (ProCESA7:miRNA CCR1) was used to silence monolignol biosynthesis specifically in cells developing lignified secondary cell walls. When monolignol biosynthesis in ProCESA7:miRNA CCR1 lines was silenced in the lignifying cells themselves, but not in the neighboring cells, lignin was still deposited in the xylem secondary cell walls. Surprisingly, a dramatic reduction in cell wall lignification of extraxylary fiber cells demonstrates that extraxylary fibers undergo cell autonomous lignification.

Mediation of clathrin-dependent trafficking during cytokinesis and cell expansion by Arabidopsis stomatal cytokinesis defective proteins

Stomatal cytokinesis defective1 (SCD1) encodes a putative Rab guanine nucleotide exchange factor that functions in membrane trafficking and is required for cytokinesis and cell expansion in Arabidopsis thaliana. Here, we show that the loss of SCD2 function disrupts cytokinesis and cell expansion and impairs fertility, phenotypes similar to those observed for scd1 mutants. Genetic and biochemical analyses showed that SCD1 function is dependent upon SCD2 and that together these proteins are required for plasma membrane internalization. Further specifying the role of these proteins in membrane trafficking, SCD1 and SCD2 proteins were found to be associated with isolated clathrin-coated vesicles and to colocalize with clathrin light chain at putative sites of endocytosis at the plasma membrane. Together, these data suggest that SCD1 and SCD2 function in clathrin-mediated membrane transport, including plasma membrane endocytosis, required for cytokinesis and cell expansion.

A pepper (Capsicum annuum L.) metacaspase 9 (Camc9) plays a role in pathogen-induced cell death in plants

Metacaspases, which belong to the cysteine-type C14 protease family, are most structurally similar to mammalian caspases than any other caspase-like protease in plants. Atmc9 (Arabidopsis thaliana metacaspase 9) has a unique domain structure, and distinct biochemical characteristics, such as Ca²⁺ binding, pH, redox status, S-nitrosylation and specific protease inhibitors. However, the biological roles of Atmc9 in plant-pathogen interactions remain largely unknown. In this study, a metacaspase gene present as a single copy in the pepper genome, and sharing 54% amino acid sequence identity with Atmc9, was isolated and named Capsicum annuum metacaspase 9 (Camc9). Camc9 encodes a 318-amino-acid polypeptide with an estimated molecular weight of 34.6 kDa, and shares approximately 40% amino acid sequence identity with known type II metacaspases in plants. Quantitative reverse transcription-polymerase chain reaction analyses revealed that the expression of Camc9 was induced by infections of Xanthomonas campestris pv. vesicatoria race 1 and race 3 and treatment with methyl jasmonate. Suppression of Camc9 expression using virus-induced gene silencing enhanced disease resistance and suppressed cell death symptom development following infection with virulent bacterial pathogens. By contrast, overexpression of Camc9 by transient or stable transformation enhanced disease susceptibility and pathogen-induced cell death by regulation of reactive oxygen species production and defence-related gene expression. These results suggest that Camc9 is a possible member of the metacaspase gene family and plays a role as a positive regulator of pathogen-induced cell death in the plant kingdom.

The Arabidopsis thaliana MYB60 promoter provides a tool for the spatio-temporal control of gene expression in stomatal guard cells

Plants have evolved different strategies to resist drought, of which the best understood is the abscisic acid (ABA)-induced closure of stomatal pores to reduce water loss by transpiration. The availability of useful promoters that allow for precise spatial and temporal control of gene expression in stomata is essential both for investigating stomatal regulation in model systems and for biotechnological applications in field crops. Previous work indicated that the regulatory region of the transcription factor AtMYB60 specifically drives gene expression in guard cells of Arabidopsis, although its activity is rapidly down-regulated by ABA. Here, the activity of the full-length and minimal AtMYB60 promoters is reported in rice (Oryza sativa), tobacco (Nicotiana tabacum), and tomato (Solanum lycopersicum), using a reporter gene approach. In rice, the activity of both promoters was completely abolished, whereas it was spatially restricted to guard cells in tobacco and tomato. To overcome the negative effect of ABA on the AtMYB60 promoter, a chimeric inducible system was developed, which combined the cellular specificity of the AtMYB60 minimal promoter with the positive responsiveness to dehydration and ABA of the rd29A promoter. Remarkably, the synthetic module specifically up-regulated gene expression in guard cells of Arabidopsis, tobacco, and tomato in response to dehydration or ABA. The comparative analysis of different native and synthetic regulatory modules derived from the AtMYB60 promoter offers new insights into the functional conservation of the cis-mechanisms that mediate gene expression in guard cells in distantly related dicotyledonous species and provides novel tools for modulating stomatal activity in plants.

Release of proteins from intact chloroplasts induced by reactive oxygen species during biotic and abiotic stress

Plastids sustain life on this planet by providing food, feed, essential biomolecules and oxygen. Such diverse metabolic and biosynthetic functions require efficient communication between plastids and the nucleus. However, specific factors, especially large molecules, released from plastids that regulate nuclear genes have not yet been fully elucidated. When tobacco and lettuce transplastomic plants expressing GFP within chloroplasts, were challenged with Erwinia carotovora (biotic stress) or paraquat (abiotic stress), GFP was released into the cytoplasm. During this process GFP moves gradually towards the envelope, creating a central red zone of chlorophyll fluorescence. GFP was then gradually released from intact chloroplasts into the cytoplasm with an intact vacuole and no other visible cellular damage. Different stages of GFP release were observed inside the same cell with a few chloroplasts completely releasing GFP with detection of only red chlorophyll fluorescence or with no reduction in GFP fluorescence or transitional steps between these two phases. Time lapse imaging by confocal microscopy clearly identified sequence of these events. Intactness of chloroplasts during this process was evident from chlorophyll fluorescence emanated from thylakoid membranes and in vivo Chla fluorescence measurements (maximum quantum yield of photosystem II) made before or after infection with pathogens to evaluate their photosynthetic competence. Hydrogen peroxide and superoxide anion serve as signal molecules for generation of reactive oxygen species and Tiron, scavenger of superoxide anion, blocked release of GFP from chloroplasts. Significant increase in ion leakage in the presence of paraquat and light suggests changes in the chloroplast envelope to facilitate protein release. Release of GFP-RC101 (an antimicrobial peptide), which was triggered by Erwinia infection, ceased after conferring protection, further confirming this export phenomenon. These results suggest a novel signaling mechanism, especially for participation of chloroplast proteins (e.g. transcription factors) in retrograde signaling, thereby offering new opportunities to regulate pathways outside chloroplasts.

Development and disintegration of tapetum-specific lipid-accumulating organelles, elaioplasts and tapetosomes, in Arabidopsis thaliana and Brassica napus

The pollen coat covering the surface of pollen grains has many important roles for pollination. In Brassicaceae plants, the pollen coat components are synthesized and temporarily accumulated in two tapetum-specific organelles, the elaioplast and the tapetosome. Although many biochemical and electron microscopic analyses have been attempted, the structure and biogenesis of these organelles have not been fully elucidated. To resolve this problem, we performed live imaging of these organelles using two markers, FIB1a-GFP and GRP17-GFP. FIB1a is an Arabidopsis fibrillin, a structural protein of elaioplast plastoglobules. In transgenic Arabidopsis, fluorescence of FIB1a-GFP appeared in young elaioplasts, in which small plastoglobules were developing. However, the fluorescence disappeared in later stages, while enlargement of plastoglobules continued. GRP17 is an Arabidopsis oleopollenin, an oleosin-like protein in tapetosomes. Fluorescence microscopy of GRP17-GFP expressed in Arabidopsis and Brassica napus revealed that tapetosomes do not contain oleopollenin-coated vesicles but have an outer envelope, indicating that the tapetosome structure is distinct from seed oil bodies. Visualization of GRP17-GFP also demonstrated that the tapetal cells become protoplasts and migrate into locules before pollen coat formation, and provided live imaging of the foot formation between pollen grains and stigmatic papilla cells.

Analysis of gamete membrane dynamics during double fertilization of Arabidopsis

Angiosperms have a unique sexual reproduction system called "double fertilization." One sperm cell fertilizes the egg and another sperm cell fertilizes the central cell. To date, plant gamete membrane dynamics during fertilization has been poorly understood. To analyze this unrevealed gamete subcellular behavior, live cell imaging analyses of Arabidopsis double fertilization were performed. We produced female gamete membrane marker lines in which fluorescent proteins conjugated with PIP2a finely visualized egg cell and central cell surfaces. Using those lines together with a sperm cell membrane marker line expressing GCS1-GFP, the double fertilization process was observed. As a result, after gamete fusion, putative sperm plasma membrane GFP signals were occasionally detected on the egg cell surface adjacent to the central cell. In addition, time-lapse imaging revealed that GCS1-GFP signals entered both the egg cell and the central cell in parallel with the sperm cell movement toward the female gametes during double fertilization. These findings suggested that the gamete fusion process based on membrane dynamics was composed of (1) plasma membrane fusion on male and female gamete surfaces, (2) entry of sperm internal membrane components into the female gametes, and (3) plasmogamy.

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

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

The grape berry-specific basic helix-loop-helix transcription factor VvCEB1 affects cell size

The development of fleshy fruits involves complex physiological and biochemical changes. After fertilization, fruit growth usually begins with cell division, continues with both cell division and expansion, allowing fruit set to occur, and ends with cell expansion only. In spite of the economical importance of grapevine, the molecular mechanisms controlling berry growth are not fully understood. The present work identified and characterized Vitis vinifera cell elongation bHLH protein (VvCEB1), a basic helix-loop-helix (bHLH) transcription factor controlling cell expansion in grape. VvCEB1 was expressed specifically in berry-expanding tissues with a maximum around veraison. The study of VvCEB1 promoter activity in tomato confirmed its specific fruit expression during the expansion phase. Overexpression of VvCEB1 in grape embryos showed that this protein stimulates cell expansion and affects the expression of genes involved in cell expansion, including genes of auxin metabolism and signalling. Taken together, these data show that VvCEB1 is a fruit-specific bHLH transcription factor involved in grape berry development.

[Actin cytoskeleton rearrangements during the gravitropic response of Arabidopsis roots]

Gravitropic response is a plant growth response against changing its position relative to the gravity vector. In the present work we studied actin cytoskeleton rearrangements during Arabidopsis root gravitropic response. Two alternative approaches were used to visualize actin microfilaments: histochemical staining of fixed roots with rhodamine-phalloidin and live imaging of microfilaments in GFP-fABD2 transgenic plants. The curvature of actin microfilaments was shown to be increased within 30-60 min of gravistimulation, the fraction of auxially oriented microfilaments decreased with a concomitant increase in the fraction of oblique and transversally oriented microfilaments. Methodological issues of actin cytoskeleton visualization in the study of Arabidopsis root gravitropic response, as well as the role of microfilaments at the stages of gravity perception, signal transduction and gravitropic bending formation are discussed. It is concluded that the actin cytoskeleton rearrangements observed are associated with the regulation of basic mechanisms of cell extension growth by which the gravitropic bending is formed.