Isolation of a full-length cDNA encoding calreticulin from a PCR library of in vitro zygotes of maize

TaCRT3 Is a Positive Regulator of Resistance to Blumeria graminis f. sp. tritici in Wheat

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is one of the most prevalent diseases of wheat worldwide and can lead to severe yield reductions. Identifying genes involved in powdery mildew resistance will be useful for disease resistance breeding and control. Calreticulin (CRT) is a member of multigene family widely found in higher plants and is associated with a variety of plant physiological functions and defense responses. However, the role of CRT in wheat resistance to powdery mildew remains unclear. TaCRT3 was identified from the proteomic sequence of an incompatible interaction between the wheat (Triticum aestivum) cultivar Xingmin 318 and the Bgt isolate E09. Following analysis of transient expression of the GFP-TaCRT3 fusion protein in Nicotiana benthamiana leaves, TaCRT3 was localized in the nucleus, cytoplasm, and cell membrane. Transcript expression levels of TaCRT3 were significantly upregulated in the wheat-Bgt incompatible interaction. More critically, knockdown of TaCRT3 using virus-induced gene silencing resulted in attenuated resistance to Bgt in wheat. Histological analysis showed a significant increase in Bgt development in TaCRT3-silenced plants, whereas the pathogen-related gene was significantly downregulated in TaCRT3-silenced leaves. In addition, overexpression of TaCRT3 in wheat enhanced the resistance to powdery mildew, the growth of Bgt was significantly inhibited, and the area of H2O2 near the infection site and the expression of defense-related genes of the salicylic acid pathway significantly increased. These findings imply that TaCRT3 may act as a disease resistance factor that positively regulates resistance to powdery mildew, during which SA signaling is probably activated.

Localization of calreticulin and calcium ions in mycorrhizal roots of Medicago truncatula in response to aluminum stress

Aluminum (Al) toxicity limits growth and symbiotic interactions of plants. Calcium plays essential roles in abiotic stresses and legume-Rhizobium symbiosis, but the sites and mechanism of Ca²⁺ mobilization during mycorrhizae have not been analyzed. In this study, the changes of cytoplasmic Ca²⁺ and calreticulin (CRT) in Medicago truncatula mycorrhizal (MR) and non-mycorrizal (NM) roots under short Al stress [50 μM AlCl₃ pH 4.3 for 3 h] were analyzed. Free Ca²⁺ ions were detected cytochemically by their reaction with potassium pyroantimonate and anti-CRT antibody was used to locate this protein in Medicago roots by immunocytochemical methods. In MR and NM roots, Al induced accumulation of CRT and free Ca²⁺. Similar calcium and CRT distribution in the MR were found at the surface of fungal structures (arbuscules and intercellular hyphae), cell wall and in plasmodesmata, and in plant and fungal intracellular compartments. Additionally, degenerated arbuscules were associated with intense Ca²⁺ and CRT accumulation. In NM roots, Ca²⁺ and CRT epitopes were observed in the stele, near wall of cortex and endodermis. The present study provides new insight into Ca²⁺ storage and mobilization in mycorrhizae symbiosis. The colocalization of CRT and Ca²⁺ suggests that CRT is essential for calcium mobilization for normal mycorrhiza development and response to Al stress.

Functional Analysis and Marker Development of TaCRT-D Gene in Common Wheat (Triticum aestivum L.)

Calreticulin (CRT), an endoplasmic reticulum (ER)-localized Ca2+-binding/buffering protein, is highly conserved and extensively expressed in animal and plant cells. To understand the function of CRTs in wheat (Triticum aestivum L.), particularly their roles in stress tolerance, we cloned the full-length genomic sequence of the TaCRT-D isoform from D genome of common hexaploid wheat, and characterized its function by transgenic Arabidopsis system. TaCRT-D exhibited different expression patterns in wheat seedling under different abiotic stresses. Transgenic Arabidopsis plants overexpressing ORF of TaCRT-D displayed more tolerance to drought, cold, salt, mannitol, and other abiotic stresses at both seed germination and seedling stages, compared with the wild-type controls. Furthermore, DNA polymorphism analysis and gene mapping were employed to develop the functional markers of this gene for marker-assistant selection in wheat breeding program. One SNP, S440 (T→C) was detected at the TaCRT-D locus by genotyping a wheat recombinant inbred line (RIL) population (114 lines) developed from Opata 85 × W7984. The TaCRT-D was then fine mapped between markers Xgwm645 and Xgwm664 on chromosome 3DL, corresponding to genetic distances of 3.5 and 4.4 cM, respectively, using the RIL population and Chinese Spring nulli-tetrasomic lines. Finally, the genome-specific and allele-specific markers were developed for the TaCRT-D gene. These findings indicate that TaCRT-D function importantly in plant stress responses, providing a gene target for genetic engineering to increase plant stress tolerance and the functional markers of TaCRT-D for marker-assistant selection in wheat breeding.

Identification and molecular characterization of 48 kDa calcium binding protein as calreticulin from finger millet (Eleusine coracana) using peptide mass fingerprinting and transcript profiling

BACKGROUND

Attempts were made to identify and characterize the calcium binding proteins (CaBPs) in grain filling stages of finger millet using proteomics, bioinformatics and molecular approaches.

RESULTS

A distinctly observed blue color band of 48 kDa stained by Stains-all was eluted and analyzed as calreticulin (CRT) using nano liquid chromatography-tandem mass spectrometry (nano LC-MS). Based on the top hits of peptide mass fingerprinting results, conserved primers were designed for isolation of the CRT gene from finger millet using calreticulin sequences of different cereals. The deduced nucleotide sequence analysis of 600 bp amplicon showed up to 91% similarity with CRT gene(s) of rice and other plant species and designated as EcCRT1. Transcript profiling of EcCRT1 showed different levels of relative expression at different stages of developing spikes. The higher expression of EcCRT1 transcripts and protein were observed in later stages of developing spikes which might be due to greater translational synthesis of EcCRT1 protein during seed maturation in finger millet.

CONCLUSIONS

Preferentially higher synthesis of this CaBP during later stages of grain filling may be responsible for the sequestration of calcium in endoplasmic reticulum of finger millet grains.

Embryogenesis and Plant Regeneration from Isolated Wheat Zygotes

Wheat zygotes can be mechanically isolated and cultivated to continue their development in vitro. Since each zygote needs to be individually isolated, only relatively few of these cells are available per experiment. To facilitate embryonic growth despite of this limitation, the zygotes are kept within a culture insert placed in a larger dish which itself contains embryogenic pollen cocultivated for continuous medium conditioning. This setup ensures that the two cultures, while being physically separated from one another, can exchange essential intercellular signal molecules passing through the bottom of the insert which is made of a permeable membrane. Thanks to the natural fate of zygotes, which is to form an embryo followed by the generation of a plant, embryogenesis and plant regeneration are achieved at much higher efficiency as compared to other single-cell systems. While the method is largely independent of the genotype, it allows for the nondestructive observation, manipulation, and individual analysis of zygotes and very young embryos.

Late progamic phase and fertilization affect calreticulin expression in the Hyacinthus orientalis female gametophyte

Calreticulin expression is upregulated during sexual reproduction of Hyacinthus orientalis, and the protein is localized both in the cytoplasm and a highly specialized cell wall within the female gametophyte. Several evidences indicate calreticulin (CRT) as an important calcium (Ca(2+))-binding protein that is involved in the generative reproduction of higher plants, including both pre-fertilization and post-fertilization events. Because CRT is able to bind and sequester exchangeable Ca(2+), it can serve as a mobile intracellular store of easily releasable Ca(2+) and control its local cytosolic concentrations in the embryo sac. This phenomenon seems to be essential during the late progamic phase, gamete fusion, and early embryogenesis. In this report, we demonstrate the differential expression of CRT within Hyacinthus female gametophyte cells before and during anthesis, during the late progamic phase when the pollen tube enters the embryo sac, and at the moment of fertilization and zygote/early endosperm activation. CRT mRNA and the protein localize mainly to the endoplasmic reticulum (ER) and Golgi compartments of the cells, which are involved in sexual reproduction events, such as those in sister synergids, the egg cell, the central cell, zygote and the developing endosperm. Additionally, immunogold research demonstrates selective CRT distribution in the filiform apparatus (FA), a highly specific component of the synergid cell wall. In the light of our previous data showing the total transcriptional activity of the Hyacinthus female gametophyte and the results presented here, we discuss the possible functions of CRT with respect to the critical role of Ca(2+) homeostasis during key events of sexual plant reproduction. Moreover, we propose that the elevated expression of CRT within the female gametophyte is a universal phenomenon in the cells involved in double fertilization in higher plants.

Overexpression of a Triticum aestivum Calreticulin gene (TaCRT1) Improves Salinity Tolerance in Tobacco

Calreticulin (CRT) is a highly conserved and abundant multifunctional protein that is encoded by a small gene family and is often associated with abiotic/biotic stress responses in plants. However, the roles played by this protein in salt stress responses in wheat (Triticum aestivum) remain obscure. In this study, three TaCRT genes were identified in wheat and named TaCRT1, TaCRT2 and TaCRT3-1 based on their sequence characteristics and their high homology to other known CRT genes. Quantitative real-time PCR expression data revealed that these three genes exhibit different expression patterns in different tissues and are strongly induced under salt stress in wheat. The calcium-binding properties of the purified recombinant TaCRT1 protein were determined using a PIPES/Arsenazo III analysis. TaCRT1 gene overexpression in Nicotiana tabacum decreased salt stress damage in transgenic tobacco plants. Physiological measurements indicated that transgenic tobacco plants showed higher activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) than non-transgenic tobacco under normal growth conditions. Interestingly, overexpression of the entire TaCRT1 gene or of partial TaCRT1 segments resulted in significantly higher tolerance to salt stress in transgenic plants compared with their WT counterparts, thus revealing the essential role of the C-domain of TaCRT1 in countering salt stress in plants.

The Maternal-to-Zygotic Transition in Flowering Plants: Evidence, Mechanisms, and Plasticity

The maternal-to-zygotic transition (MZT) defines a developmental phase during which the embryo progressively emancipates itself from a developmental control relying largely on maternal information. The MZT is a functional readout of two processes: the clearance of maternally derived information and the de novo expression of the inherited, parental alleles enabled by zygotic genome activation (ZGA). In plants, for many years the debate about whether the MZT exists at all focused on the ZGA alone. However, several recent studies provide evidence for a progressive alleviation of the maternal control over embryogenesis that is correlated with a gradual ZGA, a process that is itself maternally controlled. Yet, several examples of zygotic genes that are expressed and/or functionally required early in embryogenesis demonstrate a certain flexibility in the dynamics and kinetics of the MZT among plant species and also intraspecific hybrids.

Calreticulin expression in relation to exchangeable Ca(2+) level that changes dynamically during anthesis, progamic phase, and double fertilization in Petunia

Calcium (Ca(2+)) plays essential roles in plant sexual reproduction, but the sites and the mechanism of Ca(2+) mobile storage during pollen-pistil interactions have not been fully defined. Because the Ca(2+)-buffering protein calreticulin (CRT) is able to bind and sequester Ca(2+), it can serve as a mobile intracellular store of easily releasable Ca(2+) and control its local concentration within the cytoplasm. Our previous studies showed an enhanced expression of Petunia hybrida CRT gene (PhCRT) during pistil transmitting tract maturation, pollen germination and tube outgrowth on the stigma, gamete fusion, and early embryogenesis. Here, we demonstrate that elevated expression of CRT results in the accumulation of this protein in response to anthesis, pollination, sperm cells deposition within the receptive synergid and fertilization, when the level of exchangeable Ca(2+) changes dynamically. CRT localizes mainly to the endoplasmic reticulum and Golgi compartments in the pistil transmitting tract cells, germinated pollen/tubes, and sporophytic/gametophytic cells of the ovule and corresponds with loosely bound Ca(2+). Additionally, the immunogold research shows, for the first time, highly selective CRT distribution in specific nuclear sub-domains. On the basis of our results, we discuss the possible functions of CRT with respect to the critical role of Ca(2+) homeostasis during key events of the multi-step process of generative reproduction in angiosperms.

Molecular cloning and transcriptional activity of a new Petunia calreticulin gene involved in pistil transmitting tract maturation, progamic phase, and double fertilization

Calreticulin (CRT) is a highly conserved and ubiquitously expressed Ca²⁺-binding protein in multicellular eukaryotes. As an endoplasmic reticulum-resident protein, CRT plays a key role in many cellular processes including Ca²⁺ storage and release, protein synthesis, and molecular chaperoning in both animals and plants. CRT has long been suggested to play a role in plant sexual reproduction. To begin to address this possibility, we cloned and characterized the full-length cDNA of a new CRT gene (PhCRT) from Petunia. The deduced amino acid sequence of PhCRT shares homology with other known plant CRTs, and phylogenetic analysis indicates that the PhCRT cDNA clone belongs to the CRT1/CRT2 subclass. Northern blot analysis and fluorescent in situ hybridization were used to assess PhCRT gene expression in different parts of the pistil before pollination, during subsequent stages of the progamic phase, and at fertilization. The highest level of PhCRT mRNA was detected in the stigma-style part of the unpollinated pistil 1 day before anthesis and during the early stage of the progamic phase, when pollen is germinated and tubes outgrow on the stigma. In the ovary, PhCRT mRNA was most abundant after pollination and reached maximum at the late stage of the progamic phase, when pollen tubes grow into the ovules and fertilization occurs. PhCRT mRNA transcripts were seen to accumulate predominantly in transmitting tract cells of maturing and receptive stigma, in germinated pollen/growing tubes, and at the micropylar region of the ovule, where the female gametophyte is located. From these results, we suggest that PhCRT gene expression is up-regulated during secretory activity of the pistil transmitting tract cells, pollen germination and outgrowth of the tubes, and then during gamete fusion and early embryogenesis.

Occurrence of calreticulin during the exchange of nucleohistones into protamine-type proteins in Chara vulgaris spermiogenesis

During spermiogenesis of an alga Chara vulgaris, which resembles that of animals, nucleohistones are replaced by protamine-type proteins. This exchange takes place in a spermatid nucleus during the key V spermiogenesis stage, in which rough endoplasmic reticulum is the site of protamine-type protein synthesis and is also the pathway guiding the proteins to their destination, nucleus. In the present work, it was shown that a chaperon protein, calreticulin (CRT), abundantly present at this significant V stage of spermiogenesis in a few cellular compartments, i.e., a nucleus, lumen of cisternae, and vesicles of significantly swollen ER as well as outside these structures, e.g., in Golgi apparatus, could have taken part in the process of exchange of nuclear proteins. Colocalization of two proteins, protamine-type proteins, crucial for reproduction, and CRT, was especially visible in a nucleus, mainly on its peripheries where condensed chromatin was present. Localization of protamine-type proteins and CRT in nucleus is in agreement with our previous results showing that protamine-type proteins were twofold more labelled in the peripheral area in comparison to the nucleus center occupied by noncondensed chromatin. The role of CRT in the reproduction of both plants and animals is also discussed.

Calreticulin expression and localization in plant cells during pollen-pistil interactions

In this report, the distributions of calreticulin (CRT) and its transcripts in Haemanthus pollen, pollen tubes, and somatic cells of the hollow pistil were studied. Immunoblot analysis of protein extracts from mature anthers, dry and germinated pollen, growing pollen tubes, and unpollinated/pollinated pistils revealed a strong expression of CRT. Both in vitro and in situ studies confirmed the presence of CRT mRNA and protein in pollen/pollen tubes and somatic cells of the pistil transmitting tract. The co-localization of these molecules in ER of these cells suggests that the rough ER is a site of CRT translation. In the pistil, accumulation of the protein in pollen tubes, transmitting tract epidermis (tte), and micropylar cells of the ovule (mc) was correlated with the increased level of exchangeable calcium. Therefore, CRT as a Ca(2+)-binding/buffering protein, may be involved in mechanism of regulation calcium homeostasis in these cells. The functional role of the protein in pollen-pistil interactions, apart from its postulated function in cellular Ca(2+) homeostasis, is discussed.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Calreticulin: conserved protein and diverse functions in plants

Calreticulin (CRT) is a key Ca2+-binding protein mainly resident in the endoplasmic reticulum (ER), which is highly conserved and extensively expressed in all eukaryotic organisms investigated. The protein plays important roles in a variety of cellular processes including Ca2+ signaling and protein folding. Although calreticulin has been well characterized in mammalian systems, increased investigations have demonstrated that plant CRTs have a number of specific properties different from their animal counterparts. Recent developments on plant CRTs have highlighted the significance of CRTs in plants growth and development as well as biotic and abiotic stress responses. There are at least two distinct groups of calreticulin isoforms in higher plants. Glycosylation of CRT was uniquely observed in plants. In this article, we will describe our current understanding of plant calreticulin gene family, protein structure, cellular localization, and diverse functions in plants. We also discuss the prospects of using this information for genetic improvements of crop plants.

Isolation of egg cells and zygotes of Torenia fournieri L. and determination of their surface charge

Egg cells of Torenia fournieri were isolated from embryo sacs 1 day after anthesis using enzymatic digestion or mechanical dissection. About 5% of the egg cells and zygotes (2-3 from 50 ovules) could be mechanically dissected within 2 h. When 0.1% cellulase and 0.1% pectinase were added to the mannitol isolation solution, about 18% of the egg cells (8-10 from 50 ovules) could be isolated within 2 h. The egg cells isolated by mechanical dissection could be used for in vitro fertilization studies without any of the potentially deleterious effects of the enzymes on the plasma membrane of egg cell. The egg cells isolated using enzymatic digestion could be used in the study of the molecular biology of female gamete because more egg cells could be isolated with this technique. Using enzymatic digestion, over 10 zygotes from 50 ovules (over 20%) were isolated from the pollinated ovules. Coupled with our successful isolation of mature sperm cells, the isolation of egg cells of T. fournieri will make in vitro fertilization possible in a dicotyledon plant.

Sexual reproduction in higher plants I: fertilization and the initiation of zygotic program

Sexual plant reproduction is a critical developmental step in the life cycle of higher plants, to allow maternal and paternal genes to be transmitted in a highly regulated manner to the next generation. During evolution, a whole set of signal transduction machinery is developed by plants to ensure an error-free recognition between male and female gametes and initiation of zygotic program. In the past few years, the molecular machineries underlying this biological process have been elucidated, particularly on the importance of synergid cells in pollen tube guidance, the Ca(++) spike as the immediate response of fertilization and the epigenetic regulation of parental gene expressions in early zygotic embryogenesis. This review outlines the most recent development in this area.

In vitro fertilization with isolated higher plant gametes

Methods have been developed to isolate gametes of higher plants and to fertilize them in vitro. Zygotes, embryos, fertile plants and endosperm can now be obtained from in vitro fusion of pairs of sperm and egg cells and of pairs of sperm and central cells, respectively. This allows examination of the earliest developmental processes precisely timed after fertilization. The isolated egg and central cell, fertilized and cultured in vitro, are able to self-organize apart from each other and without mother tissue in the typically manner. Thus, this system is a powerful and unique model for studies of early zygotic embryogenesis and endosperm development. The underlying processes are now comparatively studied in detail by investigations of expression of genes and their corresponding proteins. The use of these techniques opens new avenues in fundamental and applied research in the areas of developmental and reproductive plant biology.

Calreticulin mRNA and protein are localized to protein bodies in storage maize callus cells

Maize callus cells possess numerous protein bodies which develop as sub-compartments of the endoplasmic reticulum. We localized maize calreticulin mRNAs and protein in maize callus cells using in situ hybridization and immunocytochemistry. Calreticulin mRNAs were selectively targeted to the endoplasmic reticulum (ER) subdomains surrounding protein bodies. Profilin mRNAs, used as a positive control for in situ hybridization experiments, showed distinct and rather diffuse localization pattern. Using both, immunofluorescence and immunogold electron microscopy localization techniques, calreticulin was found to be enriched around and within protein bodies in maize callus storage cells. As a positive control for reticuloplasmins, HDEL antibody revealed labelling of protein bodies and of the nuclear envelope. The identity of protein bodies was confirmed by specific binding of an alpha zein antibody. These data suggest that calreticulin mRNA is targeted towards protein body forming subdomains of the ER, and that calreticulin is localized and enriched in these protein bodies. The possibility that calreticulin plays an important role in zein retention within the ER and/or its assembly and packaging into protein bodies during protein body biogenesis in maize callus is discussed.

EST generation and analyses towards identifying female gametophyte-specific genes in Zea mays L

The embryo sac (female gametophyte) plays an important role in double fertilization. The female gametophyte is composed of four specific cell types: the synergids that attract pollen tubes, the egg cell and central cell which are fusion partners for the two sperm cells, and the antipodal cells whose function is unknown. As a resource for gene discovery and to help identify genes exhibiting cell-specific expression patterns, we constructed cDNA libraries from female gametophytes and from egg cells of maize and sequenced more than 8,500 ESTs. These libraries represent diverse transcripts, potentially corresponding to 3,850 genes (contigs and singletons) from the female gametophyte and 963 genes (contigs and singletons) from the egg cell. In each collection, 16% of the contigs/singletons have no matches in databases and 3-5% encode hypothetical proteins; novel hypothetical proteins (not found within the female gametophyte contigs) were identified among the egg cell contigs. We examined 65 contigs by RT-PCR and 19 genes that were potentially female gametophyte-specific were identified. We used in situ hybridization to determine expression specificity for seven genes: one transcript was expressed both in the egg cell and in the central cell, one was expressed in the egg cell and synergids, two were expressed in the central cell, two were expressed in the synergids, and one was expressed in the central cell and the synergids. Four of these encode small, potentially secreted peptides that are dissimilar except for a conserved triple cysteine motif near their C-terminus. These EST resources should prove useful for identifying female gametophyte or cell-specific genes.

Expression and localization of calreticulin in tobacco anthers and pollen tubes

The developmental expression pattern and localization of calreticulin were studied in Nicotiana tabacum L. anthers, pollen and pollen tubes. High transcript and protein levels were detected throughout anther development. Immunolocalization of calreticulin in the anthers showed particular dense label in tapetum and pollen at developmental stage 2, when the tapetum is highly active and the pollen tetrads are formed. Much lower transcript and protein levels were detected in dry and hydrated pollen and in pollen tubes. Immunofluorescence labeling of both chemically fixed and cryo-fixed and freeze-substituted pollen tubes showed the presence of calreticulin in Golgi apparatus and endoplasmic reticulum (ER). Calreticulin was seen throughout the stacks in the Golgi apparatus and in the areas with coated-Golgi vesicles but much less so in the ER. Calreticulin was not found in the secretory vesicles. A relatively intense label was occasionally seen adjacent to the wall of the tube. No significant label was observed in mitochondria, vacuoles, generative cells, cell wall or callose plugs. The present results are consistent with a role of calreticulin in Ca2+-dependent folding of secreted glycoproteins in tapetum, pollen and pollen tubes.

The fertilization-induced DNA replication factor MCM6 of maize shuttles between cytoplasm and nucleus, and is essential for plant growth and development

The eukaryotic genome is duplicated exactly once per cell division cycle. A strategy that limits every replication origin to a single initiation event is tightly regulated by a multiprotein complex, which involves at least 20 protein factors. A key player in this regulation is the evolutionary conserved hexameric MCM2-7 complex. From maize (Zea mays) zygotes, we have cloned MCM6 and characterized this essential gene in more detail. Shortly after fertilization, expression of ZmMCM6 is strongly induced. During progression of zygote and proembryo development, ZmMCM6 transcript amounts decrease and are low in vegetative tissues, where expression is restricted to tissues containing proliferating cells. The highest protein amounts are detectable about 6 to 20 d after fertilization in developing kernels. Subcellular localization studies revealed that MCM6 protein shuttles between cytoplasm and nucleoplasm in a cell cycle-dependent manner. ZmMCM6 is taken up by the nucleus during G1 phase and the highest protein levels were observed during late G1/S phase. ZmMCM6 is excluded from the nucleus during late S, G2, and mitosis. Transgenic maize was generated to overexpress and down-regulate ZmMCM6. Plants displaying minor antisense transcript amounts were reduced in size and did not develop cobs to maturity. Down-regulation of ZmMCM6 gene activity seems also to affect pollen development because antisense transgenes could not be propagated via pollen to wild-type plants. In summary, the transgenic data indicate that MCM6 is essential for both vegetative as well as reproductive growth and development in plants.

Expression of transgenic stilbene synthases in wheat causes the accumulation of unknown stilbene derivatives with antifungal activity

The expression of foreign phytoalexins in a new host is thought to increase fungal resistance, since host-specific pathogens have not experienced selection for detoxifying or metabolising the novel antifungal compounds. Two resveratrol synthase genes vst1 and vst2 from grapevine (Vitis vinifera L.) and the pinosylvin synthase gene pss from pine (Pinus sylvestris L.) were stably transformed into bread wheat. The expression of the target genes is regulated by stress-inducible grapevine promoters. The vst1 and vst2 promoters were functional in wheat and retained their expression profiles described for grapevine. ALL vst and pss transgenic lines accumulated stilbene derivatives upon induction by UV light. The detected stilbenes showed a remarkable similarity to resveratrol and pinosylvin, however were found to be more hydrophilic than resveratrol and pinosylvin. Upon inoculation with the biotrophic pathogen Puccinia recondita f.sp. tritici several vst expressing wheat lines showed a significant reduction of disease symptoms (19 +/- 9% to 27 +/- 8%) compared to wild-type plants. The reduction of disease symptoms was even more obvious after inoculation with the facultative biotrophic pathogen Septoria nodorum Berk. and ranged from 42 +/- 13% to 71 +/- 4%. None of the four tested pss expressing lines showed a reduction in disease incidence.

Endoplasmic reticulum-resident proteins are constitutively transported to vacuoles for degradation

Soluble endoplasmic reticulum (ER)-resident proteins have very long lives because of their ER residency. This residency depends largely on ER-retrieval signals at their C-terminus. We examined the long-term destiny of endogenous ER-resident proteins, a lumenal binding protein (BiP) and a protein disulfide isomerase (PDI), with cultured cells of Arabidopsis. ER residents, in contrast to vacuolar proteinases, were considerably degraded in cells at the stationary phase. A subcellular fractionation analysis suggested that ER residents were transported into the vacuoles, which accumulated the residents lacking the ER-retrieval signals. We showed that the PDI located in the vacuoles had high mannose glycans, but not complex glycans, which suggested that the ER resident was transported to the vacuoles independent of the medial/trans-Golgi complex. To visualize the pathway of transport of ER-resident proteins, tobacco BY-2 cells were transformed with a chimeric gene encoding an ER-targeted green fluorescent protein (30 kDa GFP-HDEL). In the transformed cells at the stationary phase, GFP fluorescence was observed in the vacuoles. A subcellular fractionation revealed that a trimmed form of 27 kDa GFP was localized in the vacuoles. Treatment with E-64d, an inhibitor of papain-type cysteine proteinases that inhibits the degradation of GFP in the vacuoles, resulted in a stable accumulation of 27 kDa GFP in the vacuoles, even in the logarithmic phase. Our results suggest that endogenous ER residents are transported constitutively to the vacuoles by bypassing the Golgi complex and are then degraded.

Some reflections on double fertilization, from its discovery to the present

The fusion of one sperm with the egg cell to form the embryo and of the other sperm with the polar fusion nucleus to give rise to the endosperm ('double fertilization') was discovered by Nawaschin in 1898 in the liliaceous plants, Lilium martagon and Fritillaria tenella. The occurrence of two fusion events analogous to double fertilization has recently been described in some gymnosperm species although the product of the second fusion is a transient embryo, rather than the endosperm as in angiosperms. Recent investigations in angiosperms describe the cell biology and nuclear cytology of double fertilization and the successful in vitro demonstration of the two fusion events using isolated egg cells, central cells, and sperm cells and the development of the fusion products into the embryo and endosperm. Molecular and genetic studies on the component elements of double fertilization have focused on the identification of mutants of Arabidopsis thaliana that display developmental patterns in the seed that result in autonomous endosperm development and even partial embryogenesis in the absence of fertilization. Characterization of the genes and their protein products has provided evidence for a predominant effect of maternal gametophytic genes and of silencing of paternal genes during double fertilization. Contents Summary 565 I. Introduction 566 II. Discovery of double fertilization 566 III. Seed development without double fertilization 568 IV. A case for double fertilization in gymnosperms 570 V. Structural and cytological perspectives on double fertilization 571 VI. In vitro double fertilization 575 VII. Genetic and molecular perspectives 576 VIII. Concluding comments 578 Acknowledgements 579 References 579.

Expression of the high capacity calcium-binding domain of calreticulin increases bioavailable calcium stores in plants

Modulation of cytosolic calcium levels in both plants and animals is achieved by a system of Ca2+-transport and storage pathways that include Ca2+ buffering proteins in the lumen of intracellular compartments. To date, most research has focused on the role of transporters in regulating cytosolic calcium. We used a reverse genetics approach to modulate calcium stores in the lumen of the endoplasmic reticulum. Our goals were two-fold: to use the low affinity, high capacity Ca2+ binding characteristics of the C-domain of calreticulin to selectively increase Ca2+ storage in the endoplasmic reticulum, and to determine if those alterations affected plant physiological responses to stress. The C-domain of calreticulin is a highly acidic region that binds 20-50 moles of Ca2+ per mole of protein and has been shown to be the major site of Ca2+ storage within the endoplasmic reticulum of plant cells. A 377-bp fragment encoding the C-domain and ER retention signal from the maize calreticulin gene was fused to a gene for the green fluorescent protein and expressed in Arabidopsis under the control of a heat shock promoter. Following induction on normal medium, the C-domain transformants showed delayed loss of chlorophyll after transfer to calcium depleted medium when compared to seedlings transformed with green fluorescent protein alone. Total calcium measurements showed a 9-35% increase for induced C-domain transformants compared to controls. The data suggest that ectopic expression of the calreticulin C-domain increases Ca2+ stores, and that this Ca2+ reserve can be used by the plant in times of stress.

ZmES genes encode peptides with structural homology to defensins and are specifically expressed in the female gametophyte of maize

All four members of a gene family, which are highly expressed in the cells of the female gametophyte (ZmES1--4: Zea mays embryo sac), were isolated from a cDNA library of maize egg cells. High expression of ZmES genes in the synergids around the micropylar region was detected in thin sections of maize ovaries. Single-cell RT--PCR analyses with the various cells of the female gametophyte confirmed the expression in synergids and also showed expression in the egg cell and central cell, and low expression in the antipodals. The expression of the whole gene family is suppressed after fertilization of the embryo sac, and expression in two-cell or later embryo stages or other tissues of maize could not be detected. In order to investigate ZmES mRNA gradients in the highly polarized and vacuolized cells of the maize embryo sac, a whole-mount in situ protocol with isolated single cells was developed: as for total RNA, ZmES transcripts are uniformly distributed in the cytoplasm of egg cell, synergids and central cell. ZmES genes encode small, cysteine-rich proteins with an N-terminal signal peptide, probably for translocation into the embryo sac cell wall. The four ZmES proteins display high sequence identity with each other, and the proposed tertiary structure of the mature peptides is similar to that of plant and animal defensins. The function of ZmES1-4 during the fertilization process is discussed.

Purification, properties, and molecular cloning of a novel Ca(2+)-binding protein in radish vacuoles

To understand the roles of plant vacuoles, we have purified and characterized a major soluble protein from vacuoles of radish (Raphanus sativus cv Tokinashi-daikon) taproots. The results showed that it is a novel radish vacuole Ca(2+)-binding protein (RVCaB). RVCaB was released from the vacuolar membrane fraction by sonication, and purified by ion exchange and gel filtration column chromatography. RVCaB is an acidic protein and migrated on sodium dodecyl sulfate-polyacrylamide gel with an apparent molecular mass of 43 kD. The Ca(2+)-binding activity was confirmed by the (45)Ca(2+)-overlay assay. RVCaB was localized in the lumen, as the protein was recovered in intact vacuoles prepared from protoplasts and was resistant to trypsin digestion. Plant vacuoles store Ca(2+) using two active Ca(2+) uptake systems, namely Ca(2+)-ATPase and Ca(2+)/H(+) antiporter. Vacuolar membrane vesicles containing RVCaB accumulated more Ca(2+) than sonicated vesicles depleted of the protein at a wide range of Ca(2+) concentrations. A cDNA (RVCaB) encoding a 248-amino acid polypeptide was cloned. Its deduced sequence was identical to amino acid sequences obtained from several peptide fragments of the purified RVCaB. The deduced sequence is not homologous to that of other Ca(2+)-binding proteins such as calreticulin. RVCaB has a repetitive unique acidic motif, but not the EF-hand motif. The recombinant RVCaB expressed in Escherichia coli-bound Ca(2+) as evidenced by staining with Stains-all and migrated with an apparent molecular mass of 44 kD. These results suggest that RVCaB is a new type Ca(2+)-binding protein with high capacity and low affinity for Ca(2+) and that the protein could function as a Ca(2+)-buffer and/or Ca(2+)-sequestering protein in the vacuole.

Molecular cloning and characterization of calreticulin, a calcium-binding protein involved in the regeneration of rice cultured suspension cells

A full-length cDNA clone encoding a phosphoprotein (pp56) involved in the regeneration of rice (Oryza sativa L.)-cultured suspension cells was isolated by screening a rice cultured suspension cell cDNA library. The 1558-bp cDNA sequence contains an ORF encoding an acidic (pI 4.38) protein of 424 amino acids (47.9 kDa), sharing 70-93% and 50-53% homology with other plant and mammalian calreticulins, respectively. Sequence analysis of the cDNA clone revealed several significant conserved motifs, including a calreticulin family repeat motif in the central domain and two calreticulin family motifs in the N-domain, indicating that this gene is a rice calreticulin (CRO1). The CRO1 gene in long-term rice cultured suspension cells shows constitutive expression in both suspension culture and regeneration media. In contrast, expression of the CRO1 gene in short-term rice cultured suspension cells, which possess regeneration potential, is increased dramatically when these cells are transferred to the regeneration medium. After approximately 2 weeks in the regeneration medium, the expression of the CRO1 gene reverts to constitutive levels. These results demonstrate the presence of calreticulin in rice cultured suspension cells and its developmental regulation during the regeneration of rice cultured suspension cells.

Protein retention and localization in the endoplasmic reticulum and the golgi apparatus

Protein transport along the secretory pathway is supported by a noria of vesicles that bud and fuse, load and unload their cargo from one compartment into the other. However, despite this constant flow-through of proteins and lipids the various compartments of the secretory pathway are able to maintain their own specific composition. Here, we discuss recent insights into mechanisms of protein retention and localization that are necessary for the maintenance of endoplasmic reticulum (ER)- and Golgi-associated typical functions such as protein folding and glycosylation in plant cells.

First evidence of a calcium transient in flowering plants at fertilization

We report here the first evidence of a transient elevation of free cytosolic Ca2+ following fusion of sperm and egg cell in a flowering plant by the use of an in vitro fertilization system recently developed in maize. Imaging changes in cytosolic Ca2+ at fertilization was undertaken by egg cell loading with the fluorescent Ca2+ indicator dye fluo-3 under controlled physiological conditions. The gamete adhesion step did not induce any cytosolic Ca2+ variation in the egg cell, whereas the fusion step triggered a transient cytosolic Ca2+ rise in the fertilized egg cell, lasting several minutes. This rise occurred after the establishment of gamete cytoplasm continuity. Through these observations, we open the way to the identification of the early signals induced by fertilization in flowering plants that give rise to the calcium transient and to investigations of the role of Ca2+ during egg activation and early zygote development in plants, as has been reported for other better characterized animal and algae systems.