Cell-cycle, phase-specific activation of Maize streak virus promoters

It is believed that geminiviral DNA replication is coupled to the cell-cycle regulatory complex of the plant cell and that the virus-early (complementary or C sense) gene products REP and REPA may be able to manipulate the regulation of the cycle. In this study, we examined expression from the promoters of Maize streak virus (MSV) in transgenic maize plants and cells to determine whether they showed cell-cycle specificity. Histochemical staining of plant roots containing "long and short" C-sense promoter sequences upstream of the GUS (beta-glucuronidase) reporter gene showed that promoter activity was restricted to the meristematic region of the roots and was enhanced by 2,4-dichlorophenoxy acetic acid (2,4-D) treatment. Analysis of reporter gene and cell-cycle-specific gene transcript levels coupled with flow cytometric data in synchronized transgenic maize cells revealed that all of the MSV promoters showed cell-cycle specificity. The coat protein gene promoter showed highest activity in early G2, whereas the C-sense promoter sequences produced two peaks of activity in the S and G2 cell-cycle phases.

A novel aldose/aldehyde reductase protects transgenic plants against lipid peroxidation under chemical and drought stresses

Rapid accumulation of toxic products from reactions of reactive oxygen species (ROS) with lipids and proteins significantly contributes to the damage of crop plants under biotic and abiotic stresses. Here we have identified a stress-activated alfalfa gene encoding a novel plant NADPH-dependent aldose/aldehyde reductase that also exhibited characteristics of the homologous human enzyme. The recombinant alfalfa enzyme is active on 4-hydroxynon-2-enal, a known cytotoxic lipid peroxide degradation product. Ectopic synthesis of this enzyme in transgenic tobacco plants provided considerable tolerance against oxidative damage caused by paraquat and heavy metal treatment. These transformants could also resist a long period of water deficiency and exhibited improved recovery after rehydration. We found a reduced production of lipid peroxidation-derived reactive aldehydes in these transformed plants under different stresses. These studies reveal a new and efficient detoxification pathway in plants.

A novel aldose/aldehyde reductase protects transgenic plants against lipid peroxidation under chemical and drought stresses

Rapid accumulation of toxic products from reactions of reactive oxygen species (ROS) with lipids and proteins significantly contributes to the damage of crop plants under biotic and abiotic stresses. Here we have identified a stress-activated alfalfa gene encoding a novel plant NADPH-dependent aldose/aldehyde reductase that also exhibited characteristics of the homologous human enzyme. The recombinant alfalfa enzyme is active on 4-hydroxynon-2-enal, a known cytotoxic lipid peroxide degradation product. Ectopic synthesis of this enzyme in transgenic tobacco plants provided considerable tolerance against oxidative damage caused by paraquat and heavy metal treatment. These transformants could also resist a long period of water deficiency and exhibited improved recovery after rehydration. We found a reduced production of lipid peroxidation-derived reactive aldehydes in these transformed plants under different stresses. These studies reveal a new and efficient detoxification pathway in plants.

Nuclear localization of a hypoxia-inducible novel non-symbiotic hemoglobin in cultured alfalfa cells

We have isolated a 483-bp-long full-length cDNA clone encoding a non-symbiotic hemoglobin called Mhb1, the first one found in alfalfa. This non-symbiotic hemoglobin is a single copy gene localized in linkage group 4 in diploid Medicago genome. The Mhb1 mRNA was found only in the roots of alfalfa plants. The Mhb1 gene was inducible by hypoxia and showed no induction by cold stress treatment. The Mhb1 transcript level increased at the G2/M boundary in a synchronized alfalfa cell suspension culture. The majority of Mhb1 protein was shown to be localized in the nucleus and smaller amounts were detected in the cytoplasm. A potential link to the nitric oxide signalling pathway is also discussed.

Nuclear localization of a hypoxia-inducible novel non-symbiotic hemoglobin in cultured alfalfa cells

We have isolated a 483-bp-long full-length cDNA clone encoding a non-symbiotic hemoglobin called Mhb1, the first one found in alfalfa. This non-symbiotic hemoglobin is a single copy gene localized in linkage group 4 in diploid Medicago genome. The Mhb1 mRNA was found only in the roots of alfalfa plants. The Mhb1 gene was inducible by hypoxia and showed no induction by cold stress treatment. The Mhb1 transcript level increased at the G2/M boundary in a synchronized alfalfa cell suspension culture. The majority of Mhb1 protein was shown to be localized in the nucleus and smaller amounts were detected in the cytoplasm. A potential link to the nitric oxide signalling pathway is also discussed.

Multiple cyclin-dependent kinase complexes and phosphatases control G2/M progression in alfalfa cells

Reversible phosphorylation of proteins by kinases and phosphatases plays a key regulatory role in several eukaryotic cellular functions including the control of the division cycle. Increasing numbers of sequence and biochemical data show the involvement of cyclin-dependent kinases (CDKs) and cyclins in regulation of the cell cycle progression in higher plants. The complexity represented by different types of CDKs and cyclins in a single species such as alfalfa, indicates that multicomponent regulatory pathways control G2/M transition. A set of cdc2-related genes (cdc2Ms A, B, D and F) was expressed in G2 and M cells. Phosphorylation assays also revealed that at least three kinase complexes (Cdc2Ms A/B, D and F) were successively active in G2/M cells after synchronization. Interaction between alfalfa mitotic cyclin (Medsa;CycB2;1) and a kinase partner has been reported previously. The present yeast two-hybrid analyses showed differential interaction between defined D-type cyclins and Cdc2Ms kinases functioning in G2/M phases. Localization of Cdc2Ms F kinase to the preprophase band (PPB), the perinuclear ring in early prophase, the mitotic spindle and the phragmoplast indicated a pivotal role for this kinase in mitotic plant cells. So far limited research efforts have been devoted to the functions of phosphatases in the control of plant cell division. A homologue of dual phosphatase, cdc25, has not been cloned yet from alfalfa; however tyrosine phosphorylation was indicated in the case of Cdc2Ms A kinase and the p(13suc1)-bound kinase activity was increased by treatment of this complex with recombinant Drosophila Cdc25. The potential role of serine/threonine phosphatases can be concluded from inhibitor studies based on okadaic acid or endothall. Endothall elevated the kinase activity of p(13suc1)-bound fractions in G2-phase alfalfa cells. These biochemical data are in accordance with observed cytological abnormalities. The present overview with selected original data outlines a conclusion that emphasizes the complexity of G2/M regulatory events in flowering plants.

Targeted inactivation of the plastid ndhB gene in tobacco results in an enhanced sensitivity of photosynthesis to moderate stomatal closure

The ndh genes encoding for the subunits of NAD(P)H dehydrogenase complex represent the largest family of plastid genes without a clearly defined function. Tobacco (Nicotiana tabacum) plastid transformants were produced in which the ndhB gene was inactivated by replacing it with a mutant version possessing translational stops in the coding region. Western-blot analysis indicated that no functional NAD(P)H dehydrogenase complex can be assembled in the plastid transformants. Chlorophyll fluorescence measurements showed that dark reduction of the plastoquinone pool by stromal reductants was impaired in ndhB-inactivated plants. Both the phenotype and photosynthetic performance of the plastid transformants was completely normal under favorable conditions. However, an enhanced growth retardation of ndhB-inactivated plants was revealed under humidity stress conditions causing a moderate decline in photosynthesis via stomatal closure. This distinctive phenotype was mimicked under normal humidity by spraying plants with abscisic acid. Measurements of CO(2) fixation demonstrated an enhanced decline in photosynthesis in the mutant plants under humidity stress, which could be restored to wild-type levels by elevating the external CO(2) concentration. These results suggest that the plastid NAD(P)H:plastoquinone oxidoreductase in tobacco performs a significant physiological role by facilitating photosynthesis at moderate CO(2) limitation.

Plants ectopically expressing the iron-binding protein, ferritin, are tolerant to oxidative damage and pathogens

Transgenic tobacco plants that synthesize alfalfa ferritin in vegetative tissues--either in its processed form in chloroplasts or in the cytoplasmic nonprocessed form--retained photosynthetic function upon free radical toxicity generated by iron excess or paraquat treatment. Progeny of transgenic plants accumulating ferritin in their leaves exhibited tolerance to necrotic damage caused by viral (tobacco necrosis virus) and fungal (Alternaria alternata, Botrytis cinerea) infections. These transformants exhibited normal photosynthetic function and chlorophyll content under greenhouse conditions. We propose that by sequestering intracellular iron involved in generation of the very reactive hydroxyl radicals through a Fenton reaction, ferritin protects plant cells from oxidative damage induced by a wide range of stresses.

Protein phosphatase 1 catalytic subunit isoforms from alfalfa: biochemical characterization and cDNA cloning

The catalytic subunit of protein phosphatase 1 (PP1c) was purified from an alfalfa (Medicago sativa) microcallus cell culture. The preparation was inhibited by rabbit muscle inhibitor-2 and okadaic acid and had a molecular mass of 35 kDa. Five distinct cDNAs termed MsPP1alpha, -beta, -gamma, -delta, and -epsilon were cloned from a M. sativa somatic embryo library. MsPP1alpha was identical to a cDNA reported earlier [A. Páy, M. Pirck, L. Bögre, H. Hirt, and E. Heberle-Bors Mol. Gen. Genet. 244, 176-182, 1994], while the others represented novel isoforms encoded by separate genes. The predicted amino acid sequences of MsPP1alpha, -beta, -gamma, -delta, and -epsilon were highly similar to each other and to other known PP1c sequences. The GST-MsPP1ss fusion protein expressed in Escherichia coli was catalytically active and was inhibited by inhibitor-2 and okadaic acid. Affinity-purified polyclonal MsPP1antipeptide antibody detected a protein of 36 kDa in crude cell extracts. These results proved that the cDNA clone encoded an active PP1c which was very similar to the purified enzyme. The mRNA and protein concentrations of PP1c as well as the specific activity of protein phosphatase 1 did not change during the cell cycle in a synchronized alfalfa cell culture. On the other hand, the isoforms exhibited different steady-state mRNA levels in different plant organs suggesting tissue-specific functions.

Prediction of functional regions of the maize streak virus replication-associated proteins by protein-protein interaction analysis

The replication of the geminiviruses depends on the viral encoded early (complementary-sense) proteins and on host genome encoded factors. Additionally, some of the early proteins (the AL2 protein of subgroup III, and the RepA (formerly known as C1) or Rep (C1:C2) proteins of subgroup I geminiviruses) can function as transcriptional activators of virion- (V-)sense gene expression. The yeast two-hybrid system has allowed us to predict some of the functionally important regions of the maize streak virus (MSV) early proteins RepA and Rep. Defined domains of these proteins were shown to act as transactivators in yeast cells. We detected the association of the RepA and Rep proteins, and their subfragments, with the maize retinoblastoma protein (ZmRb1) which is likely to be one of the interacting host proteins. We showed the self-association capability of the MSV proteins and suggest that homo- or hetero-oligomerization may play an important role in virus replication. These results provide new insights into the role of different regions of the MSV proteins in relation to transcriptional activation and regulation of viral DNA replication.

Chloroplast transformation in plants: polyethylene glycol (PEG) treatment of protoplasts is an alternative to biolistic delivery systems

Nicotiana plumbaginifolia protoplasts were directly transformed by PEG treatment with a cloned 16S rRNA gene isolated from a double antibiotic-resistant Nicotiana tabacum plastid mutant. Putative plastid transformants were selected in cell culture by their spectinomycin resistance and identified by their unselected streptomycin resistance. Alternatively, cell lines were selected in the presence of both antibiotics. The cell line (and its regenerated plants) selected solely for spectinomycin resistance demonstrated an extensive segregation of streptomycin resistance in subsequent tests, while the double-selected line showed stable resistance for both antibiotics. The resistance markers were inherited maternally. In the putative plastid transformants the origin of the resistance mutations was identified by the absence of an AatII site, missing in the donor N. tabacum plastid gene (spectinomycin resistance site) but present in that of wild-type N. plumbaginifolia, and a sequence analysis of the particular nucleotide changes in both resistance sites. Restriction enzyme analysis of total plastid DNA (ptDNA), and the recloning and full sequencing of the fragment introduced, investigated in one of the plastid transformants, showed no DNA rearrangements accompanied with the integration process. Sequence analysis indicated a targeted, homologous integration of the DNA fragment introduced but an unexpectedly complete homology of the parental ptDNA sequences in this region prevented the location of borders. Although the frequency of plastid transformant colonies (2 x 10(-5)) should still be improved, this method for stable chloroplast DNA transformation is comparable with or more efficient than the particle bombardment techniques.

Chloroplast transformation in plants: polyethylene glycol (PEG) treatment of protoplasts is an alternative to biolistic delivery systems

Nicotiana plumbaginifolia protoplasts were directly transformed by PEG treatment with a cloned 16S rRNA gene isolated from a double antibiotic-resistant Nicotiana tabacum plastid mutant. Putative plastid transformants were selected in cell culture by their spectinomycin resistance and identified by their unselected streptomycin resistance. Alternatively, cell lines were selected in the presence of both antibiotics. The cell line (and its regenerated plants) selected solely for spectinomycin resistance demonstrated an extensive segregation of streptomycin resistance in subsequent tests, while the double-selected line showed stable resistance for both antibiotics. The resistance markers were inherited maternally. In the putative plastid transformants the origin of the resistance mutations was identified by the absence of an AatII site, missing in the donor N. tabacum plastid gene (spectinomycin resistance site) but present in that of wild-type N. plumbaginifolia, and a sequence analysis of the particular nucleotide changes in both resistance sites. Restriction enzyme analysis of total plastid DNA (ptDNA), and the recloning and full sequencing of the fragment introduced, investigated in one of the plastid transformants, showed no DNA rearrangements accompanied with the integration process. Sequence analysis indicated a targeted, homologous integration of the DNA fragment introduced but an unexpectedly complete homology of the parental ptDNA sequences in this region prevented the location of borders. Although the frequency of plastid transformant colonies (2 x 10(-5)) should still be improved, this method for stable chloroplast DNA transformation is comparable with or more efficient than the particle bombardment techniques.

Subcellular location of lincomycin resistance in Nicotiana mutants

Lincomycin-resistant Nicotiana plumbaginifolia plastid mutants were considered also to carry mitochondrial mutations on the basis of their ability to grow in the dark under selective conditions. To clarify the role of mitochondria, individual protoplasts of the green, lincomycin-resistant N. plumbaginifolia mutant LR400 were microfused with protoplasts of the N. tabacum plastid albino line 92V37, which possesses N. undulata cytoplasm. the production of lincomycin-resistant albino cybrid lines, with N. undulata plastids and recombinant mitochondria, strongly indicated a determining role for mitochondria in the lincomycin resistance. Sequence analysis of the region encompassing putative mutation sites in the 26S rRNA genes from the LR400 and several other lincomycin-resistant N. plumbaginifolia mutants revealed, however, no differences from the wild-type sequence. As an alternative source of the resistance of the fusion products, the N. tabacum fusion partner was also taken into account. Surprisingly, a natural lincomycin resistance of tobacco was detected, which was inherited as a dominant nuclear trait. This result compromises the interpretation of the fusion data suggested above. Thus, to answer the original question definitively, the mutant LR400 was crossed as a female parent with a N. plumbaginifolia line carrying streptomycin-resistant N. tabacum plastids. Calli were then induced from the seedlings. Occasional paternal plastid transmissions were selected as streptomycin-resistant calli on selective medium. These cell lines were shown by restriction enzyme analysis to contain paternal plastids and maternal mitochondria. They were tested for greening and growing ability in the presence of lincomycin. These resistance traits proved to be genetically linked and exclusively located in the plastids.

Impact of the stringency of cell selection on plastid segregation in protoplast fusion-derived Nicotiana regenerates

Vegetative segregation of a mixed plastid population in protoplast fusion-derived cell lines can be directed by a selection favouring the multiplication of one of the parental plastid types. This report defines some of the critical conditions leading to a homogeneous plastid population in cybrid plants generated by protoplast fusion between Nicotiana plumbaginifolia and an albino and streptomycin-resistant N. tabacum plastid mutant. Light (1,500 lx) conferred a strong selective advantage to chloroplasts versus albino plastids, while the lack of this effect in dim light (300 lx) indicated that a sufficient light intensity is essential to the phenomenon. Selection on streptomycin-containing medium in the dark, however, led to the preferential multiplication of resistant plastids. Streptomycin selection of resistant chloroplasts in the light, consequently, results in a plastid selection of doubled stringency. In another experiment a definite, but leaky, selection for chloroplast recombination (selection for greening on streptomycin-containing medium in dim light) was used to reveal various recombination products. Protoplast fusion in fact resulted in cybrid plants showing only simple chimeric segregation of unchanged parental plastids. These results demonstrate the essential requirement for stringent plastid selection, as defined by cell culture conditions, to precede the formation of shoots expected to possess the desired plastid genetic composition.