Shoot tip necrosis of in vitro plant cultures: a reappraisal of possible causes and solutions

Shoot tip necrosis is a physiological condition that negatively impacts the growth and development of in vitro plant shoot cultures across a wide range of species. Shoot tip necrosis is a physiological condition and disorder that can arise in plantlets or shoots in vitro that results in death of the shoot tip. This condition, which can spread basipetally and affect the emergence of axillary shoots from buds lower down the stem, is due to the cessation of apical dominance. STN can occur at both shoot multiplication and rooting stages. One of the most common factors that cause STN is nutrient deficiency or imbalance. Moreover, the presence or absence of plant growth regulators (auxins or cytokinins) at specific developmental stages may impact STN. The cytokinin to auxin ratio within an in vitro plant can be modified by varying the concentration of cytokinins used in the culture medium. The supply of nutrients to in vitro shoots or plantlets might also affect their hormonal balance, thus modifying the occurrence of STN. High relative humidity within culture vessels and hyperhydricity are associated with STN. An adequate supply of calcium as the divalent cation (Ca²⁺) can hinder STN by inhibiting the accumulation of phenolic compounds and thus programmed cell death. Moreover, the level of Ca²⁺ affects auxin transport and ethylene production, and higher ethylene production, which can occur as a result of high relative humidity in or poor ventilation of the in vitro culture vessel, induces STN. High relative humidity can decrease the mobility of Ca²⁺ within a plant, resulting in Ca²⁺ deficiency and STN. STN of in vitro shoots or plantlets can be halted or reversed by altering the basal medium, mainly the concentration of Ca²⁺, adjusting the levels of auxins or cytokinins, or modifying culture conditions. This review examines the literature related to STN, seeks to discover the associated factors and relations between them, proposes practical solutions, and attempts to better understand the mechanism(s) underlying this condition in vitro.

Selenate tolerance and selenium hyperaccumulation in the monocot giant reed (Arundo donax), a biomass crop plant with phytoremediation potential

The response of giant reed (Arundo donax L.) to selenium (Se), added as selenate, was studied. The development, stress response, uptake, translocation, and accumulation of Se were documented in three giant reed ecotypes STM (Hungary), BL (USA), and ESP (Spain), representing different climatic zones. Plantlets regenerated from sterile tissue cultures were grown under greenhouse conditions in sand supplemented with 0, 2.5, 5, and 10 mg Se kg^-1 added as sodium selenate. Total Se content was measured in different plant parts using hydride generation atomic fluorescence spectroscopy. All plants developed normally in the 0-5.0 mg Se kg^-1 concentration range regardless of ecotype, but no growth occurred at 10.0 mg Se kg^-1. There were no signs of chlorosis or necrosis, and the photosynthetic machinery was not affected as evidenced by no marked differences in the structure of thylakoid membranes. There was no change in the maximum quantum yield of photosystem II (F_v/F_m ratio) in the three ecotypes under Se stress, except for a significant negative effect in the ESP ecotype in the 5.0 mg Se kg^-1 treatment. Glutathione peroxidase (GPx) activity increased as the Se concentration increased in the growth medium. GPx activity was higher in the shoot system than the root system in all Se treatments. All ecotypes showed great capacity of take up, translocate and accumulate selenium in their stem and leaf. Relative Se accumulation is best described as leaf ˃˃ stem ˃ root. The ESP ecotype accumulated 1783 μg g^-1 in leaf, followed by BL with 1769 μg g^-1, and STM with 1606 μg g^-1 in the 5.0 mg Se kg^-1 treatment. All ecotypes showed high values of translocation and bioaccumulation factors, particularly the ESP ecotype (10.1 and 689, respectively, at the highest tolerated Se supplementation level). Based on these findings, Arundo donax has been identified as the first monocot hyperaccumulator of selenium, because Se concentration in the leaves of all three ecotypes, and also in the stem of the ESP ecotype, is higher than 0.1% (dry weight basis) under the conditions tested. Tolerance up to 5.0 mg Se kg^-1 and the Se hyperaccumulation capacity make giant reed a promising tool for Se phytoremediation.

Copper Uptake Efficiency and Its Distribution Within Bioenergy Grass Giant Reed

To evaluate copper uptake and its toxicity on bioenergy grass giant reed (Arundo donax L.), experiments were carried out using two epigenetic clonal lines - American (BL) and Hungarian (20SZ) ecotypes - grown on elevated Cu concentrations up to 26.8 mg L(-1). Neither ecotype showed any noticeable foliar symptoms of Cu toxicity at concentrations tested up to 10 mg L(-1). Dry mass of plants of both ecotypes significantly increased at the highest Cu treatment compared to control. Although the BL ecotype had greater capacity to uptake Cu than 20SZ, the dry mass and shoot length of BL was higher than that of 20SZ. Values of bioconcentration and transportation factors were higher in the BL than in the 20SZ ecotype. Almost 45 % of total Cu content within the whole plant was found in the plant root of both ecotypes. This demonstrated both ecotypes can be utilized for Cu phytoremediation alongside with significant biomass production.

High Accumulation and Subcellular Distribution of Thallium in Green Cabbage (Brassica Oleracea L. Var. Capitata L.)

The accumulation of thallium (Tl) in brassicaceous crops is widely known, but both the uptake extents of Tl by the individual cultivars of green cabbage and the distribution of Tl in the tissues of green cabbage are not well understood. Five commonly available cultivars of green cabbage grown in the Tl-spiked pot-culture trials were studied for the uptake extent and subcellular distribution of Tl. The results showed that all the trial cultivars mainly concentrated Tl in the leaves (101∼192 mg/kg, DW) rather than in the roots or stems, with no significant differences among cultivars (p = 0.455). Tl accumulation in the leaves revealed obvious subcellular fractionation: cell cytosol and vacuole >> cell wall > cell organelles. The majority (∼ 88%) of leaf-Tl was found to be in the fraction of cytosol and vacuole, which also served as the major storage site for other major elements such as Ca and Mg. This specific subcellular fractionation of Tl appeared to enable green cabbage to avoid Tl damage to its vital organelles and to help green cabbage tolerate and detoxify Tl. This study demonstrated that all the five green cabbage cultivars show a good application potential in the phytoremediation of Tl-contaminated soils.

Genetic Modification of Wetland Grasses for Phytoremediation

Wetland grasses and grass-like monocots are very important natural remediators of pollutants. Their genetic improvement is an important task because introduction of key transgenes can dramatically improve their remediation potential. Tissue culture is prerequisite for genetic manipulation, and methods are reported here for in vitro culture and micropropagation of a number of wetland plants of various ecological requirements such as salt marsh, brackish water, riverbanks, and various zones of lakes and ponds, and bogs. The monocots represent numerous genera in various families such as Poaceae, Cyperaceae, Juncaceae, and Typhaceae. The reported species are in various stages of micropropagation and Arundo donax is scaled for mass propagation for selecting elite lines for pytoremediation. Transfer of key genes for mercury phytoremediation into the salt marsh cordgrass (Spartina alterniflora) is also reported here. All but one transgenic lines contained both the organomercurial lyase (merB) and mercuric reductase (merA) sequences showing that co-introduction into Spartina of two genes from separate Agrobacterium strains is possible.

Spatial variation of contaminant elements in roadside dust of Budapest and Seoul

Roadside dusts were studied to explain the spatial variation, and present level of contaminant elements (including Pt, Pd and Ir) in Budapest and Seoul, the capitals of Hungary and Seoul. Road dust samples were obtained twice from traffic focal points in Budapest centre (four bridges, Margaret Island, two main railway stations, main roads) and the agglomeration (suburbs) of Budapest. Similarly, samples were collected from six sites having high traffic volumes in Seoul metropolitan city and from two control sites within the suburbs of Seoul, for comparison. The samples were analysed for contaminant elements by ICP-AES and for Pt, Pd and Ir by ICP-MS.The level of contaminant elements in road dusts were in the range of 67.6 g∙kg−1 and 0.6 μg∙kg−1 for Budapest; 50.2 g∙kg−1 and 0.5 μg∙kg−1 for Budapest suburbs; 43.5 g∙kg−1 and 3.3 μg∙kg−1 for Seoul samples. The Geo-Accumulation Indexes (GAI) of contaminant elements for Budapest, Budapest suburbs and Seoul ranged between 3.88 and −0.03; 2.74 and −0.13; 4.23 and 1.22; Pollution Indexes (PI) of the contaminant elements were in the range of 4.2 and 0.6; 0.9 and 0.4; 7.8 and 2.7; Contamination Indexes (CI) of the contaminant elements ranged from 27.1 to 3.3; 4.9 to 2.7; 21.8 to 10.2, respectively.Pt, Pd and Ir concentration levels (μg∙kg−1) were in the range of 133 and 1.9, 170 and 12.8, 4.5 and 0.4 for Budapest; 37 and 3.6, 39.4 and 11.3, 0.8 and 0.2 for Budapest suburbs; 98.5 and 25.6, 148 and 30.6, 5.6 and 2.1 for Seoul.Correlations between the Pollution Index (PI) and Contamination Index (CI) of the platinum metals in road dust samples were: −0.4090 (Ir), −0.3098 (Pd), −0.1994 (Pt) for Budapest centre; 0.8343 (Ir), −0.7652 (Pt), −0.7626 (Pd) for Budapest bridges; 0.5266 (Ir), −0.2863 (Pd), −0.0333 (Pt) for Budapest suburbs; and 0.3193 (Pd), 0.2874 (Ir), −0.0017 (Pt) for Seoul, respectively. The highest Pt, Pd and Ir levels in road dusts were found in samples originating from the major roads with high traffic volume. Significant differences in Pollution Index (PI) and Contamination Index (CI) between Seoul and Budapest were 4.0 and 5.9 for heavy traffic roads; 2.9 and 11.0 for roads with light traffic. Light traffic roads reflect that an important source of Pt, Pd and Ir in roadside environment is the automobile catalytic converter. Road dusts with high Pt, Pd and Ir levels were enriched with traffic-related contaminant elements as well.

Phytoaccumulation potentials of two biotechnologically propagated ecotypes of Arundo donax in copper-contaminated synthetic wastewater

An in vitro experiment was carried out to evaluate the phytoremediation potentials of two somatic embryo-derived ecotypes of Arundo donax-BL (American ecotype) and 20SZ (Hungarian ecotype)-of copper from synthetic wastewater. The two ecotypes were grown under sterile conditions in tubes containing a nutrient solution supplied with increasing doses of Cu (0, 1, 2, 3, 5, 10, and 26.8 mg L(-1)) for 6 weeks. The translocation and bioaccumulation factors and removal rate were estimated. In general, increasing Cu concentration in nutrient solution slightly decreased root, stem and leaf biomass without toxicity symptoms up to 26.8 mg L(-1). Moreover, both ecotypes showed high Cu removal efficiency from aqueous solution. However, Cu removal rate ranged between 96.6 to 98.8% for BL ecotype and 97 to 100% for 20SZ ecotype. Data illustrated that both BL and 20SZ ecotypes may be employed to treat Cu-contaminated water bodies up to 26.8 mg L(-1).

Phytoaccumulation potentials of two biotechnologically propagated ecotypes of Arundo donax in copper-contaminated synthetic wastewater

An in vitro experiment was carried out to evaluate the phytoremediation potentials of two somatic embryo-derived ecotypes of Arundo donax-BL (American ecotype) and 20SZ (Hungarian ecotype)-of copper from synthetic wastewater. The two ecotypes were grown under sterile conditions in tubes containing a nutrient solution supplied with increasing doses of Cu (0, 1, 2, 3, 5, 10, and 26.8 mg L(-1)) for 6 weeks. The translocation and bioaccumulation factors and removal rate were estimated. In general, increasing Cu concentration in nutrient solution slightly decreased root, stem and leaf biomass without toxicity symptoms up to 26.8 mg L(-1). Moreover, both ecotypes showed high Cu removal efficiency from aqueous solution. However, Cu removal rate ranged between 96.6 to 98.8% for BL ecotype and 97 to 100% for 20SZ ecotype. Data illustrated that both BL and 20SZ ecotypes may be employed to treat Cu-contaminated water bodies up to 26.8 mg L(-1).

Early post-operative thrombosis of the prosthetic mitral valve in patient with heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is one of the most common immune-mediated adverse drug reactions, with frequencies as high as 2-3% for certain groups of post-cardiac surgery patients. We report on an 50-year-old woman with early post-operative thrombosis of the prosthetic mitral valve due to heparin-induced thrombocytopenia. Non-invasive imaging (two-dimensional transesophageal echocardiography; 2D-TEE) allowed the exact localisation of thrombotic masses and revealed the increase of the mean diastolic mitral gradient. The HIT diagnosis was proved by the clinical scoring system, and with the identification of heparin platelet factor 4-induced antibodies. After the withdrawal of LMWH therapy and the start of intravenous lepirudin treatment, the patient's medical condition improved continuously. Follow-up echocardiography showed a step-wise decrease in the severity of the mean diastolic mitral valve gradient and a complete resolution of thrombus formations. Perhaps we may remind ourselves that, whilst HIT is one of the most common immune-mediated adverse drug reactions for certain groups of post-cardiac surgery patients, it can be managed successfully. We would also stress the importance of serial 2D-TEE examinations in the early post-operative period.

Phytoremediation: Enhanced cadmium (Cd) accumulation by organic manuring, EDTA and microbial inoculants (Azotobacter sp., Pseudomonas sp.) in Indian mustard (Brassica juncea L.)

Phytoremediation is an approach designed to extract excessive heavy metals from contaminated soils through plant uptake. Cadmium (Cd) is among the elements most toxic to living organisms. Health hazards associated with the lethal intake of Cd include renal (kidney) damage, anaemia, hypertension and liver damage. A greenhouse experiment was carried out with Indian mustard (Brassica juncea) grown on artificially spiked soil (100 μg Cd g−1) with EDTA (2 mmol kg−1 in 5 split doses), FYM, vermicompost (VC) and microbial inoculants (MI) such as Azotobacter sp. and Pseudomonas sp. The growth of Brassica juncea L. was better in soil amended with FYM or VC as compared to unamended Cd-polluted soil. Growth was slightly suppressed in EDTA-treated soil, whereas it was better after treatment with MI. The application of FYM and VC increased the dry matter yield of Indian mustard either alone or in combination with microbial inoculants, while that of EDTA caused a significant decrease in the biomass of Indian mustard. The application of microbial inoculants increased the dry matter yield of both the roots and shoots, but not significantly, because MI shows greater sensitivity towards cadmium. The maximum cadmium concentration was observed in the EDTA +MI treatment, but Cd uptake was maximum in the VC + MI treatment. The Cd concentration in the shoots increased by 120% in CdEDTA over the Cd100 treatment, followed by CdVC (65%) and CdFYM (42%) in the absence of microbial inoculants. The corresponding values in the presence of MI were 107, 51 and 37%, respectively. A similar trend was also observed in the roots in the order CdEDTA+M > CdVC+M > CdFYM+M>Cd100+M.MI caused an increase in Cd content of 5.5% in the roots and 4.1% in the shoots in the CdEDTA+M treatment compared with the CdEDTA treatment. FYM, VC and EDTA also increased Cd uptake significantly both in the shoots and roots with and without microbial inoculants.The results indicated that Vermicompost in combination with microbial inoculants is the best treatment for the phytoremediation of Cd-contaminated soil by Indian mustard, as revealed by the Cd uptake values in the shoots: CdVC+M (2265.7 μg/pot) followed by CdEDTA+M (2251.2 μg/pot), CdFYM+M (1485.7 μg/pot) and Cd100+M (993.1 μg/pot).

Transgenic Spartina alterniflora for phytoremediation

Perennial monoculture forming grasses are very important natural remediators of pollutants. Their genetic improvement is an important task because introduction of key transgenes can dramatically improve their remediation potential. Transfer of key genes for mercury phytoremediation into the salt marsh cordgrass (Spartina alterniflora) is reported here. S. alterniflora plays an important role in the salt marsh by cycling of elements, both nutrients and pollutants, protects the coastline from erosion, is a keystone species in the salt marsh supporting a large food web, which in turn supports a significant segment of economy, including tourism, has an impact on cloud formation and consequently on global weather, and is thus an ecologically important species relevant for our life-support systems. Embryogenic callus of S. alterniflora was co-inoculated with a pair of Agrobacterium strains LBA4404 carrying the organomercurial lyase (merB) and mercuric reductase (merA) genes, respectively, in order to co-introduce both the merA and the merB genes. Seven stable geneticin resistant lines were recovered. The presence of merA and merB genes was verified by PCR and Southern blotting. All but one transgenic lines contained both the merA and the merB sequences proving that co-introduction into Spartina of two genes from separate Agrobacterium strains is feasible and frequent, although the overall frequency of transformation is low. Northern blotting showed differences in relative expression of the two transgenes among individual transformants. The steady-state RNA levels appeared to correlate with the phenotype. Line #7 showed the highest resistance to HgCl(2) (up to 500 microM), whereas line #3 was the most resistant to phenylmercuric acetate (PMA). Wild-type (WT) callus is sensitive to PMA at 50 microM and to HgCl(2) at 225 microM.

Auxin autotrophic tobacco callus tissues resist oxidative stress: the importance of glutathione S-transferase and glutathione peroxidase activities in auxin heterotrophic and autotrophic calli

Auxin autotrophic and heterotrophic tobacco callus lines were grown on MS medium with or without 100 mmol/L NaCl and growth and some of the stress-related activities, such as GPX, SOD, CAT, GST, GSH-PX, as well as the concentration of ethylene and H2O2, were measured and compared with each other. The auxin autotrophic calli grew slower, however, on the NaCl-containing medium the growth rate was higher than that of the heterotrophic cultures after two weeks of culturing. The stress-related ethylene production was lower in the autotrophic cultures and, contrary to the heterotrophic tissues, its level did not change significantly upon NaCl treatment. The guaiacol peroxidase (GPX) activities were higher in the autotrophic tissues in all cell fractions regardless of the presence of NaCl. Treated with NaCl, the GPX activities elevated in the soluble and covalently-bound fractions in the heterotrophic calli, but were not further increased in the autotrophic line. SOD and CAT activities were higher in the heterotrophic tissues, and were increased further by 100 mmol/L NaCl treatment. The GST and GSH-PX activities were higher in the autotrophic line, which might explain their enhanced stress tolerance. In the autotrophic tissues, the elevated antioxidant activities led to reduced levels of H2O2 and malondialdehyde; under mild NaCl stress, these levels decreased further. The lower growth rate and the effective protection against NaCl stress-induced oxidative damage of the autotrophic line can be explained by the cell wall-bound peroxidase and GSH-PX activities in the auxin autotrophic tissues. Their maintained growth rate indicates that the autotropic cultures were more resistant to exogenous H2O2.

Differential mercury volatilization by tobacco organs expressing a modified bacterial merA gene

Mercury pollution is a major environmental problem accompanying industrial activities. Most of the mercury released ends up and retained in the soil as complexes of the toxic ionic mercury (Hg2+), which then can be converted by microbes into the even more toxic methylmercury which tends to bioaccumulate. Mercury detoxification of the soil can also occur by microbes converting the ionic mercury into the least toxic metallic mercury (Hg0) form, which then evaporates. The remediation potential of transgenic plants carrying the MerA gene from E. coli encoding mercuric ion reductase could be evaluated. A modified version of the gene, optimized for plant codon preferences (merApe9, Rugh et al. 1996), was introduced into tobacco by Agrobacterium-mediated leaf disk transformation. Transgenic seeds were resistant to HgCl2 at 50 microM, and some of them (10-20% ) could germinate on media containing as much as 350 microM HgCl2, while the control plants were fully inhibited or died on 50 microM HgCl2. The rate of elemental mercury evolution from Hg2+ (added as HgCl2) was 5-8 times higher for transgenic plants than the control. Mercury volatilization by isolated organs standardized for fresh weight was higher (up to 5 times) in the roots than in shoots or the leaves. The data suggest that it is the root system of the transgenic plants that volatilizes most of the reduced mercury (Hg0). It also suggests that much of the mercury need not enter the vascular system to be transported to the leaves for volatilization. Transgenic plants with the merApe9 gene may be used to mercury detoxification for environmental improvement in mercury-contaminated regions more efficiently than it had been predicted based on data on volatilization of whole plants via the upper parts only (Rugh et al. 1996).

A heartwood pigment in Dalbergia cell cultures

In an extensive survey of the genera Baphia, Caesalpinia, Dalbergia, Haematoxylon, and Pterocarpus, we have identified a number of species whose cell cultures accumulated pigments similar to those in heartwood. Thirteen rosewood (Dalbergia) species produced a purple quinonemethide pigment in the callus that was apparently identical between the species. The pigment was first purified from D. retusa cell culture and its structure was elucidated by mass, infrared, and detailed 1H and 13C NMR and NOE spectroscopic studies including 2D experiments (COSY, NOESY, HMQC, and HMBC). Retusapurpurin A (1a) is a C(30) isoflavan of novel skeleton whose formation can be rationalized to occur via regioselective oxidative coupling of an isoflavan to 4,4'-dihydroxy-2'-methoxychalcone. Retusapurpurin A was also isolated from D. parviflora heartwood and cell culture indicating that stress metabolism pathways that are shared with heartwood-type secondary metabolism subpathways are initiated in Dalbergia cell cultures. Therefore, Dalbergia cell cultures afford a good model system for studying heartwood-type metabolic differentiation.

Frequent collinear long transfer of DNA inclusive of the whole binary vector during Agrobacterium-mediated transformation

When Agrobacterium was used to transform Nicotiana plumbaginifolia protoplasts and Arabidopsis thaliana roots and seedlings, a large number of plants were found in which not only the T-region defined by the border repeat sequences but the entire binary vector was integrated, as determined by both PCR and Southern analysis techniques. N. plumbaginifolia protoplast co-cultivation experiments yielded 3 out of 5 transformants with collinear sequence past the left border. In Arabidopsis root transformation experiments, 33% (6/18) of the transformants had T-DNA which exceeded the left border repeat. Vacuum infiltration of Arabidopsis seedlings produced even a greater percentage of transformants with sequences outside the left border repeat (62%, 39/63). The long transfer DNA cosegregated with the T-region encoded hygromycin resistance in the T2 progeny eliminating the possibility that long transfer DNA was of extrachromosomal or Agrobacterium origin. The high frequency of long transfer after vacuum infiltration of A. thaliana needs to be considered when analyzing T-DNA tagged mutants.

Variable expression of the herpes simplex virus thymidine kinase gene in Nicotiana tabacum affects negative selection

The potentials and limitations of negative-selection systems based on the human herpes simplex virus thymidine kinase type-1 (HSVtk) gene, which causes sensitivity to the nucleoside analog ganciclovir, were examined in tobacco as a model system. There were great differences between individual HSVtk(+) transgenic plants in ganciclovir sensitivity. Inhibition of growth while under selection correlated with HSVtk-tianscnpt levels. Negative selection against HSVtk(+) transformants at the level of Agrobacterium-mediated transformation using a ganciclo-vir/kanamycin double-selection medium (the positive selection marker neomycin phosphotransferase-II gene was in the transformation vector) resulted in a three- to six-fold reduction in the frequency of kanamycin-resistant shoots. The efficiency of negative selection in this case was limited due to the great variation in HSVtk expression, i.e., the frequently occurring transformants with low, or no, ganciclovir sensitivity escaping negative selection. Two independently constructed HSVtk genes showed the same variability of the phenotype in Nicotiana tabacum transformants. Distinct phenotypes, ranging from no regeneration through abnormal or delayed regeneration, were observed when leaf segments were placed on shoot-inducing medium supplemented with 10(-6)-10(-3) M ganciclovir. The highest HSVtk mRNA and ganciclovir sensitivity levels were observed in plants which were transformed with the pSLJ882 chimeric construct. The pSLJ882 plant expression vector carried the coding sequence of HSVtk, whereas plasmid pCX305.1 carried an HSVtk construct retaining the untranslated 5 leader and viral 3 regions. The pCX305.1 transformants showed, at most, a delayed formation of shoots with thin stems and very narrow leaves. Ganciclovir sensitivity showed typical Mendelian segregation. A gene-dosage effect was also seen at the seedling level in the progeny of two transgenic lines.

Large-scale protoplast isolation and regeneration of Arabidopsis thaliana

A simple method for large-scale isolation and regeneration of Arabidopsis thaliana green protoplasts utilizing a novel starting material is presented. This technique takes advantage of shooty/embryogenic masses regenerating from fresh or cultured roots. There is no time-consuming mechanical isolation of starting material. Economical Bacto agar is used as an embedding agent. Plating efficiencies are between 5% and 10%. Seventy-six percent of protoplast-derived colonies produce shoots.

T-DNA-insert-independent mutations induced in transformed plant cells during Agrobacterium co-cultivation

Transformation frequencies were determined for 1n, 2n, and 4n Nicotiana plumbaginifolia protoplast cultures in Agrobacterium-mediated gene transfer experiments. An unexpected large drop (50%) in plating efficiencies was observed in the non-selected (control) 1n populations after transformation treatment with virulent strains. This effect was not observed in the 2n or 4n cultures or in the 1n cultures when treated with avirulent bacteria. The mortality was disproportionally high and could not be explained by the low (0.1-0.5%) transformation efficiency in the 1n population, indicating mutagenesis of the cell populations independently from the T-DNA insertions. Mutagenesis was also indicated in gene tagging experiments where nitrate reductase-deficient (NR-) mutants were selected from haploid Nicotiana plumbaginifolia protoplasts, as well as from leaf disc cultures or protoplasts of diploid plants that were heterozygotic for a mutation either in the NR apoenzyme gene (nia/wt) or one of the molybdenum-containing cofactor genes (cnxA/wt), after Agrobacterium co-cultivation. The chlorate-resistant isolates were tested for the T-DNA-specific kanamycin resistance trait only after NR-deficiency had been established. Thirty-nine independent NR-deficient mutants were analysed further by Southern blot hybridization. There was no indication of integrated T-DNA sequences in the mutated NR genes, despite the fact that NR-deficient cells were found more frequently in cell populations which became transformed during the treatment than in the populations which did not.(ABSTRACT TRUNCATED AT 250 WORDS)

The herpes simplex virus thymidine kinase gene as a conditional negative-selection marker gene in Arabidopsis thaliana

The human herpes simplex virus thymidine kinase type 1 gene (HSVtk) acts as a conditional lethal marker in mammalian cells. The HSVtk-encoded enzyme is able to phosphorylate certain nucleoside analogs (e.g. ganciclovir, an antiherpetic drug), thus converting them to toxic DNA replication inhibitors. The utility of HSVtk as a conditional negative-selection marker was explored in Arabidopsis thaliana (L.) Heynh. HSVtk was introduced into Arabidopsis by Agrobacterium-mediated transformation. Transgenic plants were morphologically indistinguishable from wild type and exhibited normal fertility. Ganciclovir at 10(-5) to 10(-4) M drastically reduced shoot regeneration on transgenic, HSVtk+ root explants or callus formation on HSVtk+ leaf explants but did not affect the wild-type cultures. There was a 35-fold reduction in shoot regeneration 8 d after transfer to shoot-induction medium. Negative selection against HSVtk activity along with kanamycin selection was also efficient in Agrobacterium-mediated gene transfer experiments. Shoot regeneration was 25 times lower on double-selective (ganciclovir plus kanamycin) plates than in the kanamycin control. This regeneration rate in double-selective plates is in the range of the frequency of shoots normally escaping kanamycin selection in Arabidopsis cultures.

Sustained root culture for generation and vegetative propagation of transgenic Arabidopsis thaliana

Excised roots of wild-type and nitrate-reductase deficient mutant Arabidopsis thaliana (L.) HEYNH. can be propagated as sustained root cultures in liquid medium. Culture initiation from a single seedling required a two-day indoleacetic acid treatment at 0.05 mg/l concentration. Indoleacetic acid facilitated subculture but was not essential for sustained growth. This procedure has allowed the clonal propagation of roots derived from individual wildtype and mutant seedlings for more than 21 months. The cultured roots retained their shoot regeneration ability; however, a controlled desiccation treatment was required to restore it to the level of freshly excised roots. The chromosome number remained diploid and no evidence for the accumulation of recessive mutations was observed. The cultured roots are competent for Agrobacterium-mediated transformation. The sustained root culture technology allowed the maintenance of transgenic tissues in which expression of a dominant, seed-lethal gene (seed-specific pea vicilin promoter fused to diphtheria toxin A chain gene) precluded generative propagation.

Differential manifestation of seed mortality induced by seed-specific expression of the gene for diphtheria toxin A chain in Arabidopsis and tobacco

A pea vicilin promoter-diphtheria toxin A (DTx-A) chain gene fusion was introduced into Arabidopsis and tobacco. The chimeric Dtx-A gene behaves as a dominant, seed-lethal, Mendelian factor, and the segregation ratios are consistent with the numbers of integrated copies as revealed by Southern blotting. Germination deficiency results from distinct developmental abnormalities, thus allowing genetic dissection of seed development. The endosperm is affected first in both species. In Arabidopsis, full cellularization of the initially syncytial endosperm does not take place, which results in shrinkage and a shriveled appearance of the mature dry seed. The embryo, which appears structurally normal and lacks visible lesions, ceases to develop at the partially recurved cotyledon stage and does not use the remaining endosperm. In tobacco, peripheral degeneration and premature termination of cellular endosperm development occurs at the cotyledon initiation stage. Lesions appear in the cotyledons at the advanced cotyledon stage, but the embryo continues to grow and attains nearly the same size and level of differentiation as mature wild-type embryos before degeneration and intracellular disintegration take place throughout. Accumulation of protein bodies and other cytoplasmic inclusions is very limited and occurs only in few cells. The timing and distribution of lesions follow a pattern typical for accumulation of protein bodies in wild-type seeds. These observations are consistent with expression of the vicilin promoter in the enlargement phase of cell differentiation. A novel tissue interaction arises, when the embryo uses up all the arrested endosperm: the embryo proves to be capable of absorbing the parenchyma layers of the integument, which are normally obliterated by, and incorporated into, the endosperm.(ABSTRACT TRUNCATED AT 250 WORDS)

Facile transformation of Arabidopsis

A protocol is described for the simple, rapid and efficient production of transgenic Arabidopsis plants. The procedure was developed using growth regulator regimes that promote adventitious embryogenesis during or immediately following Agrobacterium mediated transformation. Both the RLD and Columbia genotypes of Arabidopsis were transformed using slightly different growth regulator regimes. For the Columbia genotype two modifications of the protocol were identified which substantially improved regeneration. Cold treatment of the plants used as a source of root explants resulted in a three-fold increase in the number of morphogenic sectors produced. A more important modification was the inclusion of 25 mg/l silver nitrate (an inhibitor of ethylene action) to the medium used for shoot regeneration. This provided a ten-fold increase in the number of shoots produced. These procedures made it possible to obtain over 100 putative transformants of RLD or Columbia from a single 10 cm petri dish, within 2 or 4 weeks after exposure of root explants to the bacteria. When these were transferred to rooting media containing antibiotics, approximately 20% were able to root after kanamycin selection and 80% after hygromycin selection. All the rooted plantlets tested were shown to contain integrated donor DNA as determined by Southern blot analysis.

Respiratory control over photosynthetic electron transport in chloroplasts of higher-plant cells: evidence for chlororespiration

Flash-induced primary charge separation, detected as electrochromic absorbance change, the operation of the cytochrome b/f complex and the redox state of the plastoquinone pool were measured in leaves, protoplasts and open-cell preparations of tobacco (Nicotiana tabacum L.), and in isolated intact chloroplasts of peas (Pisum sativum L.). Addition of 0.5-5 mM KCN to these samples resulted in a large increase in the slow electrochromic rise originating from the electrogenic activity of the cytochrome b/f complex. The enhancement was also demonstrated by monitoring the absorbance transients of cytochrome f and b 6 between 540 and 572 nm. In isolated, intact chloroplasts with an inhibited photosystem (PS) II, low concentrations of dithionite or ascorbate rendered turnover of only 60% of the PSI reaction centers, KCN being required to reactivate the remainder. "Silent" PSI reaction centers which could be reactivated by KCN were shown to occur in protoplasts both in the absence and presence of a PSII inhibitor. Contrasting spectroscopic data obtained for chloroplasts before and after isolation indicated the existence of a continuous supply of reducing equivalents from the cytosol.Our data indicate that: (i) A respiratory electron-transport pathway involving a cyanide-sensitive component is located in chloroplasts and competes with photosynthetic electron transport for reducing equivalents from the plastoquinone pool. This chlororespiratory pathway appears to be similar to that found in photosynthetic prokaryotes and green algae. (ii) There is an influx of reducing equivalents from the cytosol to the plastoquinone pool. These may be indicative of a complex respiratory control of photosynthetic electron transport in higher-plant cells.

Organ-specific and developmental regulation of the nopaline synthase promoter in transgenic tobacco plants

Control regions of the nopaline synthase (nos) gene have been widely used to express foreign genes in plants since the promoter is active in a wide variety of plant tissues. We report here the characteristics of the nos promoter activity in transgenic tobacco (Nicotiana tabacum) plants at various developmental stages. The promoter was highly active in the lower parts of a plant and gradually decreased in the upper parts. This vertical gradient was maintained throughout plant growth until the flowering stage when the overall promoter strength decreased significantly in the vegetative organs. However, in various flower organs, the nos promoter activities increased dramatically. Higher activity was observed in calyx, corolla, and stamens although the maximum promoter activity in each organ was found at different stages of flower development. The promoter activity in pistils was low and gradually increased in the ovaries after anthesis. In developing fruits, the nos promoter activity was strongly induced during the mid-stage of embryogenesis. These results indicate that the expression of the nos promoter is developmentally regulated and organ specific in transgenic tobacco plants.

Stimulation of shoot regeneration in Triticum aestivum and Nicotiana plumbaginifolia Viv. tissue cultures using the ethylene inhibitor AgNO3

Silver nitrate effectively promoted shoot regeneration in wheat (Triticum aestivum L.) callus cultures derived from immature embryos. This effect could be observed in both weakly and strongly regenerating cultivars, and in using material from both field and greenhouse grown plants. The role of silver ions as an inhibitor of ethylene action was supported by a reversal of the inhibitory effects of 2,4-D and ethylene on morphogenesis in wheat callus cultures.Enhancement of shoot regeneration by silver nitrate was also observed in callus cultures of non-regenerating or weakly regenerating mutants of Nicotiana plumbaginifolia Viv. derived from cell cultures.

Independent integration and seed-transmission of the TR-DNA of the octopine Ti plasmid pTi Ach5 in Nicotiana plumbaginifolia

After co-cultivation of diploid Nicotiana plumbaginifolia protoplasts with an octopine-type Agrobacterium tumefaciens strain (LBA 4013) putative transformants were selected for hormone-independent growth, and were tested for T-DNA markers. The number of transformants expressing only TL-DNA markers, i.e. phytohormone autotrophy and octopine synthase, was an order of magnitude higher than that of the cell lines which were simultaneously positive for both TL- and TR-DNA markers (the latter being mannopine and agropine). In one transformant, line no. 101, only the TR-DNA markers were found. Not each of the TL-, or TR-DNA markers were expressed in each transformant resulting in a variety of phenotypes. It included the unorganized or the shoot-teratoma type of growth combined with the presence or absence of opines; e.g. agropine was absent from some of the transformants containing its precursor, mannopine. 5-Azacytidine did not induce agropine synthesis in these lines. Southern blot analysis showed that the TR-DNA region coding for agropine synthesis was rearranged or absent in one of these lines. Similar variation in the expression of agropine and mannopine production was observed in transformants obtained with the leucinopine-type strain A281.From line 101 plants could be easily regenerated with the ability to synthesize agropine and mannopine. The segregation in the self-progeny fitted to a 3:1 ratio, indicating that the TR-DNA was carried by a single chromosome. The Southern blot analysis showed that only opine-positive plants contained TR-DNA. It also confirmed the absence of the TL-DNA, demonstrating the independent integration of the TR-region of the octopine-type Ti plasmid pTi Ach5.

Co-segregation of nitrate-reductase activity and normal regeneration ability in selfed sibs of Nicotiana plumbaginifolia somatic hybrids, heterozygotes for nitrate-reductase deficiency

The nitrate-reductase (NR) defective cell lines of Nicotiana plumbaginifolia isolated in our laboratory could not be regenerated into plants on the standard medium (Márton et al. 1982 a). The normal regeneration potential, however, was restored in somatic hybrids obtained by fusing the NR(-) (green) lines with a pigment deficient (P(-)), but NR(+) line, A28. Somatic hybrid plants were fertile in two combinations (A28 + NA9 and A28 + NX9). As expected, segregation for NR(-) and P(-) was found after selfing the somatic F1 (SF1) obtained by protoplast fusion, and in the F2. The variable segregation ratios are explained by chromosome abnormalities. Co-segregation of the NR(-) phenotype and the altered response to shoot induction on standard medium suggest the involvement of the nitrate-assimilatory pathway in determining shoot regeneration ability.