Complementation in somatic hybrids indicates four types of nitrate reductase deficient lines in Nicotiana plumbaginifolia

Melatonin supplementation combats nickel-induced phytotoxicity in Trigonella foenum-graecum L. plants through metal accumulation reduction, upregulation of NO generation, antioxidant defence machinery and secondary metabolites

Nickel (Ni) at a toxic level (80 mg kg-1 of soil) adversely affects the crop performance of fenugreek (Trigonella foenum-graecum L.). Melatonin (MEL), a potent plant growth regulator, is ascribed to offer promising roles in heavy metal stress alleviation. In this study, different doses viz. 0, 25, 50, 75 and 100 μM of MEL were administered to plants through foliage under normal and Ni-stress conditions. The experiment unveiled positive roles of MEL in enhancing root-shoot lengths, fresh-dry weights, seed yield and restoring photosynthetic efficiency assessed in terms of higher Fv/Fm, YII, qP, and lower NPQ values in plants exposed to Ni (80 mg kg-1). MEL supplementation (at 75 μM) effectively restricted Ni accumulation and regulated oxidative stress via modulation of MDA, O2-, H2O2 and NO generation, most prominently. Besides, MEL at 75 μM more conspicuously perked up the activities of antioxidant enzymes like SOD, POX, CAT and APX by 15.7, 20.0, 14.5 and 16.5% higher than the Ni-exposed plants for effective ROS scavenging. Likewise, MEL at 75 μM also efficiently counteracted Ni-generated osmotic stress, through an upscaled accumulation of proline (19.6%) along with the enhancement in the concentration of total phenols (13.6%), total tannins (11.2%), total flavonoids (25.5%) and total alkaloids (19.2%) in plant's leaves. Furthermore, under 80 mg kg-1 Ni stress, MEL at 75 μM improved the seed's trigonelline content by 40.1% higher compared to Ni-disturbed plants, upgrading the pharmacological actions of the plant. Thus, the present study deciphers the envisaged roles of MEL in the alleviation of Ni stress in plants to enhance overall crop productivity.

Exogenous abscisic acid fine-tunes heavy metal accumulation and plant's antioxidant defence mechanism to optimize crop performance and secondary metabolite production in Trigonella foenum-graecum L. under nickel stress

Nickel (Ni) contamination of farming soil has become currently a recurring global menace to agriculture crop productivity. The purpose of the present study was to investigate the putative contributions of abscisic acid (ABA) to extemporize Ni tolerance in Trigonella foenum-graecum L. (fenugreek) plants. The outcomes of this study exposed that exogenous supplementation of ABA at 10, 20, 40 and 80 µM considerably enhanced the growth and physiological attributes of fenugreek under 80 mg Ni kg^-1 soil, however, 40 µM of ABA exhibited the best results under normal and Ni-stressed conditions. ABA-mediated Ni tolerance was marked by reductions in Ni accumulation and consequent lowering of reactive oxygen species (ROS) like hydrogen peroxide and superoxide radicals. Contrarily, NO (nitric oxide) level increased in response to ABA application under Ni stress conditions, accompanied by promoted antioxidant activities through improved levels of secondary metabolites, proline, and perked-up ROS-detoxification enzymes activities. Exogenous ABA at 40 µM concentration applied to Ni-exposed plants (80 mg Ni kg^-1 soil) improved the total content of alkaloids, phenolics, flavonoids and tannins by 14.3%, 10.2%, 15.4% and 7.0%, respectively, over Ni-stressed plants alone. Additionally, seed trigonelline content imparting several pharmacological actions to the fenugreek plant exhibited a remarkable escalation upto 3.6 and 2.6 mg g^-1 DW under '40 µM ABA' and '40 µM ABA + 80 mg Ni kg^-1 soil' treatments, respectively. The findings of the study suggest that ABA plays a key role in enhancing the overall performance of the fenugreek crop under excessive Ni stress.

Soil-applied Nickel Generates Differential Responses in Growth, Physiology and Oxidative Metabolism of Fenugreek (Trigonella foenum-graecum L.) Varieties

This work aims to evaluate the potential of nickel (Ni), an essential micronutrient, as an oxidative stress inducer along with associated morphological and biochemical responses in different varieties of fenugreek (Trigonella foenum-graecum L.), a chief economically cultivated crop of India. Varietal differences in crop performance upon exposure to 0, 20, 40, 60 and 80 mg Ni kg^- 1 soil reflects that Ni applied at 20 mg Ni kg^- 1 soil offers growth-promoting effects, improved photosynthesis attributes, carbonic and nitrate reductase activities more profound in PEB followed by AFg2, AFg1 and UM185 variety. This study observed a dose-dependent reduction in all the above parameters. Maximum toxic effects were noticed at 80 mg kg^- 1 Ni, manifested in the form of enhanced H₂O₂ and MDA contents, which were efficiently counteracted by augmentation in proline content, SOD, POX, CAT and APX activities in PEB over other varieties, suggesting that the Ni tolerance in fenugreek varieties can be organized as PEB > AFg2 > AFg1 > UM185.

New Insights into the Transcriptional Regulation of Genes Involved in the Nitrogen Use Efficiency under Potassium Chlorate in Rice (Oryza sativa L.)

Potassium chlorate (KClO3) has been widely used to evaluate the divergence in nitrogen use efficiency (NUE) between indica and japonica rice subspecies. This study investigated the transcriptional regulation of major genes involved in the NUE in rice treated with KClO3, which acts as an inhibitor of the reducing activity of nitrate reductase (NR) in higher plants. A set of two KClO3 sensitive nitrate reductase (NR) and two nitrate transporter (NRT) introgression rice lines (BC2F7), carrying the indica alleles of NR or NRT, derived from a cross between Saeilmi (japonica, P1) and Milyang23 (indica, P2), were exposed to KClO3 at the seedling stage. The phenotypic responses were recorded 7 days after treatment, and samples for gene expression, physiological, and biochemical analyses were collected at 0 h (control) and 3 h after KClO3 application. The results revealed that Saeilmi (P1, japonica) and Milyang23 (P2, indica) showed distinctive phenotypic responses. In addition, the expression of OsNR2 was differentially regulated between the roots, stem, and leaf tissues, and between introgression lines. When expressed in the roots, OsNR2 was downregulated in all introgression lines. However, in the stem and leaves, OsNR2 was upregulated in the NR introgression lines, but downregulation in the NRT introgression lines. In the same way, the expression patterns of OsNIA1 and OsNIA2 in the roots, stem, and leaves indicated a differential transcriptional regulation by KClO3, with OsNIA2 prevailing over OsNIA1 in the roots. Under the same conditions, the activity of NR was inhibited in the roots and differentially regulated in the stem and leaf tissues. Furthermore, the transcriptional divergence of OsAMT1.3 and OsAMT2.3, OsGLU1 and OsGLU2, between NR and NRT, coupled with the NR activity pattern in the roots, would indicate the prevalence of nitrate (NO3¯) transport over ammonium (NH4+) transport. Moreover, the induction of catalase (CAT) and polyphenol oxidase (PPO) enzyme activities in Saeilmi (P1, KClO3 resistant), and the decrease in Milyang23 (P2, KClO3 sensitive), coupled with the malondialdehyde (MDA) content, indicated the extent of the oxidative stress, and the induction of the adaptive response mechanism, tending to maintain a balanced reduction-oxidation state in response to KClO3. The changes in the chloroplast pigments and proline content propose these compounds as emerging biomarkers for assessing the overall plant health status. These results suggest that the inhibitory potential of KClO3 on the reduction activity of the nitrate reductase (NR), as well as that of the genes encoding the nitrate and ammonium transporters, and glutamate synthase are tissue-specific, which may differentially affect the transport and assimilation of nitrate or ammonium in rice.

The historical role of species from the Solanaceae plant family in genetic research

This article evaluates the main contributions of tomato, tobacco, petunia, potato, pepper and eggplant to classical and molecular plant genetics and genomics since the beginning of the twentieth century. Species from the Solanaceae family form integral parts of human civilizations as food sources and drugs since thousands of years, and, more recently, as ornamentals. Some Solanaceous species were subjects of classical and molecular genetic research over the last 100 years. The tomato was one of the principal models in twentieth century classical genetics and a pacemaker of genome analysis in plants including molecular linkage maps, positional cloning of disease resistance genes and quantitative trait loci (QTL). Besides that, tomato is the model for the genetics of fruit development and composition. Tobacco was the major model used to establish the principals and methods of plant somatic cell genetics including in vitro propagation of cells and tissues, totipotency of somatic cells, doubled haploid production and genetic transformation. Petunia was a model for elucidating the biochemical and genetic basis of flower color and development. The cultivated potato is the economically most important Solanaceous plant and ranks third after wheat and rice as one of the world's great food crops. Potato is the model for studying the genetic basis of tuber development. Molecular genetics and genomics of potato, in particular association genetics, made valuable contributions to the genetic dissection of complex agronomic traits and the development of diagnostic markers for breeding applications. Pepper and eggplant are horticultural crops of worldwide relevance. Genetic and genomic research in pepper and eggplant mostly followed the tomato model. Comparative genome analysis of tomato, potato, pepper and eggplant contributed to the understanding of plant genome evolution.

Structural and functional characterization of ferredoxin-NADP+-oxidoreductase using knock-out mutants of Arabidopsis

In Arabidopsis thaliana, the chloroplast-targeted enzyme ferredoxin-NADP+-oxidoreductase (FNR) exists as two isoforms, AtLFNR1 and AtLFNR2, encoded by the genes At5g66190 and At1g20020, respectively. Both isoforms are evenly distributed between the thylakoids and soluble stroma, and they are separated by two-dimensional electrophoresis in four distinct spots, suggesting post-translational modification of both isoforms. To reveal the functional specificity of AtLFNR1, we have characterized the T-DNA insertion mutants with an interrupted At5g66190 gene. Absence of AtLFNR1 resulted in a reduced size of the rosette with pale green leaves, which was accompanied by a low content of chlorophyll and light-harvesting complex proteins. Also the photosystem I/photosystem II (PSI/PSII) ratio was significantly lower in the mutant, but the PSII activity, measured as the F(V)/F(M) ratio, remained nearly unchanged and the excitation pressure of PSII was lower in the mutants than in the wild type. A slow re-reduction rate of P700 measured in the mutant plants suggested that AtLFNR1 is involved in PSI-dependent cyclic electron flow. Impaired function of FNR also resulted in decreased capacity for carbon fixation, whereas nitrogen metabolism was upregulated. In the absence of AtLFNR1, we found AtLFNR2 exclusively in the stroma, suggesting that AtLFNR1 is required for membrane attachment of FNR. Structural modeling supports the formation of a AtLFNR1-AtLFNR2 heterodimer that would mediate the membrane attachment of AtLFNR2. Dimer formation, in turn, might regulate the distribution of electrons between the cyclic and linear electron transfer pathways according to environmental cues.

Differential expression of the arabidopsis nia1 and nia2 genes. cytokinin-induced nitrate reductase activity is correlated with increased nia1 transcription and mrna levels

Nitrate reductase (NR) activity increased up to 14-fold in response to treatment of Arabidopsis thaliana seedlings with the cytokinin benzyladenine. NR induction was observed in seedlings germinated directly on cytokinin-containing medium, seedlings transferred to cytokinin medium, and seedlings grown in soil in which cytokinin was applied directly to the leaves. About the same level of induction was seen in both wild-type and Nia2-deletion mutants, indicating that increased NR activity is related to the expression of the minor NR gene, Nia1. The steady-state Nia1 mRNA level was increased severalfold in both wild-type and mutant seedlings after benzyladenine treatment. Transcript levels of the Nia2 gene, which is responsible for 90% of the NR activity in developing wild-type seedlings, did not show any changes upon cytokinin treatment. Nuclear run-on assays demonstrated that Nia1 gene transcription increased dramatically after cytokinin treatment.

An eight-nucleotide sequence in the potato virus X 3' untranslated region is required for both host protein binding and viral multiplication

Gel retardation and UV-cross-linking techniques were used to demonstrate that two tobacco proteins, with approximate molecular masses of 28 and 32 kDa, bind to a site within the 3' region of potato virus X (PVX) genomic RNA. The protein binding is specific, in that a 50-fold excess of unlabeled probe prevents formation of the complexes but no reduction is observed with a 2,000-fold molar excess of yeast tRNA. Complex formation is inhibited by poly(U) but is relatively unaffected by poly(A), poly(G), or poly(C-I). PVX RNA-host protein complex formation occurs in vitro at salt concentrations up to 400 mM. Deletion mapping indicates that the proteins bind within the 3' untranslated region (UTR) of PVX genomic RNA and that an 8-nucleotide U-rich sequence (5'-UAUUUUCU) is required for the binding. Deletion of the 8-nucleotide U-rich region from the 3' UTR of a sensitive PVX reporter virus that carries the luciferase gene in place of the PVX coat protein gene results in a more than 70,000-fold reduction in luciferase expression in tobacco protoplasts. RNA probes carrying the sequence GCGC in place of the central four contiguous uridines of the 8-nucleotide U-rich motif fail to bind host protein at detectable levels, and the same mutation, when introduced into the PVX reporter virus, eliminates viral multiplication. Mutations of 1 or 2 nucleotides within the same four uridines reduced both binding of host proteins and replication of reporter virus. These results indicate that the 8-nucleotide U-rich motif within the PVX 3' UTR is important for some aspect of viral multiplication and suggest that host protein binding plays a role in the process.

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)

Improved sensitivity of genetic complementation of nitrate reductase deficient mutants via protoplast fusion

An improved rapid assay for complementation testing of mutants of Nicotiana tabacum deficient in nitrate reductase is described. The test is based on measurement of in vivo nitrate reductase activity in 7 to 10 day old cultures derived from fusion-treated protoplast mixtures of the respective mutants as a criterion for complementation. It allows to detect complementing hybrids induced by the conventional droplet fusion technique in small numbers of protoplasts per assay (8×10(4)).

Isolation and characterization of Nicotiana plumbaginifolia nitrate reductase-deficient mutants: genetic and biochemical analysis of the NIA complementation group

Two hundred and eleven nitrate reductase-deficient mutants (NR-) were isolated from mutagenized Nicotiana plumbaginifolia protoplast cultures by chlorate selection and regenerated into plant. More than 40% of these clones were classified as cnx and presumed to be affected in the biosynthesis of the molybdenum cofactor, the remaining clones being classified as nia mutants. A genetic analysis of the regenerated plants confirmed this proportion of nia and cnx clones. All mutants regenerated were found to carry monogenic recessive mutations that impaired growth on nitrate as sole nitrogen source. Mutants propagated by grafting on N. tabacum systematically displayed a chlorotic leaf phenotype. This chlorosis was therefore related to the NR deficiency. The observation of leaves with NR- chlorotic sectors surrounded by NR+ wild-type tissues suggests that an NR deficiency is not corrected by diffusible factors. Periclinal chimeras between wild-type tobacco and the NR- graft were also observed. In this type of chimeric tissue chlorosis was no longer detectable when NR+ cells were in the secondmost (L2) layer, but was still detectable when NR- cells were in the secondmost layer. The genetic analysis of nia mutants revealed that they belong to a single complementation group. However three nia mutants were found to complement some of the other nia mutants. The apoenzyme of nitrate reductase was immunologically detected in several nia mutants but not in other members of this complementation group. Some of the nia mutants, although they were NR-, still displayed methylviologen-nitrate reductase activity at a high level. These data show that the nia complementation group corresponds to the structural gene of nitrate reductase. Some of the mutations affecting this structural gene result in the overproduction of an inactive nitrate reductase, suggesting a feedback regulation of the level of the apoenzyme in the wild type.

Biochemical and genetic comparison of two nitrate reductase-deficient pea mutants disturbed in the cofactor

Two nitrate reductase (NaR)-deficient mutants of pea (Pisum sativum L.), E1 and A300, both disturbed in the molybdenum cofactor function and isolated, respectively, from cv Rondo and cv Juneau, were tested for allelism and were compared in biochemical and growth characteristics. The F1 plants of the cross E1 X A300 possessed NaR and xanthine dehydrogenase (XDH) activities comparable to those of the wild types, indicating that these mutants belong to different complementation groups, representing two different loci. Therefore, mutant E1 represents, besides mutant A300 and the allelic mutants A317 and A334, a third locus governing NaR and is assigned the gene destignation nar 3. In comparison with the wild types, cytochrome c reductase activity was increased in both mutants. The mutants had different cytochrome c reductase distribution patterns, indicating that mutant A300 could be disturbed in the ability to dimerize NaR apoprotein monomers, and mutant E1 in the catalytic function of the molybdenum cofactor. In growth characteristics studied, A300 did not differ from the wild types, whereas fully grown leaves of mutant E1 became necrotic in soil and in liquid media containing nitrate.

Improved in vitro selection of nitrate reductase-deficient mutants of Nicotiana plumbaginifolia

The use of increasing knowledge on regulation of nitrate reductase activity in Nicotiana cell cultures is the basis for the described optimization of in vitro selection for nitrate reductase-deficient mutants by screening for chlorate resistance. Selection was carried out on haploid mesophyll protoplast-derived cell cultures of Nicotiana plumbaginifolia. It is demonstrated that revised selection results in high variant detectability and increased variant confirmability in comparison with the hitherto used selection scheme.

Characterization of a chlorate-hypersensitive, high nitrate reductase Arabidopsis thaliana mutant

A population of A. thaliana, produced by self-fertilization of ethylmethane sulfonate treated plants, was exposed to chlorate in the watering solution, and plants showing early susceptibility symptoms were rescued. Among the progeny lines of these plants five were shown to be repeatably chlorate-hypersusceptible. One of these lines (designated C-4) possessed elevated activity of nitrate reductase (NR). The NR activity of mutant C-4 was higher than that of normal plants throughout the life cycle. Nitrite reductase and glutamine synthetase activities of C-4 were normal, as were chlorate uptake rate and tissue nitrate content. The elevated NR activity apparently was responsible for the chlorate hypersusceptibility of C-4. Inheritance studies of NR indicated that the elevated activity of C-4 was probably controlled by a single recessive allele.

Electrofusion, a simple and reproducible technique in somatic hybridization of Nicotiana plumbaginifolia mutants

Electrofusion of protoplasts from two complementary nitrate reductase deficient mutants of Nicotiana plumbaginifolia has resulted in somatic hybrid lines. Mesophyll protoplasts isolated from the cofactor mutant CNX 20 and fluorescein diacetate stained protoplasts derived from a cell suspension culture of the NA 36 line, being defective in the apoenzyme, were used in the fusion experiments. In total, 594 lines were recovered which could proliferate on a selective medium with nitrate as the sole nitrogen source. This is including 141 putative hybrid lines which were obtained after transfer of 1048 heterokaryons with a micromanipulator one day after electrofusion. The hybrid character of some of the selected lines was confirmed by nitrate reductase activity measurements. Plants were grown from hybrid calli.

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.

Biochemical and genetical characterization of nitrate reductase deficient mutants of Petunia

Four NR(-) lines were selected by their resistance to 100 mM chlorate from X-ray irradiated protoplasts of haploid Petunia hybrida var. Mitchell. The four cell lines were characterized by the presence of xanthine dehydrogenase activity and by complementation tests via protoplast fusion. One mutant (line 1) was classified as defective in the NR apoprotein (tentatively, nia-type) and the other three (lines 2, 3, 4) in the molybdenum cofactor (tentatively, cnx-type). Some NR activity (15 %) could be restored by adding unphysiologically high concentrations of molybdate to the culture medium in two of the cnx-lines (lines 3 and 4). The third cnx-line (line 2) had no NR activity. A complementation analysis via protoplast fusion confirmed that the mutants comprised 3 non-allelic groups. From these results it can be concluded that these NR(-) mutants are recessive and that two of the cnx-mutants (lines 3, 4) are allelic.

Isolation of biochemical mutants using haploid mesophyll protosplasts of Hyoscyamus muticus : IV. Biochemical characterisation of nitrate non-utilizing clones

Four nitrate non-utilizing clones of Hyoscyamus muticus, obtained by a total isolation method, possess all the known characteristics of cnx-type nitrate-reductase-deficient variants: 1) strict dependence on a reduced nitrogen source such as a mixture of amino acids; 2) chlorate resistance; 3) normal nitrate uptake; 4) lack of nitrate-reductase and xanthine-dehydrogenase activities, but presence of cytochrome-c-reductase and nitrite-reductase activities; 5) in vitro nitrate-reductase complementation with a molybdenum cofactor source. Two of the clones (MA-2 and I2D12) are molybdate repairable in vivo, whilst the other two clones (VIC2 and XIVE9) are not.

A heteroplasmic state induced by protoplast fusion is a necessary condition for detecting rearrangements in Nicotiana mitochondrial DNA

Mitochondrial DNA (mtDNA) restriction patterns were studied in mutant, cybrid and somatic hybrid plants regenerated from Nicotiana protoplasts.It has been shown that neither components of the culture media used for protoplast culture and plant regeneration, nor the antibiotics streptomycin and lincomycin used for the mutant selection induced alterations in the mtDNA. No rearrangements were detected in the mtDNA of plants derived from homoplasmic fusions where the mtDNA of the parents was identical as judged by mtDNA restriction patterns.There were rearrangements, however, in the mtDNA of each of the cybrid plants derived from heteroplasmic fusions. Restriction patterns generated by BamHI and SalI restriction endonucleases were different from those of both parents, and were composed of parental and non-parental fragments.

Regeneration of fully nitrate reductase - deficient mutants from protoplast culture of Nicotiana plumbaginifolia (Viviani)

Protoplast-derived colonies of haploid N. plumbaginifolia were selected for by chlorate resistance in media supplemented with casamino acids. Eighty resistant lines were confirmed by a second passage on a higher concentration of chlorate. Frequency of spontaneous mutation ranged from 10(-5) to 10(-6). Fifty of the resistant lines could be regenerated into plants, and 30 were characterized biochemically. Ninety percent were fully deficient for nitrate reductase activity. The lines were further tested for xanthine dehydrogenase activity and subsequently classified as defective in the apoenzyme (nia type, 26 lines) or the cofactor (cnx type, 4 lines). Two groups had been identified up until now within the cnx type by growth tests on high concentrations of molybdate supplied to the medium. Nitrate reductase deficiency was stably and continously expressed in both variant cell cultures and regenerants. Genetic analysis demonstrated that nitrate reductase deficiency was inherited as a single recessive nuclear gene.