How to easily detect plant NADH-glutamate dehydrogenase (GDH) activity? A simple and reliable in planta procedure suitable for tissues, extracts and heterologous microbial systems

Plant NADH glutamate dehydrogenase (GDH) is an intriguing enzyme, since it is involved in different metabolic processes owing to its reversible (anabolic/catabolic) activity and due to the oligomeric nature of the enzyme, that gives rise to several isoforms. The complexity of GDH isoenzymes pattern and the variability of the spatial and temporal localization of the different isoforms have limited our comprehension of the physiological role of GDH in plants. Genetics, immunological, and biochemical approaches have been used until now in order to shed light on the regulatory mechanism that control GDH expression in different plant systems and environmental conditions. We describe here the validation of a simple in planta GDH activity staining procedure, providing evidence that it might be used, with different purposes, to determine GDH expression in plant organs, tissues, extracts and also heterologous systems.

Development of a simple and high-throughput method for detecting aflatoxins production in culture media

AIMS

To develop a simple, high-throughput and inexpensive procedure to detect and quantify aflatoxins into the culture media of growing mycelia.

METHODS AND RESULTS

Fungal conidia (Aspergillus flavus) were inoculated into the wells of a microplate containing 200 μl of different formulations of coconut-derived liquid medium. Time-dependent production of aflatoxins in the culture media was evaluated by a procedure relying on the UV-induced fluorescence emission by the toxin, using a microplate reader. These data were validated by comparison with the outputs of a conventional HPLC-based procedure. Determinations of aflatoxin concentration, according to the fluorimetric procedure, were performed either by withdrawing samples from the plates or by direct 'in situ' readings, the latter method reinforcing the high-throughput feature of the procedure. Fluorescence enhancers (cyclodextrins) did not ameliorate the sensitivity of the procedure to low concentrations of the toxin into the medium. The efficacy of the procedure was also validated by testing the effect on toxin yield of adding an antioxidant agent (α-lipoic acid) to the medium.

CONCLUSIONS

We give evidence that our improved procedure is reliable and suitable to analyse aflatoxin accumulation time course in coconut-derived culture medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that our procedure may profitably be used to give insights into the mechanisms of regulation of mycotoxin production and, consequently, to implement different strategies for the containment of aflatoxin contamination of food and feed commodities.

First and second line mechanisms of cadmium detoxification in the lichen photobiont Trebouxia impressa (Chlorophyta)

"First line" defence mechanisms, such as phytochelatin biosynthesis, and "second line" mechanisms, such as stress protein induction, were investigated in cadmium-exposed cells of Trebouxia impressa Ahmadjian, a green microalgal species that is a common photobiont of the lichen Physcia adscendens (Fr.) H. Olivier. When T. impressa cells were exposed to 0, 9 and 18 microM Cd for 6, 18 and 48 h, glutathione and phytochelatins efficiently protected the cells against Cd damage. By contrast, the highest Cd concentration (36 microM) at the longest exposure-time (48 h) caused marked drops in glutathione and phytochelatin content, several types of ultrastructural damage, and decreases in cell density and total chlorophyll concentration. In this case, induction of stress proteins was observed, but only long after the induction of phytochelatins. Thus, stress proteins could represent a "second line" mechanism to counteract Cd stress, activated when there is a decline in the "first line" mechanism of Cd detoxification given by phytochelatins.

A multiplex RT-PCR approach to detect aflatoxigenic strains of Aspergillus flavus

AIMS

To develop a multiplex reverse transciption-polymerase chain reaction (RT-PCR) protocol to discriminate aflatoxin-producing from aflatoxin-nonproducing strains of Aspergillus flavus.

METHODS AND RESULTS

The protocol was first optimized on a set of strains obtained from laboratory collections and then validated on A. flavus strains isolated from corn grains collected in the fields of the Po Valley (Italy). Five genes of the aflatoxin gene cluster of A. flavus, two regulatory (aflR and aflS) and three structural (aflD, aflO and aflQ), were targeted with specific primers to highlight their expression in mycelia cultivated under inducing conditions for aflatoxins production. 48-h-old cultures expressed the complete set of the genes analysed here whereas 24-h-old ones did not. Genomic PCR (quadruplex PCR) was also performed in parallel using chromosomal DNA extracted from the same set of strains to correlate the integrity of the genes with their expression.

CONCLUSIONS

We show that a good correlation exists between gene expression of the aflatoxin genes, here analysed by multipex RT-PCR, and aflatoxin production, except for one strain that apparently transcribed all the relevant genes but did not produce aflatoxin in the medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first example of the application of a combination of multiplex PCR and RT-PCR approaches to screen a population of A. flavus for the presence of aflatoxigenic and nonaflatoxigenic strains. The proposed protocol will be helpful in evaluating the risk posed by A. flavus in natural environments and might also be a useful tool to monitor its presence during the processing steps of food and feed commodities.

Evaluation of the genotoxicity induced by the fungicide fenarimol in mammalian and plant cells by use of the single-cell gel electrophoresis assay

Fenarimol, a systemic pyrimidine carbinol fungicide, is considered to be not genotoxic or weakly genotoxic, although the available toxicological data are controversial and incomplete. Our results obtained in vitro with leukocytes of two different rodent species (rat and mouse) show that fenarimol affects DNA, as detected by the single-cell gel electrophoresis (SCGE, Comet) assay. This fungicide is able to induce DNA damage in a dose-related manner, with significant effectiveness at 36 nM, but without significant interspecies differences. Simultaneous exposure of rat leukocytes to fenarimol (36-290 nM) and a model genotoxic compound (50 microg/ml bleomycin) produced a supra-additive cytotoxic and genotoxic effect. This supports previous findings suggesting possible co-toxic, co-mutagenic, cancer-promoting and co-carcinogenic potential of fenarimol, and modification of the effects of other xenobiotics found to be influenced by this agrotoxic chemical, with consequent different toxicological events. The potential for DNA strand breaks to act as a biomarker of genetic toxicity in plants in vivo was also considered, in view of the fact that higher plants represent reliable sensors in an ecosystem. Significant DNA breakage was observed in the nuclei of Impatiens balsamina leaves after in vivo treatment with fenarimol (145 nM, 1h). More than 50% of the cells showed such DNA damage.

Cloning of two glutamate dehydrogenase cDNAs from Asparagus officinalis: sequence analysis and evolutionary implications

Two different amplification products, termed c1 and c2, showing a high similarity to glutamate dehydrogenase sequences from plants, were obtained from Asparagus officinalis using two degenerated primers and RT-PCR (reverse transcriptase polymerase chain reaction). The genes corresponding to these cDNA clones were designated aspGDHA and aspGDHB. Screening of a cDNA library resulted in the isolation of cDNA clones for aspGDHB only. Analysis of the deduced amino acid (aa) sequence from the full-length cDNA suggests that the gene product contains all regions associated with metabolic function of NAD glutamate dehydrogenase (NAD-GDH). A first phylogenetic analysis including only GDHs from plants suggested that the two GDH genes of A. officinalis arose by an ancient duplication event, pre-dating the divergence of monocots and dicots. Codon usage analysis showed a bias towards A/T ending codons. This tendency is likely due to the biased nucleotide composition of the asparagus genome, rather than to the translational selection for specific codons. Using principal coordinate analysis, the evolutionary relatedness of plant GDHs with homologous sequences from a large spectrum of organisms was investigated. The results showed a closer affinity of plant GDHs to GDHs of thermophilic archaebacterial and eubacterial species, when compared to those of unicellular eukaryotic fungi. Sequence analysis at specific amino acid signatures, known to affect the thermal stability of GDH, and assays of enzyme activity at non-physiological temperatures, showed a greater adaptation to heat-stress conditions for the asparagus and tobacco enzymes compared with the Saccharomyces cerevisiae enzyme.

Comet assay application in environmental monitoring: DNA damage in human leukocytes and plant cells in comparison with bacterial and yeast tests

Urban airborne particulate is a complex mixture of air pollutants, many of which have not been identified. However, short-term mutagenesis tests together with chemicophysical parameter analysis are able to better assess air quality and genotoxic load. The findings of continuous monitoring (January 1991-August 1998) of urban air genotoxicity of a Po Valley town (Italy) on Salmonella typhimurium and Saccharomyces cerevisiae are reported. During this period, various measures (catalytic devices, unleaded fuels, annual vehicle overhaul, etc.) to improve air-dispersed pollutant control were enforced. However, a continuous presence of genotoxic compounds is shown and more qualitative than quantitative changes are evident. We also demonstrate the ability of the Comet assay to detect DNA-damaging agents in airborne particulate samples. We applied the test to human leukocytes and, with major improvements, to plant cells (Allium cepa roots and epigean tissues of Impatiens balsamina). The first findings on human leukocytes confirm the sensitivity of this assay, its peculiarity and its applicability in assessing genotoxicity in environmental samples. The capability of plants to show the response of multicellular organisms to environmental pollutants largely counterbalances a probable lowering in sensitivity. Moreover, application of the Comet test to epigean tissues could be useful in estimating the bioavailability of and genotoxic damage by air pollutants, including volatile compounds (ozone, benzene, nitrogen oxides, etc.) to higher plants.

Isolation and characterization of two cDNA clones encoding for glutamate dehydrogenase in Nicotiana plumbaginifolia

We have isolated two full length cDNA clones encoding Nicotiana plumbaginifolia NADH-glutamate dehydrogenase. Both clones share amino acid boxes of homology corresponding to conserved GDH catalytic domains and putative mitochondrial targeting sequence. One clone shows a putative EF-hand loop. The level of the two transcripts is affected differently by carbon source.

Linkage Arrangement of RFLP loci in progenies from crosses between doubled haploid Asparagus officinalis L. clones

A preliminary genetic map of the dioecious species Asparagus officinalis L. (2n = 20) has been constructed on the basis of restriction fragment length polymorphism (RFLP) and isozyme marker data. With DNA samples digested with either EcoRI or HindIII 61 out of 148 probes (41%) identified RFLPs in six families of doubled haploid lines obtained through anther culture. A higher level of polymorphism (65%) was observed when a single family was screened for RFLPs using six distinct restriction enzymes. Segregation analysis of the BC progenies (40-80 individuals) resulted in a 418-cM extended map comprising 43 markers: 39 RFLPs, three isozymes and one morphological (sex). These markers are clustered in 12 linkage groups and four of them exhibited significant deviations from the expected 1∶1 ratio. One isozyme and three RFLP markers were assigned to the sex chromosome.

Isozyme gene markers in the dioecious species Asparagus officinalis L

Extracts from phylloclads of Asparagus officinails were electrophoretically analyzed for isozyme polymorphism. Fourteen enzyme systems were examined using four buffer systems: seven enzymes (acid phosphatase, catalase, glutamate-oxaloacetate transaminase, isocitrate dehydrogenase, malate dehydrogenase, peroxidase, and 6-phosphogluconate dehydrogenase) exhibited clear and consistent banding patterns. Isozyme polymorphism was studied in seven pairs of male and female doubled haploids and in their male F1s. Segregation of polymorphic loci was examined in the backcross progenies and was found to be consistent with a simple Mendelian inheritance in all cases, except for three anodical peroxidases, where two factors have been hypothesized. No linkage could be found between isozyme markers that were segregating in the same cross, but association was demonstrated between one malate dehydrogenase locus and the sex determining genes. The availability of isozyme markers may be useful in breeding and, in particular, the localization of one malate dehydrogenase locus on the sex chromosomes may be helpful in mapping the sex genes.

Analysis of rho mutability in Saccharomyces cerevisiae. I. Effects of mmc and pet-ts alleles

Two additional types of nuclear determinants involved in the control of spontaneous mutability of rho in S. cerevisiae have been identified: mmc and the pet-ts 1, 2, 10, 52 and 53 genes. These genes in their mutated recessive form increase at various extents the number of respiratory deficient cytoplasmic "petite" mutants accumulated. The gene mmc does not affect the respiratory activity and is not temperature-dependent whereas the pet-ts genes determine at the non permissive temperature a respiratory deficient phenotypes even if they affect the mutability of rho at the permissive and at the non permissive temperature. The data here reported suggest that a "replicative complex" exists for the mitochondrial DNA. It is in the purpose of this paper to deal with the relative contribution that mmc and pet-ts gene products have in ensuring the fidelity of this "replicative complex".