Expression of a bacterial lysine decarboxylase gene and transport of the protein into chloroplasts of transgenic tobacco

Cadaverine: a lysine catabolite involved in plant growth and development

The cadaverine (Cad) a diamine, imino compound produced as a lysine catabolite is also implicated in growth and development of plants depending on environmental condition. This lysine catabolism is catalyzed by lysine decarboxylase, which is developmentally regulated. However, the limited role of Cad in plants is reported, this review is tempted to focus the metabolism and its regulation, transport and responses, interaction and cross talks in higher plants. The Cad varied presence in plant parts/products suggests it as a potential candidate for taxonomic marker as well as for commercial exploitation along with growth and development.

RNAi-mediated down-regulation of ornithine decarboxylase (ODC) leads to reduced nicotine and increased anatabine levels in transgenic Nicotiana tabacum L

In leaf and root tissues of Nicotiana tabacum L. (common tobacco), nicotine is by far the predominant pyridine alkaloid, with anatabine representing only a minor component of the total alkaloid fraction. The pyrrolidine ring of nicotine is derived from the diamine putrescine, which can be synthesized either directly from ornithine via the action of ODC, or from arginine via a three enzymatic step process, initiated by ADC. Previous studies in this laboratory have shown that antisense-mediated down-regulation of ADC transcript levels has only a minor effect upon the alkaloid profile of transgenic N. tabacum. In the present study, RNAi methodology was used to down-regulate ODC transcript levels in N. tabacum, using both the Agrobacterium rhizogenes-derived hairy root culture system, and also disarmed Agrobacterium tumefaciens to generate intact transgenic plants. We observed a marked effect upon the alkaloid profile of transgenic tissues, with ODC transcript down-regulation leading to reduced nicotine and increased anatabine levels in both cultured hairy roots and intact greenhouse-grown plants. Treatment of ODC-RNAi hairy roots with low levels of the wound-associated hormone methyl jasmonate, or wounding of transgenic plants by removal of apices - both treatments which normally stimulate nicotine synthesis in tobacco - did not restore capacity for normal nicotine synthesis in transgenic tissue but did lead to markedly increased levels of anatabine. We conclude that the ODC mediated route to putrescine plays an important role in determining the normal nicotine:anatabine profile in N. tabacum and is essential in allowing N. tabacum to increase nicotine levels in response to wound-associated stress.

Analysis of expressed sequence tags from the Huperzia serrata leaf for gene discovery in the areas of secondary metabolite biosynthesis and development regulation

Huperzia serrata produces various types of lycopodium alkaloids, especially the huperzine A (HupA) that is a promising drug candidate for Alzheimer's disease. Despite the medicinal importance of H. serrata, little genomic or transcriptomic data are available from the public databases. A cDNA library was thus generated from RNA isolated from the leaves of H. serrata. A total of 4012 clones were randomly selected from the library, and 3451 high-quality expressed sequence tags (ESTs) were assembled to yield 1510 unique sequences with an average length of 712 bp. The majority (79.4%) of the unique sequences were assigned to the putative functions based on the BLAST searches against the public databases. The functions of these unique sequences covered a broad set of molecular functions, biological processes and biochemical pathways according to GO and KEGG assignments. The transcripts involved in the secondary metabolite biosynthesis of alkaloids, terpenoids and flavone/flavonoids, such as cytochrome P450, lysine decarboxylase (LDC), flavanone 3-hydroxylase, squalene synthetase and 2-oxoglutarate 3-dioxygenase, were well represented by 34 unique sequences in this EST dataset. The corresponding peptide sequence of the LDC contained the Pfam 03641 domain and was annotated as a putative LDC. The unique sequences encoding transcription factors, phytohormone biosynthetic enzymes and signaling components were also found in this EST collection. In addition, a total of 501 potential SSR-motif microsatellite loci were identified from the 393 H. serrata leaf unique sequences. This set of non-redundant ESTs and the molecular markers obtained in this study will establish valuable resources for a wide range of applications including gene discovery and identification, genetic mapping and analysis of genetic diversity, cultivar identification and marker-assisted selections in this important medicinal plant.

The sweet potato ADP-glucose pyrophosphorylase gene (ibAGP1) promoter confers high-level expression of the GUS reporter gene in the potato tuber

Molecular farming refers to the process of creating bioengineered plants with the capability of producing potentially valuable products, such as drugs, vaccines, and chemicals. We have investigated the potential of the sweet potato ADP-glucose pyrophosphorylase gene (ibAGP1) promoter and its transit peptide (TP) as an expression system for the mass production of foreign proteins in potato. The ibAGP1 promoter and its TP sequence were transformed into potato along with beta-glucuronidase (GUS) as a reporter gene, and GUS activity was subsequently analyzed in the transgenic potato plants. In tuber tissues, GUS activity in transgenic plants carrying only the ibAGP1 promoter (ibAGP1::GUS) increased up to 15.6-fold compared with that of transgenic plants carrying only the CaMV35S promoter (CaMV35S::GUS). GUS activity in transgenic plants was further enhanced by the addition of the sweetpotato TP to the recombinant vector (ibAGP1::TP::GUS), with tuber tissues showing a 26-fold increase in activity compared with that in the CaMV35S::GUS-transgenic lines. In leaf tissues, the levels of GUS activity found in ibAGP1::GUS-transgenic lines were similar to those in CaMV35S::GUS-lines, but they were significantly enhanced in ibAGP1::TP::GUS-lines. GUS activity gradually increased with increasing tuber diameter in ibAGP1::GUS-transgenic plants, reaching a maximum level when the tuber was 35 mm in diameter. In contrast, extremely elevated levels of GUS activity - up to about 10-fold higher than that found in CaMV35S::GUS-lines - were found in ibAGP1::TP::GUS-transgenic lines at a much earlier stage of tuber development (diameter 4 mm), and these higher levels were maintained throughout the entire tuber developmental stage. These results suggest that the sweetpotato ibAGP1 promoter and its TP are a potentially strong foreign gene expression system that can be used for molecular farming in potato plants.

Dissected effect of a transit peptide of the ADP-glucose pyrophosphorylase gene from sweetpotato (ibAGP2) in increasing foreign protein accumulation

The transit peptide sequence of ibAGP2 (TP2) was found to be capable of targeting protein into the chloroplast in the Arabidopsis protoplasts. TP2 was fused to a beta-glucuronidase (GUS) reporter gene and expressed in Arabidopsis under the control of the ibAGP2 promoter with the aim of dissecting the effect of the transit peptide in elevating foreign protein accumulation in the transgenic plant. beta-glucuronidase protein levels were determined at three different developmental stages and in assorted tissues. TP2 dramatically elevated GUS protein accumulation regardless of developmental stage, but the level of the enhancing effect was developmental stage-dependent. This enhancing effect was strongest at the seedling stage (16-fold) and relatively moderate at the vegetative (tenfold) and reproductive (11-fold) stages. TP2 also elevated GUS protein accumulation to varying degrees (4 to 19-fold) in assorted tissues, with the effect being highest in the primary inflorescence stem and petiole (19-fold) and weakest in the root (fourfold). Although TP2 also increased GUS mRNA levels, the increased levels were not large enough to account for the elevated GUS protein levels, suggesting that the enhancing effect of TP2 does not solely result from increased levels of transcripts. Taken together, our results reveal that the TP2 significantly increased the levels of protein accumulation and that its effectiveness was developmental stage- and tissue-type-dependent in transgenic Arabidopsis. Possible differential targeting efficiencies of different transit peptides are discussed.

A strong constitutive gene expression system derived from ibAGP1 promoter and its transit peptide

To develop a strong constitutive gene expression system, the activities of ibAGP1 promoter and its transit peptide were investigated using transgenic Arabidopsis and a GUS reporter gene. The ibAGP1 promoter directed GUS expression in almost entire tissues including rosette leaf, inflorescence stem, inflorescence, cauline leaf and root, suggesting that the ibAGP1 promoter is a constitutive promoter. GUS expression mediated by ibAGP1 promoter was weaker than that by CaMV35S promoter in all tissue types, but when GUS protein was targeted to plastids with the aid of the ibAGP1 transit peptide, GUS levels increased to higher levels in lamina, petiole and cauline leaf compared to those produced by CaMV35S promoter. The enhancing effect of ibAGP1 transit peptide on the accumulation of foreign protein was tissue-specific; accumulation was high in lamina and inflorescence, but low in root and primary inflorescence stem. The transit peptide effect in the leaves was maintained highly regardless of developmental stages of plants. The ibAGP1 promoter and its transit peptide also directed strong GUS gene expression in transiently expressed tobacco leaves. These results suggest that the ibAGP1 promoter and its transit peptide are a strong constitutive foreign gene expression system for transgenesis of dicot plants.

Stress conditions applied to the interpretation of translation machinery

Gene expression is regulated at the critical steps: a regulatory event occurs at the step which has a critical effect and is responsible for the limiting rate. Enzyme activity can be regulated at several different levels: transcriptional, translational or post-translational. In this review we describe (and illustrate with experimental data) plant stress which induces regulatory mechanisms at the translational and post-translational levels. We found evidence for autorepression regulatory system of ferritin biosynthesis. Based on the knowledge of the molecular mechanism of regulation, we believe that ferritin protects the environment against heavy metal ions and supplements biological system(s) with iron. The quinolizidine alkaloids' (QA) biosynthesis is lysine decarboxylase (LDC)-dependent. The available pool of LDC limits the conversion of lysine to cadaverine. The amount of LDC depends on transcriptional and translational efficiency. However, in the light of the presented data, we have evidence for a post-translational regulatory system, i.e. the activation of LDC from low to high activity enzyme through the conversion from higher to lower molecular weight form. The plant protection system is very efficient. Understanding of the defence systems such as plant response to stress, should provide us with a possibility of applying this knowledge in practice and finding novel applications.

Improved metabolic action of a bacterial lysine decarboxylase gene in tobacco hairy root cultures by its fusion to a rbcS transit peptide coding sequence

The gene of a bacterial lysine decarboxylase (ldc) fused to a rbcS transit peptide coding sequence (tp), and under the control of the CaMV 35S promoter, was expressed in hairy root cultures of Nicotiana tabacum. The fusion of the ldc to the targeting signal sequence improved the performance of the bacterial gene in the plant cells in many respects. Nearly all transgenic hairy root cultures harbouring the 35S-tp-ldc gene contained distinctly higher lysine decarboxylase activity (from 1.5 to 30 pkat LDC per mg protein) than those which had been transformed with constructs in which the gene had been directly cloned behind the CaMV 35S promoter. The higher enzyme activity led to the accumulation of up to 0.7% cadaverine on a dry mass basis. In addition, part of the cadaverine pool was used for increased biosynthesis of anabasine, an alkaloid which was hardly detectable in control cultures. The best line contained anabasine levels of 0.5% dry mass, which could be further be enhanced by feeding of lysine.

Expression of a reporter gene is reduced by a ribozyme in transgenic plants

A chimeric gene encoding a ribozyme under the control of the cauliflower mosaic virus (CaMV) 35S promoter was introduced into transgenic tobacco plants. In vivo activity of this ribozyme, which was designed to cleave npt mRNA, was previously demonstrated by transient expression assays in plant protoplasts. The ribozyme gene was transferred into transgenic tobacco plants expressing an rbcS-npt chimeric gene as an indicator. Five double transformants out of sixteen exhibited a reduction in the amount of active NPT enzyme. To measure the amount of ribozyme produced, in the absence of its target, the ribozyme and target genes were separated by genetic segregation. The steady-state concentrations of ribozyme and target RNA were shown to be similar in the resulting single transformants. Direct evidence for a correlation between reduced npt gene expression and ribozyme expression was provided by crossing a plant containing only the ribozyme gene with a transgenic plant expressing the npt gene under control of the 35S promoter, i.e. the same promoter used to direct ribozyme expression. The expression of npt was reduced in all progeny containing both transgenes. Both steady-state levels of npt mRNA and amounts of active NPT enzyme are decreased. In addition, our data indicate that, at least in stable transformants, a large excess of ribozyme over target is not a prerequisite for achieving a significant reduction in target gene expression.

Metabolic engineering of plant secondary products

Plants interact with their environment by producing a diverse array of secondary metabolites. Many of these compounds are valued for their medicinal, industrial or agricultural properties. Other secondary products are toxic or otherwise undesirable and can reduce the commercial value of crops. Gene transfer technology offers new opportunities to modify directly plant secondary product synthesis through metabolic engineering. This article reviews some of the strategies which have been used to increase or decrease the synthesis of specific plant metabolites, as well as methods for expanding the biosynthetic capabilities of individual species.

Increased production of cadaverine and anabasine in hairy root cultures of Nicotiana tabacum expressing a bacterial lysine decarboxylase gene

Several hairy root cultures of Nicotiana tabacum varieties, carrying two direct repeats of a bacterial lysine decarboxylase (ldc) gene controlled by the cauliflower mosaic virus (CaMV) 35S promoter expressed LDC activity up to 1 pkat/mg protein. Such activity was, for example, sufficient to increase cadaverine levels of the best line SR3/1-K1,2 from ca. 50 micrograms (control cultures) to about 700 micrograms/g dry mass. Some of the overproduced cadaverine of this line was used for the formation of anabasine, as shown by a 3-fold increase of this alkaloid. In transgenic lines with lower LDC activity the changes of cadaverine and anabasine levels were correspondingly lower and sometimes hardly distinguishable from controls. Feeding of lysine to root cultures, even to those with low LDC activity, greatly enhanced cadaverine and anabasine levels, while the amino acid had no or very little effect on controls and LDC-negative lines.