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

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) impedes wound-stress stimulation of anabasine synthesis in Nicotiana glauca

Unlike most Nicotiana species, leaf tissues of the globally significant weed Nicotiana glauca Grah. (Argentinian tree tobacco) contains anabasine as the main component of its alkaloid pool, with concentrations typically increasing several fold in response to wounding of plants. The Δ(1)-piperidinium ring of anabasine is synthesised from cadaverine, via the decarboxylation of lysine, however the identity of the protein catalysing this reaction remains unknown. Recent studies indicate that ornithine decarboxylase (ODC), an enzyme involved in the synthesis of the diamine putrescine, may also possess LDC activity. Previously we found that ODC transcript is markedly up-regulated in leaves of N. glauca in response to wounding. In order to examine the role of ODC in the synthesis of anabasine in N. glauca, transcript levels were constitutively down-regulated in hairy root cultures and transgenic plants via the introduction of a CaMV35S driven ODC-RNAi construct. In addition to the anticipated marked reduction in nicotine concentrations, demonstrating that the ODC-RNAi construct was functioning in vivo, we observed that N. glauca ODC-RNAi hairy root cultures had a significantly diminished capacity to elevate anabasine synthesis in response to treatment with the wound-associated hormone methyl jasmonate, when compared to vector-only controls. We observed also that ODC-RNAi transgenic plants had significantly reduced ability to increase anabasine concentrations following removal of the plant apex. We conclude that ODC does have an important role in enabling N. glauca to elevate levels of anabasine in response to wound-associated stress.

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.

Polyamine biosynthetic diversity in plants and algae

Polyamine biosynthesis in plants differs from other eukaryotes because of the contribution of genes from the cyanobacterial ancestor of the chloroplast. Plants possess an additional biosynthetic route for putrescine formation from arginine, consisting of the enzymes arginine decarboxylase, agmatine iminohydrolase and N-carbamoylputrescine amidohydrolase, derived from the cyanobacterial ancestor. They also synthesize an unusual tetraamine, thermospermine, that has important developmental roles and which is evolutionarily more ancient than spermine in plants and algae. Single-celled green algae have lost the arginine route and are dependent, like other eukaryotes, on putrescine biosynthesis from the ornithine. Some plants like Arabidopsis thaliana and the moss Physcomitrella patens have lost ornithine decarboxylase and are thus dependent on the arginine route. With its dependence on the arginine route, and the pivotal role of thermospermine in growth and development, Arabidopsis represents the most specifically plant mode of polyamine biosynthesis amongst eukaryotes. A number of plants and algae are also able to synthesize unusual polyamines such as norspermidine, norspermine and longer polyamines, and biosynthesis of these amines likely depends on novel aminopropyltransferases similar to thermospermine synthase, with relaxed substrate specificity. Plants have a rich repertoire of polyamine-based secondary metabolites, including alkaloids and hydroxycinnamic amides, and a number of polyamine-acylating enzymes have been recently characterised. With the genetic tools available for Arabidopsis and other model plants and algae, and the increasing capabilities of comparative genomics, the biological roles of polyamines can now be addressed across the plant evolutionary lineage.

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.

Analysis of wound-induced gene expression in Nicotiana species with contrasting alkaloid profiles

We determined the capacity of three Nicotiana (Solanaceae) species with very different alkaloid profiles (Nicotiana sylvestris Speg & Comes, Nicotiana alata Link & Otto and Nicotiana glauca Grah.) to increase their alkaloid contents in both leaf and root tissues following foliage damage. We also investigated the transcriptional responses of genes encoding enzymes important for alkaloid biosynthesis, namely quinolinate phosphoribosyltransferase (QPT), putrescine N-methyltransferase (PMT), ornithine decarboxylase (ODC) and the putative alkaloid biosynthetic gene A622. In response to wounding of foliage in the well studied 'model' species N. sylvestris, a rise, approximately 2-fold, in leaf nicotine levels was observed several days after a 4-5-fold increase in the transcript levels of all genes in the roots. In contrast, leaf tissues of the ornamental tobacco N. alata showed very low levels of any pyridine alkaloid, even when analysed 1 week after wounding, correlating with a general lack of transcript abundance representing any of these genes in leaves or roots following foliage damage. However, addition of methyl jasmonate to cultured roots of N. alata did produce elevated levels of nicotine and anatabine raising the possibility that components of the leaf-root wound signalling system in N. alata are different from those in N. sylvestris. Wounding of the tree tobacco N. glauca, was followed by a 2-fold increase in anabasine levels several days later. This increase followed a large rise in transcript levels of ODC, QPT and A622, though not PMT, in wounded leaves, but not in non-wounded leaves or roots. These data support the hypothesis that N. glauca is able to produce increased anabasine levels following wounding in its foliage, setting it apart from N. sylvestris where induced alkaloid production takes place in roots. We discuss the possibility that increased transcript levels detected by ODC and A622 probes play important roles in anabasine synthesis in N. glauca.

Biotransformations using plant cells, organ cultures and enzyme systems: current trends and future prospects

Plants are valuable sources of a variety of chemicals including drugs, flavours, pigments and agrochemicals. Some of the biochemical reactions occurring in plant cells are complex and cannot be achieved by synthetic routes. In vitro plant cell and organ cultures and plant enzymes act as suitable biocatalysts to perform these complex reactions. A wide variety of chemical compounds including aromatics, steroids, alkaloids, coumarins and terpenoids can undergo biotransformations using plant cells, organ cultures and enzymes. The biocatalyst-mediated reactions are regiospecific and stereospecific. Reaction types include oxidations, reductions, hydroxylations, methylations, acetylations, isomerizations, glycosylations and esterfications. Genetic manipulation approaches to biotransformation offer great potential to express heterologous genes and to clone and overexpress genes for key enzymes. Biotransformation efficiencies can further be improved using molecular techniques involving site-directed mutagenesis and gene manipulation for substrate specificity.

Metabolic engineering of plants for alkaloid production

Alkaloids purified from plants provide many pharmacologically active compounds, including leading chemotherapy drugs. As is generally true of secondary metabolites, overall productivity is low, making commercial production expensive. Alternative production methods remain impractical, leaving the plant as the best source for these valuable chemicals. Recently, significant progress in characterizing the biosynthetic pathways leading to various alkaloids has been made, and a number of relevant genes have been cloned. Metabolic engineering employing such genes provides a promising technology for improved productivity in plant cell cultures, plant tissue cultures, or intact plants. In exploring solutions though, metabolic engineers must be careful to recognize the limitations inherent in designing plant systems.

ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell Biology, Molecular Regulation, and Metabolic Engineering Applications

Recent advances in the cell, developmental, and molecular biology of alkaloid biosynthesis have heightened our appreciation for the complexity and importance of plant secondary pathways. Several biosynthetic genes involved in the formation of tropane, benzylisoquinoline, and terpenoid indole alkaloids have now been isolated. The early events of signal perception, the pathways of signal transduction, and the function of gene promoters have been studied in relation to the regulation of alkaloid metabolism. Enzymes involved in alkaloid biosynthesis are associated with diverse subcellular compartments including the cytosol, vacuole, tonoplast membrane, endoplasmic reticulum, chloroplast stroma, thylakoid membranes, and perhaps unique "biosynthetic" or transport vesicles. Localization studies have shown that sequential alkaloid biosynthetic enzymes can also occur in distinct cell types, suggesting the intercellular transport of pathway intermediates. Isolated genes have also been used to genetically alter the accumulation of specific alkaloids and other plant secondary metabolites. Metabolic modifications include increased indole alkaloid levels, altered tropane alkaloid accumulation, elevated serotonin synthesis, reduced indole glucosinolate production, redirected shikimate metabolism, and increased cell wall-bound tyramine formation. This review discusses the biochemistry, cell biology, molecular regulation, and metabolic engineering of alkaloid biosynthesis in plants.

Active expression of the ubiA gene from E. coli in tobacco: influence of plant ER-specific signal peptides on the expression of a membrane-bound prenyltransferase in plant cells

The ubiA gene from E. coli codes for 4-hydroxybenzoate: polyprenyldiphosphate 3-polyprenyltransferase, an integral membrane protein involved in ubiquinone biosynthesis. This prokaryotic membrane protein was stably expressed in tobacco using Agrobacterium tumefaciens-mediated transformation. Transgenic lines containing a direct fusion of the ubiA structural gene to a 35S-derived promoter gave very low enzyme activity levels (average 0.16 pkat/mg). Inclusion of an N-terminal ER-specific signal peptide from a lectin gene from Phaseolus vulgaris resulted in an average activity of 1.08 pkat/mg in the transgenic tobacco lines. The additional inclusion of a C-terminal HDEL tetrapeptide, responsible for the retention of proteins in the endoplasmic reticulum of eukaryotic cells, increased the activity to 18.6 pkat/mg. When the promotor of this construct was changed from the 35S derivative to the recently described very strong plant promoter (ocs)3mas, the activity increased further to 128.6 pkat/mg. The most active tobacco line showed activities of the introduced enzyme which exceeded those of wild-type E. coli (the source of ubiA) by a factor of 1100. These results demonstrate the efficacy of plant ER-specific signal peptides for the active expression of a prokaryotic membrane protein in plants.

Use of plant cell cultures in biotechnology

Plant cell cultures are being widely used in scientific studies on the physiology, biochemistry and molecular biology of primary and secondary metabolism, developmental regulation and cellular responses to pathogens and stress. In this chapter the significance of plant cell cultures in biotechnology is discussed with special emphasis on commercial production of secondary metabolites and pharmaceuticals, the potential of genetically transformed cell cultures, photosynthetically active cell cultures, production of somatic embryos, and novel assay systems based on the use of plant cells. Future aspects of biotechnical applications with respect to the potentials and limitations of these approaches are assessed, particularly in comparison with the productivity of lower eucaryotes.