Elicitor-Induced l-Tyrosine Decarboxylase from Plant Cell Suspension Cultures : I. Induction and Purification

An Historical Review of Phenylacetic Acid

Plant hormone biology is an ever-evolving field and as such, novel avenues of research must always be sought. Technological and theoretical advancement can also allow for previously dismissed research to yield equally interesting insights into processes now that they are better understood. The auxin phenylacetic acid (PAA) is an excellent example of this. PAA is a plant auxin that also possesses substantial antimicrobial activity. It has a broad distribution and has been studied in bacteria, fungi, algae and land plants. Research on this compound in plants was prominent in the 1980s, where its bioactivity and broad distribution were frequently examined. Unfortunately, due to the strong interest in the quintessential auxin, indole-3-acetic acid (IAA), research on PAA quickly petered out. Recently, several groups have resumed investigations on this hormone in plants, yet, little is known about PAA biology and its physiological role is unclear. PAA biosynthesis from the amino acid Phe invites direct comparisons with previously studied IAA biosynthesis pathways, and recent work has shown that PAA metabolism and signaling appears to be similar to that of IAA. However, given the large gap between previous work and recent investigations, a historical review of this auxin is required to renew our understanding of PAA. Here, previous work on PAA is reassessed in light of recent research in plants and serves as a synthesis of current knowledge on PAA biology.

Investigation of a substrate-specifying residue within Papaver somniferum and Catharanthus roseus aromatic amino acid decarboxylases

Plant aromatic amino acid decarboxylases (AAADs) catalyze the decarboxylation of aromatic amino acids with either benzene or indole rings. Because the substrate selectivity of AAADs is intimately related to their physiological functions, primary sequence data and their differentiation could provide significant physiological insights. However, due to general high sequence identity, plant AAAD substrate specificities have been difficult to identify through primary sequence comparison. In this study, bioinformatic approaches were utilized to identify several active site residues within plant AAAD enzymes that may impact substrate specificity. Next a Papaver somniferum tyrosine decarboxylase (TyDC) was selected as a model to verify our putative substrate-dictating residues through mutation. Results indicated that mutagenesis of serine 372 to glycine enables the P. somniferum TyDC to use 5-hydroxytryptophan as a substrate, and reduces the enzyme activity toward 3,4-dihydroxy-L-phenylalanine (dopa). Additionally, the reverse mutation in a Catharanthus roseus tryptophan decarboxylase (TDC) enables the mutant enzyme to utilize tyrosine and dopa as substrates with a reduced affinity toward tryptophan. Molecular modeling and molecular docking of the P. somniferum TyDC and the C. roseus TDC enzymes provided a structural basis to explain alterations in substrate specificity. Identification of an active site residue that impacts substrate selectivity produces a primary sequence identifier that may help differentiate the indolic and phenolic substrate specificities of individual plant AAADs.

De Novo transcriptome sequencing reveals important molecular networks and metabolic pathways of the plant, Chlorophytum borivilianum

Chlorophytum borivilianum, an endangered medicinal plant species is highly recognized for its aphrodisiac properties provided by saponins present in the plant. The transcriptome information of this species is limited and only few hundred expressed sequence tags (ESTs) are available in the public databases. To gain molecular insight of this plant, high throughput transcriptome sequencing of leaf RNA was carried out using Illumina's HiSeq 2000 sequencing platform. A total of 22,161,444 single end reads were retrieved after quality filtering. Available (e.g., De-Bruijn/Eulerian graph) and in-house developed bioinformatics tools were used for assembly and annotation of transcriptome. A total of 101,141 assembled transcripts were obtained, with coverage size of 22.42 Mb and average length of 221 bp. Guanine-cytosine (GC) content was found to be 44%. Bioinformatics analysis, using non-redundant proteins, gene ontology (GO), enzyme commission (EC) and kyoto encyclopedia of genes and genomes (KEGG) databases, extracted all the known enzymes involved in saponin and flavonoid biosynthesis. Few genes of the alkaloid biosynthesis, along with anticancer and plant defense genes, were also discovered. Additionally, several cytochrome P450 (CYP450) and glycosyltransferase unique sequences were also found. We identified simple sequence repeat motifs in transcripts with an abundance of di-nucleotide simple sequence repeat (SSR; 43.1%) markers. Large scale expression profiling through Reads per Kilobase per Million mapped reads (RPKM) showed major genes involved in different metabolic pathways of the plant. Genes, expressed sequence tags (ESTs) and unique sequences from this study provide an important resource for the scientific community, interested in the molecular genetics and functional genomics of C. borivilianum.

Biochemical evaluation of the decarboxylation and decarboxylation-deamination activities of plant aromatic amino acid decarboxylases

Plant aromatic amino acid decarboxylase (AAAD) enzymes are capable of catalyzing either decarboxylation or decarboxylation-deamination on various combinations of aromatic amino acid substrates. These two different activities result in the production of arylalkylamines and the formation of aromatic acetaldehydes, respectively. Variations in product formation enable individual enzymes to play different physiological functions. Despite these catalytic variations, arylalkylamine and aldehyde synthesizing AAADs are indistinguishable without protein expression and characterization. In this study, extensive biochemical characterization of plant AAADs was performed to identify residues responsible for differentiating decarboxylation AAADs from aldehyde synthase AAADs. Results demonstrated that a tyrosine residue located on a catalytic loop proximal to the active site of plant AAADs is primarily responsible for dictating typical decarboxylase activity, whereas a phenylalanine at the same position is primarily liable for aldehyde synthase activity. Mutagenesis of the active site phenylalanine to tyrosine in Arabidopsis thaliana and Petroselinum crispum aromatic acetaldehyde synthases primarily converts the enzymes activity from decarboxylation-deamination to decarboxylation. The mutation of the active site tyrosine to phenylalanine in the Catharanthus roseus and Papaver somniferum aromatic amino acid decarboxylases changes the enzymes decarboxylation activity to a primarily decarboxylation-deamination activity. Generation of these mutant enzymes enables the production of unusual AAAD enzyme products including indole-3-acetaldehyde, 4-hydroxyphenylacetaldehyde, and phenylethylamine. Our data indicates that the tyrosine and phenylalanine in the catalytic loop region could serve as a signature residue to reliably distinguish plant arylalkylamine and aldehyde synthesizing AAADs. Additionally, the resulting data enables further insights into the mechanistic roles of active site residues.

Gene expression and characterization of a stress-induced tyrosine decarboxylase from Arabidopsis thaliana

Full-length tyrosine decarboxylase cDNA (TyrDC) from Arabidopsis thaliana was identified by rapid amplification of cDNA ends-PCR and isolated by RT-PCR. The TyrDC mRNA was substantially induced by drought stress and wounding, and was considerably decreased by salt stress. By using TyrDC protein fusions with green fluorescent protein, an intracellular localization to the cytoplasm was shown. Recombinant (His)(6)-TyrDC was expressed in Escherichia coli and enzymatically characterized: it exclusively catalyzed the conversion of L-tyrosine to tyramine, exhibited an optimum temperature of 50 degrees C, and an optimum pH at approximately 8.5-9. Recombinant TyrDC protein formed tetramers, as shown by blue native gel electrophoresis.

Complex evolution of orthologous and paralogous decarboxylase genes

The decarboxylases are involved in neurotransmitter synthesis in animals, and in pathways of secondary metabolism in plants. Different decarboxylase proteins are characterized for their different substrate specificities, but are encoded by homologous genes. We study, within a maximum-likelihood framework, the evolutionary relationships among dopa decarboxylase (Ddc), histidine decarboxylase (Hdc) and alpha-methyldopa hypersensitive (amd) in animals, and tryptophan decarboxylase (Wdc) and tyrosine decarboxylase (Ydc) in plants. The evolutionary rates are heterogeneous. There are differences between paralogous genes in the same lineages: 4.13 x 10(-10) nucleotide substitutions per site per year in mammalian Ddc vs. 1.95 in Hdc; between orthologous genes in different lineages, 7.62 in dipteran Ddc vs. 4.13 in mammalian Ddc; and very large temporal variations in some lineages, from 3.7 up to 54.9 in the Drosophila Ddc lineage. Our results are inconsistent with the molecular clock hypothesis.

Analysis of promoters from tyrosine/dihydroxyphenylalanine decarboxylase and berberine bridge enzyme genes involved in benzylisoquinoline alkaloid biosynthesis in opium poppy

Tyrosine/dihydroxyphenylalanine decarboxylase (TYDC) and the berberine bridge enzyme (BBE) represent the entry point and a key branch point, respectively, in the biosynthesis of benzylisoquinoline alkaloids in select species of the Papaveraceae and Fumariaceae. Genomic clones for tydc7 and bbe1 from opium poppy (Papaver somniferum L.) were isolated. Deletion analysis of tydc7 and bbe1 5'-flanking regions revealed the location of putative regulatory domains necessary for expression of the beta-glucuronidase (gus) reporter gene in a transient assay system based on the microprojectile bombardment of cultured opium poppy cells. A 105-nucleotide region between -393 and -287 of the tydc7 5'-flanking region, and a 155-nucleotide region between -355 and -200 of the bbe1 5'-flanking region, were found to be essential for promoter activity. RNA gel blot analysis showed that tydc7 and bbe1 expression is induced in cultured opium poppy cells in response to wounding or treatment with a pathogen-derived elicitor. Time-courses for the induction of tydc7 and bbe1 mRNAs in wounded cells were nearly identical to those for GUS activity in cells bombarded with select promoter-gus constructs when the -393 to -287 region of tydc7, or the -355 to -200 region of bbe1, was present. Our data suggest that the wound signal caused by the entry of DNA-coated microcarriers into opium poppy cells was sufficient to induce tydc7 and bbe1 promoter activity, and that wound-responsive regulatory elements are located within domains identified by deletion analysis.

Uncoupled defense gene expression and antimicrobial alkaloid accumulation in elicited opium poppy cell cultures

Treatment of opium poppy (Papaver somniferum L.) cell cultures with autoclaved mycelial homogenates of Botrytis sp. resulted in the accumulation of sanguinarine. Elicitor treatment also caused a rapid and transient induction in the activity of tyrosine/dopa decarboxylase (TYDC, EC 4.1.1.25), which catalyzes the conversion of L-tyrosine and L-dopa to tyramine and dopamine, respectively, the first steps in sanguinarine biosynthesis. TYDC genes were differentially expressed in response to elicitor treatment. TYDC1-like mRNA levels were induced rapidly but declined to near baseline levels within 5 h. In contrast, TYDC2-like transcript levels increased more slowly but were sustained for an extended period. Induction of TYDC mRNAs preceded that of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) mRNAs. An elicitor preparation from Pythium aphanidermatum was less effective in the induction of TYDC mRNA levels and alkaloid accumulation; however, both elicitors equally induced accumulation of PAL transcripts. In contrast, treatment with methyl jasmonate resulted in an induction of TYDC but not PAL mRNAs. The calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide and the protein kinase inhibitor staurosporine partially blocked the fungal elicitor-induced accumulation of sanguinarine. However, only staurosporine and okadaic acid, an inhibitor of protein phosphatases 1 and 2A, blocked the induction of TYDC1-like transcript levels, but they did not block the induction of TYDC2-like or PAL transcript levels. These data suggest that activation mechanisms for PAL, TYDC, and some later sanguinarine biosynthetic enzymes are uncoupled.

Stimulation of berberine secretion and growth in cell cultures of Thalictrum minus

An endo-pectate lyase (PL; EC 4.2.2.2), originally cloned fiom the phytopathogenic bacterium Erwinia chrysanthemi EC16, was expressed in recA (-) E. coli strain DK1, purified to a single band by isoelectric focusing and used to induce berberine production in established plant suspension cultures of Thalictrum minus L. subsp. saxatile. Addition of 10(-9)M pectate lyase c (PLc) stimulated berberine production and enhanced secretion of the alkaloid into the medium. A lower concentration of PLc, 10(-11)M, stimulated a transient two-fold increase in cell growth rate relative to untreated cultures. Parallel changes in L-phenylalanine ammonia lyase (PAL; EC 4.3.1.5) activity with the rate of berberine synthesis and the inverse relationship between cell growth and berberine synthesis imply that berberine synthesis is stress-related in this cell line.

Induction of berberine biosynthesis by cytokinins in Thalictrum minus cell suspension cultures

Production of berberine could be induced by adding 6-benzylaminopurine (BAP) to Thalictrum minus cells, cultured in suspension in a medium containing 2,4-dichlorophenoxyacetic acid (2,4-D), early in the growth cycle. In the presence of BAP, the precursor, L-tyrosine, was rapidly converted into berberine which was then released into the medium, whereas substantial amounts of the intermediates, tyramine and dopamine, accumulated in non-berberine-producing cells grown in the same 2,4-D-containing medium without BAP. These results suggest that BAP activated enzymatic reactions subsequent to the formation of the amines in the biosynthesis of berberine.