Light influences cytokinin biosynthesis and sensing in Nostoc (cyanobacteria).

Cytokinins are an important group of plant hormones that are also found in other organisms, including cyanobacteria. While various aspects of cytokinin function and metabolism are well understood in plants, the information is limited for cyanobacteria. In this study, we first experimentally confirmed a prenylation of tRNA by recombinant isopentenyl transferase NoIPT2 from Nostoc sp. PCC 7120, whose encoding gene we previously identified in Nostoc genome along with the gene for adenylate isopentenyl transferase NoIPT1. In contrast to NoIPT2, the transcription of NoIPT1 was strongly activated during the dark period and was followed by an increase in the cytokinin content several hours later in the light period. Dominant cytokinin metabolites detected at all time points were free bases and monophosphates of isopentenyladenine and cis-zeatin, while N-glucosides were not detected at all. Whole transcriptome differential expression analysis of cultures of the above Nostoc strain treated by cytokinin compared to untreated controls indicated that cytokinin together with light trigger expression of several genes related to signal transduction, including two-component sensor histidine kinases and two-component hybrid sensors and regulators. One of the affected histidine kinases with a cyclase/histidine kinase-associated sensory extracellular domain similar to the cytokinin-binding domain in plant cytokinin receptors was able to modestly bind isopentenyladenine. The data show that the genetic disposition allows Nostoc not only to produce free cytokinins and prenylate tRNA but also modulate the cytokinin biosynthesis in response to light, triggering complex changes in sensing and regulation.

Recent advances in molecular farming using monocot plants.

Heterologous synthesis of proteins or peptides in plant-based systems, referred to as plant molecular farming, is a practical and safe approach for the large-scale and cost-effective production of therapeutic biomolecules. In this context, monocotyledonous plants, and especially cereals, have been considered attractive vehicles for producing high-value recombinant proteins. The endosperm, as the largest grain storage compartment, offers an appropriate environment for long-lasting protein accumulation. During the last decades, fascinating progress has been achieved in the gene transfer technology and genetic manipulation of the monocot crops using either Agrobacterium tumefaciens or direct gene transfer by biolistic methods. Our group has recently expressed biologically active recombinant human peptide cathelicidin in barley grains using endosperm-specific promoter and brought such engineered lines to field cultivation under current EU regulations for genetically modified organisms. This article reviews the most recent advances and strategies for the production of biopharmaceutical proteins in transgenic monocots, highlighting various aspects involved in recombinant protein accumulation in grains, and discussing current bottlenecks and perspectives for the biosynthesis of therapeutic molecules using different monocot plant platforms.

Degradation of cytokinins by maize cytokinin dehydrogenase is mediated by free radicals generated by enzymatic oxidation of natural benzoxazinones.

Hydroxamic acid 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-one (DIMBOA) was isolated from maize phloem sap as a compound enhancing the degradation of isopentenyl adenine by maize cytokinin dehydrogenase (CKX), after oxidative conversion by either laccase or peroxidase. Laccase and peroxidase catalyze oxidative cleavage of DIMBOA to 4-nitrosoresorcinol-1-monomethyl ether (coniferron), which serves as a weak electron acceptor of CKX. The oxidation of DIMBOA and coniferron generates transitional free radicals that are used by CKX as effective electron acceptors. The function of free radicals in the CKX-catalyzed reaction was also verified with a stable free radical of 2,2'-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid. Application of exogenous cytokinin to maize seedlings resulted in an enhanced benzoxazinoid content in maize phloem sap. The results indicate a new function for DIMBOA in the metabolism of the cytokinin group of plant hormones.

Evolution of cytokinin biosynthesis and degradation.

Cytokinin hormones are important regulators of development and environmental responses of plants that execute their action via the molecular machinery of signal perception and transduction. The limiting step of the whole process is the availability of the hormone in suitable concentrations in the right place and at the right time to interact with the specific receptor. Hence, the hormone concentrations in individual tissues, cells, and organelles must be properly maintained by biosynthetic and metabolic enzymes. Although there are merely two active cytokinins, isopentenyladenine and its hydroxylated derivative zeatin, a variety of conjugates they may form and the number of enzymes/isozymes with varying substrate specificity involved in their biosynthesis and conversion gives the plant a variety of tools for fine tuning of the hormone level. Recent genome-wide studies revealed the existence of the respective coding genes and gene families in plants and in some bacteria. This review summarizes present knowledge on the enzymes that synthesize cytokinins, form cytokinin conjugates, and carry out irreversible elimination of the hormones, including their phylogenetic analysis and possible variations in different organisms.

Biochemical and Structural Aspects of Cytokinin Biosynthesis and Degradation in Bacteria.

It has been known for quite some time that cytokinins, hormones typical of plants, are also produced and metabolized in bacteria. Most bacteria can only form the tRNA-bound cytokinins, but there are examples of plant-associated bacteria, both pathogenic and beneficial, that actively synthesize cytokinins to interact with their host. Similar to plants, bacteria produce diverse cytokinin metabolites, employing corresponding metabolic pathways. The identification of genes encoding the enzymes involved in cytokinin biosynthesis and metabolism facilitated their detailed characterization based on both classical enzyme assays and structural approaches. This review summarizes the present knowledge on key enzymes involved in cytokinin biosynthesis, modifications, and degradation in bacteria, and discusses their catalytic properties in relation to the presence of specific amino acid residues and protein structure.

Overexpression of Trp-related genes in Claviceps purpurea leading to increased ergot alkaloid production.

The parasitic fungus Claviceps purpurea has been used for decades by the pharmaceutical industry as a valuable producer of ergot alkaloids. As the biosynthetic pathway of ergot alkaloids involves a common precursor L-tryptophan, targeted genetic modification of the related genes may improve production yield. In this work, the S76L mutated version of the trpE gene encoding anthranilate synthase was constitutively overexpressed in the fungus with the aim of overcoming feedback inhibition of the native enzyme by an excess of tryptophan. In another approach, the dmaW gene encoding dimethylallyltryptophan synthase, which produces a key intermediate for the biosynthesis of ergot alkaloids, was also constitutively overexpressed. Each of the above manipulations led to a significant increase (up to 7-fold) in the production of ergot alkaloids in submerged cultures.

An improved in vivo deuterium labeling method for measuring the biosynthetic rate of cytokinins.

An improved method for determining the relative biosynthetic rate of isoprenoid cytokinins has been developed. A set of 11 relevant isoprenoid cytokinins, including zeatin isomers, was separated by ultra performance liquid chromatography in less than 6 min. The iP-type cytokinins were observed to give rise to a previously-unknown fragment at m/z 69; we suggest that the diagnostic (204-69) transition can be used to monitor the biosynthetic rate of isopentenyladenine. Furthermore, we found that by treating the cytokinin nucleotides with alkaline phosphatase prior to analysis, the sensitivity of the detection process could be increased. In addition, derivatization (propionylation) improved the ESI-MS response by increasing the analytes' hydrophobicity. Indeed, the ESI-MS response of propionylated isopentenyladenosine was about 34% higher than that of its underivatized counterpart. Moreover, the response of the derivatized zeatin ribosides was about 75% higher than that of underivatized zeatin ribosides. Finally, we created a web-based calculator (IZOTOP) that facilitates MS/MS data processing and offer it freely to the research community.

New insights into auxin metabolism in Bradyrhizobium japonicum.

Bacterial metabolism of phytohormones includes several processes such as biosynthesis, catabolism, conjugation, hydrolysis and homeostatic regulation. However, only biosynthesis and occasionally catabolism are studied in depth in microorganisms. In this work, we evaluated and reconsidered IAA metabolism in Bradyrhizobiumjaponicum E109, one of the most widely used strains for soybean inoculation around the world. The genomic analysis of the strain showed the presence of several genes responsible for IAA biosynthesis, mainly via indole-3-acetonitrile (IAN), indole-3-acetamide (IAM) and tryptamine (TAM) pathways. However; in vitro experiments showed that IAA is not accumulated in the culture medium in significant amounts. On the contrary, a strong degradation activity was observed after exogenous addition of 0.1 mM of IAA, IBA or NAA to the medium. B. japonicum E109 was not able to grow in culture medium containing IAA as a sole carbon source. In YEM medium, the bacteria degraded IAA and hydrolyzed amino acid auxin conjugates with alanine (IAAla), phenylalanine (IAPhe), and leucine (IAPhe), releasing IAA which was quickly degraded. Finally, the presence of exogenous IAA induced physiological changes in the bacteria such as increased biomass and exopolysaccharide production, as well as infection effectiveness and symbiotic behavior in soybean plants.

Kinetic and chemical analyses of the cytokinin dehydrogenase-catalysed reaction: correlations with the crystal structure.

CKX (cytokinin dehydrogenase) is a flavoprotein that cleaves cytokinins to adenine and the corresponding side-chain aldehyde using a quinone-type electron acceptor. In the present study, reactions of maize (Zea mays) CKX with five different substrates (N6-isopentenyladenine, trans-zeatin, kinetin, p-topolin and N-methyl-isopentenyladenine) were studied. By using stopped-flow analysis of the reductive half-reaction, spectral intermediates were observed indicative of the transient formation of a binary enzyme-product complex between the cytokinin imine and the reduced enzyme. The reduction rate was high for isoprenoid cytokinins that showed formation of a charge-transfer complex of reduced enzyme with bound cytokinin imine. For the other cytokinins, flavin reduction was slow and no charge-transfer intermediates were observed. The binary complex of reduced enzyme and imine product intermediate decays relatively slowly to form an unbound product, cytokinin imine, which accumulates in the reaction mixture. The imine product only very slowly hydrolyses to adenine and an aldehyde derived from the cytokinin N6 side-chain. Mixing of the substrate-reduced enzyme with Cu2+/imidazole as an electron acceptor to monitor the oxidative half-reaction revealed a high rate of electron transfer for this type of electron acceptor when using N6-isopentenyladenine. The stability of the cytokinin imine products allowed their fragmentation analysis and structure assessment by Q-TOF (quadrupole-time-of-flight) MS/MS. Correlations of the kinetic data with the known crystal structure are discussed for reactions with different cytokinins.

Characterization of auxiliary iron-sulfur clusters in a radical S-adenosylmethionine enzyme PqqE from Methylobacterium extorquens AM1.

PqqE is a radical S-adenosyl-l-methionine (SAM) enzyme that catalyzes the initial reaction of pyrroloquinoline quinone (PQQ) biosynthesis. PqqE belongs to the SPASM (subtilosin/PQQ/anaerobic sulfatase/mycofactocin maturating enzymes) subfamily of the radical SAM superfamily and contains multiple Fe-S clusters. To characterize the Fe-S clusters in PqqE from Methylobacterium extorquens AM1, Cys residues conserved in the N-terminal signature motif (CX 3 CX 2C) and the C-terminal seven-cysteine motif (CX 9-15 GX 4 CX n CX 2 CX 5 CX 3 CX n C; n = an unspecified number) were individually or simultaneously mutated into Ser. Biochemical and Mössbauer spectral analyses of as-purified and reconstituted mutant enzymes confirmed the presence of three Fe-S clusters in PqqE: one [4Fe-4S]2+ cluster at the N-terminal region that is essential for the reductive homolytic cleavage of SAM into methionine and 5'-deoxyadenosyl radical, and one each [4Fe-4S]2+ and [2Fe-2S]2+ auxiliary clusters in the C-terminal SPASM domain, which are assumed to serve for electron transfer between the buried active site and the protein surface. The presence of [2Fe-2S]2+ cluster is a novel finding for radical SAM enzyme belonging to the SPASM subfamily. Moreover, we found uncommon ligation of the auxiliary [4Fe-4S]2+ cluster with sulfur atoms of three Cys residues and a carboxyl oxygen atom of a conserved Asp residue.

PqqE from Methylobacterium extorquens AM1: a radical S-adenosyl-l-methionine enzyme with an unusual tolerance to oxygen.

Methylobacterium extorquens AM1 is an aerobic facultative methylotroph known to secrete pyrroloquinoline quinone (PQQ), a cofactor of a number of bacterial dehydrogenases, into the culture medium. To elucidate the molecular mechanism of PQQ biosynthesis, we are focusing on PqqE which is believed to be the enzyme catalysing the first reaction of the pathway. PqqE belongs to the radical S-adenosyl-l-methionine (SAM) superfamily, in which most, if not all, enzymes are very sensitive to dissolved oxygen and rapidly inactivated under aerobic conditions. We here report that PqqE from M. extorquens AM1 is markedly oxygen-tolerant; it was efficiently expressed in Escherichia coli cells grown aerobically and affinity-purified to near homogeneity. The purified and reconstituted PqqE contained multiple (likely three) iron-sulphur clusters and showed the reductive SAM cleavage activity that was ascribed to the consensus [4Fe-4S](2+) cluster bound at the N-terminus region. Mössbauer spectrometric analyses of the as-purified and reconstituted enzymes revealed the presence of [4Fe-4S](2+) and [2Fe-2S](2+) clusters as the major forms with the former being predominant in the reconstituted enzyme. PqqE from M.extorquens AM1 may serve as a convenient tool for studying the molecular mechanism of PQQ biosynthesis, avoiding the necessity of establishing strictly anaerobic conditions.

Cytokinin metabolism in maize: Novel evidence of cytokinin abundance, interconversions and formation of a new trans-zeatin metabolic product with a weak anticytokinin activity.

Cytokinins (CKs) are an important group of phytohormones. Their tightly regulated and balanced levels are essential for proper cell division and plant organ development. Here we report precise quantification of CK metabolites and other phytohormones in maize reproductive organs in the course of pollination and kernel maturation. A novel enzymatic activity dependent on NADP(+) converting trans-zeatin (tZ) to 6-(3-methylpyrrol-1-yl)purine (MPP) was detected. MPP shows weak anticytokinin properties and inhibition of CK dehydrogenases due to their ability to bind to an active site in the opposite orientation than substrates. Although the physiological significance of tZ side-chain cyclization is not anticipated as the MPP occurrence in maize tissue is very low, properties of the novel CK metabolite indicate its potential for utilization in plant in vitro tissue culture. Furthermore, feeding experiments with different isoprenoid CKs revealed distinct preferences in glycosylation of tZ and cis-zeatin (cZ). While tZ is preferentially glucosylated at the N9 position, cZ forms mainly O-glucosides. Since O-glucosides, in contrast to N9-glucosides, are resistant to irreversible cleavage catalyzed by CK dehydrogenases, the observed preference of maize CK glycosyltransferases to O-glycosylate zeatin in the cis-position might be a reason why cZ derivatives are over-accumulated in different maize tissues and organs.

Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors.

The catalytic reaction of cytokinin oxidase/dehydrogenase (EC 1.5.99.12) was studied in detail using the recombinant flavoenzyme from maize. Determination of the redox potential of the covalently linked flavin cofactor revealed a relatively high potential dictating the type of electron acceptor that can be used by the enzyme. Using 2,6-dichlorophenol indophenol, 2,3-dimethoxy-5-methyl-1,4-benzoquinone or 1,4-naphthoquinone as electron acceptor, turnover rates with N6-(2-isopentenyl)adenine of approx. 150 s(-1) could be obtained. This suggests that the natural electron acceptor of the enzyme is quite probably a p-quinone or similar compound. By using the stopped-flow technique, it was found that the enzyme is rapidly reduced by N6-(2-isopentenyl)adenine (k(red)=950 s(-1)). Re-oxidation of the reduced enzyme by molecular oxygen is too slow to be of physiological relevance, confirming its classification as a dehydrogenase. Furthermore, it was established for the first time that the enzyme is capable of degrading aromatic cytokinins, although at low reaction rates. As a result, the enzyme displays a dual catalytic mode for oxidative degradation of cytokinins: a low-rate and low-substrate specificity reaction with oxygen as the electron acceptor, and high activity and strict specificity for isopentenyladenine and analogous cytokinins with some specific electron acceptors.

Biochemical Characterization of Putative Adenylate Dimethylallyltransferase and Cytokinin Dehydrogenase from Nostoc sp. PCC 7120.

Cytokinins, a class of phytohormones, are adenine derivatives common to many different organisms. In plants, these play a crucial role as regulators of plant development and the reaction to abiotic and biotic stress. Key enzymes in the cytokinin synthesis and degradation in modern land plants are the isopentyl transferases and the cytokinin dehydrogenases, respectively. Their encoding genes have been probably introduced into the plant lineage during the primary endosymbiosis. To shed light on the evolution of these proteins, the genes homologous to plant adenylate isopentenyl transferase and cytokinin dehydrogenase were amplified from the genomic DNA of cyanobacterium Nostoc sp. PCC 7120 and expressed in Escherichia coli. The putative isopentenyl transferase was shown to be functional in a biochemical assay. In contrast, no enzymatic activity was detected for the putative cytokinin dehydrogenase, even though the principal domains necessary for its function are present. Several mutant variants, in which conserved amino acids in land plant cytokinin dehydrogenases had been restored, were inactive. A combination of experimental data with phylogenetic analysis indicates that adenylate-type isopentenyl transferases might have evolved several times independently. While the Nostoc genome contains a gene coding for protein with characteristics of cytokinin dehydrogenase, the organism is not able to break down cytokinins in the way shown for land plants.

Acetic acid bacteria: A group of bacteria with versatile biotechnological applications.

Acetic acid bacteria are gram-negative obligate aerobic bacteria assigned to the family Acetobacteraceae of Alphaproteobacteria. They are members of the genera Acetobacter, Gluconobacter, Gluconacetobacter, Acidomonas, Asaia, Kozakia, Swaminathania, Saccharibacter, Neoasaia, Granulibacter, Tanticharoenia, Ameyamaea, Neokomagataea, and Komagataeibacter. Many strains of Acetobacter and Komagataeibacter have been known to possess high acetic acid fermentation ability as well as the acetic acid and ethanol resistance, which are considered to be useful features for industrial production of acetic acid and vinegar, the commercial product. On the other hand, Gluconobacter strains have the ability to perform oxidative fermentation of various sugars, sugar alcohols, and sugar acids leading to the formation of several valuable products. Thermotolerant strains of acetic acid bacteria were isolated in order to serve as the new strains of choice for industrial fermentations, in which the cooling costs for maintaining optimum growth and production temperature in the fermentation vessels could be significantly reduced. Genetic modifications by adaptation and genetic engineering were also applied to improve their properties, such as productivity and heat resistance.

Biochemical characterization of the maize cytokinin dehydrogenase family and cytokinin profiling in developing maize plantlets in relation to the expression of cytokinin dehydrogenase genes.

The cytokinin dehydrogenases (CKX; EC 1.5.99.12) are a protein family that maintains the endogenous levels of cytokinins in plants by catalyzing their oxidative degradation. The CKX family in maize (Zea mays L.) has thirteen members, only two of which--ZmCKX1 and ZmCKX10--have previously been characterized in detail. In this study, nine further maize CKX isoforms were heterologously expressed in Escherichia coli, purified by affinity and ion-exchange chromatography and biochemically characterized. ZmCKX6 and ZmCKX9 could only be expressed successfully after the removal of putative sequence-specific vacuolar sorting signals (LLPT and LPTS, respectively), suggesting that these proteins are localized to the vacuole. Substrate specificity analyses revealed that the CKX isoforms can be grouped into two subfamilies: members of the first strongly prefer cytokinin free bases while members of the second degrade a broad range of substrates. The most active isoform was found to be ZmCKX1. One of the studied isoforms, ZmCKX6, seemed to encode a nonfunctional enzyme due to a mutation in a conserved HFG protein domain at the C-terminus. Site-directed mutagenesis experiments revealed that this domain is essential for CKX activity. The roles of the maize CKX enzymes in the development of maize seedlings during the two weeks immediately after radicle emergence were also investigated. It appears that ZmCKX1 is a key regulator of active cytokinin levels in developing maize roots. However, the expression of individual CKX isoforms in the shoots varied and none of them seemed to have strong effects on the cytokinin pool.

Parasitic fungus Claviceps as a source for biotechnological production of ergot alkaloids.

Ergot alkaloids produced by the fungus Claviceps parasitizing on cereals, include three major groups: clavine alkaloids, d-lysergic acid and its derivatives and ergopeptines. These alkaloids are important substances for the pharmatech industry, where they are used for production of anti-migraine drugs, uterotonics, prolactin inhibitors, anti-Parkinson agents, etc. Production of ergot alkaloids is based either on traditional field cultivation of ergot-infected rye or on submerged cultures of the fungus in industrial fermentation plants. In 2010, the total production of these alkaloids in the world was about 20,000 kg, of which field cultivation contributed about 50%. This review covers the recent advances in understanding of the genetics and regulation of biosynthesis of ergot alkaloids, focusing on possible applications of the new knowledge to improve the production yield.

Function of plant defense secondary metabolite in cytokinin degradation.

The flavoenzyme cytokinin dehydrogenase (CKX) catalyzes an irreversible deactivation of plant hormones cytokinins through oxidative cleavage of the cytokinin side chain to yield adenine or adenosine and an aldehyde. In the catalytic cycle of CKX, the cytokinin-reduced flavin cofactor is reoxidized by a suitable electron acceptor. We have recently demonstrated that the oxidation products of natural hydroxamic acid 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) function as effective electron acceptors of apoplastic CKX from maize. The stable oxidation product of DIMBOA reacting with peroxidase or laccase was identified as 4-nitrosoresorcinol 1-monomethyl ether (coniferron), which, however, is only a weak electron acceptor of CKX. Further analyses suggested formation of transient free radicals that were estimated to reoxidize the cytokinin-reduced flavin cofactor of CKX with the rates comparable to those of flavin reduction.

Vacuolar and cytosolic cytokinin dehydrogenases of Arabidopsis thaliana: heterologous expression, purification and properties.

The catabolism of cytokinins is a vital component of hormonal regulation, contributing to the control of active forms of cytokinins and their cellular distribution. The enzyme catalyzing the irreversible cleavage of N(6)-side chains from cytokinins is a flavoprotein classified as cytokinin dehydrogenase (CKX, EC 1.5.99.12). CKXs also show low cytokinin oxidase activity, but molecular oxygen is a comparatively poor electron acceptor. The CKX gene family of Arabidopsis thaliana comprises seven members. Four code for proteins secreted to the apoplast, the remainder are not secreted. Two are targeted to the vacuoles and one is restricted to the cytosol. This study presents the purification and characterization of each of these non-secreted CKX enzymes and substrate specificities are discussed with respect to their compartmentation. Vacuolar enzymes AtCKX1 and AtCKX3 were produced in Pichia pastoris and cytosolic enzyme AtCKX7 was expressed in Escherichia coli. The recombinant proteins were purified by column chromatography. All enzymes preferred synthetic electron acceptors over oxygen, namely potassium ferricyanide and 2,3-dimetoxy-5-methyl-1,4-benzoquinone (Q(0)). In slightly acidic conditions (pH 5.0), N(6)-(2-isopentenyl)adenine 9-glucoside (iP9G) was the best substrate for AtCKX1 and AtCKX7, whereas AtCKX3 preferentially degraded N(6)-(2-isopentenyl)adenine 9-riboside-5'-monophosphate (iPMP). Moreover, vacuolar AtCKX enzymes in certain conditions degraded N(6)-(2-isopentenyl)adenine di- and triphosphates two to five times more effectively than its monophosphate.

Tissue localization of cytokinin dehydrogenase in maize: possible involvement of quinone species generated from plant phenolics by other enzymatic systems in the catalytic reaction.

The degradation of cytokinins in plants is controlled by the flavoprotein cytokinin dehydrogenase (EC 1.5.99.12). Cytokinin dehydrogenase from maize showed the ability to use oxidation products of guaiacol, 4-methylcatechol, acetosyringone and several other compounds as electron acceptors. These results led us to explore the cability for indirect production of suitable electron acceptors by different quinone-generating enzymes. The results reported here revealed that the electron acceptors may be generated in vivo from plant phenolics by other enzymatic systems such as peroxidase and tyrosinase/laccase/catechol oxidase. Histochemical localization of cytokinin dehydrogenase by activity staining and immunochemistry using optical and confocal microscopy showed that cytokinin dehydrogenase is most abundant in the aleurone layer of maize kernels and in phloem cells of the seedling shoots. Cytokinin dehydrogenase was confirmed to be present in the apoplast of cells. Co-staining of enzyme activity for laccase, an enzyme poised to function on the cell wall in the apoplast, in those tissues suggests a possible cooperation of the enzymes in cytokinin degradation. Additionally, the presence of precursors for electron acceptors of cytokinin dehydrogenase was detected in phloem exudates collected from maize seedlings, suggestive of an enzymatic capacity to control cytokinin flux through the vasculature. A putative metabolic connection between cytokinin degradation and conversion of plant phenolics by oxidases was proposed.