Synthesis of C-Glucosylated Octaketide Anthraquinones in Nicotiana benthamiana by Using a Multispecies-Based Biosynthetic Pathway.

Carminic acid is a C-glucosylated octaketide anthraquinone and the main constituent of the natural dye carmine (E120), possessing unique coloring, stability, and solubility properties. Despite being used since ancient times, longstanding efforts to elucidate its route of biosynthesis have been unsuccessful. Herein, a novel combination of enzymes derived from a plant (Aloe arborescens, Aa), a bacterium (Streptomyces sp. R1128, St), and an insect (Dactylopius coccus, Dc) that allows for the biosynthesis of the C-glucosylated anthraquinone, dcII, a precursor for carminic acid, is reported. The pathway, which consists of AaOKS, StZhuI, StZhuJ, and DcUGT2, presents an alternative biosynthetic approach for the production of polyketides by using a type III polyketide synthase (PKS) and tailoring enzymes originating from a type II PKS system. The current study showcases the power of using transient expression in Nicotiana benthamiana for efficient and rapid identification of functional biosynthetic pathways, including both soluble and membrane-bound enzymes.

The endo-arabinanase BcAra1 is a novel host-specific virulence factor of the necrotic fungal phytopathogen Botrytis cinerea.

The plant cell wall is one of the first physical interfaces encountered by plant pathogens and consists of polysaccharides, of which arabinan is an important constituent. During infection, the necrotrophic plant pathogen Botrytis cinerea secretes a cocktail of plant cell-wall-degrading enzymes, including endo-arabinanase activity, which carries out the breakdown of arabinan. The roles of arabinan and endo-arabinanases during microbial infection were thus far elusive. In this study, the gene Bcara1 encoding for a novel α-1,5-L-endo-arabinanase was identified and the heterologously expressed BcAra1 protein was shown to hydrolyze linear arabinan with high efficiency whereas little or no activity was observed against the other oligo- and polysaccharides tested. The Bcara1 knockout mutants displayed reduced arabinanase activity in vitro and severe retardation in secondary lesion formation during infection of Arabidopsis leaves. These results indicate that BcAra1 is a novel endo-arabinanase and plays an important role during the infection of Arabidopsis. Interestingly, the level of Bcara1 transcript was considerably lower during the infection of Nicotiana benthamiana compared with Arabidopsis and, consequently, the ΔBcara1 mutants showed the wild-type level of virulence on N. benthamiana leaves. These results support the conclusion that the expression of Bcara1 is host dependent and is a key determinant of the disease outcome.

Arabidopsis Deficient in Cutin Ferulate encodes a transferase required for feruloylation of ω-hydroxy fatty acids in cutin polyester.

The cuticle is a complex aliphatic polymeric layer connected to the cell wall and covers surfaces of all aerial plant organs. The cuticle prevents nonstomatal water loss, regulates gas exchange, and acts as a barrier against pathogen infection. The cuticle is synthesized by epidermal cells and predominantly consists of an aliphatic polymer matrix (cutin) and intracuticular and epicuticular waxes. Cutin monomers are primarily C(16) and C(18) unsubstituted, ω-hydroxy, and α,ω-dicarboxylic fatty acids. Phenolics such as ferulate and p-coumarate esters also contribute to a minor extent to the cutin polymer. Here, we present the characterization of a novel acyl-coenzyme A (CoA)-dependent acyl-transferase that is encoded by a gene designated Deficient in Cutin Ferulate (DCF). The DCF protein is responsible for the feruloylation of ω-hydroxy fatty acids incorporated into the cutin polymer of aerial Arabidopsis (Arabidopsis thaliana) organs. The enzyme specifically transfers hydroxycinnamic acids using ω-hydroxy fatty acids as acyl acceptors and hydroxycinnamoyl-CoAs, preferentially feruloyl-CoA and sinapoyl-CoA, as acyl donors in vitro. Arabidopsis mutant lines carrying DCF loss-of-function alleles are devoid of rosette leaf cutin ferulate and exhibit a 50% reduction in ferulic acid content in stem insoluble residues. DCF is specifically expressed in the epidermis throughout all green Arabidopsis organs. The DCF protein localizes to the cytosol, suggesting that the feruloylation of cutin monomers takes place in the cytoplasm.

ARAD proteins associated with pectic Arabinan biosynthesis form complexes when transiently overexpressed in planta.

Glycosyltransferase complexes are known to be involved in plant cell wall biosynthesis, as for example in cellulose. It is not known to what extent such complexes are involved in biosynthesis of pectin as well. To address this question, work was initiated on ARAD1 (ARABINAN DEFICIENT 1) and its close homolog ARAD2 of glycosyltransferase family GT47. Using bimolecular fluorescence complementation, Förster resonance energy transfer and non-reducing gel electrophoresis, we show that ARAD1 and ARAD2 are localized in the same Golgi compartment and form homo-and heterodimeric intermolecular dimers when expressed transiently in Nicotiana benthamiana. Biochemical analysis of arad2 cell wall or fractions hereof showed no difference in the monosaccharide composition, when compared with wild type. The double mutant arad1 arad2 had an arad1 cell wall phenotype and overexpression of ARAD2 did not complement the arad1 phenotype, indicating that ARAD1 and ARAD2 are not redundant enzymes. To investigate the cell wall structure of the mutants in detail, immunohistochemical analyses were carried out on arad1, arad2 and arad1 arad2 using the arabinan-specific monoclonal antibody LM13. In roots, the labeling pattern of arad2 was distinct from both that of wild type, arad1 and arad1 arad2. Likewise, in epidermal cell walls of inflorescence stems, LM13 binding differed between arad2 and WILD TYPE, arad1 or arad1 arad2. Altogether, these data show that ARAD2 is associated with arabinan biosynthesis, not redundant with ARAD1, and that the two glycosyltransferases may function in complexes held together by disulfide bridges.

Loss-of-function mutation of REDUCED WALL ACETYLATION2 in Arabidopsis leads to reduced cell wall acetylation and increased resistance to Botrytis cinerea.

Nearly all polysaccharides in plant cell walls are O-acetylated, including the various pectic polysaccharides and the hemicelluloses xylan, mannan, and xyloglucan. However, the enzymes involved in the polysaccharide acetylation have not been identified. While the role of polysaccharide acetylation in vivo is unclear, it is known to reduce biofuel yield from lignocellulosic biomass by the inhibition of microorganisms used for fermentation. We have analyzed four Arabidopsis (Arabidopsis thaliana) homologs of the protein Cas1p known to be involved in polysaccharide O-acetylation in Cryptococcus neoformans. Loss-of-function mutants in one of the genes, designated REDUCED WALL ACETYLATION2 (RWA2), had decreased levels of acetylated cell wall polymers. Cell wall material isolated from mutant leaves and treated with alkali released about 20% lower amounts of acetic acid when compared with the wild type. The same level of acetate deficiency was found in several pectic polymers and in xyloglucan. Thus, the rwa2 mutations affect different polymers to the same extent. There were no obvious morphological or growth differences observed between the wild type and rwa2 mutants. However, both alleles of rwa2 displayed increased tolerance toward the necrotrophic fungal pathogen Botrytis cinerea.

Glucosinolate engineering identifies a gamma-glutamyl peptidase.

Consumption of cruciferous vegetables is associated with reduced risk of developing cancer, a phenomenon attributed to glucosinolates, which are characteristic of these vegetables. We report production of the bioactive benzylglucosinolate in the noncruciferous plant Nicotiana benthamiana through metabolic engineering. The study includes identification of gamma-glutamyl peptidase 1 (GGP1), which substantially increased glucosinolate production by metabolizing an accumulating glutathione conjugate, an activity not previously described for glucosinolate biosynthesis or for proteins containing glutamine amidotransferase domains.

Controlled indole-3-acetaldoxime production through ethanol-induced expression of CYP79B2.

Indole-3-acetaldoxime (IAOx) is a key branching point between primary and secondary metabolism. IAOx serves as an intermediate in the biosynthesis of indole glucosinolates (I-GLSs), camalexin and the plant hormone indole-3-acetic acid (IAA). The cytochrome P450s CYP79B2 and CYP79B3 catalyze the conversion of tryptophan to IAOx. CYP83B1 channels IAOx into I-GLS biosynthesis, CYP71A13 channels IAOx into camalexin biosynthesis, whereas the IAOx-metabolizing enzyme in IAA biosynthesis is not known. In this report, we demonstrate controlled production of I-GLSs by introducing an ethanol (EtOH)-inducible CYP79B2 construct into double (cyp79b2 cyp79b3) or triple (cyp79b2 cyp79b3 cyp83b1) mutant lines. We show EtOH-dependent induction of camalexin and identify a number of candidate IAA homeostasis- or defense-related genes by clustered microarray analysis. The transgenic mutant lines are thus promising tools for elucidating the interplay between primary and secondary metabolism.

Heterologous production of the widely used natural food colorant carminic acid in Aspergillus nidulans.

The natural red food colorants carmine (E120) and carminic acid are currently produced from scale insects. The access to raw material is limited and current production is sensitive to fluctuation in weather conditions. A cheaper and more stable supply is therefore desirable. Here we present the first proof-of-concept of heterologous microbial production of carminic acid in Aspergillus nidulans by developing a semi-natural biosynthetic pathway. Formation of the tricyclic core of carminic acid is achieved via a two-step process wherein a plant type III polyketide synthase (PKS) forms a non-reduced linear octaketide, which subsequently is folded into the desired flavokermesic acid anthrone (FKA) structure by a cyclase and a aromatase from a bacterial type II PKS system. The formed FKA is oxidized to flavokermesic acid and kermesic acid, catalyzed by endogenous A. nidulans monooxygenases, and further converted to dcII and carminic acid by the Dactylopius coccus C-glucosyltransferase DcUGT2. The establishment of a functional biosynthetic carminic acid pathway in A. nidulans serves as an important step towards industrial-scale production of carminic acid via liquid-state fermentation using a microbial cell factory.

On the biosynthetic origin of carminic acid.

The chemical composition of the scale insect Dactylopius coccus was analyzed with the aim to discover new possible intermediates in the biosynthesis of carminic acid. UPLC-DAD/HRMS analyses of fresh and dried insects resulted in the identification of three novel carminic acid analogues and the verification of several previously described intermediates. Structural elucidation revealed that the three novel compounds were desoxyerythrolaccin-O-glucosyl (DE-O-Glcp), 5,6-didehydroxyerythrolaccin 3-O-ß-D-glucopyranoside (DDE-3-O-Glcp), and flavokermesic acid anthrone (FKA). The finding of FKA in D. coccus provides solid evidence of a polyketide, rather than a shikimate, origin of coccid pigments. Based on the newly identified compounds, we present a detailed biosynthetic scheme that accounts for the formation of carminic acid (CA) in D. coccus and all described coccid pigments which share a flavokermesic acid (FK) core. Detection of coccid pigment intermediates in members of the Planococcus (mealybugs) and Pseudaulacaspis genera shows that the ability to form these pigments is taxonomically more widely spread than previously documented. The shared core-FK-biosynthetic pathway and wider taxonomic distribution suggests a common evolutionary origin for the trait in all coccid dye producing insect species.

Design and Direct Assembly of Synthesized Uracil-containing Non-clonal DNA Fragments into Vectors by USERTM Cloning.

This protocol describes how to order and directly assemble uracil-containing non-clonal DNA fragments by uracil excision based cloning (USER cloning). The protocol was generated with the goal of making synthesized non-clonal DNA fragments directly compatible with USERTM cloning. The protocol is highly efficient and would be compatible with uracil-containing non-clonal DNA fragments obtained from any synthesizing company. The protocol drastically reduces time and handling between receiving the synthesized DNA fragments and transforming with vector and DNA fragment(s).

Interplays between the cell wall and phytohormones in interaction between plants and necrotrophic pathogens.

The plant cell wall surrounds every cell in plants. During microbial infection, the cell wall provides a dynamic interface for interaction with necrotrophic phytopathogens as a rich source of carbohydrates for the growth of pathogens, as a physical barrier restricting the progression of the pathogens, and as an integrity sensory system that can activate intracellular signaling cascades and ultimately lead to a multitude of inducible host defense responses. Studies over the last decade have provided evidence of interplays between the cell wall and phytohormone signaling. This review summarizes the current state of knowledge about the cell wall-phytohormone interplays, with the focus on auxin, cytokinin, brassinosteroids, and abscisic acid, and discuss how they impact the outcome of plant-necrotrophic pathogen interaction.

Acetylation of cell wall is required for structural integrity of the leaf surface and exerts a global impact on plant stress responses.

The epidermis on leaves protects plants from pathogen invasion and provides a waterproof barrier. It consists of a layer of cells that is surrounded by thick cell walls, which are partially impregnated by highly hydrophobic cuticular components. We show that the Arabidopsis T-DNA insertion mutants of REDUCED WALL ACETYLATION 2 (rwa2), previously identified as having reduced O-acetylation of both pectins and hemicelluloses, exhibit pleiotrophic phenotype on the leaf surface. The cuticle layer appeared diffused and was significantly thicker and underneath cell wall layer was interspersed with electron-dense deposits. A large number of trichomes were collapsed and surface permeability of the leaves was enhanced in rwa2 as compared to the wild type. A massive reprogramming of the transcriptome was observed in rwa2 as compared to the wild type, including a coordinated up-regulation of genes involved in responses to abiotic stress, particularly detoxification of reactive oxygen species and defense against microbial pathogens (e.g., lipid transfer proteins, peroxidases). In accordance, peroxidase activities were found to be elevated in rwa2 as compared to the wild type. These results indicate that cell wall acetylation is essential for maintaining the structural integrity of leaf epidermis, and that reduction of cell wall acetylation leads to global stress responses in Arabidopsis.

Arabidopsis cytochrome P450 monooxygenase 71A13 catalyzes the conversion of indole-3-acetaldoxime in camalexin synthesis.

Camalexin (3-thiazol-2-yl-indole) is an indole alkaloid phytoalexin produced by Arabidopsis thaliana that is thought to be important for resistance to necrotrophic fungal pathogens, such as Alternaria brassicicola and Botrytis cinerea. It is produced from Trp, which is converted to indole acetaldoxime (IAOx) by the action of cytochrome P450 monooxygenases CYP79B2 and CYP79B3. The remaining biosynthetic steps are unknown except for the last step, which is conversion of dihydrocamalexic acid to camalexin by CYP71B15 (PAD3). This article reports characterization of CYP71A13. Plants carrying cyp71A13 mutations produce greatly reduced amounts of camalexin after infection by Pseudomonas syringae or A. brassicicola and are susceptible to A. brassicicola, as are pad3 and cyp79B2 cyp79B3 mutants. Expression levels of CYP71A13 and PAD3 are coregulated. CYP71A13 expressed in Escherichia coli converted IAOx to indole-3-acetonitrile (IAN). Expression of CYP79B2 and CYP71A13 in Nicotiana benthamiana resulted in conversion of Trp to IAN. Exogenously supplied IAN restored camalexin production in cyp71A13 mutant plants. Together, these results lead to the conclusion that CYP71A13 catalyzes the conversion of IAOx to IAN in camalexin synthesis and provide further support for the role of camalexin in resistance to A. brassicicola.

CYP71B15 (PAD3) catalyzes the final step in camalexin biosynthesis.

Camalexin represents the main phytoalexin in Arabidopsis (Arabidopsis thaliana). The camalexin-deficient phytoalexin deficient 3 (pad3) mutant has been widely used to assess the biological role of camalexin, although the exact substrate of the cytochrome P450 enzyme 71B15 encoded by PAD3 remained elusive. 2-(Indol-3-yl)-4,5-dihydro-1,3-thiazole-4-carboxylic acid (dihydrocamalexic acid) was identified as likely intermediate in camalexin biosynthesis downstream of indole-3-acetaldoxime, as it accumulated in leaves of silver nitrate-induced pad3 mutant plants and it complemented the camalexin-deficient phenotype of a cyp79b2/cyp79b3 double-knockout mutant. Recombinant CYP71B15 heterologously expressed in yeast catalyzed the conversion of dihydrocamalexic acid to camalexin with preference of the (S)-enantiomer. Arabidopsis microsomes isolated from leaves of CYP71B15-overexpressing and induced wild-type plants were capable of the same reaction but not microsomes from induced leaves of pad3 mutants. In conclusion, CYP71B15 catalyzes the final step in camalexin biosynthesis.

Hypersensitivity of abscisic acid-induced cytosolic calcium increases in the Arabidopsis farnesyltransferase mutant era1-2.

Cytosolic calcium increases were analyzed in guard cells of the Arabidopsis farnesyltransferase deletion mutant era1-2 (enhanced response to abscisic acid). At low abscisic acid (ABA) concentrations (0.1 microM), increases of guard cell cytosolic calcium and stomatal closure were activated to a greater extent in the era1-2 mutant compared with the wild type. Patch clamping of era1-2 guard cells showed enhanced ABA sensitivity of plasma membrane calcium channel currents. These data indicate that the ERA1 farnesyltransferase targets a negative regulator of ABA signaling that acts between the points of ABA perception and the activation of plasma membrane calcium influx channels. Experimental increases of cytosolic calcium showed that the activation of S-type anion currents downstream of cytosolic calcium and extracellular calcium-induced stomatal closure were unaffected in era1-2, further supporting the positioning of era1-2 upstream of cytosolic calcium in the guard cell ABA signaling cascade. Moreover, the suppression of ABA-induced calcium increases in guard cells by the dominant protein phosphatase 2C mutant abi2-1 was rescued partially in era1-2 abi2-1 double mutant guard cells, further reinforcing the notion that ERA1 functions upstream of cytosolic calcium and indicating the genetic interaction of these two mutations upstream of ABA-induced calcium increases.

Plasma membrane-associated ROP10 small GTPase is a specific negative regulator of abscisic acid responses in Arabidopsis.

Abscisic acid (ABA) is an important plant hormone that modulates seed germination and plant growth and stress responses, but its signaling remains poorly understood. We investigated the role of ROP10, a member of the Arabidopsis Rop subfamily of Rho GTPases, in ABA signaling. A null rop10 mutant exhibits enhanced responses to ABA in seed germination, root elongation, and stomatal closure assays and in the induction of expression of the transcription factor MYB2, but it shows wild-type levels of ABA and normal responses to other hormones. Consistently, transgenic expression of a constitutively active form of ROP10 reduces ABA inhibition of seed germination, whereas dominant-negative mutants of ROP10 enhance ABA response and partially suppress abi2. Furthermore, ABA specifically downregulates ROP10 transcription in root tips. ROP10 is localized to the plasma membrane (PM), and PM localization is crucial for its function. These results suggest that ROP10 is a PM-localized signaling molecule that is involved specifically in the negative regulation of ABA signaling.