Wounding Induces One of Two Isoenzymes of 3-Deoxy-d-arabino-Heptulosonate 7-Phosphate Synthase in Solanum tuberosum L

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase as the gatekeeper of plant aromatic natural product biosynthesis

The shikimate pathway connects the central carbon metabolism with the biosynthesis of aromatic amino acids-l-tyrosine, l-phenylalanine, and l-tryptophan-which play indispensable roles as precursors of numerous aromatic phytochemicals. Despite the importance of the shikimate pathway-derived products for both plant physiology and human society, the regulatory mechanism of the shikimate pathway remains elusive. This review summarizes the recent progress and current understanding on the plant 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHP synthase or DHS) enzymes that catalyze the committed reaction of the shikimate pathway. We particularly focus on how the DHS activity is regulated in plants in comparison to those of microbes and discuss potential roles of DHS as the critical gatekeeper for the production of plant aromatic compounds.

Revisiting the dual pathway hypothesis of Chorismate production in plants

The shikimate pathway, the seven enzymatic steps that synthesize chorismate from phosphoenolpyruvate and erythrose 4-phosphate, produces the last common precursor of the three aromatic amino acids. It is firmly established that all seven enzymes are present in plastids, and it is generally accepted that this organelle is likely the sole location for production of chorismate in plants. However, recently a growing body of evidence has provided support for a previous proposal that at least portions of the pathway are duplicated in the cytosol, referred to as the Dual Pathway Hypothesis. Here I revisit this obscure hypothesis by reviewing the findings that provided the original basis for its formulation as well as more recent results that provide fresh support for a possible extra-plastidial shikimate pathway duplication. Similarities between this possible intercompartmental metabolic redundancy and that of terpenoid metabolism are used to discuss potential advantages of pathway duplication, and the translational implications of the Dual Pathway Hypothesis for metabolic engineering are noted.

Proteomic Analysis of Lipid Droplets from Arabidopsis Aging Leaves Brings New Insight into Their Biogenesis and Functions

Lipid droplets (LDs) are cell compartments specialized for oil storage. Although their role and biogenesis are relatively well documented in seeds, little is known about their composition, structure and function in senescing leaves where they also accumulate. Here, we used a label free quantitative mass spectrometry approach to define the LD proteome of aging Arabidopsis leaves. We found that its composition is highly different from that of seed/cotyledon and identified 28 proteins including 9 enzymes of the secondary metabolism pathways involved in plant defense response. With the exception of the TRIGALACTOSYLDIACYLGLYCEROL2 protein, we did not identify enzymes implicated in lipid metabolism, suggesting that growth of leaf LDs does not occur by local lipid synthesis but rather through contact sites with the endoplasmic reticulum (ER) or other membranes. The two most abundant proteins of the leaf LDs are the CALEOSIN3 and the SMALL RUBBER PARTICLE1 (AtSRP1); both proteins have structural functions and participate in plant response to stress. CALEOSIN3 and AtSRP1 are part of larger protein families, yet no other members were enriched in the LD proteome suggesting a specific role of both proteins in aging leaves. We thus examined the function of AtSRP1 at this developmental stage and found that AtSRP1 modulates the expression of CALEOSIN3 in aging leaves. Furthermore, AtSRP1 overexpression induces the accumulation of triacylglycerol with an unusual composition compared to wild-type. We demonstrate that, although AtSRP1 expression is naturally increased in wild type senescing leaves, its overexpression in senescent transgenic lines induces an over-accumulation of LDs organized in clusters at restricted sites of the ER. Conversely, atsrp1 knock-down mutants displayed fewer but larger LDs. Together our results reveal that the abundancy of AtSRP1 regulates the neo-formation of LDs during senescence. Using electron tomography, we further provide evidence that LDs in leaves share tenuous physical continuity as well as numerous contact sites with the ER membrane. Thus, our data suggest that leaf LDs are functionally distinct from seed LDs and that their biogenesis is strictly controlled by AtSRP1 at restricted sites of the ER.

Overexpression of 3-deoxy-7-phosphoheptulonate synthase gene from Gossypium hirsutum enhances Arabidopsis resistance to Verticillium wilt

Expression of DHS1 in cotton is induced upon infection by Verticillium dahliae , and overexpression of GhDHS1 endows transgenic Arabidopsis plants excellent Verticillium resistance. Verticillium wilt is caused by a soil-borne fungus Verticillium dahliae. Resistance in most cotton cultivars is either scarce or unavailable, making Verticillium wilt a major obstacle in cotton production. Here, we identified a 3-deoxy-7-phosphoheptulonate synthase (DHS, EC 4.1.2.15) gene from Gossypium hirsutum, named GhDHS1. Its 1620 bp open reading frame encodes a putative 59.4 kDa protein. Phylogenetic analysis indicated that GhDHS1 is clustered in a clade with potato and tomato DHSs that can be induced by wounding and elicitors, respectively. Expression analysis demonstrated that GhDHS1 is constitutively expressed in cotton roots and stems, but transcripts are rare or non-existent in the leaves. Subcellular localization showed that GhDHS1 occurs in the plastids. When plants of three cultivars were inoculated with V. dahliae, DHS1 expression was more significantly up-regulated in the roots of resistant G. barbadense cv. Pima90-53 and G. hirsutum cv. Jimian20 than in the susceptible G. hirsutum cv. Han208. This suggested that DHS1 is involved in the cotton resistance to Verticillium wilt. Furthermore, GhDHS1 overexpressing transgenic lines of Arabidopsis were developed via Agrobacterium tumefaciens-mediated transformation. Compared with the untransformed WT (wild type), these transgenic plants showed excellent Verticillium wilt resistance with a significantly lower disease index. The overexpressing transgenic lines also had significantly longer primary roots and greatly increased xylem areas under V. dahliae infection. Overall, our results indicate that GhDHS1 performs a role in the cotton resistance to V. dahliae and would be potential to breeding cottons of Verticillium wilt resistance.

UV-B induced transcript accumulation of DAHP synthase in suspension-cultured Catharanthus roseus cells

The enzyme 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (EC 4.1.2.15) catalyzes the first committed step in the shikimate pathway of tryptophan synthesis, an important precursor for the production of terpenoid indole alkaloids (TIAs). A full-length cDNA encoding nuclear coded chloroplast-specific DAHP synthase transcript was isolated from a Catharanthus roseus cDNA library. This had high sequence similarity with other members of plant DAHP synthase family. This transcript accumulated in suspension cultured C. roseus cells on ultraviolet (UV-B) irradiation. Pretreatment of C.roseus cells with variety of agents such as suramin, N-acetyl cysteine, and inhibitors of calcium fluxes and protein kinases and MAP kinase prevented this effect of UV-B irriadiation. These data further show that the essential components of the signaling pathway involved in accumulation DAHP synthase transcript in C. roseus cells include suramin-sensitive cell surface receptor, staurosporine-sensitive protein kinase and MAP kinase.

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase is regulated for the accumulation of polysaccharide-linked hydroxycinnamoyl esters in rice (Oryza sativa L.) internode cell walls

Polysaccharide-linked hydroxycinnamoyl esters (PHEs) over-accumulate in the internodes of a rice (Oryza sativa L.) mutant, Fukei 71 (F71). This accumulation is accompanied by over-expression of phenylalanine ammonialyase (PAL). In this study, we show that only one member of the 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHPS) family expresses in close correlation with PAL. Furthermore, substrate availability to DAHPS is promoted by down-regulating the expression of plastidic pyruvate kinase (PKp), a competitor of DAHPS. Since the over-production of PHEs is caused by D50 gene disruption, these results suggest that specific enzymes in the phenylpropanoid and shikimate pathways are coordinately up-regulated. In addition, the results indicate that carbon-flow into the shikimate pathway is modified for the synthesis of PHEs, and is probably controlled by D50.

Marine tannins: the importance of a mechanistic framework for predicting ecological roles

Since chemical ecology emerged as a field of marine science, it has been strongly influenced by studies of chemically mediated interactions in land-based systems. Marine chemical ecologists, like their terrestrial counterparts, initially focused on identifying natural products and evaluating the potential ecological roles of these products as defenses, attractants, or other cues. Now, like our land-based colleagues, we must increase our focus on the physiological and biochemical mechanisms that underlie the chemical interactions, paying particular attention to regulation of biosynthetic pathways, within-plant and between-plant signaling cues, and comparative and functional genomics. Here, we review the current state of knowledge regarding a heterogenous group of macrophyte natural products, the marine tannins and simple phenolics, to illustrate how such information is critical to future attempts to predict their ecological roles.

Identification, characterization and comparative analysis of a novel chorismate mutase gene in Arabidopsis thaliana

Phenylalanine, tyrosine, and tryptophan have a dual biosynthetic role in plants; they are required for protein synthesis and are also precursors to a number of aromatic secondary metabolites critical to normal development and stress responses. Whereas much has been learned in recent years about the genetic control of tryptophan biosynthesis in Arabidopsis and other plants, relatively little is known about the genetic regulation of phenylalanine and tyrosine synthesis. We have isolated, characterized and determined the expression of Arabidopsis thaliana genes encoding chorismate mutase, the enzyme catalyzing the first committed step in phenylalanine and tyrosine synthesis. Three independent Arabidopsis chorismate mutase cDNAs were isolated by functional complementation of a Saccharomyces cerevisiae mutation. Two of these cDNAs have been reported independently (Eberhard et al., 1993. FEBS 334, 233-236; Eberhard et al., 1996. Plant J. 10, 815-821), but the third (designated CM-3) represents a novel gene. The different organ-specific expression patterns of these cDNAs, their regulation in response to pathogen infiltration, as well as the different enzymatic characteristics of the proteins they encode are also described. Together, these data suggest that each isoform may play a distinct physiological role in coordinating chorismate mutase activity with developmental and environmental signals.

THE SHIKIMATE PATHWAY

The shikimate pathway links metabolism of carbohydrates to biosynthesis of aromatic compounds. In a sequence of seven metabolic steps, phosphoenolpyruvate and erythrose 4-phosphate are converted to chorismate, the precursor of the aromatic amino acids and many aromatic secondary metabolites. All pathway intermediates can also be considered branch point compounds that may serve as substrates for other metabolic pathways. The shikimate pathway is found only in microorganisms and plants, never in animals. All enzymes of this pathway have been obtained in pure form from prokaryotic and eukaryotic sources and their respective DNAs have been characterized from several organisms. The cDNAs of higher plants encode proteins with amino terminal signal sequences for plastid import, suggesting that plastids are the exclusive locale for chorismate biosynthesis. In microorganisms, the shikimate pathway is regulated by feedback inhibition and by repression of the first enzyme. In higher plants, no physiological feedback inhibitor has been identified, suggesting that pathway regulation may occur exclusively at the genetic level. This difference between microorganisms and plants is reflected in the unusually large variation in the primary structures of the respective first enzymes. Several of the pathway enzymes occur in isoenzymic forms whose expression varies with changing environmental conditions and, within the plant, from organ to organ. The penultimate enzyme of the pathway is the sole target for the herbicide glyphosate. Glyphosate-tolerant transgenic plants are at the core of novel weed control systems for several crop plants.

Purification and cDNA cloning of isochorismate synthase from elicited cell cultures of Catharanthus roseus

Isochorismate is an important metabolite formed at the end of the shikimate pathway, which is involved in the synthesis of both primary and secondary metabolites. It is synthesized from chorismate in a reaction catalyzed by the enzyme isochorismate synthase (ICS; EC 5.4.99.6). We have purified ICS to homogeneity from elicited Catharanthus roseus cell cultures. Two isoforms with an apparent molecular mass of 64 kD were purified and characterized. The Km values for chorismate were 558 and 319 microM for isoforms I and II, respectively. The isoforms were not inhibited by aromatic amino acids and required Mg2+ for enzyme activity. Polymerase chain reaction on a cDNA library from elicited C. roseus cells with a degenerated primer based on the sequence of an internal peptide from isoform II resulted in an amplification product that was used to screen the cDNA library. This led to the first isolation, to our knowledge, of a plant ICS cDNA. The cDNA encodes a protein of 64 kD with an N-terminal chloroplast-targeting signal. The deduced amino acid sequence shares homology with bacterial ICS and also with anthranilate synthases from plants. Southern analysis indicates the existence of only one ICS gene in C. roseus.

Impaired Wound Induction of 3-Deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) Synthase and Altered Stem Development in Transgenic Potato Plants Expressing a DAHP Synthase Antisense Construct

Potato (Solanum tuberosum L.) cells were transformed with an antisense DNA construct encoding part of 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase (EC 4.1.2.15), the first enzyme of the shikimate pathway, to examine the role(s) of this protein in plant growth and development. Chimeric DNA constructs contained the transcript start site, the first exon, and part of the first intron of the shkA gene in antisense or sense orientations under the control of the cauliflower mosaic virus 35S promoter. Some, but not all, of the transgenic plants expressing antisense DAHP synthase RNA showed reduced levels of wound-induced DAHP synthase enzyme activity, polypeptide, and mRNA 12 and 24 h after wounding. No alteration in the wound induction of DAHP synthase gene expression was observed in transgenic potato tubers containing the chimeric sense construct. Reduced steady-state levels of DAHP synthase mRNA were observed in stem and shoot tip tissue. Some plants with the chimeric antisense construct had reduced stem length, stem diameter, and reduced stem lignification.

Differential expression of tomato (Lycopersicon esculentum L.) genes encoding shikimate pathway isoenzymes. I. 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

Tomato (Lycopersicon esculentum L. cv. UC82b) was found to contain two distinct 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase genes that are differentially expressed. The corresponding cDNAs were isolated and characterized. Both genes code for putative plastidic DAHP synthase isoforms. The deduced amino acid sequences are 79% identical. A comparison of the known Solanaceae DAHP synthases indicates two distinct conserved isoforms. The steady-state levels of transcripts of the two tomato genes differ in all organs analysed.