Subcellular Localization of Anthocyanin Methyltransferase in Flowers of Petunia hybrida

Subcellular compartmentalization in the biosynthesis and engineering of plant natural products

Plant natural products (PNPs) are specialized metabolites with diverse bioactivities. They are extensively used in the pharmaceutical, cosmeceutical and food industries. PNPs are synthesized in plant cells by enzymes that are distributed in different subcellular compartments with unique microenvironments, such as ions, co-factors and substrates. Plant metabolic engineering is an emerging and promising approach for the sustainable production of PNPs, for which the knowledge of the subcellular compartmentalization of their biosynthesis is instrumental. In this review we describe the state of the art on the role of subcellular compartments in the biosynthesis of major types of PNPs, including terpenoids, phenylpropanoids, alkaloids and glucosinolates, and highlight the efforts to target biosynthetic pathways to subcellular compartments in plants. In addition, we will discuss the challenges and strategies in the field of plant synthetic biology and subcellular engineering. We expect that newly developed methods and tools, together with the knowledge gained from the microbial chassis, will greatly advance plant metabolic engineering.

Purification, molecular cloning, and characterization of glutathione S-transferases (GSTs) from pigmented Vitis vinifera L. cell suspension cultures as putative anthocyanin transport proteins

The ligandin activity of specific glutathione S-transferases (GSTs) is necessary for the transport of anthocyanins from the cytosol to the plant vacuole. Five GSTs were purified from Vitis vinifera L. cv. Gamay Fréaux cell suspension cultures by glutathione affinity chromatography. These proteins underwent Edman sequencing and mass spectrometry fingerprinting, with the resultant fragments aligned with predicted GSTs within public databases. The corresponding coding sequences were cloned, with heterologous expression in Escherichia coli used to confirm GST activity. Transcriptional profiling of these candidate GST genes and key anthocyanin biosynthetic pathway genes (PAL, CHS, DFR, and UFGT) in cell suspensions and grape berries against anthocyanin accumulation demonstrated strong positive correlation with two sequences, VvGST1 and VvGST4, respectively. The ability of VvGST1 and VvGST4 to transport anthocyanins was confirmed in the heterologous maize bronze-2 complementation model, providing further evidence for their function as anthocyanin transport proteins in grape cells. Furthermore, the differential induction of VvGST1 and VvGST4 in suspension cells and grape berries suggests functional differences between these two proteins. Further investigation of these candidate ligandins may identify a mechanism for manipulating anthocyanin accumulation in planta and in vitro suspension cells.

Primary structure and expression of a 24-kD vacuolar protein (VP24) precursor in anthocyanin-producing cells of sweet potato in suspension culture

A 24-kD vacuolar protein (VP24) accumulates abundantly in intravacuolar pigmented globules in anthocyanin-containing sweet potato (Ipomoea batatas) cells in suspension culture. A cDNA clone encoding VP24 was isolated from a cDNA library constructed from light-irradiated suspension-cultured cells. Sequence analysis revealed that a 2.9-kbp VP24 cDNA encodes a protein of 893 amino acid residues with a molecular mass of 96.3 kD. According to the deduced amino acid sequence of VP24 cDNA, VP24 is probably synthesized as a large precursor protein with an N-terminal extension composed of a signal peptide and a propeptide, plus the polypeptide of the mature VP24 and its C-terminal propeptide, which contains the multiple transmembrane domains. A search in the ProDom database revealed the mature VP24 domain belongs to the zinc metalloprotease family. Northern analysis revealed that the single 2.9-kb VP24 mRNA increases rapidly after light irradiation, whereas VP24 mRNA was undetectable in the dark-cultured cells or in the presence of a high concentration of 2,4-dichlorophenoxyacetic acid. Light-induced VP24 gene expression closely correlated with the accumulation of anthocyanin in the vacuoles. These results suggested that proteins derived from the VP24 precursor protein may be involved in vacuolar transport and/or accumulation of anthocyanin synthesized in the cytosol.

Expression of a vacuolar protein (VP24) in anthocyanin-producing cells of sweet potato in suspension culture

VP24, an abundant protein of 24 kD, was found to accumulate in the anthocyanin-containing vacuoles of cells of sweet potato (Ipomoea batatas) in suspension culture. Light-induced expression of VP24 was analyzed by immunoblotting in three different cell lines that produced anthocyanins at different rates. The expression of VP24 was closely correlated with the accumulation of anthocyanin in these cell lines. Immunocytochemical detection of VP24 with specific antibodies on thin sections showed that VP24 was localized in the intravacuolar pigmented globules (cyanoplasts) in the anthocyanin-containing vacuoles and not in the tonoplast. No VP24 immunogold labeling was detected in the vacuoles of the cell line that does not produce anthocyanin. We suggest that VP24 may be involved in the formation of the cyanoplast via an interaction with anthocyanin, and that it may play an important role in the trapping in vacuoles of large amounts of anthocyanins that have been transported into these vacuoles.

Synthesis of Phytoalexins in Sorghum as a Site-Specific Response to Fungal Ingress

Sorghum produces phytoalexins that are 3-deoxyanthocyanidin flavonoids. The compounds inhibit the growth of phytopathogenic fungi in vitro. The phytoalexins appear to be synthesized in subcellular inclusions within a host epidermal cell that is about to be penetrated by a fungus. This site-restricted synthesis suggests that the phytoalexin response occurs initially in the first cells that come under fungal attack and is not simply a response of cells that surround the original infection site.

The uptake of acylated anthocyanin into isolated vacuoles from a cell suspension culture of Daucus carota

Anthocyanin-containing vacuoles were isolated from protoplasts of a cell suspension culture of Daucus carota. The vacuoles were stable for at least 2 h as demonstrated by the fact that they showed no efflux of anthocyanin. The uptake of radioactively labelled anthocyanin was time-dependent with a pH optimum at 7.5, and could be inhibited by the protonophore carbonylcyanide m-chlorophenylhydrazone. Furthermore, the transport was specific, since vacuoles from other plant species showed no uptake of labelled anthocyanin, and strongly depended on acylation with sinapic acid, as deacylated glycosides were not taken up by isolated vacuoles. Hence, it is suggested that the acylation of anthocyanin, which is also required for the stabilization of colour in vacuoles, is important for transport, and that acylated anthocyanin is transported by a selective carrier and might be trapped by a pH-dependent conformational change of the molecule inside the acid vacuolar sap.

Vacuolar localization of 1-sinapolglucose: L-malate sinapoyltransferase in protoplasts from cotyledons of Raphanus sativus

The distribution of L-malate, sinapic acid esters and 1-sinapoylglucose: L-malate sinapoyltransferase (SMT) which catalyzes the synthesis of sinapoyl-L-malate were examined in preparations of protoplasts obtained from cotyledons of red radish (Raphanus sativus L. var. sativus). Vacuoles isolated from the protoplasts contained all of the SMT activity, all of the accumulated sinapic acid esters and about 50% of free L-malate present initially in the protoplasts. An esterase activity, acting on 1-sinapoyglucose, was found to be exclusively localized in the cytoplasm and a large proportion was found to be recoverable in a 100 000-g pellet obtained from protoplast lysates. The vacuoles were obtained after lysis of the protoplasts by osmotic shock and purification on a Ficoll gradient. The cytoplasmic contamination of vacuole preparations was found to be about 10%, as judged by enzymatic markers and microscopic inspection. No SMT activity was found in a 100 000-g pellet obtained from vacuole lysates. The results indicate that biosynthesis of sinapoyl-L-malate takes place within the central vacuoles of redradish cotyledons.

On the origin of anthocyanin methyltransferase isozymes of Petunia hybrida and their role in regulation of anthocyanin methylation

Four genes controlling anthocyanin methylation in flowers of Petunia hybrida have been described. Three of them, Mt2, Mf1 and Mf2, caused a dosage effect on anthocyanin methyltransferase activity and degree of methylation of anthocyanins. Antiserum raised against partially purified Mf2-enzyme precipitated three of the four anthocyanin methyltransferases. In two subspecies of one of the ancestral species of P. hybrida: Petunia integrifolia, different anthocyanin methyltransferases were found as determined by immunoprecipitation. The methyltransferase isozymes showed no differences in subcellular or tissue location, and had no physiologically important difference in time course of activity during bud development. The methylation-system in Petunia is discussed with regard to anthocyanin methylation in other plant species.