VvGONST-A and VvGONST-B are Golgi-localised GDP-sugar transporters in grapevine (Vitis vinifera L.)

Prunus Hexokinase 3 genes alter primary C-metabolism and promote drought and salt stress tolerance in Arabidopsis transgenic plants

Hexokinases (HXKs) and fructokinases (FRKs) are the only two families of enzymes in plants that have been identified as able to phosphorylate Glucose (Glc) and Fructose (Fru). Glc can only be phosphorylated in plants by HXKs, while Fru can be phosphorylated by either HXKs or FRKs. The various subcellular localizations of HXKs in plants indicate that they are involved in diverse functions, including anther dehiscence and pollen germination, stomatal closure in response to sugar levels, stomatal aperture and reducing transpiration. Its association with modulating programmed cell death, and responses to oxidative stress and pathogen infection (abiotic and biotic stresses) also have been reported. To extend our understanding about the function of HXK-like genes in the response of Prunus rootstocks to abiotic stress, we performed a detailed bioinformatic and functional analysis of hexokinase 3-like genes (HXK3s) from two Prunus rootstock genotypes, 'M.2624' (Prunus cerasifera Ehrh × P. munsoniana W.Wight & Hedrick) and 'M.F12/1' (P. avium L.), which are tolerant and sensitive to hypoxia stress, respectively. A previous large-scale transcriptome sequencing of roots of these rootstocks, showed that this HXK3-like gene that was highly induced in the tolerant genotype under hypoxia conditions. In silico analysis of gene promoters from M.2624 and M.F12/1 genotypes revealed regulatory elements that could explain differential transcriptional profiles of HXK3 genes. Subcellular localization was determinates by both bioinformatic prediction and expression of their protein fused to the green fluorescent protein (GFP) in protoplasts and transgenic plants of Arabidopsis. Both approaches showed that they are expressed in plastids. Metabolomics analysis of Arabidopsis plants ectopically expressing Prunus HXK3 genes revealed that content of several metabolites including phosphorylated sugars (G6P), starch and some metabolites associated with the TCA cycle were affected. These transgenic Arabidopsis plants showed improved tolerance to salt and drought stress under growth chamber conditions. Our results suggest that Prunus HXK3 is a potential candidate for enhancing tolerance to salt and drought stresses in stone fruit trees and other plants.

Solanum lycopersicum (tomato) possesses mitochondrial and plastidial lipoyl synthases capable of increasing lipoylation levels when expressed in bacteria

Lipoic acid (LA) and its reduced form (dihydrolipoic acid, DHLA) have unique antioxidant properties among such molecules. Moreover, after a process termed lipoylation, LA is an essential prosthetic group covalently-attached to several key multi-subunit enzymatic complexes involved in primary metabolism, including E2 subunits of pyruvate dehydrogenase (PDH). The metabolic pathway of lipoylation has been extensively studied in Escherichia coli and Arabidopsis thaliana in which protein modification occurs via two routes: de novo synthesis and salvage. Common to both pathways, lipoyl synthase (LIP1 in plants, LipA in bacteria, EC 2.8.1.8) inserts sulphur atoms into the molecule in a final, activating step. However, despite the detection of LA and DHLA in other plant species, including tomato (Solanum lycopersicum), no plant LIP1s have been characterised to date from species other than Arabidopsis. In this work, we present the identification and characterisation of two LIPs from tomato, SlLIP1 and SlLIP1p. Consistent with in silico data, both are widely-expressed, particularly in reproductive organs. In line with bioinformatic predictions, we determine that yellow fluorescent protein tagged versions of SlLIP1 and SlLIP1p are mitochondrially- and plastidially-localised, respectively. Both possess the molecular hallmarks and domains of well-characterised bacterial LipAs. When heterologously-expressed in an E. coli lipA mutant, both are capable of complementing specific growth phenotypes and increasing lipoylation levels of E2 subunits of PDH in vivo, demonstrating that they do indeed function as lipoyl synthases.

The GDP-mannose transporter gene (DoGMT) from Dendrobium officinale is critical for mannan biosynthesis in plant growth and development

Dendrobium officinale is a precious traditional Chinese medicinal herb because it is abundant in mannose-containing polysaccharides (MCPs). GDP-mannose transporter (GMT), which translocates GDP-mannose into the Golgi lumen, is indispensable for the biosynthesis of MCPs. In this study, we found that the dominant polysaccharides in D. officinale were MCPs in a range of varieties and different physiological phases. After a positive correlation between the accumulation of mannose and the transcript levels of candidate GMT genes was found, three GMT genes (DoGMT1-3) were identified in D. officinale. DoGMT1, DoGMT2 and DoGMT3 exhibited the highest transcript level in stem that an organ for MCPs storage. All three DoGMT proteins were targeted to Golgi apparatus, and had a GDP binding domain (GXL/VNK) that was homologous to a specially characterized GMT protein GONST1 in Arabidopsis thaliana. Moreover, DoGMT1, DoGMT2 and DoGMT3 complemented a GDP-mannose transport-defective yeast mutant (vrg4-2), meanwhile they also demonstrated a higher GDP-mannose uptake activity. Therefore, we conclude that DoGMT1, DoGMT2 and DoGMT3 are able to transport GDP-mannose while the expression patterns of these genes correspond to the accumulation of MCPs in D. officinale. These findings support the importance of GMT genes from D. officinale in the biosynthesis of MCPs.

Unraveling the induction of phytoene synthase 2 expression by salt stress and abscisic acid in Daucus carota

Phytoene synthase (PSY) is the first committed enzyme of the carotenoid biosynthesis pathway and the most important point of regulation. Carotenoids are precursors of abscisic acid (ABA), which mediates abiotic stress tolerance responses in plants. ABA activates the synthesis of its own precursors through induction of PSY expression. Carrot, a species that accumulates very high amounts of carotenoids in its reserve root, has two PSY paralog genes that are expressed differentially in the root. Here, we determined that DcPSY2 expression is induced by salt stress and ABA. A DcPSY2 promoter fragment was obtained and characterized. Bioinformatic analysis showed the presence of three ABA responsive elements (ABREs). Through overexpressing pPSY2:GFP in Nicotiana tabacum we determined that all three ABREs are necessary for the ABA response. In the carrot transcriptome, we identified three ABRE binding protein (DcAREB) transcription factor candidates that localized in the nucleus, but only one, DcAREB3, was induced under ABA treatment in carrot roots. We found that AREB transcription factors bind to the carrot DcPSY2 promoter and transactivate the expression of reporter genes. We conclude that DcPSY2 is involved in ABA-mediated salt stress tolerance in carrot through the binding of AREB transcription factors to its promoter.

A catechol oxidase AcPPO from cherimoya (Annona cherimola Mill.) is localized to the Golgi apparatus

Cherimoya (Annona cherimola) is an exotic fruit with attractive organoleptic characteristics. However, it is highly perishable and susceptible to postharvest browning. In fresh fruit, browning is primarily caused by the polyphenol oxidase (PPO) enzyme catalyzing the oxidation of o-diphenols to quinones, which polymerize to form brown melanin pigment. There is no consensus in the literature regarding a specific role of PPO, and its subcellular localization in different plant species is mainly described within plastids. The present work determined the subcellular localization of a PPO protein from cherimoya (AcPPO). The obtained results revealed that the AcPPO- green fluorescent protein co-localized with a Golgi apparatus marker, and AcPPO activity was present in Golgi apparatus-enriched fractions. Likewise, transient expression assays revealed that AcPPO remained active in Golgi apparatus-enriched fractions obtained from tobacco leaves. These results suggest a putative function of AcPPO in the Golgi apparatus of cherimoya, providing new perspectives on PPO functionality in the secretory pathway, its effects on cherimoya physiology, and the evolution of this enzyme.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

The Arabidopsis Golgi-localized GDP-L-fucose transporter is required for plant development

Nucleotide sugar transport across Golgi membranes is essential for the luminal biosynthesis of glycan structures. Here we identify GDP-fucose transporter 1 (GFT1), an Arabidopsis nucleotide sugar transporter that translocates GDP-L-fucose into the Golgi lumen. Using proteo-liposome-based transport assays, we show that GFT preferentially transports GDP-L-fucose over other nucleotide sugars in vitro, while GFT1-silenced plants are almost devoid of L-fucose in cell wall-derived xyloglucan and rhamnogalacturonan II. Furthermore, these lines display reduced L-fucose content in N-glycan structures accompanied by severe developmental growth defects. We conclude that GFT1 is the major nucleotide sugar transporter for import of GDP-L-fucose into the Golgi and is required for proper plant growth and development.

Nucleotide sugars are transported from the cytoplasm to the Golgi lumen where they are incorporated into cell wall polysaccharides and used for glycosylation of proteins and lipids. Here the authors identify GFT1, an Arabidopsis Golgi-localized GDP-fucose transporter that is required for plant growth and development

FcLDP1, a Gene Encoding a Late Embryogenesis Abundant (LEA) Domain Protein, Responds to Brassinosteroids and Abscisic Acid during the Development of Fruits in Fragaria chiloensis

White Chilean strawberries (Fragaria chiloensis) are non-climacteric fruits, with an exotic color and aroma. In order to discover genes involved in the development of these fruits, we identified a fragment of a gene encoding a late embryogenesis abundant domain protein, FcLDP1, that was expressed in early stages of fruit development, particularly in receptacles. Hormones play key roles in regulating the development of non-climacteric fruits. We show that the brassinosteroid content of the white strawberry varies during development. Additionally, FcLDP1 as well as the closest ortholog in the woodland strawberry, F. vesca (FvLDP1) possess multiple brassinosteroid, as well as abscisic acid (ABA) response motifs in the promoter region, consistent with the response of transiently expressed FcLDP1 promoter-GFP fusions to these hormones, and the rise in FcLDP1 transcript levels in white strawberry fruits treated with brassinosteroids or ABA. These findings suggest that both hormones regulate FcLDP1 expression during the development of white strawberries.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.