Glycosyltransferases of lipophilic small molecules

Glycosyltransferases of small molecules transfer sugars to a wide range of acceptors, from hormones and secondary metabolites to biotic and abiotic chemicals and toxins in the environment. The enzymes are encoded by large multigene families and can be identified by a signature motif in their primary sequence, which classifies them as a subset of Family 1 glycosyltransferases. The transfer of a sugar onto a lipophilic acceptor changes its chemical properties, alters its bioactivity, and enables access to membrane transporter systems. In vitro studies have shown that a single gene product can glycosylate multiple substrates of diverse origins; multiple enzymes can also glycosylate the same substrate. These features suggest that in a cellular context, substrate availability is a determining factor in enzyme function, and redundancy depends on the extent of coordinate gene regulation. This review discusses the role of these glycosyltransferases in underpinning developmental and metabolic plasticity during adaptive responses.

Spreading of RNA targeting and DNA methylation in RNA silencing requires transcription of the target gene and a putative RNA-dependent RNA polymerase

RNA silencing is a sequence-specific RNA degradation process that follows the recognition of double-stranded RNA. Here, we show that virus vectors carrying parts of a green fluorescent protein (GFP) transgene targeted RNA silencing in Nicotiana benthamiana and Arabidopsis against the entire GFP RNA. These results indicate that there was spreading of RNA targeting from the initiator region into the adjacent 5' and 3' regions of the target gene. Spreading was accompanied by methylation of the corresponding GFP DNA. It also was dependent on transcription of the transgene and on the putative RNA-dependent RNA polymerase, SDE1/SGS2. These findings indicate that SDE1/SGS2 produces double-stranded RNA using the target RNA as a template. RNA silencing of ribulose-1,5-bisphosphate carboxylase/oxygenase and phytoene desaturase was not associated with the spreading of RNA targeting or DNA methylation, indicating that these endogenous RNAs are not templates for SDE1/SGS2.

A Papaver somniferum 10-gene cluster for synthesis of the anticancer alkaloid noscapine

Noscapine is an antitumor alkaloid from opium poppy that binds tubulin, arrests metaphase, and induces apoptosis in dividing human cells. Elucidation of the biosynthetic pathway will enable improvement in the commercial production of noscapine and related bioactive molecules. Transcriptomic analysis revealed the exclusive expression of 10 genes encoding five distinct enzyme classes in a high noscapine-producing poppy variety, HN1. Analysis of an F(2) mapping population indicated that these genes are tightly linked in HN1, and bacterial artificial chromosome sequencing confirmed that they exist as a complex gene cluster for plant alkaloids. Virus-induced gene silencing resulted in accumulation of pathway intermediates, allowing gene function to be linked to noscapine synthesis and a novel biosynthetic pathway to be proposed.

12-oxo-phytodienoic acid accumulation during seed development represses seed germination in Arabidopsis

Arabidopsis thaliana COMATOSE (CTS) encodes an ABC transporter involved in peroxisomal import of substrates for β-oxidation. Various cts alleles and mutants disrupted in steps of peroxisomal β-oxidation have previously been reported to exhibit a severe block on seed germination. Oxylipin analysis on cts, acyl CoA oxidase1 acyl CoA oxidase2 (acx1 acx2), and keto acyl thiolase2 dry seeds revealed that they contain elevated levels of 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and JA-Ile. Oxylipin and transcriptomic analysis showed that accumulation of these oxylipins occurs during late seed maturation in cts. Analysis of double mutants generated by crossing cts with mutants in the JA biosynthesis pathway indicate that OPDA, rather than JA or JA-Ile, contributes to the block on germination in cts seeds. We found that OPDA was more effective at inhibiting wild-type germination than was JA and that this effect was independent of CORONATINE INSENSITIVE1 but was synergistic with abscisic acid (ABA). Consistent with this, OPDA treatment increased ABA INSENSITIVE5 protein abundance in a manner that parallels the inhibitory effect of OPDA and OPDA+ABA on seed germination. These results demonstrate that OPDA acts along with ABA to regulate seed germination in Arabidopsis.

The UGT73C5 of Arabidopsis thaliana glucosylates brassinosteroids

Steroid hormones are essential for development, and the precise control of their homeostasis is a prerequisite for normal growth. UDP-glycosyltransferases (UGTs) are considered to play an important regulatory role in the activity of steroids in mammals and insects. This study provides an indication that a UGT accepting plant steroids as substrates functions in brassinosteroid (BR) homeostasis. The UGT73C5 of Arabidopsis thaliana catalyses 23-O-glucosylation of the BRs brassinolide (BL) and castasterone. Transgenic plants overexpressing UGT73C5 displayed BR-deficient phenotypes and contained reduced amounts of BRs. The phenotype, which was already apparent in seedlings, could be rescued by application of BR. In feeding experiments with BL, wild-type seedlings converted BL to the 23-O-glucoside; in the transgenic lines silenced in UGT73C5 expression, no 23-O-glucoside was detected, implying that this UGT is the only enzyme that catalyzes BL-23-O-glucosylation in seedlings. Plant lines in which UGT73C5 expression was altered also displayed hypocotyl phenotypes previously described for seedlings in which BR inactivation by hydroxylation was changed. These data support the hypothesis that 23-O-glucosylation of BL is a function of UGT73C5 in planta, and that glucosylation regulates BR activity.

Use of the glucosyltransferase UGT71B6 to disturb abscisic acid homeostasis in Arabidopsis thaliana

A glucosyltransferase (GT) of Arabidopsis, UGT71B6, recognizing the naturally occurring enantiomer of abscisic acid (ABA) in vitro, has been used to disturb ABA homeostasis in planta. Transgenic plants constitutively overexpressing UGT71B6 (71B6-OE) have been analysed for changes in ABA and the related ABA metabolites abscisic acid glucose ester (ABA-GE), phaseic acid (PA), dihydrophaseic acid (DPA), 7'-hydroxyABA and neo-phaseic acid. Overexpression of the GT led to massive accumulation of ABA-GE and reduced levels of the oxidative metabolites PA and DPA, but had marginal effect on levels of free ABA. The control of ABA homeostasis, as reflected in levels of the different metabolites, differed in the 71B6-OEs whether the plants were grown under standard conditions or subjected to wilt stress. The impact of increased glucosylation of ABA on ABA-related phenotypes has also been assessed. Increased glucosylation of ABA led to phenotypic changes in post-germinative growth. The use of two structural analogues of ABA, known to have biological activity but to differ in their capacity to act as substrates for 71B6 in vitro, confirmed that the phenotypic changes arose specifically from the increased glucosylation caused by overexpression of 71B6. The phenotype and profile of ABA and related metabolites in a knockout line of 71B6, relative to wild type, has been assessed during Arabidopsis development and following stress treatments. The lack of major changes in these parameters is discussed in the context of functional redundancy of the multigene family of GTs in Arabidopsis.

Characterization of Mbb1, a nucleus-encoded tetratricopeptide-like repeat protein required for expression of the chloroplast psbB/psbT/psbH gene cluster in Chlamydomonas reinhardtii

Genetic analysis has revealed that the accumulation of several chloroplast mRNAs of the green alga Chlamydomonas reinhardtii requires specific nucleus-encoded functions. To gain insight into this process, we have cloned the nuclear gene encoding the Mbb1 factor by genomic rescue of a mutant specifically deficient in the accumulation of the mRNAs of the psbB/psbT/psbH chloroplast transcription unit. Mbb1 is a soluble protein in the stromal phase of the chloroplast. It consists of 662 amino acids with a putative chloroplast-transit peptide at its N-terminal end. A striking feature is the presence of 10 tandemly arranged tetratricopeptide-like repeats that account for half of the protein sequence and are thought to be involved in protein-protein interactions. The Mbb1 protein seems to have a homologue in higher plants and is part of a 300-kDa complex that is associated with RNA. This complex is most likely involved in psbB mRNA processing, stability, and/or translation.

A cytosolic acyltransferase contributes to triacylglycerol synthesis in sucrose-rescued Arabidopsis seed oil catabolism mutants

Triacylglycerol (TAG) levels and oil bodies persist in sucrose (Suc)-rescued Arabidopsis (Arabidopsis thaliana) seedlings disrupted in seed oil catabolism. This study set out to establish if TAG levels persist as a metabolically inert pool when downstream catabolism is disrupted, or if other mechanisms, such as fatty acid (FA) recycling into TAG are operating. We show that TAG composition changes significantly in Suc-rescued seedlings compared with that found in dry seeds, with 18:2 and 18:3 accumulating. However, 20:1 FA is not efficiently recycled back into TAG in young seedlings, instead partitioning into the membrane lipid fraction and diacylglycerol. In the lipolysis mutant sugar dependent1and the β-oxidation double mutant acx1acx2 (for acyl-Coenzyme A oxidase), levels of TAG actually increased in seedlings growing on Suc. We performed a transcriptomic study and identified up-regulation of an acyltransferase gene, DIACYLGLYCEROL ACYLTRANSFERASE3 (DGAT3), with homology to a peanut (Arachis hypogaea) cytosolic acyltransferase. The acyl-Coenzyme A substrate for this acyltransferase accumulates in mutants that are blocked in oil breakdown postlipolysis. Transient expression in Nicotiana benthamiana confirmed involvement in TAG synthesis and specificity toward 18:3 and 18:2 FAs. Double-mutant analysis with the peroxisomal ATP-binding cassette transporter mutant peroxisomal ABC transporter1 indicated involvement of DGAT3 in the partitioning of 18:3 into TAG in mutant seedlings growing on Suc. Fusion of the DGAT3 protein with green fluorescent protein confirmed localization to the cytosol of N. benthamiana. This work has demonstrated active recycling of 18:2 and 18:3 FAs into TAG when seed oil breakdown is blocked in a process involving a soluble cytosolic acyltransferase.

The glucosyltransferase UGT72E2 is responsible for monolignol 4-O-glucoside production in Arabidopsis thaliana

The phenylpropanoid pathway in plants leads to the synthesis of a wide range of soluble secondary metabolites, many of which accumulate as glycosides. In Arabidopsis, a small cluster of three closely related genes, UGT72E1-E3, encode glycosyltransferases shown to glucosylate several phenylpropanoids in vitro, including monolignols, hydroxycinnamic acids and hydroxycinnamic aldehydes. The role of these genes in planta has now been investigated through genetically downregulating the expression of individual genes or silencing the entire cluster. Analysis of these transgenic Arabidopsis plants showed that the levels of coniferyl and sinapyl alcohol 4-O-glucosides that accumulate in light-grown roots were significantly reduced. A 50% reduction in both glucosides was observed in plants in which UGT72E2 was downregulated, whereas silencing the three genes led to a 90% reduction, suggesting some redundancy of function within the cluster. The gene encoding UGT72E2 was constitutively overexpressed in transgenic Arabidopsis to determine whether increased glucosylation of monolignols could influence flux through the soluble phenylpropanoid pathway. Elevated expression of UGT72E2 led to increased accumulation of monolignol glucosides in root tissues and also the appearance of these glucosides in leaves. In particular, coniferyl alcohol 4-O-glucoside accumulated to massive amounts (10 micromol g(-1) FW) in root tissues of these plants. Increased glucosylation of other phenylpropanoids also occurred in plants overexpressing this glycosyltransferase. Significantly changing the pattern of glycosides in the leaves also led to a pronounced change in accumulation of the hydroxycinnamic ester sinapoyl malate. The data demonstrate the plasticity of phenylpropanoid metabolism and the important role that glucosylation of secondary metabolites can play in cellular homeostasis.

Compromised virus-induced gene silencing in RDR6-deficient plants

RNA silencing in plants serves as a potent antiviral defense mechanism through the action of small interfering RNAs (siRNAs), which direct RNA degradation. siRNAs can be derived directly from the viral genome or via the action of host-encoded RNA-dependent RNA polymerases (RDRs). Plant genomes encode multiple RDRs, and it has been demonstrated that plants defective for RDR6 hyperaccumulate several classes of virus. In this study, we compared the effectiveness of virus-induced gene silencing (VIGS) and RNA-directed DNA methylation (RdDM) in wild-type and RDR6-deficient Nicotiana benthamiana plants. For the potexvirus Potato virus X (PVX) and the potyvirus Plum pox virus (PPV), the efficiency of both VIGS and RdDM were compromised in RDR6-defective plants despite accumulating high levels of viral siRNAs similar to infection of wild-type plants. The reduced efficiency of VIGS and RdDM was unrelated to the size class of siRNA produced and, at least for PVX, was not dependent on the presence of the virus-encoded silencing suppressor protein, 25K. We suggest that primary siRNAs produced from PVX and PPV in the absence of RDR6 may not be good effectors of silencing and that RDR6 is required to produce secondary siRNAs that drive a more effective antiviral response.

Stability determinants in the chloroplast psbB/T/H mRNAs of Chlamydomonas reinhardtii

The chloroplast gene psbB encodes the chlorophyll-a binding protein P5 (CP47), one of the core subunits of photosystem II (PSII). The psbB mRNA and the downstream psbT and psbH transcripts fail to accumulate in the Chlamydomonas reinhardtii nuclear mutant 222E affected in the Mbb1 gene (Monod et al. 1992, Mol. Gen. Genet. 231, 449-459). By introducing chimeric genes consisting of sequences from psbB and the reporter gene aadA into the chloroplast, the target site of Mbb1 was mapped in the psbB 5' untranslated region (UTR). Primer extension analysis indicates that the psbB RNA exists in a less abundant long form and a more abundant short form, with 5' ends at positions -147 and -35 relative to the AUG initiation codon, respectively. The longer transcript is present both in the wild type (WT) and 222E mutant, but the shorter one accumulates only in the WT. Two putative stem-loop structures in the longer 5' UTR can be deleted individually without affecting psbB mRNA accumulation. Insertion of a poly G cassette in the long leader stabilizes a chimeric psbB transcript in the 222E mutant, suggesting the involvement of a 5'-3' exonuclease. We also show that psbH and psbT are transcribed from the upstream psbB gene promoter, and that the psbH mRNA has its own target sequence for Mbb1 function. We discuss the role of this nucleus-encoded factor, required for specific chloroplast gene expression, in the assembly of the multi-protein PSII complex.

A DELLA in disguise: SPATULA restrains the growth of the developing Arabidopsis seedling

The period following seedling emergence is a particularly vulnerable stage in the plant life cycle. In Arabidopsis thaliana, the phytochrome-interacting factor (PIF) subgroup of basic-helix-loop-helix transcription factors has a pivotal role in regulating growth during this early phase, integrating environmental and hormonal signals. We previously showed that SPATULA (SPT), a PIF homolog, regulates seed dormancy. In this article, we establish that unlike PIFs, which mainly promote hypocotyl elongation, SPT is a potent regulator of cotyledon expansion. Here, SPT acts in an analogous manner to the gibberellin-dependent DELLAs, REPRESSOR OF GA1-3 and GIBBERELLIC ACID INSENSITIVE, which restrain cotyledon expansion alongside SPT. However, although DELLAs are not required for SPT action, we demonstrate that SPT is subject to negative regulation by DELLAs. Cross-regulation of SPT by DELLAs ensures that SPT protein levels are limited when DELLAs are abundant but rise following DELLA depletion. This regulation provides a means to prevent excessive growth suppression that would result from the dual activity of SPT and DELLAs, yet maintain growth restraint under DELLA-depleted conditions. We present evidence that SPT and DELLAs regulate common gene targets and illustrate that the balance of SPT and DELLA action depends on light quality signals in the natural environment.

Virus-induced gene silencing of argonaute genes in Nicotiana benthamiana demonstrates that extensive systemic silencing requires Argonaute1-like and Argonaute4-like genes

Several distinct pathways of RNA silencing operate in plants with roles including the suppression of virus accumulation, control of endogenous gene expression, and direction of DNA and chromatin modifications. Proteins of the Dicer-Like and Argonaute (AGO) families have key roles within these silencing pathways and have distinct biochemical properties. We are interested in the relationships between different silencing pathways and have used Nicotiana benthamiana as a model system. While not being an amenable plant for traditional genetics, N. benthamiana is extensively used for RNA-silencing studies. Using virus-induced gene silencing technology we demonstrate that both NbAGO1- and NbAGO4-like genes are required for full systemic silencing but not for silencing directed by an inverted repeat transgene. Phenotypic differences between the virus-induced gene silencing plants indicate that NbAGO1 and NbAGO4 like act at different stages of the silencing pathways. Suppression of NbAGO1 expression recapitulated the hypomorphic mutant phenotype of certain Arabidopsis (Arabidopsis thaliana) ago1 alleles, however, suppression of NbAgo4 like resulted in phenotypes differing in some respects from those reported for Arabidopsis ago4. We suggest that the small interfering RNA amplification step required for full systemic silencing is dependent upon a nuclear event requiring the activity of NbAGO4 like.

Reduced expression of FatA thioesterases in Arabidopsis affects the oil content and fatty acid composition of the seeds

Acyl-acyl carrier protein (ACP) thioesterases are enzymes that control the termination of intraplastidial fatty acid synthesis by hydrolyzing the acyl-ACP complexes. Among the different thioesterase gene families found in plants, the FatA-type fulfills a fundamental role in the export of the C18 fatty acid moieties that will be used to synthesize most plant glycerolipids. A reverse genomic approach has been used to study the FatA thioesterase in seed oil accumulation by screening different mutant collections of Arabidopsis thaliana for FatA knockouts. Two mutants were identified with T-DNA insertions in the promoter region of each of the two copies of FatA present in the Arabidopsis genome, from which a double FatA Arabidopsis mutant was made. The expression of both forms of FatA thioesterases was reduced in this double mutant (fata1 fata2), as was FatA activity. This decrease did not cause any evident morphological changes in the mutant plants, although the partial reduction of this activity affected the oil content and fatty acid composition of the Arabidopsis seeds. Thus, dry mutant seeds had less triacylglycerol content, while other neutral lipids like diacylglycerols were not affected. Furthermore, the metabolic flow of the different glycerolipid species into seed oil in the developing seeds was reduced at different stages of seed formation in the fata1 fata2 line. This diminished metabolic flow induced increases in the proportion of linolenic and erucic fatty acids in the seed oil, in a similar way as previously reported for the wri1 Arabidopsis mutant that accumulates oil poorly. The similarities between these two mutants and the origin of their phenotype are discussed in function of the results.

Redirection of flux through the phenylpropanoid pathway by increased glucosylation of soluble intermediates

The phenylpropanoid pathway is used in biosynthesis of a wide range of soluble secondary metabolites including hydroxycinnamic acid esters, flavonoids and the precursors of lignin and lignans. In Arabidopsis thaliana a small cluster of three closely related genes, UGT72E1-E3, encode glycosyltransferases (GTs) that glucosylate phenylpropanoids in vitro. This study explores the effect of constitutively over-expressing two of these GTs (UGT72E1 and E3) in planta using the CaMV-35S promoter to determine whether phenylpropanoid homeostasis can be altered in a similar manner to that achieved by over-expression of UGT72E2 as previously reported. The data show that impact of over-expressing UGT72E3 in leaves is highly similar to that of UGT72E2 in that the production of massive levels of coniferyl and sinapyl alcohol 4-O-glucosides and a substantial loss in sinapoyl malate. In contrast, the over-expression of UGT72E1 in leaves led only to minimal changes in coniferyl alcohol 4-O-glucoside and no effect was observed on sinapoyl malate levels. In roots, over-expression of both UGTs led to some increase in the accumulation of the two glucosides. The cell specificity expression of the whole UGT72E gene cluster was investigated and interestingly only UGT72E3 was found to be wound and touch responsive.

NRPD1a and NRPD1b are required to maintain post-transcriptional RNA silencing and RNA-directed DNA methylation in Arabidopsis

In plants, both transcriptional (TGS) and post-transcriptional gene silencing (PTGS) can be self-reinforcing, and this allows maintenance of silencing once the initiator has been removed or suppressed. For TGS, this can be accomplished by the generation of small interfering RNAs (siRNAs) from methylated DNA templates by RNA polymerase IV (PolIV), RNA-dependent RNA polymerase 2 (RDR2), DICER-LIKE 3 (DCL3), and the RNA-directed DNA methylation (RdDM) machinery. Maintenance of PTGS requires RNA-dependent RNA polymerase 6 (RDR6), and may be associated with DNA methylation and transitive production of secondary siRNAs. In this work, mutants defective for the NRPD1a and NRPD1b alternative largest subunits of PolIV were tested for their ability to undergo RdDM, transitive RNA silencing and maintenance of PTGS. PTGS could be initiated in both nrpd1a and nrpd1b mutants, and this was associated with production of secondary siRNAs; silencing was not maintained however. nrpd1a mutants could support RdDM although this was lost upon reversal of silencing, as was methylation in rdr6 mutants. We conclude that components of the machinery that maintain TGS are required for maintenance of PTGS, and that RDR6 uses distinct templates in the initiation and maintenance phases of RNA silencing.

A chromosome-level Amaranthus cruentus genome assembly highlights gene family evolution and biosynthetic gene clusters that may underpin the nutritional value of this traditional crop

Traditional crops have historically provided accessible and affordable nutrition to millions of rural dwellers but have been neglected, with most modern agricultural systems over-reliant on a small number of internationally traded crops. Traditional crops are typically well-adapted to local agro-ecological conditions and many are nutrient-dense. They can play a vital role in local food systems through enhanced nutrition (particularly where diets are dominated by starch crops), food security and livelihoods for smallholder farmers, and a climate-resilient and biodiverse agriculture. Using short-read, long-read and phased sequencing technologies, we generated a high-quality chromosome-level genome assembly for Amaranthus cruentus, an under-researched crop with micronutrient- and protein-rich leaves and gluten-free seed, but lacking improved varieties, with respect to productivity and quality traits. The 370.9 Mb genome demonstrates a shared whole genome duplication with a related species, Amaranthus hypochondriacus. Comparative genome analysis indicates chromosomal loss and fusion events following genome duplication that are common to both species, as well as fission of chromosome 2 in A. cruentus alone, giving rise to a haploid chromosome number of 17 (versus 16 in A. hypochondriacus). Genomic features potentially underlying the nutritional value of this crop include two A. cruentus-specific genes with a likely role in phytic acid synthesis (an anti-nutrient), expansion of ion transporter gene families, and identification of biosynthetic gene clusters conserved within the amaranth lineage. The A. cruentus genome assembly will underpin much-needed research and global breeding efforts to develop improved varieties for economically viable cultivation and realization of the benefits to global nutrition security and agrobiodiversity.

An Oleuropein β-Glucosidase from Olive Fruit Is Involved in Determining the Phenolic Composition of Virgin Olive Oil

Phenolic composition of virgin olive oil is determined by the enzymatic and/or chemical reactions that take place during olive fruit processing. Of these enzymes, β-glucosidase activity plays a relevant role in the transformation of the phenolic glycosides present in the olive fruit, generating different secoiridoid derivatives. The main goal of the present study was to characterize olive fruit β-glucosidase genes and enzymes responsible for the phenolic composition of virgin olive oil. To achieve that, we have isolated an olive β-glucosidase gene from cultivar Picual (OepGLU), expressed in Nicotiana benthamiana leaves and purified its corresponding recombinant enzyme. Western blot analysis showed that recombinant OepGLU protein is detected by an antibody raised against the purified native olive mesocarp β-glucosidase enzyme, and exhibits a deduced molecular mass of 65.0 kDa. The recombinant OepGLU enzyme showed activity on the major olive phenolic glycosides, with the highest levels with respect to oleuropein, followed by ligstroside and demethyloleuropein. In addition, expression analysis showed that olive GLU transcript level in olive fruit is spatially and temporally regulated in a cultivar-dependent manner. Furthermore, temperature, light and water regime regulate olive GLU gene expression in olive fruit mesocarp. All these data are consistent with the involvement of OepGLU enzyme in the formation of the major phenolic compounds present in virgin olive oil.

Regulation of Arabidopsis thaliana seed dormancy and germination by 12-oxo-phytodienoic acid

We previously demonstrated that the oxylipin 12-oxo-phytodienoic acid (OPDA) acts along with abscisic acid to regulate seed germination in Arabidopsis thaliana, but the mechanistic details of this synergistic interaction remain to be elucidated. Here, we show that OPDA acts through the germination inhibition effects of abscisic acid, the abscisic acid-sensing ABI5 protein, and the gibberellin-sensing RGL2 DELLA protein. We further demonstrate that OPDA also acts through another dormancy-promoting factor, MOTHER-OF-FT-AND-TFL1 (MFT). Both abscisic acid and MFT positively feed back into the OPDA pathway by promoting its accumulation. These results confirm the central role of OPDA in regulating seed dormancy and germination in A. thaliana and underline the complexity of interactions between OPDA and other dormancy-promoting factors such as abscisic acid, RGL2, and MFT.

Effect of a mutagenized acyl-ACP thioesterase FATA allele from sunflower with improved activity in tobacco leaves and Arabidopsis seeds

The substrate specificity of the acyl-acyl carrier protein (ACP) thioesterases significantly determines the type of fatty acids that are exported from plastids. Thus, designing acyl-ACP thioesterases with different substrate specificities or kinetic properties would be of interest for plant lipid biotechnology to produce oils enriched in specialty fatty acids. In the present work, the FatA thioesterase from Helianthus annuus was used to test the impact of changes in the amino acids present in the binding pocket on substrate specificity and catalytic efficiency. Amongst all the mutated enzymes studied, Q215W was especially interesting as it had higher specificity towards saturated acyl-ACP substrates and higher catalytic efficiency compared to wild-type H. annuus FatA. Null, wild type and high-efficiency alleles were transiently expressed in tobacco leaves to check their effect on lipid biosynthesis. Expression of active FatA thioesterases altered the composition of leaf triacylglycerols but did not alter total lipid content. However, the expression of the wild type and the high-efficiency alleles in Arabidopsis thaliana transgenic seeds resulted in a strong reduction in oil content and an increase in total saturated fatty acid content. The role and influence of acyl-ACP thioesterases in plant metabolism and their possible applications in lipid biotechnology are discussed.

Suppression of microRNA accumulation via RNA interference in Arabidopsis thaliana

MicroRNAs (miRNAs) are key regulatory molecules in plants. These small RNAs are processed in the nucleus from longer precursor transcripts that form distinct secondary structures. The miRNAs target specific messenger RNAs (mRNAs) and consequently down-regulate gene expression. The importance of these regulatory molecules is wide-ranging, however, few loss-of-function mutants have been identified in miRNA genes and understanding the biology of miRNA-target pairings has largely depended upon creating alterations in the sequences of the target genes. Here we demonstrate using Arabidopsis thaliana, that it is possible to use RNA interference (RNAi) to suppress accumulation of miRNAs. Significantly reduced accumulation of miR163 and miR171a was achieved using hairpin RNAi constructs that were designed to target both the primary miRNA transcripts and their promoters. The presence of DNA methylation in the targeted promoter regions suggests that inhibition of transcription of the miRNA precursors is responsible. Reduction of miRNA accumulation resulted in an increase in accumulation of the mRNA targets of these miRNAs. This demonstrates that knock-down of miRNA expression can be achieved, thereby providing a straightforward approach for disrupting miRNA-target pairings and studying miRNA functions.

cis-12-Oxo-phytodienoic acid represses Arabidopsis seed germination in shade conditions

Light-dependent seed germination is induced by gibberellins (GA) and inhibited by abscisic acid (ABA). The widely accepted view of the GA/ABA ratio controlling germination does not, however, explain the fact that seeds deficient in ABA still germinate poorly under shade conditions that repress germination. In Arabidopsis, MOTHER-OF-FT-AND-TFL1 (MFT) acts as a key negative regulator of germination, modulating GA and ABA responses under shade conditions. Under full light the oxylipin cis-12-oxo-phytodienoic acid (OPDA), a precursor of the stress-related phytohormone jasmonic acid, interacts with ABA and MFT to repress germination. Here, we show that under shade conditions both OPDA and ABA repress germination to varying extents. We demonstrate that the level of shade-induced MFT expression influences the ability of OPDA and/or ABA to fully repress germination. We also found that MFT expression decreases with seed age and this again correlates with the response of seeds to OPDA and ABA. We conclude that OPDA plays an essential role alongside ABA in repressing germination in response to shade and the combined effect of these phytohormones is integrated to a significant extent through MFT.

Generalist endophyte Phomopsis liquidambaris colonization of Oryza sativa L. promotes plant growth under nitrogen starvation

Fungal endophytes establish symbiotic relationships with host plants, which results in a mutual growth benefit. However, little is known about the plant genetic response underpinning endophyte colonization. Phomopsis liquidambaris usually lives as an endophyte in a wide range of asymptomatic hosts and promotes biotic and abiotic stress resistance. In this study, we show that under low nitrogen conditions P. liquidambaris promotes rice growth in a hydroponic system, which is free of other microorganisms. In order to gain insights into the mechanisms of plant colonization by P. liquidambaris under low nitrogen conditions, we compared root and shoot transcriptome profiles of root-inoculated rice at different colonization stages. We determined that genes related to plant growth promotion, such as gibberellin and auxin related genes, were up-regulated at all developmental stages both locally and systemically. The largest group of up-regulated genes (in both roots and shoots) were related to flavonoid biosynthesis, which is involved in plant growth as well as antimicrobial compounds. Furthermore, genes encoding plant defense-related endopeptidase inhibitors were strongly up-regulated at the early stage of colonization. Together, these results provide new insights into the molecular mechanisms of plant-microbe mutualism and the promotion of plant growth by a fungal endophyte under nitrogen-deficient conditions.

Impact on Arabidopsis growth and stress resistance of depleting the Maf1 repressor of RNA polymerase III

Maf1 is a transcription factor that is conserved in sequence and structure between yeasts, animals and plants. Its principal molecular function is also well conserved, being to bind and repress RNA polymerase (pol) III, thereby inhibiting synthesis of tRNAs and other noncoding RNAs. Restrictions on tRNA production and hence protein synthesis can provide a mechanism to preserve resources under conditions that are suboptimal for growth. Accordingly, Maf1 is found in some organisms to influence growth and/or stress survival. Because of their sessile nature, plants are especially vulnerable to environmental changes and molecular adaptations that enhance growth under benign circumstances can increase sensitivity to external challenges. We tested if Maf1 depletion in the model plant Arabidopsis affects growth, pathogen resistance and tolerance of drought or soil salinity, a common physiological challenge that imposes both osmotic and ionic stress. We find that disruption of the Maf1 gene or RNAi-mediated depletion of its transcript is well-tolerated and confers a modest growth advantage without compromising resistance to common biotic and abiotic challenges.