Diverging cell wall strategies for drought adaptation in two maize inbreds with contrasting lodging resistance.

The plant cell wall is a plastic structure of variable composition that constitutes the first line of defence against environmental challenges. Lodging and drought are two stressful conditions that severely impact maize yield. In a previous work, we characterised the cell walls of two maize inbreds, EA2024 (susceptible) and B73 (resistant) to stalk lodging. Here, we show that drought induces distinct phenotypical, physiological, cell wall, and transcriptional changes in the two inbreds, with B73 exhibiting lower tolerance to this stress than EA2024. In control conditions, EA2024 stalks had higher levels of cellulose, uronic acids and p-coumarate than B73. However, upon drought EA2024 displayed increased levels of arabinose-enriched polymers, such as pectin-arabinans and arabinogalactan proteins, and a decreased lignin content. By contrast, B73 displayed a deeper rearrangement of cell walls upon drought, including modifications in lignin composition (increased S subunits and S/G ratio; decreased H subunits) and an increase of uronic acids. Drought induced more substantial changes in gene expression in B73 compared to EA2024, particularly in cell wall-related genes, that were modulated in an inbred-specific manner. Transcription factor enrichment assays unveiled inbred-specific regulatory networks coordinating cell wall genes expression. Altogether, these findings reveal that B73 and EA2024 inbreds, with opposite stalk-lodging phenotypes, undertake different cell wall modification strategies in response to drought. We propose that the specific cell wall composition conferring lodging resistance to B73, compromises its cell wall plasticity, and renders this inbred more susceptible to drought.

Loose Morphology and High Dynamism of OSER Structures Induced by the Membrane Domain of HMG-CoA Reductase.

The membrane domain of eukaryotic HMG-CoA reductase (HMGR) has the conserved capacity to induce endoplasmic reticulum (ER) proliferation and membrane association into Organized Smooth Endoplasmic Reticulum (OSER) structures. These formations develop in response to overexpression of particular proteins, but also occur naturally in cells of the three eukaryotic kingdoms. Here, we characterize OSER structures induced by the membrane domain of Arabidopsis HMGR (1S domain). Immunochemical confocal and electron microscopy studies demonstrate that the 1S:GFP chimera co-localizes with high levels of endogenous HMGR in several ER compartments, such as the ER network, the nuclear envelope, the outer and internal membranes of HMGR vesicles and the OSER structures, which we name ER-HMGR domains. After high-pressure freezing, ER-HMGR domains show typical crystalloid, whorled and lamellar ultrastructural patterns, but with wide heterogeneous luminal spaces, indicating that the native OSER is looser and more flexible than previously reported. The formation of ER-HMGR domains is reversible. OSER structures grow by incorporation of ER membranes on their periphery and progressive compaction to the inside. The ER-HMGR domains are highly dynamic in their formation versus their disassembly, their variable spherical-ovoid shape, their fluctuating borders and their rapid intracellular movement, indicating that they are not mere ER membrane aggregates, but active components of the eukaryotic cell.

An engineered extraplastidial pathway for carotenoid biofortification of leaves.

Carotenoids are lipophilic plastidial isoprenoids highly valued as nutrients and natural pigments. A correct balance of chlorophylls and carotenoids is required for photosynthesis and therefore highly regulated, making carotenoid enrichment of green tissues challenging. Here we show that leaf carotenoid levels can be boosted through engineering their biosynthesis outside the chloroplast. Transient expression experiments in Nicotiana benthamiana leaves indicated that high extraplastidial production of carotenoids requires an enhanced supply of their isoprenoid precursors in the cytosol, which was achieved using a deregulated form of the main rate-determining enzyme of the mevalonic acid (MVA) pathway. Constructs encoding bacterial enzymes were used to convert these MVA-derived precursors into carotenoid biosynthetic intermediates that do not normally accumulate in leaves, such as phytoene and lycopene. Cytosolic versions of these enzymes produced extraplastidial carotenoids at levels similar to those of total endogenous (i.e. chloroplast) carotenoids. Strategies to enhance the development of endomembrane structures and lipid bodies as potential extraplastidial carotenoid storage systems were not successful to further increase carotenoid contents. Phytoene was found to be more bioaccessible when accumulated outside plastids, whereas lycopene formed cytosolic crystalloids very similar to those found in the chromoplasts of ripe tomatoes. This extraplastidial production of phytoene and lycopene led to an increased antioxidant capacity of leaves. Finally, we demonstrate that our system can be adapted for the biofortification of leafy vegetables such as lettuce.

Proliferation and Morphogenesis of the Endoplasmic Reticulum Driven by the Membrane Domain of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase in Plant Cells.

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis and is composed of an endoplasmic reticulum (ER)-anchoring membrane domain with low sequence similarity among eukaryotic kingdoms and a conserved cytosolic catalytic domain. Organized smooth endoplasmic reticulum (OSER) structures are common formations of hypertrophied tightly packed ER membranes devoted to specific biosynthetic and secretory functions, the biogenesis of which remains largely unexplored. We show that the membrane domain of plant HMGR suffices to trigger ER proliferation and OSER biogenesis. The proliferating membranes become highly enriched in HMGR protein, but they do not accumulate sterols, indicating a morphogenetic rather than a metabolic role for HMGR. The N-terminal MDVRRRPP motif present in most plant HMGR isoforms is not required for retention in the ER, which was previously proposed, but functions as an ER morphogenic signal. Plant OSER structures are morphologically similar to those of animal cells, emerge from tripartite ER junctions, and mainly build up beside the nuclear envelope, indicating conserved OSER biogenesis in high eukaryotes. Factors other than the OSER-inducing HMGR construct mediate the tight apposition of the proliferating membranes, implying separate ER proliferation and membrane association steps. Overexpression of the membrane domain of Arabidopsis (Arabidopsis thaliana) HMGR leads to ER hypertrophy in every tested cell type and plant species, whereas the knockout of the HMG1 gene from Arabidopsis, encoding its major HMGR isoform, causes ER aggregation at the nuclear envelope. Our results show that the membrane domain of HMGR contributes to ER morphogenesis in plant cells.

Determination of 3-hydroxy-3-methylglutaryl CoA reductase activity in plants.

The enzyme 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase catalyzes the NADPH-mediated reductive deacylation of HMG-CoA to mevalonic acid, which is the first committed step of the mevalonate pathway for isoprenoid biosynthesis. In agreement with its key regulatory role in the pathway, plant HMG-CoA reductase is modulated by many diverse external stimuli and endogenous factors and can be detected to variable levels in every plant tissue. A fine determination of HMG-CoA reductase activity levels is required to understand its contribution to plant development and adaptation to changing environmental conditions. Here, we report a procedure to reliably determine HMG-CoA reductase activity in plants. The method includes the sample collection and homogenization strategies as well as the specific activity determination based on a classical radiochemical assay.

The protein quality control system manages plant defence compound synthesis.

Jasmonates are ubiquitous oxylipin-derived phytohormones that are essential in the regulation of many development, growth and defence processes. Across the plant kingdom, jasmonates act as elicitors of the production of bioactive secondary metabolites that serve in defence against attackers. Knowledge of the conserved jasmonate perception and early signalling machineries is increasing, but the downstream mechanisms that regulate defence metabolism remain largely unknown. Here we show that, in the legume Medicago truncatula, jasmonate recruits the endoplasmic-reticulum-associated degradation (ERAD) quality control system to manage the production of triterpene saponins, widespread bioactive compounds that share a biogenic origin with sterols. An ERAD-type RING membrane-anchor E3 ubiquitin ligase is co-expressed with saponin synthesis enzymes to control the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the rate-limiting enzyme in the supply of the ubiquitous terpene precursor isopentenyl diphosphate. Thus, unrestrained bioactive saponin accumulation is prevented and plant development and integrity secured. This control apparatus is equivalent to the ERAD system that regulates sterol synthesis in yeasts and mammals but that uses distinct E3 ubiquitin ligases, of the HMGR degradation 1 (HRD1) type, to direct destruction of HMGR. Hence, the general principles for the management of sterol and triterpene saponin biosynthesis are conserved across eukaryotes but can be controlled by divergent regulatory cues.

Functional and evolutionary analysis of DXL1, a non-essential gene encoding a 1-deoxy-D-xylulose 5-phosphate synthase like protein in Arabidopsis thaliana.

The synthesis of 1-deoxy-D-xylulose 5-phosphate (DXP), catalyzed by the enzyme DXP synthase (DXS), represents a key regulatory step of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for isoprenoid biosynthesis. In plants DXS is encoded by small multigene families that can be classified into, at least, three specialized subfamilies. Arabidopsis thaliana contains three genes encoding proteins with similarity to DXS, including the well-known DXS1/CLA1 gene, which clusters within subfamily I. The remaining proteins, initially named DXS2 and DXS3, have not yet been characterized. Here we report the expression and functional analysis of A. thaliana DXS2. Unexpectedly, the expression of DXS2 failed to rescue Escherichia coli and A. thaliana mutants defective in DXS activity. Coherently, we found that DXS activity was negligible in vitro, being renamed as DXL1 following recent nomenclature recommendation. DXL1 is targeted to plastids as DXS1, but shows a distinct expression pattern. The phenotypic analysis of a DXL1 defective mutant revealed that the function of the encoded protein is not essential for growth and development. Evolutionary analyses indicated that DXL1 emerged from DXS1 through a recent duplication apparently specific of the Brassicaceae lineage. Divergent selective constraints would have affected a significant fraction of sites after diversification of the paralogues. Furthermore, amino acids subjected to divergent selection and likely critical for functional divergence through the acquisition of a novel, although not yet known, biochemical function, were identified. Our results provide with the first evidences of functional specialization at both the regulatory and biochemical level within the plant DXS family.

Modulation of plant HMG-CoA reductase by protein phosphatase 2A: positive and negative control at a key node of metabolism.

The enzyme HMG-CoA reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis, critical not only for normal plant development, but also for the adaptation to demanding environmental conditions. Consistent with this notion, plant HMGR is modulated by many diverse endogenous signals and external stimuli. Protein phosphatase 2A (PP2A) is involved in auxin, abscisic acid, ethylene and brassinosteroid signaling and now emerges as a positive and negative multilevel regulator of plant HMGR, both during normal growth and in response to a variety of stress conditions. The interaction with HMGR is mediated by B" regulatory subunits of PP2A, which are also calcium binding proteins. The new discoveries uncover the potential of PP2A to integrate developmental and calcium-mediated environmental signals in the control of plant HMGR.

Multilevel control of Arabidopsis 3-hydroxy-3-methylglutaryl coenzyme A reductase by protein phosphatase 2A.

Plants synthesize a myriad of isoprenoid products that are required both for essential constitutive processes and for adaptive responses to the environment. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a key regulatory step of the mevalonate pathway for isoprenoid biosynthesis and is modulated by many endogenous and external stimuli. In spite of that, no protein factor interacting with and regulating plant HMGR in vivo has been described so far. Here, we report the identification of two B'' regulatory subunits of protein phosphatase 2A (PP2A), designated B''α and B''ß, that interact with HMGR1S and HMGR1L, the major isoforms of Arabidopsis thaliana HMGR. B''α and B''ß are Ca²âº binding proteins of the EF-hand type. We show that HMGR transcript, protein, and activity levels are modulated by PP2A in Arabidopsis. When seedlings are transferred to salt-containing medium, B''α and PP2A mediate the decrease and subsequent increase of HMGR activity, which results from a steady rise of HMGR1-encoding transcript levels and an initial sharper reduction of HMGR protein level. In unchallenged plants, PP2A is a posttranslational negative regulator of HMGR activity with the participation of B''ß. Our data indicate that PP2A exerts multilevel control on HMGR through the five-member B'' protein family during normal development and in response to a variety of stress conditions.

Characterisation of the gene family encoding acetoacetyl-CoA thiolase in Arabidopsis.

Thiolases are ubiquitous enzymes involved in many essential biochemical processes. Biosynthetic thiolases, also known as acetoacetyl-CoA thiolases (AACT), catalyse a reversible Claisen-type condensation of two acetyl-CoA molecules to form acetoacetyl-CoA. Here, we report the characterisation of two genes from Arabidopsis thaliana L., ACT1 and ACT2, which encode two closely related AACT isoforms (AACT1 and AACT2, respectively). Transient expression of constructs encoding AACT1 and AACT2 fused to GFP revealed that the two proteins show a different subcellular localisation. While AACT1 is found in peroxisomes, AACT2 localises in the cytosol and the nucleus. The peroxisomal localisation of AACT1 depends on the presence of a C-terminal peroxisomal targeting sequence (PTS1) motif (Ser-Ala-Leu) not previously found in other organisms. ACT1 and ACT2 genes are also differentially expressed. Whereas ACT2 is expressed at relatively high level in all plant tissues, the expression of ACT1 is restricted to roots and inflorescences and its transcript is present at very low levels. The obtained results are in agreement with the involvement of AACT2 in catalysing the first step of the mevalonate pathway. The metabolic function of AACT1 is not clear at present, although its particular peroxisomal localisation might exclude a role in isoprenoid biosynthesis.

Enhanced flux through the methylerythritol 4-phosphate pathway in Arabidopsis plants overexpressing deoxyxylulose 5-phosphate reductoisomerase.

The methylerythritol 4-phosphate (MEP) pathway synthesizes the precursors for an astonishing diversity of plastid isoprenoids, including the major photosynthetic pigments chlorophylls and carotenoids. Since the identification of the first two enzymes of the pathway, deoxyxylulose 5-phoshate (DXP) synthase (DXS) and DXP reductoisomerase (DXR), they both were proposed as potential control points. Increased DXS activity has been shown to up-regulate the production of plastid isoprenoids in all systems tested, but the relative contribution of DXR to the supply of isoprenoid precursors is less clear. In this work, we have generated transgenic Arabidopsis thaliana plants with altered DXS and DXR enzyme levels, as estimated from their resistance to clomazone and fosmidomycin, respectively. The down-regulation of DXR resulted in variegation, reduced pigmentation and defects in chloroplast development, whereas DXR-overexpressing lines showed an increased accumulation of MEP- derived plastid isoprenoids such as chlorophylls, carotenoids, and taxadiene in transgenic plants engineered to produce this non-native isoprenoid. Changes in DXR levels in transgenic plants did not result in changes in DXS gene expression or enzyme accumulation, confirming that the observed effects on plastid isoprenoid levels in DXR-overexpressing lines were not an indirect consequence of altering DXS levels. The results indicate that the biosynthesis of MEP (the first committed intermediate of the pathway) limits the production of downstream isoprenoids in Arabidopsis chloroplasts, supporting a role for DXR in the control of the metabolic flux through the MEP pathway.

Arabidopsis thaliana expresses two functional isoforms of Arvp, a protein involved in the regulation of cellular lipid homeostasis.

Arv1p is involved in the regulation of cellular lipid homeostasis in the yeast Saccharomyces cerevisiae. Here, we report the characterization of the two Arabidopsis thaliana ARV genes and the encoded proteins, AtArv1p and AtArv2p. The functional identity of AtArv1p and AtArv2p was demonstrated by complementation of the thermosensitive phenotype of the arv1Delta yeast mutant strain YJN1756. Both A. thaliana proteins contain the bipartite Arv1 homology domain (AHD), which consists of an NH(2)-terminal cysteine-rich subdomain with a putative zinc-binding motif followed by a C-terminal subdomain of 33 amino acids. Removal of the cysteine-rich subdomain has no effect on Arvp activity, whereas the presence of the C-terminal subdomain of the AHD is critical for Arvp function. Localization experiments of AtArv1p and AtArv2p tagged with green fluorescent protein (GFP) and expressed in onion epidermal cells demonstrated that both proteins are exclusively targeted to the endoplasmic reticulum. Analysis of beta-glucuronidase (GUS) activity in transgenic A. thaliana plants carrying chimeric ARV1::GUS and ARV2::GUS genes showed that ARV gene promoters direct largely overlapping patterns of expression that are restricted to tissues in which cells are actively dividing or expanding. The results of this study support the notion that plants, yeast and mammals share common molecular mechanisms regulating intracellular lipid homeostasis.

Subcellular localization of Arabidopsis 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

Plants produce diverse isoprenoids, which are synthesized in plastids, mitochondria, endoplasmic reticulum (ER), and the nonorganellar cytoplasm. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the synthesis of mevalonate, a rate-limiting step in the cytoplasmic pathway. Several branches of the pathway lead to the synthesis of structurally and functionally varied, yet essential, isoprenoids. Several HMGR isoforms have been identified in all plants examined. Studies based on gene expression and on fractionation of enzyme activity suggested that subcellular compartmentalization of HMGR is an important intracellular channeling mechanism for the production of the specific classes of isoprenoids. Plant HMGR has been shown previously to insert in vitro into the membrane of microsomal vesicles, but the final in vivo subcellular localization(s) remains controversial. To address the latter in Arabidopsis (Arabidopsis thaliana) cells, we conducted a multipronged microscopy and cell fractionation approach that included imaging of chimeric HMGR green fluorescent protein localizations in transiently transformed cell leaves, immunofluorescence confocal microscopy in wild-type and stably transformed seedlings, immunogold electron microscopy examinations of endogenous HMGR in seedling cotyledons, and sucrose density gradient analyses of HMGR-containing organelles. Taken together, the results reveal that endogenous Arabidopsis HMGR is localized at steady state within ER as expected, but surprisingly also predominantly within spherical, vesicular structures that range from 0.2- to 0.6-microm diameter, located in the cytoplasm and within the central vacuole in differentiated cotyledon cells. The N-terminal region, including the transmembrane domain of HMGR, was found to be necessary and sufficient for directing HMGR to ER and the spherical structures. It is believed, although not directly demonstrated, that these vesicle-like structures are derived from segments of HMGR-ER. Nevertheless, they represent a previously undescribed subcellular compartment likely capable of synthesizing mevalonate, which provides new evidence for multiorganelle compartmentalization of the isoprenoid biosynthetic pathways in plants.

Expression and molecular analysis of the Arabidopsis DXR gene encoding 1-deoxy-D-xylulose 5-phosphate reductoisomerase, the first committed enzyme of the 2-C-methyl-D-erythritol 4-phosphate pathway.

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) catalyzes the first committed step of the 2-C-methyl-D-erythritol 4-phosphate pathway for isoprenoid biosynthesis. In Arabidopsis, DXR is encoded by a single-copy gene. We have cloned a full-length cDNA corresponding to this gene. A comparative analysis of all plant DXR sequences known to date predicted an N-terminal transit peptide for plastids, with a conserved cleavage site, and a conserved proline-rich region at the N terminus of the mature protein, which is not present in the prokaryotic DXR homologs. We demonstrate that Arabidopsis DXR is targeted to plastids and localizes into chloroplasts of leaf cells. The presence of the proline-rich region in the mature Arabidopsis DXR was confirmed by detection with a specific antibody. A proof of the enzymatic function of this protein was obtained by complementation of an Escherichia coli mutant defective in DXR activity. The expression pattern of beta-glucuronidase, driven by the DXR promoter in Arabidopsis transgenic plants, together with the tissue distribution of DXR transcript and protein, revealed developmental and environmental regulation of the DXR gene. The expression pattern of the DXR gene parallels that of the Arabidopsis 1-deoxy-D-xylulose 5-phosphate synthase gene, but the former is slightly more restricted. These genes are expressed in most organs of the plant including roots, with higher levels in seedlings and inflorescences. The block of the 2-C-methyl-D-erythritol 4-phosphate pathway in Arabidopsis seedlings with fosmidomycin led to a rapid accumulation of DXR protein, whereas the 1-deoxy-D-xylulose 5-phosphate synthase protein level was not altered. Our results are consistent with the participation of the Arabidopsis DXR gene in the control of the 2-C-methyl-D-erythritol 4-phosphate pathway.

Functional analysis of the Arabidopsis thaliana GCPE protein involved in plastid isoprenoid biosynthesis.

Plastid isoprenoids are synthesized via the 2-C-methyl-D-erythritol 4-phosphate pathway. A few years after its discovery, most of the Escherichia coli genes involved in the pathway have been identified, including gcpE. In this work, we have identified an Arabidopsis thaliana protein with homology to the product of this gene. The plant polypeptide, GCPE, contains two structural domains that are absent in the E. coli protein: an N-terminal extension and a central domain of 30 kDa. We demonstrate that the N-terminal region targets the Arabidopsis protein to chloroplasts in vivo, consistent with its role in plastid isoprenoid biosynthesis. Although the presence of the internal extra domain may have an effect on activity, the Arabidopsis mature GCPE was able to complement a gcpE-defective E. coli strain, indicating the plant protein is a true functional homologue of the bacterial gcpE gene product.