Higher sterol content regulated by CYP51 with concomitant lower phospholipid content in membranes is a common strategy for aluminium tolerance in several plant species

Advances and Challenges in Plant Sterol Research: Fundamentals, Analysis, Applications and Production

Plant sterols (PS) are cholesterol-like terpenoids widely spread in the kingdom Plantae. Being the target of extensive research for more than a century, PS have topped with evidence of having beneficial effects in healthy subjects and applications in food, cosmetic and pharmaceutical industries. However, many gaps in several fields of PS's research still hinder their widespread practical applications. In fact, many of the mechanisms associated with PS supplementation and their health benefits are still not fully elucidated. Furthermore, compared to cholesterol data, many complex PS chemical structures still need to be fully characterized, especially in oxidized PS. On the other hand, PS molecules have also been the focus of structural modifications for applications in diverse areas, including not only the above-mentioned but also in e.g., drug delivery systems or alternative matrixes for functional foods and fats. All the identified drawbacks are also superimposed by the need of new PS sources and technologies for their isolation and purification, taking into account increased environmental and sustainability concerns. Accordingly, current and future trends in PS research warrant discussion.

A biochemical and lipidomic approach to perceive Halimione portulacoides (L.) response to mercury: An environmental perspective

The impact of hazardous materials, such as Hg, on life is far from being understood and due to the high number of polluted sites it has generated great concern. A biochemical and lipidomic approach was used to assess the effects of Hg on the saltmarsh halophyte Halimione portulacoides. Plants were collected at two sites of a Hg contaminated saltmarsh. Hg accumulation and distribution in the plant, biochemical parameters (antioxidant and metabolic) and lipid profiles were determined and compared between plant organs and sites (s1 and s2). Hg did not induce antioxidant enzyme activity. Lipid profiles changed under Hg exposure, especially in leaves, decreasing the unsaturation level, the membrane fluidity and stability, and evidencing that membrane lipid remodeling influences plant tolerance to Hg. This knowledge can help select the most appropriate methodologies for the restoration of Hg polluted hotspots, curtailing a serious environmental problem threatening saltmarshes.

Response mechanism of Chlamydomonas reinhardtii to nanoscale bismuth oxyiodide (nano-BiOI): Integrating analysis of mineral nutrient metabolism and metabolomics

Nanoscale bismuth oxyiodide (nano-BiOI) is widely studied and applied in environmental applications and biomedical fields, with the consequence that it may be deposited into aquatic environments. However, the impact of nano-BiOI on aquatic ecosystems, especially freshwater microalga, remains limited. Herein, the nano-BiOI was synthesized and its response mechanism towards microalga Chlamydomonas reinhardtii was evaluated. Results showed that a low concentration of nano-BiOI (5 mg/L) could stimulate algal growth at the early stage of stress. With the increase in concentration, the growth rate of algal cells was inhibited and showed a dose effect. Intracellular reactive oxygen species (ROS) were significantly induced and accompanied by enhanced lipid peroxidation, decreased nonspecific esterase activity, and significantly upregulated glutathione S-transferase activity (GST) activity. Mineral nutrient metabolism analysis showed that nano-BiOI significantly interfered with the mineral nutrients of the algae. Non-targeted metabolomics identified 35 different metabolites (DEMs, 22 upregulated, and 13 downregulated) under 100 mg/L BiOI stress. Metabolic pathway analysis demonstrated that a high concentration of nano-BiOI significantly induced metabolic pathways related to amino acid biosynthesis, lipid biosynthesis, and glutathione biosynthesis, and significantly inhibited the sterol biosynthesis pathway. This finding will contribute to understanding the toxicological mechanisms of nano-BiOI on C. reinhardtii.

Arabidopsis mgd mutants with reduced monogalactosyldiacylglycerol contents are hypersensitive to aluminium stress

Aluminium (Al) is a key element that plays a major role in inhibiting plant growth and productivity under acidic soils. While lipids may be involved in plant tolerance/sensitivity to Al, the role of monogalactosyldiacylglycerol (MGDG) in Al response remains unknown. In this study, Arabidopsis MGDG synthase (AtMGD) mutants (mgd1, mgd2 and mgd3) and wild-type (Col-0) plants were treated with AlCl₃; the effect of aluminium on root growth, aluminium distribution, plasma membrane integrity, lipid peroxidation, hydrogen peroxide content and membrane lipid compositions were analysed. Under Al stress, mgd mutants exhibited a more severe root growth inhibition, plasma membrane integrity damage and lipid peroxidation compared to Col-0. Al accumulation in root tips showed no difference between Col-0 and mutants under Al stress. Lipid analysis demonstrated that under Al treatment the MGDG content in all plants and MGDG/DGDG (digalactosyldiacylglycerol) remarkably reduced, especially in mutants impairing the stability and permeability of the plasma membrane. These results indicate that the Arabidopsis mgd mutants are hypersensitive to Al stress due to the reduction in MGDG content, and this is of great significance in the discovery of effective measures for plants to inhibit aluminium toxicity.

Plant Glycosides and Glycosidases: A Treasure-Trove for Therapeutics

Plants contain numerous glycoconjugates that are metabolized by specific glucosyltransferases and hydrolyzed by specific glycosidases, some also catalyzing synthetic transglycosylation reactions. The documented value of plant-derived glycoconjugates to beneficially modulate metabolism is first addressed. Next, focus is given to glycosidases, the central theme of the review. The therapeutic value of plant glycosidases is discussed as well as the present production in plant platforms of therapeutic human glycosidases used in enzyme replacement therapies. The increasing knowledge on glycosidases, including structure and catalytic mechanism, is described. The novel insights have allowed the design of functionalized highly specific suicide inhibitors of glycosidases. These so-called activity-based probes allow unprecedented visualization of glycosidases cross-species. Here, special attention is paid on the use of such probes in plant science that promote the discovery of novel enzymes and the identification of potential therapeutic inhibitors and chaperones.

Mining candidate gene for rice aluminum tolerance through genome wide association study and transcriptomic analysis

BACKGROUND

The genetic mechanism of aluminum (Al) tolerance in rice is great complicated. Uncovering genetic mechanism of Al tolerance in rice is the premise for Al tolerance improvement. Mining elite genes within rice landrace is of importance for improvement of Al tolerance in rice.

RESULTS

Genome-wide association study (GWAS) performed in EMMAX for rice Al tolerance was carried out using 150 varieties of Ting's core collection constructed from 2262 Ting's collections with more than 3.8 million SNPs. Within Ting's core collection of clear population structure and kinship relatedness as well as high rate of linkage disequilibrium (LD) decay, 17 genes relating to rice Al tolerance including cloned genes like NRAT1, ART1 and STAR1 were identified in this study. Moreover, 13 new candidate regions with high LD and 69 new candidate genes were detected. Furthermore, 20 of 69 new candidate genes were detected with significant difference between Al treatment and without Al toxicity by transcriptome sequencing. Interestingly, both qRT-PCR and sequence analysis in CDS region demonstrated that the candidate genes in present study might play important roles in rice Al tolerance.

CONCLUSIONS

The present study provided important information for further using these elite genes existing in Ting's core collection for improvement of rice Al tolerance.

Alleviation by abscisic acid of Al toxicity in rice bean is not associated with citrate efflux but depends on ABI5-mediated signal transduction pathways

Under conditions of aluminum (Al) toxicity, which severely inhibits root growth in acidic soils, plants rapidly alter their gene expression to optimize physiological fitness for survival. Abscisic acid (ABA) has been suggested as a mediator between Al stress and gene expression, but the underlying mechanisms remain largely unknown. Here, we investigated ABA-mediated Al-stress responses, using integrated physiological and molecular biology approaches. We demonstrate that Al stress caused ABA accumulation in the root apex of rice bean (Vigna umbellata [Thunb.] Ohwi & Ohashi), which positively regulated Al tolerance. However, this was not associated with known Al-tolerance mechanisms. Transcriptomic analysis revealed that nearly one-third of the responsive genes were shared between the Al-stress and ABA treatments. We further identified a transcription factor, ABI5, as being positively involved in Al tolerance. Arabidopsis abi5 mutants displayed increased sensitivity to Al, which was not related to the regulation of AtALMT1 and AtMATE expression. Functional categorization of ABI5-mediated genes revealed the importance of cell wall modification and osmoregulation in Al tolerance, a finding supported by osmotic stress treatment on Al tolerance. Our results suggest that ABA signal transduction pathways provide an additional layer of regulatory control over Al tolerance in plants.

Dark conditions enhance aluminum tolerance in several rice cultivars via multiple modulations of membrane sterols

Aluminum-sensitive rice (Oryza sativa L.) cultivars showed increased Al tolerance under dark conditions, because less Al accumulated in the root tips (1 cm) under dark than under light conditions. Under dark conditions, the root tip concentration of total sterols, which generally reduce plasma membrane permeabilization, was higher in the most Al-sensitive japonica cultivar, Koshihikari (Ko), than in the most Al-tolerant cultivar, Rikuu-132 (R132), but the phospholipid content did not differ between the two. The Al treatment increased the proportion of stigmasterol (which has no ability to reduce membrane permeabilization) out of total sterols similarly in both cultivars under light conditions, but it decreased more in Ko under dark conditions. The carotenoid content in the root tip of Al-treated Ko was significantly lower under dark than under light conditions, indicating that isopentenyl diphosphate transport from the cytosol to plastids was decreased under dark conditions. HMG2 and HMG3 (encoding the key sterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase) transcript levels in the root tips were enhanced under dark conditions. We suggest that the following mechanisms contribute to the increase in Al tolerance under dark conditions: inhibition of stigmasterol formation to retain membrane integrity; greater partitioning of isopentenyl diphosphate for sterol biosynthesis; and enhanced expression of HMGs to increase sterol biosynthesis.

Phytohormone inhibitor treatments phenocopy brassinosteroid-gibberellin dwarf mutant interactions in maize

Phytohormone biosynthesis produces metabolites with profound effects on plant growth and development. Modulation of hormone levels during developmental events, in response to the environment, by genetic polymorphism, or by chemical application, can reveal the plant processes most responsive to a phytohormone. Applications of chemical inhibitors and subsequent measurements of specific phytohormones can determine whether, and which, phytohormone is affected by a molecule. In many cases, the sensitivity of biochemical testing has determined multiple pathways affected by a single inhibitor. Genetic studies are not subject to this problem, and a wealth of data about the morphological impacts of hormone biosynthetic inhibition have accumulated through the study of enzyme mutants. In this work, we sought to assess the specificity of three triazole inhibitors of cytochrome P450s by determining their abilities to recapitulate the phenotypes of single and double mutants affected in the production of brassinosteroid (BR) and gibberellin (GA) biosynthesis. The GA biosynthetic inhibitors uniconazole (UCZ) and paclobutrazol (PAC) were applied to the BR biosynthetic mutant nana plant2 (na2), and all double-mutant phenotypes were recovered in the UCZ treatment. PAC was unable to suppress the retention of pistils in the tassels of na2 mutant plants. The BR biosynthetic inhibitor propiconazole (PCZ) suppressed tiller outgrowth in the GA biosynthetic mutant dwarf5 (d5). All treatments were additive with genetic mutants for effects on plant height. Due to additional measurements performed here but not in previous studies of the double mutants, we detected new interactions between GA and BR biosynthesis affecting the days to tassel emergence and tassel branching. These experiments, a refinement of our previous model, and a discussion of the extension of this type of work are presented.

An RNA-Seq transcriptome analysis revealing novel insights into aluminum tolerance and accumulation in tea plant

The tea plant ( Camellia sinensis L. O. Kuntze) is a high aluminum (Al) tolerant and accumulator species. Candidate genes related to Al tolerance in tea plants were assembled based on de novo transcriptome analysis. The homologs implied some common and distinct Al-tolerant mechanism between tea plants and rice, Arabidopsis and buckwheat. In addition to high Al tolerance, the tea plant exhibits good performance exposure to a proper Al level, and accumulates high Al in the leaves without any toxicity symptom. Therefore, Al was considered as a hyperaccumulator and beneficial element for tea plants. However, the whole-genome molecular mechanisms accounting for Al-tolerance and accumulation remain unknown in tea plants. In this study, transcriptome analysis by RNA-Seq following a gradient Al-level exposure was assessed to further reveal candidate genes involved. Totally more than 468 million high-quality reads were generated and 213,699 unigenes were de novo assembled, among which 8922 unigenes were all annotated in the seven databases used. A large number of transporters, transcription factors, cytochrome P450, ubiquitin ligase, organic acid biosynthesis, heat shock proteins differentially expressed in response to high Al (P ≤ 0.05) were identified, which were most likely ideal candidates involved in the Al tolerance or accumulation. Furthermore, a few of the candidate Al-responsive genes related to Al sequestration, cell wall modification and organic acid excretion have been well elucidated as was already found in Arabidopsis, rice, and buckwheat. Thus, some consistent Al-tolerance mechanisms across the species are indicated. In conclusion, the transcriptome data provided useful insights of promising candidates for further characterizing the functions of genes involved in Al tolerance and accumulation in tea plants.

Contribution of constitutive characteristics of lipids and phenolics in roots of tree species in Myrtales to aluminum tolerance

High aluminum (Al) concentration in soil solution is the most important factor restricting plant growth in acidic soils. However, various plant species naturally grow in such soils. Generally, they are highly tolerant to Al, but organic acid exudation, the most common Al tolerance mechanism, cannot explain their tolerance. Lower phospholipid and higher sterol proportions in root plasma membrane enhance Al tolerance. Other cellular components, such as cell walls and phenolics, may also be involved in Al tolerance mechanisms. In this study, the relationships between these cellular components and the Al tolerance mechanisms in Melastoma malabathricum and Melaleuca cajuputi, both highly Al-tolerant species growing in strongly acidic soils, were investigated. Both species contained lower proportions of phospholipids and higher proportions of sterols in roots, respectively. Concentrations of phenolics in roots of both species were higher than that of rice; their phenolics could form chelates with Al. In these species, phenolic concentrations and composition were the same irrespective of the presence or absence of Al in the medium, suggesting that a higher concentration of phenolics is not a physiological response to Al but a constitutive characteristic. These characteristics of cellular components in roots may be cooperatively involved in their high Al tolerance.

Localization of Sphingolipid Enriched Plasma Membrane Regions and Long-Chain Base Composition during Mature-Fruit Abscission in Olive

Sphingolipids, found in membranes of eukaryotic cells, have been demonstrated to carry out functions in various processes in plant cells. However, the roles of these lipids in fruit abscission remain to be determined in plants. Biochemical and fluorescence microscopy imaging approach has been adopted to investigate the accumulation and distribution of sphingolipids during mature-fruit abscission in olive (Olea europaea L. cv. Picual). Here, a lipid-content analysis in live protoplasts of the olive abscission zone (AZ) was made with fluorescent dyes and lipid analogs, particularly plasma membrane sphingolipid-enriched domains, and their dynamics were investigated in relation to the timing of mature-fruit abscission. In olive AZ cells, the measured proportion of both polar lipids and sphingolipids increased as well as endocytosis was stimulated during mature-fruit abscission. Likewise, mature-fruit abscission resulted in quantitative and qualitative changes in sphingolipid long-chain bases (LCBs) in the olive AZ. The total LCB increase was due essentially to the increase of t18:1(8E) LCBs, suggesting that C-4 hydroxylation and Δ8 desaturation with a preference for (E)-isomer formation were quantitatively the most important sphingolipids in olive AZ during abscission. However, our results also showed a specific association between the dihydroxylated LCB sphinganine (d18:0) and the mature-fruit abscission. These results indicate a clear correlation between the sphingolipid composition and mature-fruit abscission. Moreover, measurements of endogenous sterol levels in the olive AZ revealed that it accumulated sitosterol and campesterol with a concomitant decrease in cycloartenol during abscission. In addition, underlying the distinct sterol composition of AZ during abscission, genes for key biosynthetic enzymes for sterol synthesis, for obtusifoliol 14α-demethylase (CYP51) and C-24 sterol methyltransferase2 (SMT2), were up-regulated during mature-fruit abscission, in parallel to the increase in sitosterol content. The differences found in AZ lipid content and the relationships established between LCB and sterol composition, offer new insights about sphingolipids and sterols in abscission.

Aluminum, a Friend or Foe of Higher Plants in Acid Soils

Aluminum (Al) is the most abundant metal in the earth's crust, but its availability depends on soil pH. Despite this abundance, Al is not considered an essential element and so far no experimental evidence has been put forward for a biological role. In plants and other organisms, Al can have a beneficial or toxic effect, depending on factors such as, metal concentration, the chemical form of Al, growth conditions and plant species. Here we review recent advances in the study of Al in plants at physiological, biochemical and molecular levels, focusing mainly on the beneficial effect of Al in plants (stimulation of root growth, increased nutrient uptake, the increase in enzyme activity, and others). In addition, we discuss the possible mechanisms involved in improving the growth of plants cultivated in soils with acid pH, as well as mechanisms of tolerance to the toxic effect of Al.

Tomato UDP-Glucose Sterol Glycosyltransferases: A Family of Developmental and Stress Regulated Genes that Encode Cytosolic and Membrane-Associated Forms of the Enzyme

Sterol glycosyltransferases (SGTs) catalyze the glycosylation of the free hydroxyl group at C-3 position of sterols to produce sterol glycosides. Glycosylated sterols and free sterols are primarily located in cell membranes where in combination with other membrane-bound lipids play a key role in modulating their properties and functioning. In contrast to most plant species, those of the genus Solanum contain very high levels of glycosylated sterols, which in the case of tomato may account for more than 85% of the total sterol content. In this study, we report the identification and functional characterization of the four members of the tomato (Solanum lycopersicum cv. Micro-Tom) SGT gene family. Expression of recombinant SlSGT proteins in E. coli cells and N. benthamiana leaves demonstrated the ability of the four enzymes to glycosylate different sterol species including cholesterol, brassicasterol, campesterol, stigmasterol, and β-sitosterol, which is consistent with the occurrence in their primary structure of the putative steroid-binding domain found in steroid UDP-glucuronosyltransferases and the UDP-sugar binding domain characteristic for a superfamily of nucleoside diphosphosugar glycosyltransferases. Subcellular localization studies based on fluorescence recovery after photobleaching and cell fractionation analyses revealed that the four tomato SGTs, like the Arabidopsis SGTs UGT80A2 and UGT80B1, localize into the cytosol and the PM, although there are clear differences in their relative distribution between these two cell fractions. The SlSGT genes have specialized but still largely overlapping expression patterns in different organs of tomato plants and throughout the different stages of fruit development and ripening. Moreover, they are differentially regulated in response to biotic and abiotic stress conditions. SlSGT4 expression increases markedly in response to osmotic, salt, and cold stress, as well as upon treatment with abscisic acid and methyl jasmonate. Stress-induced SlSGT2 expression largely parallels that of SlSGT4. On the contrary, SlSGT1 and SlSGT3 expression remains almost unaltered under the tested stress conditions. Overall, this study contributes to broaden the current knowledge on plant SGTs and provides support to the view that tomato SGTs play overlapping but not completely redundant biological functions involved in mediating developmental and stress responses.

Suppressing Farnesyl Diphosphate Synthase Alters Chloroplast Development and Triggers Sterol-Dependent Induction of Jasmonate- and Fe-Related Responses

Farnesyl diphosphate synthase (FPS) catalyzes the synthesis of farnesyl diphosphate from isopentenyl diphosphate and dimethylallyl diphosphate. Arabidopsis (Arabidopsis thaliana) contains two genes (FPS1 and FPS2) encoding FPS. Single fps1 and fps2 knockout mutants are phenotypically indistinguishable from wild-type plants, while fps1/fps2 double mutants are embryo lethal. To assess the effect of FPS down-regulation at postembryonic developmental stages, we generated Arabidopsis conditional knockdown mutants expressing artificial microRNAs devised to simultaneously silence both FPS genes. Induction of silencing from germination rapidly caused chlorosis and a strong developmental phenotype that led to seedling lethality. However, silencing of FPS after seed germination resulted in a slight developmental delay only, although leaves and cotyledons continued to show chlorosis and altered chloroplasts. Metabolomic analyses also revealed drastic changes in the profile of sterols, ubiquinones, and plastidial isoprenoids. RNA sequencing and reverse transcription-quantitative polymerase chain reaction transcriptomic analysis showed that a reduction in FPS activity levels triggers the misregulation of genes involved in biotic and abiotic stress responses, the most prominent one being the rapid induction of a set of genes related to the jasmonic acid pathway. Down-regulation of FPS also triggered an iron-deficiency transcriptional response that is consistent with the iron-deficient phenotype observed in FPS-silenced plants. The specific inhibition of the sterol biosynthesis pathway by chemical and genetic blockage mimicked these transcriptional responses, indicating that sterol depletion is the primary cause of the observed alterations. Our results highlight the importance of sterol homeostasis for normal chloroplast development and function and reveal important clues about how isoprenoid and sterol metabolism is integrated within plant physiology and development.

Regulation of Galactolipid Biosynthesis by Overexpression of the Rice MGD Gene Contributes to Enhanced Aluminum Tolerance in Tobacco

Membrane lipid alterations affect Al tolerance in plants, but little is known about the regulation of membrane lipid metabolism in response to Al stress. Transgenic tobacco (Nicotiana tabacum) overexpressing rice monogalactosyldiacylglycerol (MGDG) synthase (OsMGD) gene and wild-type tobacco plants were exposed to AlCl3, and the impact of Al toxicity on root growth, Al accumulation, plasma membrane integrity, lipid peroxidation and membrane lipid composition were investigated. Compared with the wild type, the transgenic plants exhibited rapid regrowth of roots after removal of Al and less damage to membrane integrity and lipid peroxidation under Al stress, meanwhile, the Al accumulation showed no difference between wild-type and transgenic plants. Lipid analysis showed that Al treatment dramatically decreased the content of MGDG and the ratio of MGDG to digalactosyldiacylglycerol (DGDG) in wild-type plants, while it was unchanged in transgenic plants. The stable of MGDG level and the ratio of MGDG/DGDG contribute to maintain the membrane stability and permeability. Moreover, Al caused a significant increase in phospholipids in wild-type plants, resulting in a high proportion of phospholipids and low proportion of galactolipids, but these proportions were unaffected in transgenic plants. The high proportion of phospholipids could contribute to a higher rate of Al(3+) binding in the membrane and thereby leads to more membrane perturbation and damage. These results show that the regulation of galactolipid biosynthesis could play an important role in maintaining membrane structure and function under Al stress.