Molecular genetics, seed morphology and fatty acids diversity in castor (Ricinus communis L., Euphorbiaceae) Iranian populations

BACKGROUND

Castor (Ricinus communis L.) seeds contain a large amount of oil that has several biological activities. In the current research, phytogeographic distribution, seed morphological characteristics, molecular genetic diversity and structure, and fatty acid composition were investigated in nine Iranian castor populations.

METHODS AND RESULTS

The cetyltrimethylammonium bromide (CTAB) protocol was used to extract the nuclear genomes. These were later amplified using 13 SCoT molecular primers. The phytogeographic distribution was determined based on the Zohary mapping, GC apparatus determined the fatty acid composition of the seeds. GenAlex, STRUCTURE, GenoDive, PopGene, and PopART software were used for the statistical analyzes. On phytogeographic mapping, the harvested populations belonged to different districts of the Euro-Siberian and Irano-Turanian regions (Holarctic kingdom). Most of the quantitative morphological traits of the seeds differed significantly (P ≤ 0.05) between the populations. The AMOVA test demonstrated a large proportion of significant genetic diversity assigned among populations, which were approved by some estimated parameters of genetic diversity such as Nm, Ht, Hs, and Gst. Nei's genetic distance and structure analysis confirmed the existence of two main genotype groups and some intermediates. However, there was no isolation by distance between the genotypes. Unsaturated fatty acids were detected as the main component of seed oil with linoleic and ricinoleic acids. Significant correlations were detected between the main fatty acids of seed oil with seed morphological traits, geographic distance and the geographic parameters of habitats. According to the composition of the seed fatty acids, four chemotypes groups were detected.

CONCLUSIONS

The classification patterns of the populations based on molecular genetic data, fatty acid composition, and phytogeographic mapping were not identical. These findings indicated that Iranian castor populations had unusual seed fatty acid composition which strongly depended on habitat geographic factors and seed morphological traits. However, the identified chemotypes and genotypes can be used in future breeding programs.

Genome-wide identification of core components of ABA signaling and transcriptome analysis reveals gene circuits involved in castor bean (Ricinus communis L.) response to drought

Castor bean (Ricinus communis L.) can withstand long periods of water deficit and high temperatures, and therefore has been recognized as a drought-resistant plant species, allowing the study of gene networks involved in drought response and tolerance. The identification of genes networks related to drought response in this plant may yield important information in the characterization of molecular mechanisms correlating changes in the gene expression with the physiological adaptation processes. In this context, gene families related to abscisic acid (ABA) signaling play a crucial role in developmental and environmental adaptation processes of plants to drought stress. However, the families that function as the core components of ABA signaling, as well as genes networks related to drought response, are not well understood in castor bean. In this study 7 RcPYL, 63 RcPP2C, and 6 RcSnRK2 genes were identified in castor bean genome, which was further supported by chromosomal distribution, gene structure, evolutionary relationships, and conserved motif analyses. The castor bean general expression profile was investigated by RNAseq in root and leaf tissues in response to drought stress. These analyses allowed the identification of genes differentially expressed, including genes from the ABA signaling core, genes related to photosynthesis, cell wall, energy transduction, antioxidant response, and transcription factors. These analyses provide new insights into the core components of ABA signaling in castor bean, allow the identification of several molecular responses associated with the high physiological adaptation of castor bean to drought stress, and contribute to the identification of candidate genes for genetic improvement.

Substrate profiling of the Arabidopsis Ca2+-dependent protein kinase AtCPK4 and its Ricinus communis ortholog RcCDPK1

AtCPK4 and AtCPK11 are Arabidopsis thaliana Ca²⁺-dependent protein kinase (CDPK) paralogs that have been reported to positively regulate abscisic acid (ABA) signal transduction by phosphorylating ABA-responsive transcription factor-4 (AtABF4). By contrast, RcCDPK1, their closest Ricinus communis ortholog, participates in the control of anaplerotic carbon flux in developing castor oil seeds by catalyzing inhibitory phosphorylation of bacterial-type phosphoenolpyruvate carboxylase at Ser451. LC-MS/MS revealed that AtCPK4 and RcCDPK1 transphosphorylated several common, conserved residues of AtABF4 and its castor ortholog, TRANSCRIPTION FACTOR RESPONSIBLE FOR ABA REGULATON. Arabidopsis atcpk4/atcpk11 mutants displayed an ABA-insensitive phenotype that corroborated the involvement of AtCPK4/11 in ABA signaling. A kinase-client assay was employed to identify additional AtCPK4/RcCDPK1 targets. Both CDPKs were separately incubated with a library of 2095 peptides representative of Arabidopsis protein phosphosites; five overlapping targets were identified including PLANT INTRACELLULAR RAS-GROUP-RELATED LEUCINE-RICH REPEAT PROTEIN-9 (AtPIRL9) and the E3-ubiquitin ligase ARABIDOPSIS TOXICOS EN LEVADURA 6 (AtATL6). AtPIRL9 and AtATL6 residues phosphorylated by AtCPK4/RcCDPK1 conformed to a CDPK recognition motif that was conserved amongst their respective orthologs. Collectively, this study provides evidence for novel AtCPK4/RcCDPK1 substrates, which may help to expand regulatory networks linked to Ca²⁺- and ABA-signaling, immune responses, and central carbon metabolism.

Regulation of capsule spine formation in castor

Castor (Ricinus communis L.) is a dicotyledonous oilseed crop that can have either spineless or spiny capsules. Spines are protuberant structures that differ from thorns or prickles. The developmental regulatory mechanisms governing spine formation in castor or other plants have remained largely unknown. Herein, using map-based cloning in 2 independent F2 populations, F2-LYY5/DL01 and F2-LYY9/DL01, we identified the RcMYB106 (myb domain protein 106) transcription factor as a key regulator of capsule spine development in castor. Haplotype analyses demonstrated that either a 4,353-bp deletion in the promoter or a single nucleotide polymorphism leading to a premature stop codon in the RcMYB106 gene could cause the spineless capsule phenotype in castor. Results of our experiments indicated that RcMYB106 might target the downstream gene RcWIN1 (WAX INDUCER1), which encodes an ethylene response factor known to be involved in trichome formation in Arabidopsis (Arabidopsis thaliana) to control capsule spine development in castor. This hypothesis, however, remains to be further tested. Nevertheless, our study reveals a potential molecular regulatory mechanism underlying the spine capsule trait in a nonmodel plant species.

Phosphatidylcholine:diacylglycerol cholinephosphotransferase's unique regulation of castor bean oil quality

Castor bean (Ricinus communis) seed oil (triacylglycerol [TAG]) is composed of ∼90% of the industrially important ricinoleoyl (12-hydroxy-9-octadecenoyl) groups. Here, phosphatidylcholine (PC):diacylglycerol (DAG) cholinephosphotransferase (PDCT) from castor bean was biochemically characterized and compared with camelina (Camelina sativa) PDCT. DAGs with ricinoleoyl groups were poorly used by Camelina PDCT, and their presence inhibited the utilization of DAG with "common" acyl groups. In contrast, castor PDCT utilized DAG with ricinoleoyl groups similarly to DAG with common acyl groups and showed a 10-fold selectivity for DAG with one ricinoleoyl group over DAG with two ricinoleoyl groups. Castor DAG acyltransferase2 specificities and selectivities toward different DAG and acyl-CoA species were assessed and shown to not acylate DAG without ricinoleoyl groups in the presence of ricinoleoyl-containing DAG. Eighty-five percent of the DAG species in microsomal membranes prepared from developing castor endosperm lacked ricinoleoyl groups. Most of these species were predicted to be derived from PC, which had been formed by PDCT in exchange with DAG with one ricinoleoyl group. A scheme of the function of PDCT in castor endosperm is proposed where one ricinoleoyl group from de novo-synthesized DAG is selectivity transferred to PC. Nonricinoleate DAG is formed and ricinoleoyl groups entering PC are re-used either in de novo synthesis of DAG with two ricinoleoyl groups or in direct synthesis of triricinoleoyl TAG by PDAT. The PC-derived DAG is not used in TAG synthesis but is proposed to serve as a substrate in membrane lipid biosynthesis during oil deposition.

Different acyl-CoA:diacylglycerol acyltransferases vary widely in function, and a targeted amino acid substitution enhances oil accumulation

Triacylglycerols (TAGs) are the major component of plant storage lipids such as oils. Acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyzes the final step of the Kennedy pathway, and is mainly responsible for plant oil accumulation. We previously found that the activity of Vernonia DGAT1 was distinctively higher than that of Arabidopsis and soybean DGAT1 in a yeast microsome assay. In this study, the DGAT1 cDNAs of Arabidopsis, Vernonia, soybean, and castor bean were introduced into Arabidopsis. All Vernonia DGAT1-expressing lines showed a significantly higher oil content (49% mean increase compared with the wild-type) followed by soybean and castor bean. Most Arabidopsis DGAT1-overexpressing lines did not show a significant increase. In addition to these four DGAT1 genes, sunflower, Jatropha, and sesame DGAT1 genes were introduced into a TAG biosynthesis-defective yeast mutant. In the yeast expression culture, DGAT1s from Arabidopsis, castor bean, and soybean only slightly increased the TAG content; however, DGAT1s from Vernonia, sunflower, Jatropha, and sesame increased TAG content >10-fold more than the former three DGAT1s. Three amino acid residues were characteristically common in the latter four DGAT1s. Using soybean DGAT1, these amino acid substitutions were created by site-directed mutagenesis and substantially increased the TAG content.

Effect of arsenate toxicity on antioxidant enzymes and expression of nicotianamine synthase in contrasting genotypes of bioenergy crop Ricinus communis

Arsenic (As) is a toxic environmental pollutant. Growing Ricinus communis (castor) on As-contaminated land has the advantage that in addition to revegetation of contaminated land, it can produce bioenergy. To date, As tolerance mechanisms of this plant are not fully understood. In our previous study, we screened tolerant and sensitive genotypes of castor and reported higher total As concentration, enhanced reactive oxygen species (ROS) generation, and oxidative stress in sensitive genotypes of castor GCH 2 and GCH 4 in comparison to tolerant genotypes WM and DCH 177. In the present study, we compared the activity, isoenzyme profile, and gene expression of ROS-scavenging enzymes, proline content, and expression of nicotianamine synthase genes (RcNAS1, RcNAS2, and RcNAS3) in As-tolerant and As-sensitive genotypes of castor. SOD and GPX activity increased significantly in roots of tolerant genotype WM but remained the same or decreased in sensitive genotype GCH 2 and GCH 4 at 200 μM arsenate [As(V)] treatment indicating their important role in As tolerance in castor. CAT activity and proline content increased in sensitive genotypes but remained the same in tolerant genotypes due to As(V) treatment. APX activity showed no significant change in roots and leaves of both tolerant and sensitive genotypes. NAS genes (RcNAS1, RcNAS2, and RcNAS3) encode enzymes that catalyze trimerization of S-adenosylmethionine to form nicotianamine and are critical for metal chelation and heavy metal tolerance. Differential responses of RcNAS1, RcNAS2, and RcNAS3 genes in WM and GCH 2 due to As(V) treatment suggest their role in As(V) tolerance.

Physaria fendleri and Ricinus communis lecithin:cholesterol acyltransferase-like phospholipases selectively cleave hydroxy acyl chains from phosphatidylcholine

Production of hydroxy fatty acids (HFAs) in transgenic crops represents a promising strategy to meet our demands for specialized plant oils with industrial applications. The expression of Ricinus communis (castor) OLEATE 12-HYDROXYLASE (RcFAH12) in Arabidopsis has resulted in only limited accumulation of HFAs in seeds, which probably results from inefficient transfer of HFAs from their site of synthesis (phosphatidylcholine; PC) to triacylglycerol (TAG), especially at the sn-1/3 positions of TAG. Phospholipase As (PLAs) may be directly involved in the liberation of HFAs from PC, but the functions of their over-expression in HFA accumulation and distribution at TAG in transgenic plants have not been well studied. In this work, the functions of lecithin:cholesterol acyltransferase-like PLAs (LCAT-PLAs) in HFA biosynthesis were characterized. The LCAT-PLAs were shown to exhibit homology to LCAT and mammalian lysosomal PLA₂ , and to contain a conserved and functional Ser/His/Asp catalytic triad. In vitro assays revealed that LCAT-PLAs from the HFA-accumulating plant species Physaria fendleri (PfLCAT-PLA) and castor (RcLCAT-PLA) could cleave acyl chains at both the sn-1 and sn-2 positions of PC, and displayed substrate selectivity towards sn-2-ricinoleoyl-PC over sn-2-oleoyl-PC. Furthermore, co-expression of RcFAH12 with PfLCAT-PLA or RcLCAT-PLA, but not Arabidopsis AtLCAT-PLA, resulted in increased occupation of HFA at the sn-1/3 positions of TAG as well as small but insignificant increases in HFA levels in Arabidopsis seeds compared with RcFAH12 expression alone. Therefore, PfLCAT-PLA and RcLCAT-PLA may contribute to HFA turnover on PC, and represent potential candidates for engineering the production of unusual fatty acids in crops.

The genome of Ricinus communis encodes a single glycolate oxidase with different functions in photosynthetic and heterotrophic organs

The biochemical characterization of glycolate oxidase in Ricinus communis hints to different physiological functions of the enzyme depending on the organ in which it is active. Enzymatic activities of the photorespiratory pathway are not restricted to green tissues but are present also in heterotrophic organs. High glycolate oxidase (GOX) activity was detected in the endosperm of Ricinus communis. Phylogenetic analysis of the Ricinus L-2-hydroxy acid oxidase (Rc(L)-2-HAOX) family indicated that Rc(L)-2-HAOX1 to Rc(L)-2-HAOX3 cluster with the group containing streptophyte long-chain 2-hydroxy acid oxidases, whereas Rc(L)-2-HAOX4 clusters with the group containing streptophyte GOX. Rc(L)-2-HAOX4 is the closest relative to the photorespiratory GOX genes of Arabidopsis. We obtained Rc(L)-2-HAOX4 as a recombinant protein and analyze its kinetic properties in comparison to the Arabidopsis photorespiratory GOX. We also analyzed the expression of all Rc(L)-2-HAOXs and conducted metabolite profiling of different Ricinus organs. Phylogenetic analysis indicates that Rc(L)-2-HAOX4 is the only GOX encoded in the Ricinus genome (RcGOX). RcGOX has properties resembling those of the photorespiratory GOX of Arabidopsis. We found that glycolate, the substrate of GOX, is highly abundant in non-green tissues, such as roots, embryo of germinating seeds and dry seeds. We propose that RcGOX fulfills different physiological functions depending on the organ in which it is active. In autotrophic organs it oxidizes glycolate into glyoxylate as part of the photorespiratory pathway. In fast growing heterotrophic organs, it is most probably involved in the production of serine to feed the folate pathway for special demands of those tissues.

Castor LPCAT and PDAT1A Act in Concert to Promote Transacylation of Hydroxy-Fatty Acid onto Triacylglycerol

Oilseeds produce abundant triacylglycerol (TAG) during seed maturation to fuel the establishment of photoautotrophism in the subsequent generation. Commonly, TAG contains 18-carbon polyunsaturated fatty acids (FA), but plants also produce oils with unique chemical properties highly desirable for industrial processes. Unfortunately, plants that produce such oils are poorly suited to agronomic exploitation, leading to a desire to reconstitute novel oil biosynthesis in crop plants. Here, we studied the production and incorporation of hydroxy-fatty acids (HFA) onto TAG in Arabidopsis (Arabidopsis thaliana) plants expressing the castor (Ricinus communis) FAH12 hydroxylase. One factor limiting HFA accumulation in these plants is the inefficient removal of HFA from the site of synthesis on phosphatidylcholine (PC). In Arabidopsis, lysophosphatidic acid acyltransferase (LPCAT) cycles FA to and from PC for modification. We reasoned that the castor LPCAT (RcLPCAT) would preferentially remove HFA from PC, resulting in greater incorporation onto TAG. However, expressing RcLPCAT in Arabidopsis expressing FAH12 alone (line CL37) or together with castor acyl:coenzyme A:diacylglycerol acyltransferase2 reduced HFA and total oil yield. Detailed analysis indicated that RcLPCAT reduced the removal of HFA from PC, possibly by competing with the endogenous LPCAT isozymes. Significantly, coexpressing RcLPCAT with castor phospholipid:diacylglycerol acyltransferase increased novel FA and total oil contents by transferring HFA from PC to diacylglycerol. Our results demonstrate that a detailed understanding is required to engineer modified FA production in oilseeds and suggest that phospholipase A2 enzymes rather than LPCAT mediate the highly efficient removal of HFA from PC in castor seeds.

Castor Stearoyl-ACP Desaturase Can Synthesize a Vicinal Diol by Dioxygenase Chemistry

In previous work, we identified a triple mutant of the castor (Ricinus communis) stearoyl-Acyl Carrier Protein desaturase (T117R/G188L/D280K) that, in addition to introducing a double bond into stearate to produce oleate, performed an additional round of oxidation to convert oleate to a trans allylic alcohol acid. To determine the contributions of each mutation, in this work we generated individual castor desaturase mutants carrying residue changes corresponding to those in the triple mutant and investigated their catalytic activities. We observed that T117R, and to a lesser extent D280K, accumulated a novel product, namely erythro-9,10-dihydroxystearate, that we identified via its methyl ester through gas chromatography-mass spectrometry and comparison with authentic standards. The use of ¹⁸O₂ labeling showed that the oxygens of both hydroxyl moieties originate from molecular oxygen rather than water. Incubation with an equimolar mixture of ¹⁸O₂ and ¹⁶O₂ demonstrated that both hydroxyl oxygens originate from a single molecule of O₂, proving the product is the result of dioxygenase catalysis. Using prolonged incubation, we discovered that wild-type castor desaturase is also capable of forming erythro-9,10-dihydroxystearate, which presents a likely explanation for its accumulation to ∼0.7% in castor oil, the biosynthetic origin of which had remained enigmatic for decades. In summary, the findings presented here expand the documented constellation of di-iron enzyme catalysis to include a dioxygenase reactivity in which an unactivated alkene is converted to a vicinal diol.

Introduction to the Toxins Special Issue "Ricin Toxins"

Ricin toxin isolated from the castor bean (Ricinus communis) is one of the most potent and lethal molecules known [...].

Selection and validation of castor bean (Ricinus communis) reference genes for quantitative PCR (RT-qPCR) in developing and germinating seeds and expression pattern of four ricin-family genes

This study aims to expand the set of internal control genes used for RT-qPCR experiments with Castor bean (Ricinus communis) seeds by evaluating candidate genes across several seed tissues and developmental stages. Nine reference genes were selected, including actin-11 (ACT11), tubulin alpha-2 (Tα2), elongation factor 1-alpha (EF1-α), protein phosphatase 2A-2 (PP2A2), polyubiquitin-3 (PUB3) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Biological samples consisted of R. communis seeds in 15 stages of maturation and germination. We demonstrate that PP2A2, PUB3 and EF1-α are the most stably expressed genes across the tested conditions and therefore appropriate for RT-qPCR. Subsequently, those reference genes were used for the analysis of the expression of four R. communis ricin-family genes. In developing seeds, the highest ricin expression levels was seen in the nucellus and in the endosperm, whereas in germinating seeds a peak expression occurs 4-6 days after germination. The four tested ricin isoforms exhibited differential expression patterns across tissues and seed developmental stages, which may indicate distinct biological roles for each ricin gene.

WRINKLED1 homologs highly and functionally express in oil-rich endosperms of oat and castor

Oat (Avena sativa) and castor (Ricinus communis) accumulate a large amount of lipids in their endosperms, however the molecular mechanism remains unknown. In this study, differences in oil regulators between oat and wheat (Triticum aestivum) as well as common features between oat and castor were tested by analyzing their transcriptomes with further q-PCR analysis. Results indicated that WRINKLED1 (WRI1) homologs and their target genes highly expressed in the endosperms of oat and castor, but not in the starchy endosperms of wheat. Expression pattern of WRI1s was in agreement with that of oil accumulation. Three AsWRI1s (AsWRI1a, AsWRI1b and AsWRI1c) and one RcWRI1 were identified in the endosperms of oat and castor, respectively. AsWRI1c lacks VYL motif, which is different from the other three WRI1s. Expressions of these four WRI1s all complemented the phenotypes of Arabidopsis wri1-1 mutant. Overexpression of these WRI1s in Arabidopsis and tobacco BY2 cells increased oil contents of seeds and total fatty acids of the cells, respectively. Moreover, this overexpression also resulted in up-regulations of WRI1 target genes, such as PKp-β1. Taken together, our results suggest that high and functional expression of WRI1 play a key role in the oil-rich endosperms and the VYL motif is dispensable for WRI1 function.

SRAP analysis of the genetic diversity of wild castor (Ricinus communis L.) in South China

Castor bean is an important seed oil crop. Castor oil is a highly demanded oil for several industrial uses. Currently, castor bean varieties suffer from low productivity and high risk of insect pests and diseases. It is in urgent need to mine elite genes from wild materials for castor breeding. 29 pairs of polymorphic SRAP primers out of 361 pairs were used to analyse the genetic diversity of 473 wild castor materials from South China. 203 bands were amplified by the 29 pairs of primers, of which 169 bands were polymorphic, with a polymorphic percentage of 83.25%. With an average number of alleles per locus (Ap) of 1.801, average number of effective alleles per locus (Ae) of 1.713 and average percentage of polymorphic loci (P) of 90.04%, these primers were proven to be useful and effective. Nei' genetic distance between the materials ranged from 1.04 to 25.02, with an average of 13.03. At the genetic distance of 25.02, the materials clustered into two major groups, consistent with the result of population structure analysis. However, more subgroups existed between 5.21 and 13.32. Although not all the materials from the same region were clustered in the same group, an obvious trend existed where the groups were related to regions to a great extent. Based on multiple indices, the genetic diversity of materials from Hainan was the lowest. However, there was not much difference between West Guangdong and Guangxi, although the former was slightly higher. Moderate genetic differentiation was observed in wild materials in South China. The genetic differentiation mainly occurred within population, with maximum differentiation in Guangxi, followed by West Guangdong and the minimum in Hainan. Nonetheless, there was an extensive geneflow between populations. The above results provided a direction for the conservation and breeding application of these materials.

RcPAL, a key gene in lignin biosynthesis in Ricinus communis L

BACKGROUND

Castor (Ricinus communis L.) is an important seed oil crop. Castor oil is a highly demanded oil for several industrial uses. Current castor bean varieties suffer from low productivity and high risk of insect pests and diseases. High productive and pest/disease resistance varieties are needed. Lignin has been associated to the resistance for pest, disease and lodging. Lignin is produced from several metabolites of the phenylpropanoid pathway. PAL is the key enzyme of the phenylpropanoid pathway. The gene PAL may assist in the improvement of resistance of castor bean.

RESULTS

The RcPAL CDs was amplified and its function was examined by transgenic overexpression and antisense expression, lignin histochemical staining, real-time PCR, lignin content measurement and morphological investigation. Its full length was 2145 bp, encoding 714 amino acids. The overexpression of RcPAL (7.2 times) increased significantly the PAL activity, dyeing depth of xylem cells and lignin content (14.44%), resulting in a significantly lower plant height, deeper and thicker blade, more green leaves, shorter internode, thicker stem diameter, and opposite in antisense expression plants (lignin content lowered by 27.1%), demonstrated that the gene RcPAL was a key gene in castor lignin biosynthesis.

CONCLUSIONS

The gene RcPAL is a key gene in castor lignin biosynthesis and can be induced to express under mechanical damage stress. When up-regulated, it increased the lignin content significantly and dwarfed the plant height, and opposite when down-regulated. The gene RcPAL may assist in the improvement of resistance and plant type of castor bean.

Tri-Hydroxy-Triacylglycerol Is Efficiently Produced by Position-Specific Castor Acyltransferases

Understanding the biochemistry of triacylglycerol (TAG) assembly is critical in tailoring seed oils to produce high-value products. Hydroxy-fatty acid (HFA) is one such valuable modified fatty acid, which can be produced at low levels in Arabidopsis (Arabidopsis thaliana) seed through transgenic expression of the castor (Ricinus communis) hydroxylase. The resulting plants have low seed oil content and poor seedling establishment, indicating that Arabidopsis lacks efficient metabolic networks for biosynthesis and catabolism of hydroxy-containing TAG. To improve utilization of such substrates, we expressed three castor acyltransferase enzymes that incorporate HFA at each stereochemical position during TAG synthesis. This produced abundant tri-HFA TAG and concentrated 44% of seed HFA moieties into this one TAG species. Ricinoleic acid was more abundant than any other fatty acid in these seeds, which had 3-fold more HFA by weight than that in seeds following simple hydroxylase expression, the highest yet measured in a nonnative plant. Efficient utilization of hydroxy-containing lipid substrates increased the rate of TAG synthesis 2-fold, leading to complete relief of the low-oil phenotype. Partition of HFA into specific TAG molecules increased the storage lipid available for mobilization during seedling development, resulting in a 1.9-fold increase in seedling establishment. Expression of a complete acyltransferase pathway to efficiently process HFA establishes a benchmark in the quest to successfully produce modified oils in plants.

Quantitative Proteomic Analysis of Castor (Ricinus communis L.) Seeds During Early Imbibition Provided Novel Insights into Cold Stress Response

Early planting is one of the strategies used to increase grain yield in temperate regions. However, poor cold tolerance in castor inhibits seed germination, resulting in lower seedling emergence and biomass. Here, the elite castor variety Tongbi 5 was used to identify the differential abundance protein species (DAPS) between cold stress (4 °C) and control conditions (30 °C) imbibed seeds. As a result, 127 DAPS were identified according to isobaric tag for relative and absolute quantification (iTRAQ) strategy. These DAPS were mainly involved in carbohydrate and energy metabolism, translation and posttranslational modification, stress response, lipid transport and metabolism, and signal transduction. Enzyme-linked immunosorbent assays (ELISA) demonstrated that the quantitative proteomics data collected here were reliable. This study provided some invaluable insights into the cold stress responses of early imbibed castor seeds: (1) up-accumulation of all DAPS involved in translation might confer cold tolerance by promoting protein synthesis; (2) stress-related proteins probably protect the cell against damage caused by cold stress; (3) up-accumulation of key DAPS associated with fatty acid biosynthesis might facilitate resistance or adaptation of imbibed castor seeds to cold stress by the increased content of unsaturated fatty acid (UFA). The data has been deposited to the ProteomeXchange with identifier PXD010043.

Tissue-specific differences in metabolites and transcripts contribute to the heterogeneity of ricinoleic acid accumulation in Ricinus communis L. (castor) seeds

INTRODUCTION

Castor (Ricinus communis L.) seeds are valued for their production of oils which can comprise up to 90% hydroxy-fatty acids (ricinoleic acid). Castor oil contains mono-, di- and tri- ricinoleic acid containing triacylglycerols (TAGs). Although the enzymatic synthesis of ricinoleic acid is well described, the differential compartmentalization of these TAG molecular species has remained undefined.

OBJECTIVES

To examine the distribution of hydroxy fatty acid accumulation within the endosperm and embryo tissues of castor seeds.

METHODS

Matrix assisted laser desorption/ionization mass spectrometry imaging was used to map the distribution of triacylglycerols in tissue sections of castor seeds. In addition, the endosperm and embryo (cotyledons and embryonic axis) tissues were dissected and extracted for quantitative lipidomics analysis and Illumina-based RNA deep sequencing.

RESULTS

This study revealed an unexpected heterogeneous tissue distribution of mono-, di- and tri- hydroxy-triacylglycerols in the embryo and endosperm tissues of castor seeds. Pathway analysis based on transcript abundance suggested that distinct embryo- and endosperm-specific mechanisms may exist for the shuttling of ricinoleic acid away from phosphatidylcholine (PC) and into hydroxy TAG production. The embryo-biased mechanism appears to favor removal of ricinoleic acid from PC through phophatidylcholine: diacylglycerol acyltransferase while the endosperm pathway appears to remove ricinoleic acid from the PC pool by preferences of phospholipase A (PLA2α) and/or phosphatidylcholine: diacylglycerol cholinephosphotransferase.

CONCLUSIONS

Collectively, a combination of lipidomics and transcriptomics analyses revealed previously undefined spatial aspects of hydroxy fatty acid metabolism in castor seeds. These studies underscore a need for tissue-specific studies as a means to better understand the regulation of triacylglycerol accumulation in oilseeds.

CBF/DREB transcription factor genes play role in cadmium tolerance and phytoaccumulation in Ricinus communis under molybdenum treatments

The C-repeat binding factor/dehydration responsive element binding proteins (CBF/DREB) constitute a large group of transcriptional factors. Their role in abiotic stresses such as drought, salinity and low temperature tolerance in plants have been well established, while little information about their role in metals stress tolerance is available. Transcriptomic analyses of four genes (DREB-1A, DREB-1B, DREB-1F and CBF) were carried out in industrially important plant Ricinus communis under cadmium (Cd) and molybdenum (Mo) treatments. Cadmium (in soil) and Mo (as foliar spray) were used separate as well as in combinations. All the genes (except DREB 1A) expressed under Cd stress, while Mo further enhanced their expression. The proline (55.68 ± 5.51 ppm) and phenolic (120.00 ± 14.40 ppm) contents were significant increase in combination treatments of Cd and Mo. Positive and significant correlations of DREB 1B, DREB 1F and CBF genes expressions with free proline (0.92, 0.93 and 0.88 respectively), phenolic (075, 0.77 and 0.62 respectively) contents and Cd accumulation were demonstrated. Nucleotide sequence of R. comunis DREB1F and CBF genes showed more than 80% homology with related genes of other flowering plants. Predicted amino acids sequence of R. communis DREB 1F and CBF protein fragment demonstrated more than 75% homology with related proteins from other flowering plants.

Development Defects of Hydroxy-Fatty Acid-Accumulating Seeds Are Reduced by Castor Acyltransferases

Researchers have long endeavored to produce modified fatty acids in easily managed crop plants where they are not natively found. An important step toward this goal has been the biosynthesis of these valuable products in model oilseeds. The successful production of such fatty acids has revealed barriers to the broad application of this technology, including low seed oil and low proportion of the introduced fatty acid and reduced seed vigor. Here, we analyze the impact of producing hydroxy-fatty acids on seedling development. We show that germinating seeds of a hydroxy-fatty acid-accumulating Arabidopsis (Arabidopsis thaliana) line produce chlorotic cotyledons and suffer reduced photosynthetic capacity. These seedlings retain hydroxy-fatty acids in polar lipids, including chloroplast lipids, and exhibit decreased fatty acid synthesis. Triacylglycerol mobilization in seedling development also is reduced, especially for lipids that include hydroxy-fatty acid moieties. These developmental defects are ameliorated by increased flux of hydroxy-fatty acids into seed triacylglycerol created through the expression of either castor (Ricinus communis) acyltransferase enzyme ACYL-COA:DIACYLGLYCEROL ACYLTRANSFERASE2 or PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1A. Such expression increases both the level of total stored triacylglycerol and the rate at which it is mobilized, fueling fatty acid synthesis and restoring photosynthetic capacity. Our results suggest that further improvements in seedling development may require the specific mobilization of triacylglycerol-containing hydroxy-fatty acids. Understanding the defects in early development caused by the accumulation of modified fatty acids and providing mechanisms to circumvent these defects are vital steps in the development of tailored oil crops.

Synthesis of Novel Amino Acid-Fipronil Conjugates and Study on Their Phloem Loading Mechanism

To develop a new pesticide with phloem mobility, a series of new amino acid–fipronil conjugates were designed and synthesized based on derivatization at the 3-position of the pyrazole ring of fipronil. Experiments using a Ricinus communis seedling system showed that all tested conjugates were phloem mobile except for the isoleucine–fipronil conjugate, and that the serine–fipronil conjugate (4g) exhibited the highest concentration in phloem sap (52.00 ± 5.80 μM). According to prediction with log Cf values and uptake experiments with Xenopus oocytes, the phloem loading process of conjugate 4g involved both passive diffusion and an active carrier system (RcANT15). In particular, compared with for a previously reported glycinergic–fipronil conjugate (GlyF), passive diffusion played a more important role for conjugate 4g in the enhancement of phloem mobility. This study suggests that associating a nutrient at a different position of an existing pesticide structure could still be effective in obtaining phloem-mobile derivatives, but the distinct physicochemical properties of resultant conjugates may lead to different phloem loading mechanisms.

Gene structure, expression pattern and interaction of Nuclear Factor-Y family in castor bean (Ricinus communis)

Nuclear Factor-Y transcription factors, which function in regulating seed development (including storage reservoir accumulation) and responding to abiotic stresses, were identified and characterized in castor bean. Nuclear Factor-Y (NF-Y) transcription factors in plants contain three subunits (NF-YA, NF-YB and NF-YC), and function as a heterodimer or heterotrimer complex in regulating plant growth, development and response to stresses. Castor bean (Ricinus communis, Euphorbiaceae) one of the most economically important non-edible oilseed crops, able to grow in diverse soil conditions and displays high tolerance to abiotic stresses. Due to increasing demands for its seed oils, it is necessary to elucidate the molecular mechanism underlying the regulation of growth and development. Based on the available genome data, we identified 25 RcNF-Y members including six RcNF-YAs, 12 RcNF-YBs and seven RcNF-YCs, and characterized their gene structures. Yeast two-hybrid assays confirmed the protein-protein interactions among three subunits. Using transcriptomic data from different tissues, we found that six members were highly or specifically expressed in endosperms (in particular, two LEC1-type members RcNF-YB2 and RcNF-YB12), implying their involvement in regulating seed development and storage reservoir accumulation. Further, we investigated the expression changes of RcNF-Y members in two-week-old seedlings under drought, cold, hot and salt stresses. We found that the expression levels of 20 RcNF-Y members tested were changed and three RcNF-Y members might function in response to abiotic stresses. This study is the first reported on genomic characterization of NF-Y transcription factors in the family Euphorbiaceae. Our results provide the basis for improved understanding of how NF-Y genes function in the regulation of seed development and responses to abiotic stresses in both castor bean and other plants in this family.

Splice Variants of the Castor WRI1 Gene Upregulate Fatty Acid and Oil Biosynthesis When Expressed in Tobacco Leaves

The plant-specific WRINKLED1 (WRI1) is a member of the AP2/EREBP class of transcription factors that positively regulate oil biosynthesis in plant tissues. Limited information is available for the role of WRI1 in oil biosynthesis in castor bean (Ricinus connunis L.), an important industrial oil crop. Here, we report the identification of two alternatively spliced transcripts of RcWRI1, designated as RcWRI1-A and RcWRI1-B. The open reading frames of RcWRI1-A (1341 bp) and RcWRI1-B (1332 bp) differ by a stretch of 9 bp, such that the predicted RcWRI1-B lacks the three amino acid residues "VYL" that are present in RcWRI1-A. The RcWRI1-A transcript is present in flowers, leaves, pericarps and developing seeds, while the RcWRI1-B mRNA is only detectable in developing seeds. When the two isoforms were individually introduced into an Arabidopsiswri1-1 loss-of-function mutant, total fatty acid content was almost restored to the wild-type level, and the percentage of the wrinkled seeds was largely reduced in the transgenic lines relative to the wri1-1 mutant line. Transient expression of each RcWRI1 splice isoform in N. benthamiana leaves upregulated the expression of the WRI1 target genes, and consequently increased the oil content by 4.3-4.9 fold when compared with the controls, and RcWRI1-B appeared to be more active than RcWRI1-A. Both RcWRI1-A and RcWRI1-B can be used as a key transcriptional regulator to enhance fatty acid and oil biosynthesis in leafy biomass.

Coimmunoprecipitation of reversibly glycosylated polypeptide with sucrose synthase from developing castor oilseeds

The sucrose synthase (SUS) interactome of developing castor oilseeds (COS; Ricinus communis) was assessed using coimmunoprecipitation (co-IP) with anti-(COS RcSUS1)-IgG followed by proteomic analysis. A 41-kDa polypeptide (p41) that coimmunoprecipitated with RcSUS1 from COS extracts was identified as reversibly glycosylated polypeptide-1 (RcRGP1) by LC-MS/MS and anti-RcRGP1 immunoblotting. Reciprocal Far-western immunodot blotting corroborated the specific interaction between RcSUS1 and RcRGP1. Co-IP using anti-(COS RcSUS1)-IgG and clarified extracts from other developing seeds as well as cluster (proteoid) roots of white lupin and Harsh Hakea consistently recovered 90 kDa SUS polypeptides along with p41/RGP as a SUS interactor. The results suggest that SUS interacts with RGP in diverse sink tissues to channel UDP-glucose derived from imported sucrose into hemicellulose and/or glycoprotein/glycolipid biosynthesis.

Family of Ricinus communis Monosaccharide Transporters and RcSTP1 in Promoting the Uptake of a Glucose-Fipronil Conjugate

Enhancing the systemic distribution of a bioactive compound by exploiting the vascular transport system of a plant presents a means of reducing both the volume and frequency of pesticide/fungicide application. The foliar uptake of the glucose-fipronil conjugate N-[3-cyano-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[(trifluoromethyl)sulfinyl]-1H-pyrazol-5-yl]-1-(β-d-glucopyranosyl)-1H-1,2,3-triazole-4-methanamine (GTF) achieved in castor bean (Ricinus communis) and its transport via the phloem are known to be mediated by monosaccharide transporter(s) [MST(s)], although neither the identity of the key MST(s) involved nor the mechanistic basis of its movement have yet to be described. On the basis of homology with Arabidopsis thaliana sugar transporters, the castor bean genome was concluded to harbor 53 genes encoding a sugar transporter, falling into the eight previously defined subfamilies INT, PMT, VGT, STP, ERD6, pGlucT, TMT, and SUT. Transcriptional profiling identified the product of RcSTP1 as a candidate for mediating GTF uptake, because this gene was induced by exposure of the plant to GTF. When RcSTP1 was transiently expressed in onion epidermis cells, the site of RcSTP1 deposition was shown to be the plasma membrane. A functional analysis based on RcSTP1 expression in Xenopus laevis oocytes demonstrated that its product has a high affinity for GTF. The long-distance root-to-shoot transport of GTF was enhanced in a transgenic soybean chimera constitutively expressing RcSTP1.

Regulatory Phosphorylation of Bacterial-Type PEP Carboxylase by the Ca2+-Dependent Protein Kinase RcCDPK1 in Developing Castor Oil Seeds

Phosphoenolpyruvate carboxylase (PEPC) is a tightly controlled cytosolic enzyme situated at a crucial branch point of central plant metabolism. In developing castor oil seeds (Ricinus communis) a novel, allosterically desensitized 910-kD Class-2 PEPC hetero-octameric complex, arises from a tight interaction between 107-kD plant-type PEPC and 118-kD bacterial-type (BTPC) subunits. The native Ca²⁺-dependent protein kinase (CDPK) responsible for in vivo inhibitory phosphorylation of Class-2 PEPC's BTPC subunit's at Ser-451 was highly purified from COS and identified as RcCDPK1 (XP_002526815) by mass spectrometry. Heterologously expressed RcCDPK1 catalyzed Ca²⁺-dependent, inhibitory phosphorylation of BTPC at Ser-451 while exhibiting: (i) a pair of Ca²⁺ binding sites with identical dissociation constants of 5.03 μM, (ii) a Ca²⁺-dependent electrophoretic mobility shift, and (iii) a marked Ca²⁺-independent hydrophobicity. Pull-down experiments established the Ca²⁺-dependent interaction of N-terminal GST-tagged RcCDPK1 with BTPC. RcCDPK1-Cherry localized to the cytosol and nucleus of tobacco bright yellow-2 cells, but colocalized with mitochondrial-surface associated BTPC-enhanced yellow fluorescent protein when both fusion proteins were coexpressed. Deletion analyses demonstrated that although its N-terminal variable domain plays an essential role in optimizing Ca²⁺-dependent RcCDPK1 autophosphorylation and BTPC transphosphorylation activity, it is not critical for in vitro or in vivo target recognition. Arabidopsis (Arabidopsis thaliana) CPK4 and soybean (Glycine max) CDPKβ are RcCDPK1 orthologs that effectively phosphorylated castor BTPC at Ser-451. Overall, the results highlight a potential link between cytosolic Ca²⁺ signaling and the posttranslational control of respiratory CO₂ refixation and anaplerotic photosynthate partitioning in support of storage oil and protein biosynthesis in developing COS.

Genome-Wide Identification, Evolutionary Analysis, and Stress Responses of the GRAS Gene Family in Castor Beans

Plant-specific GRAS transcription factors play important roles in regulating growth, development, and stress responses. Castor beans (Ricinus communis) are important non-edible oilseed plants, cultivated worldwide for its seed oils and its adaptability to growth conditions. In this study, we identified and characterized a total of 48 GRAS genes based on the castor bean genome. Combined with phylogenetic analysis, the castor bean GRAS members were divided into 13 distinct groups. Functional divergence analysis revealed the presence of mostly Type-I functional divergence. The gene structures and conserved motifs, both within and outside the GRAS domain, were characterized. Gene expression analysis, performed in various tissues and under a range of abiotic stress conditions, uncovered the potential functions of GRAS members in regulating plant growth development and stress responses. The results obtained from this study provide valuable information toward understanding the potential molecular mechanisms of GRAS proteins in castor beans. These findings also serve as a resource for identifying the genes that allow castor beans to grow in stressful conditions and to enable further breeding and genetic improvements in agriculture.

Glycinergic-Fipronil Uptake Is Mediated by an Amino Acid Carrier System and Induces the Expression of Amino Acid Transporter Genes in Ricinus communis Seedlings

Phloem-mobile insecticides are efficient for piercing and sucking insect control. Introduction of sugar or amino acid groups to the parent compound can improve the phloem mobility of insecticides, so a glycinergic-fipronil conjugate (GlyF), 2-(3-(3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-5-yl)ureido) acetic acid, was designed and synthesized. Although the "Kleier model" predicted that this conjugate is not phloem mobile, GlyF can be continually detected during a 5 h collection of Ricinus communis phloem sap. Furthermore, an R. communis seedling cotyledon disk uptake experiment demonstrates that the uptake of GlyF is sensitive to pH, carbonyl cyanide m-chlorophenylhydrazone (CCCP), temperature, and p-chloromercuribenzenesulfonic acid (pCMBS) and is likely mediated by amino acid carrier system. To explore the roles of amino acid transporters (AATs) in GlyF uptake, a total of 62 AAT genes were identified from the R. communis genome in silico. Phylogenetic analysis revealed that AATs in R. communis were organized into the ATF (amino acid transporter) and APC (amino acid, polyaminem and choline transporter) superfamilies, with five subfamilies in ATF and two in APC. Furthermore, the expression profiles of 20 abundantly expressed AATs (cycle threshold (Ct) values <27) were analyzed at 1, 3, and 6 h after GlyF treatment by RT-qPCR. The results demonstrated that expression levels of four AAT genes, RcLHT6, RcANT15, RcProT2, and RcCAT2, were induced by the GlyF treatment in R. communis seedlings. On the basis of the observation that the expression profile of the four candidate genes is similar to the time course observation for GlyF foliar disk uptake, it is suggested that those four genes are possible candidates involved in the uptake of GlyF. These results contribute to a better understanding of the mechanism of GlyF uptake as well as phloem loading from a molecular biology perspective and facilitate functional characterization of candidate AAT genes in future studies.

High light exposure on seed coat increases lipid accumulation in seeds of castor bean (Ricinus communis L.), a nongreen oilseed crop

Little was known on how sunlight affects the seed metabolism in nongreen seeds. Castor bean (Ricinus communis L.) is a typical nongreen oilseed crop and its seed oil is an important feedstock in industry. In this study, photosynthetic activity of seed coat tissues of castor bean in natural conditions was evaluated in comparison to shaded conditions. Our results indicate that exposure to high light enhances photosynthetic activity in seed coats and consequently increases oil accumulation. Consistent results were also reached using cultured seeds. High-throughput RNA-Seq analyses further revealed that genes involved in photosynthesis and carbon conversion in both the Calvin-Benson cycle and malate transport were differentially expressed between seeds cultured under light and dark conditions, implying several venues potentially contributing to light-enhanced lipid accumulation such as increased reducing power and CO2 refixation which underlie the overall lipid biosynthesis. This study demonstrated the effects of light exposure on oil accumulation in nongreen oilseeds and greatly expands our understanding of the physiological roles that light may play during seed development in nongreen oilseeds. Essentially, our studies suggest that potential exists to enhance castor oil yield through increasing exposure of the inflorescences to sunlight either by genetically changing the plant architecture (smart canopy) or its growing environment.

Expression of Castor LPAT2 Enhances Ricinoleic Acid Content at the sn-2 Position of Triacylglycerols in Lesquerella Seed

Lesquerella is a potential industrial oilseed crop that makes hydroxy fatty acid (HFA). Unlike castor its seeds are not poisonous but accumulate lesquerolic acid mostly at the sn-1 and sn-3 positions of triacylglycerol (TAG), whereas castor contains ricinoleic acid (18:1OH) at all three positions. To investigate whether lesquerella can be engineered to accumulate HFAs in the sn-2 position, multiple transgenic lines were made that express castor lysophosphatidic acid acyltransferase 2 (RcLPAT2) in the seed. RcLPAT2 increased 18:1OH at the sn-2 position of TAGs from 2% to 14%–17%, which resulted in an increase of tri-HFA-TAGs from 5% to 13%–14%. Our result is the first example of using a LPAT to increase ricinoleic acid at the sn-2 position of seed TAG. This work provides insights to the mechanism of HFA-containing TAG assembly in lesquerella and directs future research to optimize this plant for HFA production.

Expression profiles of genes related to carbohydrate metabolism provide new insights into carbohydrate accumulation in seeds and seedlings of Ricinus communis in response to temperature

Ricinus communis possesses a specific metabolic signature to adjust growth and developmental processes in response to temperature: carbohydrates are accumulated at low temperatures, whereas amino acids are accumulated at elevated temperatures. Our objective was to assess tissue-specific changes in transcript levels of genes related with carbohydrate biosynthesis and catabolism in response to temperature. For that, we measured transcript levels of genes encoding enzymes involved in starch biosynthesis, starch catabolism, and gluconeogenesis in R. communis leaves, roots, and seeds grown at 20 °C and 35 °C. Transcript levels of genes involved in starch catabolism were higher in leaves grown at 20 °C than at 35 °C, but up-regulation of genes involved in starch biosynthesis seems to compensate for this and, therefore, are the likely explanation for higher levels of starch in leaves grown at 20 °C. Higher levels of soluble carbohydrates in leaves grown at 20 °C may be caused by a coordinated increase in transcript level of genes associated with starch catabolism and gluconeogenesis pathways. In roots, transcript levels of genes associated with starch catabolism and gluconeogenesis seem to be enhanced at elevated temperatures. Higher levels of starch in seeds germinated at low temperatures is associated with higher transcript levels of genes involved in starch biosynthesis. Similarly, higher transcript levels of RcPEPCK and RcFBPase are most likely causal for fructose and glucose accumulation in seeds germinated at 20 °C. This study provides important insights in the understanding of the plasticity of R. communis in response to temperature that may apply to other species as well.

Effect of temperature on biomass allocation in seedlings of two contrasting genotypes of the oilseed crop Ricinus communis

Ricinus communis is becoming an important crop for oil production, and studying the physiological and biochemical aspects of seedling development may aid in the improvement of crop quality and yield. The objective of this study was to assess the effect of temperature on biomass allocation in two R. communis genotypes. Biomass allocation was assessed by measuring dry weight of roots, stems, and cotyledons of seedlings grown at three different temperatures. Root length of each seedling was measured. Biomass allocation was strongly affected by temperature. Seedlings grown at 25°C and 35°C showed greater biomass than seedlings grown at 20°C. Cotyledon and stem dry weight increased for both genotypes with increasing temperature, whereas root biomass allocation showed a genotype-dependent behavior. Genotype MPA11 showed a continuous increase in root dry weight with increasing temperature, while genotype IAC80 was not able to sustain further root growth at higher temperatures. Based on metabolite and gene expression profiles, genotype MPA11 increases its level of osmoprotectant molecules and transcripts of genes encoding for antioxidant enzymes and heat shock proteins to a higher extent than genotype IAC80. This might be causal for the ability to maintain homeostasis and support root growth at elevated temperatures in genotype MPA11.

Production of bioactive diterpenoids in the euphorbiaceae depends on evolutionarily conserved gene clusters

The Euphorbiaceae produce a diverse range of diterpenoids, many of which have pharmacological activities. These diterpenoids include ingenol mebutate, which is licensed for the treatment of a precancerous skin condition (actinic keratosis), and phorbol derivatives such as resiniferatoxin and prostratin, which are undergoing investigation for the treatment of severe pain and HIV, respectively. Despite the interest in these diterpenoids, their biosynthesis is poorly understood at present, with the only characterized step being the conversion of geranylgeranyl pyrophosphate into casbene. Here, we report a physical cluster of diterpenoid biosynthetic genes from castor (Ricinus communis), including casbene synthases and cytochrome P450s from the CYP726A subfamily. CYP726A14, CYP726A17, and CYP726A18 were able to catalyze 5-oxidation of casbene, a conserved oxidation step in the biosynthesis of this family of medicinally important diterpenoids. CYP726A16 catalyzed 7,8-epoxidation of 5-keto-casbene and CYP726A15 catalyzed 5-oxidation of neocembrene. Evidence of similar gene clustering was also found in two other Euphorbiaceae, including Euphorbia peplus, the source organism of ingenol mebutate. These results demonstrate conservation of gene clusters at the higher taxonomic level of the plant family and that this phenomenon could prove useful in further elucidating diterpenoid biosynthetic pathways.

Evaluation of automated and manual DNA purification methods for detecting Ricinus communis DNA during ricin investigations

In April of 2013, letters addressed to the President of United States and other government officials were intercepted and found to be contaminated with ricin, heightening awareness about the need to evaluate laboratory methods for detecting ricin. This study evaluated commercial DNA purification methods for isolating Ricinus communis DNA as measured by real-time polymerase chain reaction (PCR). Four commercially available DNA purification methods (two automated, MagNA Pure compact and MagNA Pure LC, and two manual, MasterPure complete DNA and RNA purification kit and QIAamp DNA blood mini kit) were evaluated. We compared their ability to purify detectable levels of R. communis DNA from four different sample types, including crude preparations of ricin that could be used for biological crimes or acts of bioterrorism. Castor beans, spiked swabs, and spiked powders were included to simulate sample types typically tested during criminal and public health investigations. Real-time PCR analysis indicated that the QIAamp kit resulted in the greatest sensitivity for ricin preparations; the MasterPure kit performed best with spiked powders. The four methods detected equivalent levels by real-time PCR when castor beans and spiked swabs were used. All four methods yielded DNA free of PCR inhibitors as determined by the use of a PCR inhibition control assay. This study demonstrated that DNA purification methods differ in their ability to purify R. communis DNA; therefore, the purification method used for a given sample type can influence the sensitivity of real-time PCR assays for R. communis.

Characterization of an oleate 12-desaturase from Physaria fendleri and identification of 5'UTR introns in divergent FAD2 family genes

Mining of an EST sequence collection representing genes expressed during seed development in Physaria fendleri identified abundant sequences encoding apparent homologues of the Arabidopsis oleate 12-desaturase (AtFAD2 At3g12120). Of the 62 sequenced clones, 59 were identified as encoding the previously characterized bifunctional oleate 12-hydroxylase/desaturase (LFAH12/PfFAH12). The remaining 3 clones encoded a second FAD2 homologue. Isolation of a full length ORF and heterologous expression in yeast revealed that this sequence, designated PfFAD2, is the first full length sequence from any Physaria species that encodes an oleate 12-desaturase. PfFAD2 was expressed in both leaf and developing seed with activity on palmitate (16:1(Δ9)) and oleate (18:1(Δ9)). Sequence comparison revealed that PfFAD2 shares 93% amino acid identity with Arabidopsis FAD2 and only 84% identity with PfFAH12. By comparison of EST and genomic sequences it was revealed that the PfFAD2 gene encodes a transcript with a single intron of 1120 bp in the 5'-untranslated region (5'UTR). A short intron, 81 bp in length, was also discovered in the 5'UTR of the PfFAH12 gene, 16 bp upstream of the translation initiation codon. In silico examination of FAD2 like genes from the genome of castor (Ricinus communis) identified putative 5'UTR introns in genes encoding the castor oleate 12-desaturase (RcFAD2) and oleate 12-hydroxylase (CFAH12). By sequencing of genomic DNA the presence of single 5'UTR introns in each gene, and the size of these introns, was confirmed. These findings suggest that 5'UTR introns may be a characteristic feature of FAD2 genes and also of divergent FAD2 genes encoding fatty acid modifying enzymes, and that the selection pressure maintaining these introns is very different.

The multigene family of lysophosphatidate acyltransferase (LPAT)-related enzymes in Ricinus communis: cloning and molecular characterization of two LPAT genes that are expressed in castor seeds

The multigene family encoding proteins related to lysophosphatidyl-acyltransferases (LPATs) has been analyzed in the castor plant Ricinus communis. Among them, two genes designated RcLPAT2 and RcLPATB, encoding proteins with LPAT activity and expressed in the developing seed, have been cloned and characterized in some detail. RcLPAT2 groups with well characterized members of the so-called A-class LPATs and it shows a generalized expression pattern in the plant and along seed development. Enzymatic assays of RcLPAT2 indicate a preference for ricinoleoyl-CoA over other fatty acid thioesters when ricinoleoyl-LPA is used as the acyl acceptor, while oleoyl-CoA is the preferred substrate when oleoyl-LPA is employed. RcLPATB groups with B-class LPAT enzymes described as seed specific and selective for unusual fatty acids. However, RcLPATB exhibit a broad specificity on the acyl-CoAs, with saturated fatty acids (12:0-16:0) being the preferred substrates. RcLPATB is upregulated coinciding with seed triacylglycerol accumulation, but its expression is not restricted to the seed. These results are discussed in the light of a possible role for LPAT isoenzymes in the channelling of ricinoleic acid into castor bean triacylglycerol.

Transformation of nuclear DNA in meristematic and embryogenic tissues

Particle bombardment/biolistic delivery is a very popular method of genetic transformation of diverse targets including cells and intact tissues. Delivery of DNA through particle bombardment is genotype and species independent, nevertheless, an efficient protocol for large-scale generation of transgenic plants through embryogenic tissues with a high (≥80%) shoot regeneration efficiency is a prerequisite. Young embryogenic tissues or multiple shoot buds in early stages of induction are the most suited target tissues for recovery of transgenic plants. We describe the protocol for delivery of foreign genes using particle delivery system (Biorad gene gun, PDS-1000/He) in to the meristematic tissues of embryonic axes derived from mature seeds of castor. With the optimized physical and biological parameters, putative transformants were obtained at a frequency of 1.4% through particle gun bombardment of castor embryo axes. Also, transformation of embryogenic calli of sorghum using particle inflow gun (PIG) is described.

Comparative deep transcriptional profiling of four developing oilseeds

Transcriptome analysis based on deep expressed sequence tag (EST) sequencing allows quantitative comparisons of gene expression across multiple species. Using pyrosequencing, we generated over 7 million ESTs from four stages of developing seeds of Ricinus communis, Brassica napus, Euonymus alatus and Tropaeolum majus, which differ in their storage tissue for oil, their ability to photosynthesize and in the structure and content of their triacylglycerols (TAG). The larger number of ESTs in these 16 datasets provided reliable estimates of the expression of acyltransferases and other enzymes expressed at low levels. Analysis of EST levels from these oilseeds revealed both conserved and distinct species-specific expression patterns for genes involved in the synthesis of glycerolipids and their precursors. Independent of the species and tissue type, ESTs for core fatty acid synthesis enzymes maintained a conserved stoichiometry and a strong correlation in temporal profiles throughout seed development. However, ESTs associated with non-plastid enzymes of oil biosynthesis displayed dissimilar temporal patterns indicative of different regulation. The EST levels for several genes potentially involved in accumulation of unusual TAG structures were distinct. Comparison of expression of members from multi-gene families allowed the identification of specific isoforms with conserved function in oil biosynthesis. In all four oilseeds, ESTs for Rubisco were present, suggesting its possible role in carbon metabolism, irrespective of light availability. Together, these data provide a resource for use in comparative and functional genomics of diverse oilseeds. Expression data for more than 350 genes encoding enzymes and proteins involved in lipid metabolism are available at the 'ARALIP' website (http://aralip.plantbiology.msu.edu/).

Tissue-specific expression and post-translational modifications of plant- and bacterial-type phosphoenolpyruvate carboxylase isozymes of the castor oil plant, Ricinus communis L

This study employs transcript profiling together with immunoblotting and co-immunopurification to assess the tissue-specific expression, protein:protein interactions, and post-translational modifications (PTMs) of plant- and bacterial-type phosphoenolpyruvate carboxylase (PEPC) isozymes (PTPC and BTPC, respectively) in the castor plant, Ricinus communis. Previous studies established that the Class-1 PEPC (PTPC homotetramer) of castor oil seeds (COS) is activated by phosphorylation at Ser-11 and inhibited by monoubiquitination at Lys-628 during endosperm development and germination, respectively. Elimination of photosynthate supply to developing COS by depodding caused the PTPC of the endosperm and cotyledon to be dephosphorylated, and then subsequently monoubiquitinated in vivo. PTPC monoubiquitination rather than phosphorylation is widespread throughout the castor plant and appears to be the predominant PTM of Class-1 PEPC that occurs in planta. The distinctive developmental patterns of PTPC phosphorylation versus monoubiquitination indicates that these two PTMs are mutually exclusive. By contrast, the BTPC: (i) is abundant in the inner integument, cotyledon, and endosperm of developing COS, but occurs at low levels in roots and cotyledons of germinated COS, (ii) shows a unique developmental pattern in leaves such that it is present in leaf buds and young expanding leaves, but undetectable in fully expanded leaves, and (iii) tightly interacts with co-expressed PTPC to form the novel and allosterically-desensitized Class-2 PEPC heteromeric complex. BTPC and thus Class-2 PEPC up-regulation appears to be a distinctive feature of rapidly growing and/or biosynthetically active tissues that require a large anaplerotic flux from phosphoenolpyruvate to replenish tricarboxylic acid cycle C-skeletons being withdrawn for anabolism.

Endoplasmic reticulum-located PDAT1-2 from castor bean enhances hydroxy fatty acid accumulation in transgenic plants

Ricinoleic acid (12-hydroxy-octadeca-9-enoic acid) is a major unusual fatty acid in castor oil. This hydroxy fatty acid is useful in industrial materials. This unusual fatty acid accumulates in triacylglycerol (TAG) in the seeds of the castor bean (Ricinus communis L.), even though it is synthesized in phospholipids, which indicates that the castor plant has an editing enzyme, which functions as a phospholipid:diacylglycerol acyltransferase (PDAT) that is specific to ricinoleic acid. Transgenic plants containing fatty acid Δ12-hydroxylase encoded by the castor bean FAH12 gene produce a limited amount of hydroxy fatty acid, a maximum of around 17% of TAGs present in Arabidopsis seeds, and this unusual fatty acid remains in phospholipids of cell membranes in seeds. Identification of ricinoleate-specific PDAT from castor bean and manipulation of the phospholipid editing system in transgenic plants will enhance accumulation of the hydroxy fatty acid in transgenic seeds. The castor plant has three PDAT genes; PDAT1-1 and PDAT2 are homologs of PDAT, which are commonly found in plants; however, PDAT1-2 is newly grouped as a castor bean-specific gene. PDAT1-2 is expressed in developing seeds and localized in the endoplasmic reticulum, similar to FAH12, indicating its involvement in conversion of ricinoleic acid into TAG. PDAT1-2 significantly enhances accumulation of total hydroxy fatty acid up to 25%, with a significant increase in castor-like oil, 2-OH TAG, in seeds of transgenic Arabidopsis, which is an identification of the key gene for oilseed engineering in production of unusual fatty acids.

Development of a novel set of microsatellite markers for castor bean, Ricinus communis (Euphorbiaceae)

PREMISE OF STUDY

Microsatellite primers were developed for castor bean (Ricinus communis L.) to investigate genetic diversity and population structure, and to provide support to germplasm management.

METHODS AND RESULTS

Eleven microsatellite loci were isolated using an enrichment cloning protocol and used to characterize castor bean germplasm from the collection at the Instituto Agronômico de Campinas (IAC). In a survey of 76 castor bean accessions, the investigated loci displayed polymorphism ranging from two to five alleles.

CONCLUSIONS

The information derived from microsatellite markers led to significant gains in conserved allelic richness and provides support to the implementation of several molecular breeding strategies for castor bean.

Characterization of the 11S globulin gene family in the castor plant Ricinus communis L

The 11S globulin (legumin) gene family has been characterized in the castor plant Ricinus communis L. Phylogenetic analysis reveals the presence of two diverged subfamilies (RcLEG1 and RcLEG2) comprising a total of nine genes and two putative pseudogenes. The expression of castor legumin genes has been studied, indicating that it is seed specific and developmentally regulated, with a maximum at the stage when cellular endosperm reaches its full expansion (around 40-45 DAP). However, conspicuous differences are appreciated in the expression timing of individual genes. A characterization of the 5'-proximal regulatory regions for two genes, RcLEG1-1 and RcLEG2-1, representative of the two legumin subfamilies, has also been performed by fusion to the GUS reporter gene. The results obtained from heterologous expression in tobacco and transient expression in castor, indicating seed-specific regulation, support the possible utility of these promoters for biotechnological purposes.

Temporal and spatial expression of 2S albumin in castor (Ricinus communis L.)

We studied the temporal and spatial expression of the 2S albumin in castor (Ricinus communis L.) during seed development, germination, post-germination, and plant development. Quantitative polymerase chain reaction analysis showed that the 2S albumin transcript accumulated to a maximum level at the middle of seed development, showing a bell-shaped temporal pattern. Residual levels of the transcript were present in the mature seed and degraded rapidly upon germination. Immunodetection analysis was performed using an anti-2S albumin antibody under reducing conditions. During seed development, the 2S albumin precursor pro-protein began to be synthesized at 26 days after pollination (DAP); the pro-protein was thereafter processed to mature proteins at 40 DAP, suggesting that the post-translation modification of 2S albumin takes place during this time period. Both the 2S albumin precursor pro-protein and the mature proteins accumulated throughout seed maturation and desiccation stages. During seed germination, both forms of the 2S albumin proteins were present in endosperm and cotyledon until the completion of germination and degraded rapidly afterwards. However, the antibody also detected a group of proteins/peptides in endosperm and cotyledon when the seeds progressed to germination and post-germination stages. A 14 kDa protein in the leaves of fully developed seedlings and mature plants also reacted to the anti-2S albumin antibody. The identity of the proteins accumulated in germinating seed and leaf remains unknown.

RiVax, a recombinant ricin subunit vaccine, protects mice against ricin delivered by gavage or aerosol

Ricin is a plant toxin that is a CDC level B biothreat. Our recombinant ricin A chain vaccine (RiVax), which contains mutations in both known toxic sites, has no residual toxicity at doses at least 800 times the immunogenic dose. RiVax without adjuvant given intramuscularly (i.m.) protected mice against intraperitoneally administered ricin. Furthermore the vaccine without alum was safe and immunogenic in human volunteers. Here we describe the development of gavage and aerosol ricin challenge models in mice and demonstrate that i.m. vaccination protects mice against ricin delivered by either route. Also RiVax protects against aerosol-induced lung damage as determined by histology and lung function tests.

Expression profiles of genes involved in fatty acid and triacylglycerol synthesis in castor bean (Ricinus communis L.)

Castor seed triacylglycerols (TAGs) contain 90% ricinoleate (12-hydroxy-oleate) which has numerous industrial applications. Due to the presence of the toxin ricin and potent allergenic 2S albumins in the seed, it is desirable to produce ricinoleate from temperate oilseeds. To identify regulatory genes or genes for enzymes that may up-regulate multiple activities or entire pathways leading to the ricinoleate and TAG synthesis, we have analyzed expression profiles of 12 castor genes involved in fatty acid and TAG synthesis using quantitative reverse transcription-polymerase chain reaction technology. A collection of castor seeds with well-defined developmental stages and morphologies was used to determine the levels of mRNA, ricinoleate and TAG. The synthesis of ricinoleate and TAG occurred when seeds progressed to stages of cellular endosperm development. Concomitantly, most of the genes increased their expression levels, but showed various temporal expression patterns and different maximum inductions ranging from 4- to 43,000-fold. Clustering analysis of the expression data indicated five gene groups with distinct temporal patterns. We identified genes involved in fatty acid biosynthesis and transport that fell into two related clusters with moderate flat-rise or concave-rise patterns, and others that were highly expressed during seed development that displayed either linear-rise or bell-shaped patterns. Castor diacylglycerol acyltransferase 1 was the only gene having a higher expression level in leaf and a declining pattern during cellular endosperm development. The relationships among gene expression, cellular endosperm development and ricinoleate/TAG accumulation are discussed.

A mutation in Arabidopsis cytochrome b5 reductase identified by high-throughput screening differentially affects hydroxylation and desaturation

As a model for analyzing the production of novel fatty acids in oilseeds, we used the genetic and molecular techniques available for Arabidopsis to characterize modifying mutations affecting the accumulation of hydroxy fatty acids in the seeds of Arabidopsis plants that express a transgene for the castor bean fatty acid hydroxylase, FAH12. We developed a high-throughput analytical system and used it to identify three complementation classes of mutations with reduced hydroxy fatty acid accumulation from among Arabidopsis M3 seed samples derived from chemical mutagenesis. We identified one of the mutations by positional cloning as a single base pair change in a gene encoding NADH:cytochrome b5 reductase (CBR1, At5g17770). When expressed in yeast, the mutant form of the enzyme was less active and less stable than the wild-type enzyme. Characterization of homozygous mutant lines with and without the FAH12 transgene (FAH12 cbr1-1 and cbr1-1, respectively) indicated that the only detectable consequence of the cbr1-1 mutation was on desaturase and hydroxylase reactions in the developing seed. The leaf and root fatty compositions, as well as the growth, development and seed production of mutant plants were indistinguishable from wild type. Interestingly, while the cbr1-1 mutation reduced the accumulation of hydroxy fatty acids in seeds by 85%, the effects on 18:1 and 18:2 desaturation reactions were much less (<25% and <60%, respectively). These results suggest that there is competition in developing seeds among the several reactions that utilize reduced cytochrome b5.

Cloning and characterization of a lupeol synthase involved in the synthesis of epicuticular wax crystals on stem and hypocotyl surfaces of Ricinus communis

Pentacyclic triterpenoids are a large group of secondary metabolites found in many different plant species, either as glycoside conjugates or as aglycones. The latter in many cases accumulate to high amounts in the cuticular wax and hence at the surface of plant organs. In the present work, the cuticle-specific formation of triterpenoids was investigated in Ricinus communis stems, combining analytical and molecular genetic methods. Two phenotypes of castor bean could be distinguished based on the glaucous or glossy appearance of the surfaces of all stem portions including the hypocotyls, and were due to the presence or absence of thread-shaped epicuticular wax crystals, respectively. Comparative studies showed that these crystals are formed by the triperpenoid lupeol, present in high amounts on all stem surfaces. On the hypocotyl portion of stems, lupeol was found to accumulate rapidly during early development of the surface (10-15 days after emergence). Mature hypocotyls of glossy individuals were covered with 12.5 microg/cm2 of wax containing approximately 1% of lupeol, whereas the glaucous phenotype had a wax load of 51.9 microg/cm2 with 56% of lupeol. Two oxidosqualene cyclases from castor bean were cloned, functionally expressed in yeast, and characterized as a cycloartenol synthase (RcCAS) and a lupeol synthase (RcLUS). Phylogenetic analyses revealed that RcLUS is similar to two clades of known lupeol synthases, but also exhibits some similarities with beta-amyrin synthases. Both the organ-specific expression of RcLUS and the expression pattern during hypocotyl development exactly matched the accumulation of cuticular lupeol in castor bean. In contrast, RcCAS was constitutively expressed in all organs at various times. We conclude that the RcLUS enzyme is responsible for formation of the cuticular lupeol, and thus for the characteristic surface properties of R. communis stems.

A phloem-enriched cDNA library from Ricinus: insights into phloem function

The aim of this study was to identify genes that are expressed in the phloem. Increased knowledge of phloem regulation will contribute to our understanding of its many roles, from transport of solutes to information about interactions with pathogens. A cDNA library constructed from phloem-enriched sap exuding from cut Ricinus communis (L.) hypocotyls was sequenced. To assess contamination from other tissues, two libraries were constructed: one using the first 15 min of exudation and the other from sap collected after 120 min of exudation had elapsed. Of 1012 clones sequenced, 158 unique transcripts were identified. The presence of marker molecules such as profilin, the low occurrence of chloroplast-related mRNAs, and the sieve element localization of constituent mRNA using in situ hybridization were consistent with a phloem origin of the sap. Functional analysis of the cDNAs revealed classifications including ribosomal function, interaction with the environment, transport, DNA/RNA binding, and protein turnover. An analysis of the closest Arabidopsis thaliana (L.) homologue for each clone indicated that genes involved in cell localization, protein synthesis, tissue localization, organ localization, organ differentiation, and cell fate were represented at twice the level occurring in the whole Arabidopsis genome. The transcripts found in this phloem-enriched library are discussed in the context of phloem function and the relationship between the companion cell and sieve element.

In vivo regulatory phosphorylation of novel phosphoenolpyruvate carboxylase isoforms in endosperm of developing castor oil seeds

Our previous research characterized two phosphoenolpyruvate (PEP) carboxylase (PEPC) isoforms (PEPC1 and PEPC2) from developing castor oil seeds (COS). The association of a shared 107-kD subunit (p107) with an immunologically unrelated bacterial PEPC-type 64-kD polypeptide (p64) leads to marked physical and kinetic differences between the PEPC1 p107 homotetramer and PEPC2 p107/p64 heterooctamer. Here, we describe the production of antiphosphorylation site-specific antibodies to the conserved p107 N-terminal serine-6 phosphorylation site. Immunoblotting established that the serine-6 of p107 is phosphorylated in COS PEPC1 and PEPC2. This phosphorylation was reversed in vitro following incubation of clarified COS extracts or purified PEPC1 or PEPC2 with mammalian protein phosphatase type 2A and is not involved in a potential PEPC1 and PEPC2 interconversion. Similar to other plant PEPCs examined to date, p107 phosphorylation increased PEPC1 activity at pH 7.3 by decreasing its K(m)(PEP) and sensitivity to L-malate inhibition, while enhancing glucose-6-P activation. By contrast, p107 phosphorylation increased PEPC2's K(m)(PEP) and sensitivity to malate, glutamic acid, and aspartic acid inhibition. Phosphorylation of p107 was promoted during COS development (coincident with a >5-fold increase in the I(50) [malate] value for total PEPC activity in desalted extracts) but disappeared during COS desiccation. The p107 of stage VII COS became fully dephosphorylated in planta 48 h following excision of COS pods or following 72 h of dark treatment of intact plants. The in vivo phosphorylation status of p107 appears to be modulated by photosynthate recently translocated from source leaves into developing COS.