Molecular basis of the key regulator WRINKLED1 in plant oil biosynthesis

Integration of BSA-seq and high-resolution mapping reveals genomic regions and candidate genes controlling seed oil accumulation in peanut (Arachis hypogaea L.)

KEY MESSAGE

Three environmentally stable major QTL controlling seed oil content in peanut were identified, and their genetic effects were evaluated in near‑isogenic lines and a peanut germplasm panel. Increasing seed oil content (SOC) is a primary objective in peanut (Arachis hypogaea L.) breeding, meeting the rising global demand for edible oil. Quantitative trait loci (QTL) mapping can help to identify genes underlying SOC variation and develop markers to enhance selection efficiency by marker-assisted breeding. In the present study, three major and stable QTL for SOC were identified on peanut chromosome Arahy.08, using a bulked segregant analysis (BSA) approach based on whole-genome sequencing of F8 recombinant inbred lines (RILs). The QTL qSOCA08-1, explaining 11.41-20.97% of phenotypic variation, was mapped on a 0.65-Mb genomic region. The QTL qSOCA08-2, accounting 25.57-39.40% of phenotypic variation, was located on a 1.04-Mb physical interval. Finally, qSOCA08-3 explaining up to 17.31% of the phenotypic variation was mapped in a genomic region of 1.02-Mb. The genetic effects of these three QTL were assessed using near‑isogenic lines (NILs), derived from residual heterozygous individuals, and a tetraploid peanut germplasm panel. Potential candidate genes within the physical intervals of corresponding major QTL were predicted to participate in the oil biosynthesis in peanut. In summary, our study provides valuable genetic resources and tightly linked molecular markers for peanut molecular breeding aimed at improving SOC.

Impact of molecular regulation on plant oil synthesis

The synthesis of lipids in plants is essential for their growth and development, and it has wide-ranging applications in various fields, including diet and industry. In the majority of plants, the principal unsaturated fatty acids (UFAs) are three C18 varieties: oleic acid (18:1), linoleic acid (18:2), and α-linolenic acid (18:3). Despite the clear delineation of the principal biosynthetic pathways of fatty acids in plants, numerous unresolved issues persist. The regulation of transcription factors can significantly influence the rate of fatty acid synthesis in plants. Consequently, several transcription factors associated with oil synthesis have been identified in recent years, among which the WRINKLED1 (WRI1) and V-myb avian myeloblastosis viral oncogene homolog (MYB) transcription factors play central roles. This study will explain how plants make essential lipids, bring up many unanswered questions, and describe the regulatory network of many transcription factors involved in oil production, with a focus on recent progress in research related to WRI1 and MYB1. The aim is to provide insights for the biological cultivation of high-quality oilseed crops.

The comprehensive regulatory network in seed oil biosynthesis

Plant oils play a crucial role in human nutrition, industrial applications and biofuel production. While the enzymes involved in fatty acid (FA) biosynthesis are well-studied, the regulatory networks governing these processes remain largely unexplored. This review explores the intricate regulatory networks modulating seed oil biosynthesis, focusing on key pathways and factors. Seed oil content is determined by the efficiency of de novo FA synthesis as well as influenced by sugar transport, lipid metabolism, FA synthesis inhibitors and fine-tuning mechanisms. At the center of this regulatory network is WRINKLED1 (WRI1), which plays a conserved role in promoting seed oil content across various plant species. WRI1 interacts with multiple proteins, and its expression level is regulated by upstream regulators, including members of the LAFL network. Beyond the LAFL network, we also discuss a potential nuclear factor-Y (NF-Y) regulatory network in soybean with an emphasis on NF-YA and NF-YB and their associated proteins. This NF-Y network represents a promising avenue for future efforts aimed at enhancing oil accumulation and improving stress tolerance in soybean. Additionally, the application of omics-based approaches is of great significance. Advances in omics technologies have greatly facilitated the identification of gene resources, opening new opportunities for genetic improvement. Importantly, several transcription factors involved in oil biosynthesis also participate in stress responses, highlighting a potential link between the two processes. This comprehensive review elucidates the complex mechanisms underlying the regulation of oil biosynthesis, offering insights into potential biotechnological strategies for improving oil production and stress tolerance in oil crops.

Phase separation of MYB73 regulates seed oil biosynthesis in Arabidopsis

MYB family transcription factors (TFs) play crucial roles in plant development, metabolism, and responses to various stresses. However, whether MYB TFs are involved in regulating fatty acid biosynthesis in seeds remains largely elusive. Here, we demonstrated that transgenic Arabidopsis (Arabidopsis thaliana) plants overexpressing MYB73 exhibit altered FATTY ACID ELONGATION1 (FAE1) expression, seed oil content, and seed fatty acid composition. Electrophoretic mobility shift assays showed that FAE1 is a direct target of MYB73, and functional assays revealed that MYB73 represses FAE1 promoter activity. Transcriptomic analysis of the MYB73-overexpressing plants detected significant changes in the expression of genes involved in fatty acid biosynthesis and triacylglycerol assembly. Furthermore, MYB73 expression was responsive to abscisic acid (ABA), and ABA-responsive element binding factor 2 directly bound to the ABA-responsive element in the MYB73 promoter to activate its expression. Additionally, we determined that MYB73 exhibits the hallmarks of an intrinsically disordered protein and forms phase-separated condensates with liquid-like characteristics, which are important in regulating target gene expression. Together, our findings suggest that MYB73 condensate formation likely fine-tunes seed oil biosynthesis.

Plasticity of the Arabidopsis leaf lipidome and proteome in response to pathogen infection and heat stress

Plants must cope with a variety of stressors during their life cycle, and the adaptive responses to these environmental cues involve all cellular organelles. Among them, comparatively little is known about the contribution of cytosolic lipid droplets (LDs) and their core set of neutral lipids and associated surface proteins to the rewiring of cellular processes in response to stress. Here, we analyzed the changes that occur in the lipidome and proteome of Arabidopsis (Arabidopsis thaliana) leaves after pathogen infection with Botrytis cinerea or Pseudomonas syringae, or after heat stress. Analyses were carried out in wild-type plants and the oil-rich double mutant trigalactosyldiacylglycerol1-1 sugar dependent 1-4 (tgd1-1 sdp1-4) that allowed for an allied study of the LD proteome in stressed leaves. Using liquid chromatography-tandem mass spectrometry-based methods, we showed that a hyperaccumulation of the primary LD core lipid TAG is a general response to stress and that acyl chain and sterol composition are remodeled during cellular adaptation. Likewise, comparative analysis of the LD protein composition in stress-treated leaves highlighted the plasticity of the LD proteome as part of the general stress response. We further identified at least two additional LD-associated proteins, whose localization to LDs in leaves was confirmed by confocal microscopy of fluorescent protein fusions. Taken together, these results highlight LDs as dynamic contributors to the cellular adaptation processes that underlie how plants respond to environmental stress.

Transcriptional engineering for value enhancement of oilseed crops: a forward perspective

Plant-derived oils provide 20%-35% of dietary calories and are a primary source of essential omega-6 (linoleic) and omega-3 (α-linolenic) fatty acids. While traditional breeding has significantly increased yields in key oilseed crops like soybean, sunflower, canola, peanut, and cottonseed, overall gains have plateaued over the past few decades. Oilseed crops also experience substantial yield losses in both prime and marginal agricultural areas due to biotic and abiotic stresses and shifting agro-climates. Recent genomic, transcriptomic, and metabolomics research has expanded our understanding of the genetic and physiological control of fatty acid biosynthesis and composition. Many oilseed species have inherent stress-combating mechanisms, including transcription factor regulation. Advances in genome editing tools like CRISPR/Cas9 offer precise genetic modifications, targeting transcription factors and binding sites to enhance desirable traits, such as the nutritional profile and chemical composition of fatty acids. This review explores the application of genome editing in oilseed improvement, covering recent progress, challenges, and future potential to boost yield and oil content. These advancements could play a transformative role in developing resilient, nutritious crop varieties essential for sustainable food security in a changing climate.

Endosperm-specific expressed transcription factor protein WRINKLED1-mediated oil accumulative mechanism in woody oil peony Paeonia ostii var. lishizhenii

Paeonia ostii var. lishizhenii exhibits superiority of high α-linolenic acid in seed oils, yet, the low yield highlights the importance of enhancing oil accumulation in seeds for edible oil production. The transcription factor protein WRINKLED1 (WRI1) plays crucial roles in modulating oil content in higher plants; however, its functional characterization remains elusive in P. ostii var. lishizhenii. Herein, based on a correlation analysis of transcription factor transcript levels, FA accumulation rates, and interaction assay of FA biosynthesis associated proteins, a WRI1 homologous gene (PoWRI1) that potentially regulated oil content in P. ostii var. lishizhenii seeds was screened. The PoWRI1 exhibited an endosperm-specific and development-depended expression pattern, encoding a nuclear-localized protein with transcriptional activation capability. Notably, overexpressing PoWRI1 upregulated certain key genes relevant to glycolysis, FA biosynthesis and desaturation, and improved seed development, oil body formation and oil accumulation in Arabidopsis seeds, resulting an enhancement of total seed oil weight by 9.47-18.77 %. The defective impacts on seed phenotypes were rescued through ectopic induction of PoWRI1 in wri1 mutants. Our findings highlight the pivotal role of PoWRI1 in controlling oil accumulation in P. ostii var. lishizhenii, offering bioengineering strategies to increase seed oil accumulation and enhance its potential for edible oil production.

Genome-wide profiling of soybean WRINKLED1 transcription factor binding sites provides insight into seed storage lipid biosynthesis

Understanding the regulatory mechanisms controlling storage lipid accumulation will inform strategies to enhance seed oil quality and quantity in crop plants. The WRINKLED1 transcription factor (WRI1 TF) is a central regulator of lipid biosynthesis. We characterized the genome-wide binding profile of soybean (Gm)WRI1 and show that the TF directly regulates genes encoding numerous enzymes and proteins in the fatty acid and triacylglycerol biosynthetic pathways. GmWRI1 binds primarily to regions downstream of target gene transcription start sites. We showed that GmWRI1-bound regions are enriched for the canonical WRI1 DNA binding element, the ACTIVATOR of Spomin::LUC1/WRI1 (AW) Box (CNTNGNNNNNNNCG), and another DNA motif, the CNC Box (CNCCNCC). Functional assays showed that both DNA elements mediate transcriptional activation by GmWRI1. We also show that GmWRI1 works in concert with other TFs to establish a regulatory state that promotes fatty acid and triacylglycerol biosynthesis. In particular, comparison of genes targeted directly by GmWRI1 and by GmLEC1, a central regulator of the maturation phase of seed development, reveals that the two TFs act in a positive feedback subcircuit to control fatty acid and triacylglycerol biosynthesis. Together, our results provide unique insights into the genetic circuitry in which GmWRI1 participates to regulate storage lipid accumulation during seed development.

Natural variation in GhKASI_A05 modulates cottonseed oil content in Gossypium hirsutum L

Cotton is of great economic value because of its fiber that is used in natural textile commodities and its seeds that contain an edible oil with a high content of unsaturated fatty acids and biodiesel applications. Here, we reported that GhKASI_A05 was associated with the cottonseed oil content (SOC) in a natural population via candidate gene association analysis. An 11-bp Indel located in the GhKASI_A05 promoter was found to contribute to SOC and differential expression in upland cotton inbred accessions. Interaction analysis showed that GhWRI1, an AP2/EREBP family transcription factor, that reportedly functions in plant seed oil and fatty acids (FAs) accumulation, directly bound to AW-box cis-elements in two haplotypes of the GhKASI_A05 promoter and activated the expression of GhKASI_A05 at different levels. The seed-specific overexpression of GhKASI_A05 resulted in increased seed size, weight, and protein content, and C16:0 and C18:1 contents but reduced SOC. Our results provide new insights into the biological function of GhKASI in SOC and effective strategies for cotton breeding in the future.

Increasing oil content in Brassica oilseed species

Brassica oilseed species are the third most important in the world, providing approximately 15 % of the total vegetable oils. Three species (Brassica rapa, B. juncea, B. napus) dominate with B. napus being the most common in Canada, China and Europe. Originally, B. napus was a crop producing seed with high erucic acid content, which still persists today, to some extent, and is used for industrial purposes. In contrast, cultivars which produce seed used for food and feed are low erucic acid cultivars which also have reduced glucosinolate content. Because of the limit to agricultural land, recent efforts have been made to increase productivity of oil crops, including Brassica oilseed species. In this article, we have detailed research in this regard. We have covered modern genetic, genomic and metabolic control analysis approaches to identifying potential targets for the manipulation of seed oil content. Details of work on the use of quantitative trait loci, genome-wide association and comparative functional genomics to highlight factors influencing seed oil accumulation are given and functional proteins which can affect this process are discussed. In summary, a wide variety of inputs are proving useful for the improvement of Brassica oilseed species, as major sources of global vegetable oil.

DNA sequence and chromatin differentiate sequence-specific transcription factor binding in the human malaria parasite Plasmodium falciparum

Development of the malaria parasite, Plasmodium falciparum, is regulated by a limited number of sequence-specific transcription factors (TFs). However, the mechanisms by which these TFs recognize genome-wide binding sites is largely unknown. To address TF specificity, we investigated the binding of two TF subsets that either bind CACACA or GTGCAC DNA sequence motifs and further characterized two additional ApiAP2 TFs, PfAP2-G and PfAP2-EXP, which bind unique DNA motifs (GTAC and TGCATGCA). We also interrogated the impact of DNA sequence and chromatin context on P. falciparum TF binding by integrating high-throughput in vitro and in vivo binding assays, DNA shape predictions, epigenetic post-translational modifications, and chromatin accessibility. We found that DNA sequence context minimally impacts binding site selection for paralogous CACACA-binding TFs, while chromatin accessibility, epigenetic patterns, co-factor recruitment, and dimerization correlate with differential binding. In contrast, GTGCAC-binding TFs prefer different DNA sequence context in addition to chromatin dynamics. Finally, we determined that TFs that preferentially bind divergent DNA motifs may bind overlapping genomic regions due to low-affinity binding to other sequence motifs. Our results demonstrate that TF binding site selection relies on a combination of DNA sequence and chromatin features, thereby contributing to the complexity of P. falciparum gene regulatory mechanisms.

Possible crosstalk between the Arabidopsis TSPO-related protein and the transcription factor WRINKLED1

This study uncovers a regulatory interplay between WRINKLED1 (WRI1), a master transcription factor for glycolysis and lipid biosynthesis, and Translocator Protein (TSPO) expression in Arabidopsis thaliana seeds. We identified potential WRI1-responsive elements upstream of AtTSPO through bioinformatics, suggesting WRI1's involvement in regulating TSPO expression. Our analyses showed a significant reduction in AtTSPO levels in wri1 mutant seeds compared to wild type, establishing a functional link between WRI1 and TSPO. This connection extends to the coordination of seed development and lipid metabolism, with both WRI1 and AtTSPO levels decreasing post-imbibition, indicating their roles in seed physiology. Further investigations into TSPO's impact on fatty acid synthesis revealed that TSPO misexpression alters WRI1's post-translational modifications and significantly enhances seed oil content. Additionally, we noted a decrease in key reserve proteins, including 12 S globulin and oleosin 1, in seeds with TSPO misexpression, suggesting a novel energy storage strategy in these lines. Our findings reveal a sophisticated network involving WRI1 and AtTSPO, highlighting their crucial contributions to seed development, lipid metabolism, and the modulation of energy storage mechanisms in Arabidopsis.

Variety of Plant Oils: Species-Specific Lipid Biosynthesis

Plant oils represent a large group of neutral lipids with important applications in food, feed and oleochemical industries. Most plants accumulate oils in the form of triacylglycerol within seeds and their surrounding tissues, which comprises three fatty acids attached to a glycerol backbone. Different plant species accumulate unique fatty acids in their oils, serving a range of applications in pharmaceuticals and oleochemicals. To enable the production of these distinctive oils, select plant species have adapted specialized oil metabolism pathways, involving differential gene co-expression networks and structurally divergent enzymes/proteins. Here, we summarize some of the recent advances in our understanding of oil biosynthesis in plants. We compare expression patterns of oil metabolism genes from representative species, including Arabidopsis thaliana, Ricinus communis (castor bean), Linum usitatissimum L. (flax) and Elaeis guineensis (oil palm) to showcase the co-expression networks of relevant genes for acyl metabolism. We also review several divergent enzymes/proteins associated with key catalytic steps of unique oil accumulation, including fatty acid desaturases, diacylglycerol acyltransferases and oleosins, highlighting their structural features and preference toward unique lipid substrates. Lastly, we briefly discuss protein interactomes and substrate channeling for oil biosynthesis and the complex regulation of these processes.

ACRE, a class of AP2/ERF transcription factors, activates the expression of sweet potato ß-amylase and sporamin genes through the sugar-responsible element CMSRE-1

Sugars, synthesized by photosynthesis in source organs, are loaded and utilized as an energy source and carbon skeleton in sink organs, and also known to be important signal molecules regulating gene expression in higher plants. The expression of genes coding for sporamin and β-amylase, the two most abundant proteins in storage roots of sweet potato, is coordinately induced by sugars. We previously reported on the identification of the carbohydrate metabolic signal-responsible element-1 (CMSRE-1) essential for the sugar-responsible expression of two genes. However, transcription factors that bind to this sequence have not been identified. In this study, we performed yeast one-hybrid screening using the sugar-responsible minimal promoter region of the ß-amylase gene as bait and a library composed only transcription factor cDNAs of Arabidopsis. Two clones, named Activator protein binding to CMSRE-1 (ACRE), encoding AP2/ERF transcription factors were isolated. ACRE showed transactivation activity of the sugar-responsible minimal promoter in a CMSRE-1-dependent manner in Arabidopsis protoplasts. Electric mobility shift assay (EMSA) using recombinant proteins and transient co-expression assay in Arabidopsis protoplasts revealed that ACRE could actually act to the CMSRE-1. Among the DEHYDRATION -RESPONSIVE ELEMENT BINDING FACTOR (DREB) subfamily, almost all homologs including ACRE, could act on the DRE, while only three ACREs could act to the CMSRE-1. Moreover, ACRE-homologs of Japanese morning glory also have the same property of DNA-binding preference and transactivation activity through the CMSRE-1. These findings suggested that ACRE plays an important role in the mechanism regulating the sugar-responsible gene expression through the CMSRE-1 conserved across plant species.

Molecular characterization of oleosin genes in Cyperus esculentus, a Cyperaceae plant producing oil in underground tubers

KEY MESSAGE

CeOLE genes exhibit a tuber-predominant expression pattern and their mRNA/protein abundances are positively correlated with oil accumulation during tuber development. Overexpression could significantly increase the oil content of tobacco leaves. Oleosins (OLEs) are abundant structural proteins of lipid droplets (LDs) that function in LD formation and stabilization in seeds of oil crops. However, little information is available on their roles in vegetative tissues. In this study, we present the first genome-wide characterization of the oleosin family in tigernut (Cyperus esculentus L., Cyperaceae), a rare example accumulating high amounts of oil in underground tubers. Six members identified represent three previously defined clades (i.e. U, SL and SH) or six out of seven orthogroups (i.e. U, SL1, SL2, and SH1-3) proposed in this study. Comparative genomics analysis reveals that lineage-specific expansion of Clades SL and SH was contributed by whole-genome duplication and dispersed duplication, respectively. Moreover, presence of SL2 and SH3 in Juncus effuses implies their appearance sometime before Cyperaceae-Juncaceae divergence, whereas SH2 appears to be Cyperaceae specific. Expression analysis showed that CeOLE genes exhibit a tuber-predominant expression pattern and transcript levels are considerably more abundant than homologs in the close relative Cyperus rotundus. Moreover, CeOLE mRNA and protein abundances were shown to positively correlate with oil accumulation during tuber development. Additionally, two dominant isoforms (i.e. CeOLE2 and -5) were shown to locate in LDs as well as the endoplasmic reticulum of tobacco (Nicotiana benthamiana) leaves, and are more likely to function in homo and heteromultimers. Furthermore, overexpression of CeOLE2 and -5 in tobacco leaves could significantly increase the oil content, supporting their roles in oil accumulation. These findings provide insights into lineage-specific family evolution and putative roles of CeOLE genes in oil accumulation of vegetative tissues, which facilitate further genetic improvement for tigernut.

Transcription factor bZIP52 modulates Arabidopsis seed oil biosynthesis through interaction with WRINKLED1

Transcriptional regulation mediated by combinatorial interaction of transcription factors (TFs) is a key molecular mechanism modulating plant development and metabolism. Basic leucine zipper (bZIP) TFs play important roles in various plant developmental and physiological processes. However, their involvement in fatty acid biosynthesis is largely unknown. Arabidopsis (Arabidopsis thaliana) WRINKLED1 (WRI1) is a pivotal TF in regulation of plant oil biosynthesis and interacts with other positive and negative regulators. In this study, we identified two bZIP TFs, bZIP21 and bZIP52, as interacting partners of AtWRI1 by yeast-two-hybrid (Y2H)-based screening of an Arabidopsis TF library. We found that coexpression of bZIP52, but not bZIP21, with AtWRI1 reduced AtWRI1-mediated oil biosynthesis in Nicotiana benthamiana leaves. The AtWRI1-bZIP52 interaction was further verified by Y2H, in vitro pull-down, and bimolecular fluorescence complementation assays. Transgenic Arabidopsis plants overexpressing bZIP52 showed reduced seed oil accumulation, while the CRISPR/Cas9-edited bzip52 knockout mutant exhibited increased seed oil accumulation. Further analysis revealed that bZIP52 represses the transcriptional activity of AtWRI1 on the fatty acid biosynthetic gene promoters. Together, our findings suggest that bZIP52 represses fatty acid biosynthesis genes through interaction with AtWRI1, resulting in a reduction of oil production. Our work reports a previously uncharacterized regulatory mechanism that enables fine-tuning of seed oil biosynthesis.

WRINKLED1 Positively Regulates Oil Biosynthesis in Carya cathayensis

Hickory (Carya cathayensis Sarg.) is a kind of important woody oil tree species, and its nut has high nutritional value. Previous gene coexpression analysis showed that WRINKLED1 (WRI1) may be a core regulator during embryo oil accumulation in hickory. However, its specific regulatory mechanism on hickory oil biosynthesis has not been investigated. Herein, two hickory orthologs of WRI1 (CcWRI1A and CcWRI1B) containing two AP2 domains with AW-box binding sites and three intrinsically disordered regions (IDRs) but lacking the PEST motif in the C-terminus were characterized. They are nucleus-located and have self-activated ability. The expression of these two genes was tissue-specific and relatively high in the developing embryo. Notably, CcWRI1A and CcWRI1B can restore the low oil content, shrinkage phenotype, composition of fatty acid, and expression of oil biosynthesis pathway genes of Arabidopsis wri1-1 mutant seeds. Additionally, CcWRI1A/B were shown to modulate the expression of some fatty acid biosynthesis genes in the transient expression system of nonseed tissues. Transcriptional activation analysis further indicated that CcWRI1s directly activated the expression of SUCROSE SYNTHASE2 (SUS2), PYRUVATE KINASE β SUBUNIT 1 (PKP-β1), and BIOTIN CARBOXYL CARRIER PROTEIN2 (BCCP2) involved in oil biosynthesis. These results suggest that CcWRI1s can promote oil synthesis by upregulating some late glycolysis- and fatty acid biosynthesis-related genes. This work reveals the positive function of CcWRI1s in oil accumulation and provides a potential target for improving plant oil by bioengineering technology.

Tree peony transcription factor PrWRI1 enhances seed oil accumulation

BACKGROUND

WRINKLED1 (WRI1) encodes a transcription factor, belonging to the APETALA2 (AP2) family, and plays a key role in regulating plant oil biosynthesis. As a newly woody oil crop, tree peony (Paeonia rockii) was notable for the abundant unsaturated fatty acids in its seed oil. However, the role of WRI1 during the accumulation of P. rockii seeds oil remains largely unknown.

RESULTS

In this study, a new member of the WRI1 family was isolated from P. rockii and was named PrWRI1. The ORF of PrWRI1 consisted of 1269 nucleotides, encoding a putative protein of 422 amino acids, and was highly expressed in immature seeds. Subcellular localization analysis in onion inner epidermal cells showed that PrWRI1 was located at the nucleolus. Ectopic overexpression of PrWRI1 could significantly increase the total fatty acid content in Nicotiana benthamiana leaf tissue and even PUFAs in transgenic Arabidopsis thaliana seeds. Furthermore, the transcript levels of most genes related to fatty acids (FA) synthesis and triacylglycerol (TAG) assembly were also up-regulated in transgenic Arabidopsis seeds.

CONCLUSIONS

Together, PrWRI1 could push carbon flow to FA biosynthesis and further enhance the TAG amount in seeds with a high proportion of PUFAs.