Stable, Environmental Specific and Novel QTL Identification as Well as Genetic Dissection of Fatty Acid Metabolism in Brassica napus

Genetic dissection of Brassica napus seed vigor after aging

KEY MESSAGE

Stable and novel QTLs that affect seed vigor under different storage durations were discovered, and BnaOLE4, located in the interval of cqSW-C2-3, increased seed vigor after aging. Seed vigor is an important trait in crop breeding; however, the underlying molecular regulatory mechanisms governing this trait in rapeseed remain largely unknown. In the present study, vigor-related traits were analyzed in seeds from a doubled haploid (DH) rapeseed (Brassica napus) population grown in 2 different environments using seeds stored for 7, 5, and 3 years under natural storage conditions. A total of 229 quantitative trait loci (QTLs) were identified and were found to explain 3.78%-17.22% of the phenotypic variance for seed vigor-related traits after aging. We further demonstrated that seed vigor-related traits were positively correlated with oil content (OC) but negatively correlated with unsaturated fatty acids (FAs). Some pleiotropic QTLs that collectively regulate OC, FAs, and seed vigor, such as uq.A8, uq.A3-2, uq.A9-2, and uq.C3-1, were identified. The transcriptomic results from extreme pools of DH lines with distinct seed vigor phenotypes during accelerated aging revealed that various biological pathways and metabolic processes (such as glutathione metabolism and reactive oxygen species) were involved in seed vigor. Through integration of QTL analysis and RNA-Seq, a regulatory network for the control of seed vigor was constructed. Importantly, a candidate (BnaOLE4) from cqSW-C2-3 was selected for functional analysis, and transgenic lines overexpressing BnaOLE4 showed increased seed vigor after artificial aging. Collectively, these results provide novel information on QTL and potential candidate genes for molecular breeding for improved seed storability.

Lipid Droplets in Endosymbiotic Symbiodiniaceae spp. Associated with Corals

Symbiodiniaceae species is a dinoflagellate that plays a crucial role in maintaining the symbiotic mutualism of reef-building corals in the ocean. Reef-building corals, as hosts, provide the nutrition and habitat to endosymbiotic Symbiodiniaceae species and Symbiodiniaceae species transfer the fixed carbon to the corals for growth. Environmental stress is one of the factors impacting the physiology and metabolism of the corals-dinoflagellate association. The environmental stress triggers the metabolic changes in Symbiodiniaceae species resulting in an increase in the production of survival organelles related to storage components such as lipid droplets (LD). LDs are found as unique organelles, mainly composed of triacylglycerols surrounded by phospholipids embedded with some proteins. To date, it has been reported that investigation of lipid droplets significantly present in animals and plants led to the understanding that lipid droplets play a key role in lipid storage and transport. The major challenge of investigating endosymbiotic Symbiodiniaceae species lies in overcoming the strategies in isolating lesser lipid droplets present in its intercellular cells. Here, we review the most recent highlights of LD research in endosymbiotic Symbiodiniaceae species particularly focusing on LD biogenesis, mechanism, and major lipid droplet proteins. Moreover, to comprehend potential novel ways of energy storage in the symbiotic interaction between endosymbiotic Symbiodiniaceae species and its host, we also emphasize recent emerging environmental factors such as temperature, ocean acidification, and nutrient impacting the accumulation of lipid droplets in endosymbiotic Symbiodiniaceae species.

Hub Genes in Stable QTLs Orchestrate the Accumulation of Cottonseed Oil in Upland Cotton via Catalyzing Key Steps of Lipid-Related Pathways

Upland cotton is the fifth-largest oil crop in the world, with an average supply of nearly 20% of vegetable oil production. Cottonseed oil is also an ideal alternative raw material to be efficiently converted into biodiesel. However, the improvement in kernel oil content (KOC) of cottonseed has not received sufficient attention from researchers for a long time, due to the fact that the main product of cotton planting is fiber. Previous studies have tagged QTLs and identified individual candidate genes that regulate KOC of cottonseed. The regulatory mechanism of oil metabolism and accumulation of cottonseed are still elusive. In the current study, two high-density genetic maps (HDGMs), which were constructed based on a recombinant inbred line (RIL) population consisting of 231 individuals, were used to identify KOC QTLs. A total of forty-three stable QTLs were detected via these two HDGM strategies. Bioinformatic analysis of all the genes harbored in the marker intervals of the stable QTLs revealed that a total of fifty-one genes were involved in the pathways related to lipid biosynthesis. Functional analysis via coexpression network and RNA-seq revealed that the hub genes in the co-expression network that also catalyze the key steps of fatty acid synthesis, lipid metabolism and oil body formation pathways (ACX4, LACS4, KCR1, and SQD1) could jointly orchestrate oil accumulation in cottonseed. This study will strengthen our understanding of oil metabolism and accumulation in cottonseed and contribute to KOC improvement in cottonseed in the future, enhancing the security and stability of worldwide food supply.

Rice lipases: a conundrum in rice bran stabilization: a review on their impact and biotechnological interventions

Rice is a primary food and is one of the most important constituents of diets all around the world. Rice bran is a valuable component of rice, containing many oil-soluble vitamins, minerals, and oil. It is known for its ability to improve the economic value of rice. Further, it contains substantial quantities of minerals like potassium, calcium, magnesium, iron and antioxidants like tocopherols, tocotrienols, and γ-oryzanol, indicating that rice bran can be utilized effectively against several life-threatening disorders. It is difficult to fully utilize the necessary nutrients due to the presence of lipases in rice bran. These lipases break down lipids, specifically Triacylglycerol, into free fatty acids and glycerol. This review discusses physicochemical properties, mechanism of action, distribution, and activity of lipases in various components of rice seeds. The phylogenetic and gene expression analysis helped to understand the differential expression pattern of lipase genes at different growth phases of rice plant. Further, this review discusses various genetic and biotechnological approaches to decrease lipase activity in rice and other plants, which could potentially prevent the degradation of bran oil. The goal is to establish whether lipases are a major contributor to this issue and to develop rice varieties with improved bran stability. This information sets the stage for upcoming molecular research in this area.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01343-3.

Meta-analysis of seed weight QTLome using a consensus and highly dense genetic map in Brassica napus L

KEY MESSAGE

We report here the discovery of high-confidence MQTL regions and of putative candidate genes associated with seed weight in B. napus using a highly dense consensus genetic map and by comparing various large-scale multiomics datasets. Seed weight (SW) is a direct determinant of seed yield in Brassica napus and is controlled by many loci. To unravel the main genomic regions associated with this complex trait, we used 13 available genetic maps to construct a consensus and highly dense map, comprising 40,401 polymorphic markers and 9191 genetic bins, harboring a cumulative length of 3047.8 cM. Then, we performed a meta-analysis using 639 projected SW quantitative trait loci (QTLs) obtained from studies conducted since 1999, enabling the identification of 57 meta-QTLS (MQTLs). The confidence intervals of our MQTLs were 9.8 and 4.3 times lower than the average CIs of the original QTLs for the A and C subgenomes, respectively, resulting in the detection of some key genes and several putative novel candidate genes associated with SW. By comparing the genes identified in MQTL intervals with multiomics datasets and coexpression analyses of common genes, we defined a more reliable and shorter list of putative candidate genes potentially involved in the regulation of seed maturation and SW. As an example, we provide a list of promising genes with high expression levels in seeds and embryos (e.g., BnaA03g04230D, BnaC03g08840D, BnaA10g29580D and BnaA03g27410D) that can be more finely studied through functional genetics experiments or that may be useful for MQTL-assisted breeding for SW. The high-density genetic consensus map and the single nucleotide polymorphism (SNP) physical map generated from the latest B. napus cv. Darmor-bzh v10 assembly will be a valuable resource for further mapping and map-based cloning of other important traits.

New insight into the genetic basis of oil content based on noninvasive three-dimensional phenotyping and tissue-specific transcriptome in Brassica napus

BACKGROUND

Increasing seed oil content is the most important breeding goal in Brassica napus, and phenotyping is crucial to dissect its genetic basis in crops. To date, QTL mapping for oil content has been based on whole seeds, and the lipid distribution is far from uniform in different tissues of seeds in B. napus. In this case, the phenotype based on whole seeds was unable to sufficiently reveal the complex genetic characteristics of seed oil content.

RESULTS

Here, the three-dimensional (3D) distribution of lipid was determined for B. napus seeds by magnetic resonance imaging (MRI) and 3D quantitative analysis, and ten novel oil content-related traits were obtained by subdividing the seeds. Based on a high-density genetic linkage map, 35 QTLs were identified for 4 tissues, the outer cotyledon (OC), inner cotyledon (IC), radicle (R) and seed coat (SC), which explained up to 13.76% of the phenotypic variation. Notably, 14 tissue-specific QTLs were reported for the first time, 7 of which were novel. Moreover, haplotype analysis showed that the favorable alleles for different seed tissues exhibited cumulative effects on oil content. Furthermore, tissue-specific transcriptomes revealed that more active energy and pyruvate metabolism influenced carbon flow in the IC, OC and R than in the SC at the early and middle seed development stages, thus affecting the distribution difference in oil content. Combining tissue-specific QTL mapping and transcriptomics, 86 important candidate genes associated with lipid metabolism were identified that underlie 19 unique QTLs, including the fatty acid synthesis rate-limiting enzyme-related gene CAC2, in the QTLs for OC and IC.

CONCLUSIONS

The present study provides further insight into the genetic basis of seed oil content at the tissue-specific level.

Further insight into decreases in seed glucosinolate content based on QTL mapping and RNA-seq in Brassica napus L

The QTL hotspots determining seed glucosinolate content instead of only four HAG1 loci and elucidation of a potential regulatory model for rapeseed SGC variation. Glucosinolates (GSLs) are amino acid-derived, sulfur-rich secondary metabolites that function as biopesticides and flavor compounds, but the high seed glucosinolate content (SGC) reduces seed quality for rapeseed meal. To dissect the genetic mechanism and further reduce SGC in rapeseed, QTL mapping was performed using an updated high-density genetic map based on a doubled haploid (DH) population derived from two parents that showed significant differences in SGC. In 15 environments, a total of 162 significant QTLs were identified for SGC and then integrated into 59 consensus QTLs, of which 32 were novel QTLs. Four QTL hotspot regions (QTL-HRs) for SGC variation were discovered on chromosomes A09, C02, C07 and C09, including seven major QTLs that have previously been reported and four novel major QTLs in addition to HAG1 loci. SGC was largely determined by superimposition of advantage allele in the four QTL-HRs. Important candidate genes directly related to GSL pathways were identified underlying the four QTL-HRs, including BnaC09.MYB28, BnaA09.APK1, BnaC09.SUR1 and BnaC02.GTR2a. Related differentially expressed candidates identified in the minor but environment stable QTLs indicated that sulfur assimilation plays an important rather than dominant role in SGC variation. A potential regulatory model for rapeseed SGC variation constructed by combining candidate GSL gene identification and differentially expressed gene analysis based on RNA-seq contributed to a better understanding of the GSL accumulation mechanism. This study provides insights to further understand the genetic regulatory mechanism of GSLs, as well as the potential loci and a new route to further diminish the SGC in rapeseed.

Construction of a Quantitative Genomic Map, Identification and Expression Analysis of Candidate Genes for Agronomic and Disease-Related Traits in Brassica napus

Rapeseed is the second most important oil crop in the world. Improving seed yield and seed oil content are the two main highlights of the research. Unfortunately, rapeseed development is frequently affected by different diseases. Extensive research has been made through many years to develop elite cultivars with high oil, high yield, and/or disease resistance. Quantitative trait locus (QTL) analysis has been one of the most important strategies in the genetic deciphering of agronomic characteristics. To comprehend the distribution of these QTLs and to uncover the key regions that could simultaneously control multiple traits, 4,555 QTLs that have been identified during the last 25 years were aligned in one unique map, and a quantitative genomic map which involved 128 traits from 79 populations developed in 12 countries was constructed. The present study revealed 517 regions of overlapping QTLs which harbored 2,744 candidate genes and might affect multiple traits, simultaneously. They could be selected to customize super-rapeseed cultivars. The gene ontology and the interaction network of those candidates revealed genes that highly interacted with the other genes and might have a strong influence on them. The expression and structure of these candidate genes were compared in eight rapeseed accessions and revealed genes of similar structures which were expressed differently. The present study enriches our knowledge of rapeseed genome characteristics and diversity, and it also provided indications for rapeseed molecular breeding improvement in the future.

Plant monounsaturated fatty acids: Diversity, biosynthesis, functions and uses

Monounsaturated fatty acids are straight-chain aliphatic monocarboxylic acids comprising a unique carbon‑carbon double bond, also termed unsaturation. More than 50 distinct molecular structures have been described in the plant kingdom, and more remain to be discovered. The evolution of land plants has apparently resulted in the convergent evolution of non-homologous enzymes catalyzing the dehydrogenation of saturated acyl chain substrates in a chemo-, regio- and stereoselective manner. Contrasted enzymatic characteristics and different subcellular localizations of these desaturases account for the diversity of existing fatty acid structures. Interestingly, the location and geometrical configuration of the unsaturation confer specific characteristics to these molecules found in a variety of membrane, storage, and surface lipids. An ongoing research effort aimed at exploring the links existing between fatty acid structures and their biological functions has already unraveled the importance of several monounsaturated fatty acids in various physiological and developmental contexts. What is more, the monounsaturated acyl chains found in the oils of seeds and fruits are widely and increasingly used in the food and chemical industries due to the physicochemical properties inherent in their structures. Breeders and plant biotechnologists therefore develop new crops with high monounsaturated contents for various agro-industrial purposes.

Breeding Canola (Brassica napus L.) for Protein in Feed and Food

Interest in canola (Brassica napus L.). In response to this interest, scientists have been tasked with altering and optimizing the protein production chain to ensure canola proteins are safe for consumption and economical to produce. Specifically, the role of plant breeders in developing suitable varieties with the necessary protein profiles is crucial to this interdisciplinary endeavour. In this article, we aim to provide an overarching review of the canola protein chain from the perspective of a plant breeder, spanning from the genetic regulation of seed storage proteins in the crop to advancements of novel breeding technologies and their application in improving protein quality in canola. A review on the current uses of canola meal in animal husbandry is presented to underscore potential limitations for the consumption of canola meal in mammals. General discussions on the allergenic potential of canola proteins and the regulation of novel food products are provided to highlight some of the challenges that will be encountered on the road to commercialization and general acceptance of canola protein as a dietary protein source.

Multi-locus genome-wide association mapping for spike-related traits in bread wheat (Triticum aestivum L.)

BACKGROUND

Bread wheat (Triticum aestivum L.) is one of the most important cereal food crops for the global population. Spike-layer uniformity (the consistency of the spike distribution in the vertical space)-related traits (SLURTs) are quantitative and have been shown to directly affect yield potential by modifying the plant architecture. Therefore, these parameters are important breeding targets for wheat improvement. The present study is the first genome-wide association study (GWAS) targeting SLURTs in wheat. In this study, a set of 225 diverse spring wheat accessions were used for multi-locus GWAS to evaluate SLURTs, including the number of spikes per plant (NSPP), spike length (SL), number of spikelets per spike (NSPS), grain weight per spike (GWPS), lowest tiller height (LTH), spike-layer thickness (SLT), spike-layer number (SLN) and spike-layer uniformity (SLU).

RESULTS

In total, 136 significant marker trait associations (MTAs) were identified when the analysis was both performed individually and combined for two environments. Twenty-nine MTAs were detected in environment one, 48 MTAs were discovered in environment two and 59 MTAs were detected using combined data from the two environments. Altogether, 15 significant MTAs were found for five traits in one of the two environments, and four significant MTAs were detected for the two traits, LTH and SLU, in both environments i.e. E1, E2 and also in combined data from the two environments. In total, 279 candidate genes (CGs) were identified, including Chaperone DnaJ, ABC transporter-like, AP2/ERF, SWEET sugar transporter, as well as genes that have previously been associated with wheat spike development, seed development and grain yield.

CONCLUSIONS

The MTAs detected through multi-locus GWAS will be useful for improving SLURTs and thus yield in wheat production through marker-assisted and genomic selection.

Identification and Development of KASP Markers for Novel Mutant BnFAD2 Alleles Associated With Elevated Oleic Acid in Brassica napus

The fatty acid desaturase FAD2 genes are the main contributors to oleic acid content, and different FAD2 alleles can result in different oleic acid contents in rapeseed oil. Hence, identification of allelic variation in FAD2 is an extremely desirable breeding goal. By performing QTL mapping using 190 F_2:3 lines genotyped by genome-wide single nucleotide polymorphism (SNP) markers assayed by the Brassica 60 K Infinium BeadChip Array, four quantitative trait loci (QTL) for C18:1 content were mapped on chromosomes A01, A05, A09 and C05 over 3 years in a population segregating for oleic acid content. Two BnFAD2 genes on A05 and C05 were anchored within the QTL intervals, explaining 45-52 and 15-44% of the observed variation for C18:1 content. Sequence polymorphisms between the corresponding coding regions of the parental lines found two single-nucleotide polymorphisms (SNPs) in BnFAD2.A05 and BnFAD2.C05, respectively, which led to the amino acid changes (C421T and G1073E) in the corresponding proteins. The mutation sites of Bnfad2.A05 and Bnfad2.C05 alleles were located within the second H-box and near the third H-box motif of the protein, respectively, and were found to be novel mutant alleles. Lines resulting from the combination of these two alleles contained up to 88% oleic acid in their seed oil, compared with 63% in wild-type controls. Two competitive allele-specific PCR (KASP) markers based on these two mutation sites were successfully developed and validated in segregating F₂ populations. These markers will facilitate breeding for ultra-high seed oleic acid content in oilseed rape.

New Insights Into the Role of Seed Oil Body Proteins in Metabolism and Plant Development

Oil bodies (OBs) are ubiquitous dynamic organelles found in plant seeds. They have attracted increasing attention recently because of their important roles in plant physiology. First, the neutral lipids stored within these organelles serve as an initial, essential source of energy and carbon for seed germination and post-germinative growth of the seedlings. Secondly, they are involved in many other cellular processes such as stress responses, lipid metabolism, organ development, and hormone signaling. The biological functions of seed OBs are dependent on structural proteins, principally oleosins, caleosins, and steroleosins, which are embedded in the OB phospholipid monolayer. Oleosin and caleosin proteins are specific to plants and mainly act as OB structural proteins and are important for the biogenesis, stability, and dynamics of the organelle; whereas steroleosin proteins are also present in mammals and play an important role in steroid hormone metabolism and signaling. Significant progress using new genetic, biochemical, and imaging technologies has uncovered the roles of these proteins. Here, we review recent work on the structural or metabolic roles of these proteins in OB biogenesis, stabilization and degradation, lipid homeostasis and mobilization, hormone signal transduction, stress defenses, and various aspects of plant growth and development.

A novel quantitative trait locus on chromosome A9 controlling oleic acid content in Brassica napus

One of the most important goals in the breeding of oilseed crops, including Brassica napus, is to improve the quality of edible vegetable oil, which is mainly determined by the seed fatty acid composition, particularly the C18:1 content. Previous studies have indicated that the C18:1 content is a polygenic trait, and no stable quantitative trait loci (QTLs) except for FAD2 have been reported. By performing a GWAS using 375 low erucic acid B. napus accessions genotyped with the Brassica 60K SNP array and constructing a high-density SNP-based genetic map of a 150 DH population, we identified a novel QTL on the A9 chromosome. The novel locus could explain 11.25%, 5.72% and 6.29% of phenotypic variation during three consecutive seasons and increased the C18:1 content by approximately 3%-5%. By fine mapping and gene expression analysis, we found three potential candidate genes and verified the fatty acids in a homologous gene mutant of Arabidopsis. A metal ion-binding protein was found to be the most likely candidate gene in the region. Thus, the C18:1 content can be further increased to about 80% with this novel locus together with FAD2 mutant allele without compromise of agronomic performance. A closely linked marker, BnA129, for this novel QTL (OLEA9) was developed so that we can effectively identify materials with high C18:1 content at an early growth stage by marker-assisted selection. Our results may also provide new insight for understanding the complex genetic mechanism of fatty acid metabolism.

Integration of QTL Mapping and Gene Fishing Techniques to Dissect the Multi-Main Stem Trait in Rapeseed (Brassica napus L.)

Rapeseed is one of the most important oilseed crops in the world. Improving the production of rapeseed is beneficial to relieve the shortage of edible vegetable oil. As the organ of support and transport, the main stem of rapeseed controls the plant architecture, transports the water and nutrients, and determines the number of inflorescence. Increasing the number of main stems would be helpful for the yield improvement in Brassica napus (B. napus). This attractive multi-main stem (MMS) trait was observed in the KN DH population. We investigated not only the frequency of MMS traits but also dissected the genetic basis with QTL mapping analysis and Gene-Fishing technique. A total of 43 QTLs were identified for MMS based on high-density linkage map, which explained 2.95-14.9% of the phenotypic variation, among which two environmental stable QTLs (cqMMS.A3-2 and cqMMS.C3-5) were identified in winter and semi-winter environments. Epistatic interaction analysis indicated cqMMS.C3-5 was an important loci for MMS. According to the functional annotation, 159 candidate genes within QTL confidence intervals, corresponding to 148 Arabidopsis thaliana (A. thaliana) homologous genes, were identified, which regulated lateral bud development and tiller of stem, such as shoot meristemless (STM), WUSCHEL-regulated-related genes, cytokinin response factors (CRF5), cytokinin oxidase (CKX4), gibberellin-regulated (RDK1), auxin-regulated gene (ARL, IAR4), and auxin-mediated signaling gene (STV1). Based on Gene-Fishing analysis between the natural plants and the double-main stem (DMS) plant, 31 differentially expressed genes (DEGs) were also obtained, which were related to differentiation and formation of lateral buds, biotic stimulus, defense response, drought and salt-stress responses, as well as cold-response functional genes. In addition, by combining the candidate genes in QTL regions with the DEGs that were obtained by Gene-Fishing technique, six common candidate genes (RPT2A, HLR, CRK, LRR-RLK, AGL79, and TCTP) were identified, which might probably be related to the formation of MMS phenotype. The present results not only would give a new insight into the genetic basis underlying the regulation of MMS but also would provide clues for plant architecture breeding in rapeseed.