Herbaceous Edible Oilseed-Bearing Plants: Origin, Botanical Insights, Constraints, and Recent Progress

1.1 Human, balanced diet, and plants

Plants and/or plant-based (food and non-food) resources fundamentally sustain and nourish almost all life-forms on the Earth. In particular context of human nutrition (and diet), carbohydrates, lipids (mostly fats and oils), proteins, vitamins, minerals, and water are the six classes of nutrients found in foods. The first three components together constitute the bulk of the human diet. Notably, falling in the category of lipids, oils stand second to carbohydrates as an important source of major calories required in the human diet. Oils act as a vehicle for some of the important vitamins and also provide a range of nutrients and bio-active compounds with numerous metabolic functions and health benefits (Figure 1).

Interestingly, edible oils are obtained to varying extents from the seeds, pulps, fruits, and plumules of certain plants, which can be categorized into non-woody (herbaceous) and woody (non-herbaceous) oil-bearing plants. In particular, edible oils are extracted from oil-yielding edible seeds (edible oilseeds). The list of major non-woody edible oilseed-bearing herbaceous (crop)plants includes pea(ground)nut (Arachis hypogaea L.), Indian mustard (Brassica juncea L. Czern & Coss.), rapeseed (Brassica campestris L.) (syn. B. rapa) (toria, sarson, summer turnip rape or Polish rape), soybean (Glycine max L.), flaxseed (Linum usitatissimum L.), sunflower (Helianthus annuus L.), sesame (Sesamum indicum L.), safflower (Carthamus tinctorius L.), and niger (Guizotia abyssinica L.). The oils obtained from the mentioned oil-yielding edible oilseeds significantly vary in their fatty acid compositions (Figure 2A) [1]. In particular, edible oils obtained from A. hypogaea and B. campestris (syn. B. rapa), possess the highest (about 80%) unsaturated fatty acids. On the other hand, oil palm (Elaeis guineensis Jacq.), tea-oil tree (Camellia oleifera Abel.), olive (Olea europaea L.), and coconut (Cocos nucifera L.) are four well-known woody edible oil-bearing plants (trees) with high oil content. Other important woody edible oil-bearing plants are walnut (Juglans regia L.), pecan (Carya cathayensis Sarg.), shiny-leaved yellow-horn (Xanthoceras sorbifolium Bunge.), and hazelnut (Corylus heterophylla Fisch. ex Trautv.) (Figure 2B) [1]. Interestingly, human diet involving non-animal products (such as vegetable/rice, sugar, and nuts oils) minimizes the emission/relative footprints of greenhouse gases (heat-trapping gases in the Earth’s atmosphere) [2]. There has been an increasing trend in the cultivation of oil-bearing (crop)plants on the globe. Unfortunately, edible oil-bearing (crop)plants are sensitive to most stress and extreme climatic factors. Most of these stresses or factors, in isolation or combination, varyingly impact the health of edible oil-bearing (crop)plants and thereby also limit their quantitative and qualitative productivity.

Taking into account major herbaceous edible oilseed (crop)plants (A. hypogaea, B. juncea, B. campestris (syn. B. rapa), G. max, L. usitatissimum, H. annuus, S. indicum, C. tinctorius, and Guizotia abyssinica), (i) their origin, key botanical insights, and the major health benefits are introduced; (ii) the major constraints and their impacts are provided and enlightens important mitigation strategies are overviewed; (iii) the major progress made in important breeding programmes for edible oilseed (crop)plants are highlighted; and also (iv) important aspects so far least or unexplored in the present context are mentioned as follows.

2.1 Pea(ground)nut

Believed to have originated in the Southern Bolivia and Brazil to the northern Argentina region of South America, pea(ground)nut (Arachis hypogaea L.) (hypogaea = under the Earth) is a perennial, herbaceous, self-pollinated, monoecious (having both the male and female reproductive organs in the same plant), and a leguminous edible allotetraploid (AABB-type genome; 2n = 4x = 40) oilseed crop and belongs to family Leguminosae (sub-family: Fabaceae). A. hypogaea usually attains 15-70 cm height, and its root system consists of a well-developed taproot with many lateral roots. Interestingly, A. hypogaea is a unique edible oilseed crop, which produces flowers above ground, and exhibits geocarpy, as it develops the fruits (pods containing edible oilseeds) below ground (earth). It is worldwide grown (as an annual crop) in the tropics and subtropics under rainfed conditions [3]. Moreover, A. hypogaea is grown mainly for the production of oil (seed-oil 43-55%), which is full of biologically active compounds with widely reported human health benefits (Figure 3). Given this, the consumption of A. hypogaea-based ‘ready-to-use therapeutic foods’ for community-based treatment of severe malnutrition has been encouraged by the World Health Organization of the United Nations [7].

2.2 Cruciferous oilseeds

The cruciferous oilseeds (oilseed Brassicas) are herbaceous, self-pollinated, monoecious and annual economically important edible oilseed crops. Moreover, cruciferous oilseeds stand as the second largest oilseed crops after Glycine max in the world oilseed production [8]. The cruciferous oilseeds belong to a plant family Brassicaceae (Cruciferae), and are represented by a very complex genus Brassica. The genus Brassica possesses several taxonomic and classification problems because it has been entirely modified during its domestication by humans and contains many cultivated plants and wild species, entirely different from their ancestor(s). To his end, based mainly on the cytogenetic relationships, Nagaharu [9] attempted to systematically explain the major cultivated interrelated species of genus Brassica. Brassica species contain one or two of the three genome types A, B, and C. As per the relationships explained: Brassica campestris (syn. B. rapa) (2n = 20, with genome formula AA) of rapeseed and B. nigra (2n = 16, BB) and B. oleracea (2n = 18, CC) are the primary species. However, B. napus (2n = 28, AACC) and B. carinata (2n = 34, BBCC) of rapeseed and B. juncea (2n = 36, AABB) are the amphidiploids resulting from paired crossings between the primary species. A triangle of U [9] represents the botanical and genomic relationships between these six species.

The most widely distributed oilseed crop among the oilseed Brassicas, B. campestris (syn. B. rapa) is presently native to throughout Europe, Russia, Central Asia, and Near East; and has northern India (Himalayan region) as its primary centre of origin, and the European-Mediterranean area and Asia as its secondary centre of origin. Spring types of B. campestris are usually preferred and are largely grown in Sweden, Finland, some parts of Canada, and northwest China [10, 11]. Generally thought to have originated in the Middle East, B. juncea may also have arisen by independent hybridization at secondary centres in India, China, and Caucasus [12, 13]. B. juncea is the predominant Brassica in Asia and the Indian sub-continent. However, Canadian-grown Oriental mustard (golden yellow seed coat colored version of B. juncea) is used in Japan for about half of its edible oil-requirement [11]. Believed to be originated in India over 3000 years ago, rapeseed (B. napus) (also known as Argentine rape, Swede rape, and colza) was introduced into China and Japan, carried out to Europe in the nineteenth century, and was eventually introduced into Canada in 1936 by Polish travelers [14]. B. napus is of commercial value in Canada and Australia. In particular, Winter types of B. napus are largely grown under north European, Chinese, and Canadian conditions [8].

2.3 Soybean

Native to East Asia (central China) and recognized as the ‘golden bean’ or the ‘miracle bean’, soybean (Glycine max L.) is an erect, bushy, herbaceous, annual, self-pollinated, and diploid legume, which belongs to family Leguminosae (sub-family: Fabaceae). Morphological, cytogenetic, and molecular analyses have indicated the domestication of G. max from its wild relative (G. soja) in East Asia 6000-9000 years ago. Additionally, before its spread to huge G. max yield-producing America and Brazil (in the last 100 years), G. max was first introduced into Japan, Korea, and Southeast Asia [15, 16]. G. max seed-oil (18–20% of the dry weight of seed) is nutritionally very important as it possess five most abundant fatty acids (palmitic, stearic, oleic, linoleic, and linolenic acids) and exhibits arachidic acid (1–3%) and polyunsaturated/saturated ratio of 4:1. Lecithin (phosphatidylcholine), phospholipids, tocopherols, and phytosterols also occur in G. max seed-oil. Given numerous human health benefits, G. max is in its high demand particularly in Northeast Asian countries including China, Japan, the Republic of Korea, and Russia [17].

2.4 Flax(lin)seed

An ancient annual herbaceous and a self-pollinated plant, linseed or flaxseed (Linum usitatissimum L.) belongs to Linaceae family and has its uncertain origin. However, east of the Mediterranean Sea toward India (having a very diverse forms of L. usitatissimum) is argued as the centre of its origin. L. usitatissimum has also been considered a native of West Asia and the Mediterranean coastal lands, Asia Minor, Egypt, Algeria, Tunis, Spain, Italy, and Greece, where fiber types (fiber type L. usitatissimum) of this plant were cultivated. On the other hand, seed-type L. usitatissimum for expressible oil was grown in Southwest Asia, including Turkestan, Afghanistan, and India [18]. L. usitatissimum attains 60-100 cm length, and has non-shattering capsules. The seed-type L. usitatissimum is shorter and more branched, when compared to slender and more branched fiber type L. usitatissimum. L. usitatissimum is widely distributed in the northern hemisphere (Canada, United States, Russia, China and India), where it is cultivated for its seed-oil (40% of its seed weight) and stem fiber (linen) [19]. Though L. usitatissimum seed-oil (linseed oil) components depend on seed cultivar, location, environmental condition, and methods of analysis, its seed-oil is enriched in nutritionally important omega-3 fatty acids including α-linolenic acid (an n-3 polyunsaturated fatty acid), and a good content of the important bioactive components (Figure 4). Linseed oil is particularly rich in total unsaturated fatty acids (87.8-89.8%). Hence, linseed oil is in its great demand as a dietary supplement and is also recognized as nutraceutical and functional food owing to numerous known and potential human health benefits of bioactive components therein [20, 21].

2.5 Sunflower

Originated from a Mexico native wild sunflower, Helianthus lenticularis Douglas (Greek term, Helios = sun; Anthos: a flower), sunflower (Helianthus annuus L.) is a fast-growing diploid species (2n = 2x = 34) annual herb and is thought indigenous to central and eastern North America (and/or south-west United States-Mexico area and western Canada). Mainly commercialized in Russia, H. annuus is cultivated worldwide, extensively in sub-tropical and temperate regions, and its cultivation is spreading to tropical regions. A representative member of family Compositae (Asteraceae), H. annuus, is an erect, hardy, often unbranched, coarse, and stout-stemmed having a varying height up to 4.2 m and with a decorative flower head composed of disc florets in its centre and surrounded ray flowers [22, 23]. H. annuus seed-oil (22–55% of seed dry weight) is nutritionally valued for its high content of approximately 15% saturated and 85% unsaturated fatty acids. Its oil has multifaceted therapeutic benefits including attenuation of cardiovascular and inflammatory diseases, and antimicrobial, antioxidant, and antihypertensive benefits, among others [22].

2.6 Sesame

A native of Africa, China, and India, sesame (Sesamum indicum L.) is considered as the oldest known oilseed crop. It was domesticated nearly 3000 years ago and was the first known oil consumed by humans [24]. Domestication of S. indicum in South India has taken place about 1500 years ago [25]. It belongs to family Pedaliaceae and is recognized as the queen of oilseeds owing to its high value seed-oil quality and plethora of nutritive substances [26]. The genus Sesamum is one among the 13 genera of the family Pedaliaceae. In particular, S. indicum, together with S. capense, S. malabaricum and S. schenckii, has the somatic number 2n = 26; S. laciniatum, 2n = 28; S. angolase and S. prostratum, 2n = 32; S. occidentale; and S. radiatum, 2n = 64 [27]. S. indicum is an annual or biannual and self-pollinated herb with tall and branched (having an indeterminate growth habit) ridges bearing stems and mostly with solitary, axillary, shortly pedicellate, and zygomorphic flowers in white color (pink, purple, or various shades of purple-white flowers also occur). The fatty acid composition of S. indicum seed-oil comprises oleic acid (43%), linoleic acid (35%), palmitic acid (11%), and stearic acid (7%), which together contribute to 96% of total fatty acid content [4]. Given these, S. indicum seed-oil promotes health and also provides relief from hypertension, oxidative stress, and neurodegenerative diseases [28].

2.7 Safflower

An annual, erect, herbaceous, highly branched, spiny (leaves and bracts), thistle-like plant, safflower (Carthamus tinctorius L.) belongs to the family Asteraceae (Compositae), attains from 30 to 150 cm height, and has a chromosome number of 2n = 24 [29]. A minor, underutilized, and xerophilous oilseed crop, C. tinctorius is the only cultivated representative of genus Carthamus. Based on the archaeological records, C. tinctorius was established in north-western India during the Chalcolithic culture, and thereafter, it was distributed to the other regions. Three major centres: Ethiopia, the Iran-Afghanistan area, and India were assumed by Nikolai Vavilov (1887-1943) as the centres of origin of Carthamus sp. [30]. Moreover seven centres of similarity: (i) The Far East (China and Korea), (ii) India-Pakistan, (iii) Middle East, (iv) Egypt, (v) Sudan, (vi) Ethiopia, and (vii) Europe were the regions, where the Carthamus spp. crop was mainly cultivated [31]. The hypothesis of these seven centres of similarity were also corroborated with molecular characterization of genetic diversity, population structure, and similarity centres for 131 Carthamus sp. accessions obtained from 28 countries using 12 Inter Simple Sequence Repeat markers [32]. In particular, C. tinctorius was domesticated from its putative wild ancestor, C. palestinus, about 4500 years ago. C. tinctorius is thought to be native to parts of Asia (northern China and India), the Middle East (Iran and Iraq), and Africa and Mediterranean basin and is grown in arid and semi-arid regions in the world [33]. Moreover, based on phylogenetic analysis, C. tinctorius has diverged from artichokes (Cynara cardunculus) and Helianthus annuus, approximately 30.7 and 60.5 million years ago, respectively [34]. C. tinctorius yields high quality 32-40% seed-oil, which is rich in bioactive components including polyunsaturated fatty acids (with linoleic acid being predominant at 70%), monounsaturated oleic acid (10%), small amounts of stearic acid, flavonoids, alkaloids, and poly-alkenes. These bioactive components are valued for human health reasons, and helps in significantly reducing blood cholesterol levels [35]. C. tinctorius seed-oil also helps in the management of coronary heart disease, hypertension, dysmenorrhoea, and amenorrhoea [35].

2.8 Niger

Another minor, neglected or underutilized crop, niger (Guizotia abyssinica L.) is native to the Ethiopian highlands [36]. The genus Guizotia has its centre of origin, distribution, and genetic diversity in Ethiopia, where G. abyssinica has been domesticated, and is only cultivated out of the six species in genus Guizotia. A stout, erect annual, and moderately to well-branched dicotyledonous herb, G. abyssinica belongs to the Compositae (Asteraceae) family and has soft, hairy, and hollow stem and a well-developed taproot system (with many lateral roots); grows up to 2 m tall; has hermaphroditic disk florets; and has a diploid genome constituting (2n = 2x = 30) chromosome number. G. abyssinica is mainly planted for its edible oil in India and Ethiopia. It is also cultivated at small scale in several other African and Asian countries [37]. As a good source of oil, G. abyssinica exhibits its oil-yielding capacity between 28 and 50%. The occurrence of a high amount of linoleic acid (C 18:2) as the main fatty acid in its oil makes G. abyssinica nutritionally valuable and attractive. Additionally, G. abyssinica seed-oil has been a good source of phytochemicals and antioxidants, known for busting many life-threatening diseases including cancer and cardiovascular ailments [38].

3.1 Pea(ground)nut

A. hypogaea seed yield and quality are mainly limited by both abiotic stresses (extreme temperatures, drought, and extreme soil factors) and biotic stresses (pod borers, aphids, mites as insect pests and leaf spots, rusts and toxin producing Aspergillus fungus among the diseases) [39]. Drought stress is known to impact stabilization and productivity of Fabaceae crops including A. hypogaea [40]. Improved growth, yields, quality, and economics were observed in summer A. hypogaea, employing agronomic management, which comprised irrigation scheduling, mulching and integrated nutrient management [41]. The supply of auxiliary combined applications of cobalt and chelated zinc to drought-stressed A. hypogaea increased the capacity of this crop for nutrient absorption and water-use-efficiency and eventually improved its yield [42]. Identifying the most sensitive growth stage to water stress and understanding its consequences were argued essential for developing technologies aimed at enhancing the performance of crops (such as A. hypogaea) in water-limited environments [43]. Several genetic manipulations have been done in A. hypogaea for its improved tolerance to major abiotic stresses including drought [44, 45] and high salinity [46, 47]. Molecular genetic efforts have also been made to enhance to a great extent the resistance of A. hypogaea to the major biotic stresses such as pests/insects Spodoptera litura [48], and Holotrichia parallela larvae [49].

3.2 Cruciferous oilseeds

Abiotic stresses known impact the growth and productivity of Brassica oilseed crops including drought, temperature, and soil salinity [50, 51]. On the other hand, Brassica oilseed crop-health and productivity can also be impacted by biotic stresses such as fungal pathogens (Alternaria brassicae, which causes Alternaria leaf blight; Albugo candida, which causes white rust; Sclerotinia sclerotiorum, which causes Sclerotinia stem rot; Erysiphe cruciferarum, which causes powdery mildew; Leptosphaeria maculans and L. biglobosa, which cause blackleg disease; and Hyaloperonospora parasitica, which causes downy mildew), bacterial diseases (black rot, caused by Xanthomonas campestris, and leaf blight, caused by Pseudomonas syringae), and viral diseases (caused by Turnip yellows virus, cauliflower mosaic virus, and turnip mosaic virus) [50, 51]. Conventional breeding is difficult to develop varieties resistant to the mentioned fungal diseases due to unavailability of disease-resistant germplasm and also due to lack of sufficient innate resistance to these fungal diseases in most oilseed Brassicas such as B. juncea. Transgenic B. juncea cultivars produced by insertion of the WRR4 gene, exhibited their broad-spectrum resistance to white rust [52]. Earlier, using a B. juncea-B. napus line and a B. carinata-B. napus line, possible genomic regions of interest for blackleg resistance genes were analyzed in the B. juncea genome [53]. B. juncea, B. napus, and B. carinata exhibited more tolerance to Alternaria blight when compared to B. rapa [54]. Overexpression of NPR1 (non-expressor of pathogen-related gene 1) in B. juncea was conferred the broad spectrum resistance of this oilseed crop to major fungal pathogens such as Alternaria brassicae and Erysiphe cruciferarum [50]. Sclerotinia stem rot (caused by a plant pathogenic fungus, Sclerotinia sclerotiorum) causes extensive yield losses world-wide in Brassica oilseed crops. To this end, marker trait associations for resistance against Sclerotinia sclerotiorum were detected in B. juncea-Erucastrum cardaminoides introgression lines [55]. An effective qualitative and quantitative resistance against the globally predominant Xanthomonas campestris races (such as R1 and R4) was observed in B. juncea [56, 57]. Resistance to Turnip Mosaic Virus was shown to be enhanced by BjAOX1a (an oxidase gene), which was cloned from B. juncea [58].

Drought stress was reported to cause significant yield losses in B. juncea [59]. The supply of methyl jasmonate and Pseudomonas fluorescens to drought stress-exposed B. juncea improved its drought tolerance by strengthening antioxidative defense mechanisms, elevating secondary metabolite production, and promoting osmolyte accumulation [59]. A differential regulation of certain genes and transcription factors was observed in drought-exposed B. juncea [60, 61]. The study also enlightened the coding transcripts, which, in turn, could be associated with drought tolerance traits in B. juncea [60, 61]. Salinity stress can also impact the growth, development, and productivity in oilseed Brassicas including B. juncea [62, 63, 64]. Nitrogen availability was found to regulate proline and ethylene production, which eventually alleviated salinity stress in B. juncea [62]. A salt-tolerant Pseudomonas species was found to ameliorate salinity stress and stimulate the growth of B. juncea [64]. In a recent study, 24-epibrassinolide-mediated alleviation of salinity stress in B. juncea involved 24-epibrassinolide-mediated regulation of functional components of nitric oxide (NO) signaling and antioxidant defense pathways [63]. Ectopic overexpression of cytosolic ascorbate peroxidase gene (Apx1) improved salinity stress tolerance in B. juncea by strengthening antioxidative defense mechanisms [65]. In B. carinata-derived B. juncea introgression lines, quantitative trait loci (QTL)-conferring drought tolerance was performed, and over 15 candidate genes related to biotic and abiotic stresses were reported in the genomic regions of identified QTLs [66].

Elevated temperature regimes (such as heat stress) were also reported to limit the growth and productivity of oilseed Brassica crops [67]. Differentially expressed genes and key pathways were analyzed in heat-stressed B. napus using bioinformatics tools [67]. Genome-Wide Association Studies (GWAS) have enlightened genomic regions and QTLs controlling various traits known to be affected by heat stress in B. napus [68] and B. rapa [69]. In particular, QTLs for heat stress tolerance in B. rapa were found distributed across the genome [69]. Moreover, the occurrence of QTL was also found in diverse genetic groups, flowering phenologies, and morphotypes [69]. Several transcription factors known to play regulatory roles in heat stress responses were also identified in B. napus [70, 71]. Additionally, major gene families involved in controlling different hormonal signaling networks during heat stress were also explored in B. napus [72]. Utilization of a number of crop wild relatives (CWRs) of B. juncea was recommended for biotic and abiotic stress management in B. juncea (Figure 5) [51, 73]. It was argued that these CWRs may serve as resourceful gene pools for introgression of stress resilience traits.

3.3 Soybean

The list of important abiotic stresses reported to impact the productivity and chemical composition of Glycine max oilseeds includes drought, temperature, and salinity. Drought stress decreased linoleic and linolenic acids; whereas, an elevated accumulation of oleic acid argued an adaptive mechanism for the maintenance of cell turgor in drought-stressed G. max [74]. Yield of seed and its oil and also the seed-oil fatty acid composition can be impacted by elevated soil salinity [75]. The major effects of drought and salinity were ascertained in G. max employing transcriptomics studies [76]. Agrobacterium rhizogenes-mediated hairy-root transformation was employed for G. max gene functional research [77]. Eventually, CRISPR/Cas9 system was extensively used in G. max gene functional study [78, 79, 80]. It was also employed in improving the major G. max traits, including abiotic stress (herbicide) tolerance and disease resistance [78, 79, 80]. Actin depolymerization factor (GmADF13) was functionally characterized in G. max, where GmADF13 was found responsible for drought stress resistance [81]. Recently, the major genetics underlying red crown rot disease-resistance was identified and dissected in a diverse G. max germplasm population [82].

3.4 Flax(lin)seed

In Asian countries (particularly India), oil-type L. usitatissimum mainly suffers from drought, salinity, and heat in addition to biotic stress factors such as Fusarium oxysporum, Septoria linicola, and Melampsora lini. Interestingly, due to its hardiness, L. usitatissimum is known to tolerate drought when compared to many other oil and food crops. However, two major reasons, namely, the occurrence of a shallow root system and the transpiration of a high amount of water owing to high transpiration coefficient (value of 787-1093), make this plant more susceptible to drought stress [83]. Drought stress (water scarcity) continues to be a significant impediment to L. usitatissimum production. To this end, understanding the ‘root system architecture’ can be pivotal in order to improve water-acquisition in L. usitatissimum. However, knowledge is still meager in L. usitatissimum [84, 85]. Research is also meager on the identification of drought-resilient genotypes and genome-wide analysis of drought-induced gene expression in L. usitatissimum [86, 87]. Soil salinity-alkalinity was reported to impact L. usitatissimum germination and emergence, seedling survival, growth, and productivity [88]. Identification of salinity-tolerant lines has been done in few studies [89, 90]. Additionally, important genes responsible for conferring salt tolerance were also identified [91].

Notably, L. usitatissimum is a cool season crop. The production and viability of pollen and seed set, and the quality and quantity of seed-oil in L. usitatissimum can be impacted by heat stress (40°C) [92, 93]. Fusarium oxysporum f. sp. lini was reported to cause wilt disease in L. usitatissimum and can cause 80–100% yield loss [94]. Genetic improvement and adaptive breeding for high temperature stress tolerance in L. usitatissimum can be accelerated with a comprehensive characterization of heat shock factor (HSF) candidate genes [95]. Genomic insights into HSFs were enlightened a candidate gene (LusHSF) for high temperature stress-adaptation and gene editing with minimal off-target effects in L. usitatissimum [95]. Drought-, salinity-, cold- and heat-associated NAC-domain transcription factor genes (LuNACs) were discovered and functionally characterized in L. usitatissimum [93]. A nuclear magnetic resonance-metabolomics-based tool was developed to select L. usitatissimum varieties with better nutrient profile [96]. The de novo genome of L. usitatissimum-rust pathogen Melampsora lini was sequenced, and putative protein coding genes were identified [97]. Important insights into the genetic architecture and genomic prediction of powdery mildew-resistance were enlightened in L. usitatissimum [98]. Drought and salinity impacts were ascertained in L. usitatissimum employing transcriptomics studies [99]. In order to efficiently exploit CRISPR-Cas9 technology in L. usitatissimum, a guideline was made available for designing guide RNA, assembling DNA fragments, and also for preparing a detailed protocol for protoplast isolation, transfection, and mutation detection [100].

3.5 Sunflower

Temperature and water regimes and exposure to soil salinity during the phenological development of Helianthus annuus plants were reported to impact its seed yield and also the seed-oil quality [101, 102, 103]. For its optimal growth and development, H. annuus plants require 15–35°C mean daily temperatures [101]. Unfortunately, H. annuus shows susceptibility to heat stress (elevated temperature). H. annuus exposed to elevated threshold temperature during its reproductive stage exhibited negative impacts on fertilization, grain-filling, embryo growth, seed number, seed growth, seed weight, and seed-oil characteristics [102]. A constant seven-day exposure of H. annuus to high temperatures of ≥35°C, during grain-filling, strongly modified seed-oil fatty acid composition [103]. H. annuus is considered a moderately drought-resistant oilseed crop and is adapted to low water-input regimes in warm to semi-arid zones, owing to its well-developed tap roots [104]. Water deficit reduced seed yield (16.3%), oil yield (22.5%), and linolenic and palmitic acid (29.8 and 28.5%, respectively) [35]. Soil salinity can cause over 55% reduction in H. annuus production worldwide [105, 106]. Enhanced growth and production were observed in salinity-exposed H. annuus supplied with Pseudomonas aeruginosa PF23 [105] and fluorescent Pseudomonas sp. PF17 [106]. Transcriptomics studies ascertained the effects of drought and salinity stresses in H. annuus [107]. Recently, the characterization of the genetic control of abiotic stress-related specialized metabolites was performed in H. annuus [108]. Putatively annotated 30 compounds were found to be associated to seven most likely candidate genes with varied functions [108]. An integrated omics approach (comprising genomics, epigenomics, transcriptomics, proteomics, metabolomics and phenomics) was recommended for efficient genetic improvement in H. annuus breeding [109].

Two major diseases, namely, downy mildew (caused by Plasmopara halstedii) and H. annuus rust (caused by Puccinia helianthin), and a holo-parasitic flowering plant species, broomrape (Orobanche cumana) cause significant damages to H. annuus growth and productivity [110, 111, 112, 113]. In particular, downy mildew constitutes a significant risk factor during H. annuus production [111]. An integrated pest management approach against H. annuus downy mildew was recommended [111]. Moreover, among the downy mildew resistance inducers tested (azadirachtin, AZA; benzothiadiazole, BTH; Trichoderma asperellum) in in vivo experiments, BTH, T. asperellum, and the highest dose of AZA (0.2%) significantly reduced downy mildew symptoms in H. annuus [113]. H. annuus rust is another robust and economically important disease, which is known to cause significant damages and yield reductions in infected fields [110, 112]. Mutation and sequencing-based cloning and functional studies of a rust resistance gene were performed in H. annuus [112]. Most of the rust resistance genes (R genes), described so far, were found monogenic dominant. Orobanche cumana infects H. annuus roots in large areas of Europe and Asia, where it caused significant yield loss (up to 100%) [110, 114]. Seed pre-treatment with brassinosteroids was reported to stimulate H. annuus immunity against O. cumana infection [115].

3.6 Sesame

Due to changes in consumer lifestyle and health awareness programmes, the consumption of S. indicum is increasing globally, which is forecasted to be 100 million metric tonnes by 2030 [116]. However, a number of factors including: a narrow genetic base; the shattering of the seed-bearing capsule of S. indicum at harvest and eventual unsuitability of this crop for mechanized harvesting; cultivation in marginal lands with poor management practices; and lack of production strategies has caused perceptible declines in both the global cultivated area of sesame and its yield potential [24, 117]. This list also includes exposure of sesame crops to many biotic and abiotic stresses; inefficient processing technology; and also the shifting in cultivation to other cash crops. The cultivated S. indicum is sensitive to salinity [118], drought [119], and waterlogging [120]. An elevated accumulation of an osmolyte proline (a good stabilizer of proteins and a regulator of cytosolic pH) was linked to a higher tolerance to salinity (50 and 100 mM NaCl) in S. indicum cv. Cumhuriyet [118]. Omics technologies have also differentially improved S. indicum-tolerance to major biotic and abiotic stresses [121, 122].

The growth and productivity of S. indicum may also be limited by phyllody (replacement of parts of a flower or the entire flower with leafy structures), which is associated with phloem-limiting phytoplasma, intracellular pathogens in Mollicutes class [24, 123]. S. indicum phyllody primarily spreads via leafhopper species, with Orosius orientalis as the major vector. Unfortunately, complete phytoplasma genome sequencing has not been undertaken yet [24]. Dry root rot (caused by a soil-borne Deuteromycetes fungus, Macrophomina phaseolina) is another widespread and destructive disease of S. indicum [24, 124]. Major disease-resistance mechanism against M. phaseolina involved 52 differentially expressed genes associated with different signaling pathways [125]. Earlier, Fusarium wilt-resistant genotype of S. indicum (Sanliurfa-63,189) was also identified [126]. Moreover, out of 28 S. indicum genotypes screened under field conditions, resistance of S2 and H4 cultivars of S. indicum to Fusarium wilt was reported [127].

Predicted through RNA-seq data analysis, a well-annotated data resource for transcriptomic signatures of abiotic and biotic stress responses, ‘Sesame Genomic Web Resource (SesameGWR)’ was recently developed in S. indicum [117]. SesameGWR is expected to boost the mining of various stress-associated genes and the molecular breeding of S. indicum. It will also help in developing climate-resilient S. indicum varieties and eventually in enhancing its growth and productivity (Figure 6) [117].

3.7 Safflower

Naturally gifted with a strong and deep tap root (which can grow to a depth of 2-3 m), C. tinctorius thrives under dry/xerophytic climates and marginal lands through extracting water from deeper soil layers (up to 5 feet). Mainly doing this, C. tinctorius satisfies its water requirements. Hence, C. tinctorius is considered a moderately tolerant oilseed crop to drought stress [128]. Despite this, varied abiotic stresses (including temperature and the soil moisture) were reported to severely impact C. tinctorius growth, development and the yield of oilseeds and seed-oil, and seed-oil quality. Exposure of C. tinctorius to drought stress during its flowering and seed filling stages was detrimental on its seed yield [129, 130]. Similarly, drought stress at the heading stage decreased seed and oil yields in C. tinctorius [131]. Moreover, drought stress was reported to decrease in C. tinctorius the yield of seed (9.4%) and oil (10.2%) and also linolenic acid (29.8%) and palmitic acids (28.5%) [35]. A higher proportion of oleic acid as compared to linoleic acid was observed in C. tinctorius exposed to a terminal heat stress [132]. The sowing dates, genotype characteristics, and the climate variability were also reported to significantly impact C. tinctorius yield, oil, and fatty acids [133, 134]. Several innovative approaches for genetic improvement of C. tinctorius stress resistance were also explored [135, 136].

Important biotic stresses including leaf spot disease (caused by Alternaria carthami, Cercospora carthami, and Ramularia carthami), wilts (caused by Fusarium oxysporum and Verticillium alboatrum), powdery mildew (caused by Leveillula taurica), Phytophthora root rot (caused by several fungus-like organisms in the genus Phytophthora, including P. cactorum and P. cryptogea), and insects such as aphids and C. tinctorius fly are known to hamper C. tinctorius growth and productivity [137, 138]. Several insect pests also impact C. tinctorius health and productivity [139, 140]. To this end, Acanthiophilus helianthi, Helicoverpa armigera, and Uroleucon carthami were mainly found to diminish the productivity of C. tinctorius up to 35–80% [139]. Genetic mapping revealed two major QTLs (QUc-Ct3.1 and QUc-Ct5.1) associated with tolerance to the aphid, Uroleucon compositae, in C. tinctorius [140].

3.8 Niger

Major factors contributing to decreased growth and productivity in G. abyssinica include abiotic stresses, pathogens/diseases and parasitic weeds, low response to inputs, seed shattering due to indeterminate growth habit, lodging, insect pests, and diseases [36, 141, 142, 143, 144]. Drought stress-caused differential growth and physiological alterations were observed in G. abyssinica cultivars [145]. Soil salinity is also a major constraint for the cultivation of G. abyssinica [141, 143]. Genomic identification of salt-induced microRNAs was done in G. abyssinica, which helped to get insights into the post-transcriptional regulatory aspect of the applied salt stress responses [143]. G. abyssinica is an outcrossing edible oilseed crop and has highly limited genomic resources. To this end, RNA-Seq-based transcriptome sequencing of G. abyssinica genotypes provided novel genomic resources for this crop and also revealed microsatellite frequency and distribution in its transcriptome [146]. The list of important diseases of G. abyssinica includes Alternaria blight (caused by Alternaria porii and A. alternata), leaf spot (caused by Cercospora guizoticola), seedling blight (caused by Alternaria tenuis), seed rot (caused by Rhizoctonia bataticola), rust (caused by Puccinia guizotiae), powdery mildew (caused by Sphaerotheca sp.), and root rot (caused by Macrophomina phaseolina) [142, 144]. Association of Golovinomyces ambrosiae and Podosphaera xanthii was investigated in causing powdery mildew disease of G. abyssinica in India [144].

4.1 Pea(ground)nut

A. hypogaea is a self-pollinated crop with cleistogamous flowers and exhibits limited genetic variation. To this end, gamma irradiation-mediated induced mutations was reported to develop A. hypogaea mutants (M1 generation) with improved morphological and quantitative characteristics and also with seeds having enhanced biochemical composition [147]. Moreover, a large number of mutant varieties (3402) (via gamma irradiation) was released through breeding in the world [148]. Induced biofortification of Fe and Zn metal ions in A. hypogaea oilseeds through a range of molecular-genetic and other approaches was also explored [149, 150]. Genetic, genomic, and breeding efforts have improved the A. hypogaea health and productivity and also the nutritional quality of its oilseeds [7]. Efforts have been made to address the major key challenges of A. hypogaea grain nutrient quality, minimizing carcinogenic and immunosuppressive aflatoxin contamination [151] and allergens [152], and improving oleic acid profile in its oil [153]. Quality traits (e.g., seed-oil and protein) were reported to have predominantly qualitative inheritance [154]. However, quantitative inheritances were also reported for seed-oil content and quality in A. hypogaea [155]. A. hypogaea has also been the focus of breeding programmes focused on developing its short duration and high-yielding varieties/hybrids [156, 157]. Fatty acid desaturase 2 (FAD2) enzymes are involved in the catalysis of the conversion of oleic acid to linoleic acid. At the same time, the oleic acid to linoleic acid conversion is argued as an important regulatory point linked both to improved abiotic stress responses, and the ratio of these two components decides the seed-oil quality [158]. Undertaking a CRISPR/Cas9-based site-specific genome modification approach, FAD2 cis-regulatory motifs were modified in A. hypogaea [158].

4.2 Cruciferous oilseeds

The development of cultivars with varied fatty acid composition to suit specific nutritional and industrial needs has been in focus of breeding programmes in cruciferous oilseeds. Notably, the pod-shattering trait has been reported to negatively affect the crop yield in oilseed Brassicas especially under dry conditions. Hence, identification of key genes and unraveling the pod-shattering mechanism thoroughly was argued which is very important for both understanding the trait better and also for cultivating higher resistance varieties [159]. Novel quantitative trait loci from an interspecific B. rapa derivative improved pod-shatter resistance in B. napus [160]. Moreover, major genes associated with senescence and polygalacturonase activity involved in pod shattering were unveiled in B. napus employing a comparative transcriptome and co-expression network analysis [160]. Expression divergence of FRUITFULL homeologs enhanced pod shatter resistance in B. napus [161]. Introgression of the B. rapa genome was exploited to expand the genetic variation of B. juncea [162]. Phenotypic variations for pod shattering, pod length and number of seeds per pod were investigated in large germplasm collections of B. juncea and its progenitor species, B. rapa and B. nigra [163].

Breeding programmes in cruciferous oilseeds are also focused on a very-long-chain monounsaturated fatty acid, erucic acid (C22:1, ω-9). An optimum level of erucic acid (low-erucic acid) (nationwide range: 2-5%) is important for health safety reasons. Low erucic acid B. napus seed-oil possess a large amount of unsaturated fatty acids (90%) and vitamin E. B. napus seed-oil has unique absorption rate by the human body (99%). Hence, the low erucic acid B. napus seed-oil is a nutritionally valuable edible oil and also occupies an important position in the food industry [164, 165, 166]. Low erucic acid B. napus seed-oil has also been granted Substances Generally Recognized as Safe status in the United States [164, 165, 166]. Notably, before the development of low erucic acid B. napus (with 0.36-3.0%), erucic acid content of B. napus seed-oil generally ranged from 45 to 50%. The loss-of-function mutations in the FAE1 (Fatty Acid Elongase 1) gene and the FAE2 (Fatty Acid Elongase 2) gene was revealed as the current selection for low erucic acid B. napus [166, 167]. A targeted mutation of the 845th base of FAE1 resulted in a dramatic decrease in erucic acid content, which led to the development of world’s first zero-erucic acid B. napus ‘Oro’ [168]. RNA interference (RNAi)-mediated inhibition of FAE1 gene expression helped in decreasing erucic acid content of B. napus from 40% to less than 3% [169]. Intron-spliced hairpin RNA (ihpRNA)-mediated inhibition of FAE1 gene expression decreased the erucic acid content of B. napus from 40% to 0.36% [170]. RNAi-mediated inhibition of BnaFAE1 and BnaFAD2 (Brassica napus fatty acidΔ12-desaturase 2) expression decreased erucic acid content of low erucic acid B. napus from 0.87% to undetectable levels [171].

Since the first application of CRISPR/Cas9, B. napus genome editing was initiated, a critical mass of its genes were mutated [172, 173]. Eventually a valuable germplasm resource was generated for B. napus fundamental biological research and novel variety cultivation [172, 173]. For instance, CRISPR/Cas9 technology-assisted creation of targeted mutations on BnaFAE1 (BnaA08.FAE1 and BnaC03.FAE1) helped in the reduction of erucic acid content to nearly zero in B. napus [171]. Moreover, elevated seed-oil and protein content and altered fatty acid composition were obtained in B. napus mutants by knocking out the BnTT8 gene mediated by CRISPR/Cas9 [172].

4.3 Soybean

The major breeding programmes for G. max aimed at improving the yield of its seed and seed-oil, protein and seed-oil composition and quality and also at its tolerance to the major biotic and abiotic stresses [174]. GWAS were performed for understanding insights into the modulators of seed-oil (and seed protein) G. max [175, 176]. A comprehensive GWAS identified QTLs for seed-oil contents and identified five genomic regions associated with seed-oil content [176]. A large portion of a selected region in the G. max genome was found occupied by overlapping QTL regions related to oil content [177]. G. max lines exhibiting stable oleic acid and linolenic acid across environments may be used as a source of germplasms with a desirable fatty acid composition [178]. G. max genetic transformation has become a valuable tool for the functional study of genes and the production of agronomically improved plants [77].

CRISPR/Cas9 system has enlightened G. max gene functions and has also improved important G. max traits, including yield, quality, and stress tolerance [78, 79, 80]. Based on the CRISPR/Cas9 system, stable male sterile lines were produced in G. max using the ABORTED MICROSPORES (AMS) homologs, where GmAMS1 produced male sterile lines, whereas GmAMS2 failed to produce [79]. Genetic regulatory networks of the G. max traits (seed size, oil and protein contents), and also the major strategies for improving the same, were also explored [179, 180]. GWAS and RNA-seq have identified GmWRI1-like transcription factor related to the seed weight in G. max [181]. Recently, GWAS was performed, and candidate gene mining of G. max seed size traits was done [182]. Major fatty acid desaturase genes were manipulated in G. max employing different molecular techniques such as induced mutagenesis [183], RNA interference [184], transcription activator-like effector nucleases genome editing [185], and CRISPR-cas9-mediated genome editing [186, 187]. G. max triacylglycerol lipase GmSDP1 gene was reported to regulate the quality and quantity of seed-oil [188]. Transcription regulation of oil accumulation in G. max seeds was also enlightened [189, 190]. G. max WRINKLED1 transcription factor, GmWRI1a was found to positively regulate seed-oil accumulation [190]. Moreover, oil accumulation in G. max seeds was altered by modifying biosynthetic genes [191, 192]. A high-monounsaturated fatty acid G. max seed-oil was also successfully produced using GmPDCTs knockout via a CRISPR-Cas9 genome editing system [193].

4.4 Flax(lin)seed

The breeding programmes for L. usitatissimum largely aimed at improving yield of seed and oil and the quality of seed-oil [194, 195, 196, 197]. Several QTLs for fatty acid composition and yield in L. usitatissimum were unveiled [195]. Three QTL each for oleic acid and stearic acid and two QTL each for linoleic acid and iodine value and one each for palmitic acid, linolenic acid, oil content were discovered [195]. These agronomic- and quality traits-associated QTLs may help in marker-assisted breeding as well as map-based cloning of key genes in L. usitatissimum. Earlier, GWAS have helped in the identification of single-nucleotide polymorphisms related to oil quality attributes [196, 197]. An association of seed color variation with yield and quality traits was observed in a diversity panel of L. usitatissimum [198]. Altered fatty acid content and composition (especially the increase of oleic acid content) were reported in L. usitatissimum employing the approaches based on RNA interference of FAD2 gene encoding the enzyme Fatty Acid Desaturase 2 [194]. Introduction of Primula vialii-sourced Δ6-desaturase gene in L. usitatissimum increased therein the synthesis of ω-3 fatty acids [199]. Introduction of the Solanum sogarandinum glycosyltransferase (SsGT1) gene into L. usitatissimum genome elevated the accumulation of unsaturated fatty acids in L. usitatissimum seeds [200]. Significantly increased polyunsaturated fatty acids levels were observed in L. usitatissimum plants transformed with a chalcone synthase gene from Petunia hybrida [201]. The genetic diversity of stearoyl-ACP desaturase and fatty acid desaturase genes responsible for the fatty acid composition in L. usitatissimum cultivars and lines were investigated [202]. Heterosis for seed yield and its attributing traits in L. usitatissimum was also explored [203]. An apical meristem-targeted in planta transformation method can be promising for the successful development of transgenics in L. usitatissimum [204]. Genetic enhancement of major nutraceuticals was also attempted in L. usitatissimum [205].

4.5 Sunflower

The development of breeding H. annuus lines suitable for hybrid breeding, resistance to stresses (diseases, abiotic, biotic, and herbicides), and seed and oil yield and quality traits are mainly focused in H. annuus breeding [110, 206]. The fatty acid residue composition of the main oil lipids, triacylglycerides, and some of the minor lipids are the major determinants of the nutritional properties (and industrial use) of H. annuus seed-oil [207, 208]. QTLs controlling seed-oil content, oleic acid and linoleic acid content in H. annuus were mapped [209]. An F2 mapping population from cytoplasmic male-sterile line COSF 7A (33-35% oleic acid) was developed. Additionally, high oleic acid inbred line HO 5-13 (88-90% oleic acid) was also phenotyped for oil content and oleic acid and linoleic acid content at the F2 seed level [209]. The obtained QTLs may be useful in the marker-assisted selection breeding programme aimed at improving oil quality. Heterotic hybrids are required for the major improvements in H. annuus. These hybrids can be achieved by tapping combining ability of F1 hybrids developed from crossing of two genetically diverse female lines with male testers [210]. Mutation breeding has also been argued rewarding for changed oil quality in H. annuus oilseeds [211]. Recommendation has also been made to employ ‘pollen irradiation-mediated induction of haploid plants’ for speeding-up breeding in H. annuus [212]. Combined high-resolution lipidome phenotyping and genome-wide genotyping has unveiled novel genetic determinants of seed-oil fatty acid content in inbred H. annuus lines [208]. Nutragenomic approaches for understanding metabolic pathways of various quality traits in H. annuus was also explored [213].

4.6 Sesame

Over 27,000 genes and high genetic diversity in genes related to oil biosynthesis, resulting in variation in seed-oil content, were revealed through de novo-based assembly of the S. indicum genome [214]. Key genes associated with S. indicum seed-oil composition were identified in 705 accessions, where variation in seed-oil content was found associated with seed color [215]. GWAS have identified novel genes, SiLTP3 and SiACS8, which were found related to seed yield traits, such as capsule length and number [214, 216]. The Ethiopian Institute of Agricultural Research (Addis Ababa, Ethiopia) developed and released a total of 32 improved S. indicum varieties through mass selection from among the local germplasm collections since 1976 [217]. Earlier, eight gamma ray (300-750 Gy)-induced closed capsule (indehiscent) mutants were obtained from four different Turkish cultivars of S. indicum [218]. Moreover, hybridization has helped in a successful recovery of high-yielding S. indicum plants from a progeny of 103 crosses [219]. Important variants in S. indicum (with improved seed-oil yield, composition and quality) were generated employing genome editing technology [220]. Several favorable genes, QTLs, and genotypes were also discovered, which would act as a valuable genetic resource for S. indicum improvement [221]. CRISPR/Cas9 system and CRISPR/Cas9 coupled with hairy root transformation-mediated efficient targeted mutagenesis was efficiently created, and the major gene functions were assessed in S. indicum [222]. With the highlighted research progress, S. indicum has been transformed from an ‘orphan crop’ to a ‘genomic resource-rich crop’.

4.7 Safflower

Unfortunately, the occurrence of low genetic diversity in C. tinctorius-breeding lines and cultivars restricts their utility in breeding programmes [223]. To overcome this issue, the prevalent genetic and phenotypic diversity among the global germplasm of C. tinctorius was extensively characterized. This has facilitated the identification of genetic determinants of trait variability and effective utilization of the prevalent diversity [224]. The genetic determinants of trait were also used in development of effective improvement programmes for C. tinctorius [224]. A scarcity of identified marker-trait associations is limiting the development of successful marker-assisted breeding programmes in C. tinctorius. To this end, a C. tinctorius panel (CartAP) comprising 124 accessions derived from two core collections was assayed for its suitability for association mapping [225]. The marker-trait associations were also identified, which are expected to facilitate marker-assisted breeding and identification of genetic determinants of trait variability in C. tinctorius [225]. Moreover, genetic variation among 131 C. tinctorius germplasm was argued to help in parental selection and the future breeding programmes for C. tinctorius [33]. Enhancements of γ-linolenic acid, α-linolenic acid, oleic acid, bioactive peptide, bioactive flavonoid, and stress resistance were also achieved [135, 136]. The miRNA expression pattern studied and novel miRNAs identified in developing seeds enlightened the role of miRNA in seed development and oil metabolism [226]. Genetic modification of C. tinctorius was also done for improved production of oils with distinct properties [227, 228]. The genetic origin and diversity of FAD2-1 were identified and characterized in C. tinctorius [229]. This result could aid C. tinctorius breeders in reducing population size and generations required for the development of new high oleic acid varieties using perfect molecular marker-assisted selection [229].

4.8 Niger

G. abyssinica is known for its multiuse and numerous human health benefits. However, the average productivity of G. abyssinica in most world regions (including Ethiopia and Indian subcontinent) is very low. This may be due to the natural indeterminate growth habit, shattering lodging, and self-incompatibility (strict out-crossing nature) of G. abyssinica and/or due to its sensitivity to weeds, pests, diseases, and insects [230, 231]. G. abyssinica has been the focus of the major oilseed breeding programmes [232, 233, 234]. Mutation breeding was adopted for isolating mutants with desirable traits including seed yield and oil content [234, 235]. Individual and combined levels of gamma rays (24, 26, and 28 kR) and ethyl methane sulphonate (0.2 and 0.4% v/v) tested in G. abyssinica seeds (cultivar N-71) varyingly increased grain yield and yield-related components [234]. G. abyssinica has a wide genetic basis, which may be used for the improvement of the species through conventional breeding and/or marker-assisted selection [230, 236]. Induced polyploidy has improved the major agronomic traits in economically important crop plants [237]. To this end, the tetraploidization in G. abyssinica plants (with larger capitula, larger seeds, and a greater number of seeds per capitula) was achieved by treating apical portion of in vitro-raised shoots with colchicine (0.02%) [233]. A highly efficient protocol for in vitro double haploid production was achieved employing anther culture, and Indian G. abyssinica varieties JNS 9 and JNS 28 were developed [238]. Moreover, double haploid was argued to develop inbred lines in a shorter time in problematic plants including G. abyssinica. Earlier, repeated selection and breeding increased oleic acid content from approximately 5-11% to 80-86% in G. abyssinica materials from Ethiopia [232]. Despite the highlighted above studies, G. abyssinica remained as an underutilized oilseed crop.