Development of molecular resources for the genetic improvement of noug (Guizotia abyssinica (L.f) Cass): a mini review

Abstract

Noug (Guizotia abyssinica (L.f) Cass) is an important edible oil-producing crop. Ethiopia is the center of origin and diversity for noug and thousands of noug accessions are being maintained at the Gene Bank in Ethiopian Biodiversity Institute (EBI). The crop is grown mainly for edible oil and the by-product named as noug-cake is widely used as animal feed. However, the production and productivity of noug is too low compared to other oilseed crops mainly due to the self-incompatible nature of the crop species, shattering, lodging, indeterminate growth habit, pests, and diseases. The development and application of molecular resources and tools have played a great role in the improvement of oilseed crops worldwide. Although conventional breeding has been used to develop commercial noug varieties, the application of modern genomic tools to enhance the use of noug germplasm resources is very limited. In this work, we have reviewed the scientific literature available on the development and application of molecular resources on oil-producing crops and specifically reveal research gaps on noug genetic improvement and highlight broadly applicable and affordable short-term strategic interventions.

Introduction

Oilseed crops are predominantly grown worldwide for the production of edible oil being used as an important source of fatty acids and nutrients (Yadav et al. 2012). Additionally, oilseed crops are used for biofuel production, livestock feed, pharmaceuticals, soap production, hair oils, textiles, and paints (Rahman and Jiménez 2016; York and Garden 2016). The total worldwide oilseed production for the year 2019/20 was 574.62 million metric tons from 279.2 million hectares, only from major oilseed crops (USDA 2019). In Ethiopia, about 7,850,196.94 quintals from 747, 803.78 hectares were obtained for the year 2018/19 (CSA 2019). The leading oil crop-producing countries are Brazil, USA, Argentina, China, and India (USDA 2019). Some of the oilseed crops produced worldwide are; soybean (Glycine max L.), sunflower (Helianthus annus L.), cottonseed (Gossypium hirsutum L), coconut (Cocos nucifera), peanut (Arachis hypogaea L.), groundnut (Archis hypogaea) safflower (Carthamus tinctorius L), rapeseed (Brassica napus L.), palm (Elaeis guineensis Jacq.), castor bean (Ricinus communis L.), mustard (Brassica carinatea) (Farooq et al. 2016) and noug/Niger seed (Guizotia abyssinica (L.f) Cass). From these; noug is considered as one of the minor and underutilized oilseed crops due mainly to limited research short of what the crop deserves for its improvements.

Noug (Guizotia abyssinica) is an edible oilseed crop indigenous to Ethiopia (Baagoe 1974) which has been cultivated for approximately 5000 years (Ramadan 2012). Guizotia abyssinica has a diploid genome constituting (2n = 2x = 30) chromosome number and belongs to the family composite/Asteraceae/ (Dagne 1995). Out of the six species in the genus Guizotia; Guizotia abyssinica is the only cultivated one. Noug is a dicotyledonous herb with moderate to well branch and grows up to 2 m in height (Fig. 1A and B). The stems are soft, hairy, hollow with a diameter up to 2 cm, and branched. The leaves are opposite or alternate at the stem apices, with lanceolate to obviate leaf blades, 3–23 cm × 1–6 cm, variable in shape, with an entire or toothed margin, ciliate, and softly hairy on both surfaces. The root system is well developed, with a taproot with numerous lateral roots, especially in the top 5 cm. As well as the seeds are small achenes (fruits), 3–6 mm long × 1.5–4 mm wide and glossy black in color (Bulcha 2007). The hermaphroditic disk florets, usually 40–60 per capitulum are arranged in three whorls. Those at the edge opening first, followed progressively by the next in line to the center of the head (Alemaw and Wold 1995; Getinet and Sharma 1996). These are some of the physiological distinguishing characteristics from the closest species.

Fig. 1

Field cultivation status (A), Flowering stage (B), Harvested seed (C), and Packaged pure edible oil (5L) (D) of Noug/Niger seed. Source: (https://www.landwirtschaftskammer.de/fotos/zoom/r/ramtillkraut.jpg; https://www.springhausagro.com/niger?pgid=k76pue4k-c1d1968d-2640-410b-abf5-07bb829e7b9f; https://twobrothersindiashop.com/blogs/farmers-kitaab/niger-seeds-khurashni-ram-til-long-forgotten-oil-seed-with-applications-in-tribal-medicine; https://cnetcommerce.com/dukem-pure-niger-seed-oil-5l)

Noug grows best on poorly drained, heavy clay soil, and is not dependent on highly valued agricultural inputs (Alemaw and Wold 1995; Geleta and Ortiz 2013; Tadele 2018). It’s optimally grown in Ethiopia in the temperature range of 15–23 °C and 1600–2200 m asl with an annual rainfall of around 500–1000 mm (Getinet and Sharma 1996) but also there are indications that it can be grown as low as 1200 m asl and above 2700 m asl (Geleta and Ortiz 2013). Major growing areas in Ethiopia are Gonder, Shewa, Wollo, Gojjam, Wollega, Jimma, Tigray, and Illubabur (Getinet and Sharma 1996). It’s extensively produced in Ethiopia and India and also on a small scale in several other countries. India ranks first in the area, production, and export of noug in the world followed by Ethiopia (Rai et al. 2016). In Ethiopia; it is the second oilseed crop next to sesame (Sesamum indicum L.) in terms of production; from 257,950.4 hectares obtained about 2,963,227.47 quintals which contributes about 2.03% of the national grain total production (CSA 2019). It is cultivated, consumed, and sold by several smallholders with high-value socio-economic importance (Geleta et al. 2002). The Ethiopian Biodiversity Institute (EBI) conserved germplasm collections from different agro-ecological regions in the country. The country is the major source of noug germplasms.

Noug is mainly used as human food. It contributes up to 50% of the Ethiopian oilseed crop (MoA 2016) and 3% of Indian national production (Getinet and Sharma 1996). However; both Ethiopia and India are heavily reliant on oil imports for their domestic use. The noug oil is used to protect against cardiovascular disorders and to treat burns (Adarsh et al. 2014), and also for other cultural and medicinal values in Ethiopia and as the main birds feed in the USA (Geleta et al. 2002). The seed is warmed in a kettle over an open fire, crushed with a pestle in a mortar, and then mixed with crushed pulse seeds to prepare a stew in Ethiopia and ‘Chibto’ and ‘litlit’ are also the preferred food for young boys (Getinet and Sharma, 1996). The pressed cake left after oil extraction is used for livestock feed as a good source of protein, carbohydrate, and fiber (Kandel and Porter 2002).

The noug seed contains 17–30% protein, 34–39% carbohydrate, and 9–13% fiber (Ramadan 2009).The oil content of the Ethiopian noug seed ranges between 27- 56% (Geleta et al. 2011) and its predominant fatty acid composition vary with linoleic acid (C18:2) 54–85%, oleic acid (C18:1) 3.3–31.1%, palmitic acid (C16:0) 7.8–10%, and stearic acid (C18:0) 5–8% (Dagne and Jonsson 1997). The noug seed also contains essential minerals and nutrients for food (Tsehay et al. 2021).

Despite its multiuse, the national average productivity is very low, which is only 1.1ton/ha (CSA 2019; Fig. 2). This might be due to individual or combined factor/s contribution of; the plants' indeterminate growth habit, shattering lodging, and self-incompatibility (strict out-crossing nature) (Geleta et al. 2007), weeds, pests, diseases, and insects (Getinet and Sharma 1996). Until very recently noug has attracted little research attention and mainly remained as an underutilized oilseed crop (Dempewolf et al. 2015).

Fig. 2

Average yield increment of noug for the last 10 years in Ethiopia (qt/ha). Source: CSA (2019)

Breeding noug (research) in Ethiopia started in 1961 at Debrezeit experimental station, Debreziet, and continued at Holeta Research Station, Holeta (Alemaw and Alemayehu, 1992). The overall breeding objective was to develop high-yielding and disease-resistance varieties that are adapted to vertisols, with synchronized flowering and maturity, semi-dwarf stature, and thin hull (Alemaw and Alemayehu, 1992). So far, only five improved varieties are under production (Table 1) which are released in different years (MoA 2016), much less than twenty-six (26) sesame improved varieties.

Table 1 Lists of improved varieties year of release, yield and oil contents

These varieties were developed using conventional breeding approaches mostly selection. With the availability of molecular resources, however, the efforts of noug breeders can be complemented to enhance the efficiency of varietal development and increase genetic gain.

Ethiopia is the center of origin for noug and is expected to have highly diversified genetic resources that are essential to improve specific traits. However, the development, utilization, and conservation of genetic resources depend on the understanding of their genetic information. This paper reviews molecular resources adopted and developed for noug so far and highlights research gaps for future improvement and utilization of noug, special emphasis on tools for genetic improvement and application of different genetic marker systems for diversity study of noug is given.

Tools for the genetic improvement of oilseed crops

In oilseed crops, limited research focus has been vested as compared to cereal crops in the context of developing new varieties. Studies on the new cultivars' development in the oilseed crops primarily focus to increase yield, oil content or quality, and biotic and abiotic stress resistance. In the ensuing sections tools employed for the genetic improvement of major oilseed crops are given with special emphasis on noug.

Molecular markers for diversity analysis and marker-assisted breeding in oilseed crops

Reliable molecular markers have significantly speeded up modern plant breeding by enhancing genetic gains and reducing the breeding cycles. Different types of molecular marker systems are widely applied to oilseed crops for genotyping and breeding efforts. For instance, the genetic diversity of some major oilseed crops was studied and assisted the breeding efforts using RAPD in Caster bean (Gajera et al. 2010), AFLP in Safflower (Zhang et al. 2006), RFLP in Sunflower (Gentzbittel et al. 1994), ISSR in groundnut (Mondal et al. 2009), SSR, and/or EST-SSR in Soybean (Liu et al. 2010) and SNPs in Cotton (Song et al. 2018). The progress in the exploitation of genetic diversity and utilization of the genetic gains on major oilseed crops shifted the interest also to minor-oilseed crops.

The molecular understanding of noug germplasms is imperative; this could be significant in the varietal identification and implementation of improved breeding and conservation programs. Attempts at genetic characterization of noug using a few molecular markers revealed considerable variation. For instance, using RAPD (Dagne and Jonsson 1997), AFLP (Geleta et al. 2008), ISSR (Hussain et al. 2015; Petros et al. 2007), SSR (Dempewolf et al. 2015; Abebaw and Solomon 2017; Aboye et al. 2020) markers (Table 2). However, they were limited in the high resolution genetic analysis, and they should be complemented with other high-throughput markers.

Table 2 Utilization of molecular markers for diversity and phylogenetic analysis of noug (Guizotia abyssinica)

Markers play a great role in the identification and selection of QTLs that control economically important traits. Marker-assisted breeding (MAB) can be used to increase the efficiency of selecting parents and the effectiveness of backcross (Semagn et al. 2006). This technique has been successfully applied in different oilseed crops such as disease resistance hybrid selection in Sunflower (Sahin et al. 2018), marker-assisted selection for oleic acid controlling genes in groundnut, marker-assisted introgression of fatty acid desaturase mutant allele in peanut (Janila et al. 2016), and marker-assisted backcross selection for seed oleic acid content in peanut (Huang et al. 2019).

In noug, breeding was based on phenotype screening. Subsequently, few molecular markers were developed and applied for genetic diversity study in noug (Table 2), but there is a need for linkage and QTL mapping of traits of interest and marker-assisted selection and/or breeding. To use MAS/MAB, markers closely linked to genes/QTLs controlling target agronomic traits should be identified (Janila et al. 2016). A wide range of traits might be targeted by the breeders for genetic improvement of noug i.e. abiotic stress tolerance, disease resistance, and yield and quality types which might be controlled by one or more genes. In the future, marker-assisted selection and introgression of desirable traits using tightly linked molecular markers may be a breakthrough to boost the production and productivity of noug.

Overall, further understanding of the existing variation using more molecular marker tools will help the available genetic resources for further improvement and conservation.

Linkage and QTL mapping

Linkage and quantitative trait loci (QTL) mapping help to identify the relative location of various genetic markers present in the chromosome of crops (Singh et al. 2015) and to identify regions of the genome contributing to variation in desirable traits. High-density genetic maps are particularly important for genome assembly and precise mapping of interest agronomic traits (genes) for marker-assisted selection (Song et al. 2018). This approach has been applied in oilseed crops like identification of QTL markers contributing to plant growth, oil yield, and fatty acid composition in Jatropha (King et al. 2015), SSR based genetic mapping and identification of QTLs in Sesame (Wang et al. 2017), high-density genetic map construction, and identification of QTLs controlling oleic acid and linoleic acid in peanut using SSR and specific length amplified fragment sequencing (SLAF-seq markers (Hu et al. 2018) and mapping of quantitative trait loci for yield-related traits in peanut using high-resolution SNPs markers (Liang et al. 2018).

Noug is a complete out-crossing crop that makes it difficult to develop inbred lines that are the basis for linkage and QTL map construction. Attempts on the development of self-compatible lines were made by Geleta and Bryngelsson (2010) through reciprocal cross-pollination. As a result, completely self-compatible genotypes have been developed for the first time with various associated advantages. Interestingly, two of the compatible lines (C19-1 and K13-1) were utilized for transcriptome sequencing (Tsehay et al. 2020). Additionally, self-compatible genotypes were developed through crossbreeding and selfing and were used for RNA-seq based sequencing (Gebeyehu et al. 2022). Once self-compatible genotypes are identified, it is simple to generate pure-lines maintaining traits for generations. However, there was no further report on pure-line development attempts for linkage and QTL mapping or other genomic studies in noug. This implies the need for generation of pure-lines, QTL mapping, and the identification of important agronomic traits. One possible explanation for the crop's limited studies on QTL mapping and molecular mechanisms of noug could be lack of research vigilance.

Genome-wide association studies and genomic selection

Genome-wide association studies (GWAS) and genomic selections (GS) are powerful tools to understand the regulatory loci and the genetic architecture of complex traits of interest in plants at the whole genome level (Kushwaha et al. 2017; Werner et al. 2018). The ultimate goal of GWAS also called association mapping or linkage disequilibrium (LD) mapping is to dissect the association of markers/QTLs and complex quantitative traits involved in phenotypic variation. Whereas GS refers, to the genome-wide based selection of traits-associated markers by capturing the genetic variance (de Koning 2016). With the development of high-resolution SSR and SNPs markers, a large number of GWAS and GS studies have been reported in oilseed crops such as in Safflower (Ambreen et al. 2018), soybean (Zhang et al. 2016), Sesame (Li et al. 2014) and Cotton (Gapare et al. 2017).

A GWAS and GS study in noug is scarce and, both GWAS and GS will help to clearly understand the genetic basis of complex traits that are productive for noug.

In general, noug offers very plausible genomic tools and techniques to increase its production and productivity, which is now very low as compared to other oilseed crops. The pioneering tools were developed and applied in noug with the primary aim of determining the taxonomic classification, genetic diversity, and its relationship with wild relatives (Tables 2, 3).

Table 3 Molecular tools and resources developed in noug (Guizotia abyssinica)

Induced mutagenesis and genetic transformation of oilseed crops

Induced mutation, in which mutation occurs as a result of chemical (ethyl methane sulfate (EMS), physical (X-rays, gamma rays, UV and ion beam), or biological (site-directed mutation or genome editing) mutagenic agents to create variability (Oladosu et al. 2016). Mutation breeding in crops relies on altering traits of interest using diverse mutagens (Tadele 2016). This mutation breeding program has been the base for the release of different new crop varieties globally. For example in oilseed crops, non-shattering Sesame capsules using gamma-ray and EMS (Wongyai et al. 2001), increased oil quality of soybean using site-directed mutagenesis (Haun et al. 2014) and EMS-based mutation of desaturase gene (FADH2-2) which resulted in high oleic acid content in rapeseed (Won et al. 2018). Mutation breeding along with transgenic breeding becomes a pillar for modern breeding programs (Oladosu et al. 2016). Genetic transformation has been described as the fundamental tool for the genetic improvement of crops including major oilseed crops. For instance, herbicide-resistance (roundup ready) in Soybean (Padgette et al. 1995), disease resistance transgenic castor (Sood and Chauhan 2018), and oil quality in sunflower (Cveji, 2020).

After a multidisciplinary research approach has been started on noug in 1979, induced mutation research was initiated using gamma rays targeting for dwarfing genes (Alemaw and Alemayehu 1992) and showed promising results at F1 generation. However, it fails to transfer to the next generations. Since then, no more progress was made to implement induced-mutation breeding on noug. In another hand, the first report on the genetic transformation of noug was only protocol development for Agrobacterium-mediated genetic transformation in India (Yadava et al. 2012). However, it was not clearly defined and reproducible. Establishing a reproducible transformation protocol is a priority task to implement genetic transformation on noug.

Conclusion and future prospectives

Molecular resources have played a vital role in the genetic improvement of oilseed crops. Noug, which is a multi-purposed oilseed crop should benefit from these. Interestingly, despite being domesticated since the BCs and having a long history of cultivation, it is still referred to as a semi-domesticated crop for a variety of reasons (Dempewolf et al. 2015). This is critical for acquiring important genes from wild types and adapting to a new area in resisting harsh environments. The crop has genetic resources due to phenotype plasticity and molecular polymorphism, particularly in primary origin areas, which will be used for genetic improvements.

As demand for its oil consumption and industrial applications increases over time, its yield, oil content, and quality should be improved. However, the crop is facing different production and breeding constraints. The advancement of molecular biology tools, as well as the presence of high genetic variation, reinforces the opportunity to improve the crop with the available resources. Remarks for general genetic improvement of noug to make it more competitive includes (i) further molecular characterization of noug landraces with more efficient molecular markers for breeding and conservation; (ii) utilization of techniques and means to consolidate noug improvement intents such as induced mutagenesis breeding for specific traits and QTL mapping of desirable traits (iii) Developing and applying highly advanced molecular tools and resources for further crop exploration, like as genome sequencing, genotyping by sequencing (GBS), transcriptome analysis for important gene function prediction, gene editing or manipulation, and genetic transformation, especially on proteins essential for fatty acid synthesis.

The utmost utilization of its genetic resources and giving research attention will have a great role in fulfilling the demand for edible oil worldwide and contribute to ensuring food security.