Introduction

The sesame (Sesamum indicum L.; 2n = 26) belongs to the family Pedaliaceae. According to Kobayashi et al. (1990), genus sesame comprises 36 species found mainly in Africa and Asia. Depending on the chromosome number, the thirty-six sesame species fall in three groups including 2n = 26, 32, and 64; however, the cytology of 12 species still needs to be studied (Nimmakayala et al. 2011). The archaeological findings revealed that cultivated sesame was derived from the wild species S. malabaricum. Sesame cultivation was established in South Asia at the time of the Harappan civilization and later spread west to Mesopotamia before 2000 B.C. (Fuller 2003). Sesame is cultivated worldwide, and the top ten sesame countries in terms of production are Sudan, India, China, Myanmar, Sudan (former), Nigeria, the United Republic of Tanzania, South Sudan, Ethiopia, and Uganda (FAO 2020). Globally, sesame is cultivated in 13.96 million hectares (mha) with a total production of 6.8 million tons. The sesame seed production index in Asia, Africa, and America is 53.1%, 42.5%, and 4.3%, respectively. Sudan ranks first with a production of 0.79 million tons, whereas India ranks second with 0.67 million tons (FAO 2021). India is the largest exporter of sesame seeds (Kumaraswamy et al. 2015), and the maximum sesame production was found in West Bengal followed by Gujarat state.

Sesame is an annual herb, completing its life cycle in 90 to 150 d, and reaches 60 to 120 cm in height with moderate branching or unbranched, ovate to lanceolate leaves with or without hairs and campanulate flowers varying from purple to white in color. The capsule size varies and possesses > 100 seeds, with seed color varying from black to white (Andrade et al. 2014). Sesame seeds contain 50 to 60% oil, 20% protein, and 13 to 14% carbohydrate (Morris 2002). Sesame, referred to as the “Queen of Oilseeds” (Lakhanpaul et al. 2012), is rich in oil and essential minerals and, thus, is largely (70%) employed for oil production and forms an important ingredient in cuisines, cosmetics, and medicines. The presence of antioxidants, lignins (sesamin and sesamolin), and tocopherols in sesame impart health-promoting attributes. The availability of antioxidants increases its demand as a food, enhances the shelf life, and reduces rancidity of the sesame oil (Dossa et al. 2017). The oil contains 35% monounsaturated fatty acids and 44% polyunsaturated fatty acids (Islam et al. 2016); it also contains other fatty acids, including oleic and linoleic acids, which constitute almost 40 to 45% each (Zhang et al. 2013), and palmitic and stearic acids (Kamal-Eldin et al. 1992). The roasting of sesame converts the sesamolin to sesamol, enhancing its antioxidant anticancer activities (Islam et al. 2016). The traditional methods of medicines in ayurveda, Chinese, and Tibetan forms consider sesame to be a vital source of anti-inflammatory, anti-proliferative, anti-hypertensive, anti-neurodegenerative, and anticancer constituents. Sesame has also attained interest in a wide commercial sector, as its leaves and roots are used in the production of hair dyes, in emollient plaster in Sri Lanka (Wesis 1971), and as a cattle feed from the oil cake that is produced.

Sesame production suffers heavy yield losses due to biotic factors (Tiwari et al. 2011), abiotic factors (Rao and Ravishankar 2002), indeterminate growth, uneven capsule ripening, and seed shattering. The pathogenic diseases like root or stem rot and phytophthora blight (Gangopadhyay et al. 1998) and charcoal rot by Macrophomina phaseolina fungus (Silme and Cagirgan 2010; Enikuomehin et al. 2011; Chowdhury et al. 2014) are serious threats to sesame production. It has been reported that by 2030 the sesame consumption would reach 100 million metric tons (Troncoso-Ponce et al. 2011). Therefore, it is important to develop sesame varieties for increasing seed yield, oil or fatty acid quality and quantity, functional bioactive compounds, and biotic and abiotic stress tolerance (Rao and Ravishankar 2002). Breeding efforts in sesame have focused on developing improved cultivars with traits, such as higher yield, disease resistance, drought tolerance, improved oil quality, and desirable agronomic characteristics (Teklu et al. 2022). The important aspect of breeding is genetic diversity; however, sesame is known for its narrow genetic base (Bhat et al. 1999), which poses challenges for breeders. To overcome this limitation, various approaches have been used to introduce genetic diversity into breeding populations, including the use of wild relatives, landraces, and diverse germplasm collections (Yermanos et al. 1972). These diverse genetic resources provide a valuable pool of traits that can be incorporated into cultivated sesame.

Traditional breeding methods, such as mass selection and pedigree breeding, have been employed to improve sesame cultivars (Tripathy et al. 2019). More recently, molecular breeding techniques have been applied to sesame breeding, allowing for more precise selection and faster progress. Molecular markers, such as simple sequence repeats (SSRs, Dixit et al. 2005; Spandana et al. 2012; Badri et al. 2014; Yan-Xin et al. 2014; Uncu et al. 2015; Wang et al. 2017) and single nucleotide polymorphisms (SNPs, Wei et al. 2014; Du et al. 2019), have been used for genetic analysis and marker-assisted selection (MAS; Wei et al. 2009; Ali et al. 2007) in sesame breeding programs. These techniques enable breeders to select plants with desired traits at the molecular level, facilitating more efficient and targeted breeding efforts.

The advancements in genomic technologies, including next-generation sequencing and genotyping-by-sequencing, have provided valuable genomic resources for sesame breeding. With the availability of assembled whole genome sequence of the S. indicum and detailed information of the oil biosynthetic pathways and stress-responsive genes (Wang et al. 2014, 2015, 2021; Dossa et al. 2017; Wei et al. 2017), it is possible to understand the different regulatory mechanisms and employ different genetic engineering techniques to enhance the economic potential of the sesame crop. The advantage of this highly important genome data can be very useful for the improvement of sesame crop either by overexpression for gain of function or editing advantageous genes by CRISPR-Cas9 or other molecular tools.

The recalcitrant nature of sesame significantly reduces the regeneration and genetic transformation efficiency (Baskaran and Jayabalan 2006; Zimik and Arumugam 2017). To date, the regeneration protocols with different cultivars of sesame are reported; however, only few reports with limited success on genetic transformation are available. In this review, we have highlighted the regeneration protocols developed with different varieties of sesame. The effect of major factors on regeneration efficiency in the sesame, like type of explants, age of explants, medium compositions, basal medium, and different hormone compositions, is discussed (Fig. 1). The review also emphasizes the efforts on sesame transformation, which are very miniscule and have limited success. The problems in sesame regeneration and genetic transformation are also discussed.

Figure 1.
figure 1

Schematic representation factors affecting callus formation, somatic embryogenesis, shoot regeneration and genetic transformation in Sesamum indicum L. # denotes following cultivars: Busia, Ex-El, Indian, Koyonzo, Mbale, McWhite, Mtwara-2, Siaya, Dasak, HT -1, TKG-55, Pb No. 1, GT-2 Himalayan, Sohag 1, SVPR – 1, DS-1, E-8, GLW, TLB, WII, DS-1, DSS-9, KNL, RT-273, TNL, WII, VRI-1, HT-1, DS-1, DSS-9, KNL, RT-273, TNL, WII, SVPR-1, TMV-7, AKT 64, Rajeshwari, RT127, TC 25, TMV 3, TMV 4, TMV 5, TMV 6, UMA, VRI1, TMV-3, JK-1, JT-7, PADMA, PHULE-TIL1, RT-103, TAPI-A, Rama, GT-3, GT-4.

Full size image

Plant regeneration

Plant tissue culture is the in vitro technique used to regenerate plants from different tissues and organs in sterile condition for better traits or a large number of genetically similar plants. Since the last couple of decades from the advent of the totipotency phenomenon, the tissue culture process has shown successful results in a large number of plants. Different methods, like micropropagation, somatic embryogenesis (SE), anther culture, meristem culture, and somaclonal and gametoclonal variations, have provided opportunities to develop the new varieties of plants. The regeneration of whole plant from single cell was reported over 50 yr ago by Steward et al. (1970). The success for regeneration of whole plant varies from species to species where the medium compositions, plant growth hormones, and other adjuvants play a major role. The high-frequency regeneration is a prerequisite for the improvement of different crops by genetic engineering. The regeneration protocols in different oleiferous species are developed for enhanced agriculture traits. In sesame, the regeneration protocols are highly dependent on different cultivars. The proper regeneration system is still a major drawback, and, therefore, genetic engineering of this crop has limited success (Were et al. 2006; Seo et al. 2007; Yadav et al. 2010; Chowdhury et al. 2014; Zimic and Arumugam 2017). Lee et al. (1985) published the first report on shoot regeneration from the shoot tip explant. Furthermore, George et al. (1987) showed regeneration in sesame via SE and multiple shoot production from shoot tips. Since then, sesame tissue culture has been reported using different explants to enhance shoot formation capacity (leaf, cotyledon, shoot tips, mature embryo, anther, hypocotyl, de-embryonated cotyledon, and plumule explant). The details of the regeneration protocols are discussed in the following sections.

Plant regeneration via somatic embryogenesis

In vitro SE is a developmental process in which a somatic cell can differentiate into non-zygotic embryos that can develop into a new plant under appropriate conditions. SE follows two main stages called induction and expression (Jimenez 2005). During the induction phase, cells pass through the physiological changes and altered gene expression for acquiring the embryogenic characteristics (Feher et al. 2002). Furthermore, in appropriate culture medium and plant growth regulators (PGRs), the induced cell develops into full embryos (Jimenez 2005). The somatic embryos (SEs) can be formed indirectly via callusing phase or directly without callusing phase. In sesame, different cultivars, like TMV 6, Nigrum, Darab 1, and E8, showed SEs. Different explants, like cotyledons, shoot tips, hypocotyls, and PGRs alone or in combinations, were used for the formation of SEs. In sesame until now, only five reports were published on SEs formation, where four reports show the SE via callus phase (Mary and Jayabalan 1997; Xu et al. 1997; Shashidhara et al. 2011; Chamandoosti 2016). Until now, the highest SE frequency was observed by Honnale and Rao (2013) directly from cotyledonary and hypocotyl explants from 5-d-old seedlings. The cotyledon explants showed a higher number of SEs compared to hypocotyls (Honnale and Rao 2013). The 95% cotyledon explants showed SE with a large number of SEs (59.16 ± 4.30) on 3.0 mg L−1 2,4-D + 1.0 mg L−1 BAP (Honnale and Rao 2013; Table 1). The 2,4-D was found to be very efficient for induction of SEs compared to other auxins, and further addition of low concentrations of BAP enhanced the frequency of SE (Honnale and Rao 2013). It was observed that 2,4-D was the key PGR used in all the studies for embryo formation. In addition to 2,4-D, BAP, Kinetin, and NAA also helped in SEs formation in sesame. The addition of both 2,4-D and BAP is suggested in several reports as an important factor for inducing and developing SEs in different crops (Jimenez 2005).

Table 1. Effect of explant, hormones, and media adjuvants on somatic embryogenesis in different cultivars of Sesamum indicum L
Full size table

Plant regeneration via callus induction

In sesame, different cultivars, like E-8, G-1, Giza 32, Rama, Sohag 1, Shandweel 3, and TMV-7, showed only callus induction, which eventually did not regenerate in shoot buds (Al-Shafeay et al. 2011; Pusadkar et al. 2015; Gayatri and Basu 2020; Table 2). Al-Shafeay et al. (2011) reported high-frequency callus induction from embryo, cotyledon, and hypocotyl explants in Giza 32, Sohag 1, and Shandweel 3 varieties. The callus cultures, when transferred on MS (Murashige and Skoog 1962) medium with BAP, Kinetin, and IAA, turned brown and did not support shoot induction. However, the de-embryonated cotyledon explants cultured on MS medium with BA and IAA directly regenerated shoots without callus phase (Al-Shafeay et al. 2011). Pusadkar et al. (2016) obtained callus in TMV 7 and G-1 varieties in both hypocotyl and cotyledon on MS medium with different combinations of PGRs. NAA (0.5 mg L−1) and TDZ (1.0 mg L−1) showed the highest callus percentage in both TMV 7 (93.3%) and G 1 (90%). Similarly, Gayatri and Basu (2020) reported the callus production using different concentrations of 2,4-D, Kinetin, and BAP in the variety Rama. The highest frequency of callus formation (approximately 89%) was found on MS medium supplemented with 2.0 mg L−1 2,4-D and 1.5 mg L−1 BAP. In this case, also the cotyledon explants showed the highest callus formation as reported by Al-Shafeay et al. (2011). According to the findings presented in Table 2, it can be observed that the varieties listed do not yield shoots through callus. However, exceptions were found in the cases of Sohag 1 (Al-Shafeay et al. 2011) and Rama (Gayatri and Basu 2020) varieties, where direct regeneration was successfully accomplished (Table 4). It appears that PGRs are not solely responsible for further shoot regeneration in these varieties as callus obtained on combination of 2,4-D and BAP has shown shoot regeneration in other sesame varieties as discussed in the next section (Kwon et al. 1993; Saravanan and Nadarajan 2005). Therefore, it is apparent that the differential callus response is highly genotype dependent.

Table 2. Callus formation from different cultivars of Sesamum indicum L
Full size table

In sesame, different cultivars, like Ansan, AHT 123, CO-1, Danbeak, DS-1, DSS-9, E-8, GLW, KNL, RT-273, SVPR-1, TMV 3, TL, TNL, VRI 1, VRII, and W-II, showed callus induction, and, subsequently, shoot regeneration (Table 3). The callus induction was reported from 0- to 10-d-old seedlings, cotyledons, embryos, leaf, shoot tips, and hypocotyl explants (Table 3). In all the reports, the calluses were obtained on different auxins (2,4-D, IAA, and NAA) at variable concentrations. When these calluses were transferred on low concentration of auxin along with high concentration of BAP, the multiple shoots were observed. Kwon et al. (1993) reported the highest number of shoots per explant (2 to 13) from the callus explant on casein hydrolysate (CH)-supplemented medium. In contrast, other studies have reported significantly low rates of regeneration (1 to 2 shoots per callus explant) were observed, indicating the highly challenging and recalcitrant nature of sesame towards plant regeneration. Casein hydrolysate is considered rich in minerals, microelements, and vitamins and eliminates the problem of phosphate deficiency in the regenerating explants. The variable response towards shoot regeneration could be due to varietal differences and the endogenous auxin concentrations between genotypes (Khemkladngoen et al. 2011; Zimik and Arumugam 2017). The regeneration of shoots from the callus tissue after transferring on BAP medium is in agreement with other previously published reports from different crops species, like Malus domestica (Caboni et al. 2000), Holostemma ada-kodien (Martin 2002), and S. indicum cultivar SVPR-1 (Raja and Jayabalan 2010).

Table 3. Effect of different hormones and other factors on shoot regeneration via callus formation in different cultivars of Sesamum indicum L
Full size table

Plant regeneration via direct shoot induction/shoot morphogenesis

The regeneration via callus formation can lead to somaclonal variation, and moreover, the transformation is difficult via callus phase; therefore, direct regeneration was attempted using different cultivars in sesame (SVPR-1, WII, DS-1, E-8, GLW, TLB, and RT-273; Table 4). Explants, like cotyledons, de-embryonated cotyledons, cotyledonary nodes, nodes, shoot tips, plumule, hypocotyls, and PGRs, alone or in combinations were used for direct shoot induction (Fig. 1). For the majority, regeneration was achieved from cotyledons and de-embryonated cotyledons (Table 4). From the plumule explants, high-frequency direct shoot regeneration (94.44%) and the maximum number of shoots per explant (15.96) were achieved in the Rama cultivar (Gayatri and Basu 2020). Malaghan et al. (2013) achieved the highest shoot regeneration, for example, 100% (½ MS with 20.0 µM TDZ, 2.5 µM IAA, and 25.0 µM BAP) with 8.15 shoots per cotyledon in RT-273 cultivar. Despite reports of regeneration in various media supplemented with TDZ and IAA (Were et al. 2006; Seo et al. 2007; Al-Shafeay et al. 2011; Wadeyar et al. 2013) as well as BAP and IAA (Were et al. 2006), the overall frequency of regeneration remains low. As a result, researchers have also explored the use of adjuvants to enhance the efficiency of regeneration.

Table 4. Effect of different hormones and other factors on direct in vitro shoot regeneration in different cultivars of Sesamum indicum L
Full size table

Effect of AgNO3 on regeneration

In this context, fortification of MS medium with various concentrations of AgNO3 along with BAP and IAA improved the frequency of regeneration (Table 4). AgNO3 is used as an ethylene action blocker in in vitro cultures (Mohiuddin et al. 1997). Many plants, including sesame (Seo et al. 2007; Abdellatef et al. 2010; Al-Shafeay et al. 2011), have shown the ability of AgNO3 to enhance in vitro regeneration (Williams et al. 1990; Ashwani et al. 2017; Panigrahi et al. 2017). Low concentrations of silver ions inhibited the impact of ethylene on plant cells by likely substituting the copper co-factor of the cell’s ethylene receptor (ETR1) and rendering it resistant to ethylene, as previously demonstrated (Moshkov et al. 2008; Kumar et al. 2009).

The application of AgNO3 dramatically accelerated regeneration in sesame (Seo et al. 2007; Chowdhury et al. 2014; Zimik and Arumugam 2017; Debnath et al. 2018; Rajput et al. 2022; Table 4). In addition, several other crops, including Coffea camephora (Kumar et al. 2007), Zea mays (Songstad et al. 1988), Cucumis melo (Roustan et al. 1992), Brassica campestris (Zhang et al. 1998), Brassica juncea (Pua and Chi 1993), and Capsicum annuum (Hyde and Phillips 1996), showed improved organogenesis from the addition of AgNO3.

Effect of sucrose on regeneration

In the development of shoots, carbohydrates have been shown to serve as both an osmoticum and an energy source (Brown et al. 1979). Different investigations have shown the specific physiological role of sucrose in the induction of shoots (Brown et al. 1979; Strickland et al. 1987; Ramage and Leung 1996). In Capsicum annuum hypocotyl explants, an external source of sucrose is required for at least the first 4 d of cultivation (Ramage and Leung 1996). In Aloe saponaria, shoot organogenesis required a 4% sugar content (Kim et al. 2016). In sesame, the regeneration frequency improved in the presence of high sucrose concentrations in the medium (6 to 9%; Seo et al. 2007; Chowdhury et al. 2014; Pusadkar et al. 2016; Debnath et al. 2018; Table 4). High sucrose pre-treatment in these studies has enhanced adventitious shoot production by resulting in high sucrose uptake in explants. Generally, the explants are pre-cultured on high sucrose (6 to 9%) medium for 7 to 15 d and then transferred on 3% sucrose. According to Seo et al. (2007), pre-culturing cotyledon explants for 2 wk on 9% sucrose and then sub-culturing to medium containing 3% sucrose significantly improved adventitious shoots production in sesame; however, prolonged incubation on 9% was not useful and explants turned brown. According to Chowdhury et al. (2014) and Debnath et al. (2018), the addition of sucrose along with ABA in the culture medium promoted regeneration. It was discussed that ABA might be facilitating the uptake of sucrose, and interestingly, both ABA and sucrose synergistically improved adventitious shoot formation (Seo et al. 2007). However, the detailed experiments are required to prove and justify the role of ABA in sucrose uptake in sesame explants.

Effect of ABA on regenerations

Abscisic acid (ABA) is an important plant hormone, which plays a pivotal role in embryo and seed maturation (Zeevaart and Creelman 1988). It prevents the precocious germination of embryos in developing seeds. According to Choi and Jeong (2002) and García-Martín et al. (2005), ABA pre-treatment prevents premature germination of somatic embryos and decreases the production of secondary embryos. ABA helps the plants by protecting cells from desiccation during water limitation by regulating different genes and stomatal regulation and provides drought tolerance (Wasilewska et al. 2008). The role of ABA in promoting regeneration frequency in sesame has been reported by different researchers. In sesame, a brief pre-treatment of different durations with ABA increased the development of adventitious shoots (Seo et al. 2007; Chowdhury et al. 2014; Pusadkar et al. 2016; Debnath et al. 2018; Rajput et al. 2022; Table 4). The addition of ABA further to AgNO3-supplemented medium showed enhanced regeneration efficiency in some reports (Debnath et al. 2018; Rajput et al. 2022). In most of the cases, the ABA pre-treatment for 1 wk promoted the regeneration efficiency (Table 4). The presence of ABA attributed to high sucrose uptake in explants and increased cellular sucrose content (Debnath et al. 2018). This is in agreement to the report of Seo et al. (2007) where the initial incubation of cultures for 2 wk at high sucrose concentration increased adventitious shoot formation in sesame. The presence of ABA synchronizing the sucrose uptake is also reported in strawberry (Archbold 1988), pea (Estruch et al. 1989), and tobacco and beet (Saftner and Wyse 1984). It is well known that ABA promotes the uptake of sucrose by Beta vulgaris in root tissue (Saftner and Wyse 1984).

Root induction

The in vitro raised adventitious shoots of different cultivars of sesame that were transferred on a rooting medium supplemented with different auxins, like IBA, IAA, and NAA, at different concentrations formed roots (Table 5). Different cultivars showed different root frequency; in some, the rooting percentage was < 50% whereas in other cultivars, like Dasak, Uma, JK-1, TMV3, GT-3, and GT-4, the root frequency was > 50%. In cultivar Dasak, high-frequency root formation (98%) was observed on MS media supplemented with 2.69 µM NAA (Seo et al. 2007), but the highest number of roots per explant (13.81) was obtained by Saravanan and Nadarajan (2005). Gayatri and Basu (2020) observed that the addition of NAA or IAA to basal medium did not help in root formation, but it did support callus formation, which affected shoot growth negatively followed by shoot fatality. Similarly, in the present study, no root formation was observed in transgenic plants of cultivars GT3 and GT4, and the basal end of the plants turned brown leading to plant death during subsequent culturing. Gayatri and Basu (2020) explained that half concentration of MS macronutrients in combination with SH (Schenk and Hildebrandt 1972) micronutrient and vitamins supported better root frequency compared to full-strength MS major. This suggests that reduced salt concentration is necessary for high frequency of root formation in the cultivar Rama.

Table 5. Root induction response on different PGRs of Sesamum indicum L
Full size table

Agrobacterium-mediated transformation in sesame

As mentioned above, sesame is a highly important oil seed crop, and it faces a major problem of abiotic and biotic stresses. Therefore, a need for genetic improvement lies in this crop towards the development of disease-resistant and nutritionally enriched varieties. The sexual incompatibility between the cultivated and wild species in sesame crop limits its improvement via conventional breeding method (Tiwari et al. 2011; Kulkarni et al. 2017). Genetic engineering of crops via overexpression, CRISPR-Cas9, and RNAi, has emerged as a potential tool for targeted changes for crops improvement.

The limited success of regeneration was discussed in the previous section. Because of unavailability of suitable and reproducible regeneration and transformation protocols, the success of genetic engineering in this crop remains a bottleneck (Zimik and Arumugam 2017). Until now, only a few research publications have reported on the genetic transformation of the sesame crop using Agrobacterium-mediated transformation (Table 6). Taskin et al. (1999) attempted the first Agrobacterium-mediated transformation in cultivar Ozberk using pBI121 vector; however, no success was achieved. Later, Yadav et al. (2010) succeeded in achieving the fertile transgenic plants (1.01%) using pCAMBIA2301 in cultivar HT-1 through the use of cotyledon explants. Yadav et al. (2010) observed that a total of 60% of the explants showed GUS activity; furthermore, by the addition of thiol compounds, like L-Cystine and dithiothreitol, the higher number of explants showed an increase in GUS activity. The thiol compounds are known to act as an inhibitor of antioxidants and have a wounding effect in plants (Svabova and Griga 2008). Al-Shafeay et al. (2011) achieved fertile transgenic plants via Agrobacterium-mediated transformation using pBI121 vector in cultivar Sohag1 with 1.67% frequency. Out of the various parameters tested (bacterial concentration, co-cultivation time, and explants), the co-cultivation for 1 or 2d was found better compared to longer incubation. Thereafter, Chowdhury et al. (2014) reported transformation efficiency of 42.66% in sesame cultivar VRI 1 using de-embryonated cotyledon explants. Chowdhury et al. (2014) also observed that co-cultivation of explants with Agrobacterium for 1 d has higher transformation efficiency compared to longer incubation with bacterial density of 1.6 OD600 as reported earlier by Al-Shafeay et al. (2011).

Table 6. Transgenic plants of Sesamum indicum L. using the optimized conditions for transformation
Full size table

Along with this high concentration of BAP (30.0 µM), sucrose (9.0%) and acetosyringone helped in higher frequency of transformation in this cultivar. Gayatri and Basu (2020) reported an improved transformation protocol using Agrobacterium harboring pCAMBIA vector in cultivar Rama. Southern analysis revealed 1.33% transformation efficiency with low bacterial concentration (OD600 < 0.6) and for a 72-hr co-cultivation duration. The low Agrobacterium culture density and extended co-cultivation duration were more effective for transformation in Rama as well as other sesame cultivars (Yadav et al. 2010; Al-Shafeay et al. 2011). Overall, the transformation efficiency using A. tumefaciencs-mediated transformation is highly dependent on different cultivars. Therefore, as minor variations in protocols lead to variation in transformation efficiency, therefore, high scientific precision and careful handling should be followed to get the success in other sesame varieties. Chowdhury et al. (2017) for the first time developed the sesame transgenic lines using osmotin-like proteins and achieved both abiotic stress-tolerant and biotic stress-tolerant transgenic plants. The transgenic plants showed regulation of different biochemical parameters responsible for the regulation of combined stress (Chowdhury et al. 2017).

In sesame, hairy root transformation was also attempted, and some success was made by Agrobacterium rhizogenes-mediated system using CRISPR/Cas9. Two sesame cytochrome P450 genes responsible for sesamin and sesamolin were targeted using A. rhizogenes-mediated transformation, showing the feasibility of CRISPR-based genome editing in sesame (You et al. 2022). The particle bombardment in cultivar Rama, the genetic transformation used in apical meristematic tissue, showed approximately 16% transformation efficiency (Bhattacharyya et al. 2015), which is almost double the Agrobacterium-mediated transformation efficiency in the same cultivar (Gayatri and Basu 2020). The enhanced transformation efficiency could be due to the reduced toxicity or browning of the tissue caused by Agrobacterium; however, the particle bombardment method needs to be evaluated for successful transformation on other sesame varieties.

Future prospects and conclusions

In the last few decades, many efforts have been made in the area of tissue culture of the oil seed crop, sesame. The major challenges of the cultivation are yield loss due to biotic and abiotic stresses. Therefore, new research approaches are required for genetic and nutritional improvement of oil quality as well as higher yield in sesame. Although numerous regeneration protocols are reported by various researchers using different parameters and factors, but still the challenge remains because of recalcitrant nature of sesame for regeneration. It has been demonstrated that regeneration is highly genotype dependent. The regeneration protocols have been reported both directly and through the callus phase. It has been observed that a low salt concentration in the basal medium and a high concentration of BAP favor regeneration in this plant. The addition of AgNO3 improved the regeneration frequency in some cultivars. Furthermore, it has been observed that the addition of ABA to AgNO3-supplemented medium enhanced the regeneration. High sucrose pre-treatment and then sub-culturing to medium containing 3% sucrose were also found to be beneficial for shoot production. Sesame exhibits significant challenges on genetic transformation, as following co-cultivation with Agrobacterium, sesame often experiences a prevalent occurrence of browning and necrosis, further emphasizing its highly recalcitrant nature. The “altruistic transformation” approach was successfully used in promoting genetic transformation efficiency in different monocots and dicots using the Wus2 and Bbm transcription factor in maize and sorghum (Hoerster et al. 2020; Nelson-Vasilchik et al. 2022), and AtGRF5 and its orthologs in soybean and sunflower (Kong et al. 2020) can be applied for the future success of sesame transformation. Different methods, like particle gun or electroporation, should be also attempted at large in the future for the genetic transformation in this crop.