Solanum torvum of the family Solanaceae, a wild relative of eggplant (Solanum melongena), has been identified to carry multiple traits of resistance to the most serious biological and abiotic stress (i.e., bacteria, fungal wilts, and root-knot nematodes; salt or cadmium stress) (Bagnaresi et al., 2013; Gousset et al., 2005). Transgenic technology, which relies on efficient genetic transformation systems, is the best way to solve these problems, and the target genes, which are strongly resistant and high yielding, can be imported into the cultivated plant to obtain high-quality varieties through genetic engineering (Jin et al., 2004). Regeneration systems have been successfully established in many varieties of Solanum L., including eggplant, potato, and tomato. However, compared with plants of the same genus, the callus differentiation frequency (%) of S. torvum is still relatively low, and there is no stable plant regeneration system for the genetic transformation of S. torvum. The purpose of this study was therefore to develop an efficient and reproducible in vitro regeneration protocol for explants of S. torvum as a necessary first step for subsequent biotechnological studies and applications.
Choosing appropriate explants is the first step in building an efficient plant regeneration system. Several explants have been used to establish regeneration systems in the Solanum genus, such as anthers, protoplasts, stem segments, leaves, cotyledons, and hypocotyls (Alicchio et al., 1984; Jia and Potrykus, 1981; Matsuoka and Hinata, 1979; Raina and Iyer, 1973; Saxena et al., 1981; Xing et al., 2010). In eggplant, anther culture to obtain transgenic plants from microspore-derived embryos has been studied since the 1980s, and plantlets are regenerated at a satisfactory rate from the callus of eggplant following anther culture (Rotino, 2016). Regenerated plants have also been obtained from shoots of different eggplant varieties such as S. khasianum, S. indicum, and S. sisymbrifolium (Bhatt et al., 1979; Fassuliotis, 1975).
Several experiments in eggplant regeneration have been performed using MS medium supplemented with different plant growth regulators. For instance, MS with thiadiazuron is used for in vitro organogenesis (Magioli et al., 1998), kinetin (KT) is used for anther culture (Rotino, 2016), and NAA is used for somatic embryogenesis (Sharma and Rajam, 1995). In different stages of tissue culture, the hormones and concentrations used are different. Bud induction of cotyledon explants in eggplant is improved by favorable adjustment of ZT, IAA, and sucrose, and shoot elongation occurred in response to MS supplemented with gibberellic acid and AgNO3 (Xing et al., 2010). This article reports a highly efficient callus induction and plant regeneration system for a wild relative of eggplant Solanum torvum through the use of stem segment and leaf explants. The effects of different hormone concentrations on the construction of a high-frequency regeneration system of S. torvum were studied to provide theoretical guidance for the rapid development of S. torvum tissue culture and genetic engineering breeding.
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