Abstract
Stevia rebaudiana Bertoni is a commercially important zero calorie natural-sweetener herb which produce sweet compounds known as steviol glycosides. Rising demands of steviol glycosides by food and beverage industries has led to an increase in its cultivation in various countries. Unfortunately, stevia cultivation faces 2–25% yield penalty due to weeds which further adds to its cultivation cost. To resolve this major challenge, Agrobacterium-mediated genetic transformation of in vitro derived stevia-nodal explants using herbicide resistance gene (bar) has been optimized, for the production of stable transgenic stevia plants. Several parameters including explant type, pre-incubation duration, acetosyringone (As) concentration, Agrobacterium cell density, Agro-inoculation duration, co-cultivation duration, selection regime and plant growth regulators (PGRs) combination and concentration, have been successfully optimized. Among the two types of explants used, nodal explants showed a higher regeneration response of 82.85%, with an average of 25 shoots/explant. The best PGRs combination and concentration for shoot-induction, shoot-elongation and root-induction was found to be 6-benzyladenine (1.0 mg l−1) + naphthalene acetic acid (0.5 mg l−1), gibberellic acid (1.0 mg l−1), and half-strength MS medium, respectively. The two-step selection (phosphinothricin) regime resulted in an average transformation efficiency of 40.48% with nodal explants. Molecular characterization of putative transformants through PCR, RT-PCR, qRT-PCR and Southern-blot hybridization confirmed the presence, stability, expression as well as copy number of bar gene respectively. Compared to the non-transgenic plants, the T0 transgenic plants successfully tolerated 8 mg l−1 glufosinate ammonium sprays. Thus, the optimized protocol can be useful for the introduction of other genes (inter-kingdom transfer) into stevia genome.
Highlights
Stevia rebaudiana Bertoni is a commercially important zero calorie natural-sweetener herb which produce sweet compounds known as steviol glycosides
The objectives of our study were (1) optimization of in vitro regeneration of stevia, (2) optimization of Agrobacterium-mediated genetic transformation of stevia with herbicide tolerant bar gene, (3) molecular characterization and (4) herbicide tolerance assay of the T 0 transgenic plants
Leaf discs were most efficient in callus formation and maximum callus induction was achieved on MS2 medium [2,4-D (2 mg l−1) and KIN (1 mg l−1)] (Supplementary Table 1)
Summary
Stevia rebaudiana Bertoni is a commercially important zero calorie natural-sweetener herb which produce sweet compounds known as steviol glycosides. Stevia cultivation faces 2–25% yield penalty due to weeds which further adds to its cultivation cost To resolve this major challenge, Agrobacterium-mediated genetic transformation of in vitro derived stevia-nodal explants using herbicide resistance gene (bar) has been optimized, for the production of stable transgenic stevia plants. Several parameters including explant type, pre-incubation duration, acetosyringone (As) concentration, Agrobacterium cell density, Agro-inoculation duration, co-cultivation duration, selection regime and plant growth regulators (PGRs) combination and concentration, have been successfully optimized. The conventional strategies used for stevia cultivation are not reliable because of poor viability of seeds and germination percentage[8] To overcome such limitations, in vitro propagation is the only remedy that can facilitate large-scale production of genetically identical stevia plants. Implementation of a robust weed control strategy is crucial for sustainable stevia cultivation
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