Fig. 1.

A schematic of the plant tissue culture and transformation process. Left: a typical plant tissue culture/micropropagation cycle is represented with its different stages: collection of explants from a mature donor plant (Stage 0, S0), sterilization and initiation of explants on shoot proliferation medium (Stage 1, S1), repeated shoot multiplication and elongation (Stage 2-S2), rooting (Stage 3, S3), and acclimatization to ex vitro growing conditions (Stage 4, S4). The circular arrow at the multiplication stage represents the iteration of in vitro multiplication cycles for large-scale production of TC plantlets. In vitro recalcitrance can occur at any stage (S1–S4). Right: genetic transformation of a plant species involves transferring a piece of DNA, such as a gene(s) of interest (GOI), with a selectable marker (Sel), into cells within the explants either through co-cultivation with Agrobacterium cells carrying the transformation vector or directly through biolistics. The transformed explants are proliferated on a callus initiation medium with selection pressure (+selection) to select only those cells which have the GOI integrated into their genome. The proliferated calli are then transferred to regeneration medium for embryo development and shoot regeneration with continued selection pressure. The regenerated plantlets enter the usual micropropagation stages (S2–S4). The circular arrow at the regeneration stage represents the continued multiplication of transformed plants. Alternatively, transformed calli can be used to initiate sterile cell suspension cultures, bypassing the need to generate a transgenic plant. In vitro recalcitrance can be encountered during genetic transformation (1) or through the process of transgenic plant regeneration at callus proliferation, embryo development, and/or plantlet growth stages (2, 3).