Meta-Topolin as an Aromatic Cytokinin for In Vitro Propagation of Thymus vulgaris L.

Effective in vitro propagation of medicinal and aromatic plants such as Thymus vulgaris is often limited by the choice of optimal cytokinin sources, which are critical for successful shoot proliferation and overall regeneration. Since their discovery, topolins have been recognized as alternatives to traditional cytokinins (CKs) such as benzyladenine (BA) and kinetin (Kin) in plant tissue culture (PTC). This study investigated the influence of three aromatic CKs (meta-topolin (mT), BA, and Kin), on shoot proliferation of thyme, with the goal of improving current PTC protocols. Of all the tested treatments, the highest shoot proliferation (7.25 ± 0.72 shoots per nodal explant) was observed in the treatment with 1 µM mT, superior to BA and Kin. Increasing mT and BA concentrations from 1 to 15 µM had an inverse effect on shoot production. The addition of indole-3-butyric acid (IBA) at concentrations ranging from 1.0 to 5.0 µM in combination with 0.5 and 1 µM mT did not increase the mean shoot numbers per explant. Regenerated shoots exhibited a strong propensity for root development even in the absence of plant growth regulators with 100% survival ex vitro. This study demonstrated that mT is an effective, sustainable and cost-effective alternative to traditional CKs for in vitro propagation of T. vulgaris, achieving an improved shoot proliferation with 1 μM mT application.