Exogenous implications of silver nitrate on direct and indirect somatic embryogenesis and germination of cold stored synseeds of Vanilla planifolia Jacks. ex Andrews

Vanilla planifolia – a natural source of vanillin is the world's highest-valued spice crop with endangered status and the only hand-pollinated crop globally. The present study evaluates the effect of exogenous silver nitrate (AgNO₃) on effective direct and indirect somatic embryogenesis in V. planifolia for the first time. The direct and indirect somatic embryogenic capacity of leaf explants were studied using Murashige and Skoog (MS) medium containing 6-benzylaminopurine (BAP), kinetin (KN), 2,4-Dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), and α-naphthalene acetic acid (NAA) alone and in combinations. Additionally, the efficacy of silver nitrate (SN) in promoting somatic embryogenesis was tested with optimized growth regulators. Non-embryogenic callus was obtained from the explants when cultured on 3.0 mg L⁻¹ 2,4-D. The embryogenic potential was developed while culturing the non-embryogenic callus on 1.0 mg L⁻¹ each of BAP and KN combined with 2.5 mg L⁻¹ SN. On the other hand, direct somatic embryogenesis (92.0%) was obtained when culturing the explants directly on MS medium containing 1.0 mg L⁻¹ each of BAP and KN combined with 2.5 mg L⁻¹ SN, and a maximum of 30.0 somatic embryos (SEs) were induced per explant. Microscopic analysis revealed that SEs were originated externally as cell division initiated on the leaf surface and cut ends of explants, and have no vascular connections with the explant. Actively dividing embryonic cells possessed dense cytoplasm with prominent nuclei. The mature SEs were encapsulated and stored at - 4 °C for one year. The synseeds showed maximum germination and proliferation (17.2 shoots per SE) on MS medium containing 0.5 mg L⁻¹ BAP, 0.25 mg L⁻¹ KN, and 2.5 mg L⁻¹ SN. The results indicated that AgNO₃ in the medium promoted somatic embryogenesis and plantlet regeneration in V. planifolia.