Using synergistic exogenous phytohormones to enhance somatic embryogenesis from leaf explants of a <i>Eucalyptus grandis</i> clone

Abstract

Somatic embryogenesis (SE) in Eucalyptus spp. has been limited to germinated seeds, flowers, lignotubers or zygotic embryos. The low yield of somatic embryos from leaf explants has hampered progress, even though leaves offer a more viable source of clonal explants from superior selected genotypes. It was hypothesised that SE from leaf explants could be enhanced through pairing of synergistic  exogenous plant growth regulators, such as natural auxins with natural cytokinins. Callus and embryo induction using 2,4-dichlorophenoxyacetic acid (2,4-D), α-naphthalene acetic acid (NAA), indole acetic acid (IAA), and indole butyric acid (IBA), each at either 1.0 or 3.0 mg L⁻¹, indicated that IAA and IBA favoured significantly higher numbers of embryos compared with 2,4-D or NAA. Hence, IAA and IBA were used for subsequent experiments, combining them (at 1.0 mg L⁻¹) with either the synthetic cytokinin, kinetin, or the natural cytokinin, trans-zeatin, both at 0.1 mg L⁻¹. The combination of trans-zeatin and either IAA or IBA resulted in a significant increase in SE (e.g. 86 ± 17.2% and 23 ± 3.2% for IAA with trans-zeatin and kinetin, respectively), compared with kinetin, or with these auxins alone. Embryo maturation and plantlet regeneration was highest in those calli that were induced with IAA and trans-zeatin, indicating that maturation was dependent on auxin depletion, based on the stability of the analogue used for induction. For the E. grandis clone under study, the use of synergistic plant growth regulators  significantly enhanced SE from leaf explants, thus presenting the opportunity to benefit from the advantages that SE offers over conventional organogenesis.

Keywords: embryogenesis, Eucalyptus, leaf explants, plant growth regulator, synergistic interactions