Histocytological changes and reserve accumulation during somatic embryogenesis in Eucalyptus globulus

Abstract

We recently described a protocol for Eucalyptus globulus somatic embryogenesis (SE). For its immediate use at industrial levels, some stages of the process require better control. In particular, SE germination rates are variable, decreasing SE efficacy. As reserves may play a central role in embryogenic processes, we followed histocytological changes and reserve fluctuations, during SE. For SE induction, explants of mature zygotic embryos were grown on Murashige and Skoog (MS) medium with 3 mg l⁻¹ α-naphthalene acetic acid and later transferred to MS without growth regulators (MS_WH). Samples of zygotic embryo cotyledons (explants), of globular and dicotyledonar somatic embryos, and of embling leaves were analysed for reserve accumulation and histocytological profiles. Cotyledon cells of zygotic embryos were rich in lipid and protein bodies, having almost no starch. After 3 weeks of induction, starch grain density increased in differentiated mesophyll regions, while in meristematic regions their occurrence was diffuse. In globular somatic embryos, starch accumulation increased with time (in amyloplasts), but protein bodies were absent. Cotyledonary somatic embryos had lower density of starch grains and absence of lipid and protein bodies. Embling leaves showed typical histological organisation. This is the first comprehensive study on histological and cytological changes during Eucalyptus SE with emphasis in reserve accumulation. With this work we demonstrate that the presently available SE protocol for E. globulus leads to reserve fluctuations during the process. Moreover, the reserves of somatic embryo cotyledons differ from those of their zygotic embryo counterparts, which reinforce the importance of reserves in the embryogenic process and suggests that manipulating external conditions, SE may be optimised giving suitable emblings production for industrial purposes.