Abstract
Conifer somatic embryogenesis (SE) is a process driven by exogenously supplied plant growth regulators (PGRs). Exogenous PGRs and endogenous phytohormones trigger particular ontogenetic events. Complex mechanisms involving a number of endogenous phytohormones control the differentiation of cells and tissues, as well as the establishment of structures and organs. Most of the mechanisms and hormonal functions in the SE of conifers have not yet been described. With the aim to better understand these mechanisms, we provided detailed analysis of the spectrum of endogenous phytohormones over the course of SE in Norway spruce (Picea abies). Concentrations of endogenous phytohormones including auxins, cytokinins (CKs), abscisic acid (ABA), jasmonates, and salicylic acid (SA) in somatic P. abies embryos were analyzed by HPLC-ESI-MS/MS. The results revealed that the concentrations of particular phytohormone classes varied substantially between proliferation, maturation, desiccation, and germination. Endogenous ABA showed a maximum concentration at the maturation stage, which reflected the presence of exogenous ABA in the medium and demonstrated its efficient perception by the embryos as a prerequisite for their further development. Auxins also had concentration maxima at the maturation stage, suggesting a role in embryo polarization. Endogenous jasmonates were detected in conifer somatic embryos for the first time, and reached maxima at germination. According to our knowledge, we have presented evidence for the involvement of the non-indole auxin phenylacetic acid, cis-zeatin- and dihydrozeatin-type CKs and SA in SE for the first time. The presented results represent the currently most comprehensive overview of plant hormone levels in embryos throughout the whole process of conifer SE. The differences in concentrations of various classes of phytohormones over the proliferation, maturation, desiccation, and germination in somatic P. abies embryos clearly indicate correlations between endogenous phytohormone profiles and particular developmental stages of the SE of conifers.
Highlights
Somatic embryogenesis is a five-step developmental process during which somatic cells dedifferentiate and divide to initiate embryogenic development
The endogenous phytohormone profile is a proportional representation of individual embryogenic suspensor mass (ESM) components and shows the concentrations of various exogenous plant growth regulator (PGR) that are necessary for successful proliferation
Somatic embryogenesis in conifers is driven by a complex network of hormonal, metabolic and signaling pathways that respond to the strict regulation of particular developmental steps, from early somatic embryos to emblings, by exogenous PGR treatments
Summary
Somatic embryogenesis is a five-step developmental process during which somatic cells dedifferentiate and divide to initiate embryogenic development. The early embryo can mature and even recreate a whole plant. Embryogenic cultures have been induced in many conifer species and different cell lines. Most plants require similar physical conditions (temperature, light regime) for the induction of SE, medium composition can have a large impact on SE outcome. Especially in exogenously applied PGRs, are decisive for the progression of embryos through the steps of SE (von Arnold et al, 2002). ABA is necessary during the maturation process, whereas desiccation and germination represent the PGR-independent steps of SE (von Arnold et al, 1996)
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