Hormonal regulation of somatic embryogenesis in Medicago spp.

Abstract

In contrast to zygotic embryogenesis, somatic embryogenesis culture systems, not limited in tissue quantity and accessibility, were found to be useful models with which to investigate the role of phytohormones during induction, development and maturation of somatic embryos. Artificial seed technology is one of the important efficient methods of in vitro propagation of a number of agronomically important plants, including Medicago species, with Medicago truncatula serving as a Fabaceae experimental model. The review will focus on the role of exogenous and endogenous plant growth regulators /phytohormones in different phases of somatic embryogenesis in Medicago species. There is evidence that induction and development of M. sativa somatic embryos require endogenous levels not only of growth stimulants (e.g. gibberellins, auxins and cytokinins), but also of phytohormones known as growth inhibitors, i.e. ABA, jasmonates and ethylene. Any alterations in the contents of these plant growth inhibitors during the distinct phases of M. sativa SE, proved unfavorable for the subsequent somatic embryo production, suggesting that the level of biosynthetic activity in tissues is optimal for sustaining an appropriate course of this developmental process. It turned out that, in the case of ethylene, its synthesis in and activity towards M. sativa is not linked to the induction, but is very important in the proliferation and differentiation phases. An ABA content lower than that of active GAs (GA4, GA7, GA1, GA5, GA3, GA6) and IAA was proven to enable the embryo formation in the M. truncatula callus. The importance of some phytohormones in the germination and conversion of somatic embryos is also discussed.