AUXIN PULSE IN THE INDUCTION OF SOMATIC EMBRYOS OF Eucalyptus

Abstract

The objective of this study was to evaluate the effect of auxin pulse intervals on the induction of somatic embryos of Eucalyptus grandis x E. urophylla and to describe the embryogenic behavior of callus under the effect of auxinic stress. Cotyledons were inoculated in culture medium containing 207.07 µM picloram, a treatment considered as auxin pulse. Explants that were in the auxin pulse treatment were transferred to semisolid or liquid medium containing 20.71 µM picloram after one, two, four or eight days of auxin pulse. In a second experiment, explants that were on auxin pulse treatment were transferred to semi-solid medium containing 20.71 µM picloram after one, two or three days of auxin pulse. Auxiliary picloram pulse treatments (207.02 µM) can be used as an initial source of stress for the acquisition of embryogenic competence. The oxidation of cotyledonary explants may be considered as an indication of the formation of embryogenic calli. The presence of pectins in peripheral regions of somatic pro-embryos can be considered as a marker of somatic embryogenesis in cotyledonary explants of Eucalyptus grandis x E. urophylla.

The Prospect of Physiological Events Associated with the Micropropagation of Eucalyptus sp.

“…Although several factors directly and indirectly influence the micropropagation of Eucalyptus, the addition of different dosages of PGRs especially auxins into the media is one of the most important factors influencing the gain of cellular competence, triggering embryogenic re-differentiation, dedifferentiation, and organ(s) formation [51]. It is pertinent to mention here that prolonged exposure of the plantlets to auxins could negatively impact the morphological and physiological features of in vitro propagated plants including somaclonal variation, structure distortion, fasciation, dwarfism, and hyperhydricity [52]. Therefore, determining the optimum auxin concentration to induce somatic embryogenesis and organogenesis is a critical step, especially in recalcitrant species such as Eucalyptus.…”

“…The effectiveness and pathway of SE differentiation depends on several factors; however, optimum concentrations of PGRs play a vital function in the acquisition of cellular competence, initiating dedifferentiation, and re-differentiation [113]. At the cellular level of Eucalyptus plants, auxins are able to regulate different mechanisms such as cell division, expansion, and differentiation, and auxins can promote isodiametric expansion, lateral expansion, and axial elongation [30,52]. Furthermore, synthetic auxins such as 2,4-D, 4-amino-3,5,6-trichloropicolinic acid (picloram) and dicamba disrupt endogenous auxin metabolism, promote cellular reorganisation and induce somatic embryogenesis [10,37,53].…”

“…During SE initiation, auxins play an important role in a stress component and auxin signaling and could change the auxin content in Eucalyptus [27,103,120]. The positive interaction of the oxidation of callogenic tissue with embryogenic calli formation has been reported in cotyledonary of E. urophylla × E. grandis [52]. Among all the PGRs (especially auxins), NAA has been frequently applied for SE induction in Eucalyptus species [13,110,121,122].…”