Hormonal Control of Root Primordia Differentiation and Root Formation in Cultured Explants of Eucalyptus Globulus Seedlings.

Abstract

Auxin-induced differentiation of root primordia, and their subsequent development into growing roots, was studied in explants of 8-day-old seedlings of Eucalyptus globulus subspp. globulus. Aseptic hypocotyl or root explants were cultured at 20 ± 2ºC on modified Murashige and Skoog medium containing 2% sucrose and various combinations of auxin andfor cytokinin. Of the auxins examined, indole-butyric acid (IBA) at concentrations of 10*-4 - 10*-5 M was most effective at inducing root primordia and led to a significant (P d 0.001) increase in the density of root primordia when compared with non-auxin treated control tissues. In root explants, maximal numbers of lateral root primordia (LRP) resulted after 16-18 h contact with auxin although they were not all visible until 42-48 h after initial exposure to auxin. Efficient conversion of LRP to lateral roots (LR) required tissues to be removed from auxin-induction medium within approximately 48 h and was inhibited by 6-benzylaminopurine (BAP) concentrations of 10*-5 M or greater. Treatment of root explants with IBA only for 16 h, followed by 96 h culture on medium containing a range of BAP concentrations showed that LRP induction was not inhibited by cytokinin concentrations as high as 10*-3 M. However, pretreatment of root explants for 6 h with BAP and other cytokinins, at 10*-5 M or higher concentrations, prevented the subsequent induction of LRP by IBA. Pretreatment of root explants with either 10*-3 M actinomycin D or 10*-3 M cycloheximide for 6 h prior to exposure to M IBA, or at any time up to 42 h after initial exposure to IBA, was also inhibitory to the subsequent induction of LRP suggesting that de novo transcription and translation of an essential gene(s) islare required for LRP formation following treatment with IBA. This study serves as a valuable preliminary to a more detailed investigation into the molecular mechanisms associated with root primordia formation in eucalypts.