HORMONAL-CONTROL OF ROOT PRIMORDIA DIFFERENTIATION AND ROOT-FORMATIONIN CULTURED EXPLANTS OF EUCALYPTUS-GLOBULUS SEEDLINGS

Auxin-induced differentiation of root primordia, and their subsequent development into growing roots, was studied in explants of 8-day-old s eedlings of Eucalyptus globulus subspp. globulus. Aseptic hypocotyl or root explants were cultured at 20 +/- 2 degrees C on modified Murashi ge and Skoog medium containing 2% sucrose and various combinations of auxin and/or cytokinin. Of the auxins examined, indole-butyric acid (I BA) at concentrations of 10(-4)-10(-5) M was most effective at inducin g root primordia and led to a significant (P less than or equal to 0.0 01) increase in the density of root primordia when compared with non-a uxin treated control tissues. In root explants, maximal numbers of lat eral root primordia (LRP) resulted after 16-18 h contact with auxin al though they were not all visible until 42-48 h after initial exposure to auxin. Efficient conversion of LRP to lateral roots (LR) required t issues 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 concent rations showed that LRP induction was not inhibited by cytokinin conce ntrations 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 concentra tions, prevented the subsequent induction of LRP by IBA. Pretreatment of root explants with either 10(-3) M actinomycin D or 10(-3) M cycloh eximide for 6 h prior to exposure to 10(-5) M IBA, or at any time up t o 42 h after initial exposure to IBA, was also inhibitory to the subse quent induction of LRP suggesting that de novo transcription and trans lation of an essential gene(s) is/are required for LRP formation follo wing treatment with IBA. This study serves as a valuable preliminary t o a more detailed investigation into the molecular mechanisms associat ed with root primordia formation in eucalypts.