A. Pelosi et al., HORMONAL-CONTROL OF ROOT PRIMORDIA DIFFERENTIATION AND ROOT-FORMATIONIN CULTURED EXPLANTS OF EUCALYPTUS-GLOBULUS SEEDLINGS, Australian journal of plant physiology, 22(3), 1995, pp. 409-415
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.