The choice of auxin analogue for in vitro root induction influences post-induction root development in Eucalyptus grandis

Previous studies on in vitro rooting for improved micropropagation of eucalypts indicated that root graviperception and postacclimatisation architecture are determined by the relative exogenous auxin analogue and its stability, supplied during the pre-rooting culture stages. The specific roles of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) in the rooting medium on the in vitro root morphological processes were explored using a good-rooting clone. In vitro rooting percentage was significantly reduced when either of the auxin inhibitors 2,3,5-triiodobenzoic acid (TIBA) and ρ-chlorophenoxyisobutyric acid (PCIB) or the auxin antagonist kinetin was supplied at rooting, with or without exogenous auxin. For all treatments, at the time of root induction, shoots did not possess a vascular cambium, only procambial tissue, from where adventitious roots formed. However, when the inhibitors or the antagonist were supplied to the roots 3 days after root induction, they affected root growth and graviperception. Kinetin and PCIB significantly reduced the mean root diameter from 552.8 μm (control) to 129.2 μm and 278.6 μm, respectively, over 3 weeks. While the PCIB treatment resulted in a significant increase in Δ root length over this period, the TIBA treatment significantly decreased Δ root length and increased mean root diameter to 833.4 μm. Restricting IAA transport with TIBA further altered root vascular patterning and, as with PCIB, resulted in the collapse of the columella region. Nevertheless, only a disruption in IAA transport and subsequent auxin distribution by TIBA treatment resulted in altered root graviperception. The results suggest the necessary inclusion of IAA in eucalypt micropropagation protocols to ensure good quality roots.