Auxin stability and accumulation during in vitro shoot morphogenesis influences subsequent root induction and development in Eucalyptus grandis

Abstract

Recent results showed that after 16 months in the field, micropropagated eucalyptus plants have an inferior root system to cuttings. Such differences may be due to the plant growth regulators supplied during the culture stages of standard protocols, which are targeted at optimising plantlet yields and not root quality. This study investigated such a proposal, focusing on auxins in an easyto-root clone. Initial results showed that the auxin provided in the standard protocol (NAA for multiplication and IBA for elongation) enabled 100% rooting in auxin-free medium, where rooting was faster than on IBA-rooting media. When auxin supply was omitted from multiplication and restricted to NAA or IAA during elongation, rooting in an auxin-free medium was reduced to 68 and 31%, respectively, reflecting the stabilities of these auxins in plant tissues. Additionally, 15% of shoots from the NAA-medium and 65% from the IAA-medium produced roots with altered graviperception. GC-MS analysis of these shoots revealed a relationship between free IAA-availability and altered graviperception. This was further tested by adding the IAA-specific transport inhibitor 2,3,5-triiodobenzoic acid to rooting media with IBA, IAA or NAA, which resulted in 100, 70.9 and 20.6% rooting, respectively. At least 40% of the sampled root tips had atypical starch grain deposition and abnormal graviperception. It is proposed that, at least in this clone, while IBA and NAA can be used for in vitro root induction, IAA is necessary for development of graviresponse.

Stem Anatomy and Adventitious Root Formation in Cuttings of Angophora, Corymbia and Eucalyptus

“…Optimally-treated eucalypt shoots often produce a mean of about four adventitious roots but individual shoots can produce higher root numbers [3,4,9,17,[19][20][21][22][23][24]47,48]. These roots could have emerged along more than four radii at the same level within the shoot.…”

The Use of Auxin Quantification for Understanding Clonal Tree Propagation

“…Chromatographic methods can provide excellent hormone separation and, when coupled with mass spectrometry (MS), allow quantification of compounds from multiple hormone classes in the same sample [69,106]. These methods have greatly improved endogenous auxin identification in forestry species [107][108][109][110].…”

“…IAA levels in the cutting base and apex remained constant and were similar to those in intact seedlings, indicating that root initiation can occur without an increase in the IAA levels in the root regeneration zone [45]. Differences in auxin transport capacity may contribute directly to success in the rhizogenesis process [108], with applied auxin possibly being transported more rapidly in easy-to-root plants [141].…”

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

“…Furthermore, most eucalypt micropropagation protocols, including that of Mokotedi et al (2010), utilise one or more auxin analogues to induce roots in vitro (Jones and Van Staden, 1997;Watt et al, 2003). However, Nakhooda et al (2011) reported that the choice of auxin, along with its relative stability and concentration used in vitro, influences several aspects of root development, including post-acclimatisation root architecture. Those authors found that IAA (indole-3-acetic acid) was integral to the rooting process, regardless of the presence of other auxin analogues, such as IBA (indole-3-butyric acid) or sensitivity to exogenous auxin.…”