Auxin plays a pivotal role in various plant growth and development processes, 
including vascular differentiation. The modulation of auxin responsiveness 
through the auxin perception and signaling machinery is believed to be a major 
regulatory mechanism controlling cambium activity and wood formation. To gain 
more insights into the roles of key Aux/IAA gene regulators of the auxin 
response in these processes, we identified and characterized members of the 
Aux/IAA family in the genome of Eucalyptus grandis, a tree of worldwide economic 
importance. We found that the gene family in Eucalyptus is slightly smaller than 
that in Populus and Arabidopsis, but all phylogenetic groups are represented. 
High-throughput expression profiling of different organs and tissues highlighted 
several Aux/IAA genes expressed in vascular cambium and/or developing xylem, 
some showing differential expression in response to developmental (juvenile vs. 
mature) and/or to environmental (tension stress) cues. Based on the expression 
profiles, we selected a promising candidate gene, EgrIAA4, for functional 
characterization. We showed that EgrIAA4 protein is localized in the nucleus and 
functions as an auxin-responsive repressor. Overexpressing a stabilized version 
of EgrIAA4 in Arabidopsis dramatically impeded plant growth and fertility and 
induced auxin-insensitive phenotypes such as inhibition of primary root 
elongation, lateral root emergence and agravitropism. Interestingly, the 
lignified secondary walls of the interfascicular fibers appeared very late, 
whereas those of the xylary fibers were virtually undetectable, suggesting that 
EgrIAA4 may play crucial roles in fiber development and secondary cell wall 
deposition.