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.