Abstract:
Auxin is a crucial phytohormone regulating multiple aspects of plant growth and diferentiation, including regulation of vascular cambium activity, xylogenesis and its responsiveness towards gravitropic stress. Although the regulation of
these biological processes greatly depends on auxin and regulators of the auxin signalling pathway, many of their specifc
functions remain unclear. Therefore, the present study aims to functionally characterise Eucalyptus grandis AUX/INDOLE3-ACETIC ACID 13 (EgrIAA13), a member of the auxin signalling pathway. In Eucalyptus and Populus, EgrIAA13 and its
orthologs are preferentially expressed in the xylogenic tissues and downregulated in tension wood. Therefore, to further
investigate EgrIAA13 and its function during xylogenesis, we conducted subcellular localisation and Induced Somatic Sector Analysis experiments using overexpression and RNAi knockdown constructs of EgrIAA13 to create transgenic tissue
sectors on growing stems of Eucalyptus and Populus. Since Aux/IAAs interact with Auxin Responsive Factors (ARFs), in
silico predictions of IAA13-ARF interactions were explored and experimentally validated via yeast-2-hybrid experiments.
Our results demonstrate that EgrIAA13 localises to the nucleus and that downregulation of EgrIAA13 impedes Eucalyptus
xylem fbre and vessel development. We also observed that EgrIAA13 interacts with Eucalyptus ARF2, ARF5, ARF6 and
ARF19A. Based on these results, we conclude that EgrIAA13 is a regulator of Eucalyptus xylogenesis and postulate that
the observed phenotypes are likely to result from alterations in the auxin-responsive transcriptome via IAA13-ARF modules
such as EgrIAA13-EgrARF5. Our results provide the frst insights into the regulatory role of EgrIAA13 during xylogenesis.
