Tomato Aux/IAA3 and HOOKLESS are important actors of the interplay between auxin and ethylene during apical hook formation

Plants implement differential cell growth as an adaptation process in order to direct their development in a way that allow them to better cope with the environmental conditions. This process requires the complex integration of multiple hormone signalings, though, a lot remain to known about the mechanisms and the molecular actors take part in this hormonal dialogue. We have previously shown that Sl-IAA3, an Aux/IAA gene, is a molecular link between auxin and ethylene responses in tomato plants. We show here that the expression of Sl-IAA3 in etiolated seedlings is restricted to the inner side of the apical hook, opposite to that of the HOOKLESS gene whose loss-of-function mutation results in the absence of hook formation. We propose a model on how auxin and ethylene modulate the expression of Auxin Response Factor 2 (ARF2) via IAA3 and HLS protein to regulate hypocotyls bending.