Functional characterization of tomato Sl-IAA3 and Sl-hls genes‎. Role in auxin and ethylene cross-talk

Abstract

Plant development and survival depend on the ability of these organisms to integrate many signalling which enables them to produce an appropriate response. Ethylene and auxin are phytohormones known to regulate agonistly or antagonistly many processes of plant development but yet the key integrating molecular players remain largely undiscovered. My Ph.D project deals with the identification and characterization of molecular actors that take part in this dialogue. We report that Sl-IAA3, a member of the tomato auxin/indole-3-acetic acid (Aux/IAA) gene family, is an intersection point between auxin and ethylene signal transduction pathways. Aux/IAA genes encode short-lived transcriptional regulators that mediate auxin responses. Sl-IAA3 expression is controlled by both auxin and ethylene and is regulated on a tight tissue-specific basis. Downregulation of Sl-IAA3 via an antisense strategy results in auxin and ethylenerelated phenotypes including altered apical dominance, lower auxin sensitivity, exaggerated apical hook curvature in the dark and reduced petiole epinasty in the light. These ethylene-related phenotypes in the antisense tomato lines (AS-IAA3) position Sl-IAA3 firmly at the crossroads between auxin and ethylene signalling in tomato. The induction of apical hook offers an excellent system to study auxinethylene interplay. In Arabidopsis, ethylene acts through HOOKLESS (HLS1) to control hook formation through modulating differential cell elongation in opposite sides of the hook. Loss of function mutation in the HLS1 gene results in the absence of hook even in the presence of exogenous ethylene. In the present study, we extended the phenotypes of the Arabidopsis hls1 mutant to alteration of light sensitivity, glucose and ABA tolerance and gravitropic growth thus uncovering the importance of HLS gene in the integration of multiple signalling pathways. Two functional tomato hookless genes (Sl-HLS1 and Sl-HLS2) were isolated in this study and shown to positively complement the Arabidopsis hls1 mutant. Expression of Sl-HLS2 in the hook is restricted to the outer face, opposite to Sl-IAA3 whose expression is localized in the inner face of the hook curvature. The data suggest that Sl-HLS2 and Sl-IAA3 exert antagonist control of cell elongation in the inner and outer part of the apical hook. Overall, the two genes characterized in this study open new prospects towards addressing the role of ethylene and auxin cross-talk during fruit development and ripening.