CHARACTERISATION OF AUXIN RESPONSE FACTOR 5/MONOPTEROS POST-TRANSCRIPTIONAL AND POST-TRANSLATIONAL REGULATIONS DURING ARABIDOPSIS THALIANA DEVELOPMENT

Alternative splicing generates a MONOPTEROS isoform required for ovules development Auxin is one of the main designers of plant development and organs differentiation and its function relies on its differential accumulation with the generation of points of high and low hormone content. At cellular level, auxin signaling impacts on the transcriptomic landscape thought the modulation of the interplay between AUX/IAA repressor proteins and AUXIN RESPONSE FACTOR transcriptional factors. AUX/IAAs and ARFs compose a combinatory system in which AUX/IAAs binding to ARFs negatively regulates their ability to activate the transcription of downstream targets. Upon auxin sensing, AUX/IAAs stability is compromised thought their direct ubiquitination and degradation, allowing transcriptional activator ARFs to promote gene expression. Among transcriptional activator ARFs, ARF5/MONOPTEROS (MP) has been shown to be a key integrator of auxin signaling for many developmental events, thought the activation of the transcription of target genes above auxin threshold concentration. We show that in ovules, MP is expressed and functional in domains of low auxin content. We described that MP post-transcriptional regulations are involved in this process thought the generation of a biologically functional mRNA splicing variant, whose translation generates a protein isoform lacking the AUX/IAA interaction domain. Indeed, the alternative MPint11 isoform was able to partially complement mp mutants defects and ovule developmental process. Interestingly, also the canonical MP protein results partially functional for complementation of ovules formation, with a similar behavior to the one observed for MPint11. Our results suggest that MP functions relies on the combined activity of both its isoforms with the evidence that, during ovules development, MP domains involved in AUX/IAAs binding are not strictly required. These findings describe a novel scenario in which, not only MP is able to work uncoupled from the classical auxin signaling model, but also in which post-transcriptional regulation controls the formation of a functional isoform required for correct ovules development.