The plant hormones gibberellins (GAs) and auxin display overlapping activities in the regulation of multiple developmental processes, including the differential growth that mediates the response to tropic stimuli and the formation of the apical hook. Several mechanisms have been proposed that explain the interaction between these two hormones, such as the regulation of auxin transport by GAs, and the regulation of GA biosynthesis by auxin. GAs are known to exert their action at the transcriptional level by promoting the degradation of DELLA proteins, which in turn interact with numerous transcription factors and modulate their activity. We have identified INDOLE-3-ACETIC ACID METHYLTRANSFERASE 1 (IAMT1) as one of the earliest target genes upregulated after conditional expression of the DELLA protein GAI in Arabidopsis thaliana. In this Thesis, we have addressed two main issues: (1) the contribution of IAMT1 to auxin homeostasis and its biological relevance; and (2) the molecular mechanism by which DELLAs are able to induce the expression of IAMT1. Using combinations of iamt1 loss-of-function mutants and reporter lines for auxin accumulation and activity, we have found that IAMT1 activity is essential for proper generation and maintenance of the auxin gradients that underlie differential growth. According to our results, the role of IAMT1 would be to restrict polar auxin transport especially during the response to tropic stimuli, preventing excessive auxin accumulation in the responding tissues, and IAMT1 exerts this function, at least in part, by inhibiting the expression of the PIN genes, encoding auxin efflux carriers. Regarding the regulation of IAMT1 expression by DELLAs, dissection of the promoter, in silico analysis of putative DELLA partners, and molecular genetic analysis of reporter lines has allowed us to identify two mechanisms with different relevance depending on the environmental conditions, and through different cis elements. In etiolated seedlings, DELLA proteins are recruited by DORNRÖSCHEN (DRN) to the IAMT1 promoter to induce IAMT1 expression. In the light and in a temperature-dependent manner, DELLA proteins inhibit the DNA-binding activity of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and BRI1 EMS-SUPPRESSOR1(BES1), which act as repressors of IAMT1 expression. The work presented here highlights how GAs may affect local accumulation of auxin, being particularly relevant in processes that involve differential growth. / Abbas, M. (2014). The control of auxin homeostasis through the regulation of IAMT1 by DELLA proteins [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/39348

Growth and development of plant body is dependent on correct and effective integration of information about current deployment of its body parts, as well as on perception and transduction of inputs from environment. Multiple developmental processes within plant body are determined by specific and tightly controlled distribution of molecule with unique signaling mission within plant development - auxin. Spatial distribution of auxin is co-determined by plethora of tightly controlled processes, and the polar auxin transport plays unique role among them. PIN proteins are the plant-specific family of secondary transporters driving movement of auxin across membranes. With their frequent asymmetrical localization within cells, specific expression patterns in developing tissues and their reactiveness to external cues they secure unique, dynamic and asymmetric distribution of auxin within the plant body. This dissertation thesis is focused on characterization of the role different PIN proteins play in determining cellular auxin homeostasis and consequent formation of auxin gradients. Controlled overexpression of PIN proteins in tobacco cells showed, that PIN4 and to some extent also PIN6, function as the direct auxin efflux carriers. In the cellular auxin transport they play the role analogous to other...