Comparative study of molecular mechanisms underlying Arabidopsis and cereals root architecture

Plant roots are required to anchor the plant in the soil, acquire water and nutrients and respond to biotic and abiotic stimuli. Root branching contributes largely to the morphological plasticy of the root system. Auxin is an important plant hormone involved in lateral root (LR) development and root gravitropism. In this thesis, an in silico translational approach was employed to identify selected auxin-related genes in barely. We focused our work on auxin transporters that are involved in plant response to environmental stimuli. AUX1 and LAX3 were identified in barley and subsequently characterised. These experiments revealed the maintenance of AUX/LAX developmental function between highly divergent plant species. Reactive oxygen species (ROS) were recently proposed to contribute to auxin-mediated LR formation, however the nature of their involvement remains elusive. We show that H2O2 accumulates in middle lamellae of cells overlying LR primordia and progressively creates a fine layer around the emerging LR primordia. Our results lead us to propose that ROS contribute to LR emergence by influencing cell wall plasticity. We finally show that the Respiratory burst oxidase homologs (Rboh) gene family are likely contributors to a fine-tuned extracellular ROS balance during LR emergence. Abscisic acid (ABA) is another plant hormone known to modulate LR development and to regulate responses to environmental stimuli. We demonstrated that ABA response pathways act downstream of water deficit in the acropetal root zone and that ABA is likely to mediate LR repression. Those results lead to a model of root branching adaptation to local soil porosity