Ligand-receptor-like kinase signalling in Arabidopsis root development

Food security represents a major global issue, and it was put forward by the crop scientist Jonathan Lynch that a ‘second green revolution’ should be a priority. This should focus on root architecture, since root branching critically influences nutrient and water uptake efficiency. However, root architecture has not been a trait selected for by plant breeders in major cereal crops. In addition, in multicellular organisms, cell-cell communication plays a crucial role in patterning, growth and interaction with the environment. For example, a tight control of formative cell divisions – which are mainly asymmetric (stem) cell divisions – to produce tissues and organs and to prevent over-proliferation is crucial for the postembryonic growth and development of plants. This control of formative cell divisions is not only critical in plants, but also in animals to prevent irregular divisions or cancers.

We recently identified a protein, the membrane-associated receptor-like kinase ACR4, which is crucial in the control of root architecture and which is a key regulator of formative cell divisions in the root of the model plant Arabidopsis thaliana. ACR4 has originally been described in maize for its role in other processes, and provides an obvious starting point for the detailed analysis of the role of receptor-like kinase signaling pathways in root architecture and in registering and conveying positional information.

This research proposal aims to further characterize this ligand/receptor signaling mechanism that can integrate mobile signaling molecules to control formative cell divisions during organogenesis and, as such, provides an excellent tool to study short range, cell-cell communication during growth and development. First, we will identify those transcriptional changes that affect cell identity in the main and lateral root downstream of a ligand-driven cue from neighbouring cells. These data will provide us with a large number of transcriptional markers for other experiments. Second, unequivocal identification of the ligand is crucial to fully understand receptor-like kinase-mediated signaling pathways. One important aspect is how ligand-dependent directional cues from surrounding cells modulate receptor-like kinase signaling. We will identify and localize ligands for the ACR4 receptor-like kinase. Third, the three dimensional structure of eukaryotic membrane proteins has only been determined very limitedly. We will determine the folding and structure of purified receptor domain protein and use that for detailed analyses. Fourth, temporary and reversible phosphorylation of proteins is one of the most important post-translational modifications with a regulatory function, such as the control of transcriptional changes. Therefore, we will identify the substrates of ACR4 during main and lateral root development. Sixth, with the knowledge that we will gain on ligand-receptor-like kinase signaling and what is known about hormone signaling and transcriptional networks in the same processes, we will build a comprehensive model.