Forever young: how AHL15 delays developmental phase transitions to prevent ageing in plants

The AT-Hook Nuclear Localized 15 (AHL15) gene belongs to a plant-specific gene family of DNA-binding proteins. Overexpression of AHL15 in Arabidopsis thaliana delays developmental phase transitions whereas its loss of function causes a slight acceleration. In this thesis, the regulation of developmental phase transitions and the current knowledge on AHL genes is described in chapter 1. Chapter 2 describes the relation between the aerial rosette phenotype in several Arabidopsis ecotypes and the expression of AHL15, its close relative AHL20 and the genes FLC, SOC1, and SPL15. The results of chapter 2 show that the expression of FLC, SOC1, and AHL20 predict aerial rosette formation. In chapter 3, a simple method of chlorophyll quantification based on digital images is presented, which is applied in chapter 4 to study how AHL15 represses leaf senescence. Chapter 4 describes how AHL15 directly represses leaf senescence by regulating expression of ORESARA1 and several CYTOKININ OXIDASE genes, resulting in a delay in cytokinin degradation and subsequently a delay in leaf senescence. Finally, chapter 5 investigates the genome-wide binding sites of AHL15, and shows that AHL15 can modulate gene expression without changing chromatin accessibility on a local scale, most likely via affecting the 3D structure of chromatin.<br>