Dissecting the GA regulation of cell expansion in the root Arabidopsis thaliana

Gibberellins (GAs) represent an important class of hormonal signal that regulate growth and developmental processes during the plant life cycle. GA promotes growth through the targeted degradation of the nuclear localized DELLA repressor proteins via the ubiquitin proteasome pathway. Whilst DELLAs do not appear to bind directly to DNA, recent evidence suggests that they interact with several different classes of transcription factors to control the expression of downstream genes in a GA-dependent manner. In order to pinpoint the genes targeted by GA to promote root growth, several genetic approaches have been pursued in this thesis. These approaches took advantage of the previous observation that targeting expression of a steroid regulated non-degradable form of DELLA in endodermal cells (using the SCR:gai-GR transgene) blocked root elongation (Ubeda-Tomas et al., 2008, 2009). The SCR:gai-GR line was initially mutagenized to select mutants that no longer exhibit steroid-inducible root growth inhibition. Several mutant lines have been selected, characterised and subjected to next-generation sequencing to reveal whether they disrupt novel downstream components of the GA signalling pathway. The SCR:gai-GR line has also been used in transcriptomic studies and a number of novel downstream targets identified for functional characterisation. Finally, several GA-regulated genes encoding cell wall modifying enzymes belonging to the xyloglucan endotransglucosylase/ hydrolase (XTH) family have been functionally characterised. Multiple XTH mutant combinations exhibit root elongation defects and altered cell expansion dynamics, hence providing new insight into how GAs may regulate cell wall remodelling enzymes to promote root cell expansion.