Using targeted expression of a constitutively active repressor of GA signalling Susana Ubeda-Tomas et al., (2007) demonstrated that GA action in endodermal cells is necessary for correct root growth. However, GUS studies have shown the final and penultimate GA-biosynthetic genes are not expressed in the endodermis, indicating movement of GAs may be required. This study used the targeted mis-expression of GA degrading enzymes in Col-0 and the attempted targeted rescue of GA biosynthetic and signalling mutants, using the corresponding GA metabolic or signalling component, to gain an insight into the localisation of important GA biosynthesis and signalling sites. This study has demonstrated that GA12 can be made by epidermal, cortical and endodermal cells. However, the ground tissue of the elongation zone does not contain GA12 due to the early GA biosynthetic enzymes only being expressed within cells with a close proximity to the QC. Subsequently the 20-oxidation converts GA12 to GA15, to GA24 to GA9. These reactions mobilise GA allowing it to move from the meristematic region to the elongation zone. GA20ox and GA3ox activity is required in both the meristematic region and the elongation zone for correct root growth to occur. In addition, GA metabolic components are subject to tissue specific GA feedback regulation as a result of post-transcriptional processing and/or post-translational modifications to their protein stability. GA perception in any tissue of the elongation zone can promote complete cell elongation, suggesting that any one tissue can elongate it neighbours, or that each cell is capable of releasing a signal to ensure they all elongate proportionally. The transcriptional network within the endodermis has a disproportionately important role in GAs regulation of cell division within the root proximal meristem but GA action in other cell types is also required. The cambian and bundle sheath cells in aerial tissue like the endodermis in the root contain an important transcriptional network that promotes GA dependant growth.

The formation of lateral roots is an important post-embryonic developmental process that allows plants to adapt to their environment via exploitation of soil mineral resources. New lateral roots initiate as lateral root primordia (LRP) in the pericycle cell layer adjacent to the central vascular tissue in the primary root, and must pass through the outer cell layers of endodermis, cortex and epidermis to emerge as mature roots. A key regulator of emergence is the plant hormone auxin and it has been shown previously that in Arabidopsis the auxin induced expression of the auxin influx carrier LAX3 in specific cortical cells over LRP is required for emergence to occur, as this leads to the expression of cell wall remodelling enzymes such as polygalacturonase (PG). By developing mathematical models of auxin transport and LAX3 expression the work in the thesis aims to test the existing conceptual models for lateral root emergence, and provide testable hypotheses for the existence of additional gene regulatory components. An initial single cell model demonstrates that hysteresis and bistability may explain the experimentally observed 'all-or-nothing' LAX3 spatial expression pattern in cortical cells containing a gradient of auxin concentrations. By fitting model parameters against experimental data, the model is then used to show that some auxin homeostasis mechanism is present, with both endogenous and exogenous sources of homeostasis investigated. The single cell model also investigates the validity of several alternative gene regulatory networks for LAX3, and its apparent repression by a key mediator of the auxin response, ARF19. Finally, the model is extended to a multicellular context, in which the auxin distribution from a simulated LRP source cell is used as a basis for the expression of LAX3, leading to the expression of PG in specific cells between which the LRP must pass.