Distribution and Regulation of Auxin in Arabidopsis Root Cells

The plant hormone auxin (IAA) coordinates many of the important processes in plant development. For example, IAA is critical for normal embryogenesis, root development, cell elongation, and the tropic responses such as gravitropism and phototropism. IAA gradients are established and maintained in many tissues and it is thought that these gradients act as developmental cues, determining the fate of cells and tissues. Descriptions of auxin distribution patterns with cellular resolution have previously been based solely on visual characterisation of auxin responsive reporter constructs. Measurements of the hormone gradient itself have been made in cambial sections of hybrid aspen and Pine, although even in this study, the measurements were not truly at cellular resolution. In this thesis, I describe the measurement of IAA levels in the Arabidopsis root with cellular resolution. By combining the use of Arabidopsis lines expressing cellspecific GFP, Fluorescent Activated Flow Cytometry, and high-resolution mass spectrometry, I have provided the first analytical evidence for the presence of an IAA gradient in the Arabidopsis root apex. The work has enabled the development of an auxin distribution map for the root with cellular resolution. The IAA content of cells is actively regulated in order to maintain the IAA gradient. This regulation has been shown to be achieved through a combination of active transport and de novo biosynthesis. It has been proposed that IAA catabolism and conjugation also contribute to the regulation of hormone levels. In this thesis, I present data supporting the hypothesis that catabolism, by converting IAA into its major inactive catabolite oxIAA, contributes to the regulation of the levels of free IAA. I also present data showing a role for the plant hormone cytokinin in the dynamic regulation of IAA biosynthesis in response to changing developmental and environmental conditions.