Auxin biosynthesis and homeostasis in Arabidopsis thaliana in relation to plant growth and development

摘要

The auxin indole-3-acetic acid (IAA) is a growth regulating substance important for many developmental processes during the life cycle of plants. The papers presented in this thesis address different aspects of IAA biosynthesis, metabolism and transport. The model plant Arabidopsis thaliana was used for most of the studies, but some studies were also performed on Scots pine (Pinus sylvestris). We developed very sensitive and selective mass spectrometric analytical techniques that made it possible to perform tissue specific IAA quantification and IAA biosynthesis rate measurements on small amounts of plant tissue. We observed that seeds utilised stored IAA (in the form of ester- and amide-linked conjugates) for elongation growth during the initial germination phase. IAA biosynthesis and catabolism were initiated later in the germinating seedling, and these processes appear to be tightly regulated in order to maintain IAA homeostasis in the developing tissues. High concentrations of IAA were observed in young developing leaves and tissues with high rates of cell division. Perturbation in the IAA concentration within the leaf lowered leaf expansion, and feedback inhibition of IAA biosynthesis was observed after NPA treatment to block polar auxin transport. The youngest developing leaves exhibited the highest IAA biosynthesis rates, but all parts of young seedlings, including the root, showed IAA synthesis capacity. We demonstrated that transport of IAA from the shoot to the root is essential for the emergence of lateral root primordia, and that a basipetal IAA gradient is present in the root tip. We also showed that this gradient is probably generated by the cellular localisation of auxin influx and efflux carriers, directing the flow of auxin coming from the aerial parts of the plant to specific cell types within the root tip. In addition to IAA synthesised in the shoot and then transported to the root system via polar auxin transport and/or transport in the phloem, we demonstrated that a source of IAA is located within the root tip.