[2016-09-23] PNAS has published this week at the same time three articles that focus on one enzyme, the dioxygenase for auxin oxidation 1 (DAO1). This enzyme catalyses the oxidation and thereby inactivation of auxin, a plant hormone important for the regulation of plant growth and development. Researchers from Umeå Plant Science Centre contributed to two of these articles while the third is published by a separate research group.

The plant hormone auxin is very crucial for regulating plant growth and development. The shape of the plant as well as the function of tissues and cells is controlled by auxin. It acts on cell division, elongation and differentiation and directs like this for example the growth of a plant towards the light. The spatial distribution of auxin within the plant is very inhomogeneous to allow this directed growth. It is regulated by complex interactions between different pathways for auxin transport, signalling and metabolism.

Three mechanisms control auxin metabolism: auxin biosynthesis, degradation and conjugation, i.e. the binding of auxin to amino acids or sugars. Enzymes involved in biosynthesis and conjugation of auxin are already well characterised but little was known about the enzyme which catalyses the degradation of auxin by oxidation. The three PNAS articles show for the first time that DAO1 (dioxygenase for auxin oxidation 1) is the functional auxin oxidising enzyme in Arabidopsis and they provide altogether a very detailed characterisation of this enzyme.

Ljung KarinPhoto: Mattias Pettersson
The team around Karin Ljung from the Umeå Plant Science Centre (UPSC) could show that mutants with reduced DAO1 activity have increased levels of auxin conjugates. These conjugates are considered to be storage forms of auxin with low biological activity. The researchers concluded that the missing function of DAO1 for inactivating auxin is compensated by an increased activity of auxin conjugating enzymes. This is important for keeping the auxin homeostasis balanced.

The groups of Markus Owen and Malcolm Bennett from Nottingham University chose a systems biology approach. They developed in cooperation with Karin Ljung’s group a mathematical model of auxin metabolic pathways based on experimental data. The researchers predicted with the help of their model that the auxin concentration in the DAO1 mutant is elevated in a special zone in the root tip. The root hairs in this mutant were longer than normal and the prediction gave the explanation for these findings.

These findings were complemented by results from a third team, the group of Wendy Ann Peer from the University of Maryland. They analysed the localization of DAO1 within mutant and control plants in different tissues and developmental stages and they measured the DAO1 enzyme activity. Their conclusion is that DAO1 is indeed the main enzyme catalysing auxin oxidation under normal growth conditions and that it is important for fine tuning internal auxin levels.
The detailed characterisation of DAO1 on different levels constitutes a breakthrough for studying auxin degradation and its influence on other auxin related pathways. By changing specifically DAO1 activity, the internal pool of active auxin can be modified. These new tools offer new possibilities to analyse the complex interaction between auxin metabolism, transport and signalling and will help to understand better how auxin is controlling plant growth and development.

You can find the original articles in PNAS here: 

Karin Ljungs article: 
Dioxygenase-encoding AtDAO1 gene controls IAA oxidation and homeostasis in Arabidopsis

Malcolm Bennett's and Markus Owen's article: 
Dynamic regulation of auxin oxidase and conjugating enzymes AtDAO1 and GH3 modulates auxin homeostasis

Wendy Ann Peer's article: 
DAO1 catalyzes temporal and tissue-specific oxidative inactivation of auxin in Arabidopsis thaliana

For more information, please contact:

Karin Ljung
+46 (0)90 786 8355
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