Understanding biological responses to ionising radiation in plants: a study with Lemna minor as a new molecular model system
9 March 2016
UAntwerp, Campus Middelheim, A.143 - Middelheimlaan 1 - 2020 Antwerp
Arne Van Hoeck
Ronny Blust & Nele Horemans
PhD defence Arne Van Hoeck - Faculty of Science, Department of Biology
All living organisms, including plants, are continuously exposed to low levels of ionising radiation. Anthropogenic activities on natural radionuclides can lead to higher levels of ionising radiation in the environment which can be harmful for organisms. Studies focusing on short-term treatments already revealed that high levels of ionising radiation could induce adverse effects. However to provide a broader understanding and to predict the possible effects due to ionising radiation in the environment, dose rate dependent responses of long-term irradiation exposures need to be unraveled on different levels of biological complexity.
This thesis aimed at improving the understanding of stress responses due to chronic ionising radiation of both γ- and β-radiation on multiple levels of biological complexity in vascular plants. The floating plant Lemna minor was chosen as model system since this plant species is widely used as test organism in ecotoxicological research. Since a molecular platform was absent for this plant, we first sequenced and annotated the full genome of L. minor.
Results revealed that both chronic γ- and β-radiation exposures significantly affected L. minor growth and development. After exposure to different dose rates levels, two distinct dose rate dependent phases in the response could be observed. Dose rates levels up to 232 mGy h-1 led to an acclimation response towards ionising radiation including a reduction of the root length coupled to an upregulation of genes related to biosynthesis pathways of flavonoids and a lack of evidence for oxidative stress. Such stress resistance and acclimation has been referred to as 'eustress'. In contrast, at a dose rate level of 423 mGy h-1 pronounced growth reduction was observed and an overwhelming amount of genes related to oxidative defense, DNA repair and cell cycle regulation were differentially transcribed.
Such physiological shift is referred to as 'distress' at which strong stress conditions cause metabolic damage. Although responses were observed at dose rate levels exceeding environmental relevant exposure conditions, it has been shown that in L. minor plants several mechanisms and pathways interplay to cope with stress induced by chronic exposures of ionising radiation.