Understanding the biological responses to ionizing radiation in plants after exposure and recovery: molecular profiling of Arabidopsis thaliana and Oryza sativa

Ionizing radiation (IR) is always present in the environment as it is constantly being emanated from natural sources (such as cosmic rays and naturally occurring radionuclides embedded within the earth’s crust) and man-made sources (such as controlled releases from the medical industry and accidental releases from nuclear power plants). The recent anthropogenic-induced increase of radioactive elements in nature has augmented the risk of exposure of living organisms. Since plants have a sessile lifestyle, they cannot escape the potentially harmful conditions. Previous studies have demonstrated that IR can affect plant growth. At the cellular level, excess production of reactive oxygen species (ROS) can be triggered in plants, which if not regulated results in the induction of oxidative stress. Excess ROS can, if not regulated properly, cause damage to important biomolecules such as DNA, proteins and membrane lipids. Nevertheless, plants are equipped with robust antioxidative defence and repair mechanisms that are activated following suitable alterations in gene expression by molecules such as the small non-coding RNAs including microRNAs (miRNAs). Despite knowledge on the responses elicited in plants shortly after exposure to IR, there is, so far, very little information on the effects elicited when plants are allowed to recover from irradiation. Such recovery studies are vital as they may help to not only determine the long-term consequences of radiation exposure in plants, but also, provide more insight into the mechanisms underpinning plant responses to IR. To this end, the first part of this study set out to investigate both irradiation and recovery responses in 1-week old Arabidopsis seedlings that were exposed to different gamma dose rates (27.2 mGy/h, 48.8 mGy/h and 103.5 mGy/h) for 2 weeks and allowed to recover for 4 days (Chapter 3). Two endpoints: rosette biomass and antioxidative capacity, were compared at the plant and metabolic levels, respectively. At the molecular level, a high-throughput mRNA sequencing analysis was carried out to gain more insight into the mechanisms triggered. From this study, it was established that Arabidopsis plants are able to recover from gamma radiation in a dose rate-dependent manner and that these plants employ different strategies at the molecular level to ensure survival both during irradiation and during recovery thereafter. Furthermore, the observed induction of early flowering during recovery in irradiated compared to control plants coupled with the differential expression of some senescence and flowering genes at the molecular level confirmed the hypothesis that early aging is induced in plants following gamma irradiation, which in the long run has an impact on development. This study also aimed at comparing the differences in radiosensitivity between Arabidopsis and the economically important rice (Oryza sativa) since the former was considered to be less sensitive to radiation compared to the latter. Therefore, responses elicited in rice plants following irradiation and recovery were also investigated by exposing 1-week old seedlings to different gamma dose rates that were comparable to those used for Arabidopsis. Rice plants were irradiated for 2 weeks and allowed to recover for another 2 weeks and sampling was done according to leaf age (Chapter 4). Leaf 4 was sampled after irradiation because it emerged during the treatment period and as a follow up, the same leaf 4 was sampled after the recovery period. In addition, leaf 7 was sampled after the recovery period as it developed during this time, thus, unlike leaf 4, the elongating/maturing cells of leaf 7 were not directly exposed to gamma radiation. The results obtained following the analysis of various endpoints in leaf 4 such as plant biomass, antioxidative potential, lignin content and the expression profiles of some important rice genes (antioxidative stress gene transcripts and DNA damage repair genes) revealed that rice plants are also able to recover from gamma radiation through the establishment of a new homeostasis. On the other hand, in leaf 7 of recovering plants, there was a radiation-induced signature even though the maturing cells of this leaf were not directly irradiated. As a result, it was proposed that a signalling mechanism comes into play during plant recovery that enables the establishment of a systemic acquired acclimation, which may serve to protect the plant in the event of future exposure to IR. The mRNA sequencing analysis conducted on Arabidopsis (Chapter 3) revealed that numerous genes were differentially expressed following irradiation and recovery. Gene expression changes have been shown to be coordinated by small non-coding RNA molecules referred to as microRNAs. So far, very few studies have identified gamma radiation-responsive miRNAs and furthermore, no efforts have been made to determine the miRNA expression profile in plants that have been allowed to recover from irradiation. Therefore, in the final part of this study, gamma radiation-responsive microRNAs in rice plants and the regulatory roles they play in mediating the responses elicited in irradiated and recovering plants, were investigated (Chapter 5). A small RNA sequencing analysis was carried out on libraries prepared from leaf 5 of irradiated and leaf 5 (follow-up) and 7 (maturing cells not directly irradiated) of recovering rice plants that had been exposed to two different gamma dose rates (48 and 107 mGy/h) for 2 weeks and allowed to recover for 2 weeks. Based on the differential expression of several rice miRNAs, this study showed for the first time that miRNAs play an essential role in regulating gene expression in both irradiated and recovering plants. In conclusion, the results of this PhD study demonstrate that both Arabidopsis and rice plants are able to recover from radiation exposure through overall molecular influence of growth, development and the physiological status. We also demonstrate that, in contrast to a statement in literature, rice plants, under the current experimental conditions used, are not more radiosensitive compared to Arabidopsis. Taken together, the information gained from this study will contribute towards a more comprehensive mechanistic understanding of IR-induced effects in living systems, which will allow for the strengthening of current legislation that has been formulated towards radiation protection of the environment.

Publication year:2019

Keywords:Ionizing radiation, Responses, Recovery, Arabidopsis, Rice

Accessibility:Embargoed