Chronic Ionizing Radiation of Plants: An Evolutionary Factor from Direct Damage to Non-Target Effects.

In present times, the levels of ionizing radiation (IR) on the surface of Earth are relatively low, posing no high challenges for the survival of contemporary life forms. IR derives from natural sources and naturally occurring radioactive materials (NORM), the nuclear industry, medical applications, and as a result of radiation disasters or nuclear tests. In the current review, we discuss modern sources of radioactivity, its direct and indirect effects on different plant species, and the scope of the radiation protection of plants. We present an overview of the molecular mechanisms of radiation responses in plants, which leads to a tempting conjecture of the evolutionary role of IR as a limiting factor for land colonization and plant diversification rates. The hypothesis-driven analysis of available plant genomic data suggests an overall DNA repair gene families' depletion in land plants compared to ancestral groups, which overlaps with a decrease in levels of radiation exposure on the surface of Earth millions of years ago. The potential contribution of chronic IR as an evolutionary factor in combination with other environmental factors is discussed.

White clover from the exclusion zone of the Chernobyl NPP: Morphological, biochemical, and genetic characteristics.

A comprehensive study of the biological effects of chronic radiation exposure (8 µGy/h) in populations of white clover (Trifolium repens L.) from the Chernobyl exclusion zone was carried out. White clover is one of the most important pasture legumes, having many agricultural applications. Studies at two reference and three radioactively contaminated plots showed no stable morphological effects in white clover at this level of radiation exposure. Increased activities of catalase and peroxidases were found in some impacted plots. Auxin concentration was enhanced in the radioactively contaminated plots. Genes involved in the maintenance of water homeostasis and photosynthetic processes (TIP1 and CAB1) were upregulated at radioactively contaminated plots.

Differential gene expression in chronically irradiated herbaceous species from the Chernobyl exclusion zone.

PURPOSE: Transcriptional activity of genes related to ionizing radiation responses in chronically irradiated plant populations at radioactively contaminated territories can be a cost-effective and precise approach for stress response evaluation. However, there are limits to studying non-model plants in field conditions. The work studies the transcriptional activity of candidate genes of adaptation to chronic radiation exposure in plant populations from radioactively contaminated territories of the Chernobyl. MATERIALS AND METHODS: In this work, we studied plant species with different sensitivity to acute irradiation: Trifolium repens L., Taraxacum officinale Wigg., and Dactylis glomerata L., sampled in the Chernobyl exclusion zone. The differential expression of several candidate genes of adaptation to chronic radiation exposure in the leaves of these species was analyzed, including homologs of Arabidopsis thaliana genes SLAC1, APX1, GPX2, CAB1, NTRB, PP2-B11, RBOH-F, HY5, SnRK2.4, PDS1, CIPK20, SIP1, PIP1, TIP1. RESULTS AND CONCLUSIONS: All studied species were characterized by upregulation of the CAB1 homolog, encoding chlorophyll a/b binding protein, at radioactively contaminated plots. An increase in the expression of genes associated with water and hydrogen peroxide transport, intensity of photosynthesis, and stress responses (homolog of aquaporin TIP1 for T. repens; homologs of aquaporin PIP1 and transcription factor HY5 for D. glomerata; homolog of CBL-interacting serine/threonine protein kinase CIPK20 for T. officinale) was revealed. The methodological approach for studying gene expression in non-model plant species is described, which may allow large-scale screening studies of candidate genes in various plant species abundant in radioactively contaminated areas.

A Pipeline for Non-model Organisms for de novo Transcriptome Assembly, Annotation, and Gene Ontology Analysis Using Open Tools: Case Study with Scots Pine.

RNA sequencing (RNA-seq) has opened up the possibility of studying virtually any organism at the whole transcriptome level. Nevertheless, the absence of a sequenced and accurately annotated reference genome may be an obstacle for applying this technique to non-model organisms, especially for those with a complex genome. While de novo transcriptome assembly can circumvent this problem, it is often computationally demanding. Furthermore, the transcriptome annotation and Gene Ontology enrichment analysis without an automatized system is often a laborious task. Here we describe step-by-step the pipeline that was used to perform the transcriptome assembly, annotation, and Gene Ontology analysis of Scots pine (Pinus sylvestris), a gymnosperm species with complex genome. Using only free software available for the scientific community and running on a standard personal computer, the pipeline intends to facilitate transcriptomic studies for non-model species, yet being flexible to be used with any organism.

Metabolic Profiling of γ-Irradiated Barley Plants Identifies Reallocation of Nitrogen Metabolism and Metabolic Stress Response.

The favorable responses of crop species to low-dose γ irradiation can help to develop cultivars with increased productivity and improved stress tolerance. In the present study, we tried to reveal the candidate metabolites involved in growth stimulation of barley seedlings after applying low-dose γ-radiation (60Co) to seeds. Stimulating doses (5-20 Gy) provided a significant increase in shoot length and biomass, while relatively high dose of 100 Gy led to significant inhibition of growth. Gas chromatography-mass spectrometry metabolomic analysis uncovered several compounds that may take part in radiation hormesis establishment in irradiated plants. This includes molecules involved in nitrogen redistribution (arginine, glutamine, asparagine, and γ-aminobutyric acid) and stress-responsive metabolites, such as ascorbate, myo-inositol and its derivates, and free amino acids (l-serine, ß-alanine, pipecolate, and GABA). These results contribute to the understanding of the molecular mechanisms of hormesis phenomenon.

The response profile to chronic radiation exposure based on the transcriptome analysis of Scots pine from Chernobyl affected zone.

Radioactive contamination of the natural areas is one of the most long-lasting anthropogenic impacts on the environment. Scots pine (Pinus sylvestris L.) is a promising organism for radiation-related research because of its high radiosensitivity, but the genome size of Pinacea species has imposed obstacles for high-throughput studies so far. In this work, we conducted the analysis of the de novo assembled transcriptome of Scots pine populations growing in the Chernobyl-affected zone, which is still today contaminated with radionuclides because of the accident at the nuclear power plant in 1986. The transcriptome profiles indicate a clear pattern of adaptive stress response, which seems to be dose-dependent. The transcriptional response indicates a continuous modulation of the cellular redox system, enhanced expression of chaperones and histones, along with the control of ions balance. Interestingly, the activity of transposable element families is inversely correlated to the exposure levels to radiation. These adaptive responses, which are triggered by radiation doses 30 times lower than the one accepted as a safe for biota species by international regulations, suggest that the environmental management in radiation protection should be reviewed.

'Omic' technologies as a helpful tool in radioecological research.

This article presents a brief review of the modern 'omic' technologies, namely genomics, epigenomics, transcriptomics, proteomics, and metabolomics, as well as the examples of their possible use in radioecology. For each technology, a short description of advances, limitations, and instrumental applications is given. In addition, the review contains examples of successful use of 'omic' technologies in the assessment of biological effects of pollutants in the field conditions.

Radiation exposure in the remote period after the Chernobyl accident caused oxidative stress and genetic effects in Scots pine populations.

Even 30 years after the Chernobyl accident, biological effects of irradiation are observed in the chronically exposed Scots pine populations. Chronic radiation exposure at dose rates above 50 mGy∙yr-1 caused oxidative stress and led to the increase of antioxidants concentrations in these populations. Genetic variability was examined for 6 enzymes and 14 enzymatic loci of 6 Scots pine populations. Dose rates over 10 mGy∙yr-1 caused the increased frequency of mutations and changes in genetic structure of Scots pine populations. However, the same dose rates had no effect on enzymatic activities. The results indicate that even relatively low dose rates of radiation can be considered as an ecological factor which should be taken into account for ecological management and radiation protection of biota species.

Genetic diversity in Scots pine populations along a radiation exposure gradient.

Polymorphisms of antioxidant enzymes were studied in the endosperm and embryos of seeds from Scots pine populations inhabiting sites in the Bryansk region of Russia radioactively contaminated as a result of the Chernobyl accident. Chronic radiation exposure at dose rates from 0.8 µGy/h led to a significant increase in the rate of enzymatic loci mutations. The main parameters of genetic variability of the affected Scots pine populations had considerably higher values than those from the reference site. Changes in the genetic makeup of Scots pine populations were observed at dose rates greater than 10.4 µGy/h. However, the higher mutation rate had no effect on the activities of antioxidant enzymes.