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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. This research received no external funding. We are thankful to Hugo Moors (Microbiology—SCK CEN) for the fruitful discussion about Earth evolution and the formation of the ozone layer.

Environmentally induced DNA methylation variants may mediate gene expression responses to environmental changes. If such induced variants are transgenerationally stable, there is potential for expression responses to persist over multiple generations. Our current knowledge in plants, however, is almost exclusively based on studies conducted in sexually reproducing species where the majority of DNA methylation changes are subject to resetting in germlines, limiting the potential for transgenerational epigenetics stress memory. Asexual reproduction circumvents germlines, and may therefore be more conducive to long-term inheritance of epigenetic marks. Taking advantage of the rapid clonal reproduction of the common duckweed Lemna minor, we hypothesize that long-term, transgenerational stress memory from exposure to high temperature can be detected in DNA methylation profiles. Using a reduced representation bisulphite sequencing approach (epiGBS), we show that temperature stress induces DNA hypermethylation at many CG and CHG cytosine contexts but not CHH. Additionally, differential methylation in CHG context that was observed was still detected in a subset of cytosines, even after 3–12 generations of culturing in a common environment. This demonstrates a memory effect of stress reflected in the methylome and that persists over multiple clonal generations. Structural annotation revealed that this memory effect in CHG methylation was enriched in transposable elements. The observed epigenetic stress memory is probably caused by stable transgenerational persistence of temperature-induced DNA methylation variants across clonal generations. To the extent that such epigenetic memory has functional consequences for gene expression and phenotypes, this result suggests potential for long-term modulation of stress responses in asexual plants.

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants. RKT is grateful to Department of Science and Technology-Science and Engineering Research Board (DST-SERB) New Delhi, for a Teachers Associateship for Research Excellence (TAR/2019/000064). Tewari, RK (corresponding author), Univ Lucknow, Dept Bot, Lucknow 226007, Uttar Pradesh, India. rktewari_bot@yahoo.com

First paragraph: Human use of radioactivity is increasing in fields such as nuclear power generation and nuclear medicine. Nuclear power continues to be a part of many countries’ energy portfolios and may increase dramatically in some Asian countries and Russia, with up to 300 new reactors are currently proposed; other countries without existing nuclear power programmes are beginning to develop them (e.g. some African nations and Persian Gulf states). Worldwide there are c.450 operating nuclear power plants (NPPs) and 60 under construction (World Nuclear Association, 2019). The continued use of nuclear power is considered, by some, as essential in the transition to low-carbon economies (e.g. Liu et al., 2013; Gibon et al., 2017). At the same time, many nations face having to develop long-term strategies, and consequent infrastructure, to manage high-level radioactive waste (as arising from nuclear power production); other nations are challenged with legacy issues associated with, for instance, large-scale accidental releases or past and on-going uranium mining/processing industries.