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| Fig. 1: Using nuclear technologies, researchers have been developing plant varieties better suited to climatic change. (Source: Wikimedia Commons) |
Nuclear weapon might be the most well-known application of nuclear energy, especially after the frantic nuclear arm race during the Cold War. However, beyond direct military applications, researchers at the time were also searching for other possibilities of using nuclear energy in a peaceful way, for example, in energy generation, medical treatment and agriculture. Very recently, the leader of Russia ordered his nuclear forces into special combat readiness. [1] It is of a great interest now to remind ourselves how nuclear power can, instead of taking lives, have important medical use to save lives through radiation therapy, and can be used in plant growth, pest control and food storage to help those who suffer from chronic undernourishment. Moreover, scientists around the world have been utilizing nuclear technology in improving plant varieties better suited to climatic change through mutation breeding (Fig. 1). Here, I will give an overview of the research done in past decades on how radiation can be used in plant growth and mutation breeding.
Many investigations has been done in the early to mid-20th century on the stimulation of plant growth by exposing seeds or the growing plants itself to low dosage of radiation. [2] However, the sensitivity varies between species, the stages of growth at which the plants were irradiated, and the type of radiation they were irradiated to, making the evidence for positive impact in yields of crop plants less conclusive. Early works on the stimulating effects of radiation were also plagued by inadequately controlled experiments. There still exists critical evidence that low levels of radiation do have a stimulating effect on certain stages of plant development, both positively and negatively impacting agricultural yield. For example, acute gamma ray irradiation can negatively impact yield of various crop plants including potato and sugar beets, reducing the yield by 50% when acute exposure of 4 krads occurred at early stage of their life cycle. In older plants, twice this dose was required to introduce the same effect. [3] On the other hand, following a range of 0.35-2 krads of acute X-ray irradiation on the seeds of radish, cabbage and peas, an increase of yield by 10- 30% was reported. [2]
Chronic exposures of plants to low dosage of radiation were also reported to significant stimulating effects. For example, chronic exposure of tobacco plants to 0.1-0.35 krads/day of gamma ray stimulated earlier flowering. Gamma irradiation of 0.0125 krads/day to oat plants for 108 days significantly increase the height of the plants but greatly reduced grain production. Tests of buckwheat with exposure of 0.002 krads/day, repeated for three years, reported a 60% increase in plant weight. [2]
It is well known nowadays that radiation can cause genetic mutation. After Stadler first reported mutations in barley induced by x-rays and radium in 1928, radiation has been widely adopted to develop new cultivars for crop production. [4,5] Compared to other breeding methods, such as cross-breeding and chemical mutagenesis, using radiation to induced mutation has its advantages, including a wider mutation spectrum and higher mutation efficiency. On the other hand, its limitation includes a relatively low occurrence of beneficial mutants, and the unpredictability in the direction and the nature of variations. [4] Up to 2021, 3,365 mutant varieties have been registered in the Mutant Variety Database of the International Atomic Energy Agency (IAEA), and more than 1,000 new varieties have been adopted worldwide. [4]
While the earlier studies of mutation were done with gamma rays and X-rays, more recent investigations were able to exploit the benefits of accelerated heavy ions beams or proton beams. Accelerated particle irradiation can maintain a higher mutation frequency and spectrum at a relatively lower dose. [4,6] This is because these particle beams can deposit more energy into the plant body and cause a larger amount of damage to DNA in a small area, which is called the clustered DNA damage. Clustered DNA damage generates many free DNA fragments, which are difficult to repair effectively and correctly. This leads to chromosome rearrangements and large deletions, which can generate more combinations of gene mutation sites.
It is important to keep in mind that the survival rate of both model plants and model microbes decreases with increasing dose. Hence, one must find the most suitable amount of dosage that balances survival and mutation. For example, Kazama et al. reported that with model plant Arabidopsis thaliana, a 300 - 400 Gy irradiation dose and a 30 keV/μm linear energy transfer (LET) carbon ion beam can generate the maximum number of mutants. [4,7]
Accelerated particle radiation can have high efficiency and frequency in generating mutation sites. By combining this benefit with various sequencing technology, one can more deeply and efficiently investigate changes in the genome and transcriptome levels in crops after irradiation, and to further clarify the mutagenic mechanism of particle radiation. [4] This enable further possibilities of a more guided and effective process of plant breeding.
The scope of study in radiation in plant growth stimulation and breeding can further expand to space radiation. [4] There is an increasing interest in space exploration and colonization of other planets in the solar system. It is important to investigate how to create habitable environment and to understand how space radiation impacts plant growth and mutations before launching a full scale of agricultural development in space, which will be necessary for eventual colonization.
© Tiffany Wang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. E. Sanger and W. J. Broad, "Putin Declares a Nuclear Alert, and Biden Seeks De-escalation," New York Times, 22 Feb 27.
[2] K. Sax, "The Stimulation of Plant Growth by Ionizing Radiation," Radiat. Bot. 3, 179 (1963).
[3] C. R. Davies, "Effects of Gamma Irradiation on Growth and Yield of Agricultural Crops - iii. Root Crops, Legumes and Grasses," Radiat. Bot. 13, 127 (1973).
[4] L. Ma et al., "From Classical Radiation to Modern Radiation: Past, Present, and Future of Radiation Mutation Breeding," Front. Public Health 9, 768071 (2021).
[5] L. J. Stadler, "Mutations in Barley Induced by X-Rays and Radium," Science 68, 186 (1928).
[6] A. Tanaka, A., N. Shikazono, and Y. Hase, "Studies on Biological Effects of Ion Beams on Lethality, Molecular Nature of Mutation, Mutation Rate, and Spectrum of Mutation Phenotype for Mutation Breeding in Higher Plants," J. Radiat. Res. 51, 223 (2010).
[7] Y. Kazama et al., "Rapid Evaluation of Effective Linear Energy Transfer in Heavy-Ion Mutagenesis of Arabidopsis thaliana," Plant Biotechnol. 29, 441 (2012).
