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Supplementary document 1. Background Plants that utilize sunlight can not escape from solar UV light. The UV light causes significant damages in plant growth, for example, decreased crop yield and leaf burn of ornamental plants, because UV light causes DNA damages. DNA damages suppress cell growth by repressing gene expression and DNA replication or inducing mutations. Recently, we are afraid that more UV light reaches Earth's surface due to the depleting ozone layer. Plants have tolerant mechanisms for UV light. Flavonoid pigment such as anthocyanin absorbs UV light and therefore protects DNA from UV light. DNA repairing mechanisms such as photoreactivation mechanism repair the UV-induced DNA damages such as pyrimidine dimers. However, we thought that there might be unidentified protection mechanism from UV light in plants. 2. Experiment To explore UV-tolerant mechanism of plants, we previously tried and succeeded in isolating UV-tolerant Arabidopsis mutant using ion beam irradiation facility (TIARA) at Takasaki Advanced Radiation Research Institute (Figure 1, 2. April, 1996). The growth of UV-resistant mutant is more than twice that of wild type under supplemental UV light condition. We identified the causal gene of this mutant, named UVI4 (Figure 3). Analyses of this mutant showed that the UVI4 gene plays a role in endoreduplication process by suppressing the increase of DNA content (Figure 4, 5). We thought that the increased gene copy number by endoreduplication conferred UV tolerance. This hypothesis is supported by the fact that the tetraploid Arabidopsis that have doubled DNA content was hyper resistant to UV light compared to diploid Arabidopsis (Figure 6). 3. Result The present result revealed that increment of DNA content by endoreduplication process is one of the important mechanisms for UV tolerance of plants. Since the endoreduplication is common process in most plant species, it is expected to produce UV-resistant wheat or soybean varieties, etc., or to protect ornamental plants from leaf burn by controlling the activity of the UVI4 gene. |
