Institut National de la Recherche Agronomique, Versailles, France.
Astrobiology. 2012 May;12(5):517-28. doi: 10.1089/ast.2011.0744.
The plausibility that life was imported to Earth from elsewhere can be tested by subjecting life-forms to space travel. Ultraviolet light is the major liability in short-term exposures (Horneck et al., 2001 ), and plant seeds, tardigrades, and lichens-but not microorganisms and their spores-are candidates for long-term survival (Anikeeva et al., 1990 ; Sancho et al., 2007 ; Jönsson et al., 2008 ; de la Torre et al., 2010 ). In the present study, plant seeds germinated after 1.5 years of exposure to solar UV, solar and galactic cosmic radiation, temperature fluctuations, and space vacuum outside the International Space Station. Of the 2100 exposed wild-type Arabidopsis thaliana and Nicotiana tabacum (tobacco) seeds, 23% produced viable plants after return to Earth. Survival was lower in the Arabidopsis Wassilewskija ecotype and in mutants (tt4-8 and fah1-2) lacking UV screens. The highest survival occurred in tobacco (44%). Germination was delayed in seeds shielded from solar light, yet full survival was attained, which indicates that longer space travel would be possible for seeds embedded in an opaque matrix. We conclude that a naked, seed-like entity could have survived exposure to solar UV radiation during a hypothetical transfer from Mars to Earth. Chemical samples of seed flavonoid UV screens were degraded by UV, but their overall capacity to absorb UV was retained. Naked DNA encoding the nptII gene (kanamycin resistance) was also degraded by UV. A fragment, however, was detected by the polymerase chain reaction, and the gene survived in space when protected from UV. Even if seeds do not survive, components (e.g., their DNA) might survive transfer over cosmic distances.
将生命形式置于太空旅行中，可以检验生命是从其他地方输入到地球的可能性。在短期暴露中，紫外线是主要的不利因素（Horneck 等人，2001 年），而植物种子、缓步动物和地衣——而不是微生物及其孢子——是长期生存的候选者（Anikeeva 等人，1990 年；Sancho 等人，2007 年；Jönsson 等人，2008 年；de la Torre 等人，2010 年）。在本研究中，植物种子在暴露于太阳紫外线、太阳和银河宇宙辐射、温度波动和国际空间站外空间真空 1.5 年后发芽。在暴露于太阳紫外线、太阳和银河宇宙辐射、温度波动和国际空间站外空间真空 1.5 年后，2100 颗暴露的野生型拟南芥和烟草（烟草）种子中有 23%在返回地球后产生了有活力的植物。在拟南芥 Wassilewskija 生态型和缺乏紫外线屏蔽的突变体（tt4-8 和 fah1-2）中，存活率较低。烟草的存活率最高（44%）。在遮蔽太阳光的种子中，发芽被延迟，但仍达到了完全存活，这表明对于嵌入不透明基质中的种子来说，更长时间的太空旅行是可能的。我们得出结论，一个裸露的、类似种子的实体在从火星到地球的假设转移过程中可能已经幸存下来，暴露于太阳紫外线辐射之下。种子类黄酮紫外线屏蔽的化学样品被紫外线降解，但它们吸收紫外线的总能力得以保留。裸露的 DNA 编码 nptII 基因（卡那霉素抗性）也被紫外线降解。然而，通过聚合酶链反应检测到一个片段，并且当基因受到紫外线保护时，它在太空中存活下来。即使种子不能存活，其组成部分（例如，它们的 DNA）也可能在穿越宇宙距离的转移中存活下来。
