Directed exospermia: II. VUV-UV spectroscopy of specialized UV screens, including plant flavonoids, suggests using metabolic engineering to improve survival in space

Abstract

We used synchrotron light to determine VUV-UV absorption spectra (125-340 nm) of thin films of substances associated with UV resistance in specific groups of organisms or across limited phylogenetic boundaries: scytonemin, mycosporine-like amino acids, dipicolinic acid, b-carotene, melanin and flavonoids (quercitrin, isoquercitrin, robinin and catechin). The objective was to extend known UV absorption spectra into the vacuum UV, and to evaluate the likely effectiveness of these molecules in shielding DNA from the unfiltered solar UV found in space, using similarity with DNA absorption spectra as the primary criterion. The spectroscopy indicated that plant flavonoids would be ideal UV screens in space. We suggest that flavonoids represent primitive UV screens, and offer explanations (including horizontal gene transfer) for their presence in plants. We also discuss the possibility of improving UV resistance by increasing flavonoid accumulation through metabolic engineering, in the hope of better adapting life for space travel, i.e. for its dissemination away from the Earth (exospermia). Finally, we propose using plant seeds as exospermia vehicles for sending life (including artificial life) into space.

Survival and DNA Damage in Plant Seeds Exposed for 558 and 682 Days outside the International Space Station

“…UV 254nm concentrates energy in a crucial part of the spectrum, since DNA and the flavonoid sunscreens have absorption peaks near 260 nm. (Zalar et al, 2007a). Germination in Arabidopsis and tobacco seeds slowed and diminished as a function of dose (Fig.…”

“…Quercitrin flavonoid mimics UV 125–340nm absorption by salmon sperm DNA (Fig. 5C) (Zalar et al, 2007a). Flavonoids exposed to space in EXPOSE-E lost structural features, but they retained their shielding functions (Tepfer et al, 2012).…”

“…UV-induced cross-linking between ribosomal RNA and proteins would inhibit protein synthesis and thus repair and recovery (Casati and Walbot, 2004). More general targets include substances with π electron systems: for example, nucleic acids, aromatic amino acids, flavonoids, and many secondary metabolites ( e.g., polyamines) (Zalar et al, 2007a, 2007b). Incoming photons not stopped in the seed coat would damage (to give a few examples) chromatin, ribosomes, membranes ( e.g., via aromatic amino acids in transmembrane proteins), and enzymes ( e.g., via conserved aromatic amino acids in ligand binding sites).…”

“…Flavonoids are also antioxidants that help cope with free radicals produced by energy absorbed by more general targets than those discussed here. We proposed that contemporary flavonoid synthesis functions were acquired from the original cyanobacterial endosymbiont and later lost in free-living cyanobacteria after the accumulation of UV-shielding oxygen in the atmosphere (Zalar et al, 2007a). …”

VUV photophysics and dissociative photoionization of pyrimidine, purine, imidazole and benzimidazole in the 7–18eV photon energy range

“…In the VUV (E photon $ 7-20 eV), the photoabsorption cross section of the DNA and RNA nucleobases is higher than in the UV [12,[16][17][18], similar to the case of polycyclic aromatic molecules and related compounds [19] and this is no doubt also the case for the molecules under study here, but which has been verified only for pyrimidine [20]. Apart from photoelectron spectral studies [21][22][23][24] and some early mass spectrometry work [25][26][27][28] there has been relatively little previous investigation of ionization phenomena of the molecules under study here.…”