The Molecular and Physiological Responses ofPhyscomitrella patensto Ultraviolet-B Radiation    

Abstract

Ultraviolet-B (UV-B) radiation present in sunlight is an important trigger of photomorphogenic acclimation and stress responses in sessile land plants. Although numerous moss species grow in unshaded habitats, our understanding of their UV-B responses is very limited. The genome of the model moss Physcomitrella patens, which grows in sun-exposed open areas, encodes signaling and metabolic components that are implicated in the UV-B response in flowering plants. In this study, we describe the response of P. patens to UV-B radiation at the morphological and molecular levels. We find that P. patens is more capable of surviving UV-B stress than Arabidopsis (Arabidopsis thaliana) and describe the differential expression of approximately 400 moss genes in response to UV-B radiation. A comparative analysis of the UV-B response in P. patens and Arabidopsis reveals both distinct and conserved pathways.Ultraviolet-B (UV-B) radiation (280-315 nm) is intrinsic to sunlight reaching the surface of the earth. During the Phanerozoic period (within the last 545 million years), the levels of UV-B reaching the biosphere generally decreased (Rozema et al., 2009). In particular, the development of a stratospheric ozone layer filtering out all UV-C (less than 280 nm) and part of the UV-B radiation was likely a prerequisite for terrestrial plant life and accompanied by the development of phenolic sunscreens in plants (Rozema et al., 1997). During early land plant evolution, the divergence of the last common ancestor of bryophytes (comprising liverworts, true mosses, and hornworts) and vascular plants (comprising lycophytes, ferns, and seed plants) occurred shortly after the water-to-landtransition in the Ordovician period, at least 450 million years ago . If the level of UV-B reaching the ground was significantly higher at that time than today, it is to be expected that protective mechanisms and their corresponding signaling pathways evolved at that time and thus should be present in all land plants.Today, ambient UV-B radiation still has a broad effect on plants relying on sunlight for photosynthesis, and its level varies strongly with season and time of day as well as with latitude and altitude (Paul and Gwyn-Jones, 2003). As an environmental stress factor, it may evoke diverse damage to a broad range of cellular constituents, including DNA (Britt, 2004). However, plants in nature are seldom visibly damaged by UV-B but are generally rather well acclimated and thus protected. This largely results from effective repair and protection mechanisms (Frohnmeyer and Staiger, 2003;Ulm and Nagy, 2005;Jenkins, 2009). A common protective measure against UV-B radiation is the synthesis of secondary metabolites acting as UV-absorbing compounds and accumulating in the vacuoles of epidermal cells. These compounds are mostly phenolic, including flavonoids and sinapate esters, and are at least partly induced by UV-B (Jenkins, 2009;Stracke et al., 2010). Next to this protective sunscreen, plants contain effective DNA repair pathways, particularly inclu...

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