Mechanism for differential desiccation tolerance in porphyra species

Abstract

Drought has been the cause of most agricultural losses, and therefore it is not surprising that desiccation has been one of the most studied stresses on plants. As a result, the mechanisms that desiccation tolerant terrestrial plants use to survive drought conditions are well understood today. This is not the case for those macrophytic marine algae (or seaweeds) that grow in the intertidal zone on rocky shores, which experience far more rapid and severe losses of water content than terrestrial plants. The goal of this study was to determine the mechanism of desiccation tolerance in seaweeds using the Red algae Porphyra umbilicalis and P. yezoensis as a model system. While these two algae have very different tolerances to desiccation, this study showed that both species lost about 95% of their water in the first two hours of dehydration and their final relative water content is virtually the same. Massive membrane leakage, reduced respiration and reduced oxygen evolution were observed in P. yezoensis after desiccation, but not in P. umbilicalis. TEM observation revealed extensive membrane disruption only in P. yezoensis after desiccation. Reactive oxygen species (ROS) defense, repression of membrane phase transition and formation of cellular glass are the three major desiccation tolerance mechanisms reported in land plants. ROS defense is not the key to the difference between P. umbilicalis and P. yezoensis for several reasons. First, desiccation in the dark did not alleviate the desiccation damage in P. yezoensis, as light has been shown to stimulate ROS damage. P. yezoensis also had higher superoxide dismutase activity. There was a small decrease in ascorbate content in P. umblicalis after desiccation, but such decrease was not found in P. yezoensis. Furthermore, neither species showed an increase in membrane peroxidation after desiccation. Repression of membrane phase transition cannot explain the different response because the membranes of both species remained in liquid crystalline when desiccated. Our data suggest that the cytoplasm of P. umbilicalis forms a more stable glass when the organism is desiccated, and that the molecular mobility is lower in the drying P. umbilicalis. A dehydrin-like protein was detected in great abundance in P. umbilicalis and could play a key role in the better desiccation tolerance of this species.