Enhanced Desiccation Tolerance In Mature Cultures Of The Streptophytic Green Alga Zygnema circumcarinatum Revealed By Transcriptomics

Desiccation tolerance is commonly regarded as one of the key features for the colonization of terrestrial habitats by green algae and the evolution of land plants. Extensive studies, focused mostly on physiology, have been carried out assessing the desiccation tolerance and resilience of the streptophytic genera Klebsormidium and Zygnema. Here we present transcriptomic analyses of Zygnema circumcarinatum exposed to desiccation stress. Cultures of Z. circumcarinatum, grown in liquid medium or on agar plates, were desiccated at ~86% relative air humidity until Y(II) ceased. In general, the response to dehydration was much more pronounced in Z. circumcarinatum, cultivated in liquid medium for one month, compared to filaments grown on agar plates for seven and twelve months. Cultivation on solid medium enables the alga to acclimate to dehydration much better and an increase in desiccation tolerance was clearly correlated to increased culture age. Moreover, gene expression analysis revealed that photosynthesis was strongly repressed upon desiccation treatment in the liquid culture while only minor effects were detected in filaments cultivated on agar plates for seven months. Otherwise, both samples showed an induction of stress protection mechanisms such as ROS scavenging (Early light-induced proteins, glutathione metabolism) and DNA repair as well as the expression of chaperones and aquaporins. Additionally, Z. circumcarinatum, cultivated in liquid medium, upregulated sucrose synthesizing enzymes and strongly induced membrane modifications in response to desiccation stress. These results corroborate the previously described hardening and associated desiccation tolerance in Zygnema in response to seasonal fluctuations in water availability.