Draparnaldia – a model alga for studying terrestrialisation in Chlorophytes
Final Report Abstract
Land plant evolution included two major events: 1. Plant terrestrialization – the transition of streptophyte algae from aquatic to terrestrial habitats and 2. Plant radiation – the subsequent spread of plants on land. Whereas plant radiation has been extensively studied, plant terrestrialisation is only poorly understood. It is generally accepted that land plants evolved from streptophyte algae. However, there are also many chlorophytes (a sister group of streptophyte algae and land plants) that moved to terrestrial habitats and even resemble mosses. Why no land plants evolved from chlorophytes? To better understand what are the evolutionary adaptations that made streptophytes so successful, I aim to establish the land adapted morphologically complex chlorophyte alga Draparnaldia (Chaetophorales, Chlorophyceae) as a model for comparative analyses between these two lineages. Because my Research Fellowship was the first step towards establishing Draparnaldia as a valuable model, I have mainly focused on two things: (1) I have developed the first transformation protocol of this alga. This protocol is based on protoplast transformation by electroporation. It yields 106 protoplasts, which is comparable with a protoplast yield of the well-established plant models Physcomitrella patens and Arabidopsis thaliana. It allows full regeneration of > 90% of protoplasts, in which transient expression of transgenes is identifiable. To complete this basic transformation toolset, I have also identified the effective selective agents (7 out of 10 tested) for future stable Draparnaldia transformation. Regeneration and transient gene expression in protoplasts of Draparnaldia (chlorophytes), an emerging model for comparative analyses with basal streptophytes. (2) I have also generated the first Draparnaldia transcriptome to investigate the desiccation stress responses. The first results have revealed several interesting and unexpected homologs of streptophyte genes/transcription factors thought to be specific for plant terrestrialization. Surprisingly, these homologues do not seem to be directly associated with pathways involved in plant desiccation stress, but rather with plant morphological adaptations. Clearly, the most interesting are genes related to filamentous rooting structures (root hairs and rhizoids) – one of the key morphological adaptations in land plant evolution. These findings together with the availability of the transformation protocol indicate that Draparnaldia has the potential to serve as an important model to compare genes and mechanisms underlying terrestrialization in chlorophytes and streptophytes. This is especially true for morphological and developmental adaptations.
