Terrestrialization: Stress Signalling Dynamics in the Algal Progenitors of Land Plants

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Land plants abound on Earth’s surface. All of this diversity arose in a singular event. The algal progenitor of land plants was a streptophyte alga and only recent phylogenomic analyses have specified the particular algal lineage that is most closely related to land plants. But why did land plants evolve only once? And what properties did the ancestors of these terrestrial organisms possess that allowed them to conquer land? Life on land involves rapid and drastic shifts in temperature, light or water availability. Hence, a prime candidate property is the ability to deal with these terrestrial stressors by dynamically responding to shifting environmental cues. My recent data highlight that the streptophyte algae closest to land plants have the genetic makeup for land plant-like stress response signalling circuits—including genes for sensing the major stress phytohormone abscisic acid (ABA). This provides us with testable candidates. To shed light on the early evolution of one of land plants’ key properties, I, here, propose to combine in-depth molecular biological analyses of these candidate stress signalling and response pathways with large-scale systems biology approaches. For this, my team and I will develop streptophyte algal model systems. We will dissect the regulatory hierarchy employed during stress signalling and the response pathways it is regulating in real-time in vivo and across evolutionary time in silico. These approaches will go beyond a view of gene evolution that is based on presence/absence to address if land plant stress dynamics have evolved from algal stress regulatory networks that became hardwired into land plant biology. The aim of this work is to infer the biology of the earliest land plants by investigating their closest algal relatives and interrogating a candidate mechanism used to deal with the challenges of life on land. Understanding this mechanism means understanding a key player that paved the way for the success of plants on land.