Algal origins of core land plant stress response subnetworks.

We computed co-expression networks from more than 2200 samples of nine species across 600 million years of divergent streptophyte evolution and infer that the streptophyte algal ancestors of land plants already had a remarkable fraction of the embryophytic stress response system. Despite its phytohormone-independent origin, homologs of all core components of the drought hormone abscisic acid (ABA) subnetwork are present, and we find that most are co-expressed in streptophyte algae and land plants; this subnetwork was thus co-opted in embryophytes by bringing it under the regime of ABA. The last common ancestor of embryophytes and Zygnematophyceae algae had ancient stress-responsive pathways, enabling it to face the stresses typical of the land environment - even before the origin of land plants - while evolution on land led to the adaptive refinement of these responses.