Abscisic acid: origins and signal integrations in plant evolution

Life has undergone profound transformations since its origin approximately 3.5 billion years ago. These evolutionary milestones, most notably terrestrialization, have left indelible marks on all extant land plants. A prime example is the sesquiterpenoid compound abscisic acid (ABA), which transitioned from a metabolic byproduct into a major phytohormone orchestrating stress responses and development in the green lineage. In this review, we examine the shaping of ABA metabolism, signaling, and downstream responses through the lens of gene family evolution and transcriptional regulation. By exploring the integration of ABA with core cellular and metabolic processes and comparing seed-free lineages and spermatophytes, we highlight the fundamental conservation and divergence of ABA-mediated responses in plant evolution.