Evolution of ABA signaling pathways

Abscisic acid (ABA) signaling first appears in the land plant lineage, regulating responses to dehydration in early-diverging plants and becoming integrated into a range of functions associated with stress-responses in vascular plants. ABA signaling is mediated by a perception-transduction-response pathway comprising "Pyrabactin Resistant/Pyrabactin Resistant-Like/Regulatory Component of ABA Response" (PYR/PYL/RCAR) receptors, protein phosphatases 2C (PP2C) and "SNF1-Related Protein Kinase 2" (SnRK2) protein kinases that activate effectors through protein phosphorylation by the SnRK2 kinases. Components of the core pathway (phosphatases, kinases and transcription factors) are found in all groups of streptophytes, from aquatic charophyte algae through to angiosperms, and the conserved functions of these components can be demonstrated by cross-species genetic complementation using the model species Physcomitrella patens (P. patens: moss) and Arabidopsis thaliana (Arabidopsis: angiosperm). ABA-receptor-like proteins were recently identified in algal species within the Zygnematophyceae, sister group to the land plants, although receptor function of these proteins has not yet been determined. Algal and bryophyte species share an additional component lost from vascular plants-the "ABA Non-Responsive/ABA Responsive Kinase/Constitutive Triple-Response-1-Like" (ANR/ARK/CTR1L) protein kinase required for the acquisition of desiccation tolerance in mosses. It is proposed that the evolution of a functional ABA receptor in an aeroterrestrial algal species enabled its establishment on land. Co-option of ABA to regulate an ancestral cellular signaling pathway required for the acquisition of desiccation tolerance-a key survival trait-accompanied by gains and losses of gene family members encoding components of ABA signaling then enabled modulation of dehydration stress responses in parallel with the evolution of anatomical complexity, allowing land plants to compete more effectively for resources in their new environment.