An ancient and conserved function for armadillo-related proteins in the control of spore and seed germination by abscisic acid

• Armadillo-related proteins regulate development throughout eukaryotic kingdoms. In the flowering plant Arabidopsis, Armadillo-related ARABIDILLO proteins promote multicellular root branching. ARABIDILLO homologues exist throughout land plants, including in early-diverging species lacking true roots, suggesting that early-evolving ARABIDILLOs had additional biological roles.
• Here we investigate, using molecular genetics, the conservation and diversification of ARABIDILLO protein function in plants separated by ~450 million years of evolution.
• We demonstrate that ARABIDILLO homologues in the moss Physcomitrella regulate a previously undiscovered inhibitory effect of abscisic acid (ABA) on spore germination. Furthermore, we show that Arabidopsis ARABIDILLOs function similarly during seed germination. Early-diverging ARABIDILLO homologues from both Physcomitrella and the lycophyte Selaginella can substitute for ARABIDILLO function during Arabidopsis root development and seed germination.
• We conclude that (i) ABA was co-opted early in plant evolution to regulate functionally analogous processes in spore- and seed-producing plants and (ii) plant ARABIDILLO germination functions were co-opted early into both gametophyte and sporophyte, with a specific rooting function evolving later in the land plant lineage.