Exploring auxin responses in streptophyte algae

Summary

To understand the evolutionary factors that shaped Earth's landscape, a key question is which traits allowed plants to colonize land. Hormone signaling pathways, particularly auxin, regulate crucial aspects of plant biology, from embryogenesis to senescence and interkingdom communication. Investigating the evolutionary origins of auxin functions can reveal ancestral traits that helped photosynthetic organisms transition from aquatic to terrestrial habitats. In this study, we examine auxin responses in streptophyte algae, the closest living relatives of land plants, at physiological, cellular, and molecular levels. Our findings show that fast auxin responses are conserved between algae and land plants, along with the molecular and biochemical functions of auxin response factor (ARF) algal orthologs. Additionally, auxin triggers long-term responses that overlap with the organic acid tryptophan. We propose that auxin responses in Penium margaritaceum may represent an ancient system for sensing extracellular organic acids. Our results also support the view that ARF regulation by auxin is a key land plant innovation.