Copper mediates auxin signalling to control cell differentiation in the copper moss Scopelophila cataractae.

The copper (Cu) moss Scopelophila cataractae (Mitt.) Broth. is often found in Cu-enriched environments, but it cannot flourish under normal conditions in nature. Excess Cu is toxic to almost all plants, and therefore how this moss species thrives in regions with high Cu concentration remains unknown. In this study, we investigated the effect of Cu on gemma germination and protonemal development in S. cataractae. A high concentration of Cu (up to 800 µM) did not affect gemma germination. In the protonemal stage, a low concentration of Cu promoted protonemal gemma formation, which is the main strategy adopted by S. cataractae to expand its habitat to new locations. Cu-rich conditions promoted auxin accumulation and induced differentiation of chloronema into caulonema cells, whereas it repressed protonemal gemma formation. Under low-Cu conditions, auxin treatment mimicked the effects of high-Cu conditions. Furthermore, Cu-induced caulonema differentiation was severely inhibited in the presence of the auxin antagonist α-(phenylethyl-2-one)-indole-3-acetic acid, or the auxin biosynthesis inhibitor l-kynurenine. These results suggest that S. cataractae flourishes in Cu-rich environments via auxin-regulated cell differentiation. The copper moss might have acquired this mechanism during the evolutionary process to benefit from its advantageous Cu-tolerance ability.