Condensation of STM empowers meristem activity and enhances salt tolerance

The shoot meristem generates the entire shoot system, and is precisely maintained throughout the life cycle under various environmental challenges. Here, we identify a prion-like domain (PrD) in the key shoot meristem regulator SHOOT MERISTEMLESS (STM) that distinguishes STM from other related KNOX1 proteins. We demonstrate that PrD stimulates STM to form nuclear condensates, which are required for maintaining the shoot meristem. STM nuclear condensate formation is stabilized by select PrD containing STM-interacting BELL proteins in vitro and in vivo. Moreover, condensation promotes the interaction between STM and Mediator to enhance transcriptional activity. Thus, condensate formation emerges as a novel mechanism of regulating shoot meristem functions. Furthermore, we find that the formation of STM condensates is enhanced upon salt stress, which allows enhanced salt tolerance and increased shoot branching. Our findings highlight the importance of transcription factor partitioning in cell fate determination, and as a tunable environmental sensing mechanism.