Condensation of STM is critical for shoot meristem maintenance and salt tolerance in Arabidopsis.

The shoot meristem generates the entire shoot system and is precisely maintained throughout the life cycle under various environmental challenges. In this study, we identified a prion-like domain (PrD) in the key shoot meristem regulator SHOOT MERISTEMLESS (STM), which distinguishes STM from other related KNOX1 proteins. We demonstrated that PrD stimulates STM to form nuclear condensates, which are required for maintaining the shoot meristem. STM nuclear condensate formation is stabilized by selected PrD-containing STM-interacting BELL proteins in vitro and in vivo. Moreover, condensation of STM promotes its interaction with the Mediator complex subunit MED8 and thereby enhances its transcriptional activity. Thus, condensate formation emerges as a novel regulatory mechanism of shoot meristem functions. Furthermore, we found that the formation of STM condensates is enhanced upon salt stress, which allows enhanced salt tolerance and increased shoot branching. Our findings highlight that the transcription factor partitioning plays an important role in cell fate determination and might also act as a tunable environmental acclimation mechanism.