PpBOR1 is critical for the excess borate tolerance of Physcomitrium patens.

Key message: Functional analysis of BORs in Physcomitrium patens indicates that both PpBOR1 and PpBOR2 possess boron efflux transporter activity, and PpBOR1 is essential for the plant's tolerance to excessive boron stress. Boron (B), an essential plant micronutrient, is crucial for achieving optimal agricultural yield. Although the function of the BOR family proteins as borate efflux transporters has been established in tracheophytes, the role of their counterparts in non-vascular plants has not been thoroughly investigated. Our phylogenetic analysis reveals that bryophyte BOR proteins originated from the basal bryophytes Takakia and Sphagnum, and can be classified into two subclasses. There are two BOR homologs in P. patens: PpBOR1 and PpBOR2, which belong to different subclades. The PpBOR1 and PpBOR2 genes are predominantly expressed in gametophores, with PpBOR1 exhibiting significantly higher expression levels than PpBOR2. Both proteins localize at the plasma membrane and can export borate from yeast cells. Disruption of PpBOR2 expression does not affect plant growth under normal conditions. However, PpBOR1-knockout gametophores exhibit stunted growth under excess boron conditions, whereas PpBOR1-overexpressing plants show enhanced tolerance compared to wild-type plants. In summary, our research suggests that BOR homologous proteins in P. patens have borate efflux activities similar to those of the BOR family members in angiosperms. PpBOR1 is critical in conferring tolerance to excessive boron stress in P. patens.