Brassinosteroids (BRs) play essential roles in growth and development in seed plants;¹ disturbances in BR homeostasis lead to altered mitotic activity in meristems^2,3 and organ boundaries^4,5 and to changes in meristem determinacy.⁶ An intricate signaling cascade linking the perception of BRs at the plasma membrane to the regulation of master transcriptional regulators belonging to the BEH, for BES1 homologues, family⁷ has been described in great detail in model angiosperms. Homologs of these transcription factors are present in streptophyte algae and in land plant lineages where BR signaling or function is absent or has not yet been characterized. The genome of the bryophyte Marchantia polymorpha does not encode for BR receptors but includes one close ortholog of Arabidopsis thaliana BRI1-EMS-SUPPRESSOR 1 (AtBES1)⁸ and Arabidopsis thaliana BRASSINAZOLE-RESISTANT 1 (AtBZR1),⁹ MpBES1. Altered levels of MpBES1 severely compromised cell division and differentiation, resulting in stunted thalli that failed to differentiate adult tissues and reproductive organs. The transcriptome of Mpbes1 knockout plants revealed a significant overlap with homologous functions controlled by AtBES1 and AtBZR1, suggesting that members of this gene family share a subset of common targets. Indeed, MpBES1 behaved as a gain-of-function substitute of AtBES1/AtBZR1 when expressed in Arabidopsis, probably because it mediates conserved functions but evades the regulatory mechanisms that native counterparts are subject to. Our results show that this family of transcription factors plays an ancestral role in the control of cell division and differentiation in plants and that BR signaling likely co-opted this function and imposed additional regulatory checkpoints upon it.

油菜素类固醇 (BRs) 在种子植物的生长和发育中发挥重要作用；¹ BR 稳态的紊乱导致分生组织 2、3 和器官边界 4、5 的有丝分裂活性改变^，并导致^{分生组织决定性}的变化。⁶一个复杂的信号级联，将质膜上 BR 的感知与属于 BEH 的主转录调节因子的调节联系起来，对于 BES1 同源物，家族⁷已在模型被子植物中详细描述。这些转录因子的同源物存在于链藻和陆生植物谱系中，其中 BR 信号或功能不存在或尚未被表征。苔藓植物Marchantia polymorpha的基因组不编码BR 受体，但包括拟南芥BRI1-EMS-SUPPRESSOR 1 (AtBES1) ⁸和拟南芥BRASSINAZOLE-RESISTANT 1 (AtBZR1), ⁹ MpBES1 的一个密切直系同源物。MpBES1 水平的改变严重损害了细胞分裂和分化，导致无法区分成体组织和生殖器官的菌体发育不良。Mp bes1的转录组敲除植物显示与 AtBES1 和 AtBZR1 控制的同源功能有显着重叠，表明该基因家族的成员共享一个共同目标的子集。实际上，当在拟南芥中表达时，MpBES1 表现为 AtBES1/AtBZR1 的功能获得替代物，可能是因为它介导了保守的功能，但避开了天然对应物所受的调节机制。我们的研究结果表明，这一转录因子家族在植物细胞分裂和分化的控制中发挥着重要作用，并且 BR 信号传导可能利用了这一功能并对其施加了额外的监管检查点。