Hormonal architects: Plant steroids shape meristems.

In plants, the shoot and root apical meristems drive post-embryonic development by tightly coordinating stem cell maintenance, cell proliferation, and differentiation. Among the hormonal regulators governing these processes, brassinosteroids (BRs) have emerged as important modulators of meristem function. Although BRs are classically associated with promoting cell expansion, accumulating evidence demonstrates that they also play crucial roles in regulating cell division and meristem activity. In this review, we provide a comparative perspective on BR function in the shoot and root apical meristems, highlighting both shared and tissue-specific regulatory mechanisms. Given the more extensive understanding of BR dynamics in the root, we focus particularly on the root apical meristem (RAM), where low BR levels and signalling activity must be precisely maintained to preserve proliferative capacity. We further discuss the possibility that analogous mechanisms operate in the shoot apical meristem (SAM), where low BR activity, maintained through local BR catabolism, has been implicated in shaping meristem organization. Together, these studies reveal that the spatial and temporal regulation of BR levels is essential for coordinating cell proliferation, expansion, and differentiation within plant meristems.