Plant hormone signaling: Is upside down right side up?

Since the early days of plant biology, small molecule hormones have held a central place in our understanding of development. A key feature of plant hormone action is the ability to regulate multiple developmental processes. Despite this pleiotropy, decades of genetic and molecular studies have shown that plant hormone signaling is often canalized through a core pathway. This raises the difficult question of how one signaling pathway produces different outputs in different tissues. Drawing on examples from gibberellin and strigolactone signaling pathways, we propose this conceptual problem arises from an upside-down perspective of hormone signaling. Recent studies have revealed hormone and core pathway-independent mechanisms of regulating downstream signaling components, which could explain multiple developmental responses to the same hormone.