Molecular Mechanisms Driving Bistable Switch Behavior in Xylem Cell Differentiation

Plant xylem cells conduct water and mineral nutrients. Although most plant cells are totipotent, xylem cells are unusual and undergo terminal differentiation. Many genes regulating this process are well characterized, including the VASCULAR-RELATED NAC DOMAIN7 (VND7), MYB46 and MYB83 transcription factors which are proposed to act in interconnected feed-forward loops. Much less is known regarding the dynamic behavior underlying the terminal transition to xylem cell differentiation. Here we utilize whole root and single cell data to mathematically model this relationship. These provide evidence for VND7 regulating bistable switching of cells in the root to a xylem cell identity, with additional features of hysteresis. We further determine that although MYB46 responds to VND7 induction, it is not inherently involved in executing the binary switch. A novel regulatory architecture is proposed that involves four downstream targets of VND7 that act in a cycle. These data provide an important model to study the emergent properties that may give rise to totipotency relative to terminal differentiation and reveal novel xylem cell subtypes.