Cambium secrets for vascular stem cell precision and adaptability

The vascular cambium, a bifacial stem cell niche, generates xylem on one side and phloem on the other, driving wood formation—the largest reservoir of terrestrial biomass. This developmental system must balance producing two distinct cell types while maintaining a reservoir of stem cells. Key questions remain: How are cambial stem cells positioned and maintained, and how does the cambium dynamically regulate its size and organization to support growth while preserving functional integrity? Auxin signaling, along with the ligand TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) and its receptor PHLOEM INTERCALATED WITH XYLEM (PXY), has been implicated in stem cell maintenance. However, the mechanisms connecting these components to precise stem cell regulation were unclear. Eswaran et al. identified CAMBIUM-EXPRESSED AINTEGUMENTA-LIKE (CAIL) transcription factors as key players in cambium stem cell identity through transcriptomic analysis of TDIF-overexpressing plants and PXY mutants. Their findings reveal that CAIL expression is restricted to cambial stem cells through a sequestration-based mechanism, where strong binding of TDIF to PXY halts TDIF diffusion at the edge of the auxin-PXY gradient. This spatial restriction creates a narrow signaling domain, ensuring robust CAIL expression and precise stem cell positioning. Disruption of this system—via TDIF overproduction or PXY knockdown—leads to an expansion of CAIL expression into the xylem domain, disrupting stem cell niche organization. This sequestration mechanism also allows dynamic adjustment of cambium size during growth. By employing opposing morphogen gradients and feedback mechanisms, plants, like animals, achieve precise control over stem cell positioning and fate decisions. This study offers a molecular framework for understanding how the cambium balances stability and adaptability in vascular development, advancing insights into stem cell niche regulation. (Summary by Hao Chen) Science  10.1126/science.adj8752