Suppression of DELLA signaling induces procambial cell formation in culture.

The post-embryonic growth of plants requires the activities of apical meristems and lateral meristems. In the meristems, self-proliferation and differentiation of stem cells is tightly modulated by plant hormone signaling networks and specific transcription factors. Despite extensive studies on stem cell maintenance in plants, the mechanism by which stem cells are initially established is largely unknown. Vascular stem cells consisting of procambial/cambial cells give rise to xylem and phloem cells. In this study, we analyzed the establishment of procambial cells using the in vitro culture system VISUAL, in which mesophyll cells rapidly differentiate into xylem tracheary elements and phloem sieve elements via procambial cells. We found that procambial cell formation in VISUAL is initiated by light, which can be replaced by application of gibberellin (GA). Gibberellin was able to promote procambial cell formation through degradation of DELLA, whereas light did not elevate the endogenous GA content. Indeed, light in combination with bikinin reduced the accumulation of DELLA protein in VISUAL. Consistently, overexpression of a constitutively active DELLA protein repressed vascular cell differentiation even under light. These combined results suggest that DELLA signaling suppresses procambial cell formation during vascular development in VISUAL.