Seminars

Vascular cambium is a secondary meristem producing xylem and phloem along thickening plant organs. By combining cell lineage tracing with molecular genetics, we recently showed that cells with a xylem identity direct adjacent vascular cambial cells to divide and function as stem cells (Smetana et al 2019). Therefore, we proposed that xylem-identity cells constitute an organizer. Molecular studies revealed that the organizer is defined by local auxin maximum, and subsequent expression of class III homeodomain-leucine zipper (HD-ZIP III) family transcription factors. HD-ZIP IIIs promote xylem identity and cellular quiescence of the organizer cells. Additionally, the organizer maintains phloem identity in a non-cell-autonomous manner. In line with this dual role of the organizer cells, xylem and phloem originate from a single, bifacial stem cell in each radial cell file, which confirms the classical theory of a uniseriate vascular cambium (Sanio 1873). Clones with high levels of ectopically induced auxin signalling differentiate as xylem vessels; these clones induce cell divisions and the expression of cambial and phloem markers in the adjacent cells. These data suggest that a local auxin signalling maximum is sufficient to specify a stem-cell organizer of vascular cambium (Smetana et al 2019). As a follow up, we have been studying whether other cambial regulators interact with the factors defining the organizer. The latest discoveries of these studies will be presented in the meeting.