Diffusible repression of cytokinin signalling produces endodermal symmetry and passage cells.

Abstract

SummaryThe root endodermis surrounds the central vasculature as a protective sheath, analogous to an animal polarised epithelium, and restricts diffusion by its ring-shaped Casparian strips (CS)1. Following a lag phase, individual endodermal cells suberise in an apparently random fashion, leading to a “patchy” suberisation that eventually gives rise to a zone of continuous deposition2. Casparian strips and suberin lamellae affect paracellular and transcellular transport, respectively. Interestingly, most angiosperms maintain some isolated cells in an unsuberised state3. These so-called “passage cells” are speculated to allow uptake across an otherwise impermeable endodermal barrier. Here, we demonstrate that these passage cells are late emanations of a meristematic patterning process that reads-out the underlying non-radial symmetry of the vasculature. This process is mediated by non-cell autonomous cytokinin repression in the root meristem, leading to distinct phloem and xylem pole-associated endodermal cells. The latter can resist ABA-dependent suberisation and give rise to passage cell formation. Our data further demonstrate that during meristematic patterning, xylem pole-associated endodermal cells can dynamically adapt passage cell numbers in response to nutrient status and that passage cells express transporters and locally impact their expression in adjacent cortical cells.