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

ABSTRACT

In vascular plants, the root endodermis surrounds the central vasculature as a protective sheath that is analogous to the polarized epithelium in animals, and contains ring-shaped Casparian strips that restrict diffusion. After an initial lag phase, individual endodermal cells suberize in an apparently random fashion to produce 'patchy' suberization that eventually generates a zone of continuous suberin deposition. Casparian strips and suberin lamellae affect paracellular and transcellular transport, respectively. Most angiosperms maintain some isolated cells in an unsuberized state as so-called 'passage cells', which have previously been suggested to enable 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 the non-cell-autonomous repression of cytokinin signalling in the root meristem, and leads to distinct phloem- and xylem-pole-associated endodermal cells. The latter cells can resist abscisic acid-dependent suberization to produce passage cells. Our data further demonstrate that, during meristematic patterning, xylem-pole-associated endodermal cells can dynamically alter passage-cell numbers in response to nutrient status, and that passage cells express transporters and locally affect the expression of transporters in adjacent cortical cells.