A Local Auxin Gradient Regulates Root Cap Self-Renewal and Size Homeostasis

Abstract

Organ size homeostasis, compensatory growth to replace lost tissue, requires constant measurement of size and adjustment of growth rates. Morphogen gradients control organ and tissue size by regulating stem cell activity, cell differentiation and removal in animals. In plants, control of organ size is of specific importance in root caps to protect the growing root tip from mechanical damage. New root cap tissue (columella cells) is formed by the columella stem cells, whose activity is regulated through the quiescent center (qc), non-dividing niche-like cells. Daughter cells in contact with the qc retain the potency to divide, while derivatives oriented towards the mature cap undergo differentiation into columella cells. The outermost columella layers are sequentially separated from the root body involving remodeling of cell walls. Factors regulating the balance between cell division, elongation and separation to keep root cap size constant are currently unknown. Here, we report that stem cell proliferation induced cell separation at the periphery of the root cap resulting in organ size homeostasis. An auxin response gradient with a maximum in the niche cells and a minimum in the detaching layer was established prior to the onset of cell separation. In agreement with a mathematical model, organ size was positively regulated by the amount of auxin released from the source. Auxin transporters localized non-polarly to plasma membranes of the inner cap isolating separating layers from the auxin source. Together, these results are in support of an auxin gradient measuring and regulating organ size.