EXPANSIN A1-mediated radial swelling of pericycle cells positions anticlinal cell divisions during lateral root initiation

Significance

Organ formation is an essential process in plants and animals, driven by cell division and cell identity establishment. Root branching where lateral roots form along the primary root axis increases the root system and aids the capture of water and nutrients. We have discovered that tight control of the cell width is necessary to coordinate asymmetric cell divisions in cells that give rise to a new lateral root organ. Although biomechanical processes have been shown to play a role in plant organogenesis, including lateral root formation, our data give mechanistic insights into cell size control during lateral root initiation.

Abstract

In plants, postembryonic formation of new organs helps shape the adult organism. This requires the tight regulation of when and where a new organ is formed and a coordination of the underlying cell divisions. To build a root system, new lateral roots are continuously developing, and this process requires the tight coordination of asymmetric cell division in adjacent pericycle cells. We identified EXPANSIN A1 (EXPA1) as a cell wall modifying enzyme controlling the divisions marking lateral root initiation. Loss of EXPA1 leads to defects in the first asymmetric pericycle cell divisions and the radial swelling of the pericycle during auxin-driven lateral root formation. We conclude that a localized radial expansion of adjacent pericycle cells is required to position the asymmetric cell divisions and generate a core of small daughter cells, which is a prerequisite for lateral root organogenesis.