Lateral root initiation in Arabidopsis thaliana: a force awakens

Abstract

Osmotically driven turgor pressure of plant cells can be higher than that of a car tire. It puts tremendous forces onto cell walls and drives cell growth and changes in cell shape. This has given rise to unique mechanisms to control organ formation compared to metazoans. The fascinating interplay between forces and local cellular reorganization is still poorly understood. Growth of lateral roots is a prominent example of a developmental process in which mechanical forces between neighboring cells are generated and must be dealt with. Lateral roots initiate from a single cell layer that resides deep within the primary root. On their way out, lateral roots grow through the overlying endodermal, cortical, and epidermal cell layers. It was recently demonstrated that endodermal cells actively accommodate lateral root formation. Interfering genetically with these accommodating responses in the endodermis completely blocks cell proliferation in the pericycle. The lateral root system provides a unique opportunity to elucidate the molecular and cellular mechanisms whereby mechanical forces and intercellular communication regulate spatial accommodation during plant development.

Figure 1.. Arabidopsis thaliana lateral root initiation

Schematic representation of the different developmental steps of lateral root initiation in Arabidopsis. Lateral root founder cells are primed at the oscillation zone in the basal root meristem. Subsequently, founder cell specification takes place through the IAA28 and IAA14/SOLITARY ROOT (SLR) auxin signaling modules. Concomitantly, lateral root founder cells increase in volume and their roundup nuclei synchronously migrate to the common cell walls. This process is dependent on endodermal volume loss or shape change mediated by an endodermal SHORT HYPOCOTYL 2 (IAA3/SHY2) auxin signaling module. After formative divisions, different auxin signaling modules, including SLR, IAA12/BODENLOS (BDL), and SHY2, and their interacting AUXIN RESPONSE FACTORS (ARFs) mediate the development and emergence of lateral roots, giving rise to a branched root system. Interfering with endodermal volume loss by expressing the stabilized repressor shy2-2 in the endodermis blocks volume increase and nuclear migration in the lateral root founder cells and results in plants without a branched root system.