Cell Death in Cells Overlying Lateral Root Primordia Contributes to Organ Growth in Arabidopsis

Plant organ growth is widely accepted to be determined by cell divisions and cell expansion, but the contribution of cell elimination has rarely been recognized. We investigated this paradigm during Arabidopsis lateral root formation when growth of the new lateral root primordia (LRP) from pericycle-derived stem cells deep inside the parent root is reportedly facilitated by remodeling of the overlying cells without apparent cell death. We observed the induction of marker genes for cell types undergoing developmental cell death in cells of the endodermis, cortex and epidermis overlying the growing LRP. We used transmission electron microscopy, combined viability and cell death stains, time-lapse confocal and light sheet microscopy techniques to establish that cell death occurred in a subset of cells overlying the growing LRP. LRP growth within the parent root was delayed in cell death deficient mutants lacking the positive cell death regulator ORESARA1/ANAC092 (ORE1). LRP growth was restored in ore1-2 knock-out plants by genetically inducing cell elimination in cells overlying the LRP, or by physically killing LRP-overlying cells by ablation with optical tweezers. Our results support that, in addition to cell shape and cell wall remodeling, cell elimination also contributes to lateral root emergence.