A lysigenic programmed cell death-dependent process shapes schizogenously formed aerenchyma in the stems of the waterweed Egeria densa

Background and Aims Plant adaptation to submergence can include the formation of prominent aerenchyma to facilitate gas exchange. The aim of this study was to characterize the differentiation of the constitutive aerenchyma in the stem of the aquatic macrophyte Egeria densa (Hydrocharitaceae) and to verify if any form of cell death might be involved.Methods Plants were collected from a pool in a botanical garden. Aerenchyma differentiation and apoptotic hallmarks were investigated by light microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) assay coupled with genomic DNA extraction and gel electrophoresis (DNA laddering assay). Cell viability and the occurrence of peroxides and nitric oxide (NO) were determined histochemically using specific fluorogenic probes.Key Results Aerenchyma differentiation started from a hexagonally packed pre-aerenchymatic tissue and, following a basipetal and centripetal developmental pattern, produced a honeycomb arrangement. After an early schizogenous differentiation process, a late lysigenous programmed cell death- (PCD) dependent mechanism occurred. This was characterized by a number of typical apoptotic hallmarks, including DNA fragmentation, chromatin condensation, apoptotic-like bodies, partial cell wall lysis and plasmolysis. In addition, local increases in H₂O₂ and NO were observed and quantified.Conclusions The differentiation of cortical aerenchyma in the stem of E. densa is a complex process, consisting of a combination of an early schizogenous differentiation mechanism and a late lysigenous PCD-dependent process. The PCD remodels the architecture of the gas spaces previously formed schizogenously, and also results in a reduction of O₂-consuming cells and in recycling of material derived from the lysigenic dismantling of the cells.