Apoptosis-like programmed cell death occurs in procambium and ground meristem of pea (Pisum sativum) root tips exposed to sudden flooding

BACKGROUND AND AIMS: Pea (Pisum sativum) primary roots form long vascular cavities when grown under wet or flooded conditions at 25 °C. It is thought that the cavities are a form of aerenchyma. At 25 °C short roots continue to grow after flooding. After roots reach 10 cm long flooding causes rapid cessation of growth, and root tips often become curled. In longer roots the cavities do not extend into the base of the roots, perhaps rendering them ineffective as aerenchyma. It was hypothesized that the resulting growth arrest was due to programmed cell death (PCD) rather than necrosis. METHODS AND KEY RESULTS: Histological examination by light microscope showed that some cells in the primary meristem (elongation) zone of the primary root tips had morphological abnormalities, including misshapen and fragmented nuclei, and cytoplasmic shrinking and fragmentation. Transmission electron microscopy revealed lobing, invagination and chromatin aggregation in nuclei. The affected cells were positive for terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling. Extracted DNA formed a 'DNA ladder' during electrophoresis. Cell death usually began in procambium at one or two protoxylem poles and seemed to spread out to nearby tissues, which asymmetrically inhibited growth and resulted in tip curling. CONCLUSIONS: The above are symptoms of apoptosis-like PCD. Programmed root tip death may rapidly reduce oxygen demand and sink strength, allowing more rapid diversion of resources to lateral roots growing in more permissive conditions.