DNA fragmentation in cereal roots indicative of programmed root cortical cell death

In cereals, a progressively increasing root cortical cell death (RCD) occurs from the root tip and upwards when measured with vital staining methods. In this study, nuclear DNA fragmentation was studied in seminal root segments of wheat and barley in order to investigate if the cell death resembled apoptosis. The fraction of cells with TUNEL-positive nuclei increased gradually with increasing root age in both the cortex and the stele. Southern analysis showed a typical ladder pattern, indicating nucleosomal fragmentation already in 2-day-old root segments, and this became more pronounced in older root segments. DNA fragmentation appeared to be more extensive in wheat than in barley roots. These results confirm earlier studies, where RCD has been found to be earlier initiated and to proceed at a faster rate in wheat. The characteristic DNA fragmentation found in the roots indicates programmed cell death with mechanistic similarities to apoptosis. Ultrastructural examination of nuclei in cortex cells with transmission electron microscopy revealed an increased chromatin condensation in older roots, particularly in wheat. In addition, we found nucleosomal DNA ladders in young leaf tissue from wheat but not from barley.