Cell death mechanisms leading to vascular cavity formation in pea (Pisum sativum) L. ‘Alaska’) primary roots

Parenchymatous cells in the center of the vascular cylinder of Pisum sativum (garden pea) primary roots degenerate in response to flooding at warm temperatures and form a long continuous cavity. Vascular cavities seem to provide a conduit for longitudinal oxygen transport in the roots. Many organisms are known to sacrifice specific cells by programmed cell death (PCD) to survive stresses. Presence of full-length cavities allows continued growth of the pea roots during flooding stress, and characteristic cytoplasmic degradation has been previously detected in the degenerating cells of cavity-forming roots. Hence, the hypothesis of this dissertation was that vascular cavity formation involves PCD.

The present study shows that this cellular degradation initiates at around 3 h after flooding and continuous cavities usually develop within 6 h after flooding. The degenerating cells had thinner primary cell walls, less electron-dense middle lamellae, and less abundant cell wall homogalacturonans (HGs) in altered patterns, compared to healthy cells of roots without cavities. Changes in cell wall HGs have been associated with PCD-induced cortical aerenchyma formation in maize. Systematic DNA fragmentation, a hallmark character of PCD, was detected in vascular cavity-forming pea roots in the current study. DNA fragmentation occurred rapidly, within 3 h after flooding. The DNA fragments produced were about 20-30 kb, possibly corresponding to the size of chromatin loops in pea chromosomes. No low-molecular weight DNA fragments (inter-nucleosomal fragments) were detected. Release of cytochrome c from mitochondria into cytosol, a characteristic feature of a common PCD pathway, was detected in the cavity-forming roots 2 h after flooding. Hence, release of cytochrome c seems to be upstream to DNA fragmentation and cellular breakdown. All the cytological changes however, remained confined to the parenchymatous cells in center of the vascular cylinders, even after 24 h of flooding. Outer vascular cylinder cells and cortical cells maintained cellular integrity and had signs of normal activity. Presumably, the cavities successfully function as aerenchyma and allow the surrounding tissues to function normally. The observations of this study suggest that vascular cavity formation in pea primary roots possibly involves mitochondria-dependent, cytochrome c- mediated PCD.