Annals of Botany

Death by drowning

In plant roots, waterlogging restricts aeration. This, in turn, affects a whole range of metabolic processes that rely on aerobic metabolism. Responses to flooding vary from species to species and according to the growth conditions and the age of the plant, as described for pea (Pisum sativum) by Dan Gladish and colleagues (Oxford, Ohio, USA and Tokyo, Japan, pp. 895–902). Previous work by this group had shown that that in plants grown at 25 °C, both 4-day-old and 5-day-old seedlings responded to flooding by forming vascular aerenchyma, a process requiring programmed cell death. However, in the 5-day-old seedlings, the aerenchyma was not extensive enough to supply the root tip with air (it did not extend far enough along the root) and the roots quickly stopped growing. Cessation of growth was associated with morphological aberrations such as tip curling, and by death of cells in the ground meristem and in the procambium. Cytological and biochemical studies of the dying cells reveal that death is an ordered process that occurs in individual cells, spreading out from the protoxylem poles. Nuclei become lobed and invaginated, and the chromatin condenses. ‘End-labelling’ techniques (‘TUNEL’) reveal extensive DNA breakage in these cells, confirmed by the observation of ‘ladders’ when extracted DNA is fractionated in agarose gels. These features are reminiscent of apoptosis in animal cells, but whether one calls this apoptosis or not may be a matter of semantics. What is clear is that this is a programmed cell death, rather than a necrotic response. Finally, it may seem rather a drastic response to kill the root tip in reaction to flooding, but the authors suggest that this leads to a diversion of resources away from the tip region and to the promotion of the growth of lateral roots from zones in the primary root that are less affected by flooding.