Post-ingestion induction of plant cell death: Regulation and implications

In addition to the role of plant cell death in determining fundamental plant characteristics such as leaf morphology, death of plant cells can also have a major impact on the environment. Recently it has been demonstrated that as a consequence of environmental conditions in the rumens of grazing cattle and sheep, ingested plant cells undergo autolysis, even in the absence of the rumen microbial population. Within 6 h of exposure to rumenal conditions (anaerobic and 39 °C) plant cells exhibit symptoms of cell death such as DNA degradation, loss of membrane integrity and increased proteolysis. Induction of these plant processes contributes to the production of nitrogenous waste by livestock agriculture. Recent evidence obtained from studies with Arabidopsis mutants (sid2, cpr5, dnd1 and npr1) suggests that death of ingested plant cells in the rumen involves the salicylic acid (SA) signalling pathway. Interestingly, the salicylic acid (SA) signalling pathway is generally associated with the hypersensitive response to pathogenic micro-organisms, although in our studies the same results were obtained both in the presence and absence of rumen micro-organisms. Hence there is a potential to exploit our understanding of control of cell death processes to address specific environmental problems. As it is estimated that one-quarter of the global land area is grassland for livestock production, it is clear that understanding the fundamental biology underlying cell death in ingested forage will have a highly significant role in minimising the impact of human activities, such as decreasing nitrogen run-off from farmland into water courses.