The phytohormone salicylic acid influences autolysis in ruminantingested plant cells

Fresh forage ingested by grazing cattle is subject to a combination of stresses including heat, anoxia and attack by micro-organisms culminating in autolysis: self-mediated breakdown of protein and DNA. Understanding the stress responses of ingested forage will allow us to breed improved forages which will release less pollutant nitrogen when used as cattle feed. The plant signalling compound salicylic acid (SA) is implicated in regulating responses to biotic and abiotic stresses such as those experienced by plant cells post-ingestion. To test the hypothesis that SA is involved in control of autolysis, Arabidopsis wild type (Col-0) has been compared with sid2 which contains a mutation in the SA biosynthetic gene isochorsimate synthase (ICS1). Incubation in an aqueous environment at 39 °C in the absence of oxygen for 6 h resulted in a decrease in leaf protein of approximately 50% in Col-0 but not in sid2. This difference was not associated with differential activities of the most abundant serine or cysteine proteases. It is possible that changes in lower abundance proteases were undetected. The SA signal transduction pathway includes induction of the oxidative burst, programmed cell death (PCD), and expression of defence-related proteins. DNA-based characterisation techniques are being used to reveal if decreased proteolysis in sid2 mutants is due to incomplete execution of an SA mediated PCD pathway when plant cells are exposed to ingestive stress.