Summary 37
I. INTRODUCTION 37
II. DIGESTION OF PLANTS IN THE RUMEN: OLD AND NEW CONCEPTS 39
III. RUMEN-INDUCED PLANT METABOLISM: CELL DEGRADATION AND DEATH 41
1. Summary of plant cell death processes 41
2. Anaerobic stress and flooding tolerance of plants 42
3. Plant cell responses to elevated temperatures 45
4. Wounding responses/pathogen attachment 45
5. Senescence in the rumen? 47
IV. FUTURE PROSPECTS 50
Acknowledgements 51
References 51
It is generally assumed that breakdown of plant material in the rumen is a process mediated by gut
microorganisms. This view arose because of the identification of a pre-gastric fermentation in the rumen, brought
about by a large and diverse microbial population. The extensive use of dried and ground feed particles in forage
evaluation might have helped to promote this assumption. However, although the assumption might be correct
in animals feeding on conserved forage (hay and silage) where the cells of ingested forage are dead, it is possible
that with grazed (living) forage, the role played by plant enzymes in the rumen has been overlooked. In a grazing
situation, plant cells that remain intact on entering the rumen are not inert, but will respond to the perceived
stresses of the rumen environment for as long as they are metabolically viable. Metabolic adjustments could
include anaerobic and heat-shock responses that could promote premature senescence, leading to remobilization
of cell components, especially proteins. Moreover, contact of plant cells with colonizing microorganisms in the
rumen might promote a type of hypersensitive response, in much the same way as it does outside the rumen. After
fresh plant material enters the rumen and prior to extensive plant cell-wall degradation, there is often a phase of
rapid proteolysis providing N in excess of that required to maintain the rumen microbial population. The
inefficient use of this ingested N results in generation of ammonia and urea in exhaled breath and urine, which
promotes welfare and environmental pollution concerns. Therefore an important research goal in livestock
agriculture is to find ways of decreasing this initial rate of proteolysis in the rumen. This will benefit the farmer
financially (through decreased use of feed supplements), but will also benefit the environment, as N pollution can
adversely affect pasture diversity and ecology. This review considers the possible responses of plant metabolism
to the rumen environment, and how such considerations could alter current thinking in ruminant agriculture.