Transformations of structural phenylpropanoids during cell wall digestion

Abstract

Phenylpropanoids limit the degradation of cell walls of roughages in herbivores but at the same time undergo transformations in the digestive tract. This review outlines the main transformations that occur in the rumen. All the monomeric aromatics tested are fully degraded under anaerobic conditions which favour electron transfer. Six main strains of bacteria degrade monomeric phenols in the rumen by several mechanisms. In addition, some fungi and bacteria are able to release, and possibly to metabolise the esterified hydroxycinnamic acids found in forage cell walls. The first step in the degradation of these acids is their reduction to non-toxic compounds, which are often growth factors. However, total degradation of monoaromatics is difficult to achieve in vivo because of the small population of organisms able to metabolise them abd the limited transit time of the substrates in the rumen. Oligolignols are also degraded to different extents depending on their size and molecular structure. Lignins are partly solubilised during cell wall degradation. They may also undergo other transformations such as demethylation and dehydroxylation. The amount of lignin that seems to be degraded in rumen fluid is low but probably higher than under other anaerobic environments over the same period of time. It is generally accepted that the digestibility of forage lignins is low. However, the wide range of values measured (from minus 0.46 to 0.64) is related either to the measurement method or to the transformations that the lignins may undergo in the digestive tract, or to both. An indigestible fraction of lignins could serve as a reliable cell wall marker but none of the fractions used to date has proved entirely satisfactory for this purpose. Future research in this field would involve better knowledge about the transformation of phenylpropanoids and the development of microbial activity on these compounds. This would improve phenolics degradation and consequently carbohydrate utilization.