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.

Effect of down-regulation of cinnamyl alcohol dehydrogenase on cell wall composition and on degradability of tobacco stems

“…Consistent with this are the changes previously reported in the bmr6 sorghum mutant (Porter et al 1978). Negative lignin disappearances have been observed in other digestibility trials (Besle et al 1995). Greater lignin disappearances and higher syringyl solubilisation observed in some tobacco transgenic populations have also been reported for bm1 maize .…”

Indigestible neutral detergent fibre in predictions of grass and red clover silage digestibility

“…() found less phenylpropanoids in faeces from sheep fed different forages than what was detected in the feed. Further, oligolignols in addition to monoaromatics can be released from the lignin molecule during ruminal cell‐wall degradation (Besle et al ., ). Soberon et al .…”

Relationship between Phenolic Acids Formed During Rumen Degradation of Maize Samples and in vitroDigestibility

“…'H NMR analysis of the liquid phase after fermentation of maize samples demonstrated that HPPA, PPA, PAA, HPAA and HBA were present as free monomeric molecules. These acids have been proposed as compounds which may be formed from ruminal degradation of PCA and FA (Besle et al 1995). The phenolic acids PCA, FA, SYRA and VA were not detected in the liquid phase by 'H NMR nor by HPLC when extraction was performed on only acidified supernatants.…”

“…Path coefficients describing the direct effects of the explanatory variates on organic matter-and cell wall digestibility compounds by rumen microorganisms (Besle et al 1995). Therefore, multiple interactions exist between many individual phenolic acids which all exert a separate effect on digestibility.…”

“…Rumen bacteria appear to rapidly detoxify PCA and F A by hydrogenation of the double bond in the propenoic side chain and yield 3-phenylpropionic acid (PPA ;Chesson et a/ 1982;Martin 1982;Chestnut et a/ 1986). Possible pathways of ruminal detoxification of PCA and FA have recently been proposed (Besle et a/ 1995).…”