Lignin Impact on Fiber Degradation:  Increased Enzymatic Digestibility of Genetically Engineered Tobacco (<i>Nicotiana tabacum</i>) Stems Reduced in Lignin Content

Abstract

Cell wall digestibility, lignin content, and lignin composition were analyzed in transgenic tobacco altered in the expression of the phenylpropanoid biosynthetic enzymes caffeic acid 3- O -methyltransferase (COMT) and l -phenylalanine ammonia-lyase (PAL). Reduction of COMT activity by antisense technology resulted in reduced lignin content accompanied by an increased syringyl (S)/guaiacyl (G) monomer ratio, as determined by pyrolysis/GC/MS and measurement of lignin methoxyl content by wet chemistry. These results resemble those obtained by reduction of flux of lignin precursors into the phenylpropanoid pathway by PAL suppression, which results in drastically reduced lignin with sharply increased methoxyl content. Enzymatic digestibility of cell walls from stem internodes was improved in the transgenic lines and was highly negatively correlated with lignin concentration ( r = −0.97). Although lignin composition was also affected, lignin concentration was the overriding factor influencing cell wall digestibility. The results provide a basis for new strategies for lignin modification to improve digestibility of forages. Keywords: Lignin; genetic engineering; digestibility; tobacco