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

Abstract

The effect of down‐regulation of tobacco cinnamyl alcohol dehydrogenase (CAD) on cell wall composition and degradability has been assessed. CAD activity was only 20, 16, 14 and 7%, relative to the controls, in four populations of plants (designated 40‐1, 40‐2, 48 and 50, respectively) transformed with CAD antisense mRNA. Cell wall residues of stem samples were analysed for polysaccharide composition, gravimetric and acetyl bromide lignins and lignin nitrobenzene oxidation products. In situ disappearance and cellulase solubility of both initial dry matter and CWR were determined. The populations of plants with depressed CAD activity showed no change in lignin content but some consistent changes in cell wall composition and digestibility were identified. The syringyl content of lignins decreased and the syringaldehyde to vanillin ratio (S/V) was consequently reduced. Dry matter degradability, as measured by both methods, was significantly improved in all CAD‐depressed samples except for population 40‐1, which was the least CAD‐depressed. Increased in situ disappearance of cell wall (ISCWD) was found in all plants exhibiting more than 80% CAD down‐regulation and was maximal (7 percentage units) in population 50 which had the greatest CAD depression. The rates of ISCWD increased slightly in some populations (40‐2 and 50). The relationship between S/V and ISCWD was significant (R = ‐0·68) only in the samples from a selected population of mature, most depleted plants. Other modifications may therefore also contribute to the improvement in degradability. However the changes in lignin composition that were observed in CAD‐depressed tobacco are largely similar to those seen in some maize and sorghum mutants with altered lignification and improved digestibility. These data therefore suggest that depressing CAD activity may be an effective method for improving digestibility in forage crops. © 1998 SCI.

Downregulation of Cinnamyl Alcohol Dehydrogenase (CAD) Leads to Improved Saccharification Efficiency in Switchgrass

“…Changes in lignin composition of CAD downregulated tobacco, alfalfa and poplar resulted in increased lignin extractability or degradability [13][14][15]17]. However, studies in alfalfa lines downregulated independently in six enzymes upstream of CAD showed that lignin content, rather than lignin composition, was the major factor affecting digestibility and saccharification efficiency [6,48].…”

“…One of the key enzymes, cinnamyl alcohol dehydrogenase (CAD), catalyzes the last step in the biosynthesis of lignin precursors, which is the reduction of cinnamaldehydes to cinnamyl alcohols [12]. Transgenic manipulation of CAD activity has been reported in several dicot species, such as tobacco [13,14], alfalfa [15,16], poplar [17][18][19], eucalyptus [20], and flax [21]. In a recent study of CAD downregulated alfalfa, two transgenic lines had increased enzymatic saccharification efficiency, although the increase was less than observed for cinnamoyl CoA reductase downregulated alfalfa lines in an otherwise identical genetic background [16].…”

Improving Saccharification Efficiency of Alfalfa Stems Through Modification of the Terminal Stages of Monolignol Biosynthesis

“…Cinnamoyl CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) catalyze the last two steps in monolignol biosynthesis, although CCR may also act earlier in the pathway [39]. Both enzymes have been targeted in a number of species in studies aimed at assessing effects on lignin content and composition [3,12,19,22,30,37,41,43], and it has been reported that CAD down-regulation in alfalfa results in small improvements in in situ digestibility of forage in sheep [1]. Strong downregulation of CCR has been shown to cause collapse of vascular tissue in tobacco, but, surprisingly, this can be overcome by simultaneously down-regulating CAD [8].…”

Lignification of plant cell walls: Impact of genetic manipulation

“…In the area of improving cell-wall digestibility of forage crops the results have been mixed. Bernard Vailhe et al (22,23) reported improved cell-wall digestibility of down-regulated O-methyltransferase (OMT) and CAD transgenic tobacco plants. The increased digestibility was attributed to an altered lignin composition (reduced S͞G ratio) because lignin concentration was not changed in these transgenics.…”