EFFECT OF MODIFICATION OF THE O-METHYLTRANSFERASE ACTIVITY ON CELL-WALL COMPOSITION, ULTRASTRUCTURE AND DEGRADABILITY OF TRANSGENIC TOBACCO

The effect of O-methyltransferase (OMT) cDNA modulation on cell wall c omposition, ultrastructure and rumen degradability was measured on tra nsgenic tobacco (Nicotiana tabacum). The expression of OMT cDNA in ant isense orientation (AS) inhibited OMT activity by 92% whereas expressi on of sense constructs led to plants either co-suppressed (CS, 98% inh ibition) or overexpressing OMT activity. The cell wall residues of ste ms were analysed for lignin content, products of nitrobenzene oxidatio n (NBO) and polysaccharide content. Degradability was determined by a cellulase method. Sections of stem were stained by acid phloroglucinol and Maule reactant. Stem samples were incubated in the rumen for 8, 2 4 and 48 h and observed by scanning electron microscopy (SEM). Compare d to controls, OMT-depleted stems showed decreased hemicellulose conte nt but unchanged lignin content. In contrast, syringyl units decreased by 40 and 90% in AS and CS samples respectively and NBO content follo wed a similar trend. Dry matter cellulase degradability was significan tly improved by 3 . 5 and 5 . 6 percentage units in AS and CS samples respectively. SEM showed a greater bacterial colonisation in these sam ples and indicated a higher rate of rumen degradability in CS tissues than in controls. Overexpressing plants had a composition and a degrad ability similar to that of controls. For all the plants studied, the i mprovements in dry matter degradability were closely linked to the syr ingyl to guaiacyl ratio or to the NBO content. The modifications obser ved in down-regulated tobacco were similar to those produced by bm3 ma ize mutation, but without lignin decrease. Genetic modifications shoul d therefore be considered for improving forage digestibility.