Decomposition in soil of tobacco plants with genetic modifications to lignin biosynthesis

Genetic modification of the amount, conformation and composition of lignin in plant materials is being explored both to understand better the process of lignin biosynthesis and with a view to enhancing forage digestibility or paper pulping properties. We have investigated the interaction between the effects of genetic modifications to lignin biosynthesis and the activity of decomposer organisms to provide information in relation to understanding the wider ecological effects of specific genetic modifications to crop plants and because the plants with modified lignin biosynthesis may be useful models in decomposition studies. The decomposition of material from the stems of four lines of tobacco (Nicotiana tabacum L.) plants, three of which had genetic modifications to lignin biosynthesis, were followed during a 77 day incubation in four different soils under laboratory conditions. The tobacco plants were either unmodified (wild-type) or had antisense or partial sense transgenes for one of three crucial enzymes [cinnamyl alcohol dehydrogenase (CAD), caffeic acid O-methyltransferase (COMT) or cinnamoyl CoA-reductase (CCR)] for lignin biosynthesis. Solid-state 13C nuclear magnetic resonance spectroscopy indicated that stem material from the unmodified plants, reduced CAD and reduced COMT plants all had similar amount of lignins, whereas stem material from the reduced CCR plants contained less lignin. Material from all of the modified plants decomposed more rapidly than material from the wild-type plants. Depending on the soil, between 11.7 and 16.3% of the C added in the plant material was lost as CO2 during a 77 day incubation from reduced CCR plants compared with between 6.1 and 9.2% for the reduced COMT plants, between 3.6 and 7.9% for the reduced CAD plants and between 3.1 and 5.9% for the wild-type plants. The increased decomposition rate of reduced CAD and reduced COMT plants compared with material from the wild-type plants was attributed primarily to differences in the degree of protection from microbial attack afforded to the polysaccharides and other relatively labile plant components by the lignin. In the reduced CAD and the reduced COMT plants, the composition and conformation but not the concentration of the lignin was altered compared to the wild-type plants. The greater rate of decomposition of reduced CCR plants compared with the wild-type plants was most likely the result of the smaller lignin content of these plants.