Decomposition in soil of soluble, insoluble and lignin-rich fractions of plant material from tobacco with genetic modifications to lignin biosynthesis

The decomposition in soil of pieces of stem and different fractions of stems from uniformly-grown tobacco plants with genetic modifications to lignin biosynthesis was investigated by measuring CO₂ production over 74 d. The fractions used were intact stems, the insoluble fraction obtained by washing the stems with water, the lignin-rich fraction obtained by dissecting away the epidermis and cortex from the stems to leave a fraction in which the vascular tissue was concentrated, and the lignin-rich, insoluble fraction obtained by washing the lignin-rich fraction in water. The genetic modifications were the introduction of partial or antisense transgenes for cinnamyl alcohol dehydrogenase (CAD), caffeic acid O-methyl transferase (COMT) and cinnamoyl CoA reductase (CCR), which are all enzymes that catalyse key steps in the monolignol pathway. The total CO₂ produced during decomposition declined in the following order: intact stem>insoluble stem fraction>lignin-rich fraction>lignin-rich, insoluble fraction, for all plant lines except the lignin-rich fraction from the reduced CCR plants. There was an initial flush of CO₂ production that peaked between 2 and 4 d and which subsided within 20 d for both the intact stems and the lignin-rich fractions from all lines. There was no such flushes of CO₂ release from the corresponding insoluble fractions. For the intact stems, the amount of CO ₂ released during the initial 20 d was significantly greater for the reduced COMT plants, than for the reduced CAD or reduced CCR plants, both of which released significantly more CO₂ than the unmodified plants. Liquid-state ¹³C nuclear magnetic resonance spectroscopy of the water-soluble material from all lines showed that it was dominated by soluble sugars, but that there were no clear differences in the composition of the soluble fractions between the different lines of plants. Furthermore, the timing of the initial flush of CO₂ release from the intact plant materials corresponded to that from glucose when incubated in the same soil indicating that sugars probably fuelled the initial flush of CO₂ from the intact stems. Over 74 d, significantly more CO₂ was released from the reduced CCR plant stems than from the reduced CAD and reduced COMT plants, which did not differ significantly from each other, but which were significantly greater than the unmodified stems. The total CO₂ production from the lignin-rich fractions did not differ significantly between the unmodified, the reduced CAD and the reduced COMT plants, but the total CO₂ production from the lignin-rich fraction from the reduced CCR plants was substantially greater than that from the other lines. 

We have shown significant effects of targeted lignin modification on decomposition in laboratory studies over a relatively short period (<74 d). The effects of genetic modification can be explained in part at least by alterations in the amount, composition and conformation of the lignin and the effect it has on other, more labile, components. However, longer-term and more detailed field-based investigations are needed before the wider ecological significance of the differences in decomposition can be properly assessed.