Potential Side Effects And Compensation Mechanisms Associated With Lignin Modifications

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One envisaged drawback related to lignin manipulation in plants could be the potential attenuation of defense mechanisms against pathogens. Indeed, lignification is considered one of the adaptive plant responses to pathogen attacks (25). Down-regulating lignin synthesis could reduce the ability of plants to protect themselves against pests. Such a hypothesis has not yet been substantiated using the transgenic plants already obtained, but the use of chemical inhibitors of lignification resulted in decreasing the hypersensitive response in wheat plants challenged by Puccinia graminis (26). In order to minimize the drop in these defense mechanisms, which usually occur at the surface of the tissues, the use of specific promoters to drive transgene expression has been envisaged. Ideally, such a promoter should be xylem specific, avoiding a decrease in stress lignins produced on the plant surfaces.

Another important aspect of lignin manipulation is the potential disturbance of plant development. Reduction or modification of the lignin con tent of plant cell walls may theoretically exert pleiotropic effects on plant functions through changes in the strength of plant organs, sap conduction through the xylem, or permeability of cell wall barriers (caspary band). Changes in plant development have been observed for strongly depressed CCR tobacco lines even though it has not been clearly demonstrated whether the resulting phenotype (reduced growth) is due to changes in cell wall properties or to changes of soluble phenolic profiles. Such a reduction in size has also been demonstrated for CCoAOMT down-regulated tobacco plants. Surprisingly, CCR/CAD double transformants exhibit a strong decrease in lignin content but no changes in morphology (at least in culture room conditions). This last observation suggests that the decrease in lignin content per se is not directly responsible for the developmental changes observed but that other unknown metabolic changes are probably involved.

It is worthwhile mentioning that in our hands the same tobacco transgenic line (CCR down-regulated) that exhibited normal growth in culture room conditions was developmentally altered when grown in field conditions. It is possible that the outside habitat provides more adverse conditions (e.g., water stress, mechanical stress) and that the transgenic line adapts itself less easily to this natural context.

It is clear that for applied purposes the transformed plants should behave as wild-type plants in terms of biomass production and agronomic performance. This has been verified for 4-year-old CAD down-regulated poplars, which appear to be promising candidates for immediate application. The same evaluation procedures at the field level should be performed in the long term for each new transformed line.

From the functional point of view, plants with altered lignin synthesis seem to adopt compensatory strategies for maintaining the integrity of their cell walls. This is clearly illustrated in the case of CAD down-regulated plants that utilize cinnamaldehydes instead of cinnamyl alcohols as building blocks of their transgenic lignins. CCR down-regulated tobacco plants partially compensate for the lack of lignin in their cell walls by a striking increase in tyramine ferulate (16). Additional reinforcement of the cell wall may also be provided in these transgenic lines by a very significant increase in wall-associated phenolic compounds (15).

The question is still open whether these new biochemical phenolic decorations of the cell wall or of the cytosolic fraction have a positive or a negative impact on the pulping process. A negative example is provided by Eucalyptus camaldulensis CAD down-regulated plants that overproduce tannins, interfering with the bleaching step during the pulping process (T. Ona, personal communication). In contrast, if the compensation strategy could concern useful components of the cell wall, the benefits would be obvious. This situation is illustrated by the exciting results obtained by Chiang's group (14) for 4CL down-regulated aspen, for which the down-regulation of lignin was accompanied by an increase in cellulose leading to a nearly doubled cellulose/lignin ratio. If the required defense, water transfer, find other properties survive, as they appear to in these particularly vigorous plants, the potential for improving plant utilization is enormous.

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