Genetic modification of cellulosic biofuel crops: Implications for pest susceptibility

Background/Question/Methods

Genetic modification is being explored as a tool for enhancement of cellulosic biofuel feedstocks. Modification of lignin content or composition is of particular interest, as a method to improve biofuel processing efficiency. Lignin, however, is not only a barrier to ethanol production, but also provides a mechanical barrier against pests, and is produced via the same metabolic pathway as principal chemical defenses in poplar. We investigated the effects of genetic modification of lignin on non-target defense traits and pest abundance in hybrid poplar (Populus alba x P. tremula).

To assess the effects of lignin modifications on pest susceptibility, we utilized multiple lines from four genotypes genetically modified for reduced total lignin content or altered lignin subunit ratios. We evaluated replicate trees from each modified line, in addition to non-modified controls, for levels of chemical resistance and pest abundance. We surveyed these trees twice yearly, for two years. During each survey, we collected leaves for analysis of compounds important to pest susceptibility (condensed tannins, phenolic glycosides, nitrogen and lignin). We also measured pest abundance via visual surveys of each tree. We predicted that modification of stem lignin would translate to changes in leaf chemical composition and thus affect pest abundance.

Results/Conclusions

Our data suggest that leaf chemistry is only significantly different among the genotypes at certain leaf collection dates. We found differences among genotypes and/or among lines within genotypes for condensed tannins (geno p<.0002; line p<.0168), phenolic glycosides (geno p<.0181), nitrogen (line p<.0087) and lignin (geno p<.0001). We know that some of these compounds vary strongly with tree age and season, however, these factors do not account for the change in phytochemical ranking we found among the genotypes at each collection date. Our results also revealed unexpected expression of altered lignin content in genotypes only modified for lignin composition.

Despite differences in leaf chemistry among genotypes at certain dates, we did not find any correlation with pest abundances. Our results suggest that although these lignin modifications did not yield the expected outcomes, effects to phytochemistry were inconsequential to pest populations. Our preliminary assessment is that these specific lignin modifications may pose little to no risk to hybrid poplar pest susceptibility over short growing periods but due to date effects, extended trials are encouraged. Results from this study will contribute to the development of genetically modified lines with higher processing potential and unchanged pest susceptibility.