Lignocellulose feedstock for biofuel production: Identification of morphological traits that could decrease lignin in crop residues

Background/Question/Methods

First generation biofuels are produced from plant biomass that is readily converted to fuel.  Second generation biofuels are produced from high lignocellulose feedstock that requires additional pretreatment steps to release sugars from the lignin prior to conversion to fuel. To decrease the economic and ecological costs associated with the production of second generation biofuels, breeding and genetic engineering programs are targeting whole-plant lignin reductions. However, these reductions often have negative consequences for vigor since lignin plays critical roles in support, water transport, and protection from herbivores and pathogens. Lignin reductions may be possible, however, if we gain a better understanding of its natural range of variation among tissues. We conducted an exploratory study to investigate the tissue-level composition of lignocellulose in stems of soybean, Glycine max L. Four non-genetically modified cultivars obtained from the USDA (Tidewater Agricultural Research Service) were planted in a field in Rockbridge County, Virginia in June 2011. Segments of soybean stems and branches were sampled twice during the growth season (at full stem height, and at senescence), and subjected to stem histology (microscopy) analysis and chemical analysis of lignocellulose.

Results/Conclusions

Soybeans in our field study allocated 49% of their biomass to lignocellulose crop residue, with 47% as stems. Stem biomass was further subdivided into main stem (62%) and branches (38%). Morphological and chemical attributes showed little variation among cultivars, cultivars were pooled to investigate possible relationships between stem tissues and chemistry. Main stems were twice as large as branches, and they contained nearly twice as much lignified tissue as branches. From a histological perspective, we discovered a significant, positive logarithmic relationship between stem diameter and lignified tissues; stem cross sectional area explained 65% of the variation in lignified tissue content. Our chemical analysis showed a significant, positive linear relationship between stem area and lignin content. Stem area explained 46% of variation in the lignin content of main stems, with maximum lignin levels reaching 20% of the dry weight. Main stems must support the weight of branches and developing fruits, so their proportionately greater amounts of lignified tissues are consistent with their load-bearing role. Our results indicate that decreasing stem diameter may reduce lignin concentration. We also found that increased branching improved seed production while stem diameter showed no significant relationship with seed production. The targeting of specific genes that increase branching or decrease stem diameter could be successful alternatives to whole-plant lignin reductions, while maintaining plant vigor.