Improving Brassica napus straw for cellulosic ethanol production
It is likely that a combination of process improvement and plant breeding will be needed to make efficient use of biomass. To achieve this, the main bottlenecks to decomposition must be identified and strategies developed to make the process more efficient. Here, the likely process and genotypic variants relevant to saccharification efficiency of Brassica napus straw were investigated using steam explosion. Screening methods were developed to gather suitable data for association mapping. Areas of the B. napus transcriptome related to processing traits were highlighted and assessed. The results show that autocatalytic pretreatments are effective, but commercial cellulases are poorly adapted to hydrolyse Brassica napus straw. Following pretreatment, saccharification was limited by uronic acids and xylose retention, which impeded cellulase performance. Surprisingly, lignin abundance correlated positively with the proportion of rapidly hydrolysable carbohydrate in the pretreated residues. Cultivars with glucan-rich straw did not necessarily produce higher saccharification or ethanol yields. Instead, variations in the non-cellulosic components were the most important differences between cultivars. Smaller scale methods were developed and rational conditions selected for screening In total, seventy-seven straw accessions were processed using four conditions which allowed processing/genotype interactions to be explored. Cultivars that produced more acetic and formic acid using suboptimal pretreatment conditions obtained higher glucose yields. Associated areas of the transcriptome suggested that genes involved in sugar nucleotide provisioning, the endogenous hydrolysis of cellulose and non-cellulosic polysaccharide synthesis were related to saccharification yields. Candidate genes aid in the development of testable hypotheses related to biomass recalcitrance and provides specific targets to conduct experiments, but molecular markers were sensitive to agronomic conditions. Arabidopsis plants deficient in a selection of candidate genes revealed subtle changes in saccharification performance. Result from this work could be the first steps towards deducing the genetic determinants of biomass recalcitrance, paving the way for further research.
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