No abstract

Genome editing is a valuable technique for gene function analysis and crop improvement. Over the past two years, the CRISPR-Cas9 system has emerged as a powerful tool for precisely targeted gene editing. In this study, we predicted 11 U6 genes in soybean (Glycine max L.). We then constructed two vectors (pCas9-GmU6-sgRNA and pCas9-AtU6-sgRNA) using the soybean U6-10 and Arabidopsis U6-26 promoters, respectively, to produce synthetic guide RNAs (sgRNAs) for targeted gene mutagenesis. Three genes, Glyma06g14180, Glyma08g02290 and Glyma12g37050, were selected as targets. Mutations of these three genes were detected in soybean protoplasts. The vectors were then transformed into soybean hairy roots by Agrobacterium rhizogenes infection, resulting in efficient target gene editing. Mutation efficiencies ranged from 3.2–9.7% using the pCas9-AtU6-sgRNA vector and 14.7–20.2% with the pCas9-GmU6-sgRNA vector. Biallelic mutations in Glyma06g14180 and Glyma08g02290 were detected in transgenic hairy roots. Off-target activities associated with Glyma06g14180 and Glyma12g37050 were also detected. Off-target activity would improve mutation efficiency for the construction of a saturated gene mutation library in soybean. Targeted mutagenesis using the CRISPR-Cas9 system should advance soybean functional genomic research, especially that of genes involved in the roots and nodules.

The bioconversion of carbohydrates in the herbaceous bioenergy crop, switchgrass (Panicum virgatum L.), is limited by the associated lignins in the biomass. The cinnamyl alcohol dehydrogenase (CAD) gene encodes a key enzyme which catalyzes the last step of lignin monomer biosynthesis. Transgenic switchgrass plants were produced with a CAD RNAi gene construct under the control of the maize ubiquitin promoter. The transgenic lines showed reduced CAD expression levels, reduced enzyme activities, reduced lignin content, and altered lignin composition. The modification of lignin biosynthesis resulted in improved sugar release and forage digestibility. Significant increases of saccharification efficiency were obtained in most of the transgenic lines with or without acid pretreatment. A negative correlation between lignin content and sugar release was found among these transgenic switchgrass lines. The transgenic materials have the potential to allow for improved efficiency of cellulosic ethanol production.

Switchgrass (Panicum virgatum L.) has been developed into an important biofuel crop. Embryogenic calli induced from caryopses or inflorescences of the lowland switchgrass cultivar Alamo were used for Agrobacteriummediated transformation. A chimeric hygromycin phosphotransferase gene (hph) was used as the selectable marker and hygromycin as the selection agent. Embryogenic calli were infected with Agrobacterium tumefaciens strain EHA105. Calli resistant to hygromycin were obtained after 5 to 8 weeks of selection. Soil-grown transgenic switchgrass plants were obtained 4 to 5 months after Agrobacterium infection. The transgenic nature of the regenerated plants was demonstrated by PCR, Southern blot hybridization analysis, and GUS staining. T1 progeny were obtained after reciprocal crosses between transgenic and untransformed control plants. Molecular analyses of the T1 progeny revealed various patterns of segregation. Transgene silencing was observed in the progeny with multiple inserts. Interestingly, reversal of the expression of the silenced transgene was found in segregating progeny with a single insert.