Ferulate‐5‐hydroxylase is a key enzyme involved in the conversion of the guaiacyl monolignol to the syringyl monolignol in angiosperms. The monolignol ratio has been proposed to affect biomass recalcitrance and the resistance to plant disease. Stem rot caused by the fungus Sclerotinia sclerotiorum in Brassica napus causes severe losses in its production. To date, there is no information about the effect of the lignin monomer ratio on the resistance to S. sclerotiorum in B. napus. Four dominantly expressed ferulate‐5‐hydroxylase genes were concertedly knocked out by CRISPR/Cas9 in B. napus, and three mutant lines were generated. The S/G lignin compositional ratio was decreased compared to that of the wild type based on the results of Mӓule staining and 2D‐NMR profiling in KO‐7. The resistance to S. sclerotiorum in stems and leaves increased for the three f5h mutant lines compared with WT. Furthermore, we found that the stem strength of f5h mutant lines was significantly increased. Overall, we demonstrate for the first time that decreasing the S/G ratio by knocking out of the F5H gene improves S. sclerotiorum resistance in B. napus and increases stem strength.
The oilseed rape production is unremittingly affected by Sclerotinia sclerotiorum, which causes stem and leaf rot and reduces the production of rapeseed oil. Lignin is an established physical barrier against pathogens, and the different lignin monomer G and S content can have different lignin structures. In this study, four dominantly expressed BnF5H genes were knocked out by CRISPR/Cas9 simultaneously in B. napus, and the f5h mutants were generated. The S‐/G‐lignin composition ratio was decreased compared to that of the wild type (WT) based on the results of Mӓule staining and 2D‐NMR profiling in the KO‐7 line. The resistance to S. sclerotiorum in stems and leaves increased in knock‐out lines. Furthermore, we found that the stem strength of mutants was significantly increased compared to that of the WT. Collectively, for the first time, we demonstrate that knockout of the lignin pathway gene F5H decreases the S/G ratio, improves S. sclerotiorum resistance in B. napus, and increases stem strength.