Targeted Mutagenesis of the Rice FW 2.2-Like Gene Family Using the CRISPR/Cas9 System Reveals OsFWL4 as a Regulator of Tiller Number and Plant Yield in Rice

The FW2.2-like (FWL) genes encode cysteine-rich proteins with a placenta-specific 8 domain. They play roles in cell division and organ size control, response to rhizobium infection, and metal ion homeostasis in plants. Here, we target eight rice FWL genes using the CRISPR/Cas9 system delivered by Agrobacterium-mediated transformation. We successfully generate transgenic T(0) lines for 15 of the 16 targets. The targeted mutations are detected in the T(0) lines of all 15 targets and the average mutation rate is found to be 81.6%. Transfer DNA (T-DNA) truncation is a major reason for the failure of mutagenesis in T(0) plants. T-DNA segregation analysis reveals that the T-DNA inserts in transgenic plants can be easily eliminated in the T(1) generation. Of the 30 putative off-target sites examined, unintended mutations are detected in 13 sites. Phenotypic analysis reveals that tiller number and plant yield of OsFWL4 gene mutants are significantly greater than those of the wild type. Flag leaves of OsFWL4 gene mutants are wider than those of the wild type. The increase in leaf width of the mutants is caused by an increase in cell number. Additionally, grain length of OsFWL1 gene mutants is higher than that of the wild type. Our results suggest that transgene-free rice plants with targeted mutations can be produced in the T(1) generation using the Agrobacterium-mediated CRISPR/Cas9 system and that the OsFWL4 gene is a negative regulator of tiller number and plant yield.