To construct the miR408 overexpression vector, genomic DNA of lettuce containing pre-miR408 was PCR amplified using the primers MIR408-F/R. This fragment was then cloned into pBI121 vector, to create the MIR408- overexpressing vector 35S:MIR408. The vector containing the overexpression cassette for MIR408 was then introduced into the Agrobacterium tumefaciens strain EHA105, then transformed to Cotyledon explants of lettuce plants. Compared to WTs, 408OE plants showed increased growth vigor, exhibiting larger leaves and longer petiole at the seedling stage and harvest stage, transgenic plants showed a significant improvement in vegetative development and size.