Genetic engineering of the biosynthesis of glycinebetaine enhances the fruit development and size of tomato

•COD and BADH transgenic tomato plants can synthesis and accumulate glycinebetaine.•Glycinebetaine enhances the fruit development and size in transgenic tomato plants.•Glycinebetaine regulates the source-sink metabolism and phytohormones to increase the fruit development and size.•Glycinebetaine affects the expression of fruit development-related genes in transgenic tomato plants.•The transcriptome analysis indicated that glycinebetaine promotes tomato fruit development in multiple ways.