Classically, enhancers mediate either direction of expression, up- and down-regulation of genes, and involve both spatial and temporal control of gene expression in a specific cell independent of genomic location [52]. On the contrary, intron-mediated enhancers (IMEs) mainly identified in plant generally act in the expression enhancement of genes and are primarily located in the first ordinary intron position within a gene. In fact, in experiments performed in Arabidopsis, rice, and even mammals, the expression level of a gene with IMEs was increased up to 100-fold [29]. Genomic location and distance from transcription start site can influence the IME activity unlike the mode of expression regulation performed by the classical enhancers [53].
