The expression of a gene or the synthesis of RNA (ribonucleic acid) from a gene by the enzyme RNA polymerase is called transcription. The Eukaryotic transcription is a way more complex pathway than prokaryotic transcription. Apart from RNA polymerase several proteins called Transcription factors and promoter (-10 and -35) and enhancer gene sequences are required for gene expression.
Both plants and humans are eukaryotes, so they have same transcription factors and promoter sequences, however they different enhancer gene domains. So, if a plant gene is inserted into the human genome, due to lack of promoter sequences of that particular plant gene in the human gene results inhibition of expression or transcription of plant gene.
To test this hypothesis, take two sets of human endothelial cells. To set-I insert the plant gene without its promoter sequence and to set-II insert the plant gene with its promoter sequence. Then check the pattern of gene expression (RNA synthesis) by blotting techniques or microarray analysis. The set-I do not express the plant gene, and whereas set-II express plant gene.