Shekoofeh Sadravi, June Lee, Jianmin Xu
Plant-based expression systems, known as molecular farming, have emerged as sustainable and scalable platforms for producing recombinant proteins used in pharmaceuticals, industrial enzymes, and agricultural products. Among the key determinants of transgene performance, promoter elements play a central role in defining transcriptional strength, specificity, and regulation. This review highlights current advances in promoter engineering tailored for plant systems, encompassing natural, synthetic, hybrid, inducible, and tissue-specific promoters used in stable transgenic plants, transient expression systems, and plant cell cultures. The structural and functional features of promoter elements are discussed, along with strategies to mitigate challenges such as transcriptional silencing, genomic context dependency, and variability cross species and production platform. Emerging synthetic biology tools, such as CRISPR-based transcriptional control, high-throughput screening, and machine learning-assisted promoter design, are enabling the creation of tunable, orthogonal promoters suited for complex multigene expression. As promoter engineering continues to evolve, it remains foundational to advancing plant molecular farming and expanding the role of plants as versatile biofactories for high-value recombinant proteins.