As mentioned in the introduction, promoters can be categorized according to the type or degree of control of gene expression: control in all or virtually all tissues or control depending on the tissue and the developmental stage of the plant. Additionally, promoters may operate in response to external and, in some cases, controllable stimuli. Thus, they can be classified as follows:
- Constitutive promoters, which induce the expression of the downstream-located coding region in all tissues irrespective of environmental or developmental factors.
- Synthetic promoters, which comprise consensus DNA sequences of common elements of natural promoter regions.
- Inducible promoters, which are only expressed under the presence of factors/compounds. Because their expression is normally restricted to certain plant tissues, they can also be considered as tissue-specific. Based on the nature of the factors that trigger their expression, they are divided into two groups:
- Chemically-regulated, where chemical compounds, usually not naturally found within plants, switch on promoter activity. Several of the types of promoters discussed in this paper involve chimeric components gathered from human, animal, fungal and bacterial sources.
- Physically-regulated, where abiotic and external factors such as light, heat, mechanical injury induce promoter activity.
- Tissue-specific promoters, which operate in particular tissues and at certain developmental stages of a plant. They may be induced by endogenous and exogenous factors, so they may be also classified as inducible.