As mentioned in the Introduction, there are promoters controlling gene expression in a tissue-dependent manner and according to the developmental stage of the plant. The transgenes driven by these type of promoters will only be expressed in tissues where the transgene product is desired, leaving the rest of the tissues in the plant unmodified by transgene expression. Tissue-specific promoters may be induced by endogenous or exogenous factors, so they can be classified as inducible promoters as well.
Unlike constitutive expression of genes, tissue-specific expression is the result of several interacting levels of gene regulation. As such, it is then preferable to use promoters from homologous or closely related plant species to achieve efficient and reliable expression of transgenes in particular tissues. This is one of the main reasons for the large amount of tissue-specific promoters isolated from particular plants and tissues found in both scientific and patent literature. Patents claiming particular tissue promoters such as beta-amylase gene or barley hordein gene promoters (for seed gene expression), tomato pz7 and pz130 gene promoters (for ovary gene expression), tobacco RD2 gene promoter (for root gene expression), banana TRX promoter and melon actin promoter (for fruit gene expression) and so forth, are the most abundant in the patent literature.
The number of promoters “tailor-made” (isolated and identified) for the expression of transgenes in particular tissues and plants is in the hundreds. Very few patents are drawn to tissue-specific promoters (e.g. seed-specific or a root-specific promoter) in general terms
The purpose of this section is to present those patents directed to plant tissue-specific promoters in broad terms. Patents related to particular p romoters derived from specific genes and plants are beyond the scope of this paper.
The analysed patents are categorised according to tissue where the promoter controls the transcription of a gene of interest. These are:
- Root promoters: Pioneer Hi-Bred has filed several patent applications directed to root promoters that enhance or suppress the expression of a linked gene in root cells. In addition, the invention comprises methods for the identification and isolation of plant tissue-specific promoters in general.
- Fruit promoters: Calgene‘s large portfolio of patents on tissue-specific promoters includes fruit specific promoters that control the expression of genes in mature ovary tissue of a fruit and in the receptacle tissue of accessory fruits such as strawberry, apple and pear. The genes driven by the promoters of Calgene’s inventions influence fruit development and ripening.
- Seed promoters: Calgene, Sapporo Breweries and the University of California have granted patents and patent applications drawn to seed-specific promoters in broad terms. Transcription cassettes having a seed-specific promoter and recombinant molecules containing a seed-maturation promoter are part of their inventions.