One of the goals of plant genetic engineering is to obtain plants having improved characteristics or traits. Many different types of characteristics or traits in plants are considered advantageous. Those of particular importance with regard to fruit bearing plants include control of fruit ripening, improvements in the nutritional characteristics of the edible portions thereof, resistance to plant diseases, resistance to insects, cold tolerance and enhanced stability or shelf-life of the ultimate consumer product obtained from the plant.
At least two key components are required to stably engineer a desired trait, or control of such a trait, into a plant. The first key component comprises identifying and isolating the gene(s) which either encode(s) or regulate(s) a particular trait. The second component comprises identifying and isolating the genetic element(s) essential for the actual expression and/or selective control of the newly isolated gene(s) so that the plant will manifest the desired trait and, ideally, manifest the trait in a controlled or controllable manner. This second component, which controls or regulates gene expression, typically comprises transcriptional control elements known as promoters. Although a generic class of promoters which drive the expression of heterologous genes in plants have been identified, a broad variety of promoters active in specific target tissues or cells of plants remain to be described. The identification of such target or tissue-specific promoters is critical to the introduction of the above-mentioned tissue-specific improvements in plants such as fruit bearing plants.
Several promoters useful in expressing heterologous genes in selected fruits have already been identified. For example, the E4 and D8 promoters (Deikman, et al.), the kiwifruit actinidin promoter (Lin, et al.) and promoter for polygalacturonase are known to be fruit specific. U.S. Pat. No. 4,943,674 (Houck et al., Jul. 24, 1990) discloses a 2All promoter as useful in expression of a heterologous gene in tomato fruit. These promoters, however, have been isolated from fruit tissue which comprises mature or maturing ovaries (hereinafter referred to as "traditional fruit"). As such, these traditional fruit promoters would be ineffective in controlling desired traits in such accessory fruit bearing plants as strawberry, apple, pear, quince and the like wherein the major portion of the edible fruit comprises receptacle tissue (see An Introduction to Plant Biology. 2nd Edition, Braungart & Arnett, eds., C. V. Mosby Co. 1965). Similarly, to date, genes thought to be active in fruit tissue have been isolated from traditional fruit tissue instead of receptacle containing tissue.
There exists a need for receptacle tissue selective promoters in the art. Access to receptacle tissue selective promoters would enable the genetic engineering of fruit tissue from commercially important plants such as strawberry, apple, and pear. Screening of DNA libraries was undertaken as a method for the identification of tissue selective promoters from strawberry. Four such sequences were identified, and the promoters and their associated structural genes sequenced. Expression of reporter genes in tobacco and tomato plants was used as an assay of the tissue specificity of the isolated promoters. Methods for the identification and isolation of analogous promoters and structural genes from other plants is described.