Genetic Engineering of Plants
The hurdle of creating successful genetically engineered plants in major crop varieties is now being overcome sequentially on a plant-by-plant basis. The term “genetic engineering” is meant to describe the manipulation of the genome of a plant, typically by the introduction of a foreign gene into a plant, or the modification of the genes of the plant, to increase or decrease the synthesis of gene products in the plant. Typically, genes are introduced into one or more plant cells which can be cultured into whole, sexually competent, viable plants which may be totally transformed or which may be chimeric, having some tissues transformed and some not. These plants can be self-pollinated or cross-pollinated with other plants of the same or compatible species so that the foreign gene or genes carried in the germ line can be bred into agriculturally useful plant varieties.
Current strategies directed toward the genetic engineering of plant lines typically involve two complementary processes. The first process involves the genetic transformation of one or more plant cells of a specifically characterized type. The term “transformation” as used herein means that a foreign gene, typically in the form of a genetic construction, is introduced into the genome of the individual plant cells.
This introduction is accomplished through the aid of a vector, which is integrated into the genome of the plant. The second process then involves the regeneration of the transformed plant cells into whole sexually competent plants. Neither the transformation nor regeneration process need to be 100% successful, but must have a reasonable degree of reliability and reproducibility so that a reasonable percentage of the cells can be transformed and regenerated into whole plants.
EP-A0342926 (the content of which is incorporated herein by reference) discloses a plant (maize) ubiquitin regulatory system comprising a heatshock element (comprising two overlapping consensus heatshock elements), a promoter, a transcription start site, an intron and a translation start site. The heatshock element component of this regulatory system is believed to confer heat inducibility of expression of associated DNA sequences in dicot or monocot cells following permissive levels of heatshock.
Plant ubiquitin regulatory system refers to the approximately 2 kb nucleotide sequence 5′ to the translation start site of the ubiquitin gene and comprises sequences that direct initiation of transcription, control of expression level, induction of stress genes and enhancement of expression in response to stress. The regulatory system, comprising both promoter (of about 1 kb nucleotide sequence) and regulatory functions, is the DNA sequence providing regulatory control or modulation of gene expression. A structural gene placed under the regulatory control of the plant ubiquitin regulatory system means that a structural gene is positioned such that the regulated expression of the gene is controlled by the sequences comprising the ubiquitin regulatory system.
Promoters are DNA elements that direct the transcription of RNA in cells. Together with other regulatory elements that specify tissue and temporal specificity of gene expression, promoters control the development of organisms.
There has been a concerted effort in identifying and isolating promoters from a wide variety of plants and animals, especially for those promoters demonstrating a high level of constitutive expression and capable of maintaining stable levels of said expression under stress conditions.
The present invention is based on modifications of the plant ubiquitin regulatory system.