Submitted to: Biologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 8, 2010
Publication Date: January 10, 2012
Citation: Chlan, C.A., Rajasekaran, K., Cary, J.W., Cleveland, T.E. 2012. Expression patterns of cotton chloroplast genes during development: implications for development of plastid transformation vectors. Biologia Plantarum. 56(1):126-130. Interpretive Summary: Commercial transgenic cotton varieties with herbicide and/or insect resistant genes are under cultivation in more than 80% of cotton growing area in the United States and they are also becoming popular worldwide. These transgenes have been expressed in the nuclear genome. Nuclear expression of transgenes in plants can be limited by the position of the gene in cotton chromosomes, the number of gene copies in the nuclear genome and the expression could be shut off by gene silencing, a process by which the host plant fights back foreign genes. In addition, nuclear expressed genes can escape by pollen transmission to related species. Transformation of plastid genome (for example, chloroplast) circumvents many of these concerns. A high level of gene expression is also possible in plastids due to their high copy numbers in each cell as opposed to one nucleus. These are some of the reasons why plastid transformation is preferable to nuclear transformation. However, to express genes in plastids of target crop species suitable cassettes (called vectors) should be developed for introduction of foreign genes. In this report, we have studied the expression patterns of 20 cotton chloroplast genes and evaluated them for use in cotton plastid transformation vectors. This information will be useful to biotechnology researchers and geneticists involved in the production of transgenic cottons via chloroplast transformation.
Technical Abstract: In order to express genes of interest in plastids, transformation vectors must be developed that include appropriate promoters to drive expression at effective levels in both green and non-green tissues. Typically, chloroplasts are transformed with vectors that contain ribosomal RNA promoters for high levels of expression. Theoretically, plastid transformation can also be used to express genes in dark-grown tissues as well as in specific stages of plant growth by using appropriate plastid promoters. Using filter dotblot hybridization, we have characterized the RNA expression patterns of 20 chloroplast genes in light and dark-grown cotton tissues at different stages of development. A subset of 6 of these genes was further characterized by real time PCR. Expression of rrn16 was the highest in all tissues and conditions tested. Expression levels of psbA approach those of rrn16 in light-gown hypocotyls, cotyledons and mature leaves. Differences in the levels of expression of all the genes tested varied between the tissues and treatments. Using the information gained in these studies, cotton plastid transformation vectors can be tailored for optimal expression in specific tissues.