Characterization of an algal phosphomannose isomerase gene and its application as a selectable marker for genetic manipulation of tomato.

Establishing a transgenic plant largely relies on a selectable marker gene that can confer antibiotic or herbicide resistance to plant cells. The existence of such selectable marker genes in genetically modified foods has long been criticized. Plant cells generally exhibit too low an activity of phosphomannose isomerase (PMI) to grow with mannose as a sole carbon source. In this study, we characterized from the green microalga sp. and assessed its feasibility as a selectable marker for plant biotechnology. sp. () was shown to be closely related to higher plants but more distant to bacterial counterparts. Overexpression of in tomato induced callus and shoot formation in media containing mannose (6 g/L) and had an average transformation rate of 3.9%. Based on this transformation system, a polycistronic gene cluster containing , , and () was co-expressed in a different tomato cultivar. Six putative transformants were achieved with a transformation rate of 1.4%, which produced significant amounts of astaxanthin due to the expression of the genes. Taken together, these findings indicate that we have established an additional tool for plant biotechnology that may be suitable for genetically modifying foods safely.