Efficient biolistic transformation of maize (Zea mays L.) and wheat (Triticum aestivum L.) using the phosphomannose isomerase gene, pmi, as the selectable marker.

A selectable marker system for plant transformation that does not require the use of antibiotics or herbicides was developed. The selectable marker consists of the manA gene from Escherichia coli under the control of a plant promoter that encodes for phosphomannose isomerase, pmi. Only transgenic plants were able to metabolize the selection agent, mannose, into a usable source of carbon, fructose. Transgenic plants were produced efficiently after delivery by Biolistics™ of the pmi gene into maize and wheat tissues, with mean transformation frequencies of 45% for maize and 20% for wheat. Adjustment of the sucrose and mannose levels in the selection medium essentially eliminated escapes. Transgenic events can be identified as early as 2 months for wheat and 4 months for maize. A simple test, a modified chlorophenol red assay, was used for early identification of transgenic events expressing the pmi gene. Transformation frequencies for both crops exceeded those obtained with the bar and pat genes with selection on either Basta or bialaphos.