Generation and Characterization of Soybean and Marker-free Tobacco Plastid Transformants Over-expressing a Bacterial 4-hydroxyphenylpyruvate Dioxygenase Which Provides Strong Herbicide Tolerance

Plant 4-hydroxyphenylpyruvate dioxygenase (HPPD) is part of the biosynthetic pathway leading to plastoquinone and vitamin E. This enzyme is also the molecular target of various new bleaching herbicides for which genetically engineered tolerant crops are being developed. We have expressed a sensitive bacterial hppd gene from Pseudomonas fluorescens in plastid transformants of tobacco and soybean and characterized in detail the recombinant lines. HPPD accumulates to approximately 5% of total soluble protein in transgenic chloroplasts of both species. As a result, the soybean and tobacco plastid transformants acquire a strong herbicide tolerance, performing better than nuclear transformants. In contrast, the over-expression of HPPD has no significant impact on the vitamin E content of leaves or seeds, quantitatively or qualitatively. A new strategy is presented and exemplified in tobacco which allows the rapid generation of antibiotic marker-free plastid transformants containing the herbicide tolerance gene only. This work reports, for the first time, the plastome engineering for herbicide tolerance in a major agronomic crop, and a technology leading to marker-free lines for this trait.