Several efforts are being made to generate engineered CMS plants (Chase 2006; Pelletier and Budar 2007). A quite promising approach was described by Ruiz and Daniell (2005) and reviewed by Khan (2005). Their approach has three advantages: (i) pollination and subsequent self-fertilisation is artificially suppressed, (ii) the trait is based on a cytoplasmic trait that cannot be transmitted via the pollen, and (iii) it allows for the selective restoration of male fertility, at least to some extent. This approach is based on inserting phaA, a gene that encodes p-ketothiolase, from the bacterium Acinetobacter into the chloroplast genome under control of the chloroplast psbA promoter. In transgenic tobacco plants, the enzyme accumulates in the leaves and anthers, altering the course of fatty acid synthesis (Fig. 14.3). By modifying lipid metabolism, pollen development is strongly impaired (Ruiz and Daniell 2005). The expression of p-ketothiolase also accelerates anther development and causes the pollen grains to collapse, leading to male sterility. Fertility restoration was achieved to some extent by growing the plants under continuous light. This effect is due to the light-sensitive gene expression controlled by the psbA promoter. Under these conditions, acetyl-CoA carboxylase gains the upper hand, thereby restoring normal fatty acid metabolism (Ruiz and Daniell 2005). However, restoration is only partial and the procedure does not appear to be applicable to field conditions.
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