With very few exceptions (e.g. alfalfa), all major food and fodder crop species fall into the large group of angiosperm plants exhibiting maternal plastid transmission. Maternal inheritance excludes plastid genes from pollen transmission. Consequently, putting transgenes into the plastid genome instead of the nuclear genome
(as done in conventional transgenic plants) can greatly reduce the risk of unwanted transgene spreading via pollen. Uncontrolled transgene transmission through pollen dispersal represents a major concern in the public debate on transgenic technologies in agriculture and plant biotechnology. In this respect, two scenarios are frequently discussed: (i) pollen flow from fields with genetically modified (GM) cultivars to neighboring fields with non-GM cultivars and (ii) unwanted transgene spreading via pollen from GM plants to related plant species (through hybridization with sexually compatible wild or weed species). As maternal transgene inheritance can potentially prevent outcrossing via pollen flow, plastid genetic engineering has recently stirred tremendous interest among plant biotechnologists (reviewed, e.g., in Bock 2001, 2007; Bock and Khan 2004; Maliga 2004).
To critically assess the level of transgene confinement attainable by chloroplast transformation technology, knowledge about the reliability of maternal inheritance and the possible frequency of paternal leakage in a given crop species is of paramount importance. In view of the many different cytological and molecular mechanisms involved in maternal plastid inheritance (Fig. 2; Table 2) and the significant variation in them even between closely related species, general conclusions and statements are inappropriate here. How strict maternal inheritance is and whether or not paternal leakage occurs must be assessed on a species-by-species basis and requires genetic analyses (crosses and phenotypic analysis of the progeny) at a very large scale (Wang et al. 2004). The possibility of occasional paternal leakage notwithstanding, it is self-evident that chloroplast transformation offers greatly increased transgene containment compared with conventional (nuclear-transgenic) plants which would transmit the transgene with every single pollen grain. However, if paternal leakage occurs in a given species and pollen transmission of the transgene must be prevented altogether, stacking of plastid transformation with other containment methods will be necessary to eliminate the residual outcrossing risk (Daniell 2002; Lee and Natesan 2006).
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