© 2008 The Author(s).
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RESEARCH PAPER |
A light and electron microscopy analysis of the events leading to male sterility in Ogu-INRA CMS of rapeseed (Brassica napus)
1Centro de Biotecnología y Genómica de Plantas, UPM-INIA, ETSI Agrónomos, Ciudad Universitaria s/n, E-28040 Madrid, Spain
2Station de Génétique et d'Amélioration des Plantes, IJPB, INRA UR254, Route de Saint-Cyr, F-78026 Versailles Cedex, France
3Instituto de Recursos Naturales, Centro de Ciencias Medioambientales, CSIC, Serrano 115-bis, E-28006 Madrid, Spain
* To whom correspondence should be addressed: E-mail: mlucas{at}ccma.csic.es
Ogura cytoplasmic male sterility (CMS) occurs naturally in radish and has been introduced into rapeseed (Brassica napus) by protoplast fusion. As with all CMS systems, it involves a constitutively expressed mitochondrial gene which induces male sterility to otherwise hermaphroditic plants (so they become females) and a nuclear gene named restorer of fertility that restores pollen production in plants carrying a sterility-inducing cytoplasm. A correlative approach using light and electron microscopy was applied to define what stages throughout development were affected and the subcellular events leading to the abortion of the developing pollen grains upon the expression of the mitochondrial protein. Three central stages of development (tetrad, mid-microspore and vacuolate microspore) were compared between fertile, restored, and sterile plants. At each stage observed, the pollen in fertile and restored plants had similar cellular structures and organization. The deleterious effect of the sterility protein expression started as early as the tetrad stage. No typical mitochondria were identified in the tapetum at any developmental stage and in the vacuolate microspores of the sterile plants. In addition, some striking ultrastructural alterations of the cell's organization were also observed compared with the normal pattern of development. The results showed that Ogu-INRA CMS was due to premature cell death events of the tapetal cells, presumably by an autolysis process rather than a normal PCD, which impairs pollen development at the vacuolate microspore stage, in the absence of functional mitochondria.
Key words: Brassica napus, cell death, light and electron microscopy, mitochondria, plastids, pollen development, Ogu-INRA cytoplasmic male sterility, transgenic-restored plants, tapetum
Received 30 September 2007; Revised 11 December 2007 Accepted 20 December 2007
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