T-DNA mediated disruption of essential gametophytic genes in Arabidopsis unexpectedly rare and cannot be inferred from segregation distortion alone

Many genes are thought to be expressed during the haploid phase in plants, however, very few haploid-specific genes have been isolated so far. T-DNA i nsertion mutagenesis is a powerful tool for generating mutations that affec t gametophyte viability and function, as disruption of a gene essential for these processes should lead to a defect in the transmission of the gametes . Mutants can therefore be screened on the basis of segregation distortion for a reporter resistance gene contained in the T-DNA. We have screened the Versailles collection of Arabidopsis transformants for 1:1 Kan(R):Kan(S) s egregation after selfing, focussing on gametophyte mutations which show nor mal transmission through one gametophyte and cause lethality or dysfunction of the other. Only 1.3% (207) of the 16,000 lines screened were scored as good candidates. Thorough genetic analysis of 38 putative T-DNA transmissio n defect lines (Ttd) identified 8 defective gametophyte mutants, which all showed 0 to 1% T-DNA transmission through the pollen. During the screen, we observed a high background of low-penetrance mutations, often affecting th e function of both gametophytes, and many lines which were likely to carry chromosomal rearrangements. The reasons for the small number of retained li nes (all male gametophytic) are discussed, as well as the finding that, for most of them, residual T-DNA transmission is obtained through the affected gametophyte.