Identification and characterization of the mutants. A) Complementation of the picup1 mutant by one of the JatY clones. B) Schematic representation of the TWR gene. Boxes indicate coding sequence. The conserved mTERF domain (pfam PF02536) in red, UTRs in light blue. twr-1: point mutation C > T causing premature stop-codon at the end of the mTERF domain. twr-2: T-DNA insertion in intron 4. C, D) Aniline blue staining of mature embryos showing a dome shaped group of cells in wt representing the shoot apical meristem (C), which appears absent in the twr-1 mutant embryo (D). E) Complementation analysis of the cross between twr-1 and twr-2 shows identical above ground phenotypes to the single mutants. A backcross of twr-1 to wt phenotypically resembles the wt.