Insertion alleles disrupting each AtRaptor locus have been described (Anderson et al. 2005; Deprost et al. 2005). Expression analysis of the two genes indicates that they show a similar pattern of expression but different relative levels. AtRaptorlB accounts for 80% of total AtRaptor transcript accumulation in most tissues. Given the high degree of similarity in their encoded proteins, single mutant homozygotes likely represent partial loss of Raptor function.
AtRaptorlA-/- insertion allele homozygotes show no phenotype (Anderson et al. 2005; Deprost et al. 2005). This phenotype is consistent with the similar pattern but low level of expression of this locus relative to AtRaptorlB.
AtRaptorlB-/- mutants described by Anderson et al. (2005) show a range of mild developmental phenotypes. Plants undergo leaf initiation more slowly than wild type and bolt later. AtRaptorlB-/- root development is mildly disrupted. Finally, AtRaptorlB-/- shoot architecture is altered. The primary shoot is shorter than wild type. Branches off of this primary shoot, as well as secondary shoots emerging from the rosette, are more abundant than wild type but do not differ from wild type in length. This phenotype points to a defect specifically in the maintenance of the primary shoot apex; the increased branching is consistent with a loss of repression of axial meristem activity upon the exhaustion of the primary apical meristem. There does not appear to be any defect in the maintenance of secondary, axial meristems in the AtRaptorlB-/- mutant.
AtRaptorlA-/- lB-/- double mutants arrest development as seedlings with minimal post-embryonic growth on soil and on agar plates (Anderson et al. 2005). Seeds germinate slowly and yield seedlings that are smaller than wild type but otherwise fully formed. Roots show minimal growth on plates. Dark-germinated seedlings show a significant lengthening of the hyopcotyl, indicating that these plants are able to undergo vacuolar-expansion driven growth. The root apical meristem is easily recognized and fully formed in these mutants, although it is smaller than wild type and there is a reduction in the number of files of cells in the root elongation zone. Primordia for leaves one and two, formed embryonically, are present but do not grow. This indicates that the shoot apical meristem has formed but is unable to initiate the post-embryonic growth that characterizes vascular plants. Both the AtRaptorlB-/- and the AtRaptorlA-/- 1B-/- phenotypes described above have been confirmed by other researchers working with progeny or sibling progeny of the plants described above.
AtRaptorlB-/- single mutants as described by Deprost et al. (2005) show a much more severe phenotype than that described above. Single mutant AtRaptorlB-/- homozygotes (called AtRaptorl) show arrest at or immediately after fertilization. The authors did not detect even the first zygotic cell division that distinguishes the suspensor precursor cell from the precursor cell of the embryo proper. This phenotype, the authors note, is much more severe than that seen with AtTOR-/- disruption (Menand et al. 2002), and it is seen in a genetic background where the AtRaptorlA locus is intact and expressed. Significantly, this early embryonic arrest phenotype is also seen in a small fraction (7%) of wild-type embryos grown under similar conditions. The phenotype reported by Deprost et al. may indicate that AtRaptorlB-/- embryos are hypersensitive to a stress specific to their lab's growth conditions, consistent with a role for TOR in stress as well as nutrient signaling.
Was this article helpful?