Disruption of Putative TORC1 Regulators and Effectors

No Arabidopsis AMPK mutant has been reported. In the moss Physcomitrella, disruption of both AMPK genes via homologous recombination-mediated gene targeting yields plants which grow normally in continuous light but starve when grown on a light-dark cycle (Thelander et al. 2004). This phe-notype is consistent with a failure to negatively regulate growth under low nutrient conditions.

There is biochemical evidence implicating AtS6K1 (Mahfouz et al. 2006) and AML1 (Anderson and Hanson 2005) as TOR effectors in Arabidopsis. Seeds of transgenic Arabidopsis plants expressing AtS6K1 driven by the viral 35S promoter germinate as wild-type under normal conditions but fail to germinate under osmotic stress. This result implicates TOR signaling via AtS6K1 in the response to osmotic stress. Combined with the biochemical work described above, the work on AtS6K suggests that TOR activation of AtS6K1

mediated by AtRaptor1B is sensitive to osmotic stress, and that an excess of inactive AtS6K1 inhibits germination under osmotic stress.

Genetic analysis of AML1 in Arabidopsis (Anderson and Hanson 2005) is complicated by the fact that it is one of five members of a gene family whose constituents show a high degree of similarity both in sequence and expression patterns (Anderson et al. 2004). Lines misexpressing AML1 have not been recovered. However, the accumulation of AML transcripts in the shoot and root meristem embryonic precursors speaks to their importance in plant development. Insertion alleles are available for all AML gene family members. Single and higher order insertion homozygotes grow at the same rate as wild-type but bolt earlier. More significantly, at a lower degree of penetrance, AML mutants show meiotic defects, sterility, defects in gametophyte development, and in some allele combinations, developmental arrest as seedlings (Kaur et al. 2006). The AML mutant meiotic defects argue strongly for a conserved role of Mei2-like proteins in meiosis regulation across a broad range of organisms. The AML seedling developmental arrest is strongly reminiscent of AtRaptor1A-/- 1B-/-double mutants, although unlike AtRaptor double mutants the AML seedling arrest is sometimes accompanied by an absence of root growth.

The larger family of Mei2-like genes in plants also includes the TELs, which share the conserved Mei2-like RRM and overall protein structure with the AMLs but are restricted to expression in the shoot and root meristems. Disruption of TE-1 in maize causes defects in leaf phylotaxy resulting in the terminal maize tassel being enclosed in an ear-like sheath of leaves (Veit et al. 1998).

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