Up and Downstream of TOR in Plants

AMPK is a conserved TORC1 regulator in yeast and mammals. AMPK has a readily identifiable homologue in Arabidopsis, At3g01090, and in other plants (Bhalerao et al. 1999; Sugden et al. 1999; Thelander et al. 2004). However, it is unclear how this energy-sensing pathway signals to TORC1.

The best-characterized mammalian TOR effectors are S6K and 4e-BP. There are two clear S6K homologues in Arabidopsis (Turck et al. 1998, 2004), raising the possibility that TOR signaling through this effector is conserved. Biochemical evidence supporting this hypothesis is discussed below. 4E-BP is not identified in the Arabidopsis genome.

Another putative TOR effector in plants is the meiosis regulator Mei2. Though not present in budding yeast or mammals, evidence from fission yeast suggests that Mei2 is intimately connected to TOR signaling.

Mei2 triggers premeiotic DNA synthesis and meiosis in conjugated diploid zygotes under low nutrient conditions (Watanabe et al. 1988; Watanabe and Yamamoto 1994). Mei2 binds fission yeast Mip1/Raptor, marking it as a likely TORC1 substrate (Shinozaki-Yabana et al. 2000). Mei2 transcription is disrupted in Ste20/Rictor mutants, implicating it in TORC2 signaling (Hilti et al. 1999). Mei2 activity is regulated by phosphorylation (Watanabe et al. 1997).

There is a small family of Mei2-like proteins in plants (Anderson et al. 2004). All members share with Mei2 a pair of weakly conserved RRM-type N-terminal RNA recognition motifs and a single highly conserved C-terminal RRM. One member, AML1 (Arabidopsis Mei2-like), suppresses Mei2 signaling defects when overexpressed in fission yeast meiosis-deficient mutants (Hirayama et al. 1997).

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