Implications for TOR Signaling in Plants

From the work discussed above, one can make a few conclusions about TOR signaling in plants. All land plants tested are rapamycin insensitive (Menand et al. 2002); in Arabidopsis, this is likely due to the failure of FKBP12 to bind rapamycin and not a refection of a fundamental difference in plant TOR proteins (Menand et al. 2002; Mahfouz et al. 2006). Plants engineered to express rapamycin-binding FKBP proteins may be a valuable means of restoring the usefulness of rapamycin in dissecting plant TOR signaling. Such an approach has been used successfully in the green alga Chlamydomonas (Crespo et al. 2005) to increase the mild rapamycin sensitivity that this organism shows.

TOR is essential for organized cell growth early in embryonic development (Menand et al. 2002). In contrast, Raptor proteins are not essential for embryonic development in the absence of stress. AtRaptor1A-/- 1B-/- mutants show slow but otherwise near wild-type embryogenesis (Anderson et al. 2005). This phenotype contrasts with that of AtTOR-/- embryos, and indicates that TOR activity in embryogenesis is Raptor-independent.

The presence of Raptor-independent TOR activity in the embryo suggests that in plants as in animals, fungi, and slime molds TOR may function in a Raptor-independent complex. AtTOR-/- mutants do not show cell cycle arrest as one sees in rapamycin-treated yeast or in yeast Tor1- Tor2- double mutants, both of which lack TORC1 activity. Instead, AtTOR-/- embryos proceed through multiple rounds of unordered cell division with little or no cell growth (Menand et al. 2002), much like TORC2 disrupted yeast Tor2- mutants. There are no genes encoding TORC2-specific components Rictor/AVO3/Ste20 or hSin1/AVO1/Sin1 in any plant genome. These proteins may not be essential for plant Raptor-independent TOR activity; alternatively, plant homologues of these genes may have diverged so that they are no longer recognizable by sequence similarity searches.

Biochemical work indicates that a core TORC1 interaction between TOR and Raptor is conserved in plants (Mahfouz et al. 2006). The failure of AtRaptor1A-/- 1B-/- mutants to make the transition to meristem-driven growth after near normal embryonic development indicates that Raptor is not essential for all cell growth, but that it is essential for post-embryonic, meristem-driven growth. Notably, plant embryonic growth is largely determinate, while meristem-driven growth is remarkably plastic in response to environmental cues, nutrient cues, and stress. Collectively, these results suggest that a nutrient-sensitive TORC1 activity essential for all cell growth in yeast and mammals has been co-opted in land plants to drive cell growth specifically in the meristem, thus playing a major role in directing the plastic, nutrient-sensitive development of land plants rather than (or in addition to) cell-level responses to nutrient and environmental stimuli.

Biochemical work has identified two putative effectors of TORC1 signaling (Anderson and Hanson 2005; Mahfouz et al. 2006). The first of these, AtS6K1, interacts with AtRaptor1B physically, marking it as a putative TORC1 substrate. AtS6K1 is the homologue of a well-characterized TORC1 effector in mammals known to regulate ribosomal processivity. Work in plants indicates that a Raptor protein regulates AtS6K1 activity, and that AtS6K1 may play a role in growth in response to osmotic stress.

The second putative effector, AML1, interacts with AtRaptor1B in a yeast two-hybrid assay. Mei2-like proteins in plants have a conserved role in meio-sis (Kaur et al. 2006) and in the transition to flowering (Anderson and Hanson 2005). A more divergent plant Mei2-like protein, TE1, is an important regulator of leaf initiation in maize (Veit et al. 1998). Mei2 in fission yeast acts by binding a noncoding, polyadenylated mRNA-like molecule that tethers it to a specific locus in the fission yeast nucleus (Watanabe and Yamamoto 1994; Yamashita et al. 1998; Shimada et al. 2003). Interestingly, a bioinformatics approach has identified a significant number of similar mRNA-like molecules in plants (MacIntosh et al. 2001), suggesting that much of this intriguing signaling pathway may be conserved from fission yeast to plants.

These interpretations present quite a few hypotheses that labs will no doubt address in the near future. The presence of distinct TORC complexes, suggested by the Raptor-independent TOR activity observed in the embryo, remains to be tested biochemically. The limited TOR expression pattern complicates biochemical analysis; however, work discussed above (Crespo et al. 2005; Mahfouz et al. 2006) suggests that tobacco transfection or algal models may prove useful tools in the biochemical dissection of TOR signaling.

Further resolution of the relative roles of Raptor-dependent vs. Raptor-independent TOR activity in plant development is also an important topic in TOR signaling. Rapamycin has thus far been of little use in the dissection of plant TOR signaling. Generation of a sensitized transgenic Arabidopsis line expressing rapamycin-binding FKBP could change this dramatically, allowing the observation of the effect of TORC1 disruption at specific stages of development. By comparing any rapamycin-induced effects with the in-ducible RNAi-mediated depletion of TOR, TOR binding partners or TOR effectors, much may be learned about how TOR has been adapted to mediate land plant growth.

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