At the whole plant level photosynthetic energy status is connected with growth control and with responses to external stresses (drought, mineral deprivation, pathogen attacks, etc.). Growth control is largely mediated through the global and specific activity of the protein synthesis machinery and through regulation of the cell cycle. Some mechanisms controlling growth are very ancient and phylogenetically well conserved. However, it is largely unknown how these mechanisms have become integrated with the energy resources specific to photosynthetic organisms, and/or to what extent they played a decisive role during evolution and the colonization of land by plants. One of the major conserved eukaryotic signaling pathways that tunes growth and metabolism is the TOR signaling pathway (Fig. 1). The TOR (Target Of Rapamycin) protein is a large phosphatidylinositol-3 kinase-like protein kinase (PIKK) that is conserved in most eukaryotes.

We study the TOR pathway in flowering plants such as Arabidopsis thaliana, but also in the moss Physcomitrella patens.  Because P. patens belongs to a basal group of land plants it will help us identify elements of the TOR pathway that are conserved in photosynthetic eukaryotes. Unlike other model plants P. patens is also amenable to gene targeting, making it a powerful model system for studying signaling system components. We are also using P. patens to investigate the relationship between two major energy signaling pathways in the cytosol and the chloroplast.

We are developing pharmacological, genetic, cellular biology and proteomic approaches to study the TOR signaling pathway in plants. The pharmacological strategy consists in using ATP-competitive inhibitors (called asTORis for active site TOR inhibitors), recently developed for the human TOR kinase, on plants (Montané and Menand 2013, Fig 2.). Using these new pharmacological tools, we have shown that plant TOR regulates growth by controlling the number of proliferating meristem cells (Fig. 3). We are currently using mutants in combination with asTORis to identify components of the plant TOR pathway at the tissue and organ level. We are also searching for new targets of plant TOR using proteomics.