# Contributions of TOR Signaling on Photosynthesis
## 1 The TOR Complex in Plants
The core components of TORC1 include TOR, regulatory-associated protein of mTOR (RAPTOR)/KOG1, and lethal with SEC13 protein 8 (LST8), while TORC2 includes TOR, SAPK-interacting protein 1 (SIN1)/AVO1, rapamycin-insensitive companion of mTOR (RICTOR)/AVO3, and LST8. In mammalian systems, they have only one _TOR_ gene. In yeast (_Saccharomyces cerevisiae_), two _TOR_ genes, _TOR1_ and _TOR2_, were identified. Either TOR1 or TOR2 can form the TORC1 complex, while only TOR2 can form the TORC2 complex [8]. Research about TOR signaling in plants lags behind that in yeast and mammals. In plant systems examined to date, most of them have only one _TOR_ gene; two _TOR_ genes are identified in polyploids _Glycine max_, _Populus trichocarpa_, and _Brassica rapa_[17; 22]. Figure 1: Structure of TOR protein and strategies to dissect TOR function. The TOR protein contains five conserved domains including HEAT repeats, FAT domain, FRB domain, kinase domain and FATC domain. The multifaceted functions of TOR kinase are dissected by integrated analysis of chemical inhibitor treatments, TOR RNAi seedlings and TOR signaling-related mutants. Abbreviations: TOR, Target of Rapamycin; HEAT repeats, Huntingtin, Elongation factor 3, subunit of protein phosphatase 2A and TOR1; FAT, FRAP, ATM and TRRAP domain; FRB, KYP12-rapamycin binding domain; FATC, Carboxyl-terminal FAT domain; RAPTOR, regulatory-associated protein of mTOR; LST8, lethal with Sec13 protein 8; FKBP12, FK506-binding protein 12; RNAi, RNA interference;miRNA, artificial microRNA. Four TOR genes are identified in allotetraploid cotton _Gossypium hirsutum_[23]. These results suggest that different plant species might have evolved unique TOR complexes with specific functions. Intriguingly, TORC2 complex components SIN1 and RICTOR appear to be absent in the plant lineage, indicating that plants may not form a conserved TORC2 [8; 24]. Scientists revealed that plants were insensitive to rapamycin and could not form a complex with RAP, a small 12 kDa protein FKBP12 and the FRB domain of TOR [13; 21; 25]. However, the growth of _Arabidopsis_ seedlings or cells in liquid culture was inhibited by RAP [26; 27]. To elucidate the function of AtTOR in plants' various cellular and developmental processes, rapamycin-sensitive transgenic _A. thaliana_ lines (BP12) expressing yeast FK506 Binding Protein12 were developed and used [28; 29; 30; 31; 32; 33]. In addition, a new generation of ATP-competitive chemical inhibitors specific to TOR kinase, such as AZD8055, Torin1, Torin2, and KU0063794, have been applied for TOR studies in plants [28; 32; 33]. In the model plant _Arabidopsis thaliana_, one copy of TOR gene, two copies of _Raptor_ (_Raptor_A and _Raptor_B) genes, and two copies of _LST8_ (_LST8-1_ and _LST8-2_) genes exist [9; 17]. Studies suggested that the null _tor_ mutant is embryo-lethal, the _raptora/b_ double mutant is unable to maintain post-embryonic meristem-driven growth, and the _lst8-1_ mutant exhibits modest dwarf growth and early senescence phenotypes [9; 34; 35; 36]. These results revealed that _Arabidopsis_ TOR, Raptor, and LST8 all play a pivotal role in regulating multiple plant growth and development, and TOR appears to regulate a much broader spectrum of biological functions than Raptor or LST8.