# Contributions of TOR Signaling on Photosynthesis
## 1 The TOR Complex in Plants  
During the long-term evolution from the Last Eukaryotic Common Ancestor (LECA), plants have diverged from animals and other eukaryotes in many ways. Their most prominent feature is the ability to using light energy to generate organic molecules (e.g., glucose (C\({}_{6}\)H\({}_{12}\)O\({}_{6}\)), sugars, starch) from carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O) and release molecular oxygen (O\({}_{2}\)) into the atmosphere, which is also called photosynthesis [1]. Photosynthesis is an essential process and facilitates the expansion of plants broadly across all of the Earth's biomes. With limited agricultural land and increasing human population, it is essential to decipher the key regulators involved in photosynthesis and enhance photosynthetic activities [1, 2, 3, 4]. Living organisms possess many mechanisms to sense when nutrient availability and environmental cues are conducive to growth and development [5]. Through identification of different external or internal cues, organisms adjust their metabolism, growth, and development to survive harsh conditions. The target of rapamycin (TOR) kinase signaling cascade is a core component of nutrient sensing in all eukaryotic organisms. Recently, [PERSON] et al. explained the importance of TOR kinase in plant development as a whole [6]. In addition, the role of the TOR signaling plants' defense against pathogen and related metabolisms was studied recently [7]. When eukaryotes are in favorable conditions, TOR signaling pathway is active, which promotes anabolic processes and inhibits catabolism and protein degradation, but when conditions are unfavorable for growth, TOR is inactivated and promotes catabolic processes [8, 9]. [PERSON] and [PERSON] explained the glucose-TOR signaling in the transcriptional control of the cell cycle and enumerated the importance of leaf photosynthesis-derived glucose in activating TOR [10]. Tremendous progress has been made in uncovering the molecular and cellular functions of TOR in regulating plant photosynthesis. In this paper, we first present a broadoverview of recent developments in the plant TOR complex. Secondly, we highlight the impact of TOR signaling on plant photosynthesis, focusing on findings on the regulation of TOR in sensing C nutrient derived from photosynthesis, light absorptions, and leaf and chloroplast development. Finally, we present some pressing questions to be addressed as we move onward in the study of TOR signaling in plant photosynthesis. Rapamycin (RAP) is an immunosuppressive macrolide antibiotic produced by the soil bacterium _Streptomyces hygroscopicus_ on Easter Island, which stops yeast division and produces cells with an N starvation phenotype [11]. Through screening for genetic mutants resistant to rapamycin, FK506-binding protein 12 (FKBP12) together with TOR were first identified, and TOR emerged as a central regulatory hub in all eukaryotic organisms, including yeasts, animals, and plants [12; 13; 14; 15; 16]. TOR is a conserved serine/threonine-protein kinase and belongs to the phosphatidylinositol 3-kinase-related kinase family and has been found in all sequenced plant and algal genomes [11; 17; 18]. The TOR protein contains five conserved domains, including HEAT (Huntington, Elongation Factor 3 regulatory, subunit A of PP2A, TOR1) repeats, FAT (FRAP-ATM-TTRAP) domain, FRB (FKBP rapamycin-binding) domain, kinase domain and FATC domain (Figure 1). The HEAT repeats mediate protein-protein interactions and membrane associations; the FAT domain, together with the FATC domains, contributes to protein interactions and kinase activation; the FRB domain is targeted by the inhibitory FKBP12-rapamycin complex [19]. In yeast and mammalian systems, TOR exists in two structurally and functionally distinct protein complexes: the rapamycin-sensitive TOR complex 1 (TORC1) and the rapamycin-insensitive TOR complex 2 (TORC2) [20; 21].