# Contributions of TOR Signaling on Photosynthesis
## 2 TOR: A Key Regulator in Photosynthesis
### TOR Regulates Leaf and Chloroplast Development  
Plant chloroplasts are essential organelles that carry out oxygenic photosynthesis. Several studies revealed the involvement of TOR signaling in the biogenesis and maturation of chloroplasts [23; 29; 33; 45; 48; 55; 56]. The suppression of TOR by active-site TOR inhibitor AZD8055 in _Arabidopsis_ eliminated greening and cotyledon expansion and remodeled the expression profile of genes associated with photosynthesis, involved in chlorophyll biosynthesis, light reactions, and CO\({}_{2}\) fixation [45]. The transcription and translation of nuclear-encoded mRNAs coding for plastid ribosomal proteins were down-regulated in TOR inactivation plants (ethanol-inducible TOR RNAi lines), which could explain the chlorotic phenotype of the TOR silenced plants [48]. Recent findings also uncovered the genes involved in plastid biogenesis were particularly sensitive to TOR inhibition by AZD8055 treatment [57]. Analysis of the _C. reinhardtii_ proteome and phosphoproteome under TOR inactivation revealed that the levels of most of the proteins related to the Calvin cycle were decreased, suggesting the involvement of TOR in photosynthesis [58]. Inhibition of TOR activity resulted in reduced chloroplast number and size, and thus impaired photoautotrophic growth in _Arabidopsis_[33]. In addition, scientists identified the brassinosterol insensitive 2 (BIN2) as a novel phosphorylation target of TOR-S6K2 and uncovered the pivotal role of TOR-S6K2-BIN2 in heterotrophic to photoautotrophic transition [33]. In addition, TOR inhibition caused altered chloroplast morphology, increased non-photochemical quenching (NPQ), damage to photosystem II (PS II) reaction centers and inhibited efficient state transitions between PSII and PSI in alga [59]. In allotetraploid cotton _Gossyipium hirsutum_, pharmacological experiments with TOR inhibitor AZD8055 and silencing _GHTOR_ genes by virus-induced gene silencing (VIGS) resulted in a significant delay in the transition from heterotrophic to photoautotrophic growth [23]. Ectopic expression of full-length ATOR increased plant photosynthetic efficiency and chlorophyll content in rice [60]. Type 2A phosphatase-associated protein of 46 kDa (TAP46), a regulatory subunit of protein phosphatase 2A, is a direct TORC1 phosphorylation substrate. Transgenic plants overexpressing TAP46 exhibited increased hypocotyl length and enlarged leaves [61; 62]. Ribosomal protein S6 kinase (S6K) functions as a key component in the target of rapamycin (TOR) pathway. In ice (_Oryza sativa L._), suppression of S6K1 caused pale yellow-green leaves and defective thylakoid grana architecture. Genetic and pharmacological evidence further demonstrated that S6K1 was vital for galactolipid biosynthesis for the thylakoid membrane [56]. Arabidopsis _raport1b_ mutation resulted in decreased CO\({}_{2}\) assimilation rate and increased stomatal conductance; however, the chloroplast development or photosynthetic electron transport efficiency was not affected [42]. In _Arabidopsis_, TOR functioned as a positive regulator of chlorophyll biosynthesis and metabolism via TRINI (_tor-inhibitor insensitive 1_)/ABI4 (abscisic acid-insensitive 4) at the photoautotrophic stage. The precise molecular connection between TOR and ABI4 remains to be elucidated [29]. The TOR complex components _lst8_ and _raport_ mutants as well as wild-type _Arabidopsis_ seedlings under TOR inhibition by AZD showed diminished ABA hormone accumulation. The diminished ABA hormone accumulation was correlated with decreased transcript levels of ABA biosynthetic enzymes including zeaxanthin epoxidase (ZEP), the 9-cis-epoxycarotenoid dioxygenase NCED3 and aldehyde oxidases AAO3, and increased transcript amount of the abscisic acid catabolism gene cytochrome P450 monooxygenases CYP707A2 [63]. Furthermore, an in vitro protein kinase screen identified that TOR kinase phosphorylated ABA receptors PYL at a conserved serine residue S119 [64].