The Plant Target of Rapamycin: A Conduc TOR of Nutrition and Metabolism in Photosynthetic Organisms

Abstract

Living organisms possess many mechanisms to sense nutrients and favorable conditions, which allow them to grow and develop. Photosynthetic organisms are very diverse, from green unicellular algae to multicellular flowering plants, but most of them are sessile and thus unable to escape from the biotic and abiotic stresses they experience. The Target of Rapamycin (TOR) signaling pathway is conserved in all eukaryotes and acts as a central regulatory hub between growth and extrinsic factors, such as nutrients or stress. However, relatively little is known about the regulations and roles of this pathway in plants and algae. Although some features of the TOR pathway seem to have been highly conserved throughout evolution, others clearly differ in plants, perhaps reflecting adaptations to different lifestyles and the rewiring of this primordial signaling module to adapt to specific requirements. Indeed, TOR is involved in plant responses to a vast array of signals including nutrients, hormones, light, stresses or pathogens. In this review, we will summarize recent studies that address the regulations of TOR by nutrients in photosynthetic organisms, and the roles of TOR in controlling important metabolic pathways, highlighting similarities and differences with the other eukaryotes.

At the Crossroads of Survival and Death: The Reactive Oxygen Species–Ethylene–Sugar Triad and the Unfolded Protein Response

“…It is important to note that TOR is not exclusively activated by sugars. Readers are referred to Ingargiola et al (2020) for a detailed overview on the regulation of TOR [40]. By contrast, stresses like nutrient starvation, pathogen attack, and oxidative stress, often lead to sugar starvation in sink tissues.…”

Control of ABA Signaling and Crosstalk with Other Hormones by the Selective Degradation of Pathway Components

“…It is negatively regulated by the Target of Rapamycin (TOR) kinase, which, along with its effectors, is active under nutrient-rich conditions when it upregulates cell growth and translation, but it is inactive during nutrient deficiency. The TOR kinase complex is modulated by diverse upstream inputs to phosphorylate different proteins in the nucleus, nucleolus, and cytosol, thus controlling transcription, the cell cycle, rRNA transcription, ribosome biogenesis, translation, and metabolism, which are pivotal to cell proliferation and growth [ 131 , 132 ].…”

Contributions of TOR Signaling on Photosynthesis

“…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]. Xiong and Sheen 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].…”

“…In the model plant Arabidopsis thaliana, one copy of TOR gene, two copies of Raptor (RaptorA and RaptorB) 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].…”