Nitric oxide participates in sucrose-TOR signaling during meristem activation in Arabidopsis thaliana.

MAIN CONCLUSION: This study provides evidence about the relationship between Target of Rapamycin (TOR) kinase and the signal molecule nitric oxide (NO) in plants. We showed that sucrose (SUC)-mediated TOR activation of root apical meristem (RAM) requires NO and that NO, in turn, participates in the regulation of TOR signaling. Nitric oxide (NO) constitutes a signal molecule that regulates important target proteins related to growth and development and also contributes to metabolic reprogramming that occurs under adverse conditions. Taking into account the important role of NO and its relationship with Target of Rapamycin (TOR) signaling in animals, we wondered about the putative link between both pathways in plants. With this aim, we studied a TOR-dependent process which is the reactivation of the root apical meristem (RAM) in Arabidopsis thaliana. We used pharmacological and genetic tools to evaluate the relationship between NO and TOR on the sugar induction of RAM, using SNP as NO donor, cPTIO as NO scavenger and the nitrate reductase (NR) mutant nia2. The results showed that sucrose (SUC)-mediated TOR activation of the RAM requires NO and that NO, in turn, participates in the regulation of TOR signaling. Interestingly, TOR activation induced by sugar increased the NO levels. We also observed that NO could mediate the repression of SnRK1 activity by SUC. By computational prediction we found putative S-nitrosylation sites in the TOR complex proteins and the catalytic subunit of SnRK1, SnRK1.1. The present work demonstrates for the first time a link between NO and TOR revealing the complex interplay between the two pathways in plants.

The tiny giant of the sea, Ostreococcus's unique adaptations.

Ostreococcus spp. are unicellular organisms with one of the simplest cellular organizations. The sequencing of the genomes of different Ostreococcus species has reinforced this status since Ostreococcus tauri has one most compact nuclear genomes among eukaryotic organisms. Despite this, it has retained a number of genes, setting it apart from other organisms with similar small genomes. Ostreococcus spp. feature a substantial number of selenocysteine-containing proteins, which, due to their higher catalytic activity compared to their selenium-lacking counterparts, may require a reduced quantity of proteins. Notably, O. tauri encodes several ammonium transporter genes, that may provide it with a competitive edge for acquiring nitrogen (N). This characteristic makes it an intriguing model for studying the efficient use of N in eukaryotes. Under conditions of low N availability, O. tauri utilizes N from abundant proteins or amino acids, such as L-arginine, similar to higher plants. However, the presence of a nitric oxide synthase (L-arg substrate) sheds light on a new metabolic pathway for L-arg in algae. The metabolic adaptations of O. tauri to day and night cycles offer valuable insights into carbon and iron metabolic configuration. O. tauri has evolved novel strategies to optimize iron uptake, lacking the classic components of the iron absorption mechanism. Overall, the cellular and genetic characteristics of Ostreococcus contribute to its evolutionary success, making it an excellent model for studying the physiological and genetic aspects of how green algae have adapted to the marine environment. Furthermore, given its potential for lipid accumulation and its marine habitat, it may represent a promising avenue for third-generation biofuels.

TOR signaling in the green picoalga Ostreococcus tauri.

Target of rapamycin (TOR) is a master regulator that controls growth and metabolism by integrating external and internal signals. Although there was a great progress in the study of TOR in plants and in the model alga Chlamydomonas, scarce data are available in other green algae. Thus, in this work we studied TOR signaling in Ostreococcus tauri, the smallest free-living eukaryote described to date. This picoalga is particularly important because it has a key site at the base of the green lineage and is part of the marine phytoplankton, contributing to global photosynthesis. We investigated OtTOR complex in silico and experimentally, by using first- and second-generation TOR inhibitors, such as rapamycin and PP242. We analyzed the effect of TOR down-regulation on cell growth and on the accumulation of carbon reserves. The results showed that O. tauri responds to TOR inhibitors more similarly to plants than to Chlamydomonas, being PP242 a valuable tool to study this pathway. Besides, Ottor expression analysis revealed that the kinase is dynamically regulated under nutritional stress. Our data indicate that TOR signaling is conserved in O. tauri and we propose this alga as a good and simple model for studying TOR kinase and its regulation.