Phosphorus Availability Regulates TORC1 Signaling via LST8 in Chlamydomonas

Abstract

Target of rapamycin complex 1 (TORC1) is a central regulator of cell growth. It balances anabolic and catabolic processes in response to nutrients, growth factors, and energy availability. Nitrogen-and carbon-containing metabolites have been shown to activate TORC1 in yeast, animals, and plants. Here, we show that phosphorus (P) regulates TORC1 signaling in the model green alga Chlamydomonas (Chlamydomonas reinhardtii) via LST8, a conserved TORC1 subunit that interacts with the kinase domain of TOR. P starvation results in a sharp decrease in LST8 abundance and downregulation of TORC1 activity. A hypomorphic lst8 mutation resulted in decreased LST8 abundance, and it both reduced TORC1 signaling and altered the cellular response to P starvation. Additionally, we found that LST8 levels and TORC1 activity were not properly regulated in a mutant defective in the transcription factor PSR1, which is the major mediator of P deprivation responses in Chlamydomonas. Unlike wild-type cells, the psr1 mutant failed to downregulate LST8 abundance and TORC1 activity when under P limitation. These results identify PSR1 as an upstream regulator of TORC1 and demonstrate that TORC1 is a key component in P signaling in Chlamydomonas.

Fixing the Broken Phosphorus Cycle: Wastewater Remediation by Microalgal Polyphosphates

“…The LST8 subunit of TOR is down-regulated under P-starvation in a PSR1 -dependent fashion. This pathway appears to link P-stress to TOR-mediated de-repression of TAG synthesis and autophagy, along with repression of translation ( Couso et al, 2020 ). Investigation of the signal transduction pathway in S deprivation in Chlamydomonas has clearly identified SNRK-family involvement ( Gonzalez-Ballester et al, 2008 ; González-Ballester et al, 2010 ) but this kinase group have also been implicated more widely in other stress responses in this alga ( Colina et al, 2019 ) ( Figure 6 ).…”

“…Functional gene categories are indicated to the left. The information and models shown are a collation of transcriptomic data and molecular work from published sources: ( Moseley et al, 2006 ; Gonzalez-Ballester et al, 2008 ; González-Ballester et al, 2010 ; Toepel et al, 2011 ; Schmollinger et al, 2014 ; Bajhaiya et al, 2016 ; Wild et al, 2016 ; Couso et al, 2018 ; Couso et al, 2020 ).…”

Algal Autophagy Is Necessary for the Regulation of Carbon Metabolism Under Nutrient Deficiency

“…In addition, these reports indicate an interaction between TOR signaling network and lipid metabolism in algae (Pérez-Pérez et al, 2017). Most recently, through the characterization of a hypomorphic mutant of the LST8 gene, lst8-1 under P deficiency, Couso et al (2020) demonstrated that P deficiency reduces the abundance of LST8 proteins and downregulates activity of TORC1 to activate autophagy and TAG synthesis in Chlamydomonas (Figure 1). In the N-deficient conditions, any significant phenotype was not observed in the lst8-1 mutant.…”

Knowing When to Self-Eat – Fine-Tuning Autophagy Through ATG8 Iso-forms in Plants

“…The mechanisms of GCN2 perception in plants are unknown, although GCN2 activation by uncharged tRNAs appears to be universal in eukaryotes (Li et al, 2013) (Figure 1A). Prolonged carbon starvation (Huang et al, 2019) and phosphate deficit which causes ER stress (Naumann et al, 2019) induce autophagy independently of TOR, although recent research in Chlamydomonas suggests LST8, the stabilizer of TOR, is involved in phosphate starvation response (Couso et al, 2020).…”