ppGpp influences protein protection, growth and photosynthesis in Phaeodactylum tricornutum

Abstract

Summary Chloroplasts retain elements of a bacterial stress response pathway that is mediated by the signalling nucleotides guanosine penta‐ and tetraphosphate ((p)ppGpp). In the model flowering plant Arabidopsis, ppGpp acts as a potent regulator of plastid gene expression and influences photosynthesis, plant growth and development. However, little is known about ppGpp metabolism or its evolution in other photosynthetic eukaryotes. Here, we studied the function of ppGpp in the diatom Phaeodactylum tricornutum using transgenic lines containing an inducible system for ppGpp accumulation. We used these lines to investigate the effects of ppGpp on growth, photosynthesis, lipid metabolism and protein expression. We demonstrate that ppGpp accumulation reduces photosynthetic capacity and promotes a quiescent‐like state with reduced proliferation and ageing. Strikingly, using nontargeted proteomics, we discovered that ppGpp accumulation also leads to the coordinated upregulation of a protein protection response in multiple cellular compartments. Our findings highlight the importance of ppGpp as a fundamental regulator of chloroplast function across different domains of life, and lead to new questions about the molecular mechanisms and roles of (p)ppGpp signalling in photosynthetic eukaryotes.

A ppGpp-mediated brake on photosynthesis is required for acclimation to nitrogen limitation in Arabidopsis

“…Our work also reveals the wide-ranging physiological effects of ppGpp on photosynthetic activity during nitrogen deprivation. These effects are reminiscent of the photosynthetic changes that occur during senescence (Krieger-Liszkay et al, 2019), and are remarkably similar to those found using the ectopic over-accumulation of ppGpp in Arabidopsis, moss and algae (Maekawa et al, 2015;Sugliani et al, 2016;Imamura et al, 2018;Honoki et al, 2018;Avilan et al, 2021;Harchouni et al, 2021). However, compared to these studies our more detailed analysis here highlights first the global effect of ppGpp on chloroplast transcript abundance (Fig.…”

“…ppGpp itself has been shown to act as an inhibitor of the expression of certain chloroplast genes in vivo (Yamburenko et al, 2015;Maekawa et al, 2015;Sugliani et al, 2016;Ono et al, 2020). Artificial accumulation of ppGpp in the chloroplast by the over-expression of RSH3 or bacterial ppGpp synthase domains has shown that ppGpp can inhibit photosynthesis in both land plants (Maekawa et al, 2015;Sugliani et al, 2016;Honoki et al, 2018;Harchouni et al, 2021) and algae (Avilan et al, 2021). Interestingly, the artificial accumulation of ppGpp also appears to protect plants against nitrogen deprivation (Maekawa et al, 2015;Honoki et al, 2018).…”

A guanosine tetraphosphate (ppGpp) mediated brake on photosynthesis is required for acclimation to nitrogen limitation in Arabidopsis

“…According to the firmicute model, guanylate kinase inhibition causes a drop in GTP concentration, which in turn results in reduced transcription from rRNA-coding genes where GTP is usually the initiating nucleotide. Over accumulation of ppGpp in the chloroplast by the overexpression of RSH3 or bacterial ppGpp synthase domains has also shown that ppGpp may act as a conserved repressor of photosynthesis in both land plants ( Maekawa et al, 2015 ; Sugliani et al, 2016 ; Honoki et al, 2018 ; Harchouni et al, 2021 ) and algae ( Avilan et al, 2021 ). Interestingly, the artificial accumulation of ppGpp also appears to protect plants against nitrogen deprivation ( Maekawa et al, 2015 ; Honoki et al, 2018 ).…”

Identification of a RelA/SpoT Homolog and Its Possible Role in the Accumulation of Astaxanthin in Haematococcus pluvialis

“…Stringent response is highly conserved and regulates many important physiological functions in plant and algae ( Kasai, 2002 ; Takahashi et al, 2004 ; Avilan et al, 2021 ). In this study, homologous protein of RelA/SpoT in H. pluvialis , HpRSH, was identified by function complement and enzyme assay in vitro ( Figure 1 ).…”

“…A growing number of studies indicated that (p)ppGpp contributes to the regulation of many aspects of bacterial biology, such as growth adaption in E. coli ( Traxler et al, 2011 ), antibiotic production in Bacillus subtilis ( Ochi and Ohsawa, 1984 ), morphological differentiation in Mycobacterium smegmatis ( Ojha et al, 2000 ), sporulation in Streptomyces ( Ochi, 1987 ), social behavior in pseudomonas aeruginosa ( Van Delden et al, 2001 ), and fruit body development in Myxococcus xanthus ( Harris et al, 1998 ). Likewise, ppGpp was also operative in plant and algae, with the identification of ppGpp synthase homologs in Arabidopsis thaliana ( Der Biezen et al, 2000 ) and Chlamydomonas reinhardtii ( Kasai, 2002 ), the detection of ppGpp in pea, wheat, rice, and spinach ( Takahashi et al, 2004 ), and the investigation of its roles in regulation of cell physiology in Synechococcus elongatus ( Puszynska and O’Shea, 2017 ), red alga ( Imamura et al, 2018 ), and Phaeodactylum tricornutum ( Avilan et al, 2021 ).…”