Nitrogen-Sparing Mechanisms in Chlamydomonas Affect the Transcriptome, the Proteome, and Photosynthetic Metabolism

Abstract

ORCID IDs: 0000-0002-7487-8014 (S.S.); 0000-0002-2594-509X (S.S.M.)Nitrogen (N) is a key nutrient that limits global primary productivity; hence, N-use efficiency is of compelling interest in agriculture and aquaculture. We used Chlamydomonas reinhardtii as a reference organism for a multicomponent analysis of the N starvation response. In the presence of acetate, respiratory metabolism is prioritized over photosynthesis; consequently, the N-sparing response targets proteins, pigments, and RNAs involved in photosynthesis and chloroplast function over those involved in respiration. Transcripts and proteins of the Calvin-Benson cycle are reduced in N-deficient cells, resulting in the accumulation of cycle metabolic intermediates. Both cytosolic and chloroplast ribosomes are reduced, but via different mechanisms, reflected by rapid changes in abundance of RNAs encoding chloroplast ribosomal proteins but not cytosolic ones. RNAs encoding transporters and enzymes for metabolizing alternative N sources increase in abundance, as is appropriate for the soil environmental niche of C. reinhardtii. Comparison of the N-replete versus N-deplete proteome indicated that abundant proteins with a high N content are reduced in N-starved cells, while the proteins that are increased have lower than average N contents. This sparing mechanism contributes to a lower cellular N/C ratio and suggests an approach for engineering increased N-use efficiency.

Two Glycerol-3-Phosphate Dehydrogenases from Chlamydomonas Have Distinct Roles in Lipid Metabolism

“…and C. reinhardtii GPDH enzymes has focused on glycerol biosynthesis and osmotic stress tolerance (Gee et al, 1993;He et al, 2007He et al, , 2009Cai et al, 2013;Casais-Molina et al, 2016). However, previous expression data showing the induction of C. reinhardtii GPD2, GPD3, and GPD4 under nutrient starvation (Goodenough et al, 2014), which in turn induces TAG accumulation (Schmollinger et al, 2014), provide indirect evidence to suggest that these three GPDH isoforms are involved in the production of G3P for entry into the Kennedy pathway for DAG and subsequently TAG biosynthesis (Fig. 1).…”

“…mechanisms to osmotic stress, to oxidative stress, and to nutrient limitation have been widely studied and elucidated in freshwater and marine microalgae (Gee et al, 1993;Tanaka et al, 2011;Urzica et al, 2012;Schmollinger et al, 2014). Many of these responses involve substantial changes to primary metabolism.…”

“…In situations where glycerolipid synthesis is highly induced, such as in response to nitrogen (N) or phosphorus (P) starvation in microalgae (Schmollinger et al, 2014), utilization of G3P by the Kennedy pathway for TAG production might be predominant rather than for glycerol production. Therefore, GPDH activity may be important for lipid biosynthesis during such lipidinducing stress conditions.…”

“…Many of these responses involve substantial changes to primary metabolism. For example, saline stress often induces the accumulation of glycerol for osmotic adjustment (Ben-Amotz et al, 1982), while nutrient starvation leads to large induction of carbon storage metabolism, including the accumulation of storage lipids, principally triacylglycerol (TAG), and the storage carbohydrate starch (Siaut et al, 2011;Schmollinger et al, 2014;Bajhaiya et al, 2016a). There is also interest in the use of microalgal metabolites as industrial products, such as a feedstock for biofuels or for nutraceutical application (Driver et al, 2014;Gangl et al, 2015;Bajhaiya et al, 2017).…”

The Regulation of Photosynthetic Structure and Function during Nitrogen Deprivation in Chlamydomonas reinhardtii

“…Using the annotated C. reinhardtii genome (Merchant et al, 2007), the C. reinhardtii transcriptome was analyzed before and after N deprivation, which revealed multiple changes in gene expression that affect diverse parts of metabolism (Miller et al, 2010;Blaby et al, 2013;Goodenough et al, 2014;Schmollinger et al, 2014). In addition, proteomics have been used to profile the changes in protein expression during N deprivation in Phaeodactylum tricornutum (Yang et al, 2014), C. reinhardtii Longworth et al, 2012;Schmollinger et al, 2014), and Nannochloropsis oceanica (Dong et al, 2013), and metabolomics have been used to assess changes in metabolite pool sizes (Bölling and Fiehn, 2005;Lee et al, 2012). These and other studies point to wide-ranging changes in the structure and operation of the metabolic and other cellular networks.…”