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Plant Sulphur Metabolism Is Stimulated by Photorespiration

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Plant Sulphur Metabolism Is Stimulated by Photorespiration ARTICLE https://doi.org/10.1038/s42003-019-0616-y OPEN Plant sulphur metabolism is stimulated by photorespiration Cyril Abadie1,2 & Guillaume Tcherkez 1* 1234567890():,; Intense efforts have been devoted to describe the biochemical pathway of plant sulphur (S) assimilation from sulphate. However, essential information on metabolic regulation of S assimilation is still lacking, such as possible interactions between S assimilation, photo- synthesis and photorespiration. In particular, does S assimilation scale with photosynthesis thus ensuring sufficient S provision for amino acids synthesis? This lack of knowledge is problematic because optimization of photosynthesis is a common target of crop breeding and furthermore, photosynthesis is stimulated by the inexorable increase in atmospheric CO2. Here, we used high-resolution 33S and 13C tracing technology with NMR and LC-MS to access direct measurement of metabolic fluxes in S assimilation, when photosynthesis and photorespiration are varied via the gaseous composition of the atmosphere (CO2,O2). We show that S assimilation is stimulated by photorespiratory metabolism and therefore, large photosynthetic fluxes appear to be detrimental to plant cell sulphur nutrition. 1 Research School of Biology, Australian National University, Canberra, ACT 2601, Australia. 2Present address: IRHS (Institut de Recherche en Horticulture et Semences), UMR 1345, INRA, Agrocampus-Ouest, Université d’Angers, SFR 4207 QuaSaV, 49071 Angers, Beaucouzé, France. *email: guillaume. [email protected] COMMUNICATIONS BIOLOGY | (2019)2:379 | https://doi.org/10.1038/s42003-019-0616-y | www.nature.com/commsbio 1 ARTICLE COMMUNICATIONS BIOLOGY | https://doi.org/10.1038/s42003-019-0616-y ulfur is a crucial microelement for plant nutrition, required experiments with glycine leaf feeding in the dark or manipula- Sby the biosynthesis of sulfolipids, antioxidants, cofactors, tion of glutathione synthesis (via enzyme overexpression) have secondary metabolites, and amino acids that are strictly or suggested that glycine provision by photorespiration might be of conditionally essential for human nutrition (cysteine and importance for glutathione synthesis and thus S metabolism24,25. methionine)1,2. Also, grain S content is essential to form protein Also, under salt stress, the increase in glutathione content has disulfide bridges and dictates flour processing properties (such as been proposed to be due to augmented photorespiration rates, dough extensibility) in bread, pasta or biscuit industry3. Sulfur which increase metabolic availability in glycine and serine26. fertilization of crops represents a demand of ≈15 Mt S and an Furthermore, the provision in redox power (in the form of expenditure of ≈$25B each year globally4. It is often assumed that NADH or via the malate valve, for nitrate and sulfate reduction), S assimilation positively correlates with photosynthesis, because and aspartate (for methionine synthesis) may in principle benefit sulfate incorporation is stimulated by sugars in plant roots5 and from photorespiration27,28. When photorespiration is low, N and inhibited under non-physiological conditions where CO2 is S assimilation is believed to be downregulated in leaves because 6 omitted from the atmosphere (CO2-free air) . Nevertheless, the there is an increase in nitrate reduction in roots resulting in a relationship with photosynthesis has never been tested experi- generally lower N/C ratio in leaves and higher xylem transport of 29 mentally and thus it is presently uncertain as to whether S organic N from roots to shoots and furthermore, C3−C4 assimilation can be impacted by environmental conditions that Flaveria interspecies grafts suggested that in C4 plants (where affect photosynthetic metabolism. photorespiration is suppressed), glutathione is preferentially As mentioned above, solving the question of a possible corre- synthesized in roots30. Conversely, the response to S-deficiency lation between photosynthesis and S metabolism in the long term includes an increase in photorespiratory transcripts and meta- ’ 31 is critically important because CO2 mole fraction in Earth s bolites and an increase in the photosynthetic CO2 compensa- atmosphere inexorably increases and favours plant photosynth- tion point32, suggesting that photorespiration participates in S esis, and also current metabolic engineering aims to increase homeostasis. photosynthesis by suppressing photorespiration7,8. In the past 20 Still, there is presently no answer to the question of short-term years, it has been found that plants cultivated in a CO2-enriched metabolic interactions between photorespiration and S assimila- 9–13 33 fl atmosphere contain less elemental S including in grains, tion . Here, we used a low S-demanding C3 crop, sun ower, and thereby affecting flour quality14,15. Accordingly, cultivation under isotopic methods to probe directly the response of the S assim- fl CO2-enriched conditions has been found to alter the content in S- ilation ux from sulfate to cysteine and methionine, in vivo. containing antioxidant (glutathione)16,17. Therefore, present Leaves from plants cultivated in S-sufficient conditions were 13 33 atmospheric changes (probable CO2 doubling by 2100) seem to labelled with both CO2 and S-sulfate under controlled atmo- be detrimental to S metabolism. This problem may worsen in the spheric O2:CO2 environment, including situations that favour or near future because of limited available S in the environment, due disfavour photorespiration compared to ambient conditions. Here, 33 to decreased pollution-driven SO2 emissions and lower utilization we used the S isotope to allow simultaneous analysis by nuclear of S-containing fertilisers (such as superphosphate). However, magnetic resonance (NMR) and mass spectrometry (34S is not possible mechanisms explaining the lower S content when plants visible using NMR and 35S is radioactive). We took advantage of 33 are grown at high CO2 have not been studied. quantitative S-NMR to measure precisely the amount of sulfur The question of a possible correlation between photosynthesis incorporated, high-resolution liquid chromatography/mass spec- and S assimilation in the short term is also important since in the trometry (LC-MS) to determine populations of isotopic species field, photosynthesis can vary considerably depending on (isotopologues), and gas chromatography/mass spectrometry environmental conditions, and thus plant S nutrition could be (GC-MS) for metabolic profiling. Because of the relatively low affected accordingly. Furthermore, S metabolism is involved in cysteine and methionine amount in leaves, the modest sensitivity electron consumption and redox metabolism in illuminated of 33S-NMR analyses and also the fact that sulfur assimilation leaves and thus can affect leaf photosynthetic capacity18.In represents a small metabolic flux (about 0.01 µmol m−2 s−1,that principle, changes in S metabolism when photosynthesis varies is, between 0.05 and 0.1% of photosynthesis), analyses required a could stem from metabolic interactions with photorespiration. considerable acquisition time, representing a total of nearly 3000 h The key enzymatic activity of photorespiratory metabolism is the for isotopic measurements. conversion of glycine to serine (Fig. 1). In this biochemically complicated reaction, a glycineÀÁ molecule is cleaved thereby lib- þ erating CO2 and ammonium NH4 , and a one-carbon unit is Results fixed onto another glycine molecule to form serine. This reaction 13C enrichment in methionine, cysteine and their precursors. involves tetrahydrofolate (H4F) as a cofactor to transfer the one- We found that leaf cysteine inherited carbon atoms derived from 19 13 carbon unit . One-carbon (C1) metabolism is thus directly photosynthesis as shown by the clear C-enrichment regardless involved in photorespiration (due to H4F requirement) and this of gas-exchange conditions, including at low photorespiration 13 is visible in, e.g., mutants affected in formyl tetrahydrofolate (inlet air at 0% O2) (Fig. 2a). Methionine was much less C- deformylase (which regenerates H4F) that have growth defects labelled (a few %) when all C-atoms are averaged. However, LC- and accumulate glycine to very high levels under photo- MS analyses with fragmentation showed that the intramolecular 20 13 respiratory conditions .Also,C1 metabolism is essential not isotopic distribution was highly heterogeneous, with a strong C only for photorespiration but also for many one-carbon requir- enrichment in the methyl group and very little 13C in other ing reactions of metabolism, including methionine synthesis21. C-atoms (Fig. 2a, Supplementary Fig. 1). Such a pattern suggests a The involvement of N-containing compounds (ammonium, higher turnover of the methyl group (demethylation to homo- glycine, serine) in photorespiration explains why photorespira- cysteine, and methionine resynthesis) and is typical of photo- tion is intimately linked to enzyme activities of N assimilation respiration, which leads to the production of highly 13C-enriched and how it may stimulate nitrate reduction22,23. There is no such glycine, serine and thus one-carbon units. In fact, the isotopic direct relationship with sulfur but presumably, photorespiration enrichment in serine and the methyl group of methionine varied may stimulate S assimilation because serine and one-carbon rather similarly (Fig. 2a–b). Also, the 13C-enrichment in the pool units are the building blocks used to synthesize S-containing of metabolically active glycine
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