BCH/PLS/PPA 609 -- Plant

Lecture Twenty-five
Nitrogen Metabolism -- Sulfur Reduction and Assimilation; N/Sulfur Compounds

Goal: Provide a fundamental understanding of sulfur reduction, assimilation and metabolism in plants. Discussion of selected important sulfur compounds of plant tissues.


  1. Introduction
  2. Sulfur reduction
  3. Sulfur assimilation
  4. Sulfur metabolism
  5. N/Sulfur compounds



Background Readings for the discussion on N/S compounds :

a)      REQUIRED:

1 -    Chapter 16 sections 16.11 - 16.18 (16.10 - 16.14 2000 version) of the Biochemistry & Molecular Biology of Plants class text.

b)      SUGGESTED:

1 - Stephane Ravenel, Bertrand Gakiere, Dominique Job and Roland Douce. 1998. The specific features of methionine biosynthesis and metabolism in plants. PNAS 95: 7805-7812.

2 - Thomas Leustek, Melinda N. Martin, Julie-Ann Bick and John P. Davies. 2000. PATHWAYS AND REGULATION OF SULFUR METABOLISM REVEALED THROUGH MOLECULAR AND GENETIC STUDIES. Annu. Rev. Plant Physiol. Plant. Mol. Biol. 51: 141-165.

3 - Naoko Yoshimoto, Eri Inoue, Kazuki Saito, Tomoyuki Yamaya and Hideki Takahashi. 2003. Phloem-Localizing Sulfate Transporter, Sultr1;3, Mediates Re-Distribution of Sulfur from Source to Sink Organs in Arabidopsis1. Plant Physiol. 131: 1511-1517.

4 - Kopriva, S., M. Suter, P. von Ballmoos, H. Hesse, U. Krahenbuhl, H. Rennenberg, and C. Brunold. 2002. Interaction of Sulfate Assimilation with Carbon and Nitrogen Metabolism in Lemna minor. Plant Physiology. 130: 1406-1413

5 -    Anna Koprivova, Marianne Suter, Roel Op den Camp, Christian Brunold, Stanislav Kopriva. 2000. Regulation of Sulfate Assimilation by Nitrogen in Arabidopsis. Plant Physiol. 122: 737-746.

6 - Shibagaki, N., A. Rose, J.P. McDermott, T. Fujiwara, H. Hayashi, T. Yoneyama and J.P. Davies. 2002. Selenate-resistant mutants of Arabidopsis thaliana identify Sultr1;2, a sulfate transporter required for efficient transport of sulfate into roots. Plant J. 29: 475-486.

About 70% of the organic sulfur in plants is associated with the 2 sulfur containing amino acids in proteins, cysteine and methionine.  The remaining 30% involves soluble amino acids and peptides, mostly glutathione.  About 1% of the organic sulfur in plants is a component of sulfolipid. 

Like nitrogen, sulfur can exist in multiple oxidation states:












Sulfur Reduction

Plants usually take up sulfur in its oxidized form, sulfate, and as the case with nitrate the sulfate must 1st be reduced before incorporation into organic molecules.  Sulfate reduction begins with internalization of sulfate by a high affinity uptake mechanism.  Sulfate is transported into chloroplasts by the triose-P translocator in exchange for phosphate. Once inside chloroplasts, sulfate reduction begins with activation in the presence of ATP forming adenosine phosphosulphate (APS) also called AMP sulfate (see Fig. 2 of Leustek and Saito, 1999):



MgATP + sulfate ->APS + MgPPi

The PPi ->2Pi   helping drive this reaction forward.


APS is further phosphorylated by APS kinase producing P-APS. In this form the sulfate of P-APS is reduced to sulfite by the free thiol (reduced) group of thioredoxin. Thioredoxin can also reduce disulfide bonds in proteins to free thiol groups:


Sulfur Assimilation

The reduced sulfur is finally transferred from the carrier to O-acetylserine (OAS) forming cysteine:

O-acetylserine + H2S (or carrier-S-S-) ->cysteine + acetate


The OAS is formed from serine and acetyl-CoA:

serine + acetyl-CoA ->O-acetylserine + CoA



This whole process is illustrated as follows:





This is summarized in Fig. 16.45 (PPt):






Sulfur Metabolism

Very little cysteine is found in vivo.  Instead it is converted to methionine upon reaction with phosphohomoserine or is rapidly converted to the tripeptide glutathione = glutamate-cysteine-glycine (ECG).  Glutathione, ascorbate and superoxide dismutase (SOD) are key components in plants (especially in plastids) for the detoxification of active oxygen species such as superoxide (O2-).  SOD converts superoxide to hydrogen peroxide:

2O2- + 2H+ ->H2O2 + O2

The hydrogen peroxide is detoxified by the combined action of ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase as mentioned in Lecture 17:

Many other toxic substances taken in by organisms from the environment (xenobiotics) are detoxified by reaction with glutathione. The reactive SH group of glutathione can form a thioether by reacting with C=C double bonds, carbonyl groups or other reactive groups forming a glutathione conjugate. The formation of glutathione conjugates is catalyzed by glutathione-S-transferases (GSH). Such glutathione conjugates are actively transported into vacuoles in an ATP-dependent process for detoxification. Crop safeners promote this process of herbicide detoxification:









Phytochelatins derived from glutathione protect against toxic heavy metals:

Cysteine provides the sulfur for methionine in a process known as transsulfuration.  In methionine synthesis cysteine reacts with the C4 skeleton of phosphohomoserine (PHS) to form cystathionine followed by removal of the original C3 skeleton of cysteine resulting in homocysteine.  Transfer of a methyl group from methyltetrahydrofolate (CH3-THF) by methionine synthase then forms methionine.  Methionine can be converted into S-adenosylmethionine (SAM) by SAM synthetase.  Sulfate reduction and cysteine, cystathionine and homocysteine syntheses occur in chloroplasts.  The homocysteine (and cystathionine in some cases) is transferred to the cytoplasm where methionine and SAM are formed (Fig. 6, Anderson, 1990; Fig. 1 Ravanel et al., 1998):



Folates are important acceptors and donors of 1-C units for all oxidation states of C except CO2 (where biotin is the relevant carrier).

folate + 4H+ ->THF




About 4 times as much methionine cycles through SAM as a methyl donor in the synthesis of numerous other molecules in plant cells vs. protein synthesis.  Of these methylated ethanolamine derivatives such as the choline of phosphatidylcholine followed by pectins are quantitatively most important.  Some of the SAM is used in polyamine synthesis as mentioned in Lecture 24.  In these reactions the reduced sulfur is recycled back into methionine as illustrated:


[skip for 2017]

N/Sulfur Compounds

The L-asparagine analog, L-3-cyanoalanine, is synthesized from L-cysteine + HCN. 

An interesting group of cysteine derivatives found in garlic and onions, Allium cepa, A. sativum and A. ursinum, are alliin and derivatives.  Alliin and propenylallin are hydrolyzed (cleaved) to allylsulfenic acid, propenyl sulfenic acid and pyruvic acid + NH3 by alliin lyase when garlic or onion tissues are damaged.  Allylsulfenic acid spontaneously condenses to the disulfides, allicin and to dialylsulfide.  Allicin and dialylsulfide have strong antimicrobial activity.  Propenyl sulfenic acid spontaneously rearranges to syn-propanethial S-oxide.  Allicin and syn-propanethial S-oxide have strong feeding deterrent activity toward herbivores such as insects.  syn-Propanethial S-oxide is a lachrymator -- the reason why slicing onions causes tears to form:




Certain plants in the dicot order Capparales such as in the Cruciferae accumulate thioglucosides known as glucosinolates in vacuoles that are similar to cyanogens in many aspects.  All plant cells that sequester glucosinolates in vacuoles also have thioglucoside glucohydrolases known as myrosinase stored in places other than vacuoles.  Upon tissue damage or wounding or large increases in membrane permeability, myrosinase and glucosinolates come together releasing the pungent, repellent and microbicidal compound, isothiocyanate:






The distinctive, pungent flavor and odor of mustard, radishes and horseradish is due to isothiocyanates.  Glucosinolates are synthesized from various protein and non-protein amino acids, cysteine and glucose in a manner very similar to the biosynthesis of cyanogenic glucosides except for the addition of sulfur from cysteine.  The precursor amino acid is converted to an aldoximine and to thiohydroximic acid after addition of a sulfate group.  A thioglucoside, desulfo-glucosinolate, is formed at the next step with addition of glucose from UDP-glucose.  Finally another sulfur group is added in the form of sulfate by transfer from phosphoadenosine-phosphosulfate (PAPS) forming a glucosinolate:




Other members of the cabbage family including cabbage itself produce some level of isothiocyanates.  Concentrations of up to 8% are seen in seeds of some Cruciferae.  One of the few developments of a major new crop in modern times is the development of canola from Brassica napus rapeseed.  Although high levels of oil and protein meal can be produced in cool areas, the use of traditional rapeseed has been limited.  The oil was not suitable for edible uses because of high levels of erucic acid, which causes cardiac lesions.  The erucic acid levels were not high enough for high value as an industrial oil.  The meal was not suitable as an animal feed because of high glucosinolate levels.  Large reductions in erucic acid and glucosinolate levels of certain rapeseed genotypes led to the development of canola by Canadian plant breeders.  In the last 30 years canola has risen to be one of the major crops of the world especially in Canada .

Background Readings for the discussion on alkaloids:

a)      REQUIRED:  

1 -    Schardl, C. L., R. B. Grossman, P. Nagabhyru, J. R. Faulkner and U. P. Mallik. 2007. "Loline alkaloids: Currencies of mutualism." Phytochemistry 68(7): 980-996.

2 -  Chapter 24 sections 24.6 - 24.8 of the Biochemistry & Molecular Biology of Plants class text.

b)      SUGGESTED:

1 -   Schardl, C. L., D. G. Panaccione and P. Tudzynski. 2007. Ergot alkaloids -- Biology and molecular biology. The Alkaloids: Chemistry and Biology 63: 45-86.

2 - Rudgers, J.A., S. Fischer, and K. Clay. 2010. Managing plant symbiosis: fungal endophyte genotype alters plant community composition. Journal of Applied Ecology 47:468-477.

2 -    Peter J. Facchini. 2001. ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell Biology, Molecular Regulation, and Metabolic Engineering Applications. Annu. Rev. Plant Physiol. Plant. Mol. Biol. 52: 29-66.

3 -    Sato, Fumihiko, Takashi Hashimoto, Akira Hachiya, Ken-ichi Tamura, Kum-Boo Choi, Takashi Morishige, Hideki Fujimoto and Yasuyuki Yamada. 2001. Metabolic engineering of plant alkaloid biosynthesis. PNAS 98: 367-372.

4 - Misako Kato, Kouichi Mizuno, Alan Crozier, Tatsuhito Fujimura, and Hiroshi Ashihara. 2000. Caffeine synthase gene from tea leaves. Nature 406: 956-957. 

5 - André Kessler and Ian T. Baldwin. 2002. PLANT RESPONSES TO INSECT HERBIVOR: The Emerging Molecular Analysis. Annu. Rev. Plant Biol. 53: 299-328.


All materials © 2018 David Hildebrand, unless otherwise noted.



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