Plant Sulfur Research Articles

Sulfur‐Enhanced Defence: Effects of Sulfur Metabolism, Nitrogen Supply, and Pathogen Lifestyle

Evidence from field experiments indicates differential roles of sulfur and nitrogen supply for plant resistance against pathogens. Dissection of these observations in defined pathosystems and controlled nutritional conditions indicates an activation of plant sulfur metabolism in several incompatible and compatible interactions. Contents of cysteine and glutathione as markers of primary sulfate assimilation and stress response show increases in ARABIDOPSIS THALIANA upon infection, coinciding with the synthesis of sulfur-containing defence compounds. Similar increases of thiols were observed with necrotrophic, biotrophic, and hemibiotrophic pathogens. Sulfate supply was found to be neutral or beneficial for tolerance against fungal but neutral for bacterial pathogens under IN VITRO conditions. According to various reports and own observations the effects of nitrogen supply appeared to be neutral or harmful, depending on the pathogen. The activation of sulfur metabolism was a consequence of activation of gene expression as revealed by macroarray analysis of an A. THALIANA/ALTERNARIA BRASSICICOLA pathosystem. This activation appeared to be largely independent from sufficient or optimal sulfate supply and from the established sulfate deficiency response. The data suggest that plant-pathogen interactions and sulfur metabolism are linked by jasmonic acid as signal.

Pungency of Spring Onion as Affected by Inoculation with Arbuscular Mycorrhizal Fungi and Sulfur Supply

ABSTRACT The influence of sulfur (S) supply and mycorrhizal colonization on the growth and pungency of spring onion (Allium fistulosum L.) seedlings grown for four months in Perlite and nutrient solution containing three levels of sulfate [0.1, 1.75, and 4 mM sulfate (SO4 2−)] were investigated. Plants were inoculated with Glomus etunicatum, Glomus intraradices, or Glomus mosseae, and uninoculated controls were included. Shoot and root dry weights of mycorrhizal and control plants supplied with 4 mM SO4 2+ were higher than with 0.1 or 1.75 mM SO4 2−. Enzyme produced pyruvic acid (EPY) and plant sulfur concentrations increased with increasing SO4 2− supply. The EPY and plant S concentrations were usually higher in mycorrhizal plants than controls irrespective of S supply, and shoot total S concentrations and EPY were strongly correlated. Arbuscular mycorrhizal colonization appeared to make a substantial contribution to the sulfur status of spring onion, and may thus have had a strong influence on its flavor characteristics.

Adenosine 5'-phosphosulphate reductase is regulated differently in Allium cepa L. and Brassica oleracea L. upon exposure to H2S

The reduction of adenosine 5'-phosphosulphate (APS) by APS reductase (APR) is considered to be one of the rate-limiting steps in the assimilation of sulphur in plants. In order to identify the mechanisms of regulation of this enzyme, the impact of atmospheric H2S exposure on mRNA expression, protein level, and activity of APR was studied in two species (Allium cepa L. and Brassica oleracea L.) with different physiological responses to H2S exposure. As expected, H2S exposure resulted in a rapid increase in thiol compounds in the shoot of both species. There was a substantial increase in total sulphur content in shoots of A. cepa, whereas it was hardly affected or even slightly decreased in B. oleracea. Sulphate uptake was only marginally affected in A. cepa, whereas it was strongly decreased in B. oleracea upon H2S exposure. Furthermore, H2S exposure resulted in a down-regulation of APR activity in shoot and roots of both species, which was probably mediated by a transcriptional mechanism of regulation by thiols, since mRNA levels also decreased. However, in contrast to B. oleracea, APR protein level was not affected by H2S exposure in A. cepa. The reduction in APR activity in onion was therefore achieved by an additional as yet unknown post-translational regulation. These results demonstrate that not only the physiological response to H2S, but also the molecular mechanisms of regulation of APR differ in the two species.

Biochemical and molecular characterization of the homocysteine S-methyltransferase from broccoli ( Brassica oleracea var. italica)

Plants are known for their unique ability to synthesize methionine from S-methylmethionine (SMM) and homocysteine using the enzyme SMM: homocysteine S-methyltransferase (HMT) in the SMM cycle. Two cDNAs exhibiting HMT activity were cloned from broccoli and functionally expressed in E. coli. One cDNA, that encodes an enzyme with high substrate specificity for homocysteine, was designated as BoHMT1. The other cDNA was the BoSMT gene that we previously characterized and encodes a selenocysteine methyltransferase (Lyi, S.M., Heller, L.I., Rutzke, M., Welch, R.M., Kochian, L.V., Li, L., 2005. Molecular and biochemical characterization of the selenocysteine Se-methyltransferase gene and Se-methylselenocysteine synthesis in broccoli. Plant Physiol. 138, 409–420). Both exist as single gene sequences in the broccoli genome. While BoSMT expression was extremely low or undetectable in broccoli plants unless the plants were exposed to selenium, the BoHMT1 mRNA accumulated in most tissues of the plant except older leaves. In contrast to BoSMT whose expression was dramatically upregulated by treating plants with selenate, the transcript levels of BoHMT1 were not markedly affected in plants exposed to selenium. BoHMT1 expression responded significantly to changes in plant sulfur status. However, its expression was not dramatically affected in plants treated with methionine, SMM, homocysteine, or the heavy metal, cadmium. The differences in the substrate specificity and gene expression in response to changes in plant sulfur and selenium status between BoHMT1 and BoSMT suggest that the enzymes encoded by these two genes play distinct roles in sulfur and selenium metabolism in broccoli.

ArabidopsisSLIM1 Is a Central Transcriptional Regulator of Plant Sulfur Response and Metabolism

Sulfur is an essential macronutrient required for plant growth. To identify key transcription factors regulating the sulfur assimilatory pathway, we screened Arabidopsis thaliana mutants using a fluorescent reporter gene construct consisting of the sulfur limitation-responsive promoter of the SULTR1;2 sulfate transporter and green fluorescent protein as a background indicator for monitoring plant sulfur responses. The isolated mutant, sulfur limitation1 (slim1), was unable to induce SULTR1;2 transcripts under low-sulfur (-S) conditions. Mutations causing the sulfur limitation responseless phenotypes of slim1 were identified in an EIL family transcription factor, ETHYLENE-INSENSITIVE3-LIKE3 (EIL3), whose functional identity with SLIM1 was confirmed by genetic complementation. Sulfate uptake and plant growth on -S were significantly reduced by slim1 mutations but recovered by overexpression of SLIM1. SLIM1 functioned as a central transcriptional regulator, which controlled both the activation of sulfate acquisition and degradation of glucosinolates under -S conditions. Metabolite analysis indicated stable accumulation of glucosinolates in slim1 mutants, even under -S conditions, particularly in the molecular species with methylsulfinylalkyl side chains beneficial to human health. Overexpression of SLIM1 and its rice (Oryza sativa) homologs, but no other EIL genes of Arabidopsis, restored the sulfur limitation responseless phenotypes of slim1 mutants, suggesting uniqueness of the SLIM1/EIL3 subgroup members as sulfur response regulators.

Sulfur uptake in the ectomycorrhizal fungus Laccaria bicolor S238N

The importance of the ectomycorrhiza symbiosis for plant acquisition of phosphorus and nitrogen is well established whereas its contribution to sulfur nutrition is only marginally understood. In a first step to investigate the role of ectomycorrhiza in plant sulfur nutrition, we characterized sulfate and glutathione uptake in Laccaria bicolor. By studying the regulation of sulfate uptake in this ectomycorrhizal fungus, we found that in contrast to bacteria, yeast, and plants, sulfate uptake in L. bicolor was not feedback-inhibited by glutathione. On the other hand, sulfate uptake was increased by sulfur starvation as in other organisms. The activity of 3'-phosphoadenosine 5'-phosphosulfate reductase, the key enzyme of the assimilatory sulfate reduction pathway in fungi, was increased by sulfur starvation and decreased after treatment with glutathione revealing an uncoupling of sulfate uptake and reduction in the presence of reduced sulfur compounds. These results support the hypothesis that L. bicolor increases sulfate supply to the plant by extended sulfate uptake and the plant provides the ectomycorrhizal fungus with reduced sulfur.

Studies on Nutrient Distribution in Some Zambian Soils with Special Reference to Sulphur Using GIS (Geographic Information Systems) II. Evaluation of Plant-Available Sulphur and Its Distribution in Major Zambian Soils

Relationship between the contents of soil sulphur and plant sulphur was studied in order to evaluate the distribution of plant-available sulphur in Zambian soils. Four soils series with distinct differences in texture and organic carbon content had been selected among representative benchmark soil series and used to cultivate maize (Zea mays L.) as a test plant in pots. From the results of the pot experiment, critical levels of sulphur deficiency in plants and soils were evaluated and the levels of available sulphur for maize nutrition were rated into four classes using data on the relative yield of maize. The optimum method to determine the available sulphur content of the soils was applied to the analysis of other benchmark soils. Satisfactory regression models for assessing the relation between the content of available soil sulphur with other soil constituents among benchmark soil could not be identified. A distribution map for available sulphur was drawn in a step-by-step format by using a different scale of the soil map, soil units and the legends of soil map databases. As a result, it was confirmed that sulphur-deficient and marginal areas were widely distributed throughout the country. Finally, soil acidity problems associated with sulphur application, the inconsistency of the content of available sulphur with that of total sulphur and limitation of the map utility were examined.

Influence of sulphur plant nutrition on oviposition and larval performance of the cabbage root fly

Abstract1 Oilseed rape plants (Brassica napus) (L.) (Brassicaceae) were grown under different levels of sulphur supply and tested for the oviposition preference and larval performance of cabbage root flies Delia radicum (L.) (Diptera: Anthomyiidae).2 Adult females laid more than three‐fold as many eggs on control Sn (normal field concentration) than on sulphur‐free S0 plants. By contrast, no significant difference was observed between control and double normal concentration (S+) plants.3 The larval performance was evaluated using three additional, intermediate sulphur levels between S0 and Sn, and the plants were infected with equal numbers of eggs. The percentage pupation at the end of larval feeding ranged from 6% (S0) to 32% (Sn or S+) and the average number of pupae, or of emerging flies, was significantly correlated with sulphur application.4 The weight of emerging males and females was correlated with plant sulphur supply.5 The duration of development from eggs to adult emergence was approximately 2 days longer in females than in males. Females originating from plants with a normal or higher sulphur supply tended to emerge 1–2 days earlier.

THE POTENTIAL FOR BIOSOLIDS AS SOIL CONDITIONERS AND FERTILISERS IN ARBORICULTURE

Summary Biosolids are used extensively as effective soil conditioners and organic fertilisers in both farming and reclamation of ex-industrial and brownfield sites. However, there has been little attempt to introduce biosolids into amenity horticultural practices. A field trial with Betula pendula Roth. (Silver Birch) carried out at the Imperial College London Field Centre, Silwood Park, Ascot has demonstrated the nutrient value and soil conditioning properties of biosolids to trees. In addition to organic matter, they also provide substantial amounts of plant available nitrogen (N), phosphate (P), sulphur (S) and micronutrients. Composted biosolids also contain significant amounts of potassium (K). Enhanced treated biosolids have potential for application to areas where there is significant public activity, as these biosolids are treated to destroy microbial pathogens. Conventionally treated materials may be used with land application and restrictions exist for this type of sludge product to control pathogen risks. Horticultural operatives in the amenity market have a preference for composted products or granules produced by thermally drying sludge because these materials are easy to handle.

Root-to-Shoot Transport of Sulfate in Arabidopsis. Evidence for the Role of SULTR3;5 as a Component of Low-Affinity Sulfate Transport System in the Root Vasculature

Xylem transport of sulfate regulates distribution of sulfur in vascular plants. Here, we describe SULTR3;5 as an essential component of the sulfate transport system that facilitates the root-to-shoot transport of sulfate in the vasculature. In Arabidopsis (Arabidopsis thaliana), SULTR3;5 was colocalized with the SULTR2;1 low-affinity sulfate transporter in xylem parenchyma and pericycle cells in roots. In a yeast (Saccharomyces cerevisiae) expression system, sulfate uptake was hardly detectable with SULTR3;5 expression alone; however, cells coexpressing both SULTR3;5 and SULTR2;1 showed substantial uptake activity that was considerably higher than with SULTR2;1 expression alone. The V(max) value of sulfate uptake activity with SULTR3;5-SULTR2;1 coexpression was approximately 3 times higher than with SULTR2;1 alone. In Arabidopsis, the root-to-shoot transport of sulfate was restricted in the sultr3;5 mutants, under conditions of high SULTR2;1 expression in the roots after sulfur limitation. These results suggested that SULTR3;5 is constitutively expressed in the root vasculature, but its function to reinforce the capacity of the SULTR2;1 low-affinity transporter is only essential when SULTR2;1 mRNA is induced by sulfur limitation. Consequently, coexpression of SULTR3;5 and SULTR2;1 provides maximum capacity of sulfate transport activity, which facilitates retrieval of apoplastic sulfate to the xylem parenchyma cells in the vasculature of Arabidopsis roots and may contribute to the root-to-shoot transport of sulfate.

Changes in the glutathione thiol-disulfide status in wheat grain by foliar sulphur fertilization: consequences for the rheological properties of dough

Storage proteins and glutathione in wheat play an important role in gluten network formation and can be modified by supplementation of nitrogen (N) and sulphur (S) in wheat plants. The glutathione thiol-disulfide status and its relationship to the molecular weight distribution wheat polymeric protein and dough rheological properties have been examined after different foliar S fertilizations (S derived from micronized elemental S and NS, a mixture of N urea and elemental S) applied at the post-anthesis stage. Changes in levels of reduced glutathione (GSH), glutathione disulfide (GSSG), polymeric protein-glutathione mixed disulfide (PPSSG) were analysed by reversed phase high performance liquid chromatography, during grain development using the wheat cultivars, Soissons and Trémie. During the grain desiccation phase, S supplementation (i) increased the GSSG/GSH ratio by 23–25% (ii) induced PPSSG accumulation, and (iii) decreased the formation of SDS-unextractable polymeric protein (UPP) and its molecular mass distribution. However, simultaneous N and S supplementation results in: (i) a decrease in PPSSG formation by 20–30% and (ii) an increase of UPP by 7–18% by enhancing both the branching of the aggregated proteins and their molecular weight. The mixograph parameters show that all forms of endogenous glutathione are linked to dough weakening and are negatively correlated with dough mixing tolerance, dough strength and consistency, while UPP is positively correlated with dough strength and consistency. These findings indicate that S nutrition influences dynamics of the glutathione forms in the grain and results in modification the degree of polymerization of storage protein. Thus both the changes in the form of glutathione and protein polymerization influence the rheological properties of dough.

Sulfur assimilatory metabolism. The long and smelling road.

Sulfur represents the ninth and least abundant essential macronutrient in plants, preceded by carbon, oxygen, hydrogen, nitrogen, potassium, calcium, magnesium, and phosphorus. The dry matter of sulfur in plants is only about one-fifteenth of that of nitrogen. Both sulfur and nitrogen are necessary to be assimilated into organic metabolites. While nitrogen is mainly used for structural macromolecules, sulfur plays critical roles in the catalytic or electrochemical functions of the biomolecules in cells.

Sulfur transport and assimilation in plants: regulation, interaction and signaling. * Davidian J-C, Grill D, De Kok LJ, Stulen I, Hawkesford MJ, Schnug E, Rennenberg H, eds. 2003. * Leiden: Backhuys Publishers. 96 (hardback). 393 pp.

Sulphur metabolism of plants (I'll stick with British English spelling here) has always received less attention and attracted less funding than has research on nitrogen and phosphorus. One of the most important reasons no doubt is that, unlike nitrogen and phosphorus, sulphur availability to plants was rarely a problem. As the post-World War II use of fertilizer in agricultural areas increased, so did inputs of sulphur derived from fossil fuels. Excess sulphur in the form of sulphur dioxide was of course recognized as one of the factors involved in acid rain, and research into sulphur from that point of view was particularly popular until the measures to reduce sulphur inputs into the atmosphere from fossil fuels started having effect. But then decreased atmospheric inputs of sulphur led to cases of sulphur deficiency in certain areas in relatively recent times, which gave rise to new initiatives in research of sulphur nutrition and metabolism in plants. Still, the sulphur research community is much smaller than that of nitrogen or phosphorus. Lack of awareness and understanding in the wider community of the importance of sulphur to plants, and of its intricate relationship with nitrogen and carbon metabolism have perhaps impeded availability of funding for this area of research. But this has not deterred the community of researchers working on sulphur in plants, which has grown into what Christian Brunold in his foreword calls the ‘sulfur family’, and which has been very effective in expanding knowledge and raising public awareness of sulphur metabolism in plants. This book is a compilation of papers presented at the 5th International Workshop on Sulfur Nutrition and Assimilation in Higher Plants, which was held in Montpellier, France, in April 2002. The first section contains 13 invited papers, the second about 65 contributed papers. As the title of the workshop suggests, most papers address the many facets of sulphur nutrition and assimilation in plants and related molecular and genetic aspects, but other topics, such as the functions of sulphur in physiology of metals, its role in determining taste in wines, and even its importance to the quality of sheep's wool, are also addressed. The sulphur research community has been very active over the past decade in producing texts on all aspects of sulphur nutrition and metabolism in plants. This book is complementary to the existing publications, partly because it takes a more holistic view. Unlike the previous books that have come to my attention, several of the invited papers in this book address the relationship between processes at the molecular scale and whole-plant integration and regulation. For example, Hawkesford and co-workers focus on the functions of sulphate transporters and integration with whole-plant nutrition, while Touraine and co-authors compare regulation of sulphate uptake with that of nitrate. Other contributions, for example Brunold and co-authors and Roje and Hanson, address the relationship between S, N and C metabolism. There are also several contributions on the sulphur-analog selenium, and on non-crop plant species, such as the common reed (Phragmites australis), algae and mosses. This book is well presented with generally good quality reproduction of tables and figures. Both author and subject indices are provided. It gives an excellent overview of the research currently being carried out on sulphur metabolism in plants. But that does not restrict its relevance to researchers in this specific subject area. Due to the intricate relationship of sulphur metabolism with nitrogen, carbon and phosphorus metabolism, anybody interested in nutrition of plants should find this book useful.

The effect of chloride and sulphate application to soil on changes in nutrient content in barley shoot biomass at an early phase of growth

In this study experiments primarily aimed at the needs of specification of an adequate soil reserve of labile sulphur were extended by investigations of the impact on interactions in nutrient uptake by a test barley plant. Vegetation (18-day) experiments under controlled conditions of cultivation were conducted on a diverse set of 48 soils from agricultural lands. Before barley sowing the experimental set of soils was divided into two variants: A – control (with NH<sub>4</sub>Cl application) and B – response variant [with (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> application], and a uniform dose of 26 mg N/kg soil was used. After the experiment terminated, concentrations of N, N-NO<sub>3</sub><sup>–</sup>, S, S-SO<sub>4</sub><sup>2–</sup>, P, K, Mg, Ca, Na, Mn and B were determined. Paired t-test revealed significant differences between the sets of data on variants A and B in barley yield and concentrations of sulphur, sulphate, nitrate, phosphorus and boron in barley plants. Sulphate variant (B) had higher yield of barley, higher concentrations of sulphur, sulphate and boron and lower concentrations of nitrate and phosphorus compared to variant A. The lower concentrations of nitrate and phosphorus could not be reasoned by the effect of dilution resulting from the higher barley yield. A substantial decrease in nitrate concentration was related to better utilisation of plant nitrogen after the nutrient status of soil was adjusted with sulphur. Phosphorus concentration in barley adequately corresponded to the soil reserve of labile phosphorus, but only after the phosphorus concentration in barley markedly decreased to the lower level in (sulphate) variant B. Higher concentration of boron in barley could potentially be related to the depression of phosphorus uptake after sulphate application.

The role of soil microbes in plant sulphur nutrition.

Chemical and spectroscopic studies have shown that in agricultural soils most of the soil sulphur (>95%) is present as sulphate esters or as carbon-bonded sulphur (sulphonates or amino acid sulphur), rather than inorganic sulphate. Plant sulphur nutrition depends primarily on the uptake of inorganic sulphate. However, recent research has demonstrated that the sulphate ester and sulphonate-pools of soil sulphur are also plant-bioavailable, probably due to interconversion of carbon-bonded sulphur and sulphate ester-sulphur to inorganic sulphate by soil microbes. In addition to this mineralization of bound forms of sulphur, soil microbes are also responsible for the rapid immobilization of sulphate, first to sulphate esters and subsequently to carbon-bound sulphur. The rate of sulphur cycling depends on the microbial community present, and on its metabolic activity, though it is not yet known if specific microbial species or genera control this process. The genes involved in the mobilization of sulphonate- and sulphate ester-sulphur by one common rhizosphere bacterium, Pseudomonas putida, have been investigated. Mutants of this species that are unable to transform sulphate esters show reduced survival in the soil, indicating that sulphate esters are important for bacterial S-nutrition in this environment. P. putida S-313 mutants that cannot metabolize sulphonate-sulphur do not promote the growth of tomato plants as the wild-type strain does, suggesting that the ability to mobilize bound sulphur for plant nutrition is an important role of this species.

Role of sulphur availability on cadmium-induced changes of nitrogen and sulphur metabolism in maize ( Zea mays L.) leaves

The interactions between sulphur nutrition and Cd exposure were investigated in maize ( Zea mays L.) plants. Plants were grown for 12 days in nutrient solution with or without sulphate. Half of the plants of each treatment were then supplied with 100 μM Cd. Leaves were collected 0, 1, 2, 3, 4 and 5 days from the beginning of Cd application and used for chemical analysis and enzyme assays. Cd exposure produced symptoms of toxicity (leaf chlorosis, growth reduction) and induced a noticeable accumulation of non-protein SH compounds. As phytochelatins are glutamate- and cysteine-rich peptides, the effect of cadmium on some enzyme activities involved in N and S metabolism of maize leaves was studied in relation to the plant sulphur supply. In vivo Cd application to S-sufficient plants resulted in a drop of all measured enzyme activities. On the other hand, S-deficient plants showed a decrease in nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) activity, and an increase in NAD-dependent glutamate dehydrogenase (GDH; EC 1.4.1.2) and phosphoenolpyruvate carboxylase (PEPc; EC 4.1.1.31) activity as a result of the Cd treatment. Furthermore, in the same plants ATP sulphurylase (ATPs; EC 2.7.7.4) and O-acetylserine sulphydrylase (OASs; EC 4.2.99.8) showed a particular pattern as both enzymes exhibited a transient maximum value of activity after 4 days from the beginning of Cd exposure. Results provide evidence that the increase of ATPs, OASs, GDH and PEPc activities, observed exclusively in S-deficient Cd-treated plants, may be part of the defence mechanism based on the production of phytochelatins.

Influence of plant sulphur nutrition on oviposition and larval performance of the diamondback moth

AbstractWe measured adult oviposition preference, larval growth, and feeding behaviour of the crucifer specialist Plutella xylostella (L.) (Lepidoptera: Plutellidae) on plants of Brassica napus (L.) cv. Express (Brassicaceae), grown under three different sulphur regimes. The nutrient solutions used were the following: one sulphur‐free (S0), one normal sulphur (Sn, normal field concentration), and one sulphur‐rich (S+, double concentration of Sn). Females laid more eggs on Sn than on S0 plants, while only a slight, non‐significant difference was observed between Sn and S+ plants. Moreover, the development time from hatching to emergence was significantly shorter, and adults were heavier on Sn than on S0 plants. Comparing these same two parameters from Sn and S+ plants, we found a shorter development time on plants rich in sulphur, although this trend was not statistically significant. Larval feeding preferences were tested in a dual choice assay using leaf discs. A significantly higher number of larvae preferred leaf discs of Sn plants than those of S0 plants. Furthermore, the larvae preferred S+ to Sn discs. An optimal supply of sulphur to oilseed rape is necessary for a good seed harvest, and it also plays an important role in acceptance by P. xylostella of the host plant. Maintaining higher levels of sulphur in the plant nutrient solution benefits insect performance, both at the adult and larval stage.

Abrol, Y. P. and Ahmad, A. Sulphur in plants

Since the publication of the still‐valuable Sulphur in agriculture volume (Tabatabai MA, ed., Madison: American Society of Agronomy) in 1986, there has been tremendous progress in knowledge of plant sulfur metabolism and the significance of sulfur metabolites in plant functioning. The growing awareness that sulfur deficiency is currently one of the major nutrient stresses in agriculture, not only in Western Europe but throughout the world, and the assessment of the significance of sulfur metabolites for food quality, protection of plants against stress and pests and their potential as phyto‐pharmaceuticals have boosted plant sulfur research. The research has also benefited from a close co‐operation between groups from various disciplines within Europe and other groups throughout the world. This co‐operation has been the basis for a series of international plant sulfur workshops and the publication of a series of proceeding volumes on ‘sulphur nutrition and assimilation in higher plants’. Most of these proceeding volumes have been published by Backhuys Publishers, Leiden, and present an overview of the ongoing developments in plant sulfur research. The aim of the editors of Sulphur in plants was ‘to put together the research contributions by experts in their respective fields to elaborate the various aspects of sulphur to be considered from the agricultural point of view’. It is a pity that the editors did not have the ambition to prepare an up‐to‐date coherent plant sulfur volume, since this could have become a reference work for plant scientists for the coming years. The Sulphur in plants book contains 21 chapters/reviews, wherein nearly all aspects of plant sulfur metabolism are discussed. Most of the chapters have been prepared by well‐recognized specialists, covering various aspects of sulfur metabolism in plants. Some of the authors also have recently written similar reviews on their topic in journals and other sulfur volumes, although most of them have really put a lot of effort into their contribution here and have chosen a different approach or included a lot of additional information. The majority of the chapters present an up‐to‐date, detailed and balanced overview of the current knowledge within the specific topics and it is also easy to read for non‐specialists in the sulfur field. For instance, the book contains comprehensive reviews on the global sulfur cycle (Haneklaus et al), the significance of sulfur in food production and agriculture (Aulakh; Pasricha and Abrol), the uptake and assimilation of sulfur (Glawischni et al; Harwood and Okanenko; Hawkesford et al; Kopriva and Koprivova; Verkleij et al), the distribution of sulfur in plants (Anderson and Fitzgerald), the significance of sulfur in adaptation to stress (Tausz et al; Sirku et al), and a valuable overview of the possible use of sulfur metabolites as phyto‐pharmaceuticals (Haq and Ali). There is some overlap between the content of different chapters, although this could hardly be avoided. However, what is missing is a coherency between the different chapters: their sequence is not always logical and they could have been more balanced with respect to different disciplines; e.g. introduction, uptake and assimilation of sulfur (essential and secondary sulfur compounds), environmental and agricultural aspects. What is also missing is a summarizing (introduction) chapter presenting a scheduled overview of sulfate metabolism, addressing the significance of cysteine as a precursor of most other sulfur metabolites in plants and the uniqueness of the thiol moiety of cysteine in the structure and function of proteins and other important sulfur‐containing metabolites. Despite the fact that most chapters end with a conclusion/prospects section, it would have been useful to include a chapter discussing the gaps in the present knowledge in plant sulfur metabolism in order to set a trend for further plant sulfur research. Furthermore, the editors should have put more effort into the layout and index of the book. For instance, the size of some of the figures has been reduced to such an extent that their content is hard to read. They are several printing/spacing errors and the index of the book is not very accurate and far from complete. In summary, the book covers all different aspects of sulfur metabolism in plants and it may be useful as a reference book for graduate and postgraduate students and senior scientists, who would like to get involved with or have an interest in plant sulfur metabolism and who do not have access to any other sulfur books. Thus it is valuable as a reference volume in the libraries of plant science departments of universities and institutes. Due to its high price, it is less attractive for individual buyers.

6th International Workshop on Plant Sulfur Metabolism

6th International Workshop on Plant Sulfur Metabolism

Sulfur assimilation in plants and weed control: Potential targets for novel herbicides and action sites of certain safeners

Sulfur is an indispensable element for plants. It is found in sulfur‐containing amino acids, cysteine and methionine, and in various other important biochemical components and processes. Inhibitors of sulfur assimilation, or cysteine and methionine synthesis, could be potential herbicides. In the present paper, the sulfur assimilation pathway in plants is described, followed by the introduction of several compounds (inhibitors and safeners) acting on this pathway. Uptake of inorganic sulfate through the roots is the first step of sulfur assimilation in plants. Sulfate is reduced mainly in chloroplasts to sulfide by a multistep process, and sulfide is then incorporated into cysteine. Cysteine is converted to cystathionine, homocysteine and methionine. Cysteine is incorporated into glutathione (GSH) by γ‐glutamylcysteine synthetase and GSH synthetase. Three enzymes involved in cysteine and methionine biosynthesis, cysteine synthase, cystathionine γ‐synthase and cystathionine β‐lyase, have been investigated as target sites for herbicides. Several inhibitors of these enzymes (e.g. rhizobitoxine and propargylglycine) were also phytotoxic, suggesting that the synthetic pathway of sulfur‐containing amino acids could be a new target site for herbicides. Some safeners for herbicides were found to act on the sulfur assimilation pathway and on GSH synthesis to increase GSH, which can be involved in herbicide metabolism and detoxification. Several safeners elevate GSH levels by increasing the activities of enzymes involved in sulfur assimilation and GSH synthesis. Further studies on plant sulfur metabolism may lead to the discovery of new herbicides and to the comprehensive understanding of the mode of action of safeners.