Phytochemistry Reviews (v.8, #1)

Phytochemistry reviews—special issue on glucosinolates by Jonathan Gershenzon; Caroline Müller (1-2).

Five members of subgroup 12 R2R3-MYB transcription factors, namely MYB51, MYB122, MYB28, MYB29 and MYB76, are novel regulators of glucosinolate biosynthesis in Arabidopsis thaliana. Overexpression of MYB51 and MYB122 led to an increased accumulation of tryptophan-derived indolic glucosinolates whereas MYB28, MYB29 and MYB76 overexpression lines showed an increase in methionine-derived aliphatic glucosinolates. Likewise, disruption of the corresponding genes caused a significant downregulation of indolic and aliphatic glucosinolates, respectively. Expression analysis of promoter-GUS fusions revealed promoter activities at the sites of glucosinolate synthesis and accumulation. Indolic glucosinolate regulators were mainly found in vegetative organs and roots, whereas aliphatic glucosinolate regulators were preferentially expressed in generative organs. Mechanical stimuli such as touch or wounding induced a transient expression of the regulators and overexpression of MYB28 and MYB51 reduced insect performance demonstrating the role of these transcription factors in plant biotic responses. The subgroup 12 R2R3-MYB transcription factors interdependently control the response to biotic challenges. For the regulation of methionine-derived glucosinolates, the coordinated activation of MYB28, MYB76 and MYB29 is required, whereas MYB51, MYB122 and the sixth member of subgroup 12 R2R3-MYB transcription factors, the previously described ATR1/MYB34, are involved in the regulation of tryptophan-derived glucosinolates. Because these two pathways are reciprocally inhibiting each other, a metabolic balance between both biosynthetic pathways can be accomplished in plants exposed to continuous biotic challenges.
Keywords: Glucosinolate biosynthesis; Gene regulation; MYB factors; Biotic stress

Transcriptome coexpression analysis, which is based on a vast amount of transcriptome data obtained by using DNA arrays, has become a routine method for functional genomics studies in Arabidopsis. This analysis enables us to predict the function of genes on the basis of a simple assumption that a set of genes involved in a particular biological process can be coexpressed under the control of a shared regulatory system. Candidate genes involved in glucosinolate biosynthesis were successfully identified by this approach. In this review, the methodology of coexpression analysis is briefly described. The advantages and disadvantages of this analysis are also discussed in the context of its ability to predict gene functions involved in glucosinolate biosynthesis.
Keywords: Coexpression; Correlation; Gene function; Network; Transcriptome; Prediction

Indolic glucosinolates at the crossroads of tryptophan metabolism by Judith Bender; John L. Celenza (25-37).
In plants the amino acid tryptophan (Trp) is used to synthesize proteins and a wide variety of compounds that control development and defense. Recent studies in the reference plant Arabidopsis thaliana have elucidated a number of tryptophan secondary metabolism pathways derived from the Trp metabolite, indole-3-acetaldoxime (IAOx), particularly the pathway for synthesis of indolic glucosinolate (IG) herbivory defense compounds. Analyses of mutants, natural variants, and transgenic strains with perturbations in the IG pathway have revealed that regulation of this pathway is networked at many levels, including interfaces with Trp synthesis, other Trp secondary metabolism pathways, other glucosinolate synthesis pathways, and sulfur metabolism. Transcriptional regulatory mechanisms have been particularly well characterized, but additional mechanisms such as metabolic channeling may also contribute to the homeostasis of Trp secondary metabolism. The IG pathway thus serves as a paradigm for regulatory cross-talk between primary and secondary metabolism and among inter-related secondary metabolic processes.
Keywords: Arabidopsis; Camalexin; Cytochrome P450; Indole-3-acetic acid; Myb transcription factor

Mathematical modelling of aliphatic glucosinolate chain length distribution in Arabidopsis thaliana leaves by Beate Knoke; Susanne Textor; Jonathan Gershenzon; Stefan Schuster (39-51).
a reversal of the relative proportions of the two shortest glucosinolates,a significant increase in the concentration of the longest glucosinolate,an increase in total glucosinolate content in the mutant. MAM3 knockout mutants on the contrary differ from wild type plants by a pronounced abundance of the second shortest glucosinolate and the depletion of the two longest glucosinolates. To clarify the contribution of the multifunctional enzymes MAM1 and MAM3 to the glucosinolate profile of Arabidopsis thaliana leaves, we simulated glucosinolate biosynthesis in a kinetic model, taking into account the structure of the pathway and measured enzymatic properties. The predicted glucosinolate profiles show all characteristics of the actual differences between wild-type and MAM1 mutants or MAM3 mutants, respectively. The model strongly supports experimental indications that the two MAM activities are not independent of each other. In particular, it showed that an elevated expression of MAM3 in the MAM1 mutant is critical in determining the glucosinolate profile of this plant line. The simulation was critical for this finding since it allowed us to assess the individual effects of two processes—the knocking out of MAM1 and the overexpression of MAM3—that are difficult to separate experimentally.
Keywords: Kinetic modelling; Plant defense metabolites; Simulation of metabolism; Methylthioalkylmalate synthase; Methionine chain elongation

Piecing together the transport pathway of aliphatic glucosinolates by Hussam Hassan Nour-Eldin; Barbara Ann Halkier (53-67).
Glucosinolates are sulfur-rich secondary metabolites characteristic of the Brassicales order. Transport of glucosinolates was suggested more than 30 years ago through a number of studies which indicated that glucosinolates are produced in maternal tissue and subsequently transported to the seed. These observations laid the foundation for numerous studies on glucosinolate transport which have provided a wealth of information on biochemical properties of glucosinolate transport, source–sink relationships between organs and on the transport routes of glucosinolates. However, most of the conclusions and hypotheses proposed in these studies have not been discussed in context of each other to provide a complete overview of the current state of knowledge on glucosinolate transport. In this review, we are thus piecing together the glucosinolate pathway by presenting and critically analyzing all data on glucosinolate research. Furthermore, the data on glucosinolate transport is considered in the light of the newest findings on glucosinolate synthesis and distribution. The aim is to provide a comprehensive and updated set of hypotheses which may prove useful in directing future research on glucosinolate transport.
Keywords: Transport; Glucosinolates; Arabidopsis thaliana ; Synthesis; Distribution; Secondary metabolites

Glucosinolates are plant secondary metabolites that are hydrolysed by the action of myrosinases into various products (isothiocyanates, thiocyanates, epithionitriles, nitriles, oxazolidines). Massive hydrolysis of glucosinolates occurs only upon tissue damage but there is also evidence indicating metabolism of glucosinolates in intact plant tissues. It was originally believed that the glucosinolate–myrosinase system in intact plants was stable due to a spatial separation of the components. This has been coined as the ‘mustard oil bomb’ theory. Proteins that form complexes with myrosinases have been described: myrosinase-binding proteins (MBPs) and myrosinase-associated proteins (MyAPs/ESM). The roles of these proteins and their biological relevance are not yet completely known. Other proteins of the myrosinase enzyme system are the epithiospecifier protein (ESP) and the thiocyanate-forming protein (TFP) that divert the glucosinolate hydrolysis from isothiocyanate production to nitrile/epithionitrile or thiocyanate production. Some glucosinolate hydrolysis products act as plant defence compounds against insects and pathogens or have beneficial health effects on humans. In this review, we survey and critically assess the available information concerning the localization, both at the tissular/cellular and subcellular level, of the different components of the myrosinase enzyme system. Data from the model plant Arabidopsis thaliana is compared to that from other glucosinolate-producing Brassicaceae in order to show common as well as divergent features of the ‘mustard oil bomb’ among these species.
Keywords: Glucosinolate hydrolysis; Plant defence; Secondary metabolites; Myrosin cell; Plant innate immunity

Regulation and function of specifier proteins in plants by Meike Burow; Ute Wittstock (87-99).
Specifier proteins are responsible for the diversification of biologically active products formed upon myrosinase-catalyzed glucosinolate hydrolysis and are therefore assumed to have an impact on the defensive function of the glucosinolate–myrosinase system. Among glucosinolate hydrolysis products, the generation of epithionitriles and organic thiocyanates requires the presence of epithiospecifier protein (ESP) and thiocyanate-forming protein (TFP), respectively, while myrosinase alone is sufficient for the production of isothiocyanates. Both ESP and TFP also promote the formation of simple nitriles upon myrosinase-catalyzed glucosinolate hydrolysis. Only little is known about the biological effects of epithionitriles and thiocyanates. Moreover, simple nitriles have repeatedly been reported to be less toxic to plant pathogens and herbivorous insects than the correponding isothiocyanates. Thus, it has remained an open question how plants benefit from the presence of specifier proteins. In this review, we survey the biological effects of different types of glucosinolate hydrolysis products on insects and pathogens as well as the current knowlegde on the developmental, organ specific and stimuli-mediated regulation of specifier proteins. Integrating these findings can help us to better understand the ecological functions of plant specifier proteins as well as the co-evolution of glucosinolate-containing plants and their insect herbivores.
Keywords: Epithionitrile; Epithiospecifier protein; Glucosinolate; Isothiocyanate; Nitrile; Thiocyanate-forming protein

Indole glucosinolate breakdown and its biological effects by Niels Agerbirk; Martin De Vos; Jae Hak Kim; Georg Jander (101-120).
Most species in the Brassicaceae produce one or more indole glucosinolates. In addition to the parent indol-3-ylmethylglucosinolate (IMG), other commonly encountered indole glucosinolates are 1-methoxyIMG, 4-hydroxyIMG, and 4-methoxyIMG. Upon tissue disruption, enzymatic hydrolysis of IMG produces an unstable aglucone, which reacts rapidly to form indole-3-acetonitrile and indol-3-ylmethyl isothiocyanate. The isothiocyanate, in turn, can react with water, ascorbate, glutathione, amino acids, and other plant metabolites to produce a variety of physiologically active indole compounds. Myrosinase-initiated breakdown of the substituted indole glucosinolates proceeds in a similar manner to that of IMG. Induction of indole glucosinolate production in response to biotic stress, experiments with mutant plants, and artificial diet assays suggest a significant role for indole glucosinolates in plant defense. However, some crucifer-feeding specialist herbivores recognize indole glucosinolates and their breakdown products as oviposition and/or feeding stimulants. In mammalian diets, IMG can have both beneficial and deleterious effects. Most IMG breakdown products induce the synthesis of phase 1 detoxifying enzymes, which may in some cases prevent carcinogenesis, but in other cases promote carcinogenesis. Recent advances in indole glucosinolate research have been fueled by their occurrence in the well-studied model plant Arabidopsis thaliana. Knowledge gained from genetic and biochemical experiments with A. thaliana can be applied to gain new insight into the ecological and nutritional properties of indole glucosinolates in other plant species.
Keywords: Ascorbigen; Brassicaceae; Cancer; Indole-3-acetonitrile; Indole-3-carbinol; Indol-3-ylmethyl isothiocyanate; Insect; Myrosinase

Several insects have specialised on using Brassicaceae as host plants. Therefore, they evolved metabolic pathways to cope with the defensive glucosinolate–myrosinase system of their diet. Larvae of the turnip sawfly, Athalia rosae L. (Hymenoptera: Tenthredinidae), incorporate various glucosinolates from their hosts into their haemolymph. The ability to sequester these metabolites makes A. rosae a useful model system to study mechanisms of glucosinolate metabolism in this species compared to other specialists, and to study effects of sawfly feeding on levels of glucosinolates and their hydrolysing enzymes in plants. The levels of plant metabolites might in turn directly affect the performance of the insect. On the one hand, costs for glucosinolate uptake and avoidance of myrosinase activity were postulated. On the other hand, sequestration of glucosinolates can be part of the insect’s defence against several predators. Here, the findings on glucosinolate metabolic pathways are compared between different herbivores and the sawfly. The impact of different glucosinolate levels and myrosinase activities on the performance of A. rosae is discussed. Furthermore, effects of feeding by A. rosae larvae on the chemical composition and enzyme activities of various Brassicaceae species are summarised. Induction patterns vary not only between different plant species and cultivars but also due to the inducing agent. Finally, the plant–herbivore interactions are discussed with regard to the sawflies’ defence abilities against different carnivore guilds.
Keywords: Easy bleeding; Metabolism; Performance; Predator defence; Sequestration

The clubroot disease is caused by the obligate biotrophic protist Plasmodiophora brassicae and is one of the most damaging for the family of Brassicaceae. Since many economically important crops belong to this plant family, the understanding of mechanisms how the disease is developing, are of high importance. Glucosinolates, a group of secondary plant products in the family of Brassicaceae, have long been associated with clubroot disease symptoms. Measurements showed that several glucosinolates are induced in root galls. While aliphatic glucosinolates are regarded as defense compounds, analysis of Brassica cultivars as well as Arabidopsis thaliana mutants provided correlative evidence between disease severity and indole glucosinolate content. The latter have been discussed as precursors for auxin biosynthesis. Since high auxin levels are associated with large root galls, indole glucosinolates could contribute directly or indirectly to the extent of disease development. Transcriptome and proteome experiments have revealed evidence for the involvement of genes from the glucosinolate and auxin pathway in gall formation. These data have been complemented by expression and mutant analysis. It can be concluded that regulation of glucosinolate and IAA biosynthesis might differ in Brassica and Arabidopsis.
Keywords: Auxin; Brassicaceae; Clubroot disease; Glucosinolates; Plasmodiophora brassicae

Like many other plant defense compounds, glucosinolates are present constitutively in plant tissues, but are also induced to higher levels by herbivore attack. Of the major glucosinolate types, indolic glucosinolates are most frequently induced regardless of the type of herbivore involved. Over 90% of previous studies found that herbivore damage to glucosinolate-containing plants led to an increased accumulation of indolic glucosinolates at levels ranging up to 20-fold. Aliphatic and aromatic glucosinolates are also commonly induced by herbivores, though usually at much lower magnitudes than indolic glucosinolates, and aliphatic and aromatic glucosinolates may even undergo declines following herbivory. The glucosinolate defense system also requires another partner, the enzyme myrosinase, to hydrolyze the parent glucosinolates into biologically active derivatives. Much less is known about myrosinase induction after herbivory compared to glucosinolate induction, and no general trends are evident. However, it is clear that insect feeding stimulates the formation of various myrosinase associated proteins whose function is not yet understood. The biochemical mechanism of glucosinolate induction involves a jasmonate signaling cascade that leads eventually to increases in the transcript levels of glucosinolate biosynthetic genes. Several recently described transcription factors controlling glucosinolate biosynthesis are activated by herbivory or wounding. Herbivore induction of glucosinolates has sometimes been demonstrated to increase protection against subsequent herbivore attack, but more research is needed to evaluate the costs and benefits of this phenomenon.
Keywords: Indolic glucosinolates; Jasmonate signaling; Myrosinase associated proteins; Systemic induction; Transcription factors

The role of glucosinolates in aboveground plant–insect and plant–pathogen interactions has been studied widely in both natural and managed ecosystems. Fewer studies have considered interactions between root glucosinolates and soil organisms. Similarly, data comparing local and systemic changes in glucosinolate levels after root- and shoot-induction are scarce. An analysis of 74 studies on constitutive root and shoot glucosinolates of 29 plant species showed that overall, roots have higher concentrations and a greater diversity of glucosinolates than shoots. Roots have significantly higher levels of the aromatic 2-phenylethyl glucosinolate, possibly related to the greater effectiveness and toxicity of its hydrolysis products in soil. In shoots, the most dominant indole glucosinolate is indol-3-ylglucosinolate, whereas roots are dominated by its methoxyderivatives. Indole glucosinolates were the most responsive after jasmonate or salicylate induction, but increases after jasmonate induction were most pronounced in the shoot. In general, root glucosinolate levels did not change as strongly as shoot levels. We postulate that roots may rely more on high constitutive levels of glucosinolates, due to the higher and constant pathogen pressure in soil communities. The differences in root and shoot glucosinolate patterns are further discussed in relation to the molecular regulation of glucosinolate biosynthesis, the within-tissue distribution of glucosinolates in the roots, and the use of glucosinolate-containing crops for biofumigation. Comparative studies of tissue-specific biosynthesis and regulation in relation to the biological interactions in aboveground and belowground environments are needed to advance investigations of the evolution and further utilization of glucosinolates in natural and managed ecosystems.
Keywords: Allelopathy; Induced responses; Isothiocyanates; Plant–environment interaction; Soil ecology

Direct and indirect plant defences are well studied, particularly in the Brassicaceae. Glucosinolates (GS) are secondary plant compounds characteristic in this plant family. They play an important role in defence against herbivores and pathogens. Insect herbivores that are specialists on brassicaceous plant species have evolved adaptations to excrete or detoxify GS. Other insect herbivores may even sequester GS and employ them as defence against their own antagonists, such as predators. Moreover, high levels of GS in the food plants of non-sequestering herbivores can negatively affect the growth and survival of their parasitoids. In addition to allelochemicals, plants produce volatile chemicals when damaged by herbivores. These herbivore induced plant volatiles (HIPV) have been demonstrated to play an important role in foraging behaviour of insect parasitoids. In addition, biosynthetic pathways involved in the production of HIPV are being unraveled using the model plant Arabidopsis thialiana. However, the majority of studies investigating the attractiveness of HIPV to parasitoids are based on experiments mainly using crop plant species in which defence traits may have changed through artificial selection. Field studies with both cultivated and wild crucifers, the latter in which defence traits are intact, are necessary to reveal the relative importance of direct and indirect plant defence strategies on parasitoid and plant fitness. Future research should also consider the potential conflict between direct and indirect plant defences when studying the evolution of plant defences against insect herbivory.
Keywords: Crucifers; Direct defence; Glucosinolates; Indirect defence; Herbivore-induced plant volatiles

Herbivorous insects identify their host plants either by structural features, chemical cues, or a combination. Some insects probe the host leaf prior feeding or oviposition, other species use olfactorial cues or compounds somewhere on the surface. Insects attacking Brassicaceae are no exception, some are attracted and stimulated by volatile isothiocyanates (ITC), many others depend fully on the non-volatile glucosinolates (GS) for host-plant recognition and acceptance. Since most insects have no access to the leaf interior investigators concluded that GS must be present on the leaf surface and ITC in the headspace. However, peelings of mechanically removed surface waxes were devoid of measurable amounts of GS, whereas solvent surface extractions revealed a correlation between stomatal conditions and GS concentrations. Both observations lead to the conclusion that the presence of GS on the top leaf surface is rather unlikely. In the experimental part we show that a chloroform/methanol/water (2:1:1 vol/vol/vol) solvent leaf extract contains GS and, in addition, thia-triaza-fluorenes (TTF), other oviposition stimulants of the cabbage root fly, Delia radicum. Electrophysiological investigations showed that both, GS and TTF stimulated specific receptor neurones of the fly. We suggest that these compounds probably originated from deeper leaf layers and that herbivorous insects may penetrate the wax layer and perceive the stimulating compounds in deeper layers or through the stomata.
Keywords: Chemoreceptor neurones; Herbivore insects; Gustation; Leaf surface wax; Olfaction

Role of glucosinolates in plant invasiveness by Caroline Müller (227-242).
Many plants have been intentionally or accidentally introduced to new habitats where some of them now cause major ecological and economic threats to natural and agricultural ecosystems. The potential to become invasive might depend on plant characteristics, as well as on specific interactions with other organisms acting as symbionts or antagonists, including other plants, microbes, herbivores, or pollinators. The invasion potential furthermore depends on abiotic conditions in the habitat. Several species of the Brassicaceae, well known for their glucosinolate–myrosinase defence system, are invasive species. Various factors are reviewed here that might explain why these species were so successful in colonising new areas. Specific emphasis is laid on the role of glucosinolates and their hydrolysis products in the invasion potential. This particular defence system is involved specifically in plant–plant, plant–microbe and plant–insect interactions. Most research has been done on the mechanisms underlying invasion success of Alliaria petiolata and Brassica spp., followed by Bunias orientalis and Lepidium draba. Some examples are also given for plants that are not necessarily considered as invasives, but which were well investigated with respect to their interference potential with their biotic environment. For each species, most likely a combination of different plant characteristics enhanced the competitive abilities and led to diverse invasive phenotypes.
Keywords: Allelopathy; Enemy release; Evolution of increased competitive ability; Life-history traits; Plant–antagonist interactions

Plants’ sessile nature has led them to develop chemical defenses, secondary metabolites, to directly cope with environmental changes rather than escape to more favorable sites. The diversity and fluctuation in biological stresses faced by a plant have generated extraordinary genetic diversity controlling the synthesis and regulation of secondary metabolites that is only now being explored. The glucosinolate secondary metabolites, amino acid derived thioglucosides specific to the order Capparales, is a model system for understanding the molecular basis of complex quantitative traits and their potential ecological role. This review focuses on the extensive progress being made towards understanding the complete molecular basis underlying the glucosinolate genetic diversity at both biosynthetic and regulatory loci. This has identified a highly interactive genetic network whereby biosynthetic loci have additional functions as regulatory loci and laid the foundation for glucosinolates to be a model system for understanding quantitative traits in a broader context.
Keywords: Glucosinolate; Quantitative trait; QTL; System biology; Maintenance of diversity

Methylthioalkylmalate synthases: genetics, ecology and evolution by Markus Benderoth; Marina Pfalz; Juergen Kroymann (255-268).
Glucosinolates display an enormous amount of structural variation, both within and between species. This diversity is thought to have evolved in response to challenges imposed on plants by their biotic environment. During the past decade, glucosinolates and myrosinase-catalyzed glucosinolate hydrolysis have become excellent examples for understanding functional diversification in plant secondary metabolism and plant defence. Methylthioalkylmalate (MAM) synthase genes and enzymes are central to the diversification of aliphatic glucosinolate structures in Arabidopsis thaliana and related plants. This review summarizes efforts to elucidate how MAM-mediated diversity in aliphatic glucosinolate structures is generated and maintained. It also attempts to put variability in methionine carbon chain elongation during glucosinolate biosynthesis into an ecological and evolutionary context.
Keywords: Complex traits; Evolutionary dynamics; Glucosinolate metabolism; Natural variation; Plant–insect interactions

Glucosinolates, isothiocyanates and human health by Maria Traka; Richard Mithen (269-282).
Concurrent with the increase in our knowledge of the genetic and environmental factors that lead to glucosinolate accumulation in plants, and the role of these compounds and their derivatives in mediating plant–herbivore interactions, there has been significant advances in our understanding of how glucosinolates and their products may contribute to a reduction in risk of carcinogenesis and heart disease when consumed as part of the diet. In this paper, we review the epidemiological evidence for the health promoting effects of cruciferous vegetables, the processes by which glucosinolates and isothiocyanates are absorbed and metabolised by humans, with particular regard to the role of glutathione S-transferases, and the biological activity of isothiocyanates towards mammalian cells and tissues.
Keywords: Brassica ; Cancer; Epidemiology; GST; Intervention studies

Physiological effects of broccoli consumption by Elizabeth H. Jeffery; Marcela Araya (283-298).
Epidemiological studies suggest that broccoli can decrease risk for cancer. Broccoli contains many bioactives, including vitamins C and E, quercetin and kaempferol glycosides and, like other members of the Brassicaceae, several glucosinolates, including glucobrassicin (3-indolylmethyl glucosinolate) and glucoraphanin (4-methylsulphinylbutyl glucosinolate). A key bioactive component responsible for much of this activity may be sulforaphane (1-isothiocyanato-4-methylsulfinylbutane), a hydrolysis product of glucoraphanin. Sulforaphane not only upregulates a number of phase II detoxification enzymes involved in clearance of chemical carcinogens and reactive oxygen species, but has anti-tumorigenic properties, causing cell cycle arrest and apoptosis of cancer cells. The bioequivalency of sulforaphane and whole broccoli have not been fully evaluated, leaving it unclear whether whole broccoli provides a similar effect to purified sulforaphane, or whether the presence of other components in broccoli, such as indole-3-carbinol from glucobrassicin, is an added health benefit. Dietary indole-3-carbinol is known to alter estrogen metabolism, to cause cell cycle arrest and apoptosis of cancer cells and, in animals, to decrease risk for breast cancer. Recent research suggests that both dietary broccoli and the individual components sulforaphane and indole-3-carbinol may offer protection from a far broader array of diseases than cancer, including cardiovascular and neurodegenerative diseases. A common link between these oxidative degenerative diseases and cancer may be aggravation by inflammation. A small body of literature is forming suggesting that both indole-3-carbinol and sulforaphane may protect against inflammation, inhibiting cytokine production. It remains to be seen whether cancer, cardiovascular disease, dementia and other diseases of aging can all benefit from a diet rich in broccoli and other crucifers.
Keywords: Broccoli; Sulforaphane; Anti-inflammatory; Indole-3-carbinol; Anti-carcinogenesis

The bioactive hydrolysis products of glucosinolates, particularly the isothiocyanates, can be used to control soil pests and weeds by incorporating glucosinolate-containing plant material in soil—a practice known as biofumigation. The fate of glucosinolates and their hydrolysis products in soil determines both the efficacy and environmental impact of biofumigation. Knowledge of the processes by which these compounds are sorbed, degraded or otherwise lost from the soil is fundamental to developing effective, but environmentally benign biofumigation strategies. Effective biofumigation relies on maximum hydrolysis of the glucosinolate in the plant tissue to generate high isothiocyanate concentrations in the soil after incorporation. This is favoured by maximum cell disruption, by addition of water, and a high soil temperature. Residual glucosinolates are very weakly sorbed, readily leached and are microbially degraded and mineralised in soil. In contrast, isothiocyanates are strongly sorbed by the organic matter in soil, react strongly with nucleophilic groups present in soil, and are prone to volatilization losses in addition to microbial degradation and mineralisation. These loss processes are influenced by soil type, water content and temperature. Using appropriate incorporation strategies, sufficiently high isothiocyanate concentrations (>100 nmol g−1) can be achieved in soil using biofumigation for effective suppression of susceptible pests. The relatively rapid sorption and degradation of the isothiocyanates in the period of days after incorporation minimizes the risks of persistence in the environment or leaching. Biofumigation is therefore a promising technique which can be further developed to form part of IPM (Integrated Pest Management) strategies to reduce reliance on synthetic pesticides with minimal unintended impacts on the environment.
Keywords: Isothiocyanates; Green manure; Brassica ; IPM; Natural products; Soil chemistry