Functional characterization of desaturases involved in the formation of the terminal double bond of an unusual 16:3Delta(9,12,150) fatty acid isolated from Sorghum bicolor root hairs

Variation on a theme: the structures and biosynthesis of specialized fatty acid natural products in plants

Plants are able to construct lineage-specific natural products from a wide array of their core metabolic pathways. Considerable progress has been made toward documenting and understanding, for example, phenylpropanoid natural products derived from phosphoenolpyruvate via the shikimate pathway, terpenoid compounds built using isopentyl pyrophosphate, and alkaloids generated by the extensive modification of amino acids. By comparison, natural products derived from fatty acids have received little attention, except for unusual fatty acids in seed oils and jasmonate-like oxylipins. However, scattered but numerous reports show that plants are able to generate many structurally diverse compounds from fatty acids, including some with highly elaborate and unique structural features that have novel bioproduct functionalities. Furthermore, although recent work has shed light on multiple new fatty acid natural product biosynthesis pathways and products in diverse plant species, these discoveries have not been reviewed. The aims of this work, therefore, are to (i) review and systematize our current knowledge of the structures and biosynthesis of fatty acid-derived natural products that are not seed oils or jasmonate-type oxylipins, specifically, polyacetylenic, very-long-chain, and aromatic fatty acid-derived natural products, and (ii) suggest priorities for future investigative steps that will bring our knowledge of fatty acid-derived natural products closer to the levels of knowledge that we have attained for other phytochemical classes.

Sorghum breeding in the genomic era: opportunities and challenges

The importance and potential of the multi-purpose crop sorghum in global food security have not yet been fully exploited, and the integration of the state-of-art genomics and high-throughput technologies into breeding practice is required. Sorghum, a historically vital staple food source and currently the fifth most important major cereal, is emerging as a crop with diverse end-uses as food, feed, fuel and forage and a model for functional genetics and genomics of tropical grasses. Rapid development in high-throughput experimental and data processing technologies has significantly speeded up sorghum genomic researches in the past few years. The genomes of three sorghum lines are available, thousands of genetic stocks accessible and various genetic populations, including NAM, MAGIC, and mutagenised populations released. Functional and comparative genomics have elucidated key genetic loci and genes controlling agronomical and adaptive traits. However, the knowledge gained has far away from being translated into real breeding practices. We argue that the way forward is to take a genome-based approach for tailored designing of sorghum as a multi-functional crop combining excellent agricultural traits for various end uses. In this review, we update the new concepts and innovation systems in crop breeding and summarise recent advances in sorghum genomic researches, especially the genome-wide dissection of variations in genes and alleles for agronomically important traits. Future directions and opportunities for sorghum breeding are highlighted to stimulate discussion amongst sorghum academic and industrial communities.

In vivo assembly of the sorgoleone biosynthetic pathway and its impact on agroinfiltrated leaves of Nicotiana benthamiana

Sorgoleone, a hydrophobic compound exuded from root hair cells of Sorghum spp., accounts for much of the allelopathic activity of the genus. The enzymes involved in the biosynthesis of this compound have been identified and functionally characterized. Here, we report the successful assembly of the biosynthetic pathway and the significant impact of in vivo synthesized sorgoleone on the heterologous host Nicotiana benthamiana. A multigene DNA construct was prepared for the expression of genes required for sorgoleone biosynthesis in planta and deployed in N. benthamiana leaf tissues via Agrobacterium-mediated transient expression. RNA-sequencing was conducted to investigate the effects of sorgoleone, via expression of its biosynthesis pathway, on host gene expression. The production of sorgoleone in agroinfiltrated leaves as detected by gas chromatography/mass spectrometry (GC/MS) resulted in the formation of necrotic lesions, indicating that the compound caused severe phytotoxicity to these tissues. RNA-sequencing profiling revealed significant changes in gene expression in the leaf tissues expressing the pathway during the formation of sorgoleone-induced necrotic lesions. Transcriptome analysis suggested that the compound produced in vivo impaired the photosynthetic system as a result of downregulated gene expression for the photosynthesis apparatus and elevated expression of proteasomal genes which may play a major role in the phytotoxicity of sorgoleone.

Two clusters of residues contribute to the activity and substrate specificity of Fm1, a bifunctional oleate and linoleate desaturase of fungal origin

Polyunsaturated fatty acids (PUFAs) have important industrial, physiological, and nutritional properties. Plants use the sequential activities of FAD2 and FAD3 desaturases to convert 18:1Δ9 to the important PUFA 18:3Δ9,12,15, whereas the fungus Fusarium verticillioides 7600 uses the bifunctional desaturase Fm1 for both reactions. Here, we used a combination of sequence comparisons, structural modeling, and mutagenesis experiments to investigate Fm1's regioselectivity and identified two functionally relevant clusters of residues that contribute to Fm1 activity. We found that cluster I (Leu¹⁵³, Phe¹⁵⁷, and His¹⁹⁴), located near the catalytic iron ions, predominantly affects activity, whereas cluster II (Tyr²⁸⁰, His²⁸⁴, and Leu²⁸⁷), located in a helix forming the entrance to the substrate-binding pocket, mainly specifies 15-desaturation. Individual or combined substitutions of cluster II residues substantially reduced 15-desaturation. The combination of F157W from cluster I with Y280L, H284V, and L287T from cluster II created an increased-activity variant that almost completely lost the ability to desaturate at C15 and acted almost exclusively as a 12-desaturase. No variants were identified in which 15-desaturation occurred in the absence of 12-desaturation. Fm1 displayed only traces of activity with C16 substrate, but several cluster I variants exhibited increased activity with both 18:1 and 16:1 substrates, converting 16:1Δ9 to 16:3Δ9,12,15, consistent with Fm1 performing sequential v + 3 desaturation reactions at C12 and then C15. We propose that cluster II residues interact with the substrate headgroup when the acyl chain contains both Δ9 and Δ12 double bonds, in which case C15 becomes positioned adjacent to the di-iron site enabling a second v + 3 desaturation.

A cytochrome P450 CYP71 enzyme expressed in Sorghum bicolor root hair cells participates in the biosynthesis of the benzoquinone allelochemical sorgoleone

Sorgoleone, a major component of the hydrophobic root exudates of Sorghum spp., is probably responsible for many of the allelopathic properties attributed to members of this genus. Much of the biosynthetic pathway for this compound has been elucidated, with the exception of the enzyme responsible for the catalysis of the addition of two hydroxyl groups to the resorcinol ring. A library prepared from isolated Sorghum bicolor root hair cells was first mined for P450-like sequences, which were then analyzed by quantitative reverse transcription-polymerase chain reaction (RT-qPCR) to identify those preferentially expressed in root hairs. Full-length open reading frames for each candidate were generated, and then analyzed biochemically using both a yeast expression system and transient expression in Nicotiana benthamiana leaves. RNA interference (RNAi)-mediated repression in transgenic S. bicolor was used to confirm the roles of these candidates in the biosynthesis of sorgoleone in planta. A P450 enzyme, designated CYP71AM1, was found to be capable of catalyzing the formation of dihydrosorgoleone using 5-pentadecatrienyl resorcinol-3-methyl ether as substrate, as determined by gas chromatography-mass spectroscopy (GC-MS). RNAi-mediated repression of CYP71AM1 in S. bicolor resulted in decreased sorgoleone contents in multiple independent transformant events. Our results strongly suggest that CYP71AM1 participates in the biosynthetic pathway of the allelochemical sorgoleone.

Classification and substrate head-group specificity of membrane fatty acid desaturases

Membrane fatty acid desaturases are a diverse superfamily of enzymes that catalyze the introduction of double bonds into fatty acids. They are essential in a range of metabolic processes, such as the production of omega-3 fatty acids. However, our structure-function understanding of this superfamily is still developing and their range of activities and substrate specificities are broad, and often overlapping, which has made their systematic characterization challenging. A central issue with characterizing these proteins has been the lack of a structural model, which has been overcome with the recent publication of the crystal structures of two mammalian fatty acid desaturases. In this work, we have used sequence similarity networks to investigate the similarity among over 5000 related membrane fatty acid desaturase sequences, leading to a detailed classification of the superfamily, families and subfamilies with regard to their function and substrate head-group specificity. This work will facilitate rapid prediction of the function and specificity of new and existing sequences, as well as forming a basis for future efforts to manipulate the substrate specificity of these proteins for biotechnology applications.

Cloning and functional expression of a cDNA encoding stearoyl-ACP Δ9-desaturase from the endosperm of coconut (Cocos nucifera L.)

Coconut (Cocos nucifera L.) is an economically tropical fruit tree with special fatty acid compositions. The stearoyl-acyl carrier protein (ACP) desaturase (SAD) plays a key role in the properties of the majority of cellular glycerolipids. In this paper, a full-length cDNA of a stearoyl-acyl carrier protein desaturase, designated CocoFAD, was isolated from cDNA library prepared from the endosperm of coconut (C. nucifera L.). An 1176 bp cDNA from overlapped PCR products containing ORF encoding a 391-amino acid (aa) protein was obtained. The coded protein was virtually identical and shared the homology to other Δ9-desaturase plant sequences (greater than 80% as similarity to that of Elaeis guineensis Jacq). The real-time fluorescent quantitative PCR result indicated that the yield of CocoFAD was the highest in the endosperm of 8-month-old coconut and leaf, and the yield was reduced to 50% of the highest level in the endosperm of 15-month-old coconut. The coding region showed heterologous expression in strain INVSc1 of yeast (Saccharomyces cerevisiae). GC-MS analysis showed that the levels of palmitoleic acid (16:1) and oleic acid (18:1) were improved significantly; meanwhile stearic acid (18:0) was reduced. These results indicated that the plastidial Δ9 desaturase from the endosperm of coconut was involved in the biosynthesis of hexadecenoic acid and octadecenoic acid, which was similar with other plants. These results may be valuable for understanding the mechanism of fatty acid metabolism and the genetic improvement of CocoFAD gene in palm plants in the future.

Δ12-Fatty acid desaturase from Candida parapsilosis is a multifunctional desaturase producing a range of polyunsaturated and hydroxylated fatty acids

Numerous Δ12-, Δ15- and multifunctional membrane fatty acid desaturases (FADs) have been identified in fungi, revealing great variability in the enzymatic specificities of FADs involved in biosynthesis of polyunsaturated fatty acids (PUFAs). Here, we report gene isolation and characterization of novel Δ12/Δ15- and Δ15-FADs named CpFad2 and CpFad3, respectively, from the opportunistic pathogenic yeast Candida parapsilosis. Overexpression of CpFad3 in Saccharomyces cerevisiae strains supplemented with linoleic acid (Δ9,Δ12-18:2) and hexadecadienoic acid (Δ9,Δ12-16:2) leads to accumulation of Δ15-PUFAs, i.e., α-linolenic acid (Δ9,Δ12,Δ15-18:3) and hexadecatrienoic acid with an unusual terminal double bond (Δ9,Δ12,Δ15-16:3). CpFad2 produces a range of Δ12- and Δ15-PUFAs. The major products of CpFad2 are linoleic and hexadecadienoic acid (Δ9,Δ12-16:2), accompanied by α-linolenic acid and hexadecatrienoic acid (Δ9,Δ12,Δ15-16:3). Using GC/MS analysis of trimethylsilyl derivatives, we identified ricinoleic acid (12-hydroxy-9-octadecenoic acid) as an additional product of CpFad2. These results demonstrate that CpFAD2 is a multifunctional FAD and indicate that detailed analysis of fatty acid derivatives might uncover a range of enzymatic selectivities in other Δ12-FADs from budding yeasts (Ascomycota: Saccharomycotina).

A paradigm shift towards low-nitrifying production systems: the role of biological nitrification inhibition (BNI)

BACKGROUND

Agriculture is the single largest geo-engineering initiative that humans have initiated on planet Earth, largely through the introduction of unprecedented amounts of reactive nitrogen (N) into ecosystems. A major portion of this reactive N applied as fertilizer leaks into the environment in massive amounts, with cascading negative effects on ecosystem health and function. Natural ecosystems utilize many of the multiple pathways in the N cycle to regulate N flow. In contrast, the massive amounts of N currently applied to agricultural systems cycle primarily through the nitrification pathway, a single inefficient route that channels much of this reactive N into the environment. This is largely due to the rapid nitrifying soil environment of present-day agricultural systems.

SCOPE

In this Viewpoint paper, the importance of regulating nitrification as a strategy to minimize N leakage and to improve N-use efficiency (NUE) in agricultural systems is highlighted. The ability to suppress soil nitrification by the release of nitrification inhibitors from plant roots is termed 'biological nitrification inhibition' (BNI), an active plant-mediated natural function that can limit the amount of N cycling via the nitrification pathway. The development of a bioassay using luminescent Nitrosomonas to quantify nitrification inhibitory activity from roots has facilitated the characterization of BNI function. Release of BNIs from roots is a tightly regulated physiological process, with extensive genetic variability found in selected crops and pasture grasses. Here, the current status of understanding of the BNI function is reviewed using Brachiaria forage grasses, wheat and sorghum to illustrate how BNI function can be utilized for achieving low-nitrifying agricultural systems. A fundamental shift towards ammonium (NH4(+))-dominated agricultural systems could be achieved by using crops and pastures with high BNI capacities. When viewed from an agricultural and environmental perspective, the BNI function in plants could potentially have a large influence on biogeochemical cycling and closure of the N loop in crop-livestock systems.

Effects of jasmonates on sorgoleone accumulation and expression of genes for sorgoleone biosynthesis in sorghum roots

This study investigated the roles of jasmonates in the regulation of sorgoleone accumulation and the expression of genes involved in sorgoleone biosynthesis in sorghum roots. Both methyl jasmonate (MeJa) and jasmonic acid (JA) substantially promoted root hair formation, secondary root development, root weight, and sorgoleone accumulation in sorghum roots. Sorgoleone content varied widely depending on the concentration of JA or MeJa and the duration of their application. Root weight and sorgoleone accumulation were highest after the application of JA or MeJa at a concentration of 5.0 μM, and then declined with increasing concentrations of jasmonates. At 5.0 μM, JA and MeJa increased sorgoleone content by 4.1 and 3.4-fold, respectively. Transcript accumulation was apparent for all genes, particularly for the O-methyltransferase 3 gene, which increased in expression levels up to 8.1-fold after a 36-h exposure to MeJa and 3.5-fold after a 48-h exposure to JA. The results of this study pave the way for more effective biosynthesis of sorgoleone, an important and useful allelochemical obtained from a variety of plant species.

Sorghum allelopathy--from ecosystem to molecule

Sorghum allelopathy has been reported in a series of field experiments following sorghum establishment. In recent years, sorghum phytotoxicity and allelopathic interference also have been well-described in greenhouse and laboratory settings. Observations of allelopathy have occurred in diverse locations and with various sorghum plant parts. Phytotoxicity has been reported when sorghum was incorporated into the soil as a green manure, when residues remained on the soil surface in reduced tillage settings, or when sorghum was cultivated as a crop in managed fields. Allelochemicals present in sorghum tissues have varied with plant part, age, and cultivar evaluated. A diverse group of sorghum allelochemicals, including numerous phenolics, a cyanogenic glycoside (dhurrin), and a hydrophobic p-benzoquinone (sorgoleone) have been isolated and identified in recent years from sorghum shoots, roots, and root exudates, as our capacity to analyze and identify complex secondary products in trace quantities in the plant and in the soil rhizosphere has improved. These allelochemicals, particularly sorgoleone, have been widely investigated in terms of their mode(s) of action, specific activity and selectivity, release into the rhizosphere, and uptake and translocation into sensitive indicator species. Both genetics and environment have been shown to influence sorgoleone production and expression of genes involved in sorgoleone biosynthesis. In the soil rhizosphere, sorgoleone is released continuously by living root hairs where it accumulates in significant concentrations around its roots. Further experimentation designed to study the regulation of sorgoleone production by living sorghum root hairs may result in increased capacity to utilize sorghum cover crops more effectively for suppression of germinating weed seedlings, in a manner similar to that of soil-applied preemergent herbicides like trifluralin.

Sorgoleone: benzoquinona lipídica de sorgo com efeitos alelopáticos na agricultura como herbicida

A cultura do sorgo cresceu rapidamente nestes últimos anos, por ser uma planta com características xerófilas, apresentando um aumento de sua produção principalmente na região nordeste devido a sua capacidade de suportar ambientes de cultivo mais secos. As ervas daninhas são um grande problema para os cultivares, pois estas podem reduzir significativamente a produção de grãos, particularmente quando surgem nas fases iniciais das culturas. Visando a obtenção de culturas resistentes às ervas daninhas, estudos têm sido realizados demonstrando que algumas plantas possuem uma defesa natural que consiste na capacidade de um organismo produzir metabólitos que atuam inibindo ou o crescimento ou o desenvolvimento de outros organismos que estão próximos; a esta capacidade dá-se o nome de alelopatia. O sorgo é uma das plantas que possuem sua alelopatia comprovada, produzindo um complexo de substâncias lipídicas e proteínas denominados genericamente de sorgoleone, tendo como seu principal composto o 2-hidroxi-5-metoxi-3-[(Z,Z)-8',11',14'-pentadecatrieno]-p-benzoquinona, que é naturalmente liberado para o solo a partir dos tricomas das suas raízes e, no momento em que entram em contato com as ervas daninhas, inibem seu crescimento. Devido a tais características inerentes à cultura do sorgo, este trabalho tem como objetivo discorrer sobre os possíveis benefícios do uso desse cereal devido a sua comprovada alelopatia, bem como informar os conhecidos mecanismos de produção e atuação dos principais compostos constituintes do sorgoleone produzidos pelas suas raízes.

Effects of auxins on sorgoleone accumulation and genes for sorgoleone biosynthesis in sorghum roots

Sorgoleone is a major component of the hydrophobic root exudate of Sorghum bicolor and is of particular interest to plant chemical ecology as well as agriculture. Sorgoleone was evaluated in this study to observe the expression levels of genes involved in its biosynthesis in response to auxins. Sorgoleone content varied widely according to the duration of application and the concentrations of the auxins. When the application time was increased, the sorgoleone content increased accordingly for all concentrations of IBA (1, 3, and 5 mg/L) and at 1 mg/L for both IAA and NAA. In this study, five different sorgoleone biosynthetic genes were observed, namely DES2, DES3, ARS1, ARS2, and OMT3, which are upregulated in response to IAA, IBA, and NAA. Transcript accumulation was apparent for all genes, but particularly for DES2, which increased up to 475-fold and 180-fold following 72 h exposure to NAA and IBA, respectively, compared to no treatment.

Caenorhabditis elegans Delta12-desaturase FAT-2 is a bifunctional desaturase able to desaturate a diverse range of fatty acid substrates at the Delta12 and Delta15 positions

Caenorhabditis elegans FAT-2 has been characterized as fatty acid Δ12-desaturase able to desaturate C16 and C18 fatty acids. However, in this report we show that when expressed in yeast cells this enzyme can also catalyze Δ15 desaturation. This results in the production of both linoleic acid (ω6 C18:2Δ9,12) and linolenic acid (ω3 C18:3Δ9,12,15) from oleic acid (C18:1Δ9) substrate, and hexadecadienoic acid (ω4 C16:2Δ9,12) and hexadecatrienoic acid (ω1 C16:3Δ9,12,15) from palmitoleic acid (C16:1Δ9) substrate. In addition, this enzyme can also produce C14:2Δ9,12, C15:2Δ9,12, C17:2Δ9,12, and C18:4Δ6,9,12,15 when C14:1Δ9, C15:1Δ9, C17:1Δ9, and C18:3Δ6,9,12 substrates are available in yeast cells. Mass spectrometry analysis of 2,4-dimethyloxazoline modification of fatty acid methyl esters confirms the positions of all newly formed double bonds. These results indicate that when expressed in yeast the C. elegans Δ12-desaturase CeFAT-2 shows a characteristic of a bifunctional Δ12/Δ15-desaturase and has a great deal of elasticity with respect to fatty acid chain length in being able to accept fatty acids ranging from C14 to C18. Interestingly, despite possessing a bifunctional Δ12/Δ15 desaturation activity, phylogenetic analysis suggests that C. elegans Δ12-desaturase CeFAT-2 might have arisen independently from other reported dual Δ12/Δ15-desaturases from fungi and protozoa.

Sorgoleone

Sorgoleone, a major component of the hydrophobic root exudate of sorghum [Sorghum bicolor (L.) Moench], is one of the most studied allelochemicals. The exudate also contains an equivalent amount of a lipid resorcinol analog as well as a number of minor sorgoleone congeners. Synthesis of sorgoleone is constitutive and compartmentalized within root hairs, which can accumulate up to 20 microg of exudate/mg root dry weight. The biosynthesis pathway involves unique fatty acid desaturases which produce an atypical 16:3 fatty acyl-CoA starter unit for an alkylresorcinol synthase that catalyzes the formation of a pentadecatrienylresorcinol intermediate. This intermediate is then methylated by SAM-dependent O-methyltransferases and dihydroxylated by cytochrome P450 monooxygenases. An EST data set derived from a S. bicolor root hair-specific cDNA library contained all the candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Sorgoleone interferes with several molecular target sites, including inhibition of photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants. Due to its hydrophobic nature, sorgoleone is strongly sorbed in soil which increases its persistence, but experiments show that it is mineralized by microorganisms over time.

Alkylresorcinol synthases expressed in Sorghum bicolor root hairs play an essential role in the biosynthesis of the allelopathic benzoquinone sorgoleone

Sorghum bicolor is considered to be an allelopathic crop species, producing phytotoxins such as the lipid benzoquinone sorgoleone, which likely accounts for many of the allelopathic properties of Sorghum spp. Current evidence suggests that sorgoleone biosynthesis occurs exclusively in root hair cells and involves the production of an alkylresorcinolic intermediate (5-[(Z,Z)-8',11',14'-pentadecatrienyl]resorcinol) derived from an unusual 16:3Delta(9,12,15) fatty acyl-CoA starter unit. This led to the suggestion of the involvement of one or more alkylresorcinol synthases (ARSs), type III polyketide synthases (PKSs) that produce 5-alkylresorcinols using medium to long-chain fatty acyl-CoA starter units via iterative condensations with malonyl-CoA. In an effort to characterize the enzymes responsible for the biosynthesis of the pentadecyl resorcinol intermediate, a previously described expressed sequence tag database prepared from isolated S. bicolor (genotype BTx623) root hairs was first mined for all PKS-like sequences. Quantitative real-time RT-PCR analyses revealed that three of these sequences were preferentially expressed in root hairs, two of which (designated ARS1 and ARS2) were found to encode ARS enzymes capable of accepting a variety of fatty acyl-CoA starter units in recombinant enzyme studies. Furthermore, RNA interference experiments directed against ARS1 and ARS2 resulted in the generation of multiple independent transformant events exhibiting dramatically reduced sorgoleone levels. Thus, both ARS1 and ARS2 are likely to participate in the biosynthesis of sorgoleone in planta. The sequences of ARS1 and ARS2 were also used to identify several rice (Oryza sativa) genes encoding ARSs, which are likely involved in the production of defense-related alkylresorcinols.

Dynamic root exudation of sorgoleone and its in planta mechanism of action

The oily droplets exuded from the root hairs of sorghum are composed of a 1:1 ratio of sorgoleone and its lipid resorcinol analogue. The production of these droplets appears to be suppressed when c. 20 microg of exudate mg(-1) root dry weight accumulates at the tip of the root hairs. However, more exudate is produced following gentle washing of the roots with water, suggesting that the biosynthesis of lipid benzoquinones and resorcinols is a dynamic process. Sorgoleone interferes with several molecular target sites, including photosynthetic electron transport, in in vitro assays. However, the in planta mechanism of action of sorgoleone remains controversial because it is not clear whether this lipid benzoquinone exuding from the roots of sorghum is taken up by roots of the receiving plants and translocated to their foliage where it must enter the chloroplast and inhibit PSII in the thylakoid membrane. Experiments designed to test the in planta mode of action of sorgoleone demonstrated that it has no effect on the photosynthesis of older plants, but inhibits photosynthesis in germinating seedlings. Sorgoleone is not translocated acropetally in older plants, but can be absorbed through the hypocotyl and cotyledonary tissues. Therefore, the mode of action of sorgoleone may be the result of inhibition of photosynthesis in young seedlings in concert with inhibition of its other molecular target sites in older plants.

Probing allelochemical biosynthesis in sorghum root hairs

Allelopathic interaction between plants is thought to involve the release of phytotoxic allelochemicals by one species, thus inhibiting the growth of neighboring species in competition for limited resources. Sorgoleone represents one of the more potent allelochemicals characterized to date, and its prolific production in root hair cells of Sorghum spp. has made the investigation of its biosynthetic pathway ideally-suited for functional genomics investigations. Through the use of a recently-released EST data set generated from isolated Sorghum bicolor root hair cells, significant inroads have been made toward the identification of genes and the corresponding enzymes involved in the biosynthesis of this compound in root hairs. Here we provide additional information concerning our recent report on the identification of a 5-n-alk(en) ylresorcinol utilizing O-methyltransferase, as well as other key enzymes likely to participate in the biosynthesis of this important allelochemical.

A functional genomics investigation of allelochemical biosynthesis in Sorghum bicolor root hairs

Sorghum is considered to be one of the more allelopathic crop species, producing phytotoxins such as the potent benzoquinone sorgoleone (2-hydroxy-5-methoxy-3-[(Z,Z)-8',11',14'-pentadecatriene]-p-benzoquinone) and its analogs. Sorgoleone likely accounts for much of the allelopathy of Sorghum spp., typically representing the predominant constituent of Sorghum bicolor root exudates. Previous and ongoing studies suggest that the biosynthetic pathway for this plant growth inhibitor occurs in root hair cells, involving a polyketide synthase activity that utilizes an atypical 16:3 fatty acyl-CoA starter unit, resulting in the formation of a pentadecatrienyl resorcinol intermediate. Subsequent modifications of this resorcinolic intermediate are likely to be mediated by S-adenosylmethionine-dependent O-methyltransferases and dihydroxylation by cytochrome P450 monooxygenases, although the precise sequence of reactions has not been determined previously. Analyses performed by gas chromatography-mass spectrometry with sorghum root extracts identified a 3-methyl ether derivative of the likely pentadecatrienyl resorcinol intermediate, indicating that dihydroxylation of the resorcinol ring is preceded by O-methylation at the 3'-position by a novel 5-n-alk(en)ylresorcinol-utilizing O-methyltransferase activity. An expressed sequence tag data set consisting of 5,468 sequences selected at random from an S. bicolor root hair-specific cDNA library was generated to identify candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Quantitative real time reverse transcription-PCR and recombinant enzyme studies with putative O-methyltransferase sequences obtained from the expressed sequence tag data set have led to the identification of a novel O-methyltransferase highly and predominantly expressed in root hairs (designated SbOMT3), which preferentially utilizes alk(en)ylresorcinols among a panel of benzene-derivative substrates tested. SbOMT3 is therefore proposed to be involved in the biosynthesis of the allelochemical sorgoleone.