Comparative transcriptome analysis reveals the differences in wound-induced agarwood formation between Chi-Nan and ordinary germplasm of Aquilaria sinensis

Agarwood is a rare and valuable heartwood derived from Aquilaria sinensis in China. Compared with ordinary germplasm, Chi-Nan, a special germplasm of A. sinensis, has a better agarwood-producing capacity. However, the mechanisms underlying their different qualities remain poorly characterized. Here, a comparative transcriptome analysis of Chi-Nan and ordinary A. sinensis was carried out to investigate the wound responses of both germplasms. A total of 198.19 Gb of clean data were obtained with an average of 6.61 Gb of clean reads for each sample. By comparing with their control groups, more differentially expressed genes (DEGs) were observed in Chi-Nan germplasm. Kyoto Encyclopedia of Genes and Genomes (KEGG) and expression profile analysis suggested that Chi-Nan possesses a stronger ability to respond to wounding. Furthermore, the enrichment of biosynthetic pathways related to sesquiterpenes and 2-(2-phenylethyl) chromones (PECs) were more significant in Chi-Nan than in ordinary germplasm, and related genes showed significantly higher up-regulation in Chi-Nan after wounding. Sixteen candidate genes presumably involved in biosynthesis of agarwood components were identified and found to exhibit higher up-regulation in Chi-Nan than in ordinary germplasm in response to wounding. Overall, these results are helpful in explaining reasons for the higher agarwood-producing properties of Chi-Nan, and contribute to a further understanding of the mechanism of agarwood formation in A. sinensis.

Genome-Wide Mining of the Tandem Duplicated Type III Polyketide Synthases and Their Expression, Structure Analysis of Senna tora

Senna tora is one of the homologous crops used as a medicinal food containing an abundance of anthraquinones. Type III polyketide synthases (PKSs) are key enzymes that catalyze polyketide formation; in particular, the chalcone synthase-like (CHS-L) genes are involved in anthraquinone production. Tandem duplication is a fundamental mechanism for gene family expansion. However, the analysis of the tandem duplicated genes (TDGs) and the identification and characterization of PKSs have not been reported for S. tora. Herein, we identified 3087 TDGs in the S. tora genome; the synonymous substitution rates (Ks) analysis indicated that the TDGs had recently undergone duplication. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that the type III PKSs were the most enriched TDGs involved in the biosynthesis of the secondary metabolite pathways, as evidenced by 14 tandem duplicated CHS-L genes. Subsequently, we identified 30 type III PKSs with complete sequences in the S. tora genome. Based on the phylogenetic analysis, the type III PKSs were classified into three groups. The protein conserved motifs and key active residues showed similar patterns in the same group. The transcriptome analysis showed that the chalcone synthase (CHS) genes were more highly expressed in the leaves than in the seeds in S. tora. The transcriptome and qRT-PCR analysis showed that the CHS-L genes had a higher expression in the seeds than in other tissues, particularly seven tandem duplicated CHS-L2/3/5/6/9/10/13 genes. The key active-site residues and three-dimensional models of the CHS-L2/3/5/6/9/10/13 proteins showed slight variation. These results indicated that the rich anthraquinones in S. tora seeds might be ascribed to the PKSs' expansion from tandem duplication, and the seven key CHS-L2/3/5/6/9/10/13 genes provide candidate genes for further research. Our study provides an important basis for further research on the regulation of anthraquinones' biosynthesis in S. tora.

The ZmMYB84-ZmPKSB regulatory module controls male fertility through modulating anther cuticle-pollen exine trade-off in maize anthers

Anther cuticle and pollen exine are two crucial lipid layers that ensure normal pollen development and pollen-stigma interaction for successful fertilization and seed production in plants. Their formation processes share certain common pathways of lipid biosynthesis and transport across four anther wall layers. However, molecular mechanism underlying a trade-off of lipid-metabolic products to promote the proper formation of the two lipid layers remains elusive. Here, we identified and characterized a maize male-sterility mutant pksb, which displayed denser anther cuticle but thinner pollen exine as well as delayed tapetal degeneration compared with its wild type. Based on map-based cloning and CRISPR/Cas9 mutagenesis, we found that the causal gene (ZmPKSB) of pksb mutant encoded an endoplasmic reticulum (ER)-localized polyketide synthase (PKS) with catalytic activities to malonyl-CoA and midchain-fatty acyl-CoA to generate triketide and tetraketide α-pyrone. A conserved catalytic triad (C171, H320 and N353) was essential for its enzymatic activity. ZmPKSB was specifically expressed in maize anthers from stages S8b to S9-10 with its peak at S9 and was directly activated by a transcription factor ZmMYB84. Moreover, loss function of ZmMYB84 resulted in denser anther cuticle but thinner pollen exine similar to the pksb mutant. The ZmMYB84-ZmPKSB regulatory module controlled a trade-off between anther cuticle and pollen exine formation by altering expression of a series of genes related to biosynthesis and transport of sporopollenin, cutin and wax. These findings provide new insights into the fine-tuning regulation of lipid-metabolic balance to precisely promote anther cuticle and pollen exine formation in plants.

Low temperature modifies seedling leaf anatomy and gene expression in Hypericum perforatum

Hypericum perforatum, commonly known as St John's wort, is a perennial herb that produces the anti-depression compounds hypericin (Hyp) and hyperforin. While cool temperatures increase plant growth, Hyp accumulation as well as changes transcript profiles, alterations in leaf structure and genes expression specifically related to Hyp biosynthesis are still unresolved. Here, leaf micro- and ultra-structure is examined, and candidate genes encoding for photosynthesis, energy metabolism and Hyp biosynthesis are reported based on transcriptomic data collected from H. perforatum seedlings grown at 15 and 22°C. Plants grown at a cooler temperature exhibited changes in macro- and micro-leaf anatomy including thicker leaves, an increased number of secretory cell, chloroplasts, mitochondria, starch grains, thylakoid grana, osmiophilic granules and hemispherical droplets. Moreover, genes encoding for photosynthesis (64-genes) and energy (35-genes) as well as Hyp biosynthesis (29-genes) were differentially regulated with an altered growing temperature. The anatomical changes and genes expression are consistent with the plant's ability to accumulate enhanced Hyp levels at low temperatures.

In silico identification of Capsicum type III polyketide synthase genes and expression patterns in Capsicum annuum

Studies have shown that abundant and various flavonoids accumulate in chili pepper (Capsicum), but there are few reports on the genes that govern chili pepper flavonoid biosynthesis. Here, we report the comprehensive identification of genes encoding type III polyketide synthase (PKS), an important enzyme catalyzing the generation of flavonoid backbones. In total, 13, 14 and 13 type III PKS genes were identified in each genome of C. annuum, C. chinense and C. baccatum, respectively. The phylogeny topology of Capsicum PKSs is similar to those in other plants, as it showed two classes of genes. Within each class, clades can be further identified. Class II genes likely encode chalcone synthase (CHS) as they are placed together with the Arabidopsis CHS gene, which experienced extensive expansions in the genomes of Capsicum. Interestingly, 8 of the 11 Class II genes form three clusters in the genome of C. annuum, which is likely the result of tandem duplication events. Four genes are not expressed in the tissues of C. annuum, three of which are located in the clusters, indicating that a portion of genes was pseudogenized after tandem duplications. Expression of two Class I genes was complementary to each other, and all the genes in Class II were not expressed in roots of C. annuum. Two Class II genes (CA00g90790 and CA05g17060) showed upregulated expression as the chili pepper leaves matured, and two Class II genes (CA05g17060 and CA12g20070) showed downregulated expression with the maturation of fruits, consistent with flavonoid accumulation trends in chili pepper as reported previously. The identified genes, sequences, phylogeny and expression information collected in this article lay the groundwork for future studies on the molecular mechanisms of chili pepper flavonoid metabolism.

Detailed characterization of Pinus ponderosa sporopollenin by infrared spectroscopy

In plant spores and pollen, sporopollenin occurs as a structural polymer with remarkable resistance to chemical degradation. This recalcitrant polymer is well-suited to analysis by non-destructive infrared spectroscopy. However, existing infrared characterization of sporopollenin has been limited in scope and occasionally contradictory. This study provides a comprehensive structural analysis of sporopollenin in the Pinus ponderosa pollen exine using infrared spectroscopy, including detailed band assignments, descriptions of chemical reactivity, and comparison to multiple reference substances. We observe that the infrared spectral characteristics of sporopollenin prepared by enzymatic digestion of the polysaccharide-based intine are largely consistent with a copolymer of aliphatic lipids and trans-4-hydroxycinnamic acid, without distinct contributions from α-pyrone or carotenoid substructures.

Expression profiles of a cytoplasmic male sterile line of Gossypium harknessii and its fertility restorer and maintainer lines revealed by RNA-Seq

The Gossypium harknessii background cytoplasmic male sterility (CMS) system has been used in cotton hybrid breeding in China. However, the mechanism underlying pollen abortion and fertility restoration in CMS remains to be determined. In this study, we used RNA-seq to identify critical genes and pathways associated with CMS in G. harknessii based CMS lines (588A), the near isogenic restorer lines (588R), and maintainer lines (588B). We performed an assembly of 80,811,676 raw reads into 89,939 high-quality unigenes with an average length of 698 bp. Among these, 72.62% unigenes were annotated in public protein databases and were classified into functional clusters. In addition, we investigated the changes in expression of genes between 588A and 588B (588R); the RNA-seq data showed 742 differentially expressed genes (DEGs) between 588A and 588B and 748 DEGs between 588A and 588R. They were mainly down-regulated in 588A and most of them distributed in metabolic and biosynthesis of secondary metabolites pathways. Further analysis revealed 23 pollen development related genes were differentially expressed between 588A and 588B. Numerous genes associated with tapetum development were down-regulated in 588A, implicating tapetum dysplasia may be a key reason for pollen abortion in CMS lines. Also, among DEGs between 588A and 588R, we identified two PPR genes which were highly up-regulated in restorer line. This study may provide assistance for detailed molecular analysis and a better understanding of harknessii based CMS in cotton.

Functional characterization and expression of GASCL1 and GASCL2, two anther-specific chalcone synthase like enzymes from Gerbera hybrida

The chalcone synthase superfamily consists of type III polyketidesynthases (PKSs), enzymes responsible for producing plant secondary metabolites with various biological and pharmacological activities. Anther-specific chalcone synthase-like enzymes (ASCLs) represent an ancient group of type III PKSs involved in the biosynthesis of sporopollenin, the main component of the exine layer of moss spores and mature pollen grains of seed plants. In the latter, ASCL proteins are localized in the tapetal cells of the anther where they participate in sporopollenin biosynthesis and exine formation within the locule. It is thought that the enzymes responsible for sporopollenin biosynthesis are highly conserved, and thus far, each angiosperm species with a genome sequenced has possessed two ASCL genes, which in Arabidopsis thaliana are PKSA and PKSB. The Gerbera hybrida (gerbera) PKS protein family consists of three chalcone synthases (GCHS1, GCHS3 and GCHS4) and three 2-pyrone synthases (G2PS1, G2PS2 and G2PS3). In previous studies we have demonstrated the functions of chalcone synthases in flavonoid biosynthesis, and the involvement of 2-pyrone synthases in the biosynthesis of antimicrobial compounds found in gerbera. In this study we expanded the gerbera PKS-family by functionally characterizing two gerbera ASCL proteins. In vitro enzymatic studies using purified recombinant proteins showed that both GASCL1 and GASCL2 were able to use medium and long-chain acyl-CoA starters and perform two to three condensation reactions of malonyl-CoA to produce tri- and tetraketide 2-pyrones, usually referred to as alpha-pyrones in sporopollenin literature. Both GASCL1 and GASCL2 genes were expressed only in floral organs, with most expression observed in anthers. In the anthers, transcripts of both genes showed strict tapetum-specific localization.

Evolutionary and functional analysis of mulberry type III polyketide synthases

Background

Type III polyketide synthases are important for the biosynthesis of flavonoids and various plant polyphenols. Mulberry plants have abundant polyphenols, but very little is known about the mulberry type III polyketide synthase genes. An analysis of these genes may provide new targets for genetic improvement to increase relevant secondary metabolites and enhance the plant tolerance to biotic and abiotic stresses.

Results

Eighteen genes encoding type III polyketide synthases were identified, including six chalcone synthases (CHS), ten stilbene synthases (STS), and two polyketide synthases (PKS). Functional characterization of four genes representing most of the MnCHS and MnSTS genes by coexpression with 4-Coumaroyl-CoA ligase in Escherichia coli indicated that their products were able to catalyze p-coumaroyl-CoA and malonyl-CoA to generate naringenin and resveratrol, respectively. Microsynteny analysis within mulberry indicated that segmental and tandem duplication events contributed to the expansion of the MnCHS family, while tandem duplications were mainly responsible for the generation of the MnSTS genes. Combining the evolution and expression analysis results of the mulberry type III PKS genes indicated that MnCHS and MnSTS genes evolved mainly under purifying selection to maintain their original functions, but transcriptional subfunctionalization occurred during long-term species evolution. Moreover, mulberry leaves can rapidly accumulated oxyresveratrol after UV-C irradiation, suggesting that resveratrol was converted to oxyresveratrol.

Conclusions

Characterizing the functions and evolution of mulberry type III PKS genes is crucial for advancing our understanding of these genes and providing the basis for further studies on the biosynthesis of relevant secondary metabolites in mulberry plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2843-7) contains supplementary material, which is available to authorized users.

The biosynthesis, composition and assembly of the outer pollen wall: A tough case to crack

The formation of the durable outer pollen wall, largely composed of sporopollenin, is essential for the protection of the male gametophyte and plant reproduction. Despite its apparent strict conservation amongst land plants, the composition of sporopollenin and the biosynthetic pathway(s) yielding this recalcitrant biopolymer remain elusive. Recent molecular genetic studies in Arabidopsis thaliana (Arabidopsis) and rice have, however, identified key genes involved in sporopollenin formation, allowing a better understanding of the biochemistry and cell biology underlying sporopollenin biosynthesis and pollen wall development. Herein, current knowledge of the biochemical composition of the outer pollen wall is reviewed, with an emphasis on enzymes with characterized biochemical activities in sporopollenin and pollen coat biosynthesis. The tapetum, which forms the innermost sporophytic cell layer of the anther and envelops developing pollen, plays an essential role in sporopollenin and pollen coat formation. Recent studies show that several tapetum-expressed genes encode enzymes that metabolize fatty acid derived compounds to form putative sporopollenin precursors, including tetraketides derived from fatty acyl-CoA starter molecules, but analysis of mutants defective in pollen wall development indicate that other components are also incorporated into sporopollenin. Also highlighted are the many uncertainties remaining in the development of a sporopollenin-fortified pollen wall, particularly in relation to the mechanisms of sporopollenin precursor transport and assembly into the patterned form of the pollen wall. A working model for sporopollenin biosynthesis is proposed based on the data obtained largely from studies of Arabidopsis, and future challenges to complete our understanding of pollen wall biology are outlined.

Hypericum perforatum hydroxyalkylpyrone synthase involved in sporopollenin biosynthesis--phylogeny, site-directed mutagenesis, and expression in nonanther tissues

Anther-specific chalcone synthase-like enzyme (ASCL), an ancient plant type III polyketide synthase, is involved in the biosynthesis of sporopollenin, the stable biopolymer found in the exine layer of the wall of a spore or pollen grain. The gene encoding polyketide synthase 1 from Hypericum perforatum (HpPKS1) was previously shown to be expressed mainly in young flower buds, but also in leaves and other tissues at lower levels. Angiosperm ASCLs, identified by sequence and phylogenetic analyses, are divided into two sister clades, the Ala-clade and the Val-clade, and HpPKS1 belongs to the Ala-clade. Recombinant HpPKS1 produced triketide and, to a lesser extent, tetraketide alkylpyrones from medium-chain (C6) to very long-chain (C24) fatty acyl-CoA substrates. Like other ASCLs, HpPKS1 also preferred hydroxyl fatty acyl-CoA esters over the analogous unsubstituted fatty acyl-CoA esters. To study the structural basis of the substrate preference, mutants of Ala200 and Ala215 at the putative active site and Arg202 and Asp211 at the modeled acyl-binding tunnel were constructed. The A200T/A215Q mutant accepted decanoyl-CoA, a poor substrate for the wild-type enzyme, possibly because of active site constriction by bulkier substitutions. The substrate preference of the A215V and A200T/A215Q mutants shifted toward nonhydroxylated, medium-chain to long-chain fatty acyl-CoA substrates. The R202L/D211V double mutant was selective for acyl-CoA with chain lengths of C16-C18, and showed a diminished preference for the hydroxylated acyl-CoA substrates. Transient upregulation by abscisic acid and downregulation by jasmonic acid and wounding suggested that HpPKS1, and possibly other Ala-clade ASCLs, may be involved in the biosynthesis of minor cell wall components in nonanther tissues.

Tapetum: regulation and role in sporopollenin biosynthesis in Arabidopsis

Pollen acts as a biological protector for protecting male sperm from various harsh conditions and is covered by an outer cell wall polymer called the exine, a major constituent of which is sporopollenin. The tapetum is in direct contact with the developing gametophytes and plays an essential role in pollen wall and pollen coat formation. The precise molecular mechanisms underlying tapetal development remain highly elusive, but molecular genetic studies have identified a number of genes that control the formation, differentiation, and programmed cell death of tapetum and interactions of genes in tapetal development. Herein, several lines of evidence suggest that sporopollenin is built up via catalytic enzyme reactions in the tapetum. Furthermore, as based on genetic evidence, we review the currently accepted understanding of the molecular regulation of sporopollenin biosynthesis and examine unanswered questions regarding the requirements underpinning proper exine pattern formation.

Sporopollenin biosynthetic enzymes interact and constitute a metabolon localized to the endoplasmic reticulum of tapetum cells

The sporopollenin polymer is the major constituent of exine, the outer pollen wall. Recently fatty acid derivatives have been shown to be the precursors of sporopollenin building units. ACYL-COA SYNTHETASE, POLYKETIDE SYNTHASE A (PKSA) and PKSB, TETRAKETIDE α-PYRONE REDUCTASE1 (TKPR1) and TKPR2 have been demonstrated to be involved in sporopollenin biosynthesis in Arabidopsis (Arabidopsis thaliana). Here all these sporopollenin biosynthetic enzymes but TKPR2 have been immunolocalized to endoplasmic reticulum of anther tapetal cells. Pull-down experiments demonstrated that tagged recombinant proteins interacted to form complexes whose constituents were characterized by immunoblotting. In vivo protein interactions were evidenced by yeast (Saccharomyces cerevisiae) two-hybrid analysis and by fluorescence lifetime imaging microscopy/Förster resonance energy transfer studies in transgenic Nicotiana benthamiana, which were used to test the possibility that the enzymes interact to form a biosynthetic metabolon. Various pairs of proteins fused to two distinct fluorochromes were coexpressed in N. benthamiana leaf tissues and fluorescence lifetime imaging microscopy/Förster resonance energy transfer measurements demonstrated that proteins interacted pairwise in planta. Taken together, these results suggest the existence of a sporopollenin metabolon.

The evolution of phenylpropanoid metabolism in the green lineage

Phenolic secondary metabolites are only produced by plants wherein they play important roles in both biotic and abiotic defense in seed plants as well as being potentially important bioactive compounds with both nutritional and medicinal benefits reported for animals and humans as a consequence of their potent antioxidant activity. During the long evolutionary period in which plants have adapted to the environmental niches in which they exist (and especially during the evolution of land plants from their aquatic algal ancestors), several strategies such as gene duplication and convergent evolution have contributed to the evolution of this pathway. In this respect, diversity and redundancy of several key genes of phenolic secondary metabolism such as polyketide synthases, cytochrome P450s, Fe(2+)/2-oxoglutarate-dependent dioxygenases and UDP-glycosyltransferases have played an essential role. Recent technical developments allowing affordable whole genome sequencing as well as a better inventory of species-by-species chemical diversity have resulted in a dramatic increase in the number of tools we have to assess how these pathways evolved. In parallel, reverse genetics combined with detailed molecular phenotyping is allowing us to elucidate the functional importance of individual genes and metabolites and by this means to provide further mechanistic insight into their biological roles. In this review, phenolic metabolite-related gene sequences (for a total of 65 gene families including shikimate biosynthetic genes) are compared across 23 independent species, and the phenolic metabolic complement of various plant species are compared with one another, in attempt to better understand the evolution of diversity in this crucial pathway.

The Botrytis cinerea type III polyketide synthase shows unprecedented high catalytic efficiency toward long chain acyl-CoAs

BPKS from Botrytis cinerea is a novel type III polyketide synthase that accepts C(4)-C(18) aliphatic acyl-CoAs and benzoyl-CoA as the starters to form pyrones, resorcylic acids and resorcinols through sequential condensation with malonyl-CoA. The catalytic efficiency (k(cat)/K(m)) of BPKS was 2.8 × 10(5) s(-1) M(-1) for palmitoyl-CoA, the highest ever reported. Substrate docking analyses addressed the unique features of BPKS such as its high activity and high specificity toward long chain acyl-CoAs.

PpASCL, a moss ortholog of anther-specific chalcone synthase-like enzymes, is a hydroxyalkylpyrone synthase involved in an evolutionarily conserved sporopollenin biosynthesis pathway

Sporopollenin is the main constituent of the exine layer of spore and pollen walls. Recently, several Arabidopsis genes, including polyketide synthase A (PKSA), which encodes an anther-specific chalcone synthase-like enzyme (ASCL), have been shown to be involved in sporopollenin biosynthesis. The genome of the moss Physcomitrella patens contains putative orthologs of the Arabidopsis sporopollenin biosynthesis genes. We analyzed available P.patens expressed sequence tag (EST) data for putative moss orthologs of the Arabidopsis genes of sporopollenin biosynthesis and studied the enzymatic properties and reaction mechanism of recombinant PpASCL, the P.patens ortholog of Arabidopsis PKSA. We also generated structure models of PpASCL and Arabidopsis PKSA to study their substrate specificity. Physcomitrella patens orthologs of Arabidopsis genes for sporopollenin biosynthesis were found to be expressed in the sporophyte generation. Similarly to Arabidopsis PKSA, PpASCL condenses hydroxy fatty acyl-CoA esters with malonyl-CoA and produces hydroxyalkyl α-pyrones that probably serve as building blocks of sporopollenin. The ASCL-specific set of Gly-Gly-Ala residues predicted by the models to be located at the floor of the putative active site is proposed to serve as the opening of an acyl-binding tunnel in ASCL. These results suggest that ASCL functions together with other sporophyte-specific enzymes to provide polyhydroxylated precursors of sporopollenin in a pathway common to land plants.

Molecular cloning, modeling, and site-directed mutagenesis of type III polyketide synthase from Sargassum binderi (Phaeophyta)

Type III polyketide synthases (PKSs) produce an array of metabolites with diverse functions. In this study, we have cloned the complete reading frame encoding type III PKS (SbPKS) from a brown seaweed, Sargassum binderi, and characterized the activity of its recombinant protein biochemically. The deduced amino acid sequence of SbPKS is 414 residues in length, sharing a higher sequence similarity with bacterial PKSs (38% identity) than with plant PKSs. The Cys-His-Asn catalytic triad of PKS is conserved in SbPKS with differences in some of the residues lining the active and CoA binding sites. The wild-type SbPKS displayed broad starter substrate specificity to aliphatic long-chain acyl-CoAs (C(6)-C(14)) to produce tri- and tetraketide pyrones. Mutations at H(331) and N(364) caused complete loss of its activity, thus suggesting that these two residues are the catalytic residues for SbPKS as in other type III PKSs. Furthermore, H227G, H227G/L366V substitutions resulted in increased tetraketide-forming activity, while wild-type SbPKS produces triketide α-pyrone as a major product. On the other hand, mutant H227G/L366V/F93A/V95A demonstrated a dramatic decrease of tetraketide pyrone formation. These observations suggest that His(227) and Leu(366) play an important role for the polyketide elongation reaction in SbPKS. The conformational changes in protein structure especially the cavity of the active site may have more significant effect to the activity of SbPKS compared with changes in individual residues.

PlaNet: combined sequence and expression comparisons across plant networks derived from seven species

The model organism Arabidopsis thaliana is readily used in basic research due to resource availability and relative speed of data acquisition. A major goal is to transfer acquired knowledge from Arabidopsis to crop species. However, the identification of functional equivalents of well-characterized Arabidopsis genes in other plants is a nontrivial task. It is well documented that transcriptionally coordinated genes tend to be functionally related and that such relationships may be conserved across different species and even kingdoms. To exploit such relationships, we constructed whole-genome coexpression networks for Arabidopsis and six important plant crop species. The interactive networks, clustered using the HCCA algorithm, are provided under the banner PlaNet (http://aranet.mpimp-golm.mpg.de). We implemented a comparative network algorithm that estimates similarities between network structures. Thus, the platform can be used to swiftly infer similar coexpressed network vicinities within and across species and can predict the identity of functional homologs. We exemplify this using the PSA-D and chalcone synthase-related gene networks. Finally, we assessed how ontology terms are transcriptionally connected in the seven species and provide the corresponding MapMan term coexpression networks. The data support the contention that this platform will considerably improve transfer of knowledge generated in Arabidopsis to valuable crop species.

LAP6/POLYKETIDE SYNTHASE A and LAP5/POLYKETIDE SYNTHASE B encode hydroxyalkyl α-pyrone synthases required for pollen development and sporopollenin biosynthesis in Arabidopsis thaliana

Plant type III polyketide synthases (PKSs) catalyze the condensation of malonyl-CoA units with various CoA ester starter molecules to generate a diverse array of natural products. The fatty acyl-CoA esters synthesized by Arabidopsis thaliana ACYL-COA SYNTHETASE5 (ACOS5) are key intermediates in the biosynthesis of sporopollenin, the major constituent of exine in the outer pollen wall. By coexpression analysis, we identified two Arabidopsis PKS genes, POLYKETIDE SYNTHASE A (PKSA) and PKSB (also known as LAP6 and LAP5, respectively) that are tightly coexpressed with ACOS5. Recombinant PKSA and PKSB proteins generated tri-and tetraketide α-pyrone compounds in vitro from a broad range of potential ACOS5-generated fatty acyl-CoA starter substrates by condensation with malonyl-CoA. Furthermore, substrate preference profile and kinetic analyses strongly suggested that in planta substrates for both enzymes are midchain- and ω-hydroxylated fatty acyl-CoAs (e.g., 12-hydroxyoctadecanoyl-CoA and 16-hydroxyhexadecanoyl-CoA), which are the products of sequential actions of anther-specific fatty acid hydroxylases and acyl-CoA synthetase. PKSA and PKSB are specifically and transiently expressed in tapetal cells during microspore development in Arabidopsis anthers. Mutants compromised in expression of the PKS genes displayed pollen exine layer defects, and a double pksa pksb mutant was completely male sterile, with no apparent exine. These results show that hydroxylated α-pyrone polyketide compounds generated by the sequential action of ACOS5 and PKSA/B are potential and previously unknown sporopollenin precursors.

Genome-wide analysis of the chalcone synthase superfamily genes of Physcomitrella patens

Enzymes of the chalcone synthase (CHS) superfamily catalyze the production of a variety of secondary metabolites in bacteria, fungi and plants. Some of these metabolites have played important roles during the early evolution of land plants by providing protection from various environmental assaults including UV irradiation. The genome of the moss, Physcomitrella patens, contains at least 17 putative CHS superfamily genes. Three of these genes (PpCHS2b, PpCHS3 and PpCHS5) exist in multiple copies and all have corresponding ESTs. PpCHS11 and probably also PpCHS9 encode non-CHS enzymes, while PpCHS10 appears to be an ortholog of plant genes encoding anther-specific CHS-like enzymes. It was inferred from the genomic locations of genes comprising it that the moss CHS superfamily expanded through tandem and segmental duplication events. Inferred exon-intron architectures and results from phylogenetic analysis of representative CHS superfamily genes of P. patens and other plants showed that intron gain and loss occurred several times during evolution of this gene superfamily. A high proportion of P. patens CHS genes (7 of 14 genes for which the full sequence is known and probably 3 additional genes) are intronless, prompting speculation that CHS gene duplication via retrotransposition has occurred at least twice in the moss lineage. Analyses of sequence similarities, catalytic motifs and EST data indicated that a surprisingly large number (as many as 13) of the moss CHS superfamily genes probably encode active CHS. EST distribution data and different light responsiveness observed with selected genes provide evidence for their differential regulation. Observed diversity within the moss CHS superfamily and amenability to gene manipulation make Physcomitrella a highly suitable model system for studying expansion and functional diversification of the plant CHS superfamily of genes.

Non-modular polyketide synthases in myxobacteria

Myxobacteria are prolific producers of a wide variety of secondary metabolites. The vast majority of these compounds are complex polyketides which are biosynthesised by multimodular polyketide synthases (PKSs). In contrast, few myxobacterial metabolites isolated to date are derived from non-modular PKSs, in particular type III PKSs. This review reports our progress on the characterisation of type III PKSs in myxobacteria. We also summarize current knowledge on bacterial type III PKSs, with a special focus on the evolutionary relationship between plant and bacterial enzymes. The biosynthesis of a quinoline alkaloid in Stigmatella aurantiaca by a non-modular PKS is also discussed.