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

Characterization and functional analysis of type III polyketide synthases in Selaginella moellendorffii

MAIN CONCLUSION

The evolutionary conservation of type III polyketide synthases (PKS) in Selaginella has been elucidated, and the critical amino acid residues of the anther-specific chalcone synthase-like enzyme (SmASCL) have been identified. Selaginella species are the oldest known vascular plants and a valuable resource for the study of metabolic evolution in land plants. Polyketides, especially flavonoids and sporopollenin precursors, are essential prerequisites for plant land colonization. Although type III polyketide synthases (PKS) are widely studied in seed plants, the related enzymes in Selaginella remain poorly characterized. Here, eight type III PKSs were identified in the Selaginella moellendorffii genome and classified into three clusters. Two PKSs were selected for further research based on their phylogenetic relationships and protein sequence similarity. Functional studies revealed that they were chalcone synthase (SmCHS) and anther-specific CHS-like enzyme (SmASCL). These enzymes are involved in the biosynthesis of flavonoids and sporopollenin, respectively. Their sequence information and enzymatic activity are similar to the orthologs in other plants. Phylogenetic analysis revealed that the ASCL and CHS enzymes were separated into two clades from the Bryophyta. These results suggest that CHS and ASCL emerged in the first land plants and then remained conserved during plant evolution. To study the structural basis of the enzymatic function of SmASCL, a series of mutants were constructed. The number of condensation reactions catalyzed by the P210L/Y211D and I200V/G201T double mutants exceeds that of the wild-type enzyme. Our study provides insight into the characteristics and functions of type III PKSs in S. moellendorffii. It also offers clues for a deeper understanding of the relationship between active sites and the enzymatic function of ASCLs.

Takakia possesses a key marker of embryophyte sporopollenin

The enigmatic moss, Takakia lepidozioides , possesses a particular type III polyketide synthase, ASCL (Anther-Specific Chalcone synthase-Like), that is an identifying marker for genuine sporopollenin in the walls of embryophyte spores and pollen grains. By contrast, a survey of all algae with sequenced genomes confirms that they do not possess ASCL and, therefore, their spore walls are not composed of sporopollenin.

A hybrid pathway for self-sustained luminescence

The fungal bioluminescence pathway can be reconstituted in other organisms allowing luminescence imaging without exogenously supplied substrate. The pathway starts from hispidin biosynthesis-a step catalyzed by a large fungal polyketide synthase that requires a posttranslational modification for activity. Here, we report identification of alternative compact hispidin synthases encoded by a phylogenetically diverse group of plants. A hybrid bioluminescence pathway that combines plant and fungal genes is more compact, not dependent on availability of machinery for posttranslational modifications, and confers autonomous bioluminescence in yeast, mammalian, and plant hosts. The compact size of plant hispidin synthases enables additional modes of delivery of autoluminescence, such as delivery with viral vectors.

Characterization and expression analysis of chalcone synthase gene family members suggested their roles in the male sterility of a wheat temperature-sensitive genic male sterile (TGMS) line

Chalcone synthase (CHS), also known as the plants-specific type III polyketide synthases (PKSs), catalyzes the first key step in the biosynthesis of plant flavonoids. Flavonoids are one of the most important secondary metabolites which participate in flower pigmentation and pollen fertility. Recent reports have demonstrated the role of the CHS family in plant pollen exine formation. This study focused on the potential roles of CHS in the pollen exine formation of wheat. In the present study, a genome-wide investigation of the CHS family was carried out, and 87 CHS genes were identified in wheat. TaCHS3, TaCHS10, and TaCHS13 are wheat orthologs of Arabidopsis LESS ADHESIVE POLLEN (LAP5); TaCHS58, TaCHS64, and TaCHS67 are wheat orthologs of AtLAP6. TaCHS3, TaCHS10, and TaCHS67 showed anther-specific patterns. The expression of TaCHS3, TaCHS10, and TaCHS67 was positively co-expressed with sporopollenin biosynthetic genes, including TaCYP703A2, TaCYP704B1, TaDRL1, TaTKPR2, and TaMS2. Coincidently, the expression of TaCHS3, TaCHS10, and TaCHS67, together with those sporopollenin biosynthetic genes, were repressed at the tetrads and uninucleate stages in the temperature-sensitive genic male-sterile (TGMS) line BS366 under sterile conditions. Wheat anther-specific CHS genes might participate in the exine formation of BS366 through co-expressing with sporopollenin biosynthetic genes, which will undoubtedly provide knowledge of the roles of CHS in wheat pollen development.

Transcriptomic and Proteomic Analyses of Celery Cytoplasmic Male Sterile Line and Its Maintainer Line

Male sterility is a common phenomenon in the plant kingdom and based on the organelles harboring the male-sterility genes, it can be classified into the genic male sterility (GMS) and the cytoplasmic male sterility (CMS). In every generation, CMS can generate 100% male-sterile population, which is very important for the breeders to take advantage of the heterosis and for the seed producers to guarantee the seed purity. Celery is a cross-pollinated plant with the compound umbel type of inflorescence which carries hundreds of small flowers. These characteristics make CMS the only option to produce the commercial hybrid celery seeds. In this study, transcriptomic and proteomic analyses were performed to identify genes and proteins that are associated with celery CMS. A total of 1255 differentially expressed genes (DEGs) and 89 differentially expressed proteins (DEPs) were identified between the CMS and its maintainer line, then 25 genes were found to differentially expressed at both the transcript and protein levels. Ten DEGs involved in the fleece layer and outer pollen wall development were identified by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses, most of which were down-regulated in the sterile line W99A. These DEGs and DEPs were mainly enriched in the pathways of "phenylpropanoid/sporopollenin synthesis/metabolism", "energy metabolism", "redox enzyme activity" and "redox processes". Results obtained in this study laid a foundation for the future investigation of mechanisms of pollen development as well as the reasons for the CMS in celery.

Dependence on clade II bHLH transcription factors for nursing of haploid products by tapetal-like cells is conserved between moss sporangia and angiosperm anthers

Clade II basic helix-loop-helix transcription factors (bHLH TFs) are essential for pollen production and tapetal nursing functions in angiosperm anthers. As pollen has been suggested to be related to bryophyte spores by descent, we characterized two Physcomitrium (Physcomitrella) patens clade II bHLH TFs (PpbHLH092 and PpbHLH098), to test if regulation of sporogenous cells and the nursing cells surrounding them is conserved between angiosperm anthers and bryophyte sporangia. We made CRISPR-Cas9 reporter and loss-of-function lines to address the function of PpbHLH092/098. We sectioned and analyzed WT and mutant sporophytes for a comprehensive stage-by-stage comparison of sporangium development. Spore precursors in the P. patens sporangium are surrounded by nursing cells showing striking similarities to tapetal cells in angiosperms. Moss clade II bHLH TFs are essential for the differentiation of these tapetal-like cells and for the production of functional spores. Clade II bHLH TFs provide a conserved role in controlling the sporophytic somatic cells surrounding and nursing the sporogenous cells in both moss sporangia and angiosperm anthers. This supports the hypothesis that such nursing functions in mosses and angiosperms, lineages separated by c. 450 million years, are related by descent.

(2'-Oxo)alkylresorcinols restore dehydration tolerance in a knockout line of PpORS, a bryophyte-specific type III polyketide synthase in Physcomitrium (Physcomitrella) patens

PpORS-produced 2'-oxo-5-pentacosylresorcinol (2'-oxo-C₂₅-RL) restored dehydration tolerance in ors-3, a knockout mutant of PpORS. Feeding experiments with [¹⁴C]-2'-oxo-C₂₅-RL suggested the role of PpORS products in cuticular polymer that confer dehydration resistance. 2'-Oxoalkylresorcinol synthase from the moss Physcomitrium (Physcomitrella) patens (PpORS) is the earliest diverged member of plant type III polyketide synthases, and produces very-long-chain 2'-oxoalkylresorcinols in vitro. Targeted knockouts of PpORS (ors) exhibited an abnormal phenotype (increased susceptibility to dehydration), and a defective cuticle in ors was suggested (Li et al., Planta 247:527-541, 2018). In the present study, we investigated chemical rescue of the ors phenotype and also metabolic fates of the PpORS products in the moss. Using C₂₄-CoA as substrate, 2'-oxo-5-pentacosylresorcinol (2'-oxo-C₂₅-RL) and two minor pyrones were first enzymatically prepared as total in vitro products. When a knockout mutant (ors-3) and control strains were grown in the presence of the total in vitro products or purified 2'-oxo-C₂₅-RL, the ability of ors-3 and the control to survive dehydration stress increased in a dose-dependent manner. Structurally analogous long-chain alkylresorcinols also rescued the ors phenotype, although less efficiently. When the moss was grown in the presence of ¹⁴C-radiolabeled 2'-oxo-C₂₅-RL, 96% of the radioactivity was recovered only after acid hydrolysis. These findings led us to propose that 2'-oxoalkylresorcinols are the functional in planta products of PpORS and are incorporated into cuticular biopolymers that confer resistance to dehydration. In addition, the earliest diverging ORS clade in phylogenetic trees of plant type III PKSs exclusively comprises bryophyte enzymes that share similar active site substitutions with PpORS. Further studies on these bryophyte enzymes may shed light on their roles in early plant evolution and offer a novel strategy for improving dehydration tolerance in plants.

Overexpression of sesame polyketide synthase A leads to abnormal pollen development in Arabidopsis

BACKGROUND

Sesame is a great reservoir of bioactive constituents and unique antioxidant components. It is widely used for its nutritional and medicinal value. The expanding demand for sesame seeds is putting pressure on sesame breeders to develop high-yielding varieties. A hybrid breeding strategy based on male sterility is one of the most effective ways to increase the crop yield. To date, little is known about the genes and mechanism underlying sesame male fertility. Therefore, studies are being conducted to identify and functionally characterize key candidate genes involved in sesame pollen development. Polyketide synthases (PKSs) are critical enzymes involved in the biosynthesis of sporopollenin, the primary component of pollen exine. Their in planta functions are being investigated for applications in crop breeding.

RESULTS

In this study, we cloned the sesame POLYKETIDE SYNTHASE A (SiPKSA) and examined its function in male sterility. SiPKSA was specifically expressed in sesame flower buds, and its expression was significantly higher in sterile sesame anthers than in fertile anthers during the tetrad and microspore development stages. Furthermore, overexpression of SiPKSA in Arabidopsis caused male sterility in transgenic plants. Ultrastructural observation showed that the pollen grains of SiPKSA-overexpressing plants contained few cytoplasmic inclusions and exhibited an abnormal pollen wall structure, with a thicker exine layer compared to the wild type. In agreement with this, the expression of a set of sporopollenin biosynthesis-related genes and the contents of their fatty acids and phenolics were significantly altered in anthers of SiPKSA-overexpressing plants compared with wild type during anther development.

CONCLUSION

These findings highlighted that overexpression of SiPKSA in Arabidopsis might cause male sterility through defective pollen wall formation. Moreover, they suggested that SiPKSA modulates vibrant pollen development via sporopollenin biosynthesis, and a defect in its regulation may induce male sterility. Therefore, genetic manipulation of SiPKSA might promote hybrid breeding in sesame and other crop species.

The Toughest Material in the Plant Kingdom: An Update on Sporopollenin

The extreme chemical and physical recalcitrance of sporopollenin deems this biopolymer among the most resilient organic materials on Earth. As the primary material fortifying spore and pollen cell walls, sporopollenin is touted as a critical innovation in the progression of plant life to a terrestrial setting. Although crucial for its protective role in plant reproduction, the inert nature of sporopollenin has challenged efforts to determine its composition for decades. Revised structural, chemical, and genetic experimentation efforts have produced dramatic advances in elucidating the molecular structure of this biopolymer and the mechanisms of its synthesis. Bypassing many of the challenges with material fragmentation and solubilization, insights from functional characterizations of sporopollenin biogenesis in planta, and in vitro, through a gene-targeted approach suggest a backbone of polyhydroxylated polyketide-based subunits and remarkable conservation of biochemical pathways for sporopollenin biosynthesis across the plant kingdom. Recent optimization of solid-state NMR and targeted degradation methods for sporopollenin analysis confirms polyhydroxylated α-pyrone subunits, as well as hydroxylated aliphatic units, and unique cross-linkage heterogeneity. We examine the cross-disciplinary efforts to solve the sporopollenin composition puzzle and illustrate a working model of sporopollenin's molecular structure and biosynthesis. Emerging controversies and remaining knowledge gaps are discussed, including the degree of aromaticity, cross-linkage profiles, and extent of chemical conservation of sporopollenin among land plants. The recent developments in sporopollenin research present diverse opportunities for harnessing the extraordinary properties of this abundant and stable biomaterial for sustainable microcapsule applications and synthetic material designs.

An Overview of the Medicinally Important Plant Type III PKS Derived Polyketides

Plants produce interesting secondary metabolites that are a valuable source of both medicines for human use, along with significant advantages for the manufacturer species. The active compounds which lead to these instrumental effects are generally secondary metabolites produced during various plant growth phases, which provide the host survival advantages while affecting human health inadvertently. Different chemical classes of secondary metabolites are biosynthesized by the plant type III polyketide synthases (PKSs). They are simple homodimeric proteins with the unique mechanistic potential to produce a broad array of secondary metabolites by utilizing simpler starter and extender units. These PKS derived products are majorly the precursors of some important secondary metabolite pathways leading to products such as flavonoids, stilbenes, benzalacetones, chromones, acridones, xanthones, cannabinoids, aliphatic waxes, alkaloids, anthrones, and pyrones. These secondary metabolites have various pharmaceutical, medicinal and industrial applications which make biosynthesizing type III PKSs an important tool for bioengineering purposes. Because of their structural simplicity and ease of manipulation, these enzymes have garnered interest in recent years due to their application in the generation of unnatural natural polyketides and modified products in the search for newer drugs for a variety of health problems. The following review covers the biosynthesis of a variety of type III PKS-derived secondary metabolites, their biological relevance, the associated enzymes, and recent research.

A study of male fertility control in Medicago truncatula uncovers an evolutionarily conserved recruitment of two tapetal bHLH subfamilies in plant sexual reproduction

Male sterility is an important tool for plant breeding and hybrid seed production. Male-sterile mutants are largely due to an abnormal development of either the sporophytic or gametophytic anther tissues. Tapetum, a key sporophytic tissue, provides nutrients for pollen development, and its delayed degeneration induces pollen abortion. Numerous bHLH proteins have been documented to participate in the degeneration of the tapetum in angiosperms, but relatively little attention has been given to the evolution of the involved developmental pathways across the phylogeny of land plants. A combination of cellular, molecular, biochemical and evolutionary analyses was used to investigate the male fertility control in Medicago truncatula. We characterized the male-sterile mutant empty anther1 (ean1) and identified EAN1 as a tapetum-specific bHLH transcription factor necessary for tapetum degeneration. Our study uncovered an evolutionarily conserved recruitment of bHLH subfamily II and III(a + c)1 in the regulation of tapetum degeneration. EAN1 belongs to the subfamily II and specifically forms heterodimers with the subfamily III(a + c)1 members, which suggests a heterodimerization mechanism conserved in angiosperms. Our work suggested that the pathway of two tapetal-bHLH subfamilies is conserved in all land plants, and likely was established before the divergence of the spore-producing land plants.

Reactive oxygen species are required for spore wall formation in Physcomitrella patens

A robust spore wall was a key requirement of terrestrialization by early plants. Sporopollenin in spore and pollen grain walls is thought to be polymerized and cross-linked to other macromolecular components partly through oxidative processes involving H2O2. Therefore, we investigated effects of scavengers of reactive oxygen species (ROS) on formation of spore walls in the moss, Physcomitrella patens. Exposure of sporophytes, containing spores in the process of forming walls, to ascorbate, dimethylthiourea or 4-hydroxy-TEMPO prevented normal wall development in a dose, chemical and stage-dependent manner. Mature spores, exposed while developing to a ROS scavenger, burst when mounted in water on a flat slide under a coverslip (a phenomenon we named "augmented osmolysis" since they did not burst in phosphate-buffered saline or in water on a depression slide). Additionally, walls of exposed spores were more susceptible to alkaline hydrolysis than those of control spores and some were characterized by discontinuities in the exine, anomalies in perine spine structure, abnormal intine and aperture and occasionally wall shedding. Our data support involvement of oxidative cross-linking in spore wall development, including sporopollenin polymerization or deposition, as well as a role for ROS in intine/aperture development.

Lipid Metabolism: Critical Roles in Male Fertility and Other Aspects of Reproductive Development in Plants

Fatty acids and their derivatives are essential building blocks for anther cuticle and pollen wall formation. Disruption of lipid metabolism during anther and pollen development often leads to genic male sterility (GMS). To date, many lipid metabolism-related GMS genes that are involved in the formation of anther cuticle, pollen wall, and subcellular organelle membranes in anther wall layers have been identified and characterized. In this review, we summarize recent progress on characterizing lipid metabolism-related genes and their roles in male fertility and other aspects of reproductive development in plants. On the basis of cloned GMS genes controlling biosynthesis and transport of anther cutin, wax, sporopollenin, and tryphine in Arabidopsis, rice, and maize as well as other plant species, updated lipid metabolic networks underlying anther cuticle development and pollen wall formation were proposed. Through bioinformatics analysis of anther RNA-sequencing datasets from three maize inbred lines (Oh43, W23, and B73), a total of 125 novel lipid metabolism-related genes putatively involved in male fertility in maize were deduced. More, we discuss the pathways regulating lipid metabolism-related GMS genes at the transcriptional and post-transcriptional levels. Finally, we highlight recent findings on lipid metabolism-related genes and their roles in other aspects of plant reproductive development. A comprehensive understanding of lipid metabolism, genes involved, and their roles in plant reproductive development will facilitate the application of lipid metabolism-related genes in gene editing, haploid and callus induction, molecular breeding and hybrid seed production in crops.

Acyl-CoA synthetases from Physcomitrella, rice and Arabidopsis: different substrate preferences but common regulation by MS188 in sporopollenin synthesis

ACOS5, OsACOS12 and PpACOS6 are all capable of fatty acyl-CoA synthetase activity but exhibit different substrate preferences. The transcriptional regulation of ACOS for sporopollenin synthesis appears to have been conserved in Physcomitrella, rice and Arabidopsis during evolution. Sporopollenin is the major constituent of spore and pollen exines. In Arabidopsis, acyl-CoA synthetase 5 (ACOS5) is an essential enzyme for sporopollenin synthesis, and its orthologues are PpACOS6 from the moss Physcomitrella and OsACOS12 from monocot rice. However, knowledge regarding the evolutionary conservation and divergence of the ACOS gene in sporopollenin synthesis remains limited. In this study, we analysed the function and regulation of PpACOS6 and OsACOS12. A complementation test showed that OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis, while PpACOS6 did not rescue the acos5 phenotype. ACOS5, PpACOS6 and OsACOS12 all complemented the acyl-CoA synthetase-deficient yeast strain (YB525) phenotype, although they exhibited different substrate preferences. To understand the conservation of sporopollenin synthesis regulation, we constructed two constructs with ACOS5 driven by the OsACOS12 or PpACOS6 promoter. Both constructs could restore the fertility of acos5 plants. The MYB transcription factor MS188 from Arabidopsis directly regulates ACOS5. We found that MS188 could also bind the promoters of OsACOS12 and PpACOS6 and activate the genes driven by the promoters, suggesting that the transcriptional regulation of these genes was similar to that of ACOS5. These results show that the ACOS gene promoter region from Physcomitrella, rice and Arabidopsis has been functionally conserved during evolution, while the chain lengths of fatty acid-derived monomers of sporopollenin vary in different plant species.

Ostkpr1 functions in anther cuticle development and pollen wall formation in rice

BACKGROUND

During pollen wall formation in flowering plants, a conserved metabolon consisting of acyl-CoA synthetase (ACOS), polyketide synthase (PKS) and tetraketide α-pyrone reductase (TKPR), is required for sporopollenin synthesis. Despite this, the precise function of each of these components in different species remains unclear.

RESULTS

In this study, we characterized the function of OsTKPR1, a rice orthologue of Arabidopsis TKPR1. Loss of function of OsTKPR1 delayed tapetum degradation, reduced the levels of anther cuticular lipids, and impaired Ubisch body and pollen exine formation, resulting in complete male sterility. In addition, the phenylpropanoid pathway in mutant anthers was remarkably altered. Localization studies suggest that OsTKPR1 accumulates in the endoplasmic reticulum, while specific accumulation of OsTKPR1 mRNA in the anther tapetum and microspores is consistent with its function in anther and pollen wall development.

CONCLUSIONS

Our results show that OsTKPR1 is indispensable for anther cuticle development and pollen wall formation in rice, providing new insights into the biochemical mechanisms of the conserved sporopollenin metabolon in flowering plants.

The Origin and Evolution of Plant Flavonoid Metabolism

During their evolution, plants have acquired the ability to produce a huge variety of compounds. Unlike the specialized metabolites that accumulate in limited numbers of species, flavonoids are widely distributed in the plant kingdom. Therefore, a detailed analysis of flavonoid metabolism in genomics and metabolomics is an ideal way to investigate how plants have developed their unique metabolic pathways during the process of evolution. More comprehensive and precise metabolite profiling integrated with genomic information are helpful to emerge unexpected gene functions and/or pathways. The distribution of flavonoids and their biosynthetic genes in the plant kingdom suggests that flavonoid biosynthetic pathways evolved through a series of steps. The enzymes that form the flavonoid scaffold structures probably first appeared by recruitment of enzymes from primary metabolic pathways, and later, enzymes that belong to superfamilies such as 2-oxoglutarate-dependent dioxygenase, cytochrome P450, and short-chain dehydrogenase/reductase modified and varied the structures. It is widely accepted that the first two enzymes in flavonoid biosynthesis, chalcone synthase, and chalcone isomerase, were derived from common ancestors with enzymes in lipid metabolism. Later enzymes acquired their function by gene duplication and the subsequent acquisition of new functions. In this review, we describe the recent progress in metabolomics technologies for flavonoids and the evolution of flavonoid skeleton biosynthetic enzymes to understand the complicate evolutionary traits of flavonoid metabolism in plant kingdom.

OsPKS1 is required for sexine layer formation, which shows functional conservation between rice and Arabidopsis

The sporopollenin precursors, as a general constituent of sexine, are synthesized in the tapetum and deposited on the pollen surface after transportation and processing. The polyketide synthase condenses the acyl-CoA into a hydroxyalkyl α-pyrone, which is predicted to be a component of the sporopollenin precursors. In this study, we found that the rice POLYKETIDE SYNTHASE 1 (OsPKS1) was the orthologue of Arabidopsis POLYKETIDE SYNTHASE A/LESS ADHESIVE POLLEN 6 (PKSA/LAP6) through sequence alignment. The OsPKS1 knockout mutants obtained by Crispr-Cas9-mediated editing exhibited a complete male sterile phenotype. Cytological observations revealed that abnormal bacula deposition and ubisch body structures for sexine formation led to pollen rupture in ospks1. The expression analysis showed that the OsPKS1 was highly expressed in tapetal cells and anther locules from stage 9 to stage 11 during anther development in rice. Subcellular localization demonstrated that the OsPKS1 protein was preferentially localized to the ER. The genomic sequence of OsPKS1 driven by the PKSA/LAP6 promoter restored the sexine pattern of Arabidopsis pksa/lap6. These results indicated that OsPKS1 is required for sexine layer formation in rice and functionally conserved in the sporopollenin synthesis pathway.

The Physcomitrella patens gene atlas project: large-scale RNA-seq based expression data

High-throughput RNA sequencing (RNA-seq) has recently become the method of choice to define and analyze transcriptomes. For the model moss Physcomitrella patens, although this method has been used to help analyze specific perturbations, no overall reference dataset has yet been established. In the framework of the Gene Atlas project, the Joint Genome Institute selected P. patens as a flagship genome, opening the way to generate the first comprehensive transcriptome dataset for this moss. The first round of sequencing described here is composed of 99 independent libraries spanning 34 different developmental stages and conditions. Upon dataset quality control and processing through read mapping, 28 509 of the 34 361 v3.3 gene models (83%) were detected to be expressed across the samples. Differentially expressed genes (DEGs) were calculated across the dataset to permit perturbation comparisons between conditions. The analysis of the three most distinct and abundant P. patens growth stages - protonema, gametophore and sporophyte - allowed us to define both general transcriptional patterns and stage-specific transcripts. As an example of variation of physico-chemical growth conditions, we detail here the impact of ammonium supplementation under standard growth conditions on the protonemal transcriptome. Finally, the cooperative nature of this project allowed us to analyze inter-laboratory variation, as 13 different laboratories around the world provided samples. We compare differences in the replication of experiments in a single laboratory and between different laboratories.

OsPKS2 is required for rice male fertility by participating in pollen wall formation

OsPKS2, the rice orthologous gene of Arabidopsis PKSB/LAP5, encodes a polyketide synthase that is involved in pollen wall formation in rice. In flowering plants, the pollen wall protects male gametes from various environmental stresses and pathogen attacks, as well as promotes pollen germination. The biosynthesis of sporopollenin in tapetal cell is critical for pollen wall formation. Recently, progress has been made in understanding sporopollenin metabolism during pollen wall development in Arabidopsis. However, little is known about the molecular mechanism that underlies the sporopollenin synthesis in pollen wall formation in rice (Oryza sativa). In this study, we identified that a point mutation in OsPKS2, a plant-specific type III polyketide synthase gene, caused male sterility in rice by affecting the normal progress of pollen wall formation. Two other allelic mutants of OsPKS2 were generated using the CRISPR/Cas9 system and are also completely male sterile. This result thus further confirmed that OsPKS2 controls rice male fertility. We also showed that OsPKS2 is an orthologous gene of Arabidopsis PKSB/LAP5 and has a tapetum-specific expression pattern. In addition, its product localizes in the endoplasmic reticulum. Results suggested that OsPKS2 is critical for pollen wall formation, and plays a conserved but differentiated role in sporopollenin biosynthesis from Arabidopsis.

PpORS, an ancient type III polyketide synthase, is required for integrity of leaf cuticle and resistance to dehydration in the moss, Physcomitrella patens

PpORS knockout mutants produced abnormal leaves with increased dye permeability and were more susceptible to dehydration, consistent with PpORS products being constituents of a cuticular structure in the moss. Type III polyketide synthases (PKSs) have co-evolved with terrestrial plants such that each taxon can generate a characteristic collection of polyketides, fine-tuned to its needs. 2'-Oxoalkylresorcinol synthase from Physcomitrella patens (PpORS) is basal to all plant type III PKSs in phylogenetic trees and may closely resemble their most recent common ancestor. To gain insight into the roles that ancestral plant type III PKSs might have played during early land plant evolution, we constructed and phenotypically characterized targeted knockouts of PpORS. Ors gametophores, unless submerged in water while they were developing, displayed various leaf malformations that included grossly misshapen leaves, missing or abnormal midribs, multicellular protuberances and localized necrosis. Ors leaves, particularly abnormal ones, showed increased permeability to the hydrophilic dye, toluidine blue. Ors gametophores lost water faster and were more susceptible to dehydration than those of the control strain. Our findings are consistent with ors leaves possessing a partially defective cuticle and implicate PpORS in synthesis of the intact cuticle. PpORS orthologs are present in a few moss species but have not been found in other plants. However, conceivably an ancestral ORS in early land plants may have contributed to their protection from dehydration.

OsLAP6/OsPKS1, an orthologue of Arabidopsis PKSA/LAP6, is critical for proper pollen exine formation

BACKGROUND

Male fertility is crucial for rice yield, and the improvement of rice yield requires hybrid production that depends on male sterile lines. Although recent studies have revealed several important genes in male reproductive development, our understanding of the mechanisms of rice pollen development remains unclear.

RESULTS

We identified a rice mutant oslap6 with complete male sterile phenotype caused by defects in pollen exine formation. By using the MutMap method, we found that a single nucleotide polymorphism (SNP) variation located in the second exon of OsLAP6/OsPKS1 was responsible for the mutant phenotype. OsLAP6/OsPKS1 is an orthologous gene of Arabidopsis PKSA/LAP6, which functions in sporopollenin metabolism. Several other loss-of-function mutants of OsLAP6/OsPKS1 generated by the CRISPR/Cas9 genomic editing tool also exhibited the same phenotype of male sterility. Our cellular analysis suggested that OsLAP6/OsPKS1 might regulate pollen exine formation by affecting bacula elongation. Expression examination indicated that OsLAP6/OsPKS1 is specifically expressed in tapetum, and its product is localized to the endoplasmic reticulum (ER). Protein sequence analysis indicated that OsLAP6/OsPKS1 is conserved in land plants.

CONCLUSIONS

OsLAP6/OsPKS1 is a critical molecular switch for rice male fertility by participating in a conserved sporopollenin precursor biosynthetic pathway in land plants. Manipulation of OsLAP6/OsPKS1 has potential for application in hybrid rice breeding.

The polyketide synthase OsPKS2 is essential for pollen exine and Ubisch body patterning in rice

Lipid and phenolic metabolism are important for pollen exine formation. In Arabidopsis, polyketide synthases (PKSs) are essential for both sporopollenin biosynthesis and exine formation. Here, we characterized the role of a polyketide synthase (OsPKS2) in male reproduction of rice (Oryza sativa). Recombinant OsPKS2 catalyzed the condensation of fatty acyl-CoA with malonyl-CoA to generate triketide and tetraketide α-pyrones, the main components of pollen exine. Indeed, the ospks2 mutant had defective exine patterning and was male sterile. However, the mutant showed no significant reduction in sporopollenin accumulation. Compared with the WT (wild type), ospks2 displayed unconfined and amorphous tectum and nexine layers in the exine, and less organized Ubisch bodies. Like the pksb/lap5 mutant of the Arabidopsis ortholog, ospks2 showed broad alterations in the profiles of anther-related phenolic compounds. However, unlike pksb/lap5, in which most detected phenolics were substantially decreased, ospks2 accumulated higher levels of phenolics. Based on these results and our observation that OsPKS2 is unable to fully restore the exine defects in the pksb/lap5, we propose that PKS proteins have functionally diversified during evolution. Collectively, our results suggest that PKSs represent a conserved and diversified biochemical pathway for anther and pollen development in higher plants.

MS26/CYP704B is required for anther and pollen wall development in bread wheat (Triticum aestivum L.) and combining mutations in all three homeologs causes male sterility

Development of anthers and pollen represents an important aspect of the life cycle in flowering plants. Genes contributing to anther and pollen development have been widely studied in many plant species. Ms26/CYP704B genes play an important role in pollen development through biosynthesis of sporopollenin for pollen exine formation. To investigate the role of Ms26/CYP704B genes in anther and pollen development of bread wheat, mutations in the A-, B-, and D-homeologs of the putative Ms26/CYP704B gene were analyzed. Single and double homozygous mutants in any of the homeologs did not affect pollen development and male fertility. Triple homozygous mutants resulted in completely male sterile plants that were defective in pollen and anther development. Additionally, double homozygous-single heterozygous mutants were also male sterile although with varying levels of residual fertility. The fertility of these triple mutants was dependent upon the homeolog contributing the wild-type allele. Two heterologous Ms26/CYP704B genes, when transformed into a triple homozygous mutant background, completely restored male fertility, whereas a single gene was unable to restore fertility. Functional analysis of Ms26/CYP704B furthers the understanding of male fertility genes which can be utilized for the development of novel hybrid seed production systems in wheat.

Sexual reproduction, sporophyte development and molecular variation in the model moss Physcomitrella patens: introducing the ecotype Reute

Rich ecotype collections are used for several plant models to unravel the molecular causes of phenotypic differences, and to investigate the effects of environmental adaption and acclimation. For the model moss Physcomitrella patens collections of accessions are available, and have been used for phylogenetic and taxonomic studies, for example, but few have been investigated further for phenotypic differences. Here, we focus on the Reute accession and provide expression profiling and comparative developmental data for several stages of sporophyte development, as well as information on genetic variation via genomic sequencing. We analysed cross-technology and cross-laboratory data to define a confident set of 15 mature sporophyte-specific genes. We find that the standard laboratory strain Gransden produces fewer sporophytes than Reute or Villersexel, although gametangia develop with the same time course and do not show evident morphological differences. Reute exhibits less genetic variation relative to Gransden than Villersexel, yet we found variation between Gransden and Reute in the expression profiles of several genes, as well as variation hot spots and genes that appear to evolve under positive Darwinian selection. We analyzed expression differences between the ecotypes for selected candidate genes in the GRAS transcription factor family, the chalcone synthase family and in genes involved in cell wall modification that are potentially related to phenotypic differences. We confirm that Reute is a P. patens ecotype, and suggest its use for reverse-genetics studies that involve progression through the life cycle and multiple generations.

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.

OsACOS12, an orthologue of Arabidopsis acyl-CoA synthetase5, plays an important role in pollen exine formation and anther development in rice

BACKGROUND

Sporopollenin is a major component of the pollen exine pattern. In Arabidopsis, acyl-CoA synthetase5 (ACOS5) is involved in sporopollenin precursor biosynthesis. In this study, we identified its orthologue, OsACOS12, in rice (Oryza sativa) and compared the functional conservation of ACOS in rice to Arabidopsis.

RESULTS

Sequence analysis showed that OsACOS12 shares 63.9 % amino acid sequence identity with ACOS5. The osacos12 mutation caused by a pre-mature stop codon in LOC_Os04g24530 exhibits defective sexine resulting in a male sterile phenotype in rice. In situ hybridization shows that OsACOS12 is expressed in tapetal cells and microspores at the transcript level. The localization of OsACOS12-GFP demonstrated that OsACOS12 protein is accumulated in tapetal cells and anther locules. OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis.

CONCLUSIONS

OsACOS12 is an orthologue of ACOS5 that is essential for sporopollenin synthesis in rice. ACOS5 and OsACOS12 are conserved for pollen wall formation in monocot and dicot species.

OsACOS12, an orthologue of Arabidopsis acyl-CoA synthetase5, plays an important role in pollen exine formation and anther development in rice

BACKGROUND

Sporopollenin is a major component of the pollen exine pattern. In Arabidopsis, acyl-CoA synthetase5 (ACOS5) is involved in sporopollenin precursor biosynthesis. In this study, we identified its orthologue, OsACOS12, in rice (Oryza sativa) and compared the functional conservation of ACOS in rice to Arabidopsis.

RESULTS

Sequence analysis showed that OsACOS12 shares 63.9 % amino acid sequence identity with ACOS5. The osacos12 mutation caused by a pre-mature stop codon in LOC_Os04g24530 exhibits defective sexine resulting in a male sterile phenotype in rice. In situ hybridization shows that OsACOS12 is expressed in tapetal cells and microspores at the transcript level. The localization of OsACOS12-GFP demonstrated that OsACOS12 protein is accumulated in tapetal cells and anther locules. OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis.

CONCLUSIONS

OsACOS12 is an orthologue of ACOS5 that is essential for sporopollenin synthesis in rice. ACOS5 and OsACOS12 are conserved for pollen wall formation in monocot and dicot species.

Phylogeny and Expression Analyses Reveal Important Roles for Plant PKS III Family during the Conquest of Land by Plants and Angiosperm Diversification

Polyketide synthases (PKSs) utilize the products of primary metabolism to synthesize a wide array of secondary metabolites in both prokaryotic and eukaryotic organisms. PKSs can be grouped into three distinct classes, types I, II, and III, based on enzyme structure, substrate specificity, and catalytic mechanisms. The type III PKS enzymes function as homodimers, and are the only class of PKS that do not require acyl carrier protein. Plant type III PKS enzymes, also known as chalcone synthase (CHS)-like enzymes, are of particular interest due to their functional diversity. In this study, we mined type III PKS gene sequences from the genomes of six aquatic algae and 25 land plants (1 bryophyte, 1 lycophyte, 2 basal angiosperms, 16 core eudicots, and 5 monocots). PKS III sequences were found relatively conserved in all embryophytes, but not exist in algae. We also examined gene expression patterns by analyzing available transcriptome data, and identified potential cis-regulatory elements in upstream sequences. Phylogenetic trees of dicots angiosperms showed that plant type III PKS proteins fall into three clades. Clade A contains CHS/STS-type enzymes coding genes with diverse transcriptional expression patterns and enzymatic functions, while clade B is further divided into subclades b1 and b2, which consist of anther-specific CHS-like enzymes. Differentiation regions, such as amino acids 196-207 between clades A and B, and predicted positive selected sites within α-helixes in late appeared branches of clade A, account for the major diversification in substrate choice and catalytic reaction. The integrity and location of conserved cis-elements containing MYB and bHLH binding sites can affect transcription levels. Potential binding sites for transcription factors such as WRKY, SPL, or AP2/EREBP may contribute to tissue- or taxon-specific differences in gene expression. Our data shows that gene duplications and functional diversification of plant type III PKS enzymes played a critical role in the ancient conquest of the land by early plants and angiosperm diversification.

Exploiting the Biosynthetic Potential of Type III Polyketide Synthases

Polyketides are structurally and functionally diverse secondary metabolites that are biosynthesized by polyketide synthases (PKSs) using acyl-CoA precursors. Recent studies in the engineering and structural characterization of PKSs have facilitated the use of target enzymes as biocatalysts to produce novel functionally optimized polyketides. These compounds may serve as potential drug leads. This review summarizes the insights gained from research on type III PKSs, from the discovery of chalcone synthase in plants to novel PKSs in bacteria and fungi. To date, at least 15 families of type III PKSs have been characterized, highlighting the utility of PKSs in the development of natural product libraries for therapeutic development.

PpASCL, the Physcomitrella patens Anther-Specific Chalcone Synthase-Like Enzyme Implicated in Sporopollenin Biosynthesis, Is Needed for Integrity of the Moss Spore Wall and Spore Viability

Sporopollenin is the main constituent of the exine layer of spore and pollen walls. The anther-specific chalcone synthase-like (ASCL) enzyme of Physcomitrella patens, PpASCL, has previously been implicated in the biosynthesis of sporopollenin, the main constituent of exine and perine, the two outermost layers of the moss spore cell wall. We made targeted knockouts of the corresponding gene, PpASCL, and phenotypically characterized ascl sporophytes and spores at different developmental stages. Ascl plants developed normally until late in sporophytic development, when the spores produced were structurally aberrant and inviable. The development of the ascl spore cell wall appeared to be arrested early in microspore development, resulting in small, collapsed spores with altered surface morphology. The typical stratification of the spore cell wall was absent with only an abnormal perine recognisable above an amorphous layer possibly representing remnants of compromised intine and/or exine. Equivalent resistance of the spore walls of ascl mutants and the control strain to acetolysis suggests the presence of chemically inert, defective sporopollenin in the mutants. Anatomical abnormalities of late-stage ascl sporophytes include a persistent large columella and an air space incompletely filled with spores. Our results indicate that the evolutionarily conserved PpASCL gene is needed for proper construction of the spore wall and for normal maturation and viability of moss spores.

Role of Lipid Metabolism in Plant Pollen Exine Development

Pollen plays important roles in the life cycle of angiosperms plants. It acts as not only a biological protector of male sperms but also a communicator between the male and the female reproductive organs, facilitating pollination and fertilization. Pollen is produced within the anther, and covered by the specialized outer envelope, pollen wall. Although the morphology of pollen varies among different plant species, the pollen wall is mainly comprised of three layers: the pollen coat, the outer exine layer, and the inner intine layer. Except the intine layer, the other two layers are basically of lipidic nature. Particularly, the outer pollen wall layer, the exine, is a highly resistant biopolymer of phenylpropanoid and lipidic monomers covalently coupled by ether and ester linkages. The precise molecular mechanisms underlying pollen coat formation and exine patterning remain largely elusive. Herein, we summarize the current genetic, phenotypic and biochemical studies regarding to the pollen exine development and underlying molecular regulatory mechanisms mainly obtained from monocot rice (Oryza sativa) and dicot Arabidopsis thaliana, aiming to extend our understandings of plant male reproductive biology. Genes, enzymes/proteins and regulatory factors that appear to play conserved and diversified roles in lipid biosynthesis, transportation and modification during pollen exine formation, were highlighted.

Anther and pollen development: A conserved developmental pathway

Pollen development is a critical step in plant development that is needed for successful breeding and seed formation. Manipulation of male fertility has proved a useful trait for hybrid breeding and increased crop yield. However, although there is a good understanding developing of the molecular mechanisms of anther and pollen anther development in model species, such as Arabidopsis and rice, little is known about the equivalent processes in important crops. Nevertheless the onset of increased genomic information and genetic tools is facilitating translation of information from the models to crops, such as barley and wheat; this will enable increased understanding and manipulation of these pathways for agricultural improvement.

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.

A novel lily anther-specific gene encodes adhesin-like proteins associated with exine formation during anther development

The anther-specific gene LLA1271 isolated from lily (Lilium longiflorum Thunb.) anthers is novel and exists in two forms. The protein encoded by LLA1271 may represent an adhesin-like protein first found in higher plants. The protein contains a typical N-terminal signal peptide followed by a highly conserved repeat domain. The LLA1271 gene is temporally expressed at the phase of microspore development. RNA blot and RNA in situ hybridization analyses demonstrated that the gene was expressed both in the tapetum and in the microspore. The gene is endo- and exogenously induced by gibberellin. Studies with the gibberellin biosynthesis inhibitor uniconazole and an inhibitor of ethylene activity, 2,5-norbornadien (NBD), revealed that LLA1271 is negatively regulated by ethylene, and a cross-talk of regulation between gibberellin and ethylene occurs in young anthers. The treatment with NBD caused the tapetum to become densely cytoplasmic and highly polarized, whereas uniconazole arrested tapetal development in a state close to that of a tapetum without treatment. The LLA1271 protein is heat stable and heterogeneous. An immunoblot of separated protein fractions of the anther revealed that the LLA1271 protein was detected in protein fraction of the microspore released from the cell wall by treatment with either 0.5% or 2% Triton X-100. Ectopic expression of LLA1271 resulted in impaired stamen and low pollen germination. Scanning electron microscopy of TAP::LLA1271 pollen showed distorted exine formation and patterning. The LLA1271 protein once synthesized in both the tapetum and microspore is secreted and deposited on the surface of microspores, moderately affecting exine formation and patterning.

Conserved metabolic steps for sporopollenin precursor formation in tobacco and rice

The development of pollen wall with proper sporopollenin deposition is essential for pollen viability and male fertility in flowering plants. Sporopollenin is a complex biopolymer synthesized from fatty acid and phenolic derivatives. Recent investigations in Arabidopsis have identified a number of anther-specific genes involved in the production of fatty-acyl monomers potentially required for exine formation. The existence of ancient biochemical pathways for sporopollenin biosynthesis has been widely proposed but experimental evidence from plant species other than Arabidopsis is not extensively available. Here, we investigated the metabolic steps catalyzed by the anther-specific acyl-CoA synthetase (ACOS), polyketide synthase (PKS) and tetraketide α-pyrone reductase (TKPR). Using fatty acids as starting substrates, sequential activities of heterologously expressed tobacco enzymes NtACOS1, NtPKS1 and NtTKPR1 resulted in the production of reduced tetraketide α-pyrones. Transgenic RNA interference lines were then generated for the different tobacco genes which were demonstrated to be indispensable for normal pollen development and male fertility. Similarly, recombinant rice OsPKS1 and OsTKPR1 were shown to function as downstream enzymes of NtACOS1. In addition, insertion mutant lines for these rice genes displayed different levels of impaired pollen and seed formation. Taken together, reduced tetraketide α-pyrones appear to represent common sporopollenin fatty-acyl precursors essential for male fertility in taxonomically distinct plant species.

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.

ABCG15 encodes an ABC transporter protein, and is essential for post-meiotic anther and pollen exine development in rice

In flowering plants, anther and pollen development is critical for male reproductive success. The anther cuticle and pollen exine play an essential role, and in many cereals, such as rice, orbicules/ubisch bodies are also thought to be important for pollen development. The formation of the anther cuticle, exine and orbicules is associated with the biosynthesis and transport of wax, cutin and sporopollenin components. Recently, progress has been made in understanding the biosynthesis of sporopollenin and cutin components in Arabidopsis and rice, but less is known about the mechanisms by which they are transported to the sites of deposition. Here, we report that the rice ATP-binding cassette (ABC) transporter, ABCG15, is essential for post-meiotic anther and pollen development, and is proposed to play a role in the transport of rice anther cuticle and sporopollenin precursors. ABCG15 is highly expressed in the tapetum at the young microspore stage, and the abcg15 mutant exhibits small, white anthers lacking mature pollen, lipidic cuticle, orbicules and pollen exine. Gas chromatography-mass spectrometry (GC-MS) analysis of the abcg15 anther cuticle revealed significant reductions in a number of wax components and aliphatic cutin monomers. The expression level of genes involved in lipid metabolism in the abcg15 mutant was significantly different from their levels in the wild type, possibly due to perturbations in the homeostasis of anther lipid metabolism. Our study provides new insights for understanding the molecular mechanism of the formation of the anther cuticle, orbicules and pollen wall, as well as the machinery for lipid metabolism in rice anthers.

Physcomitrella PpORS, basal to plant type III polyketide synthases in phylogenetic trees, is a very long chain 2'-oxoalkylresorcinol synthase

The plant type III polyketide synthases (PKSs), which produce diverse secondary metabolites with different biological activities, have successfully co-evolved with land plants. To gain insight into the roles that ancestral type III PKSs played during the early evolution of land plants, we cloned and characterized PpORS from the moss Physcomitrella. PpORS has been proposed to closely resemble the most recent common ancestor of the plant type III PKSs. PpORS condenses a very long chain fatty acyl-CoA with four molecules of malonyl-CoA and catalyzes decarboxylative aldol cyclization to yield the pentaketide 2'-oxoalkylresorcinol. Therefore, PpORS is a 2'-oxoalkylresorcinol synthase. Structure modeling and sequence alignments identified a unique set of amino acid residues (Gln(218), Val(277), and Ala(286)) at the putative PpORS active site. Substitution of the Ala(286) to Phe apparently constricted the active site cavity, and the A286F mutant instead produced triketide alkylpyrones from fatty acyl-CoA substrates with shorter chain lengths. Phylogenetic analysis and comparison of the active sites of PpORS and alkylresorcinol synthases from sorghum and rice suggested that the gramineous enzymes evolved independently from PpORS to have similar functions but with distinct active site architecture. Microarray analysis revealed that PpORS is exclusively expressed in nonprotonemal moss cells. The in planta function of PpORS, therefore, is probably related to a nonprotonemal structure, such as the cuticle.

Mapping the mechanism of the resorcinol ring formation catalyzed by ArsB, a type III polyketide synthase from Azotobacter vinelandii

Who's first? Aldol cyclization occurs before hydrolysis in the resorcinol ring formation catalyzed by the type III polyketide synthase, ArsB. Synthetic C(20)-TKA was not converted to alkylresorcinol by ArsB, but rather inhibited the enzyme activity, thus indicating that C(20)-TKA is not an intermediate in ArsB-catalyzed alkylresorcinol formation.