Role of bifunctional ammonia-lyase in grass cell wall biosynthesis

L-Phenylalanine ammonia-lyase (PAL) is the first enzyme in the biosynthesis of phenylpropanoid-derived plant compounds such as flavonoids, coumarins and the cell wall polymer lignin. The cell walls of grasses possess higher proportions of syringyl (S)-rich lignins and high levels of esterified coumaric acid compared with those of dicotyledonous plants, and PAL from grasses can also possess tyrosine ammonia-lyase (TAL) activity, the reason for which has remained unclear. Using phylogenetic, transcriptomic and in vitro biochemical analyses, we identified a single homotetrameric bifunctional ammonia-lyase (PTAL) among eight BdPAL enzymes in the model grass species Brachypodium distachyon. (13)C isotope labelling experiments along with BdPTAL1-downregulation in transgenic plants showed that the TAL activity of BdPTAL1 can provide nearly half of the total lignin deposited in Brachypodium, with a preference for S-lignin and wall-bound coumarate biosynthesis, indicating that PTAL function is linked to the characteristic features of grass cell walls. Furthermore, isotope dilution experiments suggest that the pathways to lignin from L-phenylalanine and L-tyrosine are distinct beyond the formation of 4-coumarate, supporting the organization of lignin synthesis enzymes in one or more metabolons.

Treatment of hind limb ischemia using angiogenic peptide nanofibers

For a proangiogenic therapy to be successful, it must promote the development of mature vasculature for rapid reperfusion of ischemic tissue. Whole growth factor, stem cell, and gene therapies have yet to achieve the clinical success needed to become FDA-approved revascularization therapies. Herein, we characterize a biodegradable peptide-based scaffold engineered to mimic VEGF and self-assemble into a nanofibrous, thixotropic hydrogel, SLanc. We found that this injectable hydrogel was rapidly infiltrated by host cells and could be degraded while promoting the generation of neovessels. In mice with induced hind limb ischemia, this synthetic peptide scaffold promoted angiogenesis and ischemic tissue recovery, as shown by Doppler-quantified limb perfusion and a treadmill endurance test. Thirteen-month-old mice showed significant recovery within 7 days of treatment. Biodistribution studies in healthy mice showed that the hydrogel is safe when administered intramuscularly, subcutaneously, or intravenously. These preclinical studies help establish the efficacy of this treatment for peripheral artery disease due to diminished microvascular perfusion, a necessary step before clinical translation. This peptide-based approach eliminates the need for cell transplantation or viral gene transfection (therapies currently being assessed in clinical trials) and could be a more effective regenerative medicine approach to microvascular tissue engineering.

Comparative cell-specific transcriptomics reveals differentiation of C4 photosynthesis pathways in switchgrass and other C4 lineages

Almost all C4 plants require the co-ordination of the adjacent and fully differentiated cell types, mesophyll (M) and bundle sheath (BS). The C4 photosynthetic pathway operates through two distinct subtypes based on how malate is decarboxylated in BS cells; through NAD-malic enzyme (NAD-ME) or NADP-malic enzyme (NADP-ME). The diverse or unique cell-specific molecular features of M and BS cells from separate C4 subtypes of independent lineages remain to be determined. We here provide an M/BS cell type-specific transcriptome data set from the monocot NAD-ME subtype switchgrass (Panicum virgatum). A comparative transcriptomics approach was then applied to compare the M/BS mRNA profiles of switchgrass, monocot NADP-ME subtype C4 plants maize and Setaria viridis, and dicot NAD-ME subtype Cleome gynandra. We evaluated the convergence in the transcript abundance of core components in C4 photosynthesis and transcription factors to establish Kranz anatomy, as well as gene distribution of biological functions, in these four independent C4 lineages. We also estimated the divergence between NAD-ME and NADP-ME subtypes of C4 photosynthesis in the two cell types within C4 species, including differences in genes encoding decarboxylating enzymes, aminotransferases, and metabolite transporters, and differences in the cell-specific functional enrichment of RNA regulation and protein biogenesis/homeostasis. We suggest that C4 plants of independent lineages in both monocots and dicots underwent convergent evolution to establish C4 photosynthesis, while distinct C4 subtypes also underwent divergent processes for the optimization of M and BS cell co-ordination. The comprehensive data sets in our study provide a basis for further research on evolution of C4 species.

Combining enhanced biomass density with reduced lignin level for improved forage quality

To generate a forage crop with increased biomass density that retains forage quality, we have genetically transformed lines of alfalfa (Medicago sativa L.) expressing antisense constructs targeting two different lignin pathway biosynthetic genes with a construct for down-regulation of a WRKY family transcription factor that acts as a repressor of secondary cell wall formation in pith tissues. Plants with low-level expression of the WRKY dominant repressor construct produced lignified cell walls in pith tissues and exhibited enhanced biomass and biomass density, with an increase in total sugars in the cell wall fraction; however, lines with high expression of the WRKY dominant repressor construct exhibited a very different phenotype, with loss of interfascicular fibres associated with repression of the NST1 transcription factor. This latter phenotype was not observed in transgenic lines in which the WRKY transcription factor was down-regulated by RNA interference. Enhanced and/or ectopic deposition of secondary cell walls was also seen in corn and switchgrass expressing WRKY dominant repressor constructs, with enhanced biomass in corn but reduced biomass in switchgrass. Neutral detergent fibre digestibility was not impacted by WRKY expression in corn. Cell walls from WRKY-DR-expressing alfalfa plants with enhanced secondary cell wall formation exhibited increased sugar release efficiency, and WRKY dominant repressor expression further increased sugar release in alfalfa down-regulated in the COMT, but not the HCT, genes of lignin biosynthesis. These results suggest that significant enhancements in forage biomass and quality can be achieved through engineering WRKY transcription factors in both monocots and dicots.

Role of a chalcone isomerase-like protein in flavonoid biosynthesis in Arabidopsis thaliana

Flavonoids are important natural products for plant defence and human health. Although almost all the flavonoid pathway genes have been well-documented by biochemical and/or genetic approaches, the role of the Arabidopsis chalcone isomerase-like (CHIL) gene remains unclear. Two chil mutants with a seed colour similar to that of wild-type Arabidopsis have been identified here, but in sharp contrast to the characteristic transparent testa seed phenotype associated with other known flavonoid pathway genes. CHIL loss-of-function mutations led to a strong reduction in the proanthocyanidin and flavonol levels in seeds, but not in the anthocyanin levels in leaves. CHIL over-expression could partially recover the mutant phenotype of the chil mutant and increased both proanthocyanidin and flavonol accumulation in wild-type Arabidopsis. However, the CHIL gene could not rescue the mutant phenotype of TT5 that encodes the intrinsic chalcone isomerase in Arabidopsis. Parallel phenotypical and metabolic analyses of the chil, tt5, chs, and f3h mutants revealed that, genetically, CHIL functions at the same step as TT5. Moreover, it is demonstrated that CHIL co-expresses, co-localizes, and interacts with TT5 in Arabidopsis for flavonoid production. Based on these genetic and metabolic studies, it is concluded that CHIL functions with TT5 to promote flavonoid production, which is a unique enhancer in the flavonoid pathway.

The Transcriptional Repressor MYB2 Regulates Both Spatial and Temporal Patterns of Proanthocyandin and Anthocyanin Pigmentation in Medicago truncatula

Accumulation of anthocyanins and proanthocyanidins (PAs) is limited to specific cell types and developmental stages, but little is known about how antagonistically acting transcriptional regulators work together to determine temporal and spatial patterning of pigmentation at the cellular level, especially for PAs. Here, we characterize MYB2, a transcriptional repressor regulating both anthocyanin and PA biosynthesis in the model legume Medicago truncatula. MYB2 was strongly upregulated by MYB5, a major regulator of PA biosynthesis in M. truncatula and a component of MYB-basic helix loop helix-WD40 (MBW) activator complexes. Overexpression of MYB2 abolished anthocyanin and PA accumulation in M. truncatula hairy roots and Arabidopsis thaliana seeds, respectively. Anthocyanin deposition was expanded in myb2 mutant seedlings and flowers accompanied by increased anthocyanin content. PA mainly accumulated in the epidermal layer derived from the outer integument in the M. truncatula seed coat, starting from the hilum area. The area of PA accumulation and ANTHOCYANIDIN REDUCTASE expression was expanded into the seed body at the early stage of seed development in the myb2 mutant. Genetic, biochemical, and cell biological evidence suggests that MYB2 functions as part of a multidimensional regulatory network to define the temporal and spatial pattern of anthocyanin and PA accumulation linked to developmental processes.

Computational inference of the structure and regulation of the lignin pathway in Panicum virgatum

BACKGROUND

Switchgrass is a prime target for biofuel production from inedible plant parts and has been the subject of numerous investigations in recent years. Yet, one of the main obstacles to effective biofuel production remains to be the major problem of recalcitrance. Recalcitrance emerges in part from the 3-D structure of lignin as a polymer in the secondary cell wall. Lignin limits accessibility of the sugars in the cellulose and hemicellulose polymers to enzymes and ultimately decreases ethanol yield. Monolignols, the building blocks of lignin polymers, are synthesized in the cytosol and translocated to the plant cell wall, where they undergo polymerization. The biosynthetic pathway leading to monolignols in switchgrass is not completely known, and difficulties associated with in vivo measurements of these intermediates pose a challenge for a true understanding of the functioning of the pathway.

RESULTS

In this study, a systems biological modeling approach is used to address this challenge and to elucidate the structure and regulation of the lignin pathway through a computational characterization of alternate candidate topologies. The analysis is based on experimental data characterizing stem and tiller tissue of four transgenic lines (knock-downs of genes coding for key enzymes in the pathway) as well as wild-type switchgrass plants. These data consist of the observed content and composition of monolignols. The possibility of a G-lignin specific metabolic channel associated with the production and degradation of coniferaldehyde is examined, and the results support previous findings from another plant species. The computational analysis suggests regulatory mechanisms of product inhibition and enzyme competition, which are well known in biochemistry, but so far had not been reported in switchgrass. By including these mechanisms, the pathway model is able to represent all observations.

CONCLUSIONS

The results show that the presence of the coniferaldehyde channel is necessary and that product inhibition and competition over cinnamoyl-CoA-reductase (CCR1) are essential for matching the model to observed increases in H-lignin levels in 4-coumarate:CoA-ligase (4CL) knockdowns. Moreover, competition for 4-coumarate:CoA-ligase (4CL) is essential for matching the model to observed increases in the pathway metabolites in caffeic acid O-methyltransferase (COMT) knockdowns. As far as possible, the model was validated with independent data.

Pinoresinol reductase 1 impacts lignin distribution during secondary cell wall biosynthesis in Arabidopsis

Pinoresinol reductase (PrR) catalyzes the conversion of the lignan (-)-pinoresinol to (-)-lariciresinol in Arabidopsis thaliana, where it is encoded by two genes, PrR1 and PrR2, that appear to act redundantly. PrR1 is highly expressed in lignified inflorescence stem tissue, whereas PrR2 expression is barely detectable in stems. Co-expression analysis has indicated that PrR1 is co-expressed with many characterized genes involved in secondary cell wall biosynthesis, whereas PrR2 expression clusters with a different set of genes. The promoter of the PrR1 gene is regulated by the secondary cell wall related transcription factors SND1 and MYB46. The loss-of-function mutant of PrR1 shows, in addition to elevated levels of pinoresinol, significantly decreased lignin content and a slightly altered lignin structure with lower abundance of cinnamyl alcohol end groups. Stimulated Raman scattering (SRS) microscopy analysis indicated that the lignin content of the prr1-1 loss-of-function mutant is similar to that of wild-type plants in xylem cells, which exhibit a normal phenotype, but is reduced in the fiber cells. Together, these data suggest an association of the lignan biosynthetic enzyme encoded by PrR1 with secondary cell wall biosynthesis in fiber cells.

Synthesis and quantitative analysis of plasma-targeted metabolites of catechin and epicatechin

Grape seed polyphenolic extract (GSPE) rich in the flavan-3-ols (+)-catechin and (-)-epicatechin beneficially modulates Alzheimer's Disease phenotypes in animal models. The parent molecules in the extract are converted to a series of methylated and glucuronidated derivatives. To fully characterize these metabolites and establish a robust quantitative assay of their levels in biological fluids, we have implemented a partial synthetic approach utilizing chemical methylation followed by enzymatic glucuronidation. Liquid chromatography/time-of-flight mass spectrometry (LC-TOF-MS) and nuclear magnetic resonance (NMR) spectroscopy were used to assign unequivocal structures to the compounds. An analytical method using solid-phase extraction and LC-MS/MS in selective reaction monitoring mode (SRM) was validated for their quantitation in plasma. These studies provide a basis for improvements in future work on the bioavailability, metabolism, and mechanism of action of metabolites derived from dietary flavan-3-ols in a range of interventions.

Loss of function of folylpolyglutamate synthetase 1 reduces lignin content and improves cell wall digestibility in Arabidopsis

BACKGROUND

One-carbon (C1) metabolism is important for synthesizing a range of biologically important compounds that are essential for life. In plants, the C1 pathway is crucial for the synthesis of a large number of secondary metabolites, including lignin. Tetrahydrofolate and its derivatives, collectively referred to as folates, are crucial co-factors for C1 metabolic pathway enzymes. Given the link between the C1 and phenylpropanoid pathways, we evaluated whether folylpolyglutamate synthetase (FPGS), an enzyme that catalyzes the addition of a glutamate tail to folates to form folylpolyglutamates, can be a viable target for reducing cell wall recalcitrance in plants.

RESULTS

Consistent with its role in lignocellulosic formation, FPGS1 was preferentially expressed in vascular tissues. Total lignin was low in fpgs1 plants leading to higher saccharification efficiency of the mutant. The decrease in total lignin in fpgs1 was mainly due to lower guaiacyl (G) lignin levels. Glycome profiling revealed subtle alterations in the cell walls of fpgs1. Further analyses of hemicellulosic polysaccharides by NMR showed that the degree of methylation of 4-O-methyl glucuronoxylan was reduced in the fpgs1 mutant. Microarray analysis and real-time qRT-PCR revealed that transcripts of a number of genes in the C1 and lignin pathways had altered expression in fpgs1 mutants. Consistent with the transcript changes of C1-related genes, a significant reduction in S-adenosyl-l-methionine content was detected in the fpgs1 mutant. The modified expression of the various methyltransferases and lignin-related genes indicate possible feedback regulation of C1 pathway-mediated lignin biosynthesis.

CONCLUSIONS

Our observations provide genetic and biochemical support for the importance of folylpolyglutamates in the lignocellulosic pathway and reinforces previous observations that targeting a single FPGS isoform for down-regulation leads to reduced lignin in plants. Because fpgs1 mutants had no dramatic defects in above ground biomass, selective down-regulation of individual components of C1 metabolism is an approach that should be explored further for the improvement of lignocellulosic feedstocks.

A deep transcriptomic analysis of pod development in the vanilla orchid (Vanilla planifolia)

BACKGROUND

Pods of the vanilla orchid (Vanilla planifolia) accumulate large amounts of the flavor compound vanillin (3-methoxy, 4-hydroxy-benzaldehyde) as a glucoside during the later stages of their development. At earlier stages, the developing seeds within the pod synthesize a novel lignin polymer, catechyl (C) lignin, in their coats. Genomic resources for determining the biosynthetic routes to these compounds and other flavor components in V. planifolia are currently limited.

RESULTS

Using next-generation sequencing technologies, we have generated very large gene sequence datasets from vanilla pods at different times of development, and representing different tissue types, including the seeds, hairs, placental and mesocarp tissues. This developmental series was chosen as being the most informative for interrogation of pathways of vanillin and C-lignin biosynthesis in the pod and seed, respectively. The combined 454/Illumina RNA-seq platforms provide both deep sequence coverage and high quality de novo transcriptome assembly for this non-model crop species.

CONCLUSIONS

The annotated sequence data provide a foundation for understanding multiple aspects of the biochemistry and development of the vanilla bean, as exemplified by the identification of candidate genes involved in lignin biosynthesis. Our transcriptome data indicate that C-lignin formation in the seed coat involves coordinate expression of monolignol biosynthetic genes with the exception of those encoding the caffeoyl coenzyme A 3-O-methyltransferase for conversion of caffeoyl to feruloyl moieties. This database provides a general resource for further studies on this important flavor species.

Two-year field analysis of reduced recalcitrance transgenic switchgrass

Switchgrass (Panicum virgatum L.) is a leading candidate for a dedicated lignocellulosic biofuel feedstock owing to its high biomass production, wide adaptation and low agronomic input requirements. Lignin in cell walls of switchgrass, and other lignocellulosic feedstocks, severely limits the accessibility of cell wall carbohydrates to enzymatic breakdown into fermentable sugars and subsequently biofuels. Low-lignin transgenic switchgrass plants produced by the down-regulation of caffeic acid O-methyltransferase (COMT), a lignin biosynthetic enzyme, were analysed in the field for two growing seasons. COMT transcript abundance, lignin content and the syringyl/guaiacyl lignin monomer ratio were consistently lower in the COMT-down-regulated plants throughout the duration of the field trial. In general, analyses with fully established plants harvested during the second growing season produced results that were similar to those observed in previous greenhouse studies with these plants. Sugar release was improved by up to 34% and ethanol yield by up to 28% in the transgenic lines relative to controls. Additionally, these results were obtained using senesced plant material harvested at the end of the growing season, compared with the young, green tissue that was used in the greenhouse experiments. Another important finding was that transgenic plants were not more susceptible to rust (Puccinia emaculata). The results of this study suggest that lignin down-regulation in switchgrass can confer real-world improvements in biofuel yield without negative consequences to biomass yield or disease susceptibility.

Structural studies of cinnamoyl-CoA reductase and cinnamyl-alcohol dehydrogenase, key enzymes of monolignol biosynthesis

The enzymes cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) catalyze the two key reduction reactions in the conversion of cinnamic acid derivatives into monolignol building blocks for lignin polymers in plant cell walls. Here, we describe detailed functional and structural analyses of CCRs from Medicago truncatula and Petunia hybrida and of an atypical CAD (CAD2) from M. truncatula. These enzymes are closely related members of the short-chain dehydrogenase/reductase (SDR) superfamily. Our structural studies support a reaction mechanism involving a canonical SDR catalytic triad in both CCR and CAD2 and an important role for an auxiliary cysteine unique to CCR. Site-directed mutants of CAD2 (Phe226Ala and Tyr136Phe) that enlarge the phenolic binding site result in a 4- to 10-fold increase in activity with sinapaldehyde, which in comparison to the smaller coumaraldehyde and coniferaldehyde substrates is disfavored by wild-type CAD2. This finding demonstrates the potential exploitation of rationally engineered forms of CCR and CAD2 for the targeted modification of monolignol composition in transgenic plants. Thermal denaturation measurements and structural comparisons of various liganded and unliganded forms of CCR and CAD2 highlight substantial conformational flexibility of these SDR enzymes, which plays an important role in the establishment of catalytically productive complexes of the enzymes with their NADPH and phenolic substrates.

MYB5 and MYB14 Play Pivotal Roles in Seed Coat Polymer Biosynthesis in Medicago truncatula

In Arabidopsis (Arabidopsis thaliana), the major MYB protein regulating proanthocyanidin (PA) biosynthesis is TT2, named for the transparent testa phenotype of tt2 mutant seeds that lack PAs in their coats. In contrast, the MYB5 transcription factor mainly regulates seed mucilage biosynthesis and trichome branching, with only a minor role in PA biosynthesis. We here characterize MYB5 and MYB14 (a TT2 homolog) in the model legume Medicago truncatula. Overexpression of MtMYB5 or MtMYB14 strongly induces PA accumulation in M. truncatula hairy roots, and both myb5 and myb14 mutants of M. truncatula exhibit darker seed coat color than wild-type plants, with myb5 also showing deficiency in mucilage biosynthesis. myb5 mutant seeds have a much stronger seed color phenotype than myb14. The myb5 and myb14 mutants accumulate, respectively, about 30% and 50% of the PA content of wild-type plants, and PA levels are reduced further in myb5 myb14 double mutants. Transcriptome analyses of overexpressing hairy roots and knockout mutants of MtMYB5 and MtMYB14 indicate that MtMYB5 regulates a broader set of genes than MtMYB14. Moreover, we demonstrate that MtMYB5 and MtMYB14 physically interact and synergistically activate the promoters of anthocyanidin reductase and leucoanthocyanidin reductase, the key structural genes leading to PA biosynthesis, in the presence of MtTT8 and MtWD40-1. Our results provide new insights into the complex regulation of PA and mucilage biosynthesis in M. truncatula.

Biosynthesis of Isoflavonoid Phytoalexins: Incorporation of Sodium [1,2-13C2] Acetate into Phaseollin and Kievitone

13C-NMR analysis of the isoflavonoid phytoalexins phaseollin and kievitone produced by feeding sodium [1,2-13C2]acetate to wounded bean (Phaseolus vulgaris) cotyledons has demonstrated the incorporation of intact acetate units into the aromatic A rings. Phaseollin shows a specific folding of the polyketide chain, whereas kievitone exhibits a randomisation of label in accordance with the intermediacy of a 2′,4′,6′-trihydroxylated chalcone during its formation. In neither case was sufficient label incorporated into analysis.

Lignin valorization: improving lignin processing in the biorefinery

Research and development activities directed toward commercial production of cellulosic ethanol have created the opportunity to dramatically increase the transformation of lignin to value-added products. Here, we highlight recent advances in this lignin valorization effort. Discovery of genetic variants in native populations of bioenergy crops and direct manipulation of biosynthesis pathways have produced lignin feedstocks with favorable properties for recovery and downstream conversion. Advances in analytical chemistry and computational modeling detail the structure of the modified lignin and direct bioengineering strategies for future targeted properties. Refinement of biomass pretreatment technologies has further facilitated lignin recovery, and this coupled with genetic engineering will enable new uses for this biopolymer, including low-cost carbon fibers, engineered plastics and thermoplastic elastomers, polymeric foams, fungible fuels, and commodity chemicals.

Early lignin pathway enzymes and routes to chlorogenic acid in switchgrass (Panicum virgatum L.)

Studying lignin biosynthesis in Panicum virgatum (switchgrass) has provided a basis for generating plants with reduced lignin content and increased saccharification efficiency. Chlorogenic acid (CGA, caffeoyl quinate) is the major soluble phenolic compound in switchgrass, and the lignin and CGA biosynthetic pathways potentially share intermediates and enzymes. The enzyme hydroxycinnamoyl-CoA: quinate hydroxycinnamoyltransferase (HQT) is responsible for CGA biosynthesis in tobacco, tomato and globe artichoke, but there are no close orthologs of HQT in switchgrass or in other monocotyledonous plants with complete genome sequences. We examined available transcriptomic databases for genes encoding enzymes potentially involved in CGA biosynthesis in switchgrass. The protein products of two hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyltransferase (HCT) genes (PvHCT1a and PvHCT2a), closely related to lignin pathway HCTs from other species, were characterized biochemically and exhibited the expected HCT activity, preferring shikimic acid as acyl acceptor. We also characterized two switchgrass coumaroyl shikimate 3'-hydroxylase (C3'H) enzymes (PvC3'H1 and PvC3'H2); both of these cytochrome P450s had the capacity to hydroxylate 4-coumaroyl shikimate or 4-coumaroyl quinate to generate caffeoyl shikimate or CGA. Another switchgrass hydroxycinnamoyl transferase, PvHCT-Like1, is phylogenetically distant from HCTs or HQTs, but exhibits HQT activity, preferring quinic acid as acyl acceptor, and could therefore function in CGA biosynthesis. The biochemical features of the recombinant enzymes, the presence of the corresponding activities in plant protein extracts, and the expression patterns of the corresponding genes, suggest preferred routes to CGA in switchgrass.

Early lignin pathway enzymes and routes to chlorogenic acid in switchgrass (Panicum virgatum L.)

Studying lignin biosynthesis in Panicum virgatum (switchgrass) has provided a basis for generating plants with reduced lignin content and increased saccharification efficiency. Chlorogenic acid (CGA, caffeoyl quinate) is the major soluble phenolic compound in switchgrass, and the lignin and CGA biosynthetic pathways potentially share intermediates and enzymes. The enzyme hydroxycinnamoyl-CoA: quinate hydroxycinnamoyltransferase (HQT) is responsible for CGA biosynthesis in tobacco, tomato and globe artichoke, but there are no close orthologs of HQT in switchgrass or in other monocotyledonous plants with complete genome sequences. We examined available transcriptomic databases for genes encoding enzymes potentially involved in CGA biosynthesis in switchgrass. The protein products of two hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyltransferase (HCT) genes (PvHCT1a and PvHCT2a), closely related to lignin pathway HCTs from other species, were characterized biochemically and exhibited the expected HCT activity, preferring shikimic acid as acyl acceptor. We also characterized two switchgrass coumaroyl shikimate 3'-hydroxylase (C3'H) enzymes (PvC3'H1 and PvC3'H2); both of these cytochrome P450s had the capacity to hydroxylate 4-coumaroyl shikimate or 4-coumaroyl quinate to generate caffeoyl shikimate or CGA. Another switchgrass hydroxycinnamoyl transferase, PvHCT-Like1, is phylogenetically distant from HCTs or HQTs, but exhibits HQT activity, preferring quinic acid as acyl acceptor, and could therefore function in CGA biosynthesis. The biochemical features of the recombinant enzymes, the presence of the corresponding activities in plant protein extracts, and the expression patterns of the corresponding genes, suggest preferred routes to CGA in switchgrass.

Lignin modification leads to increased nodule numbers in alfalfa

Reduction of lignin levels in the forage legume alfalfa (Medicago sativa) by down-regulation of the monolignol biosynthetic enzyme hydroxycinnamoyl coenzyme A:shikimate hydroxycinnamoyl transferase (HCT) results in strongly increased digestibility and processing ability of lignocellulose. However, these modifications are often also associated with dwarfing and other changes in plant growth. Given the importance of nitrogen fixation for legume growth, we evaluated the impact of constitutively targeted lignin modification on the belowground organs (roots and nodules) of alfalfa plants. HCT down-regulated alfalfa plants exhibit a striking reduction in root growth accompanied by an unexpected increase in nodule numbers when grown in the greenhouse or in the field. This phenotype is associated with increased levels of gibberellins and certain flavonoid compounds in roots. Although HCT down-regulation reduced biomass yields in both the greenhouse and field experiments, the impact on the allocation of nitrogen to shoots or roots was minimal. It is unlikely, therefore, that the altered growth phenotype of reduced-lignin alfalfa is a direct result of changes in nodulation or nitrogen fixation efficiency. Furthermore, HCT down-regulation has no measurable effect on carbon allocation to roots in either greenhouse or 3-year field trials.

Altering the cell wall and its impact on plant disease: from forage to bioenergy

The individual sugars found within the major classes of plant cell wall polymers are dietary components of herbivores and are targeted for release in industrial processes for fermentation to liquid biofuels. With a growing understanding of the biosynthesis of the complex cell wall polymers, genetic modification strategies are being developed to target the cell wall to improve the digestibility of forage crops and to render lignocellulose less recalcitrant for bioprocessing. This raises concerns as to whether altering cell wall properties to improve biomass processing traits may inadvertently make plants more susceptible to diseases and pests. Here, we review the impacts of cell wall modification on plant defense, as assessed from studies in model plants utilizing mutants or transgenic modification and in crop plants specifically engineered for improved biomass or bioenergy traits. Such studies reveal that cell wall modifications can indeed have unintended impacts on plant defense, but these are not always negative.

A genomics approach to deciphering lignin biosynthesis in switchgrass

It is necessary to overcome recalcitrance of the biomass to saccharification (sugar release) to make switchgrass (Panicum virgatum) economically viable as a feedstock for liquid biofuels. Lignin content correlates negatively with sugar release efficiency in switchgrass, but selecting the right gene candidates for engineering lignin biosynthesis in this tetraploid outcrossing species is not straightforward. To assist this endeavor, we have used an inducible switchgrass cell suspension system for studying lignin biosynthesis in response to exogenous brassinolide. By applying a combination of protein sequence phylogeny with whole-genome microarray analyses of induced cell cultures and developing stem internode sections, we have generated a list of candidate monolignol biosynthetic genes for switchgrass. Several genes that were strongly supported through our bioinformatics analysis as involved in lignin biosynthesis were confirmed by gene silencing studies, in which lignin levels were reduced as a result of targeting a single gene. However, candidate genes encoding enzymes involved in the early steps of the currently accepted monolignol biosynthesis pathway in dicots may have functionally redundant paralogues in switchgrass and therefore require further evaluation. This work provides a blueprint and resources for the systematic genome-wide study of the monolignol pathway in switchgrass, as well as other C4 monocot species.

Laccase is necessary and nonredundant with peroxidase for lignin polymerization during vascular development in Arabidopsis

The evolution of lignin biosynthesis was critical in the transition of plants from an aquatic to an upright terrestrial lifestyle. Lignin is assembled by oxidative polymerization of two major monomers, coniferyl alcohol and sinapyl alcohol. Although two recently discovered laccases, LAC4 and LAC17, have been shown to play a role in lignin polymerization in Arabidopsis thaliana, disruption of both genes only leads to a relatively small change in lignin content and only under continuous illumination. Simultaneous disruption of LAC11 along with LAC4 and LAC17 causes severe plant growth arrest, narrower root diameter, indehiscent anthers, and vascular development arrest with lack of lignification. Genome-wide transcript analysis revealed that all the putative lignin peroxidase genes are expressed at normal levels or even higher in the laccase triple mutant, suggesting that lignin laccase activity is necessary and nonredundant with peroxidase activity for monolignol polymerization during plant vascular development. Interestingly, even though lignin deposition in roots is almost completely abolished in the lac11 lac4 lac17 triple mutant, the Casparian strip, which is lignified through the activity of peroxidase, is still functional. Phylogenetic analysis revealed that lignin laccase genes have no orthologs in lower plant species, suggesting that the monolignol laccase genes diverged after the evolution of seed plants.

Abstract 053: Carbaprostacyclin Promotes Vascular Growth in Mouse Hindlimb Ischemia

Background: Prostacyclin and its stable analogues are vascular protective agents that have been used clinically to reduce blood vessel constriction. Recent studies indicate they stimulate angiogenesis in response to tissue ischemia, but their role in arteriolar growth remains unclear. In the current study, we test the hypothesis that the prostacyclin stable analogue carbaprostacyclin (cPGI2) facilitates arteriolar growth in ischemic hindlimbs.

Methods and Results: Catheters were attached to osmotic pumps in order to continuously deliver either cPGI2 or saline (control) locally into ischemic mouse hindlimbs for 7 and 14 days (n=5/group). At 7 days of cPGI2 delivery, live microscopic images showed more distinct structural remodeling at the arteriolar level in the cPGI2 group than in the saline group, including tortuosity of arteriolar-to-arteriolar connections and increased intersecting of adjacent arterioles. Vascular casting with a silicone radiopaque agent into the distal region of the infrarenal abdominal aorta revealed a more pronounced arterial anastomoses in the cPGI2 treated versus the saline treated ischemic legs. After immunofluorescence staining, laser scanning confocal microscopic analysis found more transmembrane proteoglycan NG2 positive vessels in cross sections of anterior thigh muscles in cPGI2-treated mice than in saline-treated mice. At 14 days, quantitative micro-computed tomography analysis indicated cPGI2-treated legs had markedly increased vascular volume (41.28 ± 2.22 vs 27.11 ± 2.85 mm3), significantly more blood vessels (0.16 ± 0.014 vs 0.09± 0.011 1/mm) and less distance between vessels (6.60 ± 0.52 vs 10.15 ±1.14 mm) than did saline-treated legs. To further verify the pro-arteriogenic effect of cPGI2, a quantitative histogram was generated and exhibited that cPGI2-treated ischemic legs have a significant increase in small vessels, with vessel diameter bins ranging from 40-60 microns. We similarly evaluated contralateral nonischemic legs and found no significant differences in vessel distribution in the cPGI2 and saline groups.

Conclusion: Multi-imaging technical analyses indicated that local delivery of carbaprostacylin promotes arteriolar growth in mouse hindlimb ischemia.

Characterization of the formation of branched short-chain fatty acid:CoAs for bitter acid biosynthesis in hop glandular trichomes

Bitter acids, known for their use as beer flavoring and for their diverse biological activities, are predominantly formed in hop (Humulus lupulus) glandular trichomes. Branched short-chain acyl-CoAs (e.g. isobutyryl-CoA, isovaleryl-CoA and 2-methylbutyryl-CoA), derived from the degradation of branched-chain amino acids (BCAAs), are essential building blocks for the biosynthesis of bitter acids in hops. However, little is known regarding what components are needed to produce and maintain the pool of branched short-chain acyl-CoAs in hop trichomes. Here, we present several lines of evidence that both CoA ligases and thioesterases are likely involved in bitter acid biosynthesis. Recombinant HlCCL2 (carboxyl CoA ligase) protein had high specific activity for isovaleric acid as a substrate (K cat /K m = 4100 s(-1) M(-1)), whereas recombinant HlCCL4 specifically utilized isobutyric acid (Kcat/K m = 1800 s(-1) M(-1)) and 2-methylbutyric acid (Kcat/K m = 6900 s(-1) M(-1)) as substrates. Both HlCCLs, like hop valerophenone synthase (HlVPS), were expressed strongly in glandular trichomes and localized to the cytoplasm. Co-expression of HlCCL2 and HlCCL4 with HlVPS in yeast led to significant production of acylphloroglucinols (the direct precursors for bitter acid biosynthesis), which further confirmed the biochemical function of these two HlCCLs in vivo. Functional identification of a thioesterase that catalyzed the reverse reaction of CCLs in mitochondria, together with the comprehensive analysis of genes involved BCAA catabolism, supported the idea that cytosolic CoA ligases are required for linking BCAA degradation and bitter acid biosynthesis in glandular trichomes. The evolution and other possible physiological roles of branched short-chain fatty acid:CoA ligases in planta are also discussed.