Redesigning lignin, the aromatic polymer fortifying plant cell walls, to be more amenable to chemical depolymerization can lower the energy required for industrial processing. We have engineered ...poplar trees to introduce ester linkages into the lignin polymer backbone by augmenting the monomer pool with monolignol ferulate conjugates. Herein, we describe the isolation of a transferase gene capable of forming these conjugates and its xylem-specific introduction into poplar. Enzyme kinetics, in planta expression, lignin structural analysis, and improved cell wall digestibility after mild alkaline pretreatment demonstrate that these trees produce the monolignol ferulate conjugates, export them to the wall, and use them during lignification. Tailoring plants to use such conjugates during cell wall biosynthesis is a promising way to produce plants that are designed for deconstruction.
Solar radiation plays an important role in carbon cycling by increasing the decomposition rates of plant litter and soil organic matter (i.e. photodegradation). Previous work suggests that exposure ...to radiation can facilitate microbial decomposition of litter by altering litter chemistry and consequently litter degradability (i.e. photopriming). However, it remains unclear to what extent photopriming contributes to litter decomposition processes and on what timescale photopriming operates. We conducted laboratory experiments to compare the effects of UV photopriming at two temporal scales (months versus days). In one experiment, we found that four months of UV exposure induced a significant but small (3–4%) mass loss in two of three litter species commonly found in California oak savanna; however, UV exposure did not alter litter degradability as measured by microbial respiration in an incubation experiment. We also found that UV exposure had limited effects on lignin and other cell wall structures, but one month of microbial decomposition (in absence of UV exposure) significantly reduced lignin β-aryl ether inter-unit linkages and acetylated xylans. These results indicate that abiotic photodegradation alone was ineffective at breaking down lignin. In another experiment, litter of a common grass was exposed to either alternating UV radiation and dark conditions or constant darkness for 128days. We found that the alternating UV exposure increased litter CO2 production in both dark and UV phases over that observed in constant darkness. This led to a 35% greater release of CO2 from the alternating UV exposure treatment between days 65 and 128 of the experiment. These results demonstrate that alternating UV exposure with dark conditions is key to enabling photopriming on a timescale of days. Overall, we identify short-term photopriming as a novel mechanism behind photodegradation. Our results also challenge the conventional hypothesis that abiotic processes are primarily responsible for degrading lignin during photodegradation.
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•Four months of continuous UV exposure increased mass loss by 3–4%.•Four months of continuous UV exposure did not affect litter degradability.•Abiotic photodegradation had limited effects on lignin chemistry.•Microbial decomposition explained UV-induced lignin changes seen in the field.•UV exposure facilitated microbial decomposition of litter on a timescale of days.
Summary
Tricin 5,7‐dihydroxy‐2‐(4‐hydroxy‐3,5‐dimethoxyphenyl)‐4H‐chromen‐4‐one, a flavone, was recently established as an authentic monomer in grass lignification that likely functions as a ...nucleation site. It is linked onto lignin as an aryl alkyl ether by radical coupling with monolignols or their acylated analogs. However, the level of tricin that incorporates into lignin remains unclear. Herein, three lignin characterization methods: acidolysis; thioacidolysis; and derivatization followed by reductive cleavage; were applied to quantitatively assess the amount of lignin‐integrated tricin. Their efficiencies at cleaving the tricin‐(4′–O–β)‐ether bonds and the degradation of tricin under the corresponding reaction conditions were evaluated. A hexadeuterated tricin analog was synthesized as an internal standard for accurate quantitation purposes. Thioacidolysis proved to be the most efficient method, liberating more than 91% of the tricin with little degradation. A survey of different seed‐plant species for the occurrence and content of tricin showed that it is widely distributed in the lignin from species in the family Poaceae (order Poales). Tricin occurs at low levels in some commelinid monocotyledon families outside the Poaceae, such as the Arecaceae (the palms, order Arecales) and Bromeliaceae (Poales), and the non‐commelinid monocotyledon family Orchidaceae (Orchidales). One eudicotyledon was found to have tricin (Medicago sativa, Fabaceae). The content of lignin‐integrated tricin is much higher than the extractable tricin level in all cases. Lignins, including waste lignin streams from biomass processing, could therefore provide a large and alternative source of this valuable flavone, reducing the costs, and encouraging studies into its application beyond its current roles.
Significance Statement
Tricin, a flavone with human health benefits, is covalently linked to lignin in monocots. Here we developed a chemical degradative method to quantify lignin‐integrated tricin. If methods to economically cleave tricin from lignin are developed, waste lignin streams from biomass processing could be used as an alternative source of tricin.
As interest in biomass utilization has grown, the manipulation of lignin biosynthesis has received significant attention, such that recent work has demanded more robust lignin analytical methods. As ...the derivatization followed by reductive cleavage (DFRC) method is particularly effective for structurally characterizing natively acylated lignins, we used an array of synthetic β‐ether γ‐acylated model compounds to determine theoretical yields for all monolignol conjugates currently known to exist in lignin, and we synthesized a new set of deuterated analogs as internal standards for quantification using GC–MS/MS. Yields of the saturated ester conjugates ranged from 40 to 90 %, and NMR analysis revealed the presence of residual unsaturated conjugates in yields of 20 to 35 %. In contrast to traditional selected‐ion‐monitoring, we demonstrated the superior sensitivity and accuracy of multiple‐reaction‐monitoring detection methods, and further highlighted the inadequacy of traditional standards relative to isotopically labeled analogs.
More than meets the eye: As plant breeding programs embrace the idea of the “biorefinery”, lignin has come to be seen as a feedstock for commodity chemicals. Without robust analytics capable of describing lignin's chemical structure, its true value will remain hidden. To this end, a powerful analytical method was developed using synthetic methodology and isotopic labeling to identify new and low‐abundance monolignols and their conjugates.
Summary
p‐Coumaroyl ester 3‐hydroxylase (C3′H) is a key enzyme involved in the biosynthesis of lignin, a phenylpropanoid polymer that is the major constituent of secondary cell walls in vascular ...plants. Although the crucial role of C3′H in lignification and its manipulation to upgrade lignocellulose have been investigated in eudicots, limited information is available in monocotyledonous grass species, despite their potential as biomass feedstocks. Here we address the pronounced impacts of C3′H deficiency on the structure and properties of grass cell walls. C3′H‐knockdown lines generated via RNA interference (RNAi)‐mediated gene silencing, with about 0.5% of the residual expression levels, reached maturity and set seeds. In contrast, C3′H‐knockout rice mutants generated via CRISPR/Cas9‐mediated mutagenesis were severely dwarfed and sterile. Cell wall analysis of the mature C3′H‐knockdown RNAi lines revealed that their lignins were largely enriched in p‐hydroxyphenyl (H) units while being substantially reduced in the normally dominant guaiacyl (G) and syringyl (S) units. Interestingly, however, the enrichment of H units was limited to within the non‐acylated lignin units, with grass‐specific γ‐p‐coumaroylated lignin units remaining apparently unchanged. Suppression of C3′H also resulted in relative augmentation in tricin residues in lignin as well as a substantial reduction in wall cross‐linking ferulates. Collectively, our data demonstrate that C3′H expression is an important determinant not only of lignin content and composition but also of the degree of cell wall cross‐linking. We also demonstrated that C3′H‐suppressed rice displays enhanced biomass saccharification.
Significance Statement
Suppression of a gene encoding p‐coumaroyl ester 3‐hydroxylase in rice resulted in altered lignins largely enriched in p‐hydroxyphenyl units, augmentation of tricin residues in lignin, and substantial reduction in wall cross‐linking ferulates. Such structural alterations reduced cell wall recalcitrance and enhanced cell wall saccharification to produce fermentable sugars from rice lignocellulose.
Operons are a hallmark of bacterial genomes, where they allow concerted expression of functionally related genes as single polycistronic transcripts. They are rare in eukaryotes, where each gene ...usually drives expression of its own independent messenger RNAs. Here, we report the horizontal operon transfer of a siderophore biosynthesis pathway from relatives of Escherichia coli into a group of budding yeast taxa. We further show that the co-linearly arranged secondary metabolism genes are expressed, exhibit eukaryotic transcriptional features, and enable the sequestration and uptake of iron. After transfer, several genetic changes occurred during subsequent evolution, including the gain of new transcription start sites that were sometimes within protein-coding sequences, acquisition of polyadenylation sites, structural rearrangements, and integration of eukaryotic genes into the cluster. We conclude that the genes were likely acquired as a unit, modified for eukaryotic gene expression, and maintained by selection to adapt to the highly competitive, iron-limited environment.
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•Some yeast species harbor enterobactin biosynthesis genes unseen in other fungi•An ancient horizontal operon transfer occurred from bacteria to a group of yeasts•The operon underwent genetic changes after transfer to a eukaryotic genome•The genes are expressed monocistronically and result in enterobactin production
Horizontal gene transfer of a full operon encoding siderophore biosynthesis genes from bacteria to a group of budding yeasts was followed by acquisition of eukaryotic genomic and transcriptional features.
Many soils in the semiarid Mediterranean Ebro Valley of Spain are prone to physical and chemical degradation due to their silty texture, low organic matter content, and presence of carbonates, gypsum ...or other soluble salts. Rainfed agriculture on these soils is also hindered by the scarcity of water. No-tillage can increase plant-available water and soil organic matter, thus helping overcome most factors limiting crop production in this area. Our objective was to determine how conventional- and no-tillage practices affected soil physical quality indicators and water availability in an on-farm study in the Ebro Valley. Soil samples were collected from 0 to 5-, 5 to 15-, and 15 to 30-cm depth increments within adjacent farmer-managed conventional- and no-tillage fields in 2007 and 2008. Both fields were managed for continuous barley (
Hordeum vulgare L.) production. The soil at both sites is a silt loam (
Haplic Calcisol). Aggregate-size distribution and stability, soil water retention characteristics
, organic carbon, and total carbonates were determined in 2007. Pore-size distribution was estimated from the water retention curve. Penetration resistance, soil bulk density and field water content during the entire crop growing season were measured for both fields in 2008. Aggregate dry mean weight diameter and stability in water were 1.2 and 2.2 times greater, respectively, under no-tillage than conventional tillage due to reduced mechanical disturbance and increased soil organic carbon content. Bulk density was 1.12 times greater (
P
<
0.1) under no-tillage only in the 0–5-cm depth. Two times greater penetration resistance to a depth of 15
cm in this treatment was related to bulk density and aggregates stability. Field water content was greater with no-tillage than conventional tillage during the driest months in 2008. The volume of equivalent diameter pores (0.2–9
μm) was 1.5 times higher under no-tillage. This increased plant-available water content and doubled barley production under no-tillage in 2008, which was a very dry year. We conclude that despite the greater penetration resistance under no-tillage, increased water availability as a result of improved structure characteristics was more important for crop yield. This suggests that producers should seriously consider adopting no-tillage practices for soil conservation in semiarid degraded areas like the one studied.
Quantum entangled states can be very delicate and easily perturbed by their external environment. This sensitivity can be harnessed in measurement technology to create a quantum sensor with a ...capability of outperforming conventional devices at a fundamental level. We compared the magnetic field sensitivity of a classical (unentangled) system with that of a 10-qubit entangled state, realized by nuclei in a highly symmetric molecule. We observed a 9.4-fold quantum enhancement in the sensitivity to an applied field for the entangled system and show that this spin-based approach can scale favorably as compared with approaches in which qubit loss is prevalent. This result demonstrates a method for practical quantum field sensing technology.
Grass lignins contain substantial amounts of p‐coumarate (pCA) that acylate the side‐chains of the phenylpropanoid polymer backbone. An acyltransferase, named p‐coumaroyl‐CoA:monolignol transferase ...(OsPMT), that could acylate monolignols with pCA in vitro was recently identified from rice. In planta, such monolignol‐pCA conjugates become incorporated into lignin via oxidative radical coupling, thereby generating the observed pCA appendages; however p‐coumarates also acylate arabinoxylans in grasses. To test the authenticity of PMT as a lignin biosynthetic pathway enzyme, we examined Brachypodium distachyon plants with altered BdPMT gene function. Using newly developed cell wall analytical methods, we determined that the transferase was involved specifically in monolignol acylation. A sodium azide‐generated Bdpmt‐1 missense mutant had no (<0.5%) residual pCA on lignin, and BdPMT RNAi plants had levels as low as 10% of wild‐type, whereas the amounts of pCA acylating arabinosyl units on arabinoxylans in these PMT mutant plants remained unchanged. pCA acylation of lignin from BdPMT‐overexpressing plants was found to be more than three‐fold higher than that of wild‐type, but again the level on arabinosyl units remained unchanged. Taken together, these data are consistent with a defined role for grass PMT genes in encoding BAHD (BEAT, AHCT, HCBT, and DAT) acyltransferases that specifically acylate monolignols with pCA and produce monolignol p‐coumarate conjugates that are used for lignification in planta.
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