Streamlining assays of glycosyltransferases activity using in vitro GT-array (i-GT-ray) platform: Application to family GT37 fucosyltransferases

Numerous putative glycosyltransferases (GTs) have been identified using bioinformatic approaches. However, demonstrating the activity of these GTs remains a challenge. Here, we describe the development of a rapid in vitro GT-array screening platform for activity of GTs. GT-arrays are generated by cell-free in vitro protein synthesis and binding using microplates precoated with a N-terminal Halo- or a C-terminal GST-tagged GT-encoding plasmid DNA and a capture antibody. These arrays are then used for screening of transferase activities and the reactions are monitored by a luminescence GLO assay. The products formed by these reactions can be analyzed directly from the microplates by mass spectrometry. Using this platform, a total of 280 assays were performed to screen 22 putative fucosyltransferases (FUTs) from family GT37 (seven from Arabidopsis and 15 from rice) for activity toward five acceptors: non-fucosylated tamarind xyloglucan (TXyG), arabinotriose (Ara3), non-fucosylated rhamnogalacturonan I (RG-I), and RG-II from the mur1-1 Arabidopsis mutant, and the celery RG-II monomer lacking Arap and MeFuc of chain B and l-Gal of chain A. Our screen showed that AtFUT2, AtFUT5, and AtFUT10 have activity toward RG-I, while AtFUT8 was active on RG-II. Five rice OsFUTs have XyG-FUT activity and four rice OsFUTs have activity toward Ara3. None of the putative OsFUTs were active on the RG-I and RG-II. However, promiscuity toward acceptors was observed for several FUTs. These findings extend our knowledge of cell wall polysaccharide fucosylation in plants. We believe that in vitro GT-array platform provides a valuable tool for cell wall biochemistry and other research fields.

Broad Specific Xyloglucan:Xyloglucosyl Transferases Are Formidable Players in the Re-Modelling of Plant Cell Wall Structures

Plant xyloglucan:xyloglucosyl transferases, known as xyloglucan endo-transglycosylases (XETs) are the key players that underlie plant cell wall dynamics and mechanics. These fundamental roles are central for the assembly and modifications of cell walls during embryogenesis, vegetative and reproductive growth, and adaptations to living environments under biotic and abiotic (environmental) stresses. XET enzymes (EC 2.4.1.207) have the β-sandwich architecture and the β-jelly-roll topology, and are classified in the glycoside hydrolase family 16 based on their evolutionary history. XET enzymes catalyse transglycosylation reactions with xyloglucan (XG)-derived and other than XG-derived donors and acceptors, and this poly-specificity originates from the structural plasticity and evolutionary diversification that has evolved through expansion and duplication. In phyletic groups, XETs form the gene families that are differentially expressed in organs and tissues in time- and space-dependent manners, and in response to environmental conditions. Here, we examine higher plant XET enzymes and dissect how their exclusively carbohydrate-linked transglycosylation catalytic function inter-connects complex plant cell wall components. Further, we discuss progress in technologies that advance the knowledge of plant cell walls and how this knowledge defines the roles of XETs. We construe that the broad specificity of the plant XETs underscores their roles in continuous cell wall restructuring and re-modelling.

CWIN-sugar transporter nexus is a key component for reproductive success

Reproductive development is critical for completion of plant life cycle and realization of crop yield potential. Reproductive organs comprise multiple distinctive or even transgenerational tissues, which are symplasmically disconnected from each other for protection and better control of nutrition and development. Cell wall invertases (CWINs) and sugar transporters are often specifically or abundantly expressed in these apoplasmic interfaces to provide carbon nutrients and sugar signals to developing pollens, endosperm and embryo. Emerging evidence shows that some of those genes were indeed targeted for selection during crop domestication. In this Opinion paper, I discuss the functional significance of the localized expression of CWINs and sugar transporters in reproductive organs followed by an analysis on how their spatial patterning may be regulated at the molecular levels and how the localized CWIN activity may be exploited for improvement of reproductive output.

Cryogenian Origin and Subsequent Diversification of the Plant Cell-Wall Enzyme XTH Family

All land plants encode large multigene families of xyloglucan endotransglucosylase/hydrolases (XTHs), plant-specific enzymes that cleave and reconnect plant cell-wall polysaccharides. Despite the ubiquity of these enzymes, considerable uncertainty remains regarding the evolutionary history of the XTH family. Phylogenomic and comparative analyses in this study traced the non-plant origins of the XTH family to Alphaproteobacteria ExoKs, bacterial enzymes involved in loosening biofilms, rather than Firmicutes licheninases, plant biomass digesting enzymes, as previously supposed. The relevant horizontal gene transfer (HGT) event was mapped to the divergence of non-swimming charophycean algae in the Cryogenian geological period. This HGT event was the likely origin of charophycean EG16-2s, which are putative intermediates between ExoKs and XTHs. Another HGT event in the Cryogenian may have led from EG16-2s or ExoKs to fungal Congo Red Hypersensitive proteins (CRHs) to fungal CRHs, enzymes that cleave and reconnect chitin and glucans in fungal cell walls. This successive transfer of enzyme-encoding genes may have supported the adaptation of plants and fungi to the ancient icy environment by facilitating their sessile lifestyles. Furthermore, several protein evolutionary steps, including coevolution of substrate-interacting residues and putative intra-family gene fusion, occurred in the land plant lineage and drove diversification of the XTH family. At least some of those events correlated with the evolutionary gain of broader substrate specificities, which may have underpinned the expansion of the XTH family by enhancing duplicated gene survival. Together, this study highlights the Precambrian evolution of life and the mode of multigene family expansion in the evolutionary history of the XTH family.

Genome-Wide Identification of the Xyloglucan endotransglucosylase/Hydrolase (XTH) and Polygalacturonase (PG) Genes and Characterization of Their Role in Fruit Softening of Sweet Cherry

Fruit firmness is an important economical trait in sweet cherry (Prunus avium L.) where the change of this trait is related to cell wall degradation. Xyloglucan endotransglycosylase/hydrolase (XTH) and polygalacturonases (PGs) are critical cell-wall-modifying enzymes that occupy a crucial position in fruit ripening and softening. Herein, we identified 18 XTHs and 45 PGs designated PavXTH1-18 and PavPG1-45 based on their locations in the genome of sweet cherry. We provided a systematical overview of PavXTHs and PavPGs, including phylogenetic relationships, conserved motifs, and expression profiling of these genes. The results showed that PavXTH14, PavXTH15 and PavPG38 were most likely to participated in fruit softening owing to the substantial increment in expression during fruit development and ripening. Furthermore, the phytohormone ABA, MeJA, and ethephon significantly elevated the expression of PavPG38 and PavXTH15, and thus promoted fruit softening. Importantly, transient expression PavXTH14, PavXTH15 and PavPG38 in cherry fruits significantly reduced the fruit firmness, and the content of various cell wall components including hemicellulose and pectin significantly changed correspondingly in the transgenic fruit. Taken together, these results present an extensive analysis of XTHs and PGs in sweet cherry and provide potential targets for breeding softening-resistant sweet cherry cultivars via manipulating cell wall-associated genes.

Early Sucrose Degradation and the Dominant Sucrose Cleavage Pattern Influence Lycoris sprengeri Bulblet Regeneration In Vitro

Bulblet formation and development determine the quantitative and qualitative traits, respectively, of bulb yield for most flowering bulbs. For Lycoris species, however, the underlying molecular mechanism remains elusive. Here, clonal bulblets of Lycoris sprengeri (Ls) derived from the same probulb were used as explants to establish efficient and inefficient in vitro regeneration systems by adjusting the 6-benzyladenine (BA) concentrations in media. BA application did not change the biological processes among groups but led to earlier decreases in sucrose and total soluble sugar (TSS) contents. Correlation analyses showed that the BA treatments changed the interaction between carbohydrate and endogenous hormone contents during bulblet regeneration. We found that two sucrose degradation enzyme-related genes, cell wall invertase (CWIN) and sucrose synthase, exhibited exactly opposite expression patterns during the competence stage. In addition, the regeneration system that obtained more bulblets showed significantly higher expression of LsCWIN2 than those that obtained fewer bulblets. Our data demonstrate the essential role of BA in accelerating sucrose degradation and the selection of a dominant sucrose cleavage pattern at the competence stage of in vitro bulblet regeneration. We propose that a relatively active CWIN-catalyzed pathway at the competence stage might promote bulblet regeneration, thus influencing bulb yield.

PtrLAC16 plays a key role in catalyzing lignin polymerization in the xylem cell wall of Populus

Plant laccases have been proposed to participate in lignin biosynthesis. However, there is no direct evidence that individual laccases in Populus can polymerize lignin monomers and alter cell wall structure. Here, a Populus laccase, PtrLAC16, was expressed and purified in a eukaryotic system. Enzymatic analysis of PtrLAC16 showed that it could polymerize lignin monomers in vitro. PtrLAC16 preferred sinapyl alcohol, and this preference is associated with an altered S/G ratio in transgenic Populus lines. PtrLAC16 was localized exclusively in the cell walls of stem vascular tissue, and a reduction in PtrLAC16 expression led to a significant decrease in lignin content and altered cell wall structure. There was a direct correlation between the inhibition of PtrLAC16 expression and structural changes in the stem cell wall of Populus. This study provides direct evidence that PtrLAC16 plays a key role in the polymerization of lignin monomers, especially for sinapyl lignin, and affects the formation of xylem cell walls in Populus.

RNA-Seq analysis reveals potential regulators of programmed cell death and leaf remodelling in lace plant (Aponogeton madagascariensis)

BACKGROUND

The lace plant (Aponogeton madagascariensis) is an aquatic monocot that develops leaves with uniquely formed perforations through the use of a developmentally regulated process called programmed cell death (PCD). The process of perforation formation in lace plant leaves is subdivided into several developmental stages: pre-perforation, window, perforation formation, perforation expansion and mature. The first three emerging "imperforate leaves" do not form perforations, while all subsequent leaves form perforations via developmentally regulated PCD. PCD is active in cells called "PCD cells" that do not retain the antioxidant anthocyanin in spaces called areoles framed by the leaf veins of window stage leaves. Cells near the veins called "NPCD cells" retain a red pigmentation from anthocyanin and do not undergo PCD. While the cellular changes that occur during PCD are well studied, the gene expression patterns underlying these changes and driving PCD during leaf morphogenesis are mostly unknown. We sought to characterize differentially expressed genes (DEGs) that mediate lace plant leaf remodelling and PCD. This was achieved performing gene expression analysis using transcriptomics and comparing DEGs among different stages of leaf development, and between NPCD and PCD cells isolated by laser capture microdissection.

RESULTS

Transcriptomes were sequenced from imperforate, pre-perforation, window, and mature leaf stages, as well as PCD and NPCD cells isolated from window stage leaves. Differential expression analysis of the data revealed distinct gene expression profiles: pre-perforation and window stage leaves were characterized by higher expression of genes involved in anthocyanin biosynthesis, plant proteases, expansins, and autophagy-related genes. Mature and imperforate leaves upregulated genes associated with chlorophyll development, photosynthesis, and negative regulators of PCD. PCD cells were found to have a higher expression of genes involved with ethylene biosynthesis, brassinosteroid biosynthesis, and hydrolase activity whereas NPCD cells possessed higher expression of auxin transport, auxin signalling, aspartyl proteases, cysteine protease, Bag5, and anthocyanin biosynthesis enzymes.

CONCLUSIONS

RNA sequencing was used to generate a de novo transcriptome for A. madagascariensis leaves and revealed numerous DEGs potentially involved in PCD and leaf remodelling. The data generated from this investigation will be useful for future experiments on lace plant leaf development and PCD in planta.

A cell wall invertase controls nectar volume and sugar composition

Nectar volume and sugar composition are key determinants of the strength of plant-pollinator mutualisms. The main nectar sugars are sucrose, glucose and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose-dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa. Moreover, a brcwinv4a null mutant: (i) has greatly reduced CWINV activity in the nectaries; (ii) produces a sucrose-rich nectar; but (iii) with significantly less volume. These results definitively demonstrate that CWINV activity is not only essential for the production of a hexose-rich nectar, but also support a hypothetical model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular-to-extracellular sucrose ratio that facilitates the continuous export of sucrose into the nectary apoplast. The extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. These results cumulatively indicate that modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition. Finally, honeybees prefer nectars with some sucrose, but wild-type B. rapa flowers were much more heavily visited than flowers of brcwinv4a, suggesting that the potentially attractive sucrose-rich nectar of brcwinv4a could not compensate for its low volume.

Functional understanding of secondary cell wall cellulose synthases in Populus trichocarpa via the Cas9/gRNA-induced gene knockouts

Plant cellulose is synthesized by a large plasma membrane-localized cellulose synthase (CesA) complex. However, an overall functional determination of secondary cell wall (SCW) CesAs is still lacking in trees, especially one based on gene knockouts. Here, the Cas9/gRNA-induced knockouts of PtrCesA4, 7A, 7B, 8A and 8B genes were produced in Populus trichocarpa. Based on anatomical, immunohistochemical and wood composition evidence, we gained a comprehensive understanding of five SCW PtrCesAs at the genetic level. Complete loss of PtrCesA4, 7A/B or 8A/B led to similar morphological abnormalities, indicating similar and nonredundant genetic functions. The absence of the gelatinous (G) layer, one-layer-walled fibres and a 90% decrease in cellulose in these mutant woods revealed that the three classes of SCW PtrCesAs are essential for multilayered SCW structure and wood G-fibre. In addition, the mutant primary and secondary phloem fibres lost the n(G + L)- and G-layers and retained the thicker S-layers (L, lignified; S, secondary). Together with polysaccharide immunolocalization data, these findings suggest differences in the role of SCW PtrCesAs-synthesized cellulose in wood and phloem fibre wall structures. Overall, this functional understanding of the SCW PtrCesAs provides further insights into the impact of lacking cellulose biosynthesis on growth, SCW, wood G-fibre and phloem fibre wall structures in the tree.

Polar Growth in Corynebacterium glutamicum Has a Flexible Cell Wall Synthase Requirement

Members of the Corynebacterineae suborder of bacteria, including major pathogens such as Mycobacterium tuberculosis, grow via the insertion of new cell wall peptidoglycan (PG) material at their poles. This mode of elongation differs from that used by Escherichia coli and other more well-studied model organisms that grow by inserting new PG at dispersed sites along their cell body. Dispersed cell elongation is known to strictly require the SEDS-type PG synthase called RodA, whereas the other major class of PG synthases called class A penicillin-binding proteins (aPBPs) are not required for this mode of growth. Instead, they are thought to be important for maintaining the integrity of the PG matrix in organisms growing by dispersed elongation. In contrast, based on prior genetic studies in M. tuberculosis and related members of the Corynebacterineae suborder, the aPBPs are widely believed to be essential for polar growth, with RodA being dispensable. However, polar growth has not been directly assessed in mycobacterial or corynebacterial mutants lacking aPBP-type PG synthases. We therefore investigated the relative roles of aPBPs and RodA in polar growth using Corynebacterium glutamicum as a model member of Corynebacterineae. Notably, we discovered that the aPBPs are dispensable for polar growth and that this growth mode can be mediated by either an aPBP-type or a SEDS-type enzyme functioning as the sole elongation PG synthase. Thus, our results reveal that the mechanism of polar elongation is fundamentally flexible and, unlike dispersed elongation, can be effectively mediated in C. glutamicum by either a SEDS-bPBP or an aPBP-type synthase. IMPORTANCE The Corynebacterineae suborder includes a number of major bacterial pathogens. These organisms grow by polar extension unlike most well-studied model bacteria, which grow by inserting wall material at dispersed sites along their length. A better understanding of polar growth promises to uncover new avenues for targeting mycobacterial and corynebacterial infections. Here, we investigated the roles of the different classes of cell wall synthases for polar growth using Corynebacterium glutamicum as a model. We discovered that the polar growth mechanism is surprisingly flexible in this organism and, unlike dispersed synthesis, can function using either of the two known types of cell wall synthase enzymes.

Identification of transcription factors controlling cell wall invertase gene expression for reproductive development via bioinformatic and transgenic analyses

Cell wall invertase (CWIN) hydrolyses sucrose into glucose and fructose in the extracellular matrix and plays crucial roles in assimilate partitioning and sugar signalling. However, the molecular regulators controlling CWIN gene transcription remain unknown. As the first step to address this issue, we performed bioinformatic and transgenic studies, which identified a cohort of transcription factors (TFs) modulating CWIN gene expression in Arabidopsis thaliana. Comprehensive bioinformatic analyses identified 18 TFs as putative regulators of the expression of AtCWIN2 and AtCWIN4 that are predominantly expressed in Arabidopsis reproductive organs. Among them, MYB21, ARF6, ARF8, AP3 and CRC were subsequently shown to be the most likely regulators of CWIN gene expression based on molecular characterization of the respective mutant of each candidate TF. More specifically, the obtained data indicate that ARF6, ARF8 and MYB21 regulate CWIN2 expression in the anthers and CWIN4 in nectaries, anthers and petals, whereas AP3 and CRC were determined primarily to regulate the transcriptional activity of CWIN4. TF-promoter interaction assays demonstrated that ARF6 and ARF8 directly control CWIN2 and CWIN4 transcription with AP3 activating CWIN4. The involvement of ARF8 in regulating CWIN4 expression was further supported by the finding that enhanced CWIN4 expression partially recovered the short silique phenotype displayed by the arf8-3 mutant. The identification of the five TFs regulating CWIN expression serves as a launching pad for future studies to dissect the upstream molecular network underpinning the transcription of CWINs and provides a new avenue, potentially, to engineer assimilate allocation and reproductive development for improving seed yield.

Waste soybean frying oil for the production, extraction, and characterization of cell-wall-associated lipases from Yarrowia lipolytica

The lipolytic yeast Yarrowia lipolytica produces cell-wall-associated lipases, namely Lip7p and Lip8p, that could have interesting properties as catalyst either in free (released lipase fraction-RLF) or cell-associated (cell-bound lipase fraction-CBLF) forms. Herein, a mixture of waste soybean frying oil, yeast extract and bactopeptone was found to favor the enzyme production. Best parameters for lipase activation and release from the cell wall by means of acoustic wave treatment were defined as: 26 W/cm2 for 1 min for CBLF and 52 W/cm2 for 2 min for RLF. Optimal pH and temperature values for lipase activity together with storage conditions were similar for both the free enzyme and cell-associated one: pH 7.0; T = 37 °C; and > 70% residual activity for 60 days at 4, - 4 °C and for 15 days at 30 °C.

Down-regulation of MANNANASE7 gene in Brassica napus L. enhances silique dehiscence-resistance

MANNANASE7 gene in Brassica napus L. encodes a hemicellulose which located at cell wall or extracellular space and dehiscence-resistance can be manipulated by altering the expression of MANNANASE7. Silique dehiscence is an important physiological process in plant reproductive development, but causes heavy yield loss in crops. The lack of dehiscence-resistant germplasm limits the application of mechanized harvesting and greatly restricts the rapeseed (Brassica napus L.) production. Hemicellulases, together with cellulases and pectinases, play important roles in fruit development and maturation. The hemicellulase gene MANNANASE7 (MAN7) was previously shown to be involved in the development and dehiscence of Arabidopsis (Arabidopsis thaliana) siliques. Here, we cloned BnaA07g12590D (BnMAN7A07), an AtMAN7 homolog from rapeseed, and demonstrate its function in the dehiscence of rapeseed siliques. We found that BnMAN7A07 was expressed in both vegetative and reproductive organs and significantly highly expressed in leaves, flowers and siliques where the abscission or dehiscence process occurs. Subcellular localization experiment showed that BnMAN7A07 was localized in the cell wall. The biological activity of the BnMAN7A07 protein isolated and purified through prokaryotic expression system was verified to catalyse the decomposition of xylan into xylose. Phenotypic studies of RNA interference (RNAi) lines revealed that down-regulation of BnMAN7A07 in rapeseed could significantly enhance silique dehiscence-resistance. In addition, the expression of upstream silique development regulators is altered in BnMAN7A07-RNAi plants, suggesting that a possible feedback regulation mechanism exists in the regulation network of silique dehiscence. Our results demonstrate that dehiscence-resistance can be manipulated by altering the expression of hemicellulase gene BnMAN7A07, which could provide an available genetic resource for breeding practice in rapeseed which is beneficial to mechanized harvest.

Horizontally acquired antibacterial genes associated with adaptive radiation of ladybird beetles

BACKGROUND

Horizontal gene transfer (HGT) has been documented in many herbivorous insects, conferring the ability to digest plant material and promoting their remarkable ecological diversification. Previous reports suggest HGT of antibacterial enzymes may have contributed to the insect immune response and limit bacterial growth. Carnivorous insects also display many evolutionary successful lineages, but in contrast to the plant feeders, the potential role of HGTs has been less well-studied.

RESULTS

Using genomic and transcriptomic data from 38 species of ladybird beetles, we identified a set of bacterial cell wall hydrolase (cwh) genes acquired by this group of beetles. Infection with Bacillus subtilis led to upregulated expression of these ladybird cwh genes, and their recombinantly produced proteins limited bacterial proliferation. Moreover, RNAi-mediated cwh knockdown led to downregulation of other antibacterial genes, indicating a role in antibacterial immune defense. cwh genes are rare in eukaryotes, but have been maintained in all tested Coccinellinae species, suggesting that this putative immune-related HGT event played a role in the evolution of this speciose subfamily of predominant predatory ladybirds.

CONCLUSION

Our work demonstrates that, in a manner analogous to HGT-facilitated plant feeding, enhanced immunity through HGT might have played a key role in the prey adaptation and niche expansion that promoted the diversification of carnivorous beetle lineages. We believe that this represents the first example of immune-related HGT in carnivorous insects with an association with a subsequent successful species radiation.

Plant Xyloglucan Xyloglucosyl Transferases and the Cell Wall Structure: Subtle but Significant

Plant xyloglucan xyloglucosyl transferases or xyloglucan endo-transglycosylases (XET; EC 2.4.1.207) catalogued in the glycoside hydrolase family 16 constitute cell wall-modifying enzymes that play a fundamental role in the cell wall expansion and re-modelling. Over the past thirty years, it has been established that XET enzymes catalyse homo-transglycosylation reactions with xyloglucan (XG)-derived substrates and hetero-transglycosylation reactions with neutral and charged donor and acceptor substrates other than XG-derived. This broad specificity in XET isoforms is credited to a high degree of structural and catalytic plasticity that has evolved ubiquitously in algal, moss, fern, basic Angiosperm, monocot, and eudicot enzymes. These XET isoforms constitute gene families that are differentially expressed in tissues in time- and space-dependent manners during plant growth and development, and in response to biotic and abiotic stresses. Here, we discuss the current state of knowledge of broad specific plant XET enzymes and how their inherently carbohydrate-based transglycosylation reactions tightly link with structural diversity that underlies the complexity of plant cell walls and their mechanics. Based on this knowledge, we conclude that multi- or poly-specific XET enzymes are widespread in plants to allow for modifications of the cell wall structure in muro, a feature that implements the multifaceted roles in plant cells.

The Role of Brachypodium distachyon Wall-Associated Kinases (WAKs) in Cell Expansion and Stress Responses

The plant cell wall plays a critical role in signaling responses to environmental and developmental cues, acting as both the sensing interface and regulator of plant cell integrity. Wall-associated kinases (WAKs) are plant receptor-like kinases located at the wall-plasma membrane-cytoplasmic interface and implicated in cell wall integrity sensing. WAKs in Arabidopsis thaliana have been shown to bind pectins in different forms under various conditions, such as oligogalacturonides (OG)s in stress response, and native pectin during cell expansion. The mechanism(s) WAKs use for sensing in grasses, which contain relatively low amounts of pectin, remains unclear. WAK genes from the model monocot plant, Brachypodium distachyon were identified. Expression profiling during early seedling development and in response to sodium salicylate and salt treatment was undertaken to identify WAKs involved in cell expansion and response to external stimuli. The BdWAK2 gene displayed increased expression during cell expansion and stress response, in addition to playing a potential role in the hypersensitive response. In vitro binding assays with various forms of commercial polysaccharides (pectins, xylans, and mixed-linkage glucans) and wall-extracted fractions (pectic/hemicellulosic/cellulosic) from both Arabidopsis and Brachypodium leaf tissues provided new insights into the binding properties of BdWAK2 and other candidate BdWAKs in grasses. The BdWAKs displayed a specificity for the acidic pectins with similar binding characteristics to the AtWAKs.

Laccases and Peroxidases Co-Localize in Lignified Secondary Cell Walls throughout Stem Development

Lignin, a critical phenolic polymer in secondary cell walls of plant cells, enables strength in fibers and water transportation in xylem vessel elements. Secreted enzymes, namely laccases (LACs) and peroxidases (PRXs), facilitate lignin polymerization by oxidizing lignin monomers (monolignols). Previous work in Arabidopsis (Arabidopsis thaliana) demonstrated that AtLAC4 and AtPRX64 localized to discrete lignified cell wall domains in fibers, although the spatial distributions of other enzymes in these large gene families are unknown. Here, we show that characteristic sets of putative lignin-associated LACs and PRXs localize to precise regions during stem development, with LACs and PRXs co-occurring in cell wall domains. AtLAC4, AtLAC17, and AtPRX72 localized to the thick secondary cell wall of xylem vessel elements and fibers, whereas AtLAC4, AtPRX64, and AtPRX71 localized to fiber cell corners. Interestingly, AtLAC4 had a transient cell corner localization early in fiber development that disappeared in the mature stem. In contrast with these secondary cell wall localizations, AtLAC10, AtPRX42, AtPRX52, and AtPRX71 were found in nonlignified tissues. Despite ubiquitous PRX occurrence in cell walls, PRX oxidative activity was restricted to lignifying regions during development, which suggested regulated production of apoplastic hydrogen peroxide. Relative amounts of apoplastic reactive oxygen species differed between lignified cell types, which could modulate PRX activity. Together, these results indicate that precise localization of oxidative enzymes and differential distribution of oxidative substrates, such as hydrogen peroxide, provide mechanisms to control spatiotemporal deposition of lignin during development.

Carbohydrase Complexes Rich in Xylanases and Arabinofuranosidases Affect the Autofluorescence Signal and Liberate Phenolic Acids from the Cell Wall Matrix in Wheat, Maize, and Rice Bran: An In Vitro Digestion Study

The high fiber content of cereal coproducts used in animal feed reduces the digestibility and nutrient availability. Therefore, the aim of this study was to elucidate the ability of two carbohydrase complexes to degrade the cell wall of wheat, maize, and rice during in vitro digestion. One complex was rich in cell-wall-degrading enzymes (NSPase 1), and the other was similar but additionally enriched with xylanases and arabinofuranosidases (NSPase 2). Degradation of arabinoxylan, the main cereal cell wall polysaccharide, was followed directly by gas-liquid chromatography (GLC) and indirectly through phenolic acid liberation as quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The effect was additionally visualized using a unique multispectral autofluorescence approach. Wheat fractions, in particular aleurone, were susceptible to degradation as judged from the redistribution of arabinoxylan (25% reduction in insoluble arabinoxylan), whereas the highest relative liberation of ferulic acid was observed in rice bran (6%). All cereal fractions, except for maize, had a higher release of ferulic acid with NSPase 2 than NSPase 1 (38% in rice and wheat bran, 30% in wheat whole grain, and 28% in wheat aleurone). Thus, the carbohydrase complexes were able to degrade important cell wall components during in vitro digestion but apparently through different mechanisms in wheat, maize, and rice.

Structural Modeling of Cell Wall Peptidase CwpFM (EntFM) Reveals Distinct Intrinsically Disordered Extensions Specific to Pathogenic Bacillus cereus Strains

The emergence of B. cereus as an opportunistic food-borne pathogen has intensified the need to distinguish strains of public health concern. The heterogeneity of the diseases associated with B. cereus infections emphasizes the versatility of these bacteria strains to colonize their host. Nevertheless, the molecular basis of these differences remains unclear. Several toxins are involved in virulence, particularly in gastrointestinal disorders, but there are currently no biological markers able to differentiate pathogenic from harmless strains. We have previously shown that CwpFM is a cell wall peptidase involved in B. cereus virulence. Here, we report a sequence/structure/function characterization of 39 CwpFM sequences, chosen from a collection of B. cereus with diverse virulence phenotypes, from harmless to highly pathogenic strains. CwpFM is homology-modeled in silico as an exported papain-like endopeptidase, with an N-terminal end composed of three successive bacterial Src Homology 3 domains (SH3b_1–3) likely to control protein–protein interactions in signaling pathways, and a C-terminal end that contains a catalytic NLPC_P60 domain primed to form a competent active site. We confirmed in vitro that CwpFM is an endopeptidase with a moderate peptidoglycan hydrolase activity. Remarkably, CwpFMs from pathogenic strains harbor a specific stretch of twenty residues intrinsically disordered, inserted between the SH3b₃ and the catalytic NLPC_P60 domain. This strongly suggests this linker as a marker of differentiation between B. cereus strains. We believe that our findings improve our understanding of the pathogenicity of B. cereus while advancing both clinical diagnosis and food safety.

How the assembly and protection of the bacterial cell envelope depend on cysteine residues

The cell envelope of Gram-negative bacteria is a multilayered structure essential for bacterial viability; the peptidoglycan cell wall provides shape and osmotic protection to the cell, and the outer membrane serves as a permeability barrier against noxious compounds in the external environment. Assembling the envelope properly and maintaining its integrity are matters of life and death for bacteria. Our understanding of the mechanisms of envelope assembly and maintenance has increased tremendously over the past two decades. Here, we review the major achievements made during this time, giving central stage to the amino acid cysteine, one of the least abundant amino acid residues in proteins, whose unique chemical and physical properties often critically support biological processes. First, we review how cysteines contribute to envelope homeostasis by forming stabilizing disulfides in crucial bacterial assembly factors (LptD, BamA, and FtsN) and stress sensors (RcsF and NlpE). Second, we highlight the emerging role of enzymes that use cysteine residues to catalyze reactions that are necessary for proper envelope assembly, and we also explain how these enzymes are protected from oxidative inactivation. Finally, we suggest future areas of investigation, including a discussion of how cysteine residues could contribute to envelope homeostasis by functioning as redox switches. By highlighting the redox pathways that are active in the envelope of Escherichia coli, we provide a timely overview of the assembly of a cellular compartment that is the hallmark of Gram-negative bacteria.

Cell Wall Invertase Is Essential for Ovule Development through Sugar Signaling Rather Than Provision of Carbon Nutrients

Ovule formation is essential for realizing crop yield because it determines seed number. The underlying molecular mechanism, however, remains elusive. Here, we show that cell wall invertase (CWIN) functions as a positive regulator of ovule initiation in Arabidopsis (Arabidopsis thaliana). In situ hybridization revealed that CWIN2 and CWIN4 were expressed at the placenta region where ovule primordia initiated. Specific silencing of CWIN2 and CWIN4 using targeted artificial microRNA driven by an ovule-specific SEEDSTICK promoter (pSTK) resulted in a substantial reduction of CWIN transcript and activity, which blocked ovule initiation and aggravated ovule abortion. There was no induction of carbon (C) starvation genes in the transgenic lines, and supplementing newly forming floral buds with extra C failed to recover the ovule phenotype. This indicates that suppression of CWIN did not lead to C starvation. A group of hexose transporters was downregulated in the transgenic plants. Among them, two representative ones were spatially coexpressed with CWIN2 and CWIN4, suggesting a coupling between CWIN and hexose transporters for ovule initiation. RNA-sequencing analysis identified differentially expressed genes encoding putative extracellular receptor-like kinases, MADS-box transcription factors, including STK, and early auxin response genes in response to CWIN-silencing. Our data demonstrate the essential role of CWIN in ovule initiation, which is most likely to occur through sugar signaling instead of C nutrient contribution. We propose that CWIN-mediated sugar signaling may be perceived by, and transmitted through, hexose transporters or receptor-like kinases to regulate ovule formation by modulating downstream auxin signaling and MADS-box transcription factors.

Microgravity Affects the Level of Matrix Polysaccharide 1,3:1,4-β-Glucans in Cell Walls of Rice Shoots by Increasing the Expression Level of a Gene Involved in Their Breakdown

The plant cell wall provides each cell with structural support and mechanical strength, and thus, it plays an important role in supporting the plant body against the gravitational force. We investigated the effects of microgravity on the composition of cell wall polysaccharides and on the expression levels of genes involved in cell wall metabolism using rice shoots cultivated under artificial 1 g and microgravity conditions on the International Space Station. The bulk amount of the cell wall obtained from microgravity-grown shoots was comparable with that from 1 g-grown shoots. However, the analysis of sugar constituents of matrix polysaccharides showed that microgravity specifically reduced the amount of glucose (Glc)-containing polysaccharides such as 1,3:1,4-β-glucans, in shoot cell walls. The expression level of a gene for endo-1,3:1,4-β-glucanase, which hydrolyzes 1,3:1,4-β-glucans, largely increased under microgravity conditions. However, the expression levels of genes involved in the biosynthesis of 1,3:1,4-β-glucans were almost the same under both gravity conditions. On the contrary, microgravity scarcely affected the level and the metabolism of arabinoxylans. These results suggest that a microgravity environment promotes the breakdown of 1,3:1,4-β-glucans, which, in turn, causes the reduced level of these polysaccharides in growing rice shoots. Changes in 1,3:1,4-β-glucan level may be involved in the modification of mechanical properties of cell walls under microgravity conditions in space.

Reactive oxygen species regulate auxin levels to mediate adventitious root induction in Arabidopsis hypocotyl cuttings

Adventitious root (AR) formation from leafy stem cuttings is critical for breeding of many forest and horticultural species. In addition to the plant hormone auxin, wound-induced signaling caused by the cutting excision is also essential for AR initiation. Here we found that reactive oxygen species (ROS) are rapidly generated at the excision site as a wound-induced signal and propagated throughout the hypocotyl cutting after excision of the Arabidopsis (Arabidopsis thaliana) primary root. ROS propagation was not observed in the presence of an NADPH oxidase inhibitor (diphenylene iodonium chloride) or in a knockout mutant of the NADPH oxidase gene respiratory burst oxidase homolog protein D (RBOHD). Respiratory burst oxidase homolog protein D was specifically upregulated in hypocotyl cuttings at 0.5 h post excision (hpe). Together, these data suggest that RBOHD mediates ROS propagation in hypocotyl cuttings. We also found that auxin levels increased significantly in the shoot apex at 5 hpe and at the base of the cutting at 6 hpe; these effects were blocked by treatment with ROS scavengers. Consistent with this, transcript levels of auxin biosynthesis and polar-transport genes generally increased between 1 to 6 hpe. Collectively, our results suggest that wound-induced ROS participate in AR induction through regulation of auxin biosynthesis and transport.

Genome mining of the citrus pathogen Elsinoë fawcettii; prediction and prioritisation of candidate effectors, cell wall degrading enzymes and secondary metabolite gene clusters

Elsinoë fawcettii, a necrotrophic fungal pathogen, causes citrus scab on numerous citrus varieties around the world. Known pathotypes of E. fawcettii are based on host range; additionally, cryptic pathotypes have been reported and more novel pathotypes are thought to exist. E. fawcettii produces elsinochrome, a non-host selective toxin which contributes to virulence. However, the mechanisms involved in potential pathogen-host interactions occurring prior to the production of elsinochrome are unknown, yet the host-specificity observed among pathotypes suggests a reliance upon such mechanisms. In this study we have generated a whole genome sequencing project for E. fawcettii, producing an annotated draft assembly 26.01 Mb in size, with 10,080 predicted gene models and low (0.37%) coverage of transposable elements. A small proportion of the assembly showed evidence of AT-rich regions, potentially indicating genomic regions with increased plasticity. Using a variety of computational tools, we mined the E. fawcettii genome for potential virulence genes as candidates for future investigation. A total of 1,280 secreted proteins and 276 candidate effectors were predicted and compared to those of other necrotrophic (Botrytis cinerea, Parastagonospora nodorum, Pyrenophora tritici-repentis, Sclerotinia sclerotiorum and Zymoseptoria tritici), hemibiotrophic (Leptosphaeria maculans, Magnaporthe oryzae, Rhynchosporium commune and Verticillium dahliae) and biotrophic (Ustilago maydis) plant pathogens. Genomic and proteomic features of known fungal effectors were analysed and used to guide the prioritisation of 120 candidate effectors of E. fawcettii. Additionally, 378 carbohydrate-active enzymes were predicted and analysed for likely secretion and sequence similarity with known virulence genes. Furthermore, secondary metabolite prediction indicated nine additional genes potentially involved in the elsinochrome biosynthesis gene cluster than previously described. A further 21 secondary metabolite clusters were predicted, some with similarity to known toxin producing gene clusters. The candidate virulence genes predicted in this study provide a comprehensive resource for future experimental investigation into the pathogenesis of E. fawcettii.

A Cell Wall Integrity-Related MAP Kinase Kinase Kinase AflBck1 Is Required for Growth and Virulence in Fungus Aspergillus flavus

Aspergillus flavus represents an important fungal pathogen, causing severe economic losses in crops. The mitogen-activated protein (MAP) kinase signaling pathway contributes to many physiological processes, but its precise role in A. flavus is not yet fully understood. In this study, we focused on the AflBck1 gene, which encodes a MAP kinase kinase kinase of the Slt2-MAPK pathway. Targeted deletion of AflBck1 led to a significant defect in growth and development, and a AflBck1-deleted mutant (∆AflBck1) showed higher sensitivity to cell-wall stress than wild type (WT). Importantly, we observed that ∆AflBck1 displayed an enhanced ability to produce aflatoxin, a potential carcinogenic mycotoxin. However, the pathogenicity of the ∆AflBck1 mutant was markedly reduced in peanut seeds. We also presented evidence that AflBck1 was genetically epistatic to AflMkk2 in the Slt2-MAPK pathway. Finally, we found that loss of the proline-rich region at the N terminus of AflBck1 affected the reproduction of A. flavus. Collectively, this study not only extended the understanding that the MAPK pathway regulated A. flavus pathogenicity but also provided a possible strategy to control A. flavus contamination.

Unique subsite specificity and potential natural function of a chitosan deacetylase from the human pathogen Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that infects ∼280,000 people every year, causing >180,000 deaths. The human immune system recognizes chitin as one of the major cell-wall components of invading fungi, but C. neoformans can circumvent this immunosurveillance mechanism by instead exposing chitosan, the partly or fully deacetylated form of chitin. The natural production of chitosans involves the sequential action of chitin synthases (CHSs) and chitin deacetylases (CDAs). C. neoformans expresses four putative CDAs, three of which have been confirmed as functional enzymes that act on chitin in the cell wall. The fourth (CnCda4/Fpd1) is a secreted enzyme with exceptional specificity for d-glucosamine at its -1 subsite, thus preferring chitosan over chitin as a substrate. We used site-specific mutagenesis to reduce the subsite specificity of CnCda4 by converting an atypical isoleucine residue in a flexible loop region to the bulkier or charged residues tyrosine, histidine, and glutamic acid. We also investigated the effect of CnCda4 deacetylation products on human peripheral blood-derived macrophages, leading to a model explaining the function of CnCda4 during infection. We propose that CnCda4 is used for the further deacetylation of chitosans already exposed on the C. neoformans cell wall (originally produced by CnChs3 and CnCda1 to 3) or released from the cell wall as elicitors by human chitinases, thus making the fungus less susceptible to host immunosurveillance. The absence of CnCda4 during infection could therefore promote the faster recognition and elimination of this pathogen.

Heterologous expression of the apple hexose transporter MdHT2.2 altered sugar concentration with increasing cell wall invertase activity in tomato fruit

Sugar transporters are necessary to transfer hexose from cell wall spaces into parenchyma cells to boost hexose accumulation to high concentrations in fruit. Here, we have identified an apple hexose transporter (HTs), MdHT2.2, located in the plasma membrane, which is highly expressed in mature fruit. In a yeast system, the MdHT2.2 protein exhibited high ¹⁴ C-fructose and ¹⁴ C-glucose transport activity. In transgenic tomato heterologously expressing MdHT2.2, the levels of both fructose and glucose increased significantly in mature fruit, with sugar being unloaded via the apoplastic pathway, but the level of sucrose decreased significantly. Analysis of enzyme activity and the expression of genes related to sugar metabolism and transport revealed greatly up-regulated expression of SlLIN5, a key gene encoding cell wall invertase (CWINV), as well as increased CWINV activity in tomatoes transformed with MdHT2.2. Moreover, the levels of fructose, glucose and sucrose recovered nearly to those of the wild type in the sllin5-edited mutant of the MdHT2.2-expressing lines. However, the overexpression of MdHT2.2 decreased hexose levels and increased sucrose levels in mature leaves and young fruit, suggesting that the response pathway for the apoplastic hexose signal differs among tomato tissues. The present study identifies a new HTs in apple that is able to take up fructose and glucose into cells and confirms that the apoplastic hexose levels regulated by HT controls CWINV activity to alter carbohydrate partitioning and sugar content.

Cellular Localization of Carbonic Anhydrase Nce103p in Candida albicans and Candida parapsilosis

Pathogenic yeasts Candida albicans and Candida parapsilosis possess a ß-type carbonic anhydrase Nce103p, which is involved in CO₂ hydration and signaling. C. albicans lacking Nce103p cannot survive in low CO₂ concentrations, e.g., in atmospheric growth conditions. Candida carbonic anhydrases are orthologous to the Saccharomyces cerevisiae enzyme, which had originally been detected as a substrate of a non-classical export pathway. However, experimental evidence on localization of C. albicans and C. parapsilosis carbonic anhydrases has not been reported to date. Immunogold labeling and electron microscopy used in the present study showed that carbonic anhydrases are localized in the cell wall and plasmatic membrane of both Candida species. This localization was confirmed by Western blot and mass spectrometry analyses of isolated cell wall and plasma membrane fractions. Further analysis of C. albicans and C. parapsilosis subcellular fractions revealed presence of carbonic anhydrases also in the cytosolic and mitochondrial fractions of Candida cells cultivated in shaken liquid cultures, under the atmospheric conditions.

Bioinformatics Analysis of Plant Cell Wall Evolution

In the past hundreds of millions of years, from green algae to land plants, cell walls have developed into a highly complex structure that is essential for plant growth and survival. Plant cell wall diversity and evolution can be directly investigated by chemically profiling polysaccharides and lignins in the cell walls of diverse plants and algae. With the increasingly low cost and high throughput of DNA sequencing technologies, cell wall evolution can also be studied by bioinformatics analysis of the occurrence of cell wall synthesis-related enzymes in the genomes and transcriptomes of different species. This chapter presents a bioinformatics workflow running on a Linux platform to process genomic data for such gene occurrence analysis. As a case study, cellulose synthase (CesA) and CesA-like (Csl) protein families are mined for in two newly sequenced organisms: the charophyte green alga Klebsormidium flaccidum (renamed as Klebsormidium nitens) and the fern Lygodium japonicum.

Activity and Action of Cell-Wall Transglycanases

Transglycanases (endotransglycosylases) are enzymes that "cut and paste" polysaccharide chains. Several transglycanase activities have been discovered which can cut (i.e., use as donor substrate) each of the major hemicelluloses [xyloglucan, mannans, xylans, and mixed-linkage β-glucan (MLG)], and, as a recent addition, cellulose. These enzymes may play interesting roles in adjusting the wall's physical properties, influencing cell expansion, stem strengthening, and fruit softening.Activities discussed include the homotransglycanases XET (xyloglucan endotransglucosylase, i.e., xyloglucan-xyloglucan endotransglycosylase), trans-β-mannanase (mannan -mannan endotransglycosylase), and trans-β-xylanase (xylan -xylan endotransglucosylase), plus the heterotransglycanases MXE (MLG -xyloglucan endotransglucosylase) and CXE (cellulose -xyloglucan endotransglucosylase).Transglycanases acting on polysaccharide donor substrates can utilize small, labeled oligosaccharides as acceptor substrates, generating easily recognizable polymeric labeled products. We present methods for extracting transglycanases from plant tissues and assaying them in vitro, either quantitatively in solution assays or by high-throughput dot-blot screens. Both radioactively and fluorescently labeled substrates are mentioned. A general procedure (glass-fiber blotting) is illustrated by which proposed novel transglycanase activities can be tested for.In addition, we describe strategies for detecting transglycanase action in vivo. These methods enable the quantification of, separately, XET and MXE action in Equisetum stems. Related methods enable the tissue distribution of transglycanase action to be visualized cytologically.

Amylopectin Chain Length Dynamics and Activity Signatures of Key Carbon Metabolic Enzymes Highlight Early Maturation as Culprit for Yield Reduction of Barley Endosperm Starch after Heat Stress

Abiotic environmental stresses have a negative impact on the yield and quality of crops. Understanding these stresses is an essential enabler for mitigating breeding strategies and it becomes more important as the frequency of extreme weather conditions increases due to climate change. This study analyses the response of barley (Hordeum vulgare L.) to a heat wave during grain filling in three distinct stages: the heat wave itself, the return to a normal temperature regime, and the process of maturation and desiccation. The properties and structure of the starch produced were followed throughout the maturational stages. Furthermore, the key enzymes involved in the carbohydrate supply to the grain were monitored. We observed differences in starch structure with well-separated effects because of heat stress and during senescence. Heat stress produced marked effects on sucrolytic enzymes in source and sink tissues. Early cessation of plant development as an indirect consequence of the heat wave was identified as the major contributor to final yield loss from the stress, highlighting the importance for functional stay-green traits for the development of heat-resistant cereals.

A Microbiological, Toxicological, and Biochemical Study of the Effects of Fucoxanthin, a Marine Carotenoid, on Mycobacterium tuberculosis and the Enzymes Implicated in Its Cell Wall: A Link Between Mycobacterial Infection and Autoimmune Diseases

This study explored the antitubercular properties of fucoxanthin, a marine carotenoid, against clinical isolates of Mycobacterium tuberculosis (Mtb). Two vital enzymes involved in Mtb cell wall biosynthesis, UDP-galactopyranose mutase (UGM) and arylamine-N-acetyltransferase (TBNAT), were selected as drug targets to reveal the mechanism underlying the antitubercular effect of fucoxanthin. The obtained results showed that fucoxanthin showed a clear bacteriostatic action against the all Mtb strains tested, with minimum inhibitory concentrations (MIC) ranging from 2.8 to 4.1 µM, along with a good degree of selectivity index (ranging from 6.1 to 8.9) based on cellular toxicity evaluation compared with standard drug isoniazid (INH). The potent inhibitory actions of fucoxanthin and standard uridine-5’-diphosphate against UGM were recorded to be 98.2% and 99.2%, respectively. TBNAT was potently inactivated by fucoxanthin (half maximal inhibitory concentration (IC₅₀) = 4.8 µM; 99.1% inhibition) as compared to INH (IC₅₀ = 5.9 µM; 97.4% inhibition). Further, molecular docking approaches were achieved to endorse and rationalize the biological findings along with envisaging structure-activity relationships. Since the clinical evidence of the last decade has confirmed the correlation between bacterial infections and autoimmune diseases, in this study we have discussed the linkage between infection with Mtb and autoimmune diseases based on previous clinical observations and animal studies. In conclusion, we propose that fucoxanthin could demonstrate great therapeutic value for the treatment of tuberculosis by acting on multiple targets through a bacteriostatic effect as well as by inhibiting UGM and TBNAT. Such outcomes may lead to avoiding or decreasing the susceptibility to autoimmune diseases associated with Mtb infection in a genetically susceptible host.

Assays for the Enzymes Catalyzing the O-Acetylation of Bacterial Cell Wall Polysaccharides

The polysaccharides that comprise bacterial cell walls are commonly O-acetylated. This modification confers resistance to hydrolases of innate immune systems and/or controls endogenous autolytic activity. Herein, we present protocols for the compositional analysis of bacterial cell wall O-acetylation, and assays for monitoring O-acetyltransferases and O-acetylesterases. The assays are amenable for the development of high-throughput screens in search of inhibitors of the respective enzymes.

Calcium chloride treatment modifies cell wall metabolism and activates defense responses in strawberry fruit (Fragaria × ananassa, Duch)

BACKGROUND

Fruit dips in calcium ions solutions have been shown as an effective treatment to extend strawberries (Fragaria × ananassa, Duch) quality during storage. In the present work, strawberry fruit were treated with 10 g L^-1 calcium chloride solution and treatment effects on cell wall enzymes activities and the expression of encoding genes, as well as enzymes involved in fruit defense responses were investigated.

RESULTS

Calcium treatment enhanced pectin methylesterase activity while inhibited those corresponding to pectin hydrolases as polygalacturonase and β-galactosidase. The expression of key genes for strawberry pectin metabolism was up-regulated (for FaPME1) and down-regulated (for FaPG1, FaPLB, FaPLC, FaβGal1 and FaAra1) by calcium dips. In agreement, a higher firmness level and ionically-bound pectins (IBPs) amount were detected in calcium-treated fruit compared with controls. The in vitro and in vivo growth rate of fungal pathogen Botrytis cinerea was limited by calcium treatment. Moreover, the activities of polyphenol oxidases, chitinases, peroxidases and β-1,3-glucanases were enhanced by calcium ion dips.

CONCLUSION

News insights concerning the biochemical and molecular basis of cell wall preservation and resistance to fungal pathogens on calcium-treated strawberries are provided. © 2019 Society of Chemical Industry.

Energy landscape of the domain movement in Staphylococcus aureus UDP-N-acetylglucosamine 2-epimerase

Staphylococcus aureus is an important cause of resistant healthcare-associated infections. It has been shown that the wall teichoic acid (WTA) may be an important drug target acting on antibiotic-resistant cells. The UDP-N-acetylglucosamine 2-epimerase, MnaA, is one of the first enzymes on the pathway for the biosynthesis of the WTA. Here, detailed molecular dynamics simulations of S. aureus MnaA were used to characterize the conformational changes that occur in the presence of UDP and UDP-GlcNac and also the energetic landscape associated with these changes. Using different simulation techniques, such as ABMD and GAMD, it was possible to assess the energetic profile for the protein with and without ligands in its active site. We found that there is a dynamic energy landscape that has its minimum changed by the presence of the ligands, with a closed structure of the enzyme being more frequently observed for the bound state while the unbound enzyme favors an opened conformation. Further structural analysis indicated that positively charged amino acids associated with UDP and UDP-GlcNac interactions play a major role in the enzyme opening movement. Finally, the energy landscape profiled in this work provides important conclusions for the design of inhibitor candidates targeting S. aureus MnaA.

Alternating temperature in postharvest cooling treatment of 'Fiano' and 'Falanghina' grapes affects cell wall enzyme rate, berry softening and polyphenols

BACKGROUND

In wine production, temperature control is important and cooling techniques are frequently used to cool down grape must. Grape compound extraction is facilitated by enzymatic cell wall disassembly. Accordingly, our hypothesis is that the use of an alternating temperature could favor enzyme activity and thus berry compound extraction.

RESULTS

White wine grapes (cvs 'Falanghina' and 'Fiano') were kept at 4 °C or at an alternating temperature (4 and 20 °C with a 6-h interval) for 24 h. During the cooling treatment, berry skin resistance, berry firmness and native enzymes from the cell wall were analyzed; sugars, total acidity, malic acid and polyphenols were also measured. In 'Falanghina', α-d-glucosidase, β-d-galactosidase and pectinmethylesterase increased in grape berry kept at an alternating temperature, and the berry firmness decreased and the polyphenol content was greater than in the other sample. In 'Fiano', polygalacturonase and pectinmethylesterase were responsable for berry softening and, overall, with an alternating temperature, the activity was higher and, consequently, sugars and polyphenol contents were higher.

CONCLUSION

Instead of just cooling wine grape, a 24-h cycle of low and high temperature on wine grape after harvest could be used to increase cell wall enzyme activity and, consequently, decrease the berry firmness, favouring an increase in sugars and polyphenol contents. © 2018 Society of Chemical Industry.

Application of sodium silicate retards apple softening by suppressing the activity of enzymes related to cell wall degradation

BACKGROUND

During the storage of apples, apple softening is one of the main problems. Sodium silicate has been used to enhance disease resistance and maintain quality of fruits. In the present study, apple fruit (cv. Golden delicious) were treated with 100 mmol L^-1 sodium silicate for 10 min and stored at 20 °C to investigate its effects on weight loss, flesh firmness, and the activity of cell wall-degrading enzymes.

RESULTS

The results indicated that 100 mmol L^-1 of sodium silicate treatment delayed the increase of weight loss and decrease of the flesh firmness in apples. Sodium silicate treatment also suppressed the activity of polygalacturonic acid transeliminase and pectin methyltranseliminase, pectin methylgalacturonase, polygalacturonase, cellulase and β-galactosidase in the fruit.

CONCLUSIONS

Delaying apple softening by sodium silicate treatment is closely related to the inhibition of the activity of cell wall-degrading enzymes and weight loss. © 2018 Society of Chemical Industry.

Calcium- and hormone-driven regulation of secondary metabolism and cell wall enzymes in grape berry cells

The efficacy of calcium sprays for improving fleshy fruit resistance to abiotic/biotic stress and enhancement of fruit shelf life has increasingly been explored. However, because calcium is a powerful secondary messenger in many signaling pathways, including those driven by abscisic acid (ABA) and jasmonates, it may interfere with the biosynthesis of specialized metabolites highly important for fruit and wine quality, such as phenolic compounds. In this study, a combination of biochemical and molecular biology approaches were applied to grape cell cultures and detached grape berries, in order to investigate the effect of calcium in the modulation of enzymes involved in the biosynthesis of phenolic compounds and in cell wall organization. Concentrations up to 10 mM CaCl₂ did not affect cell growth, size or viability, but triggered modifications in total phenolics content, particularly in anthocyanin levels in grape cell suspensions. The effects of calcium applied alone or in combination with ABA or methyl jasmonate (MeJA) were visible in several branches of specialized metabolic pathways, confirming that the calcium-hormone interplay regulates the expression of phenylalanine ammonia lyase (PAL), stilbene synthase (STS), dihydroflavonol reductase (DFR) and UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT). The activity of PAL and UFGT enzymes was also specifically modulated by calcium, ABA and MeJA. These results closely correlated to the modifications observed in the expression of VvAM1 and VvABCC1 encoding vacuolar anthocyanin transporters. Modulation of the expression and activity of pectin methyl esterases (PME) and polygalacturonases (PG) by calcium was also evident, confirming an important role of calcium in cell wall organization via the regulation of enzyme activity, besides its well-known role in the formation of cross links between pectin molecules. Overall, this study uncovers important biochemical mechanisms induced by calcium and stress hormones on grape berries, and highlights the need to consider the consequences of calcium treatments and stress for fruit quality.

Evidence for the Regulation of Gynoecium Morphogenesis by ETTIN via Cell Wall Dynamics

ETTIN (ETT) is an atypical member of the AUXIN RESPONSE FACTOR family of transcription factors that plays a crucial role in tissue patterning in the Arabidopsis (Arabidopsis thaliana) gynoecium. Though recent insights have provided valuable information on ETT's interactions with other components of auxin signaling, the biophysical mechanisms linking ETT to its ultimate effects on gynoecium morphology were until now unknown. Here, using techniques to assess cell-wall dynamics during gynoecium growth and development, we provide a coherent body of evidence to support a model in which ETT controls the elongation of the valve tissues of the gynoecium through the positive regulation of pectin methylesterase (PME) activity in the cell wall. This increase in PME activity results in an increase in the level of demethylesterified pectins and a consequent reduction in cell wall stiffness, leading to elongation of the valves. Though similar biophysical mechanisms have been shown to act in the stem apical meristem, leading to the expansion of organ primordia, our findings demonstrate that regulation of cell wall stiffness through the covalent modification of pectin also contributes to tissue patterning within a developing plant organ.

Expression analysis of Cell wall invertase under abiotic stress conditions influencing specialized metabolism in Catharanthus roseus

Catharanthus roseus is a commercial source for anti-cancer terpenoid indole alkaloids (TIAs: vincristine and vinblastine). Inherent levels of these TIAs are very low, hence research studies need to focus on enhancing their levels in planta. Since primary metabolism provides precursors for specialized-metabolism, elevating the former can achieve higher amounts of the latter. Cell Wall Invertase (CWIN), a key enzyme in sucrose-metabolism catalyses the breakdown of sucrose into glucose and fructose, which serve as carbon-skeleton for specialized-metabolites. Understanding CWIN regulation could unravel metabolic-engineering approaches towards enhancing the levels of TIAs in planta. Our study is the first to characterize CWIN at gene-expression level in the medicinal plant, C. roseus. The CWINs and their inter-relationship with sucrose and TIA metabolism was studied at gene and metabolite levels. It was found that sucrose-supplementation to C. roseus leaves significantly elevated the monomeric TIAs (vindoline, catharanthine) and their corresponding genes. This was further confirmed in cross-species, wherein Nicotiana benthamiana leaves transiently-overexpressing CrCWIN2 showed significant upregulation of specialized-metabolism genes: NbPAL2, Nb4CL, NbCHS, NbF3H, NbANS, NbHCT and NbG10H. The specialized metabolites- cinnamic acid, coumarin, and fisetin were significantly upregulated. Thus, the present study provides a valuable insight into metabolic-engineering approaches towards augmenting the levels of therapeutic TIAs.

Two Faces of CwlM, an Essential PknB Substrate, in Mycobacterium tuberculosis

Tuberculosis claims >1 million lives annually, and its causative agent Mycobacterium tuberculosis is a highly successful pathogen. Protein kinase B (PknB) is reported to be critical for mycobacterial growth. Here, we demonstrate that PknB-depleted M. tuberculosis can replicate normally and can synthesize peptidoglycan in an osmoprotective medium. Comparative phosphoproteomics of PknB-producing and PknB-depleted mycobacteria identify CwlM, an essential regulator of peptidoglycan synthesis, as a major PknB substrate. Our complementation studies of a cwlM mutant of M. tuberculosis support CwlM phosphorylation as a likely molecular basis for PknB being essential for mycobacterial growth. We demonstrate that growing mycobacteria produce two forms of CwlM: a non-phosphorylated membrane-associated form and a PknB-phosphorylated cytoplasmic form. Furthermore, we show that the partner proteins for the phosphorylated and non-phosphorylated forms of CwlM are FhaA, a fork head-associated domain protein, and MurJ, a proposed lipid II flippase, respectively. From our results, we propose a model in which CwlM potentially regulates both the biosynthesis of peptidoglycan precursors and their transport across the cytoplasmic membrane.

Effects of heat treatment on enzyme activity and expression of key genes controlling cell wall remodeling in strawberry fruit

Modification of cell wall polymers composition and structure is one of the main factors contributing to textural changes during strawberry (Fragaria x ananassa, Duch.) fruit ripening and storage. The present study aimed to provide new data to understand the molecular basis underlying the postharvest preservation of strawberry cell wall structure by heat treatment. Ripe fruit (cv. Aroma) were heat-treated in air oven (3 h at 45 °C) and then stored 8 days at 4 °C + 2 days at 20 °C, while maintaining a set of non-treated fruit as controls. The effect of heat stress on the expression pattern of key genes controlling strawberry cell wall metabolism, as well as some enzymatic activities was investigated. The expression of genes proved to be relevant for pectin disassembly and fruit softening process (FaPG1, FaPLB, FaPLC, FaAra1, FaβGal4) were down-regulated by heat treatment, while the expression of genes being involved in the reinforcement of cell wall as pectin-methylesterase (FaPME1) and xyloglucan endo-transglycosilase (FaXTH1) was up-regulated. Total cell wall amount as well as cellulose, hemicellulose, neutral sugars and ionically and covalently bounded pectins were higher in heat-stressed fruit compared to controls, which might be related to higher firmness values. Interestingly, heat stress was able to arrest the in vitro cell wall swelling process during postharvest fruit ripening, suggesting a preservation of cell wall structure, which was in agreement with a lower growth rate of Botrytis cinerea on plates containing cell walls from heat-stressed fruit when compared to controls.

Functional characterization of persimmon β-galactosidase gene DkGAL1 in tomato reveals cell wall modification related to fruit ripening and radicle elongation

Cell wall metabolism during fruit ripening is a highly organized process that involves complex interplay among various cell wall hydrolases. Among these cell wall hydrolases, β-galactosidase has been identified to participate in cell wall metabolism via its ability to catalyze galactosyl metabolism from the large and complex side chains of cell walls. In this study, the galactose content in the pericarp increased during persimmon fruit ripening, but cell wall galactosyl residues decreased, indicating a relationship between galactose metabolism and persimmon fruit ripening. Expression of a previously isolated β-galactosidase gene, DkGAL1, increased 25.01-fold during fruit ripening. Heterologous expression of DkGAL1 under the CaMV 35S promoter in tomato accelerated on-plant and postharvest fruits ripening. The fruit firmness of one of transgenic line, OE-18, was 23.83% lower than that of WT at the breaker stage. The transgenic fruits produced more ethylene by promoting the expression of ethylene synthesis-related genes and cell wall degradation-related genes. Overexpression of DkGAL1 in tomato also reduced cell-to-cell adhesion and promoted both wider intercellular spaces and less cell compaction in transgenic fruit structures. Moreover, DkGAL1 was involved in seed germination and radicle elongation in transgenic tomato seeds. These results confirm the role of DkGAL1 in fruit ripening and suggest that this gene alters galactose metabolism in the fruit, which can promote ripening and reduce cellular adhesion. In addition, the role of DkGAL1 is not limited to fruit softening; DkGAL1 was also involved in seed germination and radicle elongation in transgenic tomato seeds.

Action of selenium against Sclerotinia sclerotiorum: Damaging membrane system and interfering with metabolism

Selenium (Se) in soil is beneficial for environmental stress tolerance of plants, and it has widespread toxic effects on pathogens. Based on the fact that Se significantly inhibited the growth of Sclerotinia sclerotiorum, we set experiments with different concentrations of Se to investigate the action of Se against S. sclerotiorum in this study. The results showed that Se (>0.5 mg L^-1) changed the morphology of S. sclerotiorum mycelia, and higher Se concentrations severely damaged mycelial structures. Fourier transform infrared spectroscopy (FTIR) analysis indicated that Se treatment induced the chemical composition of mycelia with much abundance of functional groups such as alcohols, ketones, ammonium and esters, and 0.5 mg L^-1 Se maximized their concentrations. Under Se treatments, the electrical conductivity of mycelia increased in a time-dependent manner, and osmolyte concentrations of mycelia increased as well. Se supplementation significantly reduced polymethylgalacturonase (PMG) and carboxymethylcellulase (Cx) activities, which protecting plants from infection, and increased the energy expenditure in S. sclerotiorum. Combined action of Se damage on membrane system, osmoregulation, reduction of cell wall degrading enzymes activities and improvement of energy expenditure resulted in the inhibition of S. sclerotiorum growth. Findings in this study provided evidences for using Se as a potential fungicide to control S. sclerotiorum.

The Beauveria bassiana Gas3 β-Glucanosyltransferase Contributes to Fungal Adaptation to Extreme Alkaline Conditions

Fungal β-1,3-glucanosyltransferases are cell wall-remodeling enzymes implicated in stress response, cell wall integrity, and virulence, with most fungal genomes containing multiple members. The insect-pathogenic fungus Beauveria bassiana displays robust growth over a wide pH range (pH 4 to 10). A random insertion mutant library screening for increased sensitivity to alkaline (pH 10) growth conditions resulted in the identification and mapping of a mutant to a β-1,3-glucanosyltransferase gene (Bbgas3). Bbgas3 expression was pH dependent and regulated by the PacC transcription factor, which activates genes in response to neutral/alkaline growth conditions. Targeted gene knockout of Bbgas3 resulted in reduced growth under alkaline conditions, with only minor effects of increased sensitivity to cell wall stress (Congo red and calcofluor white) and no significant effects on fungal sensitivity to oxidative or osmotic stress. The cell walls of ΔBbgas3 aerial conidia were thinner than those of the wild-type and complemented strains in response to alkaline conditions, and β-1,3-glucan antibody and lectin staining revealed alterations in cell surface carbohydrate epitopes. The ΔBbgas3 mutant displayed alterations in cell wall chitin and carbohydrate content in response to alkaline pH. Insect bioassays revealed impaired virulence for the ΔBbgas3 mutant depending upon the pH of the media on which the conidia were grown and harvested. Unexpectedly, a decreased median lethal time to kill (LT50, i.e., increased virulence) was seen for the mutant using intrahemocoel injection assays using conidia grown at acidic pH (5.6). These data show that BbGas3 acts as a pH-responsive cell wall-remodeling enzyme involved in resistance to extreme pH (>9).IMPORTANCE Little is known about adaptations required for growth at high (>9) pH. Here, we show that a specific fungal membrane-remodeling β-1,3-glucanosyltransferase gene (Bbgas3) regulated by the pH-responsive PacC transcription factor forms a critical aspect of the ability of the insect-pathogenic fungus Beauveria bassiana to grow at extreme pH. The loss of Bbgas3 resulted in a unique decreased ability to grow at high pH, with little to no effects seen with respect to other stress conditions, i.e., cell wall integrity and osmotic and oxidative stress. However, pH-dependent alternations in cell wall properties and virulence were noted for the ΔBbgas3 mutant. These data provide a mechanistic insight into the importance of the specific cell wall structure required to stabilize the cell at high pH and link it to the PacC/Pal/Rim pH-sensing and regulatory system.

The Necrotrophic Fungus Macrophomina phaseolina Promotes Charcoal Rot Susceptibility in Grain Sorghum Through Induced Host Cell-Wall-Degrading Enzymes

The cell-wall-degrading enzymes (CWDE) secreted by necrotrophs are important virulence factors. Although not unequivocally demonstrated, it has been suggested that necrotrophs induce hosts to cooperate in disease development through manipulation of host CWDE. The necrotrophic fungus Macrophomina phaseolina causes charcoal rot disease in Sorghum bicolor. An RNA-seq experiment was conducted to investigate the behavior of sorghum CWDE-encoding genes after M. phaseolina inoculation. Results revealed M. phaseolina's ability to significantly upregulate pectin methylesterase-, polygalacturonase-, cellulase-, endoglucanase-, and glycosyl hydrolase-encoding genes in a charcoal rot-susceptible sorghum genotype (Tx7000) but not in a resistant genotype (SC599). For functional validation, crude enzyme mixtures were extracted from M. phaseolina- and mock-inoculated charcoal-rot-resistant (SC599 and SC35) and -susceptible (Tx7000 and BTx3042) sorghum genotype stalks. A gel diffusion assay (pectin substrate) revealed significantly increased pectin methylesterase activity in M. phaseolina-inoculated Tx7000 and BTx3042. Polygalacturonase activity was determined using a ruthenium red absorbance assay (535 nm). Significantly increased polygalacturonase activity was observed in two susceptible genotypes after M. phaseolina inoculation. The activity of cellulose-degrading enzymes was determined using a 2-cyanoacetamide fluorimetric assay (excitation and emission maxima at 331 and 383 nm, respectively). The assay revealed significantly increased cellulose-degrading enzyme activity in M. phaseolina-inoculated Tx7000 and BTx3042. These findings revealed M. phaseolina's ability to promote charcoal rot susceptibility in grain sorghum through induced host CWDE.

Pre-Harvest Treatment of Chitosan Oligosaccharides Improved Strawberry Fruit Quality

Chitosan oligosaccharide (COS), derived through hydrolysis of chitosan, has been proved to be an effective plant immunity elicitor, eco-friendly, and easily soluble in water, and influenced several secondary metabolites content to improve fruit qualities. COS are widely used in agriculture to improve the defense response in plants. The purpose of this study was to investigate the pre-harvest treatment effect of COS on the quality of strawberry (Fragaria × ananassa cv.qingxiang). COS was dissolved in distilled water at a concentration of 50 mg·L⁻¹ and sprayed at four different growth stages of strawberry plants, namely seedling stage, before flowering, fruit coloring (the stage of fruit from white to red) and full bloom. Uniform size, shape, color, without any visible damage, and disease-free fruits were harvested for determining the quality. The results showed that the fruit firmness, viscosity, lignin, sugars, protein, total soluble solid, and titratable acidity content increased in COS-treated fruits compared to control. In addition, COS pre-harvest treatment had a positive effect on anthocyanin, total phenol, flavonoid, vitamin C content and DPPH(2,2-diphenyl-1-picrylhydrazyl) scavenging activity of strawberry. Moreover, COS also increased the cell wall composition and regulated gene expression of some important enzymes involved in ethylene compound biosynthesis and cell wall degradation. The finding of this study suggests that pre-harvest application of COS is very useful for improving quality and antioxidant capacity of strawberry.

Cellulose Synthase Stoichiometry in Aspen Differs from Arabidopsis and Norway Spruce

Cellulose is synthesized at the plasma membrane by cellulose synthase complexes (CSCs) containing cellulose synthases (CESAs). Genetic analysis and CESA isoform quantification indicate that cellulose in the secondary cell walls of Arabidopsis (Arabidopsis thaliana) is synthesized by isoforms CESA4, CESA7, and CESA8 in equimolar amounts. Here, we used quantitative proteomics to investigate whether the CSC model based on Arabidopsis secondary cell wall CESA stoichiometry can be applied to the angiosperm tree aspen (Populus tremula) and the gymnosperm tree Norway spruce (Picea abies). In the developing xylem of aspen, the secondary cell wall CESA stoichiometry was 3:2:1 for PtCESA8a/b:PtCESA4:PtCESA7a/b, while in Norway spruce, the stoichiometry was 1:1:1, as observed previously in Arabidopsis. Furthermore, in aspen tension wood, the secondary cell wall CESA stoichiometry changed to 8:3:1 for PtCESA8a/b:PtCESA4:PtCESA7a/b. PtCESA8b represented 73% of the total secondary cell wall CESA pool, and quantitative polymerase chain reaction analysis of CESA transcripts in cryosectioned tension wood revealed increased PtCESA8b expression during the formation of the cellulose-enriched gelatinous layer, while the transcripts of PtCESA4, PtCESA7a/b, and PtCESA8a decreased. A wide-angle x-ray scattering analysis showed that the shift in CESA stoichiometry in tension wood coincided with an increase in crystalline cellulose microfibril diameter, suggesting that the CSC CESA composition influences microfibril properties. The aspen CESA stoichiometry results raise the possibility of alternative CSC models and suggest that homomeric PtCESA8b complexes are responsible for cellulose biosynthesis in the gelatinous layer in tension wood.

Defects in intracellular trafficking of fungal cell wall synthases lead to aberrant host immune recognition

The human fungal pathogen, Cryptococcus neoformans, dramatically alters its cell wall, both in size and composition, upon entering the host. This cell wall remodeling is essential for host immune avoidance by this pathogen. In a genetic screen for mutants with changes in their cell wall, we identified a novel protein, Mar1, that controls cell wall organization and immune evasion. Through phenotypic studies of a loss-of-function strain, we have demonstrated that the mar1Δ mutant has an aberrant cell surface and a defect in polysaccharide capsule attachment, resulting in attenuated virulence. Furthermore, the mar1Δ mutant displays increased staining for exposed cell wall chitin and chitosan when the cells are grown in host-like tissue culture conditions. However, HPLC analysis of whole cell walls and RT-PCR analysis of cell wall synthase genes demonstrated that this increased chitin exposure is likely due to decreased levels of glucans and mannans in the outer cell wall layers. We observed that the Mar1 protein differentially localizes to cellular membranes in a condition dependent manner, and we have further shown that the mar1Δ mutant displays defects in intracellular trafficking, resulting in a mislocalization of the β-glucan synthase catalytic subunit, Fks1. These cell surface changes influence the host-pathogen interaction, resulting in increased macrophage activation to microbial challenge in vitro. We established that several host innate immune signaling proteins are required for the observed macrophage activation, including the Card9 and MyD88 adaptor proteins, as well as the Dectin-1 and TLR2 pattern recognition receptors. These studies explore novel mechanisms by which a microbial pathogen regulates its cell surface in response to the host, as well as how dysregulation of this adaptive response leads to defective immune avoidance.