Ectopic Defense Gene Expression Is Associated with Growth Defects in Medicago truncatula Lignin Pathway Mutants

Adaptation mechanism of three Impatiens species to different habitats based on stem morphology, lignin and MYB4 gene

BACKGROUND

Impatiens is an important genus with rich species of garden plants, and its distribution is extremely extensive, which is reflected in its diverse ecological environment. However, the specific mechanisms of Impatiens' adaptation to various environments and the mechanism related to lignin remain unclear.

RESULTS

Three representative Impatiens species,Impatiens chlorosepala (wet, low degree of lignification), Impatiens uliginosa (aquatic, moderate degree of lignification) and Impatiens rubrostriata (terrestrial, high degree of lignification), were selected and analyzed for their anatomical structures, lignin content and composition, and lignin-related gene expression. There are significant differences in anatomical parameters among the stems of three Impatiens species, and the anatomical structure is consistent with the determination results of lignin content. Furthermore, the thickness of the xylem and cell walls, as well as the ratio of cell wall thickness to stem diameter have a strong correlation with lignin content. The anatomical structure and degree of lignification in Impatiens can be attributed to the plant's growth environment, morphology, and growth rate. Our analysis of lignin-related genes revealed a negative correlation between the MYB4 gene and lignin content. The MYB4 gene may control the lignin synthesis in Impatiens by controlling the structural genes involved in the lignin synthesis pathway, such as HCT, C3H, and COMT. Nonetheless, the regulation pathway differs between species of Impatiens.

CONCLUSIONS

This study demonstrated consistency between the stem anatomy of Impatiens and the results obtained from lignin content and composition analyses. It is speculated that MYB4 negatively regulates the lignin synthesis in the stems of three Impatiens species by regulating the expression of structural genes, and its regulation mechanism appears to vary across different Impatiens species. This study analyses the variations among different Impatiens plants in diverse habitats, and can guide further molecular investigations of lignin biosynthesis in Impatiens.

Decrypting Molecular Mechanisms Involved in Counteracting Copper and Nickel Toxicity in Jack Pine (Pinus banksiana) Based on Transcriptomic Analysis

The remediation of copper and nickel-afflicted sites is challenged by the different physiological effects imposed by each metal on a given plant system. Pinus banksiana is resilient against copper and nickel, providing an opportunity to build a valuable resource to investigate the responding gene expression toward each metal. The objectives of this study were to (1) extend the analysis of the Pinus banksiana transcriptome exposed to nickel and copper, (2) assess the differential gene expression in nickel-resistant compared to copper-resistant genotypes, and (3) identify mechanisms specific to each metal. The Illumina platform was used to sequence RNA that was extracted from seedlings treated with each of the metals. There were 449 differentially expressed genes (DEGs) between copper-resistant genotypes (RGs) and nickel-resistant genotypes (RGs) at a high stringency cut-off, indicating a distinct pattern of gene expression toward each metal. For biological processes, 19.8% of DEGs were associated with the DNA metabolic process, followed by the response to stress (13.15%) and the response to chemicals (8.59%). For metabolic function, 27.9% of DEGs were associated with nuclease activity, followed by nucleotide binding (27.64%) and kinase activity (10.16%). Overall, 21.49% of DEGs were localized to the plasma membrane, followed by the cytosol (16.26%) and chloroplast (12.43%). Annotation of the top upregulated genes in copper RG compared to nickel RG identified genes and mechanisms that were specific to copper and not to nickel. NtPDR, AtHIPP10, and YSL1 were identified as genes associated with copper resistance. Various genes related to cell wall metabolism were identified, and they included genes encoding for HCT, CslE6, MPG, and polygalacturonase. Annotation of the top downregulated genes in copper RG compared to nickel RG revealed genes and mechanisms that were specific to nickel and not copper. Various regulatory and signaling-related genes associated with the stress response were identified. They included UGT, TIFY, ACC, dirigent protein, peroxidase, and glyoxyalase I. Additional research is needed to determine the specific functions of signaling and stress response mechanisms in nickel-resistant plants.

Manipulating microRNA miR408 enhances both biomass yield and saccharification efficiency in poplar

The conversion of lignocellulosic feedstocks to fermentable sugar for biofuel production is inefficient, and most strategies to enhance efficiency directly target lignin biosynthesis, with associated negative growth impacts. Here we demonstrate, for both laboratory- and field-grown plants, that expression of Pag-miR408 in poplar (Populus alba × P. glandulosa) significantly enhances saccharification, with no requirement for acid-pretreatment, while promoting plant growth. The overexpression plants show increased accessibility of cell walls to cellulase and scaffoldin cellulose-binding modules. Conversely, Pag-miR408 loss-of-function poplar shows decreased cell wall accessibility. Overexpression of Pag-miR408 targets three Pag-LACCASES, delays lignification, and modestly reduces lignin content, S/G ratio and degree of lignin polymerization. Meanwhile, the LACCASE loss of function mutants exhibit significantly increased growth and cell wall accessibility in xylem. Our study shows how Pag-miR408 regulates lignification and secondary growth, and suggest an effective approach towards enhancing biomass yield and saccharification efficiency in a major bioenergy crop.

Systematic approaches to C-lignin engineering in Medicago truncatula

BACKGROUND

C-lignin is a homopolymer of caffeyl alcohol present in the seed coats of a variety of plant species including vanilla orchid, various cacti, and the ornamental plant Cleome hassleriana. Because of its unique chemical and physical properties, there is considerable interest in engineering C-lignin into the cell walls of bioenergy crops as a high-value co-product of bioprocessing. We have used information from a transcriptomic analysis of developing C. hassleriana seed coats to suggest strategies for engineering C-lignin in a heterologous system, using hairy roots of the model legume Medicago truncatula.

RESULTS

We systematically tested strategies for C-lignin engineering using a combination of gene overexpression and RNAi-mediated knockdown in the caffeic acid/5-hydroxy coniferaldehyde 3/5-O-methyltransferase (comt) mutant background, monitoring the outcomes by analysis of lignin composition and profiling of monolignol pathway metabolites. In all cases, C-lignin accumulation required strong down-regulation of caffeoyl CoA 3-O-methyltransferase (CCoAOMT) paired with loss of function of COMT. Overexpression of the Selaginella moellendorffii ferulate 5-hydroxylase (SmF5H) gene in comt mutant hairy roots resulted in lines that unexpectedly accumulated high levels of S-lignin.

CONCLUSION

C-Lignin accumulation of up to 15% of total lignin in lines with the greatest reduction in CCoAOMT expression required the strong down-regulation of both COMT and CCoAOMT, but did not require expression of a heterologous laccase, cinnamyl alcohol dehydrogenase (CAD) or cinnamoyl CoA reductase (CCR) with preference for 3,4-dihydroxy-substituted substrates in M. truncatula hairy roots. Cell wall fractionation studies suggested that the engineered C-units are not present in a heteropolymer with the bulk of the G-lignin.

Injectable Antioxidant and Oxygen-Releasing Lignin Composites to Promote Wound Healing

The application of engineered biomaterials for wound healing has been pursued since the beginning of tissue engineering. Here, we attempt to apply functionalized lignin to confer antioxidation to the extracellular microenvironments of wounds and to deliver oxygen from the dissociation of calcium peroxide for enhanced vascularization and healing responses without eliciting inflammatory responses. Elemental analysis showed 17 times higher quantity of calcium in the oxygen-releasing nanoparticles. Lignin composites including the oxygen-generating nanoparticles released around 700 ppm oxygen per day at least for 7 days. By modulating the concentration of the methacrylated gelatin, we were able to maintain the injectability of lignin composite precursors and the stiffness of lignin composites suitable for wound healing after photo-cross-linking. In situ formation of lignin composites with the oxygen-releasing nanoparticles enhanced the rate of tissue granulation, the formation of blood vessels, and the infiltration of α-smooth muscle actin⁺ fibroblasts into the wounds over 7 days. At 28 days after surgery, the lignin composite with oxygen-generating nanoparticles remodeled the collagen architecture, resembling the basket-weave pattern of unwounded collagen with minimal scar formation. Thus, our study shows the potential of functionalized lignin for wound-healing applications requiring balanced antioxidation and controlled release of oxygen for enhanced tissue granulation, vascularization, and maturation of collagen.

Transcriptional Profile of Soybean Seeds with Contrasting Seed Coat Color

Soybean is the primary source of vegetable protein and is used for various purposes, mainly to feed animals. This crop can have diverse seed coat colors, varying from yellow, black, brown, and green to bicolor. Black seed coat cultivars have already been assigned as favorable for both seed and grain production. Thus, this work aimed to identify genes associated with soybean seed quality by comparing the transcriptomes of soybean seeds with contrasting seed coat colors. The results from RNA-seq analyses were validated with real-time PCR using the cultivar BRS 715A (black seed coat) and the cultivars BRS 413 RR and DM 6563 IPRO (yellow seed coat). We found 318 genes differentially expressed in all cultivars (freshly harvested seeds and seeds stored in cold chamber). From the in silico analysis of the transcriptomes, the following genes were selected and validated with RT-qPCR: ACS1, ACSF3, CYP90A1, CYP710A1, HCT, CBL, and SAHH. These genes are genes induced in the black seed coat cultivar and are part of pathways responsible for ethylene, lipid, brassinosteroid, lignin, and sulfur amino acid biosynthesis. The BRSMG 715A gene has almost 4times more lignin than the yellow seed coat cultivars. These attributes are related to the BRSMG 715A cultivar's higher seed quality, which translates to more longevity and resistance to moisture and mechanical damage. Future silencing studies may evaluate the knockout of these genes to better understand the biology of soybean seeds with black seed coat.

FERONIA and wall-associated kinases coordinate defense induced by lignin modification in plant cell walls

Altering the content or composition of the cell wall polymer lignin is a favored approach to valorize lignin toward biomaterial and chemical production in the biorefinery. However, modifying lignin or cellulose in transgenic plants can induce expression of defense responses and negatively affect growth. Through genetic screening for suppressors of defense gene induction in the low lignin ccr1-3 mutant of Arabidopsis thaliana, we found that loss of function of the receptor-like kinase FERONIA, although not restoring growth, affected cell wall remodeling and blocked release of elicitor-active pectic polysaccharides as a result of the ccr1-3 mutation. Loss of function of multiple wall-associated kinases prevented perception of these elicitors. The elicitors are likely heterogeneous, with tri-galacturonic acid the smallest but not necessarily the most active component. Engineering of plant cell walls will require development of ways to bypass endogenous pectin signaling pathways.

Multi-omic characterization of bifunctional peroxidase 4-coumarate 3-hydroxylase knockdown in Brachypodium distachyon provides insights into lignin modification-associated pleiotropic effects

A bifunctional peroxidase enzyme, 4-coumarate 3-hydroxylase (C3H/APX), provides a parallel route to the shikimate shunt pathway for the conversion of 4-coumarate to caffeate in the early steps of lignin biosynthesis. Knockdown of C3H/APX (C3H/APX-KD) expression has been shown to reduce the lignin content in Brachypodium distachyon. However, like many other lignin-modified plants, C3H/APX-KDs show unpredictable pleiotropic phenotypes, including stunted growth, delayed senescence, and reduced seed yield. A system-wide level understanding of altered biological processes in lignin-modified plants can help pinpoint the lignin-modification associated growth defects to benefit future studies aiming to negate the yield penalty. Here, a multi-omic approach was used to characterize molecular changes resulting from C3H/APX-KD associated lignin modification and negative growth phenotype in Brachypodium distachyon. Our findings demonstrate that C3H/APX knockdown in Brachypodium stems substantially alters the abundance of enzymes implicated in the phenylpropanoid biosynthetic pathway and disrupt cellular redox homeostasis. Moreover, it elicits plant defense responses associated with intracellular kinases and phytohormone-based signaling to facilitate growth-defense trade-offs. A deeper understanding along with potential targets to mitigate the pleiotropic phenotypes identified in this study could aid to increase the economic feasibility of lignocellulosic biofuel production.

Expression of a Hydroxycinnamoyl-CoA Shikimate/Quinate Hydroxycinnamoyl Transferase 4 Gene from Zoysia japonica (ZjHCT4) Causes Excessive Elongation and Lignin Composition Changes in Agrostis stolonifera

Hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT) is considered to be an essential enzyme for regulating the biosynthesis and composition of lignin. To investigate the properties and function of ZjHCT4, the ZjHCT4 gene was cloned from Zoysia japonica with a completed coding sequence of 1284-bp in length, encoding 428 amino acids. The ZjHCT4 gene promoter has several methyl jasmonate (MeJA) response elements. According to analysis of expression patterns, it was up-regulated by MeJA, GA₃ (Gibberellin), and SA (Salicylic acid), and down-regulated by ABA (Abscisic acid). Ectopic ZjHCT4 expression in creeping bentgrass causes excessive plant elongation. In addition, the content of G-lingnin and H-lingnin fell in transgenic plants, whereas the level of S-lingnin increased, resulting in a considerable rise in the S/G unit ratio. Analysis of the expression levels of lignin-related genes revealed that the ectopic expression of ZjHCT4 altered the expression levels of a number of genes involved in the lignin synthesis pathway. Simultaneously, MeJA, SA, GA₃, IAA, BR (Brassinosteroid), and other hormones were dramatically enhanced in transgenic plants relative to control plants, whereas ABA concentration was significantly decreased. Expression of ZjHCT4 impacted lignin composition and plant growth via altering the phenylpropionic acid metabolic pathway and hormone response, as revealed by transcriptome analysis. HCTs may influence plant lignin composition and plant development by altering hormone content. These findings contributed to a deeper comprehension of the lignin synthesis pathway and set the stage for further investigation and application of the HCTs gene.

Genome Assembly and Population Resequencing Reveal the Geographical Divergence of Shanmei (Rubus corchorifolius)

Rubus corchorifolius (Shanmei or mountain berry, 2n = 14) is widely distributed in China, and its fruits possess high nutritional and medicinal values. Here, we reported a high-quality chromosome-scale genome assembly of Shanmei, with contig size of 215.69 Mb and 26,696 genes. Genome comparison among Rosaceae species showed that Shanmei and Fupenzi (Rubus chingii Hu) were most closely related, followed by blackberry (Rubus occidentalis), and that environmental adaptation-related genes were significantly expanded in the Shanmei genome. Further resequencing of 101 samples of Shanmei collected from four regions in the provinces of Yunnan, Hunan, Jiangxi, and Sichuan in China revealed that the Hunan population of Shanmei possessed the highest diversity and represented the more ancestral population. Moreover, the Yunnan population underwent strong selection based on the nucleotide diversity, linkage disequilibrium, and historical effective population size analyses. Furthermore, genes from candidate genomic regions that showed strong divergence were significantly enriched in the flavonoid biosynthesis and plant hormone signal transduction pathways, indicating the genetic basis of adaptation of Shanmei to the local environment. The high-quality assembled genome and the variome dataset of Shanmei provide valuable resources for breeding applications and for elucidating the genome evolution and ecological adaptation of Rubus species.

Cell Wall Signaling in Plant Development and Defense

Plant architecture fundamentally differs from that of other multicellular organisms in that individual cells serve as osmotic bricks, defined by the equilibrium between the internal turgor pressure and the mechanical resistance of the surrounding cell wall, which constitutes the interface between plant cells and their environment. The state and integrity of the cell wall are constantly monitored by cell wall surveillance pathways, which relay information to the cell interior. A recent surge of discoveries has led to significant advances in both mechanistic and conceptual insights into a multitude of cell wall response pathways that play diverse roles in the development, defense, stress response, and maintenance of structural integrity of the cell. However, these advances have also revealed the complexity of cell wall sensing, and many more questions remain to be answered, for example, regarding the mechanisms of cell wall perception, the molecular players in this process, and how cell wall-related signals are transduced and integrated into cellular behavior. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future discoveries in this exciting area of plant biology.

Developmental changes in lignin composition are driven by both monolignol supply and laccase specificity

The factors controlling lignin composition remain unclear. Catechyl (C)-lignin is a homopolymer of caffeyl alcohol with unique properties as a biomaterial and precursor of industrial chemicals. The lignin synthesized in the seed coat of Cleome hassleriana switches from guaiacyl (G)- to C-lignin at around 12 to 14 days after pollination (DAP), associated with a rerouting of the monolignol pathway. Lack of synthesis of caffeyl alcohol limits C-lignin formation before around 12 DAP, but coniferyl alcohol is still synthesized and highly accumulated after 14 DAP. We propose a model in which, during C-lignin biosynthesis, caffeyl alcohol noncompetitively inhibits oxidation of coniferyl alcohol by cell wall laccases, a process that might limit movement of coniferyl alcohol to the apoplast. Developmental changes in both substrate availability and laccase specificity together account for the metabolic fates of G- and C-monolignols in the Cleome seed coat.

Reduced apoplastic barriers in tissues of shoot-proximal rhizomes of Oryza coarctata are associated with Na+ sequestration

Oryza coarctata is the only wild rice species with significant salinity tolerance. The present work examines the role of the substantial rhizomatous tissues of O. coarctata in conferring salinity tolerance. Transition to an erect phenotype (shoot emergence) from prostrate growth of rhizome tissues is characterized by marked lignification and suberization of supporting sclerenchymatous tissue, epidermis, and bundle sheath cells in aerial shoot-proximal nodes and internodes in O. coarctata. With salinity, however, aerial shoot-proximal internodal tissues show reductions in lignification and suberization, most probably related to re-direction of carbon flux towards synthesis of the osmporotectant proline. Concurrent with hypolignification and reduced suberization, the aerial rhizomatous biomass of O. coarctata appears to have evolved mechanisms to store Na+ in these specific tissues under salinity. This was confirmed by histochemical staining, quantitative real-time reverse transcription-PCR expression patterns of genes involved in lignification/suberization, Na+ and K+ contents of internodal tissues, as well as non-invasive microelectrode ion flux measurements of NaCl-induced net Na+, K+, and H+ flux profiles of aerial nodes were determined. In O. coarctata, aerial proximal internodes appear to act as 'traffic controllers', sending required amounts of Na+ and K+ into developing leaves for osmotic adjustment and turgor-driven growth, while more deeply positioned internodes assume a Na+ buffering/storage role.

A gene-editing/complementation strategy for tissue-specific lignin reduction while preserving biomass yield

BACKGROUND

Lignification of secondary cell walls is a major factor conferring recalcitrance of lignocellulosic biomass to deconstruction for fuels and chemicals. Genetic modification can reduce lignin content and enhance saccharification efficiency, but usually at the cost of moderate-to-severe growth penalties. We have developed a method, using a single DNA construct that uses CRISPR-Cas9 gene editing to knock-out expression of an endogenous gene of lignin monomer biosynthesis while at the same time expressing a modified version of the gene's open reading frame that escapes cutting by the Cas9 system and complements the introduced mutation in a tissue-specific manner.

RESULTS

Expressing the complementing open reading frame in vessels allows for the regeneration of Arabidopsis plants with reduced lignin, wild-type biomass yield, and up to fourfold enhancement of cell wall sugar yield per plant. The above phenotypes are seen in both homozygous and bi-allelic heterozygous T1 lines, and are stable over at least four generations.

CONCLUSIONS

The method provides a rapid approach for generating reduced lignin trees or crops with one single transformation event, and, paired with a range of tissue-specific promoters, provides a general strategy for optimizing loss-of-function traits that are associated with growth penalties. This method should be applicable to any plant species in which transformation and gene editing are feasible and validated vessel-specific promoters are available.

Clavibacter michiganensis Downregulates Photosynthesis and Modifies Monolignols Metabolism Revealing a Crosstalk with Tomato Immune Responses

The gram-positive pathogenic bacterium Clavibacter michiganensis subsp. michiganensis (Cmm) causes bacterial canker disease in tomato, affecting crop yield and fruit quality. To understand how tomato plants respond, the dynamic expression profile of host genes was analyzed upon Cmm infection. Symptoms of bacterial canker became evident from the third day. As the disease progressed, the bacterial population increased in planta, reaching the highest level at six days and remained constant till the twelfth day post inoculation. These two time points were selected for transcriptomics. A progressive down-regulation of key genes encoding for components of the photosynthetic apparatus was observed. Two temporally separated defense responses were observed, which were to an extent interdependent. During the primary response, genes of the phenylpropanoid pathway were diverted towards the synthesis of monolignols away from S-lignin. In dicots, lignin polymers mainly consist of G- and S-units, playing an important role in defense. The twist towards G-lignin enrichment is consistent with previous findings, highlighting a response to generate an early protective barrier and to achieve a tight interplay between lignin recomposition and the primary defense response mechanism. Upon progression of Cmm infection, the temporal deactivation of phenylpropanoids coincided with the upregulation of genes that belong in a secondary response mechanism, supporting an elegant reprogramming of the host transcriptome to establish a robust defense apparatus and suppress pathogen invasion. This high-throughput analysis reveals a dynamic reorganization of plant defense mechanisms upon bacterial infection to implement an array of barriers preventing pathogen invasion and spread.

Growth-defense trade-offs and yield loss in plants with engineered cell walls

As a major component of plant secondary cell walls, lignin provides structural integrity and rigidity, and contributes to primary defense by providing a physical barrier to pathogen ingress. Genetic modification of lignin biosynthesis has been adopted to reduce the recalcitrance of lignified cell walls to improve biofuel production, tree pulping properties and forage digestibility. However, lignin-modification is often, but unpredictably, associated with dwarf phenotypes. Hypotheses suggested to explain this include: collapsed vessels leading to defects in water and solute transport; accumulation of molecule(s) that are inhibitory to plant growth or deficiency of metabolites that are critical for plant growth; activation of defense pathways linked to cell wall integrity sensing. However, there is still no commonly accepted underlying mechanism for the growth defects. Here, we discuss recent data on transcriptional reprogramming in plants with modified lignin content and their corresponding suppressor mutants, and evaluate growth-defense trade-offs as a factor underlying the growth phenotypes. New approaches will be necessary to estimate how gross changes in transcriptional reprogramming may quantitatively affect growth. Better understanding of the basis for yield drag following cell wall engineering is important for the biotechnological exploitation of plants as factories for fuels and chemicals.

Targeting hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase for lignin modification in Brachypodium distachyon

BACKGROUND

Hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT) is a central enzyme of the so-called "esters" pathway to monolignols. As originally envisioned, HCT functions twice in this pathway, to form coumaroyl shikimate and then, in the "reverse" direction, to convert caffeoyl shikimate to caffeoyl CoA. The discovery of a caffeoyl shikimate esterase (CSE) that forms caffeic acid directly from caffeoyl shikimate calls into question the need for the reverse HCT reaction in lignin biosynthesis. Loss of function of HCT gives severe growth phenotypes in several dicot plants, but less so in some monocots, questioning whether this enzyme, and therefore the shikimate shunt, plays the same role in both monocots and dicots. The model grass Brachypodium distachyon has two HCT genes, but lacks a classical CSE gene. This study was therefore conducted to evaluate the utility of HCT as a target for lignin modification in a species with an "incomplete" shikimate shunt.

RESULTS

The kinetic properties of recombinant B. distachyon HCTs were compared with those from Arabidopsis thaliana, Medicago truncatula, and Panicum virgatum (switchgrass) for both the forward and reverse reactions. Along with two M. truncatula HCTs, B. distachyon HCT2 had the least kinetically unfavorable reverse HCT reaction, and this enzyme is induced when HCT1 is down-regulated. Down regulation of B. distachyon HCT1, or co-down-regulation of HCT1 and HCT2, by RNA interference led to reduced lignin levels, with only modest changes in lignin composition and molecular weight.

CONCLUSIONS

Down-regulation of HCT1, or co-down-regulation of both HCT genes, in B. distachyon results in less extensive changes in lignin content/composition and cell wall structure than observed following HCT down-regulation in dicots, with little negative impact on biomass yield. Nevertheless, HCT down-regulation leads to significant improvements in biomass saccharification efficiency, making this gene a preferred target for biotechnological improvement of grasses for bioprocessing.

Expression of a bacterial 3-dehydroshikimate dehydratase (QsuB) reduces lignin and improves biomass saccharification efficiency in switchgrass (Panicum virgatum L.)

BACKGROUND

Lignin deposited in plant cell walls negatively affects biomass conversion into advanced bioproducts. There is therefore a strong interest in developing bioenergy crops with reduced lignin content or altered lignin structures. Another desired trait for bioenergy crops is the ability to accumulate novel bioproducts, which would enhance the development of economically sustainable biorefineries. As previously demonstrated in the model plant Arabidopsis, expression of a 3-dehydroshikimate dehydratase in plants offers the potential for decreasing lignin content and overproducing a value-added metabolic coproduct (i.e., protocatechuate) suitable for biological upgrading.

RESULTS

The 3-dehydroshikimate dehydratase QsuB from Corynebacterium glutamicum was expressed in the bioenergy crop switchgrass (Panicum virgatum L.) using the stem-specific promoter of an O-methyltransferase gene (pShOMT) from sugarcane. The activity of pShOMT was validated in switchgrass after observation in-situ of beta-glucuronidase (GUS) activity in stem nodes of plants carrying a pShOMT::GUS fusion construct. Under controlled growth conditions, engineered switchgrass lines containing a pShOMT::QsuB construct showed reductions of lignin content, improvements of biomass saccharification efficiency, and accumulated higher amount of protocatechuate compared to control plants. Attempts to generate transgenic switchgrass lines carrying the QsuB gene under the control of the constitutive promoter pZmUbi-1 were unsuccessful, suggesting possible toxicity issues associated with ectopic QsuB expression during the plant regeneration process.

CONCLUSION

This study validates the transfer of the QsuB engineering approach from a model plant to switchgrass. We have demonstrated altered expression of two important traits: lignin content and accumulation of a co-product. We found that the choice of promoter to drive QsuB expression should be carefully considered when deploying this strategy to other bioenergy crops. Field-testing of engineered QsuB switchgrass are in progress to assess the performance of the introduced traits and agronomic performances of the transgenic plants.

Hevea brasiliensis coniferaldehyde-5-hydroxylase (HbCAld5H) regulates xylogenesis, structure and lignin chemistry of xylem cell wall in Nicotiana tabacum

KEY MESSAGE

The HbCAld5H1 gene cloned from Hevea brasiliensis regulates the cambial activity, xylem differentiation, syringyl-guaiacyl ratio, secondary wall structure, lignification pattern and xylan distribution in xylem fibres of transgenic tobacco plants. Molecular characterization of lignin biosynthesis gene coniferaldehyde-5-hydroxylase (CAld5H) from Hevea brasiliensis and its functional validation was performed. Both sense and antisense constructs of HbCAld5H1 gene were introduced into tobacco through Agrobacterium-mediated genetic transformation for over expression and down-regulation of this key enzyme to understand its role affecting structural and cell wall chemistry. The anatomical studies of transgenic tobacco plants revealed the increase of cambial activity leading to xylogenesis in sense lines and considerable reduction in antisense lines. The ultra-structural studies showed that the thickness of secondary wall (S2 layer) of fibre had been decreased with non-homogenous lignin distribution in antisense lines, while sense lines showed an increase in S2 layer thickness. Maule color reaction revealed that syringyl lignin distribution in the xylem elements was increased in sense and decreased in antisense lines. The immunoelectron microscopy revealed a reduction in LM 10 and LM 11 labelling in the secondary wall of antisense tobacco lines. Biochemical studies showed a radical increase in syringyl lignin in sense lines without any significant change in total lignin content, while S/G ratio decreased considerably in antisense lines. Our results suggest that CAld5H gene plays an important role in xylogenesis stages such as cambial cell division, secondary wall thickness, xylan and syringyl lignin distribution in tobacco. Therefore, CAld5H gene could be considered as a promising target for lignin modification essential for timber quality improvement in rubber.

Substrate Specificity of LACCASE8 Facilitates Polymerization of Caffeyl Alcohol for C-Lignin Biosynthesis in the Seed Coat of Cleome hassleriana

Catechyl lignin (C-lignin) is a linear homopolymer of caffeyl alcohol found in the seed coats of diverse plant species. Its properties make it a natural source of carbon fibers and high-value chemicals, but the mechanism of in planta polymerization of caffeyl alcohol remains unclear. In the ornamental plant Cleome hassleriana, lignin biosynthesis in the seed coat switches from guaiacyl lignin to C-lignin at ∼12 d after pollination. Here we found that the transcript profile of the laccase gene ChLAC8 parallels the accumulation of C-lignin during seed coat development. Recombinant ChLAC8 oxidizes caffeyl and sinapyl alcohols, generating their corresponding dimers or trimers in vitro, but cannot oxidize coniferyl alcohol. We propose a basis for this substrate preference based on molecular modeling/docking experiments. Suppression of ChLAC8 expression led to significantly reduced C-lignin content in the seed coats of transgenic Cleome plants. Feeding of ¹³C-caffeyl alcohol to the Arabidopsis (Arabidopsis thaliana) caffeic acid o-methyltransferase mutant resulted in no incorporation of ¹³C into C-lignin, but expressing ChLAC8 in this genetic background led to appearance of C-lignin with >40% label incorporation. These results indicate that ChLAC8 is required for C-lignin polymerization and determines lignin composition when caffeyl alcohol is available.

An importin-beta-like protein mediates lignin-modification-induced dwarfism in Arabidopsis

Perturbation of lignin biosynthesis often results in severe growth and developmental defects in plants, which imposes practical limitations to genetic enhancement of lignocellulosic biomass for biofuel production. Currently, little information is known about the cellular and genetic mechanisms of this important phenomenon. Here we show that defects in both cell division and cell expansion underlie the dwarfism of an Arabidopsis lignin mutant ref8, and report the identification of a GROWTH INHIBITION RELIEVED 1 (GIR1) gene from a suppressor screen. GIR1 encodes an importin-beta-like protein required for the nuclear import of MYB4, a transcriptional repressor of phenylpropanoid metabolism. Disruption of GIR1 and MYB4 similarly alleviates the cellular defects and growth inhibition in ref8, suggesting that the growth rescue effect of gir1 is likely due to compromised MYB4 transport and function. Importantly, the phenylpropanoid perturbation is not alleviated in gir1 ref8 and myb4 ref8, suggesting that the function of MYB4 in growth inhibition of lignin-modified plants is likely to be distinct from its known role in transcriptional regulation of phenylpropanoid biosynthetic genes. This study also provides evidence that lignin-modification-induced dwarfism is not merely due to compromised water transport brought about by lignin deficiency, as gir1 has no effect on the growth inhibition of other lignin mutants that show the collapsed xylem phenotype.

Salicylic Acid Steers the Growth-Immunity Tradeoff

Plants possess an effective immune system to combat most microbial attackers. The activation of immune responses to biotrophic pathogens requires the hormone salicylic acid (SA). Accumulation of SA triggers a plethora of immune responses (like massive transcriptional reprogramming, cell wall strengthening, and production of secondary metabolites and antimicrobial proteins). A tradeoff of strong immune responses is the active suppression of plant growth and development. The tradeoff also works the opposite way, where active growth and developmental processes suppress SA production and immune responses. Here, we review research on the role of SA in the growth-immunity tradeoff and examples of how the tradeoff can be bypassed. This knowledge will be instrumental in resistance breeding of crops with optimal growth and effective immunity.

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1) releases latent defense signals in stems with reduced lignin content

There is considerable interest in engineering plant cell wall components, particularly lignin, to improve forage quality and biomass properties for processing to fuels and bioproducts. However, modifying lignin content and/or composition in transgenic plants through down-regulation of lignin biosynthetic enzymes can induce expression of defense response genes in the absence of biotic or abiotic stress. Arabidopsis thaliana lines with altered lignin through down-regulation of hydroxycinnamoyl CoA:shikimate/quinate hydroxycinnamoyl transferase (HCT) or loss of function of cinnamoyl CoA reductase 1 (CCR1) express a suite of pathogenesis-related (PR) protein genes. The plants also exhibit extensive cell wall remodeling associated with induction of multiple cell wall-degrading enzymes, a process which renders the corresponding biomass a substrate for growth of the cellulolytic thermophile Caldicellulosiruptor bescii lacking a functional pectinase gene cluster. The cell wall remodeling also results in the release of size- and charge-heterogeneous pectic oligosaccharide elicitors of PR gene expression. Genetic analysis shows that both in planta PR gene expression and release of elicitors are the result of ectopic expression in xylem of the gene ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE 1 (ADPG1), which is normally expressed during anther and silique dehiscence. These data highlight the importance of pectin in cell wall integrity and the value of lignin modification as a tool to interrogate the informational content of plant cell walls.

Lignin biosynthesis: old roads revisited and new roads explored

Lignin is a major component of secondarily thickened plant cell walls and is considered to be the second most abundant biopolymer on the planet. At one point believed to be the product of a highly controlled polymerization procedure involving just three potential monomeric components (monolignols), it is becoming increasingly clear that the composition of lignin is quite flexible. Furthermore, the biosynthetic pathways to the major monolignols also appear to exhibit flexibility, particularly as regards the early reactions leading to the formation of caffeic acid from coumaric acid. The operation of parallel pathways to caffeic acid occurring at the level of shikimate esters or free acids may help provide robustness to the pathway under different physiological conditions. Several features of the pathway also appear to link monolignol biosynthesis to both generation and detoxification of hydrogen peroxide, one of the oxidants responsible for creating monolignol radicals for polymerization in the apoplast. Monolignol transport to the apoplast is not well understood. It may involve passive diffusion, although this may be targeted to sites of lignin initiation/polymerization by ordered complexes of both biosynthetic enzymes on the cytosolic side of the plasma membrane and structural anchoring of proteins for monolignol oxidation and polymerization on the apoplastic side. We present several hypothetical models to illustrate these ideas and stimulate further research. These are based primarily on studies in model systems, which may or may not reflect the major lignification process in forest trees.