The polymorphism of the genes/enzymes involved in the last two reductive steps of monolignol synthesis: what is the functional significance?

Identification and Functional Analysis of CAD Gene Family in Pomegranate (Punica granatum)

[Objective] Cinnamyl alcohol dehydrogenase (CAD) is a key enzyme in lignin biosynthesis. The aim of this study was to identify CAD gene family members in pomegranate and its expression correlation with seed hardness. [Methods] Based on the reported CAD sequence of Arabidopsis, the CAD gene family of pomegranate was identified by homologous comparison, and then phylogenetic, molecular characterization, and expression profile analysis were performed. [Results] Pomegranate CAD gene family has 25 members, distributed on seven chromosomes of pomegranate. All pomegranate CAD proteins have similar physical and chemical properties. We divide the family into four groups based on evolutionary relationships. The member of group I, called bona fide CAD, was involved in lignin synthesis. Most of the members of group II were involved in stress resistance. The functions of groups III and IV need to be explored. We found four duplicated modes (whole genome duplication or segmental (WGD), tandem duplication (TD), dispersed duplication (DSD), proximal duplication (PD) in this family; TD (36%) had the largest number of them. We predicted that 20 cis-acting elements were involved in lignin synthesis, stress resistance, and response to various hormones. Gene expression profiles further demonstrated that the PgCAD gene family had multiple functions. [Conclusions] Pomegranate CAD gene family is involved in lignin synthesis of hard-seeded cultivar Hongyushizi and Baiyushizi, but its role in seed hardness of soft-seeded cultivar Tunisia needs to be further studied.

Monolignol biosynthesis is associated with resistance to Sclerotinia sclerotiorum in Camelina sativa

The ascomycete Sclerotinia sclerotiorum is a necrotrophic plant pathogen with an extremely broad host range. It causes stem rot in Camelina sativa, a crucifer with great potential as an alternative oilseed crop. Lignification is a common phenomenon in the expression of resistance against necrotrophs, but the molecular mechanisms underlying this defence response are poorly understood. We present histochemical, gene expression and biochemical data investigating the role of monolignols in the resistance of C. sativa to S. sclerotiorum. Comparative studies with resistant and susceptible lines of C. sativa revealed substantial differences in constitutive transcript levels and gene regulation patterns for members of the gene family encoding cinnamoyl-CoA reductase (CCR), the first enzyme specifically committed to the synthesis of lignin monomers. These differences were associated with anatomical and metabolic factors. While the induction of CsCCR2 expression after inoculation with S. sclerotiorum was associated with the deposition of lignin mainly derived from guaiacyl monomers, high constitutive levels of CsCCR4 paralleled a high syringyl lignin content in healthy stems of resistant plants. The results provide evidence that plant cell wall strengthening plays a role in the resistance of C. sativa to S. sclerotiorum, and that both constitutive and inducible defence mechanisms contribute to reduced symptom development in resistant germplasm. This study provides the first characterization of quantitative resistance in C. sativa to S. sclerotiorum.

Comparative and phylogenomic analyses of cinnamoyl-CoA reductase and cinnamoyl-CoA-reductase-like gene family in land plants

The biosynthesis of monolignols, the main components of lignin, involves many intermediates and enzymes. The cinnamoyl-CoA reductase (CCR) enzyme catalyzes the conversion of cinnamoyl-CoAs to cinnamaldehydes, i.e. the first specific step in lignin synthesis. The CCR and CCR-like gene family was studied partially in several plant species. This is a comprehensive study of the CCR and CCR-like gene family including genome organization, gene structure, phylogeny across land plant species, and, expression profiling in Populus. Analysis of amino acid motifs enabled the identification of sequence variations in the CCR catalytic site and annotates CCR and CCR-like genes. CCR and CCR-like genes were distributed in three major phylogenetic classes of which one includes the bona fide CCR genes. The other two classes include CCR and CCR-like, of which several genes present a high similarity to cinnamyl alcohol dehydrogenase, or dihydroflavonol reductase (DFR) genes. All CCR, CCR-like, and DFR classes were deeply rooted in the phylogeny of land plants suggesting that their evolution preceded the evolution of lycophytes. Over two thirds of CCR and CCR-like Populus genes were physically distributed on duplicated regions. This suggests that these duplication/retention processes contributed significantly to the size of the CCR and CCR-like gene family. The Populus CCR and CCR-like genes showed six expression patterns in the tissues studied with a preferential expression of PoptrCCR12 in xylem. The other genes present divergent expression profiles with some preferentially expressed in leaves, bark, or both. Several CCR and CCR-like genes were induced or repressed under various abiotic stresses suggesting that their duplication was followed by the evolution of divergent expression profiles and divergence of functions.

Cellulose and lignin biosynthesis is altered by ozone in wood of hybrid poplar (Populus tremula × alba)

Wood formation in trees is a dynamic process that is strongly affected by environmental factors. However, the impact of ozone on wood is poorly documented. The objective of this study was to assess the effects of ozone on wood formation by focusing on the two major wood components, cellulose and lignin, and analysing any anatomical modifications. Young hybrid poplars (Populus tremula × alba) were cultivated under different ozone concentrations (50, 100, 200, and 300 l l(-1)). As upright poplars usually develop tension wood in a non-set pattern, the trees were bent in order to induce tension wood formation on the upper side of the stem and normal or opposite wood on the lower side. Biosynthesis of cellulose and lignin (enzymes and RNA levels), together with cambial growth, decreased in response to ozone exposure. The cellulose to lignin ratio was reduced, suggesting that cellulose biosynthesis was more affected than that of lignin. Tension wood was generally more altered than opposite wood, especially at the anatomical level. Tension wood may be more susceptible to reduced carbon allocation to the stems under ozone exposure. These results suggested a coordinated regulation of cellulose and lignin deposition to sustain mechanical strength under ozone. The modifications of the cellulose to lignin ratio and wood anatomy could allow the tree to maintain radial growth while minimizing carbon cost.

Sequence analysis and functional characterization of the promoter of the Picea glauca Cinnamyl Alcohol Dehydrogenase gene in transgenic white spruce plants

The enzyme Cinnamyl Alcohol Dehydrogenase (CAD) catalyses the last step of lignin monomer synthesis, and is considered as a molecular marker of cell wall lignification in different plants species. Here, we report the isolation and analysis of 5' flanking genomic DNA regions upstream to the CAD gene, from two conifers, i.e. white spruce (Picea glauca (Moench) Voss) and loblolly pine (Pinus taeda L.). Sequence comparisons with available CAD gene promoters from angiosperms highlighted the conservation of cis-elements matching MYB, WRKY and bHLH binding sites. Functional characterization of the P. glauca CAD promoter used P. glauca seedlings stably transformed with a DNA fragment of 1,163 base pairs (PgCAD) fused to the beta-glucuronidase (GUS) gene. Histochemical observations of different vegetative organs of the transgenic trees showed that this sequence was sufficient to drive GUS expression in lignifying tissues, and more specifically in differentiating xylem cells. Quantitative RT-PCR experiments also indicated that the native CAD gene was preferentially expressed in differentiating xylem both in stems and roots. In addition, GUS expression driven by the PgCAD promoter was wound-inducible which was consistent with the accumulation of CAD mRNA in response to jasmonate application and mechanical wounding. The spruce CAD promoter represents a valuable tool for research and biotechnology applications related to xylem and wood.

A genomewide analysis of the cinnamyl alcohol dehydrogenase family in sorghum [Sorghum bicolor (L.) Moench] identifies SbCAD2 as the brown midrib6 gene

The content and composition of the plant cell wall polymer lignin affect plant fitness, carbon sequestration potential, and agro-industrial processing. These characteristics, are heavily influenced by the supply of hydroxycinnamyl alcohol precursors synthesized by the enzyme cinnamyl alcohol dehydrogenase (CAD). In angiosperms, CAD is encoded by a multigene family consisting of members thought to have distinct roles in different stages of plant development. Due to the high sequence similarity among CAD genes, it has been challenging to identify and study the role of the individual genes without a genome sequence. Analysis of the recently released sorghum genome revealed the existence of 14 CAD-like genes at seven genomic locations. Comparisons with maize and rice revealed subtle differences in gene number, arrangement, and expression patterns. Sorghum CAD2 is the predominant CAD involved in lignification based on the phylogenetic relationship with CADs from other species and genetic evidence showing that a set of three allelic brown midrib (bmr) lignin mutants contained mutations in this gene. The impact of the mutations on the structure of the protein was assessed using molecular modeling based on X-ray crystallography data of the closely related Arabidopsis CAD5. The modeling revealed unique changes in structure consistent with the observed phenotypes of the mutants.

From lignins to tannins: forty years of enzyme studies on the biosynthesis of phenolic compounds

In the early 1960s, enzyme studies increasingly began to replace the common 'feeding' experiments in which labeled tracers were applied to living plants or plant parts for elucidating metabolic pathways. This advanced technique allowed to gain much deeper insights into individual details of metabolic sequences, and particularly on the previously inaccessible role of activated 'energy-rich' intermediates. Based on the author's own experience for the past 40+ years in this field, principal findings and trends elucidating the pathways to lignin and lignin precursors, acyl amides and hydrolyzable tannins (gallotannins, ellagitannins) by enzyme studies are reported.

Down-regulation of cinnamoyl-CoA reductase in tomato (Solanum lycopersicum L.) induces dramatic changes in soluble phenolic pools

Health-beneficial properties of many secondary plant metabolites have created much interest into the control of their biosynthesis in crop species. Phenolic compounds, including flavonoids, hydroxycinnamates, and tannins, make up an important group of such phytonutrients. They are formed via the phenylpropanoid pathway and share common precursors with lignin, an insoluble cell wall-associated polymer. In this study, the aim was to reduce lignin biosynthesis so as to enhance the availability of these precursors and, thereby, stimulate the production of soluble, potentially health-promoting, phenolic compounds in tomato (Solanum lycopersicum L.). First two tomato genes encoding cinnamoyl-CoA reductase (CCR), a key enzyme in the formation of lignin monomers, were identified and characterized. Transgenic plants exhibiting a reduced lignin content were subsequently obtained through an RNAi strategy targeting one of these genes. As anticipated, the total level of soluble phenolics was higher in stems and leaves of the transformants as compared with control plants. This was correlated with an increased antioxidant capacity of the corresponding plant extracts. Analysis of the soluble phenolic fraction by HPLC-MS revealed that vegetative organs of CCR down-regulated plants contained higher amounts of chlorogenic acid and rutin, and accumulated new metabolites undetectable in the wild type, such as N-caffeoyl putrescine and kaempferol rutinoside. In fruits, CCR down-regulation triggered the moderate accumulation of two new compounds in the flesh, but the total phenolic content was not affected. Although the prospects of exploiting such a strategy for crop improvement are limited, the results provide further insight into the control of the phenylpropanoid pathway in the Solanaceae.

Metabolite profiling reveals a role for atypical cinnamyl alcohol dehydrogenase CAD1 in the synthesis of coniferyl alcohol in tobacco xylem

In angiosperms, lignin is built from two main monomers, coniferyl and sinapyl alcohol, which are incorporated respectively as G and S units in the polymer. The last step of their synthesis has so far been considered to be performed by a family of dimeric cinnamyl alcohol dehydrogenases (CAD2). However, previous studies on Eucalyptus gunnii xylem showed the presence of an additional, structurally unrelated, monomeric CAD form named CAD1. This form reduces coniferaldehyde to coniferyl alcohol, but is inactive on sinapaldehyde. In this paper, we report the functional characterization of CAD1 in tobacco (Nicotiana tabacum L.). Transgenic tobacco plants with reduced CAD1 expression were obtained through an RNAi strategy. These plants displayed normal growth and development, and detailed biochemical studies were needed to reveal a role for CAD1. Lignin analyses showed that CAD1 down-regulation does not affect Klason lignin content, and has a moderate impact on G unit content of the non-condensed lignin fraction. However, comparative metabolic profiling of the methanol-soluble phenolic fraction from basal xylem revealed significant differences between CAD1 down-regulated and wild-type plants. Eight compounds were less abundant in CAD1 down-regulated lines, five of which were identified as dimers or trimers of monolignols, each containing at least one moiety derived from coniferyl alcohol. In addition, 3-trans-caffeoyl quinic acid accumulated in the transgenic plants. Together, our results support a significant contribution of CAD1 to the synthesis of coniferyl alcohol in planta, along with the previously characterized CAD2 enzymes.