Eucalyptus gunnii CCR and CAD2 promoters are active in lignifying cells during primary and secondary xylem formation in Arabidopsis thaliana

Cell-specific expression patterns of the Eucalyptus gunnii cinnamoyl coenzymeA reductase (EgCCR) and cinnamyl alcohol dehydrogenase (EgCAD2) promoters were analyzed by promoter-GUS histochemistry in the primary and secondary xylem tissues from floral stems and roots of Arabidopsis thaliana. Expression patterns indicated that the EgCCR and EgCAD2 genes were expressed in a coordinated manner in primary and secondary xylem tissues of the Arabidopsis floral stem and root. Both genes were expressed in all lignifying cells (vessel elements, xylem fibers and paratracheal parenchyma cells) of xylem tissues. The capacity for long-term monolignol production appeared to be related to the cell-specific developmental processes and biological roles of different cell types. Our results suggested that lignification of short-lived vessel elements was achieved by a two-step process involving (i) monolignol production by vessel elements prior to vessel programmed cell death and (ii) subsequent monolignol production by vessel-associated living paratracheal parenchyma cells following vessel element cell death. EgCCR and EgCAD2 gene expression patterns suggested that the process of xylem cell lignification was similar in both primary and secondary xylem tissues in Arabidopsis floral stems and roots.     

Evaluation of the chemical reactivity in lignin precursors using the Fukui function

The hydroxycinnamyl alcohols: p-coumarol, coniferol and sinapol are considered the basic units and precursors of lignins models. In this work, the specific reactivity of these molecules was studied. We investigate their intrinsic chemical reactivity in terms of the Fukui function, applying the principle of hard and soft acids and bases (HSAB) in the framework of the density functional theory (DFT). Comparisons of their nucleophilic, electrophilic and free radical reactivity show their most probably sites to form linkages among them. It is found that the most reactive sites, for reactions involving free radicals, are the carbons at the β-position in the p-coumarol and sinapol molecules, whilst the regions around the carbon-oxygen bond of the phenoxyl group are the most reactive in coniferol. Figure Isocontour plots for the free radical form of the Fukui function f ⁰ (r), showing the reactive sites toward electron-rich/poor reactants     

Monolignol acylation and lignin structure in some nonwoody plants: a 2D NMR study

Lignins from three nonwoody angiosperms were analyzed by 2D NMR revealing important differences in their molecular structures. The Musatextilis milled-wood-lignin (MWL), with a syringyl-to-guaiacyl (S/G) ratio of 9, was strongly acylated (near 85% of side-chains) at the γ-carbon by both acetates and p-coumarates, as estimated from ¹H-¹³C correlations in C_γ-esterified and C_γ-OH units. The p-coumarate H_3,5-C_3,5 correlation signal was completely displaced by acetylation, and disappeared after alkali treatment, indicating that p-coumaric acid was esterified maintaining its free phenolic group. By contrast, the Cannabissativa MWL (S/G ∼0.8) was free of acylating groups, and the Agavesisalana MWL (S/G ∼4) showed high acylation degree (near 80%) but exclusively with acetates. Extensive C_γ-acylation results in the absence (in M.textilis lignin) or low abundance (4% in A.sisalana lignin) of β-β′ resinol linkages, which require free C_γ-OH to form the double tetrahydrofuran ring. However, minor signals revealed unusual acylated β-β′ structures confirming that acylation is produced at the monolignol level, in agreement with chromatographic identification of γ-acetylated sinapyl alcohol among the plant extractives. In contrast, resinol substructures involved 22% side-chains in the C.sativa MWL. The ratio between β-β′ and β-O-4′ side-chains in these and other MWL varied from 0.32 in C.sativa MWL to 0.02 in M.textilis MWL, and was inversely correlated with the degree of acylation. The opposite was observed for the S/G ratio that was directly correlated with the acylation degree. Monolignol acylation is discussed as a mechanism potentially involved in the control of lignin structure.