Lorelle Phillips

Suppression of 4-coumarate-CoA ligase in the coniferous gymnosperm Pinus radiata.

Severe suppression of 4-coumarate-coenzyme A ligase (4CL) in the coniferous gymnosperm Pinus radiata substantially affected plant phenotype and resulted in dwarfed plants with a “bonsai tree-like” appearance. Microscopic analyses of stem sections from 2-year-old plants revealed substantial morphological changes in both wood and bark tissues. This included the formation of weakly lignified tracheids that displayed signs of collapse and the development of circumferential bands of axial parenchyma. Acetyl bromide-soluble lignin assays and proton nuclear magnetic resonance studies revealed lignin reductions of 36% to 50% in the most severely affected transgenic plants. Two-dimensional nuclear magnetic resonance and pyrolysis-gas chromatography-mass spectrometry studies indicated that lignin reductions were mainly due to depletion of guaiacyl but not p-hydroxyphenyl lignin. 4CL silencing also caused modifications in the lignin interunit linkage distribution, including elevated β-aryl ether (β-O-4 unit) and spirodienone (β-1) levels, accompanied by lower phenylcoumaran (β-5), resinol (β-β), and dibenzodioxocin (5-5/β-O-4) levels. A sharp depletion in the level of saturated (dihydroconiferyl alcohol) end groups was also observed. Severe suppression of 4CL also affected carbohydrate metabolism. Most obvious was an up to approximately 2-fold increase in galactose content in wood from transgenic plants due to increased compression wood formation. The molecular, anatomical, and analytical data verified that the isolated 4CL clone is associated with lignin biosynthesis and illustrated that 4CL silencing leads to complex, often surprising, physiological and morphological changes in P. radiata.

Syringyl lignin production in conifers: Proof of concept in a Pine tracheary element system.

Significance This study shows that metabolic engineering can be used to imbue pine tracheary elements with an ability to synthesize sinapyl alcohol, a lignin monomer not normally used for lignification in conifers such as pine. The dynamic nature of the lignification process enables pines to incorporate this monolignol, allowing them to produce hardwood-like lignins that are known to facilitate refining processes such as biofuel production and chemical pulping. The potential to improve the refining of conifer-derived biomass through lignin manipulations is important, as even small improvements in yield can lead to significant environmental and economic benefits in such processes. Conifers (softwoods) naturally lack syringyl units in their lignins, rendering lignocellulosic materials from such species more difficult to process than syringyl-rich hardwood species. Using a transformable Pinus radiata tracheary element (TE) system as an experimental platform, we investigated whether metabolic engineering can be used to create syringyl lignin in conifers. Pyrolysis-GC/MS and 2D-NMR analysis of P. radiata TE cultures transformed to express ferulate 5-hydroxylase (F5H) and caffeic acid O-methyltransferase (COMT) from Liquidambar styraciflua confirmed the production and incorporation of sinapyl alcohol into the lignin polymer. Transformation with F5H was sufficient for the production of syringyl lignin in TEs, but cotransformation with COMT improved its formation. In addition, lower levels of the pathway intermediate 5-hydroxyconiferyl alcohol were evidenced in cotransformation experiments, indicating that the introduction of the COMT overcame the inefficiency of the native pine methyltransferases for supporting sinapyl alcohol production.Our results provide the proof of concept that it is possible to generate a lignin polymer that contains syringyl units in softwood species such as P. radiata, suggesting that it might be possible to retain the outstanding fiber properties of softwoods while imbuing them with the lignin characteristics of hardwoods that are more favorable for industrial processing.

Suppression of CCR impacts metabolite profile and cell wall composition in Pinus radiata tracheary elements

Suppression of the lignin-related gene cinnamoyl-CoA reductase (CCR) in the Pinus radiata tracheary element (TE) system impacted both the metabolite profile and the cell wall matrix in CCR-RNAi lines. UPLC–MS/MS-based metabolite profiling identified elevated levels of p-coumaroyl hexose, caffeic acid hexoside and ferulic acid hexoside in CCR-RNAi lines, indicating a redirection of metabolite flow within phenylpropanoid metabolism. Dilignols derived from coniferyl alcohol such as G(8-5)G, G(8-O-4)G and isodihydrodehydrodiconiferyl alcohol (IDDDC) were substantially depleted, providing evidence for CCR’s involvement in coniferyl alcohol biosynthesis. Severe CCR suppression almost halved lignin content in TEs based on a depletion of both H-type and G-type lignin, providing evidence for CCR’s involvement in the biosynthesis of both lignin types. 2D-NMR studies revealed minor changes in the H:G-ratio and consequently a largely unchanged interunit linkage distribution in the lignin polymer. However, unusual cell wall components including ferulate and unsaturated fatty acids were identified in TEs by thioacidolysis, pyrolysis-GC/MS and/or 2D-NMR in CCR-RNAi lines, providing new insights into the consequences of CCR suppression in pine. Interestingly, CCR suppression substantially promoted pyrolytic breakdown of cell wall polysaccharides, a phenotype most likely caused by the incorporation of acidic compounds into the cell wall matrix in CCR-RNAi lines.

Exploring the Ultrastructural Localization and Biosynthesis of β(1,4)-Galactan in Pinus radiata Compression Wood

Softwood species such as pines react to gravitropic stimuli by producing compression wood, which unlike normal wood contains significant amounts of β(1,4)-galactan. Currently, little is known regarding the biosynthesis or physiological function of this polymer or the regulation of its deposition. The subcellular location of β(1,4)-galactan in developing tracheids was investigated in Pinus radiata D. Don using anti-β(1,4)-galactan antibodies to gain insight into its possible physiological role in compression wood. β(1,4)-Galactan was prominent and evenly distributed throughout the S2 layer of developing tracheid cell walls in P. radiata compression wood. In contrast, β(1,4)-galactan was not detected in normal wood. Greatly reduced antibody labeling was observed in fully lignified compression wood tracheids, implying that lignification results in masking of the epitope. To begin to understand the biosynthesis of galactan and its regulation, an assay was developed to monitor the enzyme that elongates the β(1,4)-galactan backbone in pine. A β(1,4)-galactosyltransferase (GalT) activity capable of extending 2-aminopyridine-labeled galacto-oligosaccharides was found to be associated with microsomes. Digestion of the enzymatic products using a β(1,4)-specific endogalactanase confirmed the production of β(1,4)-galactan by this enzyme. This GalT activity was substantially higher in compression wood relative to normal wood. Characterization of the identified pine GalT enzyme activity revealed pH and temperature optima of 7.0 and 20°C, respectively. The β(1,4)-galactan produced by the pine GalT had a higher degree of polymerization than most pectic galactans found in angiosperms. This observation is consistent with the high degree of polymerization of the naturally occurring β(1,4)-galactan in pine.

Proteomic analysis of membrane preparations from developing Pinus radiata compression wood.

For coniferous gymnosperms, few data exist as to the contribution of the membrane-associated proteome to cell wall and wood formation. In this study, we begin to address this knowledge deficiency by examining the proteomic profile of Golgi-enriched membrane preparations derived from developing Pinus radiata compression wood. These membrane preparations were generated by a combination of discontinuous sucrose gradient centrifugation and Triton X-114-based phase separation. Fractionation by phase separation removed contaminating proteins associated with the cytoskeleton and enabled the discrimination between soluble and membrane-bound/integral proteins. The proteomic analysis of the resulting aqueous and detergent phases using high-performance liquid chromatography-tandem mass spectrometry resulted in the identification of 175 proteins. The majority of the identified proteins were membrane bound/integral and originated from cellular components such as the nucleus, plastids, endoplasmic reticulum, plasma membrane and Golgi vesicles. On the basis of bioinformatic analysis, many of the identified proteins were predicted to be involved either in the regulation of wood formation or in cell wall biosynthesis, which indicated that the proteomic analysis of non-cytosolic proteins in developing xylem is a useful strategy to investigate the molecular aspects of wood formation in pine.

Bioinformatic and phylogenetic analysis of the CLAVATA3/EMBRYO-SURROUNDING REGION (CLE) and the CLE-LIKE signal peptide genes in the Pinophyta

BackgroundThere is a rapidly growing awareness that plant peptide signalling molecules are numerous and varied and they are known to play fundamental roles in angiosperm plant growth and development. Two closely related peptide signalling molecule families are the CLAVATA3-EMBRYO-SURROUNDING REGION (CLE) and CLE-LIKE (CLEL) genes, which encode precursors of secreted peptide ligands that have roles in meristem maintenance and root gravitropism. Progress in peptide signalling molecule research in gymnosperms has lagged behind that of angiosperms. We therefore sought to identify CLE and CLEL genes in gymnosperms and conduct a comparative analysis of these gene families with angiosperms.ResultsWe undertook a meta-analysis of the GenBank/EMBL/DDBJ gymnosperm EST database and the Picea abies and P. glauca genomes and identified 93 putative CLE genes and 11 CLEL genes among eight Pinophyta species, in the genera Cryptomeria, Pinus and Picea. The predicted conifer CLE and CLEL protein sequences had close phylogenetic relationships with their homologues in Arabidopsis. Notably, perfect conservation of the active CLE dodecapeptide in presumed orthologues of the Arabidopsis CLE41/44-TRACHEARY ELEMENT DIFFERENTIATION (TDIF) protein, an inhibitor of tracheary element (xylem) differentiation, was seen in all eight conifer species. We cloned the Pinus radiata CLE41/44-TDIF orthologues. These genes were preferentially expressed in phloem in planta as expected, but unexpectedly, also in differentiating tracheary element (TE) cultures. Surprisingly, transcript abundances of these TE differentiation-inhibitors sharply increased during early TE differentiation, suggesting that some cells differentiate into phloem cells in addition to TEs in these cultures. Applied CLE13 and CLE41/44 peptides inhibited root elongation in Pinus radiata seedlings. We show evidence that two CLEL genes are alternatively spliced via 3′-terminal acceptor exons encoding separate CLEL peptides.ConclusionsThe CLE and CLEL genes are found in conifers and they exhibit at least as much sequence diversity in these species as they do in other plant species. Only one CLE peptide sequence has been 100% conserved between gymnosperms and angiosperms over 300 million years of evolutionary history, the CLE41/44-TDIF peptide and its likely conifer orthologues. The preferential expression of these vascular development-regulating genes in phloem in conifers, as they are in dicot species, suggests close parallels in the regulation of secondary growth and wood formation in gymnosperm and dicot plants. Based on our bioinformatic analysis, we predict a novel mechanism of regulation of the expression of several conifer CLEL peptides, via alternative splicing resulting in the selection of alternative C-terminal exons encoding separate CLEL peptides.