Proteomic analysis of membrane preparations from developing Pinus radiata compression wood

The Larix kaempferi genome reveals new insights into wood properties

Here, through single-molecule real-time sequencing, we present a high-quality genome sequence of the Japanese larch (Larix kaempferi), a conifer species with great value for wood production and ecological afforestation. The assembled genome is 10.97 Gb in size, harboring 45,828 protein-coding genes. Of the genome, 66.8% consists of repeat sequences, of which long terminal repeat retrotransposons are dominant and make up 69.86%. We find that tandem duplications have been responsible for the expansion of genes involved in transcriptional regulation and stress responses, unveiling their crucial roles in adaptive evolution. Population transcriptome analysis reveals that lignin content in L. kaempferi is mainly determined by the process of monolignol polymerization. The expression values of six genes (LkCOMT7, LkCOMT8, LkLAC23, LkLAC102, LkPRX148, and LkPRX166) have significantly positive correlations with lignin content. These results indicated that the increased expression of these six genes might be responsible for the high lignin content of the larches' wood. Overall, this study provides new genome resources for investigating the evolution and biological function of conifer trees, and also offers new insights into wood properties of larches.

Wood Formation under Changing Environment: Omics Approaches to Elucidate the Mechanisms Driving the Early-to-Latewood Transition in Conifers

The global change scenarios highlight the urgency of clarifying the mechanisms driving the determination of wood traits in forest trees. Coniferous xylem is characterized by the alternation between earlywood (EW) and latewood (LW), on which proportions the wood density depend, one of the most important mechanical xylem qualities. However, the molecular mechanisms triggering the transition between the production of cells with the typical features of EW to the LW are still far from being completely elucidated. The increasing availability of omics resources for conifers, e.g., genomes and transcriptomes, would lay the basis for the comprehension of wood formation dynamics, boosting both breeding and gene-editing approaches. This review is intended to introduce the importance of wood formation dynamics and xylem traits of conifers in a changing environment. Then, an up-to-date overview of the omics resources available for conifers was reported, focusing on both genomes and transcriptomes. Later, an analysis of wood formation studies using omics approaches was conducted, with the aim of elucidating the main metabolic pathways involved in EW and LW determination. Finally, the future perspectives and the urgent needs on this research topic were highlighted.

Quantitative analysis of differential dehydrin regulation in pine and spruce seedlings under water deficit

Dehydrins are well-known components of plant responses to different stresses that cause dehydration, including drought, freezing, salinity, etc. In conifers, the dehydrin gene family is very large, implying that the members of this family have important physiological functions in conifer stress tolerance. However, dehydrin gene expression displays a wide range of responses to stress, from thousand-fold increased expression to decreased expression, and it is generally unknown how regulatory systems are connected at the mRNA and protein levels. Therefore, we studied these aspects of dehydrin regulation in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst) seedlings under polyethylene glycol 6000-induced osmotic stress ranging from relatively low (culture medium water potential of -0.15 MPa) to very high (-1.0 MPa) intensities. In pine, the major dehydrin protein was Dhn1 in both the roots and needles, and in spruce, two isoforms of the Dhn4 protein were the major dehydrins; both of these proteins are AESK-type dehydrins. The genes encoding these major proteins were highly expressed even under control conditions; surprisingly, we also observed several highly expressed dehydrin genes that were not abundantly translated. Under osmotic stress, the most prominent expression changes were observed for the dehydrin genes with low basal expression levels, whereas highly expressed genes generally demonstrated rather modest changes in expression. We report proposed constitutive physiological functions of the AESK-type dehydrins in Pinaceae plants.

Mass spectrometry-based forest tree metabolomics

Research in forest tree species has advanced slowly when compared with other agricultural crops and model organisms, mainly due to the long-life cycles, large genome sizes, and lack of genomic tools. Additionally, trees are complex matrices, and the presence of interferents (e.g., oleoresins and cellulose) challenges the analysis of tree tissues with mass spectrometry (MS)-based analytical platforms. In this review, advances in MS-based forest tree metabolomics are discussed. Given their economic and ecological significance, particular focus is given to Pinus, Quercus, and Eucalyptus forest tree species to better understand their metabolite responses to abiotic and biotic stresses in the current climate change scenario. Furthermore, MS-based metabolomics technologies produce large and complex datasets that require expertize to adequately manage, process, analyze, and store the data in dedicated repositories. To ensure that the full potential of forest tree metabolomics data are translated into new knowledge, these data should comply with the FAIR principles (i.e., Findable, Accessible, Interoperable, and Re-usable). It is essential that adequate standards are implemented to annotate metadata from forest tree metabolomics studies as is already required by many science and governmental agencies and some major scientific publishers. © 2019 John Wiley & Sons Ltd. Mass Spec Rev 40:126-157, 2021.

Approaches to variant discovery for conifer transcriptome sequencing

There is a wide diversity of bioinformatic tools available for the assembly of next generation sequence and subsequence variant calling to identify genetic markers at scale. Integration of genomics tools such as genomic selection, association studies, pedigree analysis and analysis of genetic diversity, into operational breeding is a goal for New Zealand’s most widely planted exotic tree species, Pinus radiata. In the absence of full reference genomes for large megagenomes such as in conifers, RNA sequencing in a range of genotypes and tissue types, offers a rich source of genetic markers for downstream application. We compared nine different assembler and variant calling software combinations in a single transcriptomic library and found that Single Nucleotide Polymorphism (SNPs) discovery could vary by as much as an order of magnitude (8,061 SNPs up to 86,815 SNPs). The assembler with the best realignment of the packages trialled, Trinity, in combination with several variant callers was then applied to a much larger multi-genotype, multi-tissue transcriptome and identified 683,135 in silico SNPs across a predicted 449,951 exons when mapped to the Pinus taeda ver 1.01e reference.

Exploring the loblolly pine (Pinus taeda L.) genome by BAC sequencing and Cot analysis

Loblolly pine (LP; Pinus taeda L.) is an economically and ecologically important tree in the southeastern U.S. To advance understanding of the loblolly pine (LP; Pinus taeda L.) genome, we sequenced and analyzed 100 BAC clones and performed a Cot analysis. The Cot analysis indicates that the genome is composed of 57, 24, and 10% highly-repetitive, moderately-repetitive, and single/low-copy sequences, respectively (the remaining 9% of the genome is a combination of fold back and damaged DNA). Although single/low-copy DNA only accounts for 10% of the LP genome, the amount of single/low-copy DNA in LP is still 14 times the size of the Arabidopsis genome. Since gene numbers in LP are similar to those in Arabidopsis, much of the single/low-copy DNA of LP would appear to be composed of DNA that is both gene- and repeat-poor. Macroarrays prepared from a LP bacterial artificial chromosome (BAC) library were hybridized with probes designed from cell wall synthesis/wood development cDNAs, and 50 of the "targeted" clones were selected for further analysis. An additional 25 clones were selected because they contained few repeats, while 25 more clones were selected at random. The 100 BAC clones were Sanger sequenced and assembled. Of the targeted BACs, 80% contained all or part of the cDNA used to target them. One targeted BAC was found to contain fungal DNA and was eliminated from further analysis. Combinations of similarity-based and ab initio gene prediction approaches were utilized to identify and characterize potential coding regions in the 99 BACs containing LP DNA. From this analysis, we identified 154 gene models (GMs) representing both putative protein-coding genes and likely pseudogenes. Ten of the GMs (all of which were specifically targeted) had enough support to be classified as intact genes. Interestingly, the 154 GMs had statistically indistinguishable (α = 0.05) distributions in the targeted and random BAC clones (15.18 and 12.61 GM/Mb, respectively), whereas the low-repeat BACs contained significantly fewer GMs (7.08 GM/Mb). However, when GM length was considered, the targeted BACs had a significantly greater percentage of their length in GMs (3.26%) when compared to random (1.63%) and low-repeat (0.62%) BACs. The results of our study provide insight into LP evolution and inform ongoing efforts to produce a reference genome sequence for LP, while characterization of genes involved in cell wall production highlights carbon metabolism pathways that can be leveraged for increasing wood production.

Comparative "Golgi" Proteome Study of Lolium multiflorum and Populus trichocarpa

The Golgi apparatus (GA) is a crucial organelle in the biosynthesis of non-cellulosic polysaccharides, glycoproteins and proteoglycans that are primarily destined for secretion to the cell surface (plasma membrane, cell wall and apoplast). Only a small proportion of the proteins involved in these processes have been identified in plants, with the majority of their functions still unknown. The availability of a GA proteome would greatly assist plant biochemists, cell and molecular biologists in determining the precise function of the cell wall-related proteins. There has been some progress towards defining the GA proteome in the model plant system Arabidopsis thaliana, yet in commercially important species, such as either the cereals or woody species there has been relatively less progress. In this study, we applied discontinuous sucrose gradient centrifugation to partially enrich GA from suspension cell cultures (SCCs) and combined this with stable isotope labelling (iTRAQ) to determine protein sub-cellular locations. Results from a representative grass species, Italian ryegrass (Lolium multiflorum) and a dicot species, black cottonwood (Populus trichocarpa) are compared. The results confirm that membrane fractionation approaches that provide effective GA-enriched fractions for proteomic analyses in Arabidopsis are much less effective in the species examined here and highlight the complexity of the GA, both within and between species.

Biosynthesis and Metabolic Fate of Phenylalanine in Conifers

The amino acid phenylalanine (Phe) is a critical metabolic node that plays an essential role in the interconnection between primary and secondary metabolism in plants. Phe is used as a protein building block but it is also as a precursor for numerous plant compounds that are crucial for plant reproduction, growth, development, and defense against different types of stresses. The metabolism of Phe plays a central role in the channeling of carbon from photosynthesis to the biosynthesis of phenylpropanoids. The study of this metabolic pathway is particularly relevant in trees, which divert large amounts of carbon into the biosynthesis of Phe-derived compounds, particularly lignin, an important constituent of wood. The trunks of trees are metabolic sinks that consume a considerable percentage of carbon and energy from photosynthesis, and carbon is finally immobilized in wood. This paper reviews recent advances in the biosynthesis and metabolic utilization of Phe in conifer trees. Two alternative routes have been identified: the ancient phenylpyruvate pathway that is present in microorganisms, and the arogenate pathway that possibly evolved later during plant evolution. Additionally, an efficient nitrogen recycling mechanism is required to maintain sustained growth during xylem formation. The relevance of phenylalanine metabolic pathways in wood formation, the biotic interactions, and ultraviolet protection is discussed. The genetic manipulation and transcriptional regulation of the pathways are also outlined.

Proteomic Characterization of Golgi Membranes Enriched from Arabidopsis Suspension Cell Cultures

The plant Golgi apparatus has a central role in the secretory pathway and is the principal site within the cell for the assembly and processing of macromolecules. The stacked membrane structure of the Golgi apparatus along with its interactions with the cytoskeleton and endoplasmic reticulum has historically made the isolation and purification of this organelle difficult. Density centrifugation has typically been used to enrich Golgi membranes from plant microsomal preparations, and aside from minor adaptations, the approach is still widely employed. Here we outline the enrichment of Golgi membranes from an Arabidopsis cell suspension culture that can be used to investigate the proteome of this organelle. We also provide a useful workflow for the examination of proteomic data as the result of multiple analyses. Finally, we highlight a simple technique to validate the subcellular localization of proteins by fluorescent tags after their identification by tandem mass spectrometry.

An efficient two-step subcellular fractionation method for the enrichment of insulin granules from INS-1 cells

Insulin is one of the key regulators for blood glucose homeostasis. More than 99% of insulin is secreted from the pancreatic β-cells. Within each β-cell, insulin is packaged and processed in insulin secretary granules (ISGs) before its exocytosis. Insulin secretion is a complicated but well-organized dynamic process that includes the budding of immature ISGs (iISGs) from the trans-Golgi network, iISG maturation, and mature ISG (mISG) fusion with plasma membrane. However, the molecular mechanisms involved in this process are largely unknown. It is therefore crucial to separate and enrich iISGs and mISGs before determining their distinct characteristics and protein contents. Here, we developed an efficient two-step subcellular fractionation method for the enrichment of iISGs and mISGs from INS-1 cells: OptiPrep gradient purification followed by Percoll solution purification. We demonstrated that by using this method, iISGs and mISGs can be successfully distinguished and enriched. This method can be easily adapted to investigate SGs in other cells or tissues, thereby providing a useful tool for elucidating the mechanisms of granule maturation and secretion.

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

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.

Proteomic analysis during ontogenesis of secondary xylem in maritime pine

Secondary xylem (wood) is formed through an intricate biological process that results in a highly variable final product. Studies have focused on understanding the molecular events for wood formation in conifers. In this process environmental, ontogenic and genetic factors influence variation in wood characteristics, including anatomical, chemical and physical properties. The main objective of this study was to analyse the ageing (ontogenic) effect on protein accumulation in wood-forming tissues along a cambial age (CA) gradient, ranging from juvenile wood (JW) sampled at the top of the tree, to mature wood (MW) sampled at the bottom of the tree. A total of 62 proteins whose accumulation varied by at least 1.5-fold according to CA were selected and identified by ESI-MS/MS; 30 of these were more abundant in MW and 32 were more abundant in JW. Consistent with earlier findings, our results show that JW is a tissue characterized by a high energy demand with the accumulation of gene products involved in energy, protein fate and cellular transport, while proteins identified in MW (heat shock response, oxygen and radical detoxification, and the S-adenosyl methionine cycle) support the idea that this tissue undergoes extended cell-wall thickening and a delay of programmed cell death.

Golgi enrichment and proteomic analysis of developing Pinus radiata xylem by free-flow electrophoresis

Our understanding of the contribution of Golgi proteins to cell wall and wood formation in any woody plant species is limited. Currently, little Golgi proteomics data exists for wood-forming tissues. In this study, we attempted to address this issue by generating and analyzing Golgi-enriched membrane preparations from developing xylem of compression wood from the conifer Pinus radiata. Developing xylem samples from 3-year-old pine trees were harvested for this purpose at a time of active growth and subjected to a combination of density centrifugation followed by free flow electrophoresis, a surface charge separation technique used in the enrichment of Golgi membranes. This combination of techniques was successful in achieving an approximately 200-fold increase in the activity of the Golgi marker galactan synthase and represents a significant improvement for proteomic analyses of the Golgi from conifers. A total of thirty known Golgi proteins were identified by mass spectrometry including glycosyltransferases from gene families involved in glucomannan and glucuronoxylan biosynthesis. The free flow electrophoresis fractions of enriched Golgi were highly abundant in structural proteins (actin and tubulin) indicating a role for the cytoskeleton during compression wood formation. The mass spectrometry proteomics data associated with this study have been deposited to the ProteomeXchange with identifier PXD000557.

Changes in xylem tissue and laccase transcript abundance associated with posture recovery in Chamaecyparis obtusa saplings growing on an incline

Lignin is a major component of plant cell walls and is synthesised through oxidative polymerisation of monolignols. The transcription level of laccase, an enzyme implicated in monolignol polymerisation, is higher in the tissue forming compression wood than in normal wood. Compression wood, which is a special xylem tissue that develops to reorient inclined stems, also has a higher lignin content than normal wood. In the present study, Chamaecyparis obtusa Endl. saplings were grown on an incline and the following variables were tracked for 10 weeks: posture recovery of the saplings; development of xylem tissue on the lower side of inclined stems; and the transcription level of laccase. The posture of saplings approached vertical after 8 weeks, the development of compression wood reached a peak around 6 weeks and laccase transcription was the highest after 4 weeks. These results suggest a sequence of righting mechanisms. Inclination stimulates an increase in the abundance of laccase transcript and this increase encourages the formation of compression wood. The accumulation of compression wood then causes the stem to bend upward.

Accelerating the domestication of forest trees in a changing world

In light of impending water and arable land shortages, population growth and climate change, it is more important than ever to examine how forest tree domestication can be accelerated to sustainably meet future demands for wood, biomass, paper, fuel and biomaterials. Because of long breeding cycles, tree domestication cannot be rapidly achieved through traditional genetic improvement methods alone. Integrating modern genetic and genomic techniques with conventional breeding will expedite tree domestication. Breeders will only embrace these technologies if they are cost-effective and readily accessible, and forest landowners will only adopt end-products that meet with regulatory approval and public acceptance. All parties involved must work together to achieve these objectives for the benefit of society.