From the micro to the macro – a panoptic lens on wood biology

The field of wood biology, and more generally tree biology, is gaining impetus for multiple reasons – society is becoming increasingly aware of the importance of vascular plants (trees especially) in both sequestering carbon, but also forming the foundation of our daily lives (e.g. fuel, fibre, structural components, speciality chemicals and polymers). As such, there is a pressing need to develop renewable biomass to replace our fossil fuel-dependent economies with sustainable, more circular economies. Wood is also the water conducting tissue of trees, and plays a fundamental role in how trees respond to their environment, including drought and heat stress associated with climate change. Ultimately, wood biology must integrate information from multiple disciplines to address fascinating, but highly complex processes underlying traits that are critical for the conservation and management of forests and the overall landscape. The 42nd New Phytologist Symposium ‘The biology of wood: from cell to trees’ was designed with the explicit aim of uniting diverse specialists, whose academic paths may not typically cross, but who are all equally passionate about wood biology and its relevance to plant biology (https://www.newphytologist.org/symposia/42). Surrounded by aspens and conifers in the tranquil resort of Granlibakken, Lake Tahoe, the format and feel of the meeting encouraged as much informal discussion and interactions as it did thought-provoking presentations of state-of-the-art, cutting-edge research. Although the programme was divided into four broad topics, evidence of methodological depth and diversity within each speaker's research was encouraging in itself. Students and early-career researchers were encouraged to participate, and were of exceptional quality as evidenced by consistently buzzing poster sessions. Based on the early-scholar enthusiasm, passion and command of the scientific fields, there is indeed much to look forward to, and the future of the field is in good hands. The intangibles of these meetings cannot be under-estimated, and in many ways are the most important outcomes. Informally, discussions focused on the need for more interdisciplinary collaborations, ensuring the continuity of scarce skills or resources, how to increase diversity in the community, and attract more academics in related fields to specifically focus on xylem (wood) formation in trees. Several themes emerged at this meeting that traversed the scales of cell, tissue, organ, tree, population, and ecosystem (Fig. 1). Understanding the evolutionary origins and diversification of mechanisms regulating wood development is of both basic and applied importance. Taku Demura (Nara Institute of Science and Technology (NAIST), Japan) presented significant strides in updating the transcriptional regulatory machinery for key functions of xylem and phloem formation in the model plant Arabidopsis, and these findings are beginning to be examined in tree development. Inducible model systems in Arabidopsis have been developed using hormones and/or transgenes, which allow simultaneous and coordinated differentiation of cells into tracheary elements. Particularly promising for species of both commercial interest and evolutionary developmental (evo-devo) research is the potential transfer of hormonal induction of tracheary elements in leaves to nonmodel plant species. Programmed cell death (PCD) is a fundamental process involving key cell types of wood, fibres and tracheary elements. Providing an impressive cross-species comparison between Arabidopsis, Populus and Picea, Hannele Tuominen (Umeä Plant Science Centre, Sweden) gave an overview of PCD, highlighting recent work illustrating the importance of meta-caspase 9 in processing cysteine proteases for PCD in both Arabidopsis and Populus. New evidence suggests a role for several small peptides in PCD, which have traditionally been difficult to study and characterize but are increasingly proving important in multiple biological processes. In parallel, Dr Juan Du (Zhejiang University, China) described the role of a calcium-dependent DNAse that connects cell differentiation and PCD to wood development in poplar. Much of our knowledge of secondary xylem biology is dependent on model systems such as Arabidopsis, but important differences exist between how this herbaceous dicot makes secondary xylem and that produced by both gymnosperm and angiosperm trees. A notable example is the fact that some cells, such as interfascicular fibres in Arabidopsis, can have a distinct ontogeny or programme of differentiation from that of secondary xylem in trees. Existing woody models such as Populus or Eucalyptus are becoming more advanced, and newly explored nonmodel woody plants are becoming increasingly relevant in providing unique insights into wood formation. How carbon in its various forms is metabolized and localized is central to wood formation. Talks from Lacey Samuels (University of British Columbia, Canada) and Georgia Drakakaki (University of California, Davis, USA) demonstrated mechanistic insights gained from using freeze-fracture, fluorescence microscopy, and targeted subcellular component labelling and isolation techniques, into the endomembrane system's management of both primary and secondary polysaccharide synthesis. My own laboratory (University of Pretoria, South Africa) is exploring the regulation of plastid and mitochondrial biology during wood formation in Eucalyptus, elucidating the roles of the pentose phosphate and shikimate pathways on carbon partitioning between polysaccharides and phenylpropanoids. As a community, we are beginning to address long-standing questions on where, when, and how micro-compartmentation of metabolism is occurring within tissues and cells during wood formation. Zander Myburg (University of Pretoria) reported the mapping of thousands of expression quantitative trait loci (QTLs) in genetically segregating Eucalyptus hybrid populations related to cell wall synthesis, where genetical genomics is highlighting important pathways involved in xylogenesis, including the interplay among complex wood development and chemistry traits. In the context of metabolism, identifying and quantifying metabolic intermediates using candidate substrate-product pair (CSPP) networks is enabling a deeper understanding of global metabolism and significantly advancing the field of plant metabolomics as a whole, especially in the context of phenylpropanoid and lignin metabolism (Wout Boerjan, University of Ghent and Vlaams Instituut voor Biotechnologie (VIB), Belgium). Work from the Samuels laboratory (University of British Columbia, Canada) is providing new insight into how lignification is partially controlled via laccase protein localization, while recent work from both the Boerjan and Mansfield laboratories demonstrated the significant inherent plasticity in lignin deposition and highlighted unique and previously unconsidered opportunities for engineering novel traits into trees. Yaseen Mottiar (Mansfield laboratory, University of British Columbia, Canada) introduced to the symposium attendees and comprehensively examined Eastern Leatherwood (Dirca palustris) – clearly showing that this species has atypical lignification patterns and contains no lignin in the middle lamellae. This was one of several good examples of the importance of expanding the diversity of species we study. An evolutionary approach to xylem formation was also discussed. Missy Holbrook (Harvard University, Cambridge, MA, USA) discussed the complex structure–function relationships in different lineages and their ecological implications. In an innovative approach to both genetic and external perturbation to wood formation, Andrew Groover (US Forest Service and University of California, Davis, USA) is using gravitropic stimulus on the wood of multiple angiosperm lineage representatives, as well as unique Populus chromosomal indel mutants. Comparative transcriptomics across multiple dimensions (species; tension vs opposite wood; chromosomal perturbation vs wild type) is leading to unique insights into the evolution of wood development. At a cellular level, the Demura laboratory (NAIST) is testing homologues of cell identity proteins from more anciently diverging lineages such as mosses or liverworts, providing a broader context for these in all vascular plants, while Kelly Balmant (Kirst laboratory, University of Florida, Gainesville, USA) presented work on EVE, a protein demonstrably involved in vessel number and size in angiosperms – a gene and subsequent protein that appears to be of viral origin and been transferred to plants in early evolution. Through coordinated discussion groups and many informal conversations, several themes emerged repeatedly. First, understanding trees’ (especially via wood and secondary xylem) interaction with the environment is becoming of urgent importance, both in terms of biotic and abiotic stresses. Connie Millar's (US Forest Service) talk on climate change and mountain forests was particularly illuminating on this topic, and provoked important discussions about linking basic research to ongoing questions of forest management and conservation. Second, and related to this – the limitation of methodology and hitherto studied species means that we still have a fairly limited understanding of wood as a whole, since the focus is primarily on tracheids, fibres and vessel cells, and the cell walls of these cells. We do not yet have a deep knowledge of the biology of many other cell types and intercellular networks that occur in or are related to wood, including ray and axial parenchyma, as well as the diverse cell types that form secondary phloem and bark. In general, a greater understanding of cell contact points such as the middle lamellae, or symplastic and apoplastic transport systems (and the interactions between these) is of key relevance in integrating our understanding of xylem formation, especially in the context of climate change, and should be viewed as exciting areas of research for emerging researchers. An advantage of the reduction in cost of various ‘omics’ technologies and phenotyping methods is that many opportunities exist in traditional nonmodel species to emerge as models for these pressing questions. A more holistic exploration of these aspects, in terms of ecological, evolutionary, and anatomical studies of trees is of vital importance. Likewise, some valuable methodologies must be maintained in the field (and in some cases scaled up), as losing these would be of detriment to future tree and plant cell wall research. Lignin and polysaccharide two-dimensional (2D) nuclear magnetic resonance (NMR) methods, as well as polysaccharide enzyme-linked immunosorbent assay (ELISA) were highlighted in particular, but a general need for increasing accessibility, throughput and diversity of different types of wood analysis and tree species will be key to unlocking new discoveries. The increasing opportunities in phenotypic acquisition (e.g. drone and scanning technologies) will require more big data approaches such as machine learning. A broader engagement with phenomic tools across agricultural research may be of benefit to advancing the field. Uptake and maintenance of resources by the community is also important to accelerate research. These include bioinformatic resources for comparative genomics such as PLAZA and Phytozome, to multi-omics exploration platforms for trees available now on databases such as Plantgenie (http://plantgenie.org/) and BAR (http://bar.utoronto.ca/). Additionally, many clones from diverse natural tree populations are available for research – for example, Populus via University of British Columbia (Shawn Mansfield) and Oak Ridge National Laboratories (Gerald Tuskan), or University of California, Davis (Andrew Groover), and Eucalyptus via University of Pretoria (Zander Myburg) – representing natural, chromosomal indel or hybrid tree genotypes of interest, respectively. Wood biology is an exciting field of study, and provides in many ways an ideal system for studying development, evolution, ecology, and biotechnology. In the coming years we should pursue (and can anticipate) several integrative goals: first, bringing together the wealth of botanical, anatomical and adaptive trait research with more recently evolved technologies such as genomics, transcriptomics, metabolomics and phenomics. Second, building a more integrated mechanistic understanding of intracellular and intercellular metabolism, signalling and transport. Finally, and perhaps of most urgency, performing research on the earlier mentioned aspects in the context of trees as long-lived organisms in the face of rapid and unpredictable environmental change. There is an overarching need to encourage a new and diverse cadre of research leaders to advance the field of wood biology. The value of these symposia, in addition to aligning research focus areas and fostering new collaborations, are to identify gaps, comprehensively discuss the current understanding of the field, establish the possibility of employing methods in cross-discipline research, and expanding the pool of minds working together on these topical and pressing issues. In this regard, the 42nd New Phytologist Symposium was successful on all accounts. It is reasonable and encouraging to say a new community has been created, and all who attended look forward to seeing each other in a similar venue in the future. Thanks to Helen Pinfield-Wells, Holly Slater and New Phytologist Central Office, as well as the Granlibakken resort staff for immaculate organization and setting the tone for a superbly productive meeting. Also to Dr Greg Stock for his brilliant guest talk on the Geology of the Sierra Nevada and the Tahoe Basin, and (along with the guided fieldtrip) a masterful lesson on science communication and engagement. Finally, a thank you to Andrew Groover and Shawn Mansfield for putting together an insightful and provoking programme, and for bringing together this wonderful new combination of researchers and students passionate about wood development and formation.

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