Wood Formation under Severe Drought Invokes Adjustment of the Hormonal and Transcriptional Landscape in Poplar

SCPL48 regulates the vessel cell programmed cell death during xylem development in Arabidopsis thaliana

Secondary cell wall (SCW) deposition is tightly coordinated with programmed cell death (PCD) during xylem development and plays a crucial role in plant stress responses. In this study, we characterized a serine carboxypeptidase-like gene, SCPL48, which exhibits xylem cell-specific expression patterns in stem xylem during vascular development. The scpl48 plants exhibited reduced stem xylem cell numbers, particularly vessel cells, accompanied by delayed organelle degradation during PCD and increased secondary wall thickness in xylem vessel cells. In contrast, SCPL48 overexpression resulted in increased vessel cell abundance and accelerated degradation of vessel cell contents, suggesting its critical role in xylem vessel cell differentiation. Notably, SCPL48 expression was significantly up-regulated in response to ABA treatment and drought stress, with SCPL48 overexpression lines demonstrating enhanced drought resistance. Further molecular analyses revealed that SCPL48 was directly targeted and transcriptionally activated by key SCW regulators VND6, and MYB46. Furthermore, the expression of PCD-related protease genes, including XCP1, XSP1, RNS3, MC9, γVPE, and CEP1, showed compensatory changes in both scpl48 mutants and SCPL48 overexpression lines. Collectively, our findings demonstrate that SCPL48 functions as a key regulator in xylem vessel cell differentiation, PCD and drought stress responses.

Tree drought physiology: critical research questions and strategies for mitigating climate change effects on forests

Droughts of increasing severity and frequency are a primary cause of forest mortality associated with climate change. Yet, fundamental knowledge gaps regarding the complex physiology of trees limit the development of more effective management strategies to mitigate drought effects on forests. Here, we highlight some of the basic research needed to better understand tree drought physiology and how new technologies and interdisciplinary approaches can be used to address them. Our discussion focuses on how trees change wood development to mitigate water stress, hormonal responses to drought, genetic variation underlying adaptive drought phenotypes, how trees 'remember' prior stress exposure, and how symbiotic soil microbes affect drought response. Next, we identify opportunities for using research findings to enhance or develop new strategies for managing drought effects on forests, ranging from matching genotypes to environments, to enhancing seedling resilience through nursery treatments, to landscape-scale monitoring and predictions. We conclude with a discussion of the need for co-producing research with land managers and extending research to forests in critical ecological regions beyond the temperate zone.

Different ectomycorrhizal fungal species impact poplar growth but not phosphorus utilization under low P supply

Tree growth is often limited by phosphorus (P) availability. The trade-off between P homeostasis and growth is unknown. Ectomycorrhizal fungi (EMF) facilitate P availability but this trait varies among different fungal species and isolates. Here, we tested the hypotheses that (i) colonization with EMF boosts plant growth under P-limited conditions and that (ii) the poplars show P homeostasis because increased P uptake is used for growth and not for P accumulation in the tissues. We used two P treatments (high phosphate [HP]: 64 μM Pi, low phosphate [LP]: 0.64 μM Pi in the nutrient solution) and four fungal treatments (Paxillus involutus MAJ, Paxillus involutus NAU, Laccaria bicolor dikaryon LBD, Laccaria bicolor monokaryon LBM) in addition to non-inoculated poplar plants (NI) to measure growth, biomass, gas exchange and P contents. High phosphate (HP) stimulated growth compared with LP conditions. Poplars colonized with MAJ, NAU and NI showed higher growth and biomass production than those with LBD or LBM. Photosynthesis rates of poplars with lower biomass production were similar to or higher than those of plants with higher growth rates. The tissue concentrations of P were higher under HP than LP conditions and rarely affected by ectomycorrhizal colonization. Under LP, the plants produced 44% greater biomass per unit of P than under HP. At a given P supply, the tissue concentration was stable irrespective of the growth rate indicating P homeostasis. Laccaria bicolor caused growth inhibition, irrespective of P availability. These results suggest that in young poplars distinct species-specific ectomycorrhizal traits overshadowed potential growth benefits.

Xylem cell size regulation is a key adaptive response to water deficit in Eucalyptus grandis

Future climatic scenarios forecast increasingly frequent droughts that will pose substantial consequences on tree mortality. In light of this, drought-tolerant eucalypts have been propagated; however, the severity of these conditions will invoke adaptive responses, impacting the commercially valuable wood properties. To determine what mechanisms govern the wood anatomical adaptive response, highly controlled drought experiments were conducted in Eucalyptus grandis W. Hill ex Maiden, with the tree physiology and transcriptome closely monitored. In response to water deficit, E. grandis displays an isohydric stomatal response to conserve water and enable stem growth to continue, albeit at a reduced rate. Maintaining gaseous exchange is likely a critical short-term response that drives the formation of hydraulically safer xylem. For instance, the development of significantly smaller fibers and vessels was found to increase cellular density, thereby promoting drought tolerance through improved functional redundancy, as well as implosion and cavitation resistance. The transcriptome was explored to identify the molecular mechanisms responsible for controlling xylem cell size during prolonged water deficit. Downregulation of genes associated with cell wall remodeling and the biosynthesis of cellulose, hemicellulose and pectin appeared to coincide with a reduction in cellular enlargement during drought. Furthermore, transcript levels of NAC and MYB transcription factors, vital for cell wall component biosynthesis, were reduced, while those linked to lignification increased. The upregulation of EgCAD and various peroxidases under water deficit did not correlate with an increased lignin composition. However, with the elevated cellular density, a higher lignin content per xylem cross-sectional area was observed, potentially enhancing hydraulic safety. These results support the requirement for higher density, drought-adapted wood as a long-term adaptive response in E. grandis, which is largely influenced by the isohydric stomatal response coupled with cellular expansion-related molecular processes.

Integrity of xylan backbone affects plant responses to drought

Drought is a major factor affecting crops, thus efforts are needed to increase plant resilience to this abiotic stress. The overlapping signaling pathways between drought and cell wall integrity maintenance responses create a possibility of increasing drought resistance by modifying cell walls. Here, using herbaceous and woody plant model species, Arabidopsis and hybrid aspen, respectively, we investigated how the integrity of xylan in secondary walls affects the responses of plants to drought stress. Plants, in which secondary wall xylan integrity was reduced by expressing fungal GH10 and GH11 xylanases or by affecting genes involved in xylan backbone biosynthesis, were subjected to controlled drought while their physiological responses were continuously monitored by RGB, fluorescence, and/or hyperspectral cameras. For Arabidopsis, this was supplemented with survival test after complete water withdrawal and analyses of stomatal function and stem conductivity. All Arabidopsis xylan-impaired lines showed better survival upon complete watering withdrawal, increased stomatal density and delayed growth inhibition by moderate drought, indicating increased resilience to moderate drought associated with modified xylan integrity. Subtle differences were recorded between xylan biosynthesis mutants (irx9, irx10 and irx14) and xylanase-expressing lines. irx14 was the most drought resistant genotype, and the only genotype with increased lignin content and unaltered xylem conductivity despite its irx phenotype. Rosette growth was more affected by drought in GH11- than in GH10-expressing plants. In aspen, mild downregulation of GT43B and C genes did not affect drought responses and the transgenic plants grew better than the wild-type in drought and well-watered conditions. Both GH10 and GH11 xylanases strongly inhibited stem elongation and root growth in well-watered conditions but growth was less inhibited by drought in GH11-expressing plants than in wild-type. Overall, plants with xylan integrity impairment in secondary walls were less affected than wild-type by moderately reduced water availability but their responses also varied among genotypes and species. Thus, modifying the secondary cell wall integrity can be considered as a potential strategy for developing crops better suited to withstand water scarcity, but more research is needed to address the underlying molecular causes of this variability.

A systems genetic analysis identifies putative mechanisms and candidate genes regulating vessel traits in poplar wood

Wood is the water conducting tissue of tree stems. Like most angiosperm trees, poplar wood contains water-conducting vessel elements whose functional properties affect water transport and growth rates, as well as susceptibility to embolism and hydraulic failure during water stress and drought. Here we used a unique hybrid poplar pedigree carrying genomically characterized chromosomal insertions and deletions to undertake a systems genomics analysis of vessel traits. We assayed gene expression in wood forming tissues from clonal replicates of genotypes covering dosage quantitative trait loci with insertions and deletions, genotypes with extreme vessel trait phenotypes, and control genotypes. A gene co-expression analysis was used to assign genes to modules, which were then used in integrative analyses to identify modules associated with traits, to identify putative molecular and cellular processes associated with each module, and finally to identify candidate genes using multiple criteria including dosage responsiveness. These analyses identified known processes associated with vessel traits including stress response, abscisic acid and cell wall biosynthesis, and in addition identified previously unexplored processes including cell cycle and protein ubiquitination. We discuss our findings relative to component processes contributing to vessel trait variation including signaling, cell cycle, cell expansion, and cell differentiation.

An open-source machine-learning approach for obtaining high-quality quantitative wood anatomy data from E. grandis and P. radiata xylem

Quantitative wood anatomy is a subfield in dendrochronology that requires effective open-source image analysis tools. In this research, the bioimage analysis software QuPath (v0.4.4) is introduced as a candidate for accurately quantifying the cellular properties of the xylem in an automated manner. Additionally, the potential of QuPath to detect the transition of early- to latewood tracheids over the growing season was evaluated to assess a potential application in dendroecological studies. Various algorithms in QuPath were optimized to quantify different xylem cell types in Eucalyptus grandis and the transition of early- to latewood tracheids in Pinus radiata. These algorithms were coded into cell detection scripts for automatic quantification of stem microsections and compared to a manually curated method to assess the accuracy of the cell detections. The automatic cell detection approach, using QuPath, has been validated to be reproducible with an acceptable error when assessing fibers, vessels, early- and latewood tracheids. However, further optimization for parenchyma is still required. This proposed method developed in QuPath provides a scalable and accurate approach for quantifying anatomical features in stem microsections. With minor amendments to the detection and classification algorithms, this strategy is likely to be viable in other plant species.

Integrated physiological, metabolomic, and transcriptomic analyses elucidate the regulation mechanisms of lignin synthesis under osmotic stress in alfalfa leaf (Medicago sativa L.)

Alfalfa, an essential forage crop known for its high yield, nutritional value, and strong adaptability, has been widely cultivated worldwide. The yield and quality of alfalfa are frequently jeopardized due to environmental degradation. Lignin, a constituent of the cell wall, enhances plant resistance to abiotic stress, which often causes osmotic stress in plant cells. However, how lignin responds to osmotic stress in leaves remains unclear. This study explored the effects of osmotic stress on lignin accumulation and the contents of intermediate metabolites involved in lignin synthesis in alfalfa leaves. Osmotic stress caused an increase in lignin accumulation and the alteration of core enzyme activities and gene expression in the phenylpropanoid pathway. We identified five hub genes (CSE, CCR, CADa, CADb, and POD) and thirty edge genes (including WRKYs, MYBs, and UBPs) by integrating transcriptome and metabolome analyses. In addition, ABA and ethylene signaling induced by osmotic stress regulated lignin biosynthesis in a contradictory way. These findings contribute to a new theoretical foundation for the breeding of high-quality and resistant alfalfa varieties.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

The AP2/ERF transcription factor PtoERF15 confers drought tolerance via JA-mediated signaling in Populus

Drought stress is one of the major limiting factors for the growth and development of perennial trees. Xylem vessels act as the center of water conduction in woody species, but the underlying mechanism of its development and morphogenesis under water-deficient conditions remains elucidation. Here, we identified and characterized an osmotic stress-induced ETHYLENE RESPONSE FACTOR 15 (PtoERF15) and its target, PtoMYC2b, which was involved in mediating vessel size, density, and cell wall thickness in response to drought in Populus tomentosa. PtoERF15 is preferentially expressed in differentiating xylem of poplar stems. Overexpression of PtoERF15 contributed to stem water potential maintaining, thus promoting drought tolerance. RNA-Seq and biochemical analysis further revealed that PtoERF15 directly regulated PtoMYC2b, encoding a switch of JA signaling pathway. Additionally, our findings verify that three sets of homologous genes from NAC (NAM, ATAF1/2, and CUC2) gene family: PtoSND1-A1/A2, PtoVND7-1/7-2, and PtoNAC118/120, as the targets of PtoMYC2b, are involved in the regulation of vessel morphology in poplar. Collectively, our study provides molecular evidence for the involvement of the PtoERF15-PtoMYC2b transcription cascade in maintaining stem water potential through the regulation of xylem vessel development, ultimately improving drought tolerance in poplar.

Recent genome resequencing paraded COBRA-Like gene family roles in abiotic stress and wood formation in Poplar

A cell wall determines the mechanical properties of a cell, serves as a barrier against plant stresses, and allows cell division and growth processes. The COBRA-Like (COBL) gene family encodes a putative glycosylphosphatidylinositol (GPI)-anchored protein that controls cellulose deposition and cell progression in plants by contributing to the microfibril orientation of a cell wall. Despite being studied in different plant species, there is a dearth of the comprehensive global analysis of COBL genes in poplar. Poplar is employed as a model woody plant to study abiotic stresses and biomass production in tree research. Improved genome resequencing has enabled the comprehensive exploration of the evolution and functional capacities of PtrCOBLs (Poplar COBRA-Like genes) in poplar. Phylogeny analysis has discerned and classified PtrCOBLs into two groups resembling the Arabidopsis COBL family, and group I genes possess longer proteins but have fewer exons than group II. Analysis of gene structure and motifs revealed PtrCOBLs maintained a rather stable motif and exon-intron pattern across members of the same group. Synteny and collinearity analyses exhibited that the evolution of the COBL gene family was heavily influenced by gene duplication events. PtrCOBL genes have undergone both segmental duplication and tandem duplication, followed by purifying selection. Promotor analysis flaunted various phytohormone-, growth- and stress-related cis-elements (e.g., MYB, ABA, MeJA, SA, AuxR, and ATBP1). Likewise, 29 Ptr-miRNAs of 20 families were found targeting 11 PtrCOBL genes. PtrCOBLs were found localized at the plasma membrane and extracellular matrix, while gene ontology analysis showed their involvement in plant development, plant growth, stress response, cellulose biosynthesis, and cell wall biogenesis. RNA-seq datasets depicted the bulk of PtrCOBL genes expression being found in plant stem tissues and leaves, rendering mechanical strength and rejoinders to environmental cues. PtrCOBL2, 3, 10, and 11 manifested the highest expression in vasculature and abiotic stress, and resemblant expression trends were upheld by qRT-PCR. Co-expression network analysis identified PtrCOBL2 and PtrCOBL3 as hub genes across all abiotic stresses and wood developing tissues. The current study reports regulating roles of PtrCOBLs in xylem differentiating tissues, tension wood formation, and abiotic stress latency that lay the groundwork for future functional studies of the PtrCOBL genes in poplar breeding.

Transcriptional reprogramming during recovery from drought stress in Eucalyptus grandis

The importance of drought as a constraint to agriculture and forestry is increasing with climate change. Genetic improvement of plants' resilience is one of the mitigation strategies to curb this threat. Although recovery from drought stress is important to long-term drought adaptation and has been considered as an indicator of dehydration tolerance in annual crops, this has not been well explored in forest trees. Thus, we aimed to investigate the physiological and transcriptional changes during drought stress and rewatering in Eucalyptus grandis W. Hill ex Maiden. We set up a greenhouse experiment where we imposed drought stress on 2-year-old seedlings and rewatered the recovery group after 17 days of drought. Our measurement of leaf stomatal conductance (gs) showed that, while gs was reduced by drought stress, it fully recovered after 5 days of rewatering. The RNA-seq analysis from stem samples revealed that genes related to known stress responses such as phytohormone and reactive oxygen species signaling were upregulated, while genes involved in metabolism and growth were downregulated due to drought stress. We observed reprogramming of signal transduction pathways and metabolic processes at 1 day of rewatering, indicating a quick response to rewatering. Our results suggest that recovery from drought stress may entail alterations in the jasmonic acid, salicylic acid, ethylene and brassinosteroid signaling pathways. Using co-expression network analysis, we identified hub genes, including the putative orthologs of ABI1, ABF2, ABF3, HAI2, BAM1, GolS2 and SIP1 during drought and CAT2, G6PD1, ADG1 and FD-1 during recovery. Taken together, by highlighting the molecular processes and identifying key genes, this study gives an overview of the mechanisms underlying the response of E. grandis to drought stress and recovery that trees may face repeatedly throughout their long life cycle. This provides a useful reference to the identification and further investigation of signaling pathways and target genes for future tree improvement.

Responses to Drought Stress in Poplar: What Do We Know and What Can We Learn?

Poplar (Populus spp.) is a high-value crop for wood and biomass production and a model organism for tree physiology and genomics. The early release, in 2006, of the complete genome sequence of P. trichocarpa was followed by a wealth of studies that significantly enriched our knowledge of complex pathways inherent to woody plants, such as lignin biosynthesis and secondary cell wall deposition. Recently, in the attempt to cope with the challenges posed by ongoing climate change, fundamental studies and breeding programs with poplar have gradually shifted their focus to address the responses to abiotic stresses, particularly drought. Taking advantage from a set of modern genomic and phenotyping tools, these studies are now shedding light on important processes, including embolism formation (the entry and expansion of air bubbles in the xylem) and repair, the impact of drought stress on biomass yield and quality, and the long-term effects of drought events. In this review, we summarize the status of the research on the molecular bases of the responses to drought in poplar. We highlight how this knowledge can be exploited to select more tolerant genotypes and how it can be translated to other tree species to improve our understanding of forest dynamics under rapidly changing environmental conditions.

How Tree Decline Varies the Anatomical Features in Quercus brantii

Drought has serious effects on forests, especially semi-arid and arid forests, around the world. Zagros Forest in Iran has been severely affected by drought, which has led to the decline of the most common tree species, Persian oak (Quercus brantii). The objective of this study was to determine the effects of drought on the anatomical structure of Persian oak. Three healthy and three declined trees were sampled from each of two forest sites in Ilam Forest. Discs were cut at breast height, and three sapwood blocks were taken near the bark of each tree for sectioning. The anatomical characteristics measured included fiber length (FL), fiber wall thickness (FWT), number of axial parenchymal cells (NPC), ray number (RN), ray width (RW), and number of calcium oxalate crystals. Differences between healthy and declined trees were observed in the abundance of NPC and in RN, FL, and FWT, while no differences occurred in the number of oxalate crystals. The decline had uncertain effects on the FL of trees from sites A and B, which showed values of 700.5 and 837.3 μm compared with 592.7 and 919.6 μm in healthy trees. However, the decline resulted in an increase in the FWT of trees from sites A and B (9.33 and 11.53 μm) compared with healthy trees (5.23 and 9.56 μm). NPC, RN, and RW also increased in declined individuals from sites A and B (28.40 and 28.40 mm−1; 41.06 and 48.60 mm−1; 18.60 and 23.20 μm, respectively) compared with healthy trees (20.50 and 19.63 mm−2; 31.60 and 28.30 mm−2; 17.93 and 15.30 μm, respectively). Thus, drought caused measurable changes in the anatomical characteristics of declined trees compared with healthy trees.

Effect of Elevated CO2 and Drought on Biomass, Gas Exchange and Wood Structure of Eucalyptus grandis

Juvenile Eucalyptus grandis were exposed to drought and elevated CO₂ to evaluate the independent and interactive effects on growth, gas exchange and wood structure. Trees were grown in a greenhouse at ambient and elevated CO₂ (aCO₂, 410 ppm; eCO₂, 950 ppm), in combination with daily irrigation and cyclic drought during one growing season. The results demonstrated that drought stress limited intercellular CO₂ concentration, photosynthesis, stomatal conductance, and transpiration, which correlated with a lower increment in height, stem diameter and biomass. Drought also induced formation of frequent and narrow vessels accompanied by a reduction in vessel lumen area. Conversely, elevated CO₂ increased intercellular CO₂ concentration as well as photosynthesis, and partially closed stomata, leading to a more efficient water use, especially under drought. There was a clear trend towards greater biomass accumulation at eCO₂, although the results did not show statistical significance for this parameter. We observed an increase in vessel diameter and vessel lumen area at eCO₂, and, contrarily, the vessel frequency decreased. Thus, we conclude that eCO₂ delayed the effects of drought and potentialized growth. However, results on vessel anatomy suggest that increasing vulnerability to cavitation due to formation of larger vessels may counteract the beneficial effects of eCO₂ under severe drought.

Salinity induces discontinuous protoxylem via a DELLA-dependent mechanism promoting salt tolerance in Arabidopsis seedlings

Salinity is detrimental to plants and developmental adjustments limiting salt uptake and transport is therefore important for acclimation to high salt. These parameters may be influenced by xylem morphology, however how plant root xylem development is affected by salt stress remains unclear. Using molecular and genetic techniques and detailed phenotypic analyses, we demonstrate that salt causes distinct effects on Arabidopsis seedling root xylem and reveal underlying molecular mechanisms. Salinity causes intermittent inhibition of protoxylem cell differentiation, generating protoxylem gaps, in Arabidopsis and several other eudicot seedlings. The extent of protoxylem gaps in seedlings positively correlates with salt tolerance. Reduced gibberellin signalling is required for protoxylem gap formation. Mutant analyses reveal that the xylem differentiation regulator VASCULAR RELATED NAC DOMAIN 6 (VND6), along with secondary cell wall producing and cell wall modifying enzymes, including EXPANSIN A1 (EXP1), are involved in protoxylem gap formation, in a DELLA-dependent manner. Salt stress is likely to reduce levels of bioactive gibberellins, stabilising DELLAs, which in turn activates multiple factors modifying protoxylem differentiation. Salt stress impacts seedling survival and formation of protoxylem gaps may be a measure to enhance salt tolerance.

Multi-omics analysis of xylem sap uncovers dynamic modulation of poplar defenses by ammonium and nitrate

Xylem sap is the major transport route for nutrients from roots to shoots. In the present study, we investigated how variations in nitrogen (N) nutrition affected the metabolome and proteome of xylem sap and the growth of the xylem endophyte Brennaria salicis, and we also report transcriptional re-wiring of leaf defenses in poplar (Populus × canescens). We supplied poplars with high, intermediate or low concentrations of ammonium or nitrate. We identified 288 unique proteins in xylem sap. Approximately 85% of the xylem sap proteins were shared among ammonium- and nitrate-supplied plants. The number of proteins increased with increasing N supply but the major functional categories (catabolic processes, cell wall-related enzymes, defense) were unaffected. Ammonium nutrition caused higher abundances of amino acids and carbohydrates, whereas nitrate caused higher malate levels in xylem sap. Pipecolic acid and N-hydroxy-pipecolic acid increased, whereas salicylic acid and jasmonoyl-isoleucine decreased, with increasing N nutrition. Untargeted metabolome analyses revealed 2179 features in xylem sap, of which 863 were differentially affected by N treatments. We identified 124 metabolites, mainly from specialized metabolism of the groups of salicinoids, phenylpropanoids, phenolics, flavonoids, and benzoates. Their abundances increased with decreasing N, except coumarins. Brennaria salicis growth was reduced in nutrient-supplemented xylem sap of low- and high- NO₃^- -fed plants compared to that of NH₄⁺ -fed plants. The drastic changes in xylem sap composition caused massive changes in the transcriptional landscape of leaves and recruited defenses related to systemic acquired and induced systemic resistance. Our study uncovers unexpected complexity and variability of xylem composition with consequences for plant defenses.

Chemical Composition and Related Properties of Lime (Tilia cordata Mill.) Bark and Wood as Affected by Tree Growth Conditions

Tilia cordata Mill. is a favourite tree used in urban spaces. For this reason, it is important to know its sensitivity to environmental stress, which is particularly burdensome for vegetation in urban spaces. The aim of the study was to investigate the properties necessary to control the growth of these trees and their subsequent use, i.e., chemical properties (percentage contents of cellulose, holocellulose, lignin, pentosans and substances soluble in NaOH and EtOH) as well as the chemical elements (K, Na, Mg, Ca and Fe, Zn, Cu, Pb, Cd, B, Ni, Cr, Al, As and Hg) and selected hygroscopic properties (hysteresis and sorption isotherms). Trees of Tilia cordata Mill. growing in environments exposed to environmental stress of varying severity were examined. Regardless of the growth conditions, in terms of its chemical composition, bark differs significantly from wood, showing twice the contents of soluble substances in NaOH and lignin and half the content of polysaccharides. Growth conditions clearly affect the range of selected chemical components in bark, e.g., substances soluble in ethanol, cellulose, or lignin. The main inorganic elements in bark and wood are Na, K, Ca, Mg and Zn. In bark, a relationship was found between the content of most chemical elements and differing environmental growth conditions. It was shown that environmental stress influenced the hygroscopic properties of wood and bark, which are a consequence of the percentage of chemical components.

Water Stress Differentially Modulates the Expression of Tomato Cell Wall Metabolism-Related Genes in Meloidogyne incognita Feeding Sites

Microscopic observations and transcriptomic RNA-Seq analyses were applied to investigate the effect of water stress during the formation of tomato galls formation 1 and 2 weeks after inoculation with the root-knot nematode Meloidogyne incognita. Water stress affected root growth and the nematode ability to mount an efficient parasitism. The effects of water stress on the feeding site development were already observed at 1 week after nematode inoculation, with smaller giant cells, delayed development, and thinner cell walls. These features suggested changes in the expression levels of genes involved in the feeding site formation and maintenance. Gene Ontology (GO) enrichment and expression patterns were used to characterize differentially expressed genes. Water stress modified the expression profile of genes involved in the synthesis, degradation, and remodeling of the cell wall during the development of nematode feeding site. A comparison of gene expression with unstressed galls revealed that water stress intensified the up or downregulation of most genes. However, it particularly influenced the expression pattern of expansin A11 (Solyc04g081870.4.1), expansin-like B1(Solyc08g077910.3.1), a pectin acetylesterase (Solyc08g005800.4.1), and the pectin methylesterase pmeu1 (Solyc03g123630.4.1) which were upregulated in unstressed galls and repressed by water stress, at both sampling times. The expression of most members of the genes involved in cell wall metabolism, i.e., those coding for Csl, fasciclin, and COBRA proteins, were negatively influenced. Interestingly, alteration in the expression profiles of most dirigent protein genes (DIRs) and upregulation of five gene coding for Casparian strip domain protein (CASP)-like proteins were found. Gene expression analysis of galls from water stressed plants allowed us to better understand the molecular basis of M. incognita parasitism in tomato. Specific genes, including those involved in regulation of cellulose synthesis and lignification process, require further study to develop defense strategies against root-knot nematodes.

Response of Poplar Leaf Transcriptome to Changed Management and Environmental Conditions in Pure and Mixed with Black Locust Stands

Mixed cropping in short rotation coppice can be an alternative to monocultures. To design optimized mixtures, field trials are needed. Poplar, as an economically important and fast-growing species, and black locust, as a nitrogen-fixing species, are promising candidates for such studies. RNA sequencing (RNA-seq) was used to monitor effects of mixed and pure cultivations on the gene expression of poplar along with growth measurements during 2017 and 2018. Both biomass production and leaf transcriptomes revealed a strong competition pressure of black locust and the abiotic environment on poplar trees. Gene expression differed between the two study sites and pure and mixed stands. Shading effects from black locust caused the downregulation of photosynthesis and upregulation of shade avoidance genes in mixed stands in 2017. As a result of higher light availability after cutting black locust, plant organ development genes were upregulated in mixed stands in 2018. Drought conditions during the summer of 2018 and competition for water between the two species caused the upregulation of drought stress response genes in mixed stands and at the unfavorable growing site. Further investigations are required to discover the mechanisms of interspecific competition and to develop stand designs, which could increase the success and productivity of mixed plantations.